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Guidance for the Design of a Reinforced Concrete Retaining Wall
5

Guidance for the Design of a Reinforced Concrete Retaining Wall

Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)
Hello everyone, please find attached a drawing of the retaining wall i am designing according to EC7 together with an excel file of my calculations.
Its a cantilever L-shaped reinforced concrete wall comprised of a stem and foundation slab.

Backfill soil is well graded gravel with unitweight = 20kN/m3 and angle of friction = 25degrees (conservative value)
Wall height is 4.20m, Height of soil retained by wall is 2.70meters.
Surcharge load = 10kPa, A seismic force has also been considered.

According to my calculations my problem here is that the foundation of the wall needs to be at least 1.80meters wide in order to get a safety factor of 0.94 against sliding, which i think its too much for this design.
So for a 2.70m hight soil i get a 1.80m wide foundation slab in order to resist sliding. Dont you guys think that This is a really expensive design? am i doing something wrong? or did i get tit right?

The excel file is sectioned as follows:

0. Retaining Wall Properties
1. Gross Pressure Method
2. Eurocode Comb 1
3. Eurocode Comb 2
4. Seismic
5. Bending Reinforcement
6. Deflection (not complete)

Can you guys please have a look at my work and help figure this out? does the wall foundation need to be that wide?

https://res.cloudinary.com/engineering-com/image/upload/v1514841058/tips/Retaining_Wall_Details_-_forum_lrme3i.pdf

https://res.cloudinary.com/engineering-com/raw/upload/v1514841074/tips/Retaining_Wall_Design_y9vhvs.ods

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

The proportions seem fine to me.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Interesting that the surcharge stops at the "virtual face". 25 degrees friction angle for the backfill seems way too small. 30 plus would be expected.

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)

CODE --> oldestguy

Interesting that the surcharge stops at the "virtual face" 

The surcharge load is not from compaction machinery, but from a building at 3.0m away from the wall, however i have pushed the surcharge load even nearer to the wall but it stops at the virtual face because in the Eurocode approach the surcharge load is never considered as favourable load in resisting sliding or overturning.

CODE --> oldestguy

25 degrees friction angle for the backfill seems way too small. 30 plus would be expected. 

I am being conservative with the angle of friction of the backfill because this project is something like a charity, if not enough money is collected then we will have to use a fine grained backfill which is cheaper than a good granular fill and ofcourse use a drainage blanket on the wall to help with drainage.

I tried my design with a 30 degree angle of friction for the backfill and I got a safety factor of 1.15 against sliding. Do you guys feel ok with such a safety factor according to Eurocode or do you think i should try increase the safety factor ?




RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

That building just beyond the footing edge certainly applies a load on that wall footing, so sliding is very unlikely. That thrust is not likely uniform with depth, but near zero at surface, a triangular more likely.. If that building is already there, how are you going to build the wall without some significant bracing to hold it during the excavation next to it? Such bracing may have to remain in place and affect the design of the wall. It may have to be underpinning of the building. Run a lab test on your proposed backfill and my bet is no test will come under 30 degrees, even ultra loose. A simple test is a loose pile and measure the slope (at the storage pile).

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Quote (kellez)

Am i doing something wrong?

Yes, you are. To start with... you are defining the loads, trying to comply with code, and designing the retaining wall in one grand combined step - a spreadsheet that is supposed to "do it all". This approach is obviously not be working... you tell us that you do not believe the spreadsheet's results.

I'll happy to share my views, one step at a time, on the details (both technical and procedural) of what to do about this... but only if you are interested. I don't want to waste my time or yours making comments that will be ignored. My first suggestion will be to forget the spreadsheet, at least for now.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)

Quote (oldestguy)

If that building is already there, how are you going to build the wall without some significant bracing to hold it during the excavation next to it?

the building is not already there.

1. first the retaining wall will be build around the perimeter of the plot.
2. the plot will then be filled with soil to raise the level of the plot.
3. then the building will be build on top of the added soil

Quote (oldetguy)

Run a lab test on your proposed backfill

how can i test a backfill soil?

this is what I asked in another thread (see link below) and i was told that i should use a standard value from a text and that there are no lab tests for backfilled soils

http://www.eng-tips.com/viewthread.cfm?qid=433781



RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)

Quote (SlideRuleEra)

This approach is obviously not be working... you tell us that you do not believe the spreadsheet's results.

The reason i do not believe the results of the spreadsheet is because am not that experienced, this is my first retaining wall design.
Also i have seen some other retaining wall designs being build with 2.0m backfill and have a foundation slab, of b = 1.20m my design is 2.70m backfill with b = 1.80m

Quote (SlideRuleEra)

I'll happy to share my views, one step at a time, on the details (both technical and procedural) of what to do about this... but only if you are interested. I don't want to waste my time or yours making comments that will be ignored. My first suggestion will be to forget the spreadsheet, at least for now.

As I said, i am not that experienced and this is my first retaining wall design, I would really appreciate it if you could share your views with me, i am here to learn, I am guessing you are an experienced engineer and i respect your knowledge,
please guide me on how you want to proceed

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

kellez - Thanks for sharing your retaining wall experience level, that will make the discussion easier. Yes, I've been practicing a while, 48 years, 43 of them as a PE. Most of that time in heavy industry. Mentored six young engineers, one at a time, for time periods of a few months to 10 years. They have turned out well - two are vice-presidents, two are in middle management, and two are senior staff.

Back to your problem... put the spreadsheet and other concerns aside for a while. You are trying to make everything top priority... low cost backfill, low cost retaining wall, "conservative" design values. Instead of looking for precise (but not necessarily accurate) answers to specific questions, look at how math models a "real world" retaining wall. The results of your own analysis will tell you which way to proceed.

Start with Ka, your "active earth pressure coefficient". The goal is to (realistically) make that value as low a practical... that means a "high" value of φ (good granular soil). What does having soil with "high" value of φ do for the calculations (in principal... not a detailed answer)?

1. Reduces thrust on the wall.

2. Look at the soil table PEinc provided on your other thread. Tends to have a heavier unit weight... providing more load on the wall's heel slab to resist sliding and overturning.

3. Reduces the surcharge loading from the building that is 3 meters behind the wall.

4. Reduces the soil mass that is affected by seismic acceleration.

5. Probably increases the soil friction resistance for sliding. I have seen this coefficient of friction taken as tan (2/3 φ).

That's enough for now, think about it. Do you want to continue?

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Some other things to ponder:

You've assumed the conservative soil parameters in case you can't afford the good backfill, but then your wall seems expensive so you can't afford the good backfill. This is a self-fulfilling prophecy. Try with the decent backfill parameters then check whether you can afford the wall.

Notwithstanding that comment, is your base width actually out of proportion? You cite a 1.2m base for a 2.0m high wall as another example: B/H=0.6. Your design is 1.8m base for 2.7m height: B/H=0.67. Not too different at all. What do you know about the other wall? Did it have access to good fill, lower design surcharge, other factors in its favour?

Some textbooks (eg Bowles) give B/H from 0.5 to 0.7 as a guide. You're within that range. My own experience from looking at old retaining walls is that many show signs of movement (leaning and sliding) to the point that I'd say they were 'under-designed', so I'm not too confident in the lower end of any old rules of thumb.

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)

Quote (SlideRuleEra)

Start with Ka, your "active earth pressure coefficient". The goal is to (realistically) make that value as low a practical... that means a "high" value of φ (good granular soil). What does having soil with "high" value of φ do for the calculations (in principal... not a detailed answer)?

I do understand how the angle of friction affects the retaining wall but to be honest it helps a lot when you describe it the way you do here, especially when you put it down on paper.
It helps a lot with thought processing and makes it easier to understand the design problem in hand.


1. Reduces thrust on the wall. I completely understand the soil mechanics behind this

2. Look at the soil table PEinc provided on your other thread. Tends to have a heavier unit weight... providing more load on the wall's heel slab to resist sliding and overturning. ofcourse, it seems logical

3. Reduces the surcharge loading from the building that is 3 meters behind the wall. again I completely understand the soil mechanics behind this, similar to reducing thrust on wall, mentioned above in point 1.

4. Reduces the soil mass that is affected by seismic acceleration. I do understand this as well, however this is not true when using the Seed and Whitman Method to calculate the seismic force, their equation does not consider the active earth pressure coefficient and also doesnt consider the angle of friction of the soil.

However the Peudo Static and Mononobe Okabe method do consider the angle of friction in their calculation



5. Probably increases the soil friction resistance for sliding. I have seen this coefficient of friction taken as tan (2/3 φ). Could you please elaborate? Do you mean wall friction value?



That's enough for now, think about it. Do you want to continue?


Ok, I have given this some thought and my main observations is that, the design of the wall is mainly affected by the type of wall used for the retaining wall (in this case reinforced concrete cantilever wall) and the type of soil used to backfill the wall.

Good granular fill obviously will reduce the forces on the wall and create a more economical design, it will also provide good drainage and avoid any hydrostatic pressures to build against the wall.

Ok, this is very clear in my mind right now, you have helped me to take a step back and view this design with a clearer mind, and take everything step by step.


Please advise me on how you want to proceed next.

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Quote:

5. Probably increases the soil friction resistance for sliding. I have seen this coefficient of friction taken as tan (2/3 φ). Could you please elaborate? Do you mean wall friction value?



See Page 5-48 of CalTrans Retaining Walls. I'm assuming the foundation soil has properties similar to backfill.

Concerning the next step:

1. Take oldestguy's advice to determine the angle of repose (friction angle) and the unit weight of the proposed backfill. I'll bet unit weight and is less than the assumed 20 kN/m3. Do this even if you know the results will not be precise... some information is better than a guess. Use the values obtained for the design. IMHO, the time to be conservative is by increasing the safety factor, not by designing using "wrong" material properties.

2. How did you compute the surcharge load on the wall? Revisit that calculation by applying the 10 kPA load where it is (beginning 3 m from the wall) not at an assumed closer location. My simplified calcs indicate the surcharge will have minimal effect on the wall. Ignore the surcharge load contribution for resistance to overturning and sliding.

3. On your other thread, BAretired mentioned that the proportions of the wall are not right; I agree. The proposed geometry is not efficient. The loads are too near the toe (Very little lever arm to resist overturning moment. Loads too close to the toe to get proper distribution of load on the soil supporting the retaining wall.) The resultant load from supporting soil needs to the in the middle third of slab. Reproportion wall shape/dimensions for more favorable loading characteristics. Iterate for an improved design, change the initial assumed dimensions based on intermediate calculations. Don't expect to be right the first time.



4. Buy and use CRSI Design Guide for Cantilevered Retaining Walls that BAretired recommended.
Alternatively download US Army Corps of Engineers - Retaining and Flood Walls.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)

Quote (SlideRuleEra)



See Page 5-48 of CalTrans Retaining Walls. I'm assuming the foundation soil has properties similar to backfill. Ok i get that, its the angle of friction between footing and foundation soil

Concerning the next step:

1. Take oldestguy's advice to determine the angle of repose (friction angle) and the unit weight of the proposed backfill. I'll bet unit weight and is less than the assumed 20 kN/m3. Do this even if you know the results will not be precise... some information is better than a guess. Use the values obtained for the design. IMHO, the time to be conservative is by increasing the safety factor, not by designing using "wrong" material properties. Ok, i will change all my values so that my design problem represents the real conditions as much as possible

2. How did you compute the surcharge load on the wall? Revisit that calculation by applying the 10 kPA load where it is (beginning 3 m from the wall) not at an assumed closer location. My simplified calcs indicate the surcharge will have minimal effect on the wall. Ignore the surcharge load contribution for resistance to overturning and sliding. I already ignored the surcharge load contribution for resistance to overturning and sliding.

Ok, I will recalculate my surcharge load, i think this is the main reason why my wall is overdesigned, due to the combination of the seismic load and surcharge load,
I still think its good to include a surcharge load near the wall which will represent compaction machinery, but i should not include this with the seismic force
Therefore my thought is to include the surcharge load of 10kPa exactly next to the wall for the compaction machinery but i should not include that in the seismic design.


3. On your other thread, BAretired mentioned that the proportions of the wall are not right; I agree. The proposed geometry is not efficient. The loads are too near the toe (Very little lever arm to resist overturning moment. Loads too close to the toe to get proper distribution of load on the soil supporting the retaining wall.) The resultant load from supporting soil needs to the in the middle third of slab. Reproportion wall shape/dimensions for more favorable loading characteristics. Iterate for an improved design, change the initial assumed dimensions based on intermediate calculations. Don't expect to be right the first time. I agree the alternative shape of wall you suggest is more efficient, however the reason i chose an inverse L-shaped wall and not an inverse T-shaped wall is because the wall is located exactly on the boundaries of the plot, therefore its not possible to extend my footing into the neighbouring plot, in addition i dont think its good idea to move the wall further inside the plot, its a waste of land. If my only coice is an inverse L-shaped wall what would be your suggestion to overcome this?

4. Buy and use CRSI Design Guide for Cantilevered Retaining Walls that BAretired recommended.
Alternatively download US Army Corps of Engineers - Retaining and Flood Walls. I will have a look in that

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Quote (kellez)

...my thought is to include the surcharge load of 10kPa exactly next to the wall for the compaction machinery...

I do not advise doing that. You will get back to very situation which prompted your original post: The wall appears overdesigned and you don't know why.

Apply the building's 10kPa load where it is... beginning 3 meters from the wall. Then you will know how much influence the permanent 10kPa load has on the wall.

There is no limit to other loads you can voluntarily superimpose. Apply a temporary construction (compaction) load adjacent to the wall... you are not "forced" to make it 10 kPA and can see what effect it has on the design. Also, the wall loading from compaction has different characteristics than a surcharge. Finally, the compaction loading is more of a point load (not a surcharge uniform distributed load) and it is temporary - a lower safety factor can be considered for a temporary load.

See "Pressure on Retaining Walls From Compaction Effort". In particular, look at "Figure 3".

Quote:

If my only choice is an inverse L-shaped wall what would be your suggestion to overcome this?

I understand your reason of the inverse L-shaped wall, looks like the way to go.
See if you can excavate some for the wall's foundation slab. Just a depth that is reasonable without causing problems... even, say, a 0.5 meter deep excavation will help. Good idea to embed the slab in soil to minimize possible erosion problems (believe you told us frost depth is not a concern). The primary reason is to increase the amount of backfill on top of the slab... more weight to resist overturning/sliding and the centroid of this weight is located to help keep the foundation resultant force in the middle 1/3 of the slab.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)

Quote (SlideRuleEra)

I do not advise doing that. You will get back to very situation which prompted your original post: The wall appears overdesigned and you don't know why.

Apply the building's 10kPa load where it is... beginning 3 meters from the wall. Then you will know how much influence the permanent 10kPa load has on the wall.

Ok i will apply the buildings surcharge load at 3 meters from the wall, which i have changed to 20kPa (i was wrong about the 10KPa).

I am looking into text books in order to find out how i can calculate the effect the surcharge load will have on the wall but i am having some difficulties. I have never tried to calculate this with a surcharge load that is at a certain distance from the wall.

I have looked into the CalTrans Retaining Walls paper (page 39, equation (5.5.5.10.2-2)) you posted above and found this equation (see photo attached). This equation is used for a Uniformly Loaded Strip Parallel to the Wall, however my building load is not actually a strip but an RC concrete slab (raft foundation) parallel to the wall (approx, 10m x 13m) , therefore do you think I could consider this as a very wide strip or do i need to use another equation?

The value i get is 8.17kN, which seems a bit high considering that the building is 3.0 away from the wall. Also i do not know at what height (point) this acts on the wall, if you want to have a look i have attached the excel file below, its only one calculation. Am i wrong using this equation?

https://res.cloudinary.com/engineering-com/raw/upload/v1515583756/tips/Surcharge_load_at_a_destance_x_from_wall_n7kqjx


RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Equation 5.5.5.10.2-3 tells you where the resultant force acts.

The ArcelorMittal piling handbook has an alternative formula. Worth checking a few different methods since they tend to be gross approximations, then picking a number that you're comfortable with. Another alternative might be good-old Coulomb wedges: change wedge angle to find the worst case.

Is the load 3m from the wall stem, or 3m from the heel/virtual face?

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)

Quote (steveh49)

Equation 5.5.5.10.2-3 tells you where the resultant force acts.
thats a big equation, thanks, i will try it out

so i will need to determine this in order to check if the pressure actually acts on the wall, it could be lower than the base of the wall?

Quote (steveh49)

Is the load 3m from the wall stem, or 3m from the heel/virtual face?
Its 3.0meters from the wall stem

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

You might need to use equation 5.5.5.10.2-1 to determine the distribution over the height of the wall then integrate to find the resultant force. Equations 5.5.5.10.2-2 and -3 might assume that the wall extends below the horizontal pressure zone.

For a sanity check, consider that your load starts just 1.2m behind your heel/virtual face (or 1.5m, have you changed the sketch since first posting?) and extends over a large area, so isn't too different to the 'full surcharge' case shown on the sketch in your original post - ie a surcharge that continues up to the heel/vurtual face. The 'full surcharge' load is Q*Kh*H = 20kPa*0.33*2.7m = 17.8kN/m for 30degree friction angle backfill. Your first calculation (8.17kN/m) is half of that. Is it really an unreasonable load that needs to be reduced further? How accurate is the calculation since it doesn't have any factors that represent the shear strength of the soil? Is it impossible for there to be any other surcharge between the building and wall in future?


RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)

Quote (steveh49)


The 'full surcharge' load is Q*Kh*H = 20kPa*0.33*2.7m = 17.8kN/m for 30degree friction angle backfill. Your first calculation (8.17kN/m) is half of that. Is it really an unreasonable load that needs to be reduced further? How accurate is the calculation since it doesn't have any factors that represent the shear strength of the soil? Is it impossible for there to be any other surcharge between the building and wall in future?

I dont think there will be any surcharge load in the future, however, first i would like to design my wall using the most accurate load cases that best describe the real life loads, and then i can add any additional loads i want.

No, the 8.17kN/m i calculated is not an unreasonable value, however, most importantly i need to determine at what height the surcharge load will act on the wall, which i think it will act below the base of the wall. It will probably act upon the footing/foundation of the wall however i will neglect that since its a favourable action against sliding and overturning.



RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)
I had a look in ArcelorMittal piling handbook and found another equation for determining the effect of a surcharge load on the wall, a lot simpler than the one found in CalTrans Retaining Walls the equation is shown below and can be found at page 137-139.

One observation is that again the equation is not using any soil material properties. The equation is shown below with an example



I used this equation in excel and these are my results

INPUT DATA:
Surcharge, q = 20.00kPa
Distance, d = 3.00m
Strip load width, b = 2.00m
Height of wall, h = 2.70m

First I calculated the Increasing Lateral Pressure, Δσv (kN) and then I sum up the values to get the resultant lateral pressure, Is this correct?

Another observation is that if a plot the force diagram of the resultant lateral pressure, it will be triangular, which i think its not realistic in such a load case.
do you guys think i should use the The Boussinesq Equation





RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Quote (kellez)

I have never tried to calculate this with a surcharge load that is at a certain distance from the wall.

Fine, now you are making progress. You want the mathematical model to "fit" the problem... not just be application of your existing skill set. But the solution does not have to always be complicated. Since this is your first retaining wall design, suggest starting with a semi-graphical solution to get an understanding of how the remote surcharge affects the wall. I like the simplified method shown and explained on pages 28 & 29 of Allan Block Retaining Wall Engineering Manual.


The Arcelor Mittal Piling Handbook that steveh49 mentioned has a similar approach - see Chapter 4.

For your wall, I get approximately 3.3 kN thrust (assuming the foundation slab is 0.5 meters thick).



Quote (kellez)

I would like to design my wall using the most accurate load cases that best describe the real life loads, and then I can add any additional loads I want.

Good, I have a couple of suggestions for additional minor loads to check. But you should have enough info now to make a first order iterative design. Let's see what you come up with.

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RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)
I am working on it, i will post my sketch in a bit

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

For external stability, it is usual to calculate the destabilising forces that act on the virtual face of the wall. In that case, your surcharge load is 1.2~1.5m away depending on what your current base width is. This would increase the load acting on the wall compared with assessing the load based on the 3m distance of the surcharge from the stem.

The Piling Handbook gives you the distribution of load vs depth below ground. You have to work out the height of the resultant by integration. For information, the diagram and equations from your previous post [now deleted] were under the heading "Concentrated and Linear Surcharge" in previous editions of the Piling Handbook.

From a few posts ago: "First I calculated the Increasing Lateral Pressure, Δσv (kN) and then I sum up the values to get the resultant lateral pressure, Is this correct?" This isn't correct. Δσv is in kPa; it's the vertical pressure at a point in the soil due to the surcharge and varies depending on the location of the point in question relative to the surcharge load. You multiply Δσv by the horizontal pressure coefficient to get the lateral pressure, then integrate the lateral pressure distribution to get the resultant total force and location of the resultant.

The current Piling Handbook appears to give a couple of different methods for assessing the lateral pressure resulting from the area surcharge, and the Allan Block method is different again. There will be other methods out there. I hope it's clear that these equations only give order-of-magnitude estimates of the load applied to the wall. If the load due to this surcharge is a significant part of the total horizontal load, tread carefully.



RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)

Quote (steveh49)

For external stability, it is usual to calculate the destabilising forces that act on the virtual face of the wall. In that case, your surcharge load is 1.2~1.5m away depending on what your current base width is. This would increase the load acting on the wall compared with assessing the load based on the 3m distance of the surcharge from the stem.

That is a valid point you made there, i will adjust my calculations and post my sketch. I need to take this step by step. I will show you my sketch to let me know if i am correct

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)
Ok this is where i am right now

I have used the equations in The Piling Handbook (see below) in order to determine how the load is distributed



Therefore, I calculated a, c and d (see below)



And this is the shape I have come up with

CASE 1: STRIP LOADING





CASE 2: INFINITE LOADING - THIS IS MORE REALISTIC FOR MY CASE SINCE THE BUILDING IS 10m wide




My thinking now is that I should first calculate the whole surcharge load as if it was exactly next to the wall

Active thrust due to surcharge: Pq = Ka x q x h = 0.33 x 20 x 2.70 = 18.0kN

Then i should calculate the surcharge load according to triangle B

Active thrust due to surcharge from Triangle B: Pq = Ka x q x a/2 = 0.33 x 20 x (0.98/2) = 3.28 kN

now subtract B from A

Active thrust due to surcharge from Shape A: 18.0kN - 3.28 kN = 14.72 kN

Therefore the total thrust of the surcharge load is 14.72 kN

Everyone agrees with this?



RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Quote (steveh49)

For external stability, it is usual to calculate the destabilizing forces that act on the virtual face of the wall. In that case, your surcharge load is 1.2~1.5m away depending on what your current base width is. This would increase the load acting on the wall compared with assessing the load based on the 3m distance of the surcharge from the stem.

Thank you steveh49, I did not know that. It makes sense.

kellez - The 14.7kN surcharge thrust sounds good to me... for an initial trial. Make the (preliminary) calcs for all the loads, you have to start somewhere - don't "wait" until you have "better" information. The preliminary results will guide you to the next step.

Did you decide to keep the footing sitting on top of existing ground (total height 2.7m) ?

The surcharge loading will be larger (more important) than I originally estimated. Is an allowance for the building's live load included in the 20kPA value? If the building is a warehouse or for heavy equipment live load may be considerable... not so much for an office or residence.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)

Quote (SlideRuleEra)

Did you decide to keep the footing sitting on top of existing ground (total height 2.7m) ?

Yes I will keep it that way for now and carry out the design as it is, i will see how it goes an decide if i should excavate and site the wall at a lower level within the excavation.


Quote (SlideRuleEra)

Is an allowance for the building's live load included in the 20kPA value? If the building is a warehouse or for heavy equipment live load may be considerable... not so much for an office or residence.

No i did not account for a live load in the surcharge load, however this is only a 1-story residence. Its a reinforced concrete structure with a 45cm thick foundation slab/raft foundation, thats why the 20kPa surcharge load.


I am currently working on my calculations and checking everything, i did find some careless mistakes, that also contributed to the overdesign, however now my surcharge has increased as well, so i should be back at the same design as before. I will do my calculation and let you know

Any advise on how to apply the seismic load, I have used the Seed and Whitman Method to determine the seismic load, and then added it to the existing thrust from soil and thrust from surcharge.

However i think the Seed and Whitman Method is way to conservative than the Monobe Okabe method which is also suggested by the Eurocode 8, and because my main issue here is the


I have one very important question that greatly affects the design:


Load case combinations and safety factors greatly affect the design, therefore my question is, The surcharge load from the building is considered as a permanent load right?

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

I think deepening the excavation will result in diminishing returns as you end up increasing the amount of surcharge and soil pressure the wall sees. But that's just a thought.

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Quote (kellez)

Any advise on how to apply the seismic load, I have used the Seed and Whitman Method to determine the seismic load...

Seed & Whitman is as good as any. I see that the design ground acceleration = 0.25g. Take a look at Development of Improved Procedures for Seismic Design of Buried and Partially Buried Structures, published by the Pacific Earthquake Engineering Research Center. In particular, see the "Conclusions & Recommendations" on page 157:



More about that in a minute.

Quote (kellez)

Load case combinations and safety factors greatly affect the design, therefore my question is, The surcharge load from the building is considered as a permanent load right?

Yes, I would consider the building's surcharge load as permanent for horizontal thrust on the wall. As discussed earlier, no credit for the surcharge to resist sliding or overturning.

Considering load combinations - for this first order calculation I suggest applying the following loads simultaneously:

1. Thrust From Soil, with Overturning & Sliding Safety Factors of 1.5

2. Thrust From Building Surcharge, with Overturning & Sliding Safety Factors of 1.5

3. Seismic Force, with Overturning & Sliding Safety Factors of 1.0 (Recommendations given above are used as justification for no added safety factor).

Make the calcs... let's see what you get. This is a wonderful project... you'll learn a lot more by having to deal with what I believe is a first effort with "issues". There will be other concerns to address later.

jayrod12 is right, but like everything else there may be compromise advantages to an excavated foundation. First, make the above calcs.

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RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)

Quote (SlideRuleEra)



Yes, I would consider the building's surcharge load as permanent for horizontal thrust on the wall. DONE

As discussed earlier, no credit for the surcharge to resist sliding or overturning. DONE

Considering load combinations - for this first order calculation I suggest applying the following loads simultaneously:

1. Thrust From Soil, with Overturning & Sliding Safety Factors of 1.5

2. Thrust From Building Surcharge, with Overturning & Sliding Safety Factors of 1.5

3. Seismic Force, with Overturning & Sliding Safety Factors of 1.0 (Recommendations given above are used as justification for no added safety factor).

The load combination I have used is 1) Thrust from soil + 2) Thrust from building surcharge + 3) Seismic force

I am using 2 types of load combinations, according to eurocode, safety factors are shown below, Combination 1 is used to factor the loads, Combination 2 is used to factor the material properties




For the seismic force i am using a safety factor of 1.0 as you suggested


Make the calcs... let's see what you get. This is a wonderful project... you'll learn a lot more by having to deal with what I believe is a first effort with "issues". There will be other concerns to address later.

Indeed i have learned a lot up to now while trying to solve all the issues with your help and everyone elses help.

jayrod12 is right, but like everything else there may be compromise advantages to an excavated foundation. First, make the above calcs.

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)
I have done the calculations using the following properties for the wall and soil

THE Retaining Wall Foundation is still at 1.80m wide but i have more confidence in my calculations now
As you can see from the results below I still have an issue with when the seismic load is added in the calculations any advice on that?




These are the results i get

SLIDING: EUROCODE COMBINATION 1



SLIDING: EUROCODE COMBINATION 2



OVERTURNING: EUROCODE COMBINATION 1



OVERTURNING: EUROCODE COMBINATION 2



SEISMIC: EUROCODE COMBINATION 1

For the seismic load case, I have calculated the 1) Total Horizontal Force due to soil + 2)Total Horizontal Force due Surcharge, + 3) Seismic force according to the Seed and Whitman Method which i think is a bit more conservative than the Mononobe Okabe Method. I will also give Mononbe Method a try and see





As you can see my SF for seismic load case is still below 1, this is my biggest issue now, i need to concentrate on this one now





RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)
I tried calculating the total horizontal thrust using the Mononobe Okabe method but i have one issue.
The example I am following is only about a gravity mass retaining wall therefore there is no wall footing/foundation thereofe there is no soil on top of the foundation to resist sliding and overturning,

Therefore my question now is:

What do i need to do in order to take into account the resistance to sliding and overturning that is provided by the soil which sits on top of the footing/foundation.
Another way to set the question is how much of the soil sitting on top of the footing will contribute to the seismic force and how much will contribute to the resisitnace against sliding/overturning?

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)
One more question regarding the Mononobe Okabe method

Which triangle from the two below best describes the seismic force



RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

kellez - Concerning how seismic force acts on a cantilever retaining wall. Your second sketch is in accordance with US Army Corps on Engineers practice:



Seismic force comes from the "driving wedge" and acts on the "structural wedge". See Seismic Analysis of Cantilever Retaining Walls.

I'm still working on a response to your spreadsheet results. We have some more fundamental issues to look at. Will take me a while to put together an explanation... should have something later today.

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RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

kellez - Good, you made the initial cals... now you can begin to iterate the design. I'm not familiar with the Eurocodes, so can't help you on that. But the calcs indicate that "something" is wrong. Refer to my comments on January 5:

Quote (SlideRuleEra)

The resultant load from supporting soil needs to the in the middle third of slab. Reproportion wall shape/dimensions for more favorable loading characteristics.

Believe me, I don't make this stuff up. smile



You have checked overturning and sliding, but don't appear to have checked the more important soil pressure distribution under the footing. Having soil pressure distribution Case I or Case II is essential for basic loading conditions. For "extraordinary" loading, a limited amount of Case III may be considered acceptable.

Evaluate the proposed geometry (base is 1.80 meter long). Use the basic calculated active thrust from the soil and active thrust from the surcharge... NO adjustments, NO added safety factors, just the calculated values. For the time being, ignore seismic loading too.

If my math is right (I find metric to be very "difficult"... my problem, not yours), the soil pressure distribution under the footing is Class III... too much eccentricity in the loading.



The footing geometry MUST be changed for it to work properly with the existing assumptions. The wall does not fail by being Case III, but it indicates that the design is absolutely NOT right. This is your project, think about how do you want to proceed. There are several options to consider, propose what you would do.

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RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)
You are right i need to deal with this before i move to the seismic load. I did carry out the calculations for eccentricity and bearing capacity of the foundation, please see below.
You are right the vertical force of the wall is acting outside of the middle third of the wall base, which will cause an increase in pressure on the ground below the foundation.
this is due to the fact that the weight of the wall is not spread uniformly across the whole area of the foundation.

NOTES REGARDING MY CALCULATIONS:

As you will notice I have also added a small force for wind loading, what do you think about that (shall i remove it)?
Shall i also consider the surcharge acting on the base of the wall when calculating the Resisting Moment?






RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Quote (kellez)

...added a small force for wind loading, what do you think about that (shall i remove it)?

The chances of having design wind loading AND a design earthquake, at the same time are very small. The traditional approach is to pick the greater of the two loads, and ignore the other - I agree with that. Chances are the seismic loading will govern for this project.

Quote (kellez)

Shall i also consider the surcharge acting on the base of the wall when calculating the Resisting Moment?

IMHO,that is a reasonable potential solution to consider. On this project, if the heel slab was directly below the surcharge load, that should work. But the slab is not below the load, so my answer to that is no.

The surcharge load is fairly close the the heel slab, so the surcharge may put some vertical loading on the heel slab. But the soil properties are in doubt. Without detail knowledge of the soil properties and control of backfill placement, I would not consider that approach either.

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RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)

Quote (Kelles)

NOTES REGARDING MY CALCULATIONS:

As you will notice I have also added a small force for wind loading, what do you think about that (shall i remove it)?
Shall i also consider the surcharge acting on the base of the wall when calculating the Resisting Moment?

Just to be clear, both questions quoted above, are in regards to the case of determining the eccentricity of the wall.

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

The eccentricity of the wall needs to be calculated for more than one load combination. The basic combination is called "Usual Loading" by the US Army Corps of Engineers. For this project's usual loading the horizontal active thrust from the soil plus the horizontal active thrust from the surcharge are resisted the weight of the concrete stem and heel slab plus the weight of the backfill directly above the heel slab. IMHO, the horizontal active thrust from the surcharge acts on the structural wedge and the horizontal projection of the heel slab.

If the usual loading eccentricity is too large (which it is on the current design)... stop. Take the steps needed (often redesign of wall geometry) to lower the eccentricity before considering other load combinations (which may include wind loading).

This is a good time to bring up what some may consider a minor issue. The assumed weight of concrete seems high, 25 kN/m3 (159 PCF). In the US, 150 PCF (23.6 kN/m3) is considered the weight of "normal" reinforced concrete. This value is typically sightly conservative... if (per the usual case) the concrete has to be supported by falsework and forms. For uplift and buoyancy calculations, I consider 150 PCF to be overly optimistic. Note that for this retaining wall the weight of concrete is most important for resisting the overturning moment (uplift). For uplift/buoyancy I use 145 PCF (22.8 kN/m3).

Just suggesting that you use a realistic value of concrete weight for this project... whatever that value may be. What on first glance "seems" conservative, may not be.

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RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)
Going back a bit

Quote (SlideRuleEra)



1. Thrust From Soil, with Overturning & Sliding Safety Factors of 1.5 EUROCODE suggests a factor of safety of 1.35, this is what i used up to now

2. Thrust From Building Surcharge, with Overturning & Sliding Safety Factors of 1.5 if surcharge is considered a permanent load which it is in this example then the factor of safety according to Eurocode is again 1.35[ this is what i used up to now/color]

3. Seismic Force, with Overturning & Sliding Safety Factors of 1.0 (Recommendations given above are used as justification for no added safety factor). [color #EF2929]Eurocode dictates a seismic coefficient depending on the location of the structure and no factor of safety is provided therefore 1.0 is acceptable.


what are your thoughts on the eurocode factors of safety, NOTE: for variable surcharge loads Eurocode suggests a factor of safety of 1.5 but since by surcharge load is permanent i will use 1.35

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Quote (SlideRuleEra on 15 Jan 18)

I'm not familiar with the Eurocodes, so can't help you on that.

Have you taken steps to reduce the excessive eccentricity under "usual loading"?

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RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)

Quote (SlideRuleEra)

I'm not familiar with the Eurocodes, so can't help you on that.

Ok i will go with the eurocode factors of safety since i am designing in accordance with euroocde

Quote (SlideRuleEra)

Have you taken steps to reduce the excessive eccentricity under "usual loading"?

Yes i did, these are my findings:

the only way to reduce the wall eccentricity (and keep the same wall shape - L) is to increase the foundation slab width to 2.10m

or by changing the shape of the wall from L to inverse T as you previously suggested, therefore i kept the width of the wall heel at 1.80m and added another 0.20m as the toe, total of 2.0m
the eccentricity now lies with in the middle 1/3 of the slab

I noticed that even if i reduced the heel of the wall at 1.60m and add a toe of 0.20m the eccentricity still lies with in the middle 1/3 of the slab

therefore now i have an inverse T-shaped wall which solves the problem of eccentricity, however i still have an issue with the resistance against sliding both in the static and seismic conditions



UPDATE:

1) Unit weight of concrete changed from 25kN/m3 to 23.60kN/m3 as you suggested
2) After calculating the surcharge load of the building in greater detail i have increased the loading from 20kN/m2 to 26.10kN/m2
3) For the design to pass I need to increase the slab width to 2.20m for the L-shaped wall (eccentricity is fine now even with the L shaped wall due to the increase in the slab width)

and

2.40m for the T-shaped wall -> 2.20m for the heel and 0.20m for the toe

Please let me know how do you want to proceed. I do have a question regarding the wall as whole and not by analysing just one meter per run, which i think will eliminate the sliding issue of one of the walls, however i will leave that one for the end once we finish designing the wall by analysing it per meter run

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)
Some of the issues we covered here include

1) Properties of backfill - very important for Ka coefficient
2) Building surcharge load and how it acts on the wall
3) Factors of safety for different types of loads including seismic.
4) Unit weight of concrete changed from 25 to 23.6
5) Resolve wall eccentricity issue by changing wall shape or increase foundation size.

I think all the above are resolved but I still have an issue with the design against sliding. My thoughts are:

Now what exactly determines the resistance against sliding?

1) That would be the total weight of the wall (stem + foundation) in addition to the soil on top the heel.
2) Another parameter that determines the sliding resistance is the angle of friction of the soil underneath the foundation, i think this could be one of my issues now.

Up to now i have used an angle of friction of 28 degrees, but i dont really know what is the correct value.

Since this is an in-situ soil and not a compacted backfill is it possible that the angle of friction be very high?


RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Quote (kellez)

Now what exactly determines the resistance against sliding?

Factor of Safety = Driving Forces / Resisting Forces

The value of the coefficient of friction between the concrete heel and and to soil supporting it is a major factor.
Also, cohesion of the soil can be important... but probably not on this project.

See pages 79-82 of Technical Release Number 74, "Lateral Earth Pressure", US Department of Agriculture

You may have to take steps to develop passive soil pressure on the front of the heel to assist in resisting sliding.

Quote (SlideRuleEra on 12 Jan 18)

...there may be compromise advantages to an excavated foundation.

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)

Quote (SlideRuleEra)


The value of the coefficient of friction between the concrete heel and and to soil supporting it is a major factor.
Also, cohesion of the soil can be important... but probably not on this project.

I will discuss this with the client and ask for some lab testing in order to determine
a) the angle of friction and b) bearing capacity of the insitu soil - underneath retaining wall
c) sieve test to determine the type of soil



Before i start making changes i have an important question

Question 1
I think i never mentioned the fact that the backfill soil will be compacted every 30cm
Isnt this going to change the angle of friction of the material?
The angle of friction of the backfill material i am currently using is 30degrees, isnt this going to increase?

Question 2
After reading the suggested pages from the document above (U.S. DEPARTMENT OF AGRICULTURE SOIL CONSERVATION SERVICE ENGINEERING) I noticed that the safety factors are not applied directly to
the forces or materials however the document suggests a minimum safety factor to be reached. I just wanted to comment on that because for me its not necessary to reach such a high FS since my
factors of safety have already been applied to the forces and materials.

Question 3
What happens to the surcharge load from the building during an earthquake? Do i simply use the same value as with the static analysis?
I just noticed that the Mononobe Okabe method doesnt actually consider any surcharge load and therefore will need to be added thats why i was getting very low force values when using this equation. for that reason i have avoided it and i am using the Seed and Whitman method

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)

Quote (steve49)

otwithstanding that comment, is your base width actually out of proportion? You cite a 1.2m base for a 2.0m high wall as another example: B/H=0.6. Your design is 1.8m base for 2.7m height: B/H=0.67. Not too different at all. What do you know about the other wall? Did it have access to good fill, lower design surcharge, other factors in its favour?

Some textbooks (eg Bowles) give B/H from 0.5 to 0.7 as a guide. You're within that range. My own experience from looking at old retaining walls is that many show signs of movement (leaning and sliding) to the point that I'd say they were 'under-designed', so I'm not too confident in the lower end of any old rules of thumb.

The above rules of thumb give (eg Bowles) gives B/H from 0.5 to 0.7....i dont think this rule of thumb takes into consideration any surcharge loads from nearby buildings?

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Very good dialogue for this thread... one of the better threads. A couple of construction issues... I would normally have a distance of 100mm or so from the toe of the footing to the face of the wall to rest the wall formwork on. In addition, I would batter the wall into the soil by 300 or 400mm so that with any rotation, the wall becomes more vertical.

Dik

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Kellex - Good work from you. You seem to have a want to learn which is admirable. Well done to SRE, OG and others for providing great advice.

I have only looked at this thread in the last half hour, there is a lot to go through and i have only skimmed the replies so excuse me if this has been said already.

I think you have a bearing capacity problem. In your spreadsheet and drawing you has assumed (or calculated?) a bearing resistance of 250kPak, is this an ultimate or allowable?. I dont think a base soil with Phi' of 28 degrees will give you that. Your drawings done indicate if you are intending to bury your base or sit it on ground level? If you are not burying your base it reduces your available bearing capacity.

I did a quick and dirty check using Terzaghi's equation and for the Phi above, assuming 1.5m breadth by 1m width, you have an allowable bearing resistance of 156kPa. See below.

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)

Quote (dik)


Very good dialogue for this thread... one of the better threads. A couple of construction issues... I would normally have a distance of 100mm or so from the toe of the footing to the face of the wall to rest the wall formwork on. In addition, I would batter the wall into the soil by 300 or 400mm so that with any rotation, the wall becomes more vertical.

Dik

Very good suggestion thanks a lot, you are absolutely correct, I need that 100mmm distance otherwise where would i support the formwork?

About battering the wall into the soil, again thats a good approach because at some point in the future you might get some rotation especially during an earthquake. i will have to talk about this with the formwork contractor and see what he says, how expensive or difficult this might be?

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)

Quote (EireChch)



Kellex - Good work from you. You seem to have a want to learn which is admirable. Well done to SRE, OG and others for providing great advice.

I have only looked at this thread in the last half hour, there is a lot to go through and i have only skimmed the replies so excuse me if this has been said already.

I think you have a bearing capacity problem. In your spreadsheet and drawing you has assumed (or calculated?) a bearing resistance of 250kPak, is this an ultimate or allowable?. I dont think a base soil with Phi' of 28 degrees will give you that. Your drawings done indicate if you are intending to bury your base or sit it on ground level? If you are not burying your base it reduces your available bearing capacity.

I did a quick and dirty check using Terzaghi's equation and for the Phi above, assuming 1.5m breadth by 1m width, you have an allowable bearing resistance of 156kPa. See below.


Thank you, i am doing my best to get it right from the first time.

This is some great information too, and it has not been said already, thanks for bringing that up. I will look into this a bit more, Maybe i will also ask for some soil tests to determine the soil bearing pressure.

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

OP:
Both forms are tilted and there is little, if any, additional cost.

Dik

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Thanks, dik and ErieChch.

Quote (kellez)

Question 1
I think i never mentioned the fact that the backfill soil will be compacted every 30cm
Isnt this going to change the angle of friction of the material?
The angle of friction of the backfill material i am currently using is 30degrees, isnt this going to increase?

Keep in mind that the theory (Coulomb-Rankine) being used is pretty basic. No point in trying to "fine-tune" the calculations... the the numbers may be precise, but improved accuracy is missing.

Quote:

Question 2
...factors of safety have already been applied to the forces and materials.

Most of today's codes do that. I accept that as the way things are, but it obscures the physics of what actually happens as design assumptions change.

Quote:

Question 3
What happens to the surcharge load from the building during an earthquake? Do i simply use the same value as with the static analysis?

Yes, treat seismic and surcharge as two, distinct loads. Of course, they can (and will) be applied at the same time. As discussed for Question 1... keep it simple.

IMHO, the single most important thing you can do is accurately determine soil properties for ALL the soil, including backfill. Our colleagues have encouraged you to do this for some time. Accurate soil properties will allow a meaningful design that can be expected to work correctly.

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RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)

Quote (SlideRuleEra)


Keep in mind that the theory (Coulomb-Rankine) being used is pretty basic. No point in trying to "fine-tune" the calculations... the the numbers may be precise, but improved accuracy is missing.

Ok I am keeping the angle of friction as it is

Quote (SlideRuleEra)


Most of today's codes do that. I accept that as the way things are, but it obscures the physics of what actually happens as design assumptions change.

I am also simultaneously calculating the design problem without any codes or FoS applied - its called the gross pressure method

Quote (SlideRuleEra)


Yes, treat seismic and surcharge as two, distinct loads. Of course, they can (and will) be applied at the same time. As discussed for Question 1... keep it simple.

Yes i have done that already

Quote (SlideRuleEra)

IMHO, the single most important thing you can do is accurately determine soil properties for ALL the soil, including backfill. Our colleagues have encouraged you to do this for some time. Accurate soil properties will allow a meaningful design that can be expected to work correctly.

Yes thats exactly what i am planning to do now


UPDATE:

I am also designing the exact same wall but without the surcharge load of the building.
Therefore by keeping everything the same and removing the surcharge load the results i get are:

For an inverse T-shape wall the foundation slab width is = 1.60meters -> 1.50m for the heel & 0.10m for the toe

The results now seem to be reasonable and are within the rules of thumb.

I have spoken with 2 architects and 1 engineer about the wall with the surcharge load and they all think that the foundation slab i get is way to big.
I have also seen some retaining wall designs of finished projects and they look way underdesigned when compared to mine. I am really curious to see how they carry out their designs,
for this reason I will be in contact with an engineer tomorrow and hopefully he will have the time to show me his calculations and how they actually carry out their designs.
I am really curious and excited to check my calculations against his, assuming he is willing to share

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)
Eurocode suggests that any variable surcharge load should not be considered as favourable when calculating the sliding and overturning resistance.
However My surcharge load is actually a permanent load and not a variable load, its actually the load from the building nearby.

I know you also did mention not to consider the surcharge load for the resistance calculation, which i did agree with you, however i did not realise that this
was meant for the variable load and not the permanent load.

So do you think i should also consider the permanent surcharge load when calculating the resistance against sliding and overturning?

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

That is going to be a judgement decision... but you should make an informed decision. Do a sensitivity analysis:

For "usual loading" (or its' Eurocode equivalent) what is the eccentricity, and is it acceptable when the surcharge is ignored in resistance calculations?

For "usual loading" what is the eccentricity, and is it acceptable when the surcharge is included in resistance calculations?

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RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Quote (kellez)

So do you think i should also consider the permanent surcharge load when calculating the resistance against sliding and overturning?

Is the building on your property? I would not consider it. I would look to do other this first increase heal width etc.

Quote (Kellez)

EUROCODE suggests a factor of safety of 1.35, this is what i used up to now

From one of the earlier posts you said the above which isnt correct. Eurocode works on the basis that the Ed < Er. Your design action effect (Ed) should be less that a design resistance (Er). Design means factored so the factors are already built in.

Good work.

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)
After further checking my calculations i finally got my design of the foundation slab at a width of 2.0m which is final


Now i am designing the vertical reinforcement for the stem of the wall.


1. the maximum bending moment i get at the bottom of the wall were it connects with the foundations is as shown below




2. I then design the reinforcement required according to above bending moment



This is for the tension side of the wall, common practise suggests that the same amount of reinforcement is used on the compression
side as well, however i am skeptical about that. Is the steel on the compression side of the wall actually work?
can i save some steel by using 10mm bars on the compressions side instead of 12mm?









RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

You are still working on this wall?!

Must be a good budget.

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

To answer your question, technically the compression side does not require it's own layer of steel. I've been keen to omit it when possible to keep congestion to a minimum at the wall to footing connection.

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

kellez - 12mm bars with 125mm (or even 140mm) spacing? Why so "small" and close together?

For a wall of these proportions, look into saving money (reduced rebar handling/installation cost) by using fewer bars, of a larger size with greater spacing to get the same weight of rebar / meter of wall length. Rebar sizing/spacing is a compromise, don't want individual bars to be so "heavy" that a crane is needed to place them. Also, don't want the spacing to be "too large"... but bar spacing somewhat > 140mm should not be "too large" for this wall.

Agree with jayrod12 about the compression side rebar. The Eurocode (that I am not familiar with) may have guidance on the compression side (or total amount) of rebar required.

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RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)
There is no budget at all, thats why i am taking it slow. :)

Well Eurocode suggests a minimum amount of reinforcement on the compression side to avoid cracking.
After your comments i decided to increase the bar size at 14mm thick bars and increase the spacing at 160mm about 6 bars per meter run


Ok now i have a very specialised question regarding the design of the retaining wall.

Please have a look at the picture below,
Lets consider Retaining Wall 2, as you can see this wall is restrained at both ends by Retaining Walls 1 and 3, for certain their foundation slab will be connected.
which means there is no way that this wall could actually slide, right?

when designing a wall we usually design the wall by considering only 1 meter of the wall, we do not actually consider the wall as a whole.
therefore do you guys think i could reduce the size of my foundation slab by taking into consideration the fact that the wall is restrained at both ends?
My biggest issue with the wall design was resistance against sliding therefore i am thinking that this could be one way to reduce the width of the slab for only retaining wall 2

what do you guys think?

https://res.cloudinary.com/engineering-com/image/upload/v1519234569/tips/Screen_Shot_2018-02-21_at_19.16.05_bprcd4.pdf


RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)
There is no budget at all, thats why i am taking it slow. :)

Well Eurocode suggests a minimum amount of reinforcement on the compression side to avoid cracking.
After your comments i decided to increase the bar size at 14mm thick bars and increase the spacing at 160mm about 6 bars per meter run


Ok now i have a very specialised question regarding the design of the retaining wall.

Please have a look at the picture below,
Lets consider Retaining Wall 2, as you can see this wall is restrained at both ends by Retaining Walls 1 and 3, for certain their foundation slab will be connected.
which means there is no way that this wall could actually slide, right?

when designing a wall we usually design the wall by considering only 1 meter of the wall, we do not actually consider the wall as a whole.
therefore do you guys think i could reduce the size of my foundation slab by taking into consideration the fact that the wall is restrained at both ends?
My biggest issue with the wall design was resistance against sliding therefore i am thinking that this could be one way to reduce the width of the slab for only retaining wall 2

what do you guys think?





RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Quote (kellez)

...reduce the size of my foundation slab by taking into consideration the fact that the wall is restrained at both ends?

How far from the corner (horizontally) is the proposed "reduced size foundation" extended? How is that calculated (with certainty)?

IMHO, quit trying to "save money" by cutting corners on permanent materials. Real savings come from logical, simple design and reduced construction labor.

The Contractor will have wall concrete forms that he reuses many times along the length of the wall. If the design changes in the corners, he now has to do "special" (expensive) forming... just to "save" a token amount of (cheap) concrete.
Note: By "cheap" I mean to the Contractor. For example the "cost" to form, place, finish a certain concrete pour with, say 18 m3 is virtually identical to 20 m3. The only money "saved" is the Contractor's cost for 2 m3 of concrete... and it took much extra labor to do that.

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RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)
I am not just trying to reduce costs here, i am actually trying to learn.
This thought came to my mind once i started thinking about the wall as a whole and how it will react under an earthquake as whole,
i think this is only logical since when designing a wall we only think of a meter run of the wall. therefore i had to ask.
any question i ask here is only to check what other more experienced engineers would do.


Ok lets say that the point which is furthest from the restrained points, that is the middle of the wall is under loading and its about to slide,
in order for it to slide and move, the wall and foundation slab will need to bend and a bending moment will be created on the foundation slab of the wall.
As a matter of fact the foundation will act as beam supporting a floor slab. please look at the picture below where the wall foundation slab is illustrated as a beam.

while drawing the picture below i also thought that its wrong to only consider the loading only on one wall, therefore the loading on the other walls will need to be also considered.
anyway, i think i am overcomplicating this, i am already finished with the design of the wall but just discussing


RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)
what would be the use of these joints?

my guess is to disconnect the walls so that they do not affect each other,
however their foundation slab would still be connected.

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Quote (kellez (20 Feb 2018))

Can I save some steel by using 10mm bars on the compression side instead of 12mm?

I misinterpreted your 20 Feb comment about permanent material reduction, sorry about that, let's move on:

It is wrong to consider only one wall, but there are limits to what can reasonably be modeled. Consider the surcharge is a little more detail, it is different at every location on the wall and it acts in all directions simultaneously. Even this marked up sketch is a gross simplification:



Also, go back the very beginning of this thread. Just how accurate are soil properties? There is no point in, say, precise 4 significant digit calcs when the input data (including model assumptions) are estimated to 2 significant digit (at best) accuracy.

Your latest sketch shows another good point: Longitudinal steel is important. Not all support is provided by the wall resisting vertical bending.

A final point, everything interacts, just like the corner situation you described. Problem is, often the interactions are often unpredictable. I'll give you an example, sorry I did not take a picture of the following:

A timber industrial dock in seawater remained standing even though low tide revealed that far more than half the timber piling had been totally cut by marine borers. The timber stringers, tied with nails to the (suspended in midair) timber pile caps, were "sagging". These lines of stringers were acting just like the main cables of a suspension bridge to support the deck between the few (randomly spaced) piling that somehow were still standing. Could anyone predict that dock had not collapsed... I don't think so.

Make the best representative assumptions possible, then be conservative... don't look for shortcuts.

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RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Quote (OP)

my guess is to disconnect the walls so that they do not affect each other,
however their foundation slab would still be connected.

You can put control joints in the footing at the corners if you like... not likely needed... you can also put control joints in the footing at locations in the wall, but, also not likely needed.

Dik

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)
control joints it what we also call expansion joints?

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

generally to accommodate shrinkage; an expansion joint is normally formed, a control joint sawcut...

Dik

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)
Some other concerns regarding the construction of the wall:

1) I am thinking of using an expansion joint (that runs across the whole height of the wall stem) every 12m.
wall and steel reinforcement discontinuity.
2) the wall height without considering the footing is 2.7m, is it ok to pour this wall in one go?
3) is there going to be an issue pouring the concrete from a height of 2.7m?

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Quote (kellez)

I am actually trying to learn.
...and you are doing a good job, keep at it.

Before getting to your most recent 3 questions (which are good ones), keep working on the "hard" stuff. As the old saying goes, "the devil is in the details". How those 4 corners (3 exterior, 1 interior) are designed is not something to leave up to the Contractor.

The most difficult are the exterior corners, soil pressure is tending to "open" up the walls. This is your problem. We talked about the (easier) interior corner before... let's see what you have in mind on that one, too.

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RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)
Yes reinforcements at wall corners is very important i know, however before we go there, i would like to talk about the reinforcement of the wall and footing.
Below is the final design of the wall followed by two different arrangements for the reinforcement



Initially i was thinking of using bars that run straight from the top of the wall down into the footing with an angle of 90 degrees
and continue to form the reinforcement of the footing. therefore one continues bar that forms the wall reinforcement and footing lower reinforcement as shown below



However after talking to the steel fabricator, he said that it would be hard to handle this kind of bar due to the height of the wall,
therefore he suggested using starter bars so that the bars are not that big (see drawing below). i think he is right, what do you guys think is there any major issue when using starter bars?


RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Quote (Kellez)

Yes reinforcements at wall corners is very important...

I'm not talking about reinforcement in the corners. Forget the corner reinforcement for a bit, if the exterior corners are not designed correctly, the wall may collapse because of insufficient resistance to overturning moment.

Concerning the rebar detail shown today, the fabricator is right. I see that you are keeping the 10mm bars... I want you to do a simple experiment. Pickup one 10mm bar that is about 2.7 meters long and hold it only at the bottom, with the bar pointing straight up. Does it "bend" under it's own weight?

Another advantage to large reinforcing bars is that they have significant structural properties as steel members. The "stronger" bars allow the tied reinforcing cage to be more self supporting inside the wall forms (before and during concrete wall placement). IMHO, even 14mm bars (on both wall faces) are marginal (for self-support purposes on this wall). Remember the reinforcing cage has a lot of forces on it during concrete placement... and if the reinforcement ends up in the wrong place there will be "problems", as you know.

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RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)
Thank you all for the assistance.

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)
Where are the two last posts of this thread which i posted yesterday?

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Can you re-post them?

Dik

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)
Hello everyone, the construction of the wall has started, we are currently fixing the reinforcement of the footing together
with the starter bars of the wall above in order to pour the footing.

I urgently need your help regarding the distribution reinforcement of the footing at the corner between TA1 and TA2
(the corner which i am referring to is shown on the drawing below, and the arrangement of the distribution bars is shown on the pictures)
Please have a close look at the reinforcement of the footing and advice if i need additional reinforcement at the specific corner.

1) As you can see from the pictures the distribution bars from TA1 footing and the distribution bars of TA2 footing extend crossing each other.
2) In addition there is no vertical reinforcement within the specified corner between TA1 and TA2

PLAN VIEW OF RETAINING WALL:

RETAINING WALL IS ILLUSTRATED WITH CYAN COLOUR



Reinforcement at corner between TA1 and TA2




PICTURES OF REINFORCEMENT AT SPECIFIC CORNER





















RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)
anyone? we are pouring next week therefore i need your advice ASAP

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Kellez - I received your emails... take it easy. This project is in a "mess" (we will talk about that later), but there is hope.

1. I would omit all the concrete in the "interior" corner (see the image below). The wall stem stem thickness and reinforcement are minimal, the wall will deflect under soil loading. I would want each wall to move independent of the other. Fortunately, for this interior corner, the two walls will move closer together. An expansion joint between wall stems should take care of deflection.



2. The concrete forming is very "neat" but, IMHO, not adequate. Assuming the slab is 0.55m thick and the driven rebar dowels are spaced 0.7m on-center, "theoretical" concrete hydrostatic force on the dowels is high (see image below). It will probably work, if the vertical concrete rate of placement is kept low... but I would not want to rely on that. On short notice, I would just put in a lot more dowels.



3. Concerning joint in the footing - construction joints are needed. There is only so much concrete that can be placed and finished in a working day. Consider how the concrete is placed (from truck, pumped, crane/bucket, etc.), crew size / skill and don't plan on placing more concrete than should be reasonable in 9 working hours. I would split your plan into at least 3 separate placements (shown on the image below). Note: It may be possible to do 1, 2, or all 3 in one working day... if not, at least there is a logical place to stop work for the day. For a professional quality job, plan on two concrete vibrators with two independent power sources. Concrete finishing will most likely be the limiting factor... don't pour more concrete than can be finished correctly. Wet cure for 7 days, starting immediately.



You still have to deal with exterior corner design. At the exterior corners stop concrete placement where the wall heels interfere:



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RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)

Quote (SlideRuleEra)

You still have to deal with exterior corner design. At the exterior corners stop concrete placement where the wall heels interfere:



the layout of the retaining wall has been changed, wall TA4 and wall TA5 will not be build, instead we will extend
the perimeter of the backfill about 2-3 meters outwards (see drawing below). therefore the exterior corner shown on your drawing above
will not be build.

if you check the latest plan view drawing on my last post (or the drawing below) you will see that the walls TA4 and TA5 are not shown



RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)

Quote (SlideRuleEra)

1. I would omit all the concrete in the "interior" corner (see the image below). The wall stem stem thickness and reinforcement are minimal, the wall will deflect under soil loading. I would want each wall to move independent of the other. Fortunately, for this interior corner, the two walls will move closer together. An expansion joint between wall stems should take care of deflection.



Unfortunately the reinforcement and formwork of the footing at the corner shown above has already been build.

However if i add an expansion joint (as you suggested, and as shown on the drawing below) between the stem of walls
TA1 and TA2 so that the walls are independent of each other, wouldnt it still be fine? and accommodate any issues
of interaction between the two walls even if the their footings are connected at the corner?

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Quote (kellez)

Unfortunately the reinforcement and formwork of the footing at the corner shown above has already been build.

That's what acetylene torches are for. Cut the rebar and reform... or even better, postpone the concrete placement until you complete the design. Last minute changes like that are commonplace in the heavy construction industry. I would want those two structures to be entirely separate from each other - the reinforcement was designed for one thing, and only one thing - to resist soil loading. Who knows what type loading will be applied to a single slab with two walls, each loaded in a different direction. Don't dwell on what you can't do, as I said earlier, you are in a "mess"... try to get yourself out of it.

An alternate is to do something like this:

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RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)

Quote (SlideRuleEra)

That's what acetylene torches are for. Cut the rebar and reform... or even better, postpone the concrete placement until you complete the design. Last minute changes like that are commonplace in the heavy construction industry. I would want those two structures to be entirely separate from each other - the reinforcement was designed for one thing, and only one thing - to resist soil loading. Who knows what type loading will be applied to a single slab with two walls, each loaded in a different direction. Don't dwell on what you can't do, as I said earlier, you are in a "mess"... try to get yourself out of it.

An alternate is to do something like this:

Ok i see your concern, I will postpone the pouring to investigate more.

Ok lets now investigate this a bit more, so your thinking is that since i have not considered something like this
in my analysis and design then i shouldnt build it like this, since i dont know how the slab will react. That is
reasonable thinking and I totally agree with you.

However from what i see you are not 100% sure on what the issue could be by connecting
the two footings together. therefore in order to be on the safe side you choose not to
connect the two footings together and build according to the analysis and design.


Now I am trying to think what could be the problem by connecting the two footings together,
it could be just fine or it could be even better as a design than by not connecting the two footings,
or there could be lots if unforeseen issues.
How can we find out? and how can we design for this specific corner if we choose to connect the two footings??

Considering the fact that my wall has been designed so that it would not slide, overturn or failure of soil
beneath, and also designed against bending and shear of the stem and footing, then what are the possible forces
that could be created at this specific corner that could be detrimental to the wall?

my wall together with the foundation is designed not to move or slide, therefore there is not going to be a lot of
stress applied to the corner reinforcement since the soil forces are already resisted by the weight of the soil
applied upon the footing.






RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)

Quote (dik)

Can you provide joints like the attached?

the two walls indicated below in the drawing will not be build.

i think your drawing of the wall and the footings is wrong, the footing should be on the inside of
the retaining wall, therefore you should move the wall in the opposite side of the footing as shown
in the drawing below

1) what type of joints are you referring to? Expansion or contraction joints?
2) where are the joints located? on the wall, footing or both?



RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Sorry... I thought that was the soil side. You are filling the site to provide a level surface for the home?

Dik

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Quote (kellez)

...there could be lots if unforeseen issues. How can we find out?

Together with your Contractor, determine how many m3 concrete can be placed and/or how many m2 concrete can be finished in a 9 hour working day. The answer will determine how may linear meters of footing can be placed at one time, that is, the maximum spacing of construction joints. Depending on the answer, keeping the two walls/footings separate may be best for both design and construction. Tell us what numbers you come up with. For what it is worth, I believe reasonable construction overrules "clever" design.

Note: The 9 hour time frame is a reasonable limit on how long experienced workers can perform efficiently doing this type construction. Also, things go wrong... extra time may (will) often be needed. Start a sizeable concrete placement as early as possible in the day, even if it means waiting overnight to start.

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RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)
1) 1st phase we are pouring only TA1 and TA2 footings, therefore only 2 footings.
the m3 of concrete are not that high.

Footing 1 is 30m x 0.55m x 1.8 = 29.7m3
Footing 2 is 18m x 0.55m x 1.90m = 18.81m3
Total = 29.7 + 18.81 = 48.51m3

lets say 55m3 in total which is 6.11 trucks of 9m3 each,
lets say 25minutes to pour each truck, total time is 3h and 45minutes



2) *we are not going to pour TA3 footing because it is on a different level than the rest of the footings,
actually part of TA3 footing sits on top of TA2 footing

see drawing below

do i need to connect the two footings together by embedding starter bars inside TA2 footing?








RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Quote (SlideRuleEra)

Concrete finishing will most likely be the limiting factor... don't pour more concrete than can be finished correctly.

...how many m2 concrete can be finished in a 9 hour working day.

1. Placement of 55 m3 in 4 hours... plan on finishing and placement taking place at the same time.

2. Rebuild the concrete forms for full concrete hydrostatic pressure... not just the additional driven dowels I mentioned earlier. With a schedule that quick (4 hours placement) you can not risk having form trouble during placement.

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RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Kellez - Review my earlier posts today. Overlapping slabs get back to the problems with loading that is not addressed in design. Consider that what you show as overlap could be a single slab (of whatever thickness is needed) that supports two short walls. This block will have to be designed for soil pressure, overturning, sliding, etc.



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RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Do you need a guardrail at the top of the wall? requirements for public access (closer spaced rails)

Dik

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Kellez - I have given this project much thought over the past 24 hours. Have even discussed it, in private, with an experienced Geotechnical Engineer who has been following the thread. You had mentioned this was a "learning" opportunity for you. Over the past 3 months, or so, I believe you have learned a lot... the proof was yesterday when faced with actually pouring concrete, you wisely decided to postpone until design / construction issues could be resolved.

The troubling part of effort to date is that the 0.55 meter thick foundation slab is the "easy" part of the project, but the Contractor is struggling to handle it (for example, under designed concrete formwork). A 3 meter+ tall stem wall is literally exponentially more challenging.

As an example of what can go wrong, picture 55 m3 of hardened reinforced concrete that is in a misshaped, useless mass. It happens, and the cleanup costs are very high.

The Contractor needs an experienced Field Construction Manager to guide him though this project. There is no way that we (Engineering Tips members) can do this over the internet. With all due respect, I don't believe you have the field experience to do this either.

I expect many members, including me, will be happy to comment on your continuing "learning" design efforts... but ONLY if it is restricted to learning, at this time.

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