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12' Retaining Wall Question
3

12' Retaining Wall Question

12' Retaining Wall Question

(OP)
Hi everyone, after lurking around the forum here for a few years I finally made a profile. I recently broke away from the small structural engineering firm (almost exclusively light-frame residential projects) I was at to start my own firm. I hope to contribute a little back to the engineering community here since many of your answers have helped me on projects I've worked on. Anyway...

I'm designing a 12' tall retaining wall pretty much right up against the property line in the client's backyard. I have a geotechnical report for this one so I'm using all the values they gave me in the report. I have attached the design that I've come up with so far. I have two questions...

1) At what point does such a wide heel on the footing stop helping? Can I really count on a 7'-6" wide heel to act as one unit? Let's say I had a 20' wide heel, it seems obvious that at a certain point the heel is so far away from the wall itself that it's not doing anything to help with overturning, bearing, etc.
2) Are you aware of places in any relevant code books (IBC, ACI, etc.) that gives insights on drainage design for retaining walls? I feel confident in the drainage I have called out on the detail (drain rock, perforated pipe, etc.) but I'm wondering if there is any sort of standard to reference when designing for drainage?

Here's the link for the retaining wall detail: Link

RE: 12' Retaining Wall Question

1) Sure. At some point it'll stop benefiting you, but it's going to depend on the size of the wall, thickness of the heel, soil conditions, etc. Can you depend on a 7'6" heel to act as one unit? Absolutely. You'll want to consider deflection of all of the components to determine the total deflection of the top of the wall - that'll tell you how well it's acting together.

2) I'm not aware of anything. I typically call those out as "drainage by others". Sizing the drain pipe is not something I'm comfortable with as it's more storm water design than anything else. You'd have to determine the area of the drainage basin that you'll be collecting, flow rates, etc. to get the size of the pipe. Then site design to slope the pipe appropriately to a discharge. Then, based on the quantity of the discharge, do you trip any special requirements for your locality's storm water regs? I'd have no idea. If it's for ground water, you'd probably need the geotech to give you the amount of water you'd have to move based on predicted ground water levels (which most reports explicitly leave out because it requires 12 months or more of monitoring) and permeability. Could I do these things? Yes, with enough study. Can I do them quickly, efficiently, and with a level of expertise that I think my clients deserve? No, no I couldn't. So I'll put on the drawings where we need to limit the water level, and let somebody who's an expert in storm water and hydrology size it.

RE: 12' Retaining Wall Question

I didn't run any numbers, but your wall seems a bit thin (at 8" thick and 12 high). I also know there was a debate on here a while ago, but I'm not sure if those #5 J bars have the proper "hooked" into the footing.

In regards to the drainage behind the wall, I typically provide weep holes in the wall every 8'-10' o.c. With these holes I am just looking to relieve the hydrostatic pressure that may build up behind the wall. I can't say that I was taught to have as detailed a look at the drainage that phamENG suggested, maybe I'm doing it wrong? I am not sure if weep holes would work for you as you are close to the property line and the adjacent owner may object to them.

RE: 12' Retaining Wall Question

(OP)
Thanks for your input thus far. The design I have right now was calculated in RetainPro, not sure how everyone feels about that program, but what I have right now is working. I was wondering if my wall is a little thin. I do have 3 other heights of walls on this project (6', 8', and 10') so I was trying to keep the same 8" thickness throughout so it's easier to build. The active soil pressure in the geotech report was 35 psf so not too bad. I'm also interested in the discussion on the hooked bars because the distance I have them turned in the footing right now I calculated from the section on hooked bars in ACI

RE: 12' Retaining Wall Question

ohh get ready..

https://www.eng-tips.com/viewthread.cfm?qid=401855

Some great information included on stem bar embedment but you might as well sit down and get comfortable before digging in.

As far as retain pro- I have used it (along with the retaining wall modules in enercalc, which are similar) for years, but generally add some detailing/design from CRSI for taller walls.

RE: 12' Retaining Wall Question

Doesn't ASCE7 require 1.6 load factor for Earth Pressure?

Mu = (1.6 x 35 x 11.53)/6 = 14.19k-ft
d = 4.6875 in
a = 0.912 in
Phi Mn = 0.9 x 0.62 x 60 x (4.6875 - (0.912/2) = 141.7 k-in or 11.81 k-ft

The wall is too thin. You need 3" clear cover on backside. I would definitely bump that up. It's not a big deal to change wall thicknesses along the length. I would do so at your control joint locations.

Furthermore, you have #5 dowels at 6" o.c. but your vertical wall reinforcing is at 12" o.c. Are you using the extra dowels for added reinforcing only at the base as it looks like you are running them 3ft up the wall.

RE: 12' Retaining Wall Question

@ StructPono - I do not know that 3" cover is required at the backside stem unless it is cast against soil. It would however be recommended at the footing. Currently that 12" thick footing looks suspect considering 3" clear to bottom steel and 2" clear to top steel. Embedment of stem bars may be an issue as well.

@Sokka10
I have not reviewed thoroughly, but here are some thoughts
-See above link for stem hook length and extension in footing - you may want to provide a bit more horizontal length to your hook..
-Generally stem bars are hooked towards the toe. For your case where there is little/no toe the condition may merit a bit more thought/detailing
-If you have access to CRSI design handbook you might take a look at the use of "D" bars at the stem base
- A 10" or 12" stem may be a bit better suited for your case, but the 8" might be able to be made to work
-Footing thickness looks a bit thin for 2 layers of reinforcement/stem bar embedment

RE: 12' Retaining Wall Question

That is a pretty tall wall. Looks too thin, although I don't think you need 3" concrete cover on the backside; 1.5" should be enough as the surface will be formed, not poured against soil. I would have thought you would need a key to engage more passive pressure in order to provide adequate lateral resistance.

Drainage is an important issue which needs to be carefully considered, along with site grading. Weep holes in the retaining wall, if used, would drain on the neighbor's property which could raise an objection.

BA

RE: 12' Retaining Wall Question

I've not run any numbers, but your footing thickness appears to be a bit light. Your vertical wall reinforcing looks light. You might want to taper the wall from 12" at the base to 8" at the top. You might want to hook the top layer of your footing rebar into the toe. I'm not sure if there are any issues with draining your wall onto your neighbours property.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

RE: 12' Retaining Wall Question

echo...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

RE: 12' Retaining Wall Question

SteelPE - weep holes are great for bulkheads and retaining walls within a property line, but I try to avoid them on property line walls for the reason you suggested. If you're not punching through every few feet to drain the water, then I think the more in depth analysis of the drainage is needed with a wall this large as the hydrostatic build-up would get significant in a hurry.

For the record, my comments earlier were generic - I wasn't in a position to look at the attachment (sorry, should have said as much).

I agree with what others have said - your footing is way too thin and the wall looks a bit spindly, too. The hooked bar may be the right dimensions for a hook, but you lack sufficient embedment in the footing to develop it. ACI 318-14, 25.4.3.1 (a) gives 11.86" development length for a #5 hooked bar. I'd make your footing a minimum of 18" thick.

This will also be very susceptible to deflection. To use active soil pressures, the wall has to move (otherwise it's at-rest, which are much higher). A wall that high with a small angle of rotation is still going to be noticeable. It's more work in building it, but if you have room you may want to consider a slight backward slope to the outer face.

RE: 12' Retaining Wall Question

That's why you tilt the tapered wall on the outside face so that with rotation, it becomes more vertical. It may be that the drainage holes are prescriptive and that little or no water flows... I've seen lots of them that didn't 'leak'.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

RE: 12' Retaining Wall Question

I will retract my earlier statement of 3" clr. cover on backside of stem. I agree with others, running some quick numbers, the heel is way under capacity.

Sokka10, are there any other structures (house) within 12ft of this retaining wall? If so, that surcharge load would have an additional affect on your wall.

Tapered walls CAN work but need to be carefully detailed. If the wall height is not uniform across the length then forget it. You're better off going with a tapered retaining wall and changing geometry at the joints. Several years back, we were working on a project where we were trying to value engineer another firm's bridge design. As part of the bridge design there were lots of cantilevered concrete retaining walls (some as high as 24ft). It was very obvious that the EOR had designed all of the walls using Retain Pro and copied verbatim what the program had spit out as its output detail without using any judgement. This included tapered retaining walls with stepped footings which would have required all the formwork to be warped. As part of our VE package, we redesigned all the walls using a stepped wall design as the original design was unbuildable. Long story short, VE doesn't get accepted for political reasons, and original design stays. Massive contractor change order ensues due to these retaining walls. Be careful with tapered stems on retaining walls if you don't know how to detail them.

RE: 12' Retaining Wall Question

@phamENG what fc and fy did you work with to get that dev length
oo ok tried 60ksi and 4ksi

but what about the 0.7 modification factor for cover

RE: 12' Retaining Wall Question

fy=60,000psi
f'c=4000psi

ldh=[(fy*(A bunch of factors I assumed to be 1.0))/(50*λ*√f'c)]*db

ldh=(60,000psi*1.0)/(50*1.0*63.25)*(0.625in)=18.97*0.625in=11.85854123in

RE: 12' Retaining Wall Question

Quote (Tapered walls CAN work but need to be carefully detailed.)


I've used them lots of times... and much easier to construct than you would think... just a matter of tilting the form... tapered on one side only... and snap tie connections are easily 'shimmed'. Rebar cover is automatically adjusted by the tilted form... it's not really difficult to do and 10' to 15' is a good height for it...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

RE: 12' Retaining Wall Question

i think retain pro prolly uses the 0.7 factor and told him his dev length is ok

RE: 12' Retaining Wall Question

Quote (A wall that high with a small angle of rotation is still going to be noticeable.)


That's why you taper it... when it rotates, the face becomes more vertical... if it rotates more than 4", you may have other issues...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

RE: 12' Retaining Wall Question

dik - that's why I suggesting sloping the outer face in the next sentence...

wrxsti - you could be right. I don't like using that in a footing where development is absolutely critical to performance of a 12' tall retaining wall. I'd still make it a bit deeper.

RE: 12' Retaining Wall Question

Regarding the hook dimensions - Take a close look at the thread referenced above for discussion of anchorage vs development of the stem bars. I will leave the argument in the hands of others on this forum to keep from rehashing old discussion, but you might want to seriously consider providing more than a standard 90° hook at the stem base, even if you thicken the footing to provide appropriate development of the hook. As mentioned, anchorage in your case will be a bit more complicated also due to the limited toe condition.

https://www.eng-tips.com/viewthread.cfm?qid=401855

RE: 12' Retaining Wall Question

(OP)

Quote (STrctPono)

Are you using the extra dowels for added reinforcing only at the base as it looks like you are running them 3ft up the wall.
Yes, that's correct.

Quote (STrctPono)

Sokka10, are there any other structures (house) within 12ft of this retaining wall?
No, the house is set back about 25ft from the wall.

Quote (dik)

You might want to taper the wall from 12" at the base to 8" at the top.
Unfortunately where I'm at (Utah) tapered walls are never used for residential retaining walls. I can only imagine what the contractor will say if I give him a tapered wall design, but it wouldn't be anything good. And then he'd have another engineer design the wall for him lol.

Quote (wrxsti)

i think retain pro prolly uses the 0.7 factor and told him his dev length is ok
Yes, that's exactly what I used to get a development length of approximately 9.5"

Ok so I've decided I'll go with a 10" thick wall based on everyone's comments. As far as the hooked rebar issue is concerned, I read through that entire thread that RWW002 linked to and now I'm more confused than ever. I've based my hooked bar embedment and hook extension straight from ACI, but it sounds like that is not adequate? Thanks for the suggestion of looking at CSRI.

RE: 12' Retaining Wall Question

@Sokka10 if youre getting 9.5" then you're prolly on a default of 3000 psi concrete strength
and that 9.5" still wouldnt meet the req for 12" thick base and 3" cov

also phamENG made an excellent point for providing above this value for such a critical
part of the design

RE: 12' Retaining Wall Question

Quote (Sokka10)

As far as the hooked rebar issue is concerned, I read through that entire thread that RWW002 linked to and now I'm more confused than ever.

You are not alone good sir. There are a few of these such threads where it's clear something is going on that I had never previously thought about (awesome!), but at the end of the thread the only thing I am sure of is that I had it wrong and still have it wrong (not so great...my prior false confidence made for better sleeps). KootK has a couple such wowzahs when it comes to reinforcing details especially as they pertain to moment connections.

Here have been a few of my takeaways from some of those threads that pertain to your situation (note: these are my takeaways and see above for the only thing I am sure of is that I don't really understand things. So take this with a mountain of salt):

A) Although reinforcing development length calculations could take into account more global concrete conditions, they pretty much dont. Meaning all the development length tells you is the length required for being able to achieve full yield of the bar based on local bond/bearing, not that adjacent concrete wont exhibit a failure prior to the yield stress occuring.

The way I've rationalized this is to think about a simple rectangular prism made of concrete with a single rebar developed into it with some portion of the bar sticking out. The prism is some distance larger then the development length of the rebar, and has an infinitely strong clamp at the end where the bar is not sticking out. Clearly if I pull the rebar to the point of inducing the bar yield stress, it wont break from the concrete. Well, because I've developed it after all.

But that doesn't mean I wont have a tensile failure of the concrete between the end of my rebar and the infinitely strong clamp. Surely that capacity would be governed by the concrete alone and dependent on the size of my prism (after all my tensile forced needs to be resolved somehow so it needs to go through that chunk of unreinforced concrete), but the development length calculation never took that into account!

In other words, it's not a develop it and forget it kind of world out there!

B) If you agree with (A) then it becomes clear you need to check for more global concrete failures associated with your bar layout. In the case of a moment joint, such as your retaining wall, turning the wall bars into the heel induces a uplift/tensile force at a corner of concrete that is incredibly weak. In other words, that area might fail!

KootK argues that turning the bars towards the toe induces a compression strut in the concrete. Plus you get the benefit of passing your bar under more confining concrete (passes through COG of the wall). Seems reasonable. Though compressive failure of the strut could still happen and adding upper matt is ideal practice to mitigate.

C) If you cannot develop your bar into the toe because it is rather small then hooking or using a trombone will work. If you want to hook the bar into the heel you can do so but must detail the opening joint such that steel crosses it to minimize the crack opening.

RE: 12' Retaining Wall Question

Draw yourself the moment diagram for the stem and footing and ask yourself “How does this work at the base of the stem?”

Retaining walls are relatively simple. You should do this one by hand before turning over the design to a piece of software.

Using the active pressure only is pretty optimistic. All other cases (at-rest, hydro, surcharge) significantly add to your loading. Be careful about skinnying the design down.

RE: 12' Retaining Wall Question

I’ve never really trusted weep holes to not get clogged even if the neighbor wasn’t an issue.

RE: 12' Retaining Wall Question

(OP)

Quote (JLNJ)

Using the active pressure only is pretty optimistic. All other cases (at-rest, hydro, surcharge) significantly add to your loading. Be careful about skinnying the design down.
Sounds like maybe you're advocating for using the at-rest soil pressures in cantilever retaining wall design rather than the active pressure? Seems like that has been another topic of considerable debate on various threads.

Quote (Enable)

If you cannot develop your bar into the toe because it is rather small then hooking or using a trombone will work.
When you say hooking you mean the stem bars that come down into the footing and then curve back up into the other face of the stem, correct?

RE: 12' Retaining Wall Question

You say the wall "works" at 8" but....what equivalent fluid pressure are you using for the design?

I wonder what type of soil you are in. For a wall like this, I might consider an MSE wall since it looks like you're going to be moving a lot of soil anyway. Around here (Cincinnati OH), secant or tangent walls are also used (not as pretty though.

To answer your questions though:

#1. Seems like the footing will help as long as you can develop the moment in it.

#2. Regarding drainage, I'm not sure where a "code" specification might be but I'd want to make sure there is a place for the water to go. In other words, if you have weeps - Is it ok for the water to weep on to the neighbor? If you have a drain pipe behind the wall, where does it daylight.

RE: 12' Retaining Wall Question

You also lose 1 1/4” of your depth in the stem when you offset the dowels from the verts. I don’t think you want this but that’s what the cartoon looks like.

RE: 12' Retaining Wall Question

Quote (I’ve never really trusted weep holes to not get clogged even if the neighbor wasn’t an issue.)


If you put a proper rodent screen in them, I've had them work for decades.

I'd hook the vertical dowel going into the wall, the other direction, so compression is 'pressing' on the hooked part... other than that and hooked top bars in the footing... getting the As correct is the only issue... maybe added temp bars for the thickened part.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

RE: 12' Retaining Wall Question

It looks like this has been said, but I couldnt stress this enough...........
Most errors in retaining walls occur because the proper development lengths have not been addressed. Youre not even showing the minimum hook embedment to assure adequate development of your wall stem tension steel bars. Check your ACI 360. Make sure the tension steel you rely on, actually has the ability to develop its strength.

Also, detailing is everything with concrete!!! Detail your hooks so they are in a compression zone.... in other words, consider flipping your hook so the hook extension is in a compression zone, not a tensile zone.
.
Consider adding hooks to the footing steel where you rely on the steel, but do not have enough length to develop a straight bar......
.
Concrete design is about proper steel detailing, to be sure the tensile strength you rely on can actually be developed. Even bars in shear require full development in tension, or they just act like anchor bolts.....................
.
That said, I agree with comments above too... 8" is too thin. Dont go above 8ft with an 8in wall. Use a tick lower stem if you want to maintain 8" at top.
.
The footing is too thin for adequate dowel development, or to act as a rigid footing... you can check it as an elastic beam on grade.
.
Always add a water table substantially above your drain... count on poorly maintained drainage. Add filter fabric around your perforated pipe. Provide water proof membrane against the back side of wall so water doesnt penetrate and rust your steel.
.
Get use to concrete behavior.

RE: 12' Retaining Wall Question

With the dimensions shown in the detail, you'll need fairly high soil bearing capacity.

We typically hook the dowel bars into the bottom of the toe, and don't add any other transverse reinforcing in the bottom of the footing.

With the toe being that short, you'll likely need to do as dik suggested and hook the top reinforcing at the front face of the footing. In addition to checking the development of the dowel bars, you'll need to do the same check for development of the hooked top reinforcing from the critical section at the rear face of the stem wall.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: 12' Retaining Wall Question

robust

RE: 12' Retaining Wall Question

(OP)

Quote (wrxsti)

i found some doods masters thesis
Really great stuff here. Thanks for sharing. From everything that's been said here and on other threads it sounds like those 'D' bars are really important in getting full moment capacity out of the joint but most engineers aren't putting them in on smaller scale projects. In that thesis he makes it sound like having top bars in the slab is a good alternative for putting in the diagonal bars.

RE: 12' Retaining Wall Question

i dont think it is a requirement but you may not be able to get sufficient anchorage on
prescribed development lengths or geometry limitations (like your case)
NOTE: anchorage cannot be achieved by bending into the heel as your tie needs to be anchored into the node

the guy describes this when his FEM results have better performance than Nilssons experimental results

i think the takeaway though was your detailing of bending towards the heel

which D bars will not help the diagonal tension failure here (SEE FIG 3.13 where sufficient anchorage was provided)

see his quote "The use of looped reinforcement arrangement of the main tension reinforcement properly done gave the
best result. Adding a diagonal bar at the re-entrant corner improved the result even further for looped details.
However, the use of diagonal bar does not help significantly for details susceptible to diagonal tension
cracking
. However, where stirrups were used for such details, and placed in a fan-shaped arrangement (i.e.
smearing outwards from the inner corner to the outer, thus crossing the crack trajectory) improvement were
made and the joint thus reached full capacity. This is however contrary to the result Nilsson (1973) obtained
with stirrups, as the stirrups gave marginal benefit in that case, without reaching 100% efficiency. More
experiments from other authors could clarify this discrepancy.
On the impact of increasing joint size for a corner joint susceptible to failure by diagonal tension failure,
three sample of different size with comparable reinforcement ratio (for the tradition layout in figure 3.3a).
The specimen sizes were 150 × 250 𝑚𝑚2 , 150 × 350 𝑚𝑚2 and 150 × 450 𝑚𝑚2. Thus, with each
increase in specimen thickness, the lever arm increased. As expected, the failure load increased with
50 increasing specimen thickness, thus the section could carry more. However, the joint efficiency reduced
with increasing thickness, as the sections still failed by diagonal tension. Diagonal tension failure depends
on the tensile strength of the concrete (where stirrups or crossing reinforcement is not provided), and thus
increasing section thickness only increases the length of the crack path, but not concrete tensile strength.
"

the guys STP model shows how to calculate the transverse tensile force. (im not a STP expert though to confirm the accuracy)

see his quote "Thus, a tension force of 145.4𝑘𝑁 is estimated from the transverse tensile stresses within the joint. Even if cracking is not prevented, the structural detail of the wall-base connection should be capable of preventing diagonal tension cracking failure from these tensile stresses. Typical strategies used for this purpose includes bending the main reinforcement into a loop within the joint, use of inclined stirrups to control the cracks, use of bent bar to cross the strut path etc. This topic was discussed extensively in section 3.4 of this report"


the guy goes on to talk about detailing ties transverse (or inclined stirrups) to this strut to accomodate the diagonal tension

i have not seen any details like this for retaining walls (if anyone would be much obliged)
maybe just traditional vertical ties and stirrups to confine the strut?




but
as suitable quoted since not seen in many cantilever walls
"In concluding this section of the report, it should be noted that Nilsson (1973) regarded the use of stirrups
in corner joints as difficult to construct on site. It could also lead to congestion of the joint which may result
in difficulties during pouring and compacting of concrete. Unlike Nilsson however, several other
researchers recommend the use of stirrups within the corner joint to control diagonal tension crack failure.
"



it seems like the economical solution is to loop the stem steel or hairpin with additional bars SEE FIG 3.17 in my post above
or turn the bend in the direction of the toe (SEE FIG 3.14) <--- this method could suffer from insufficient anchorage hence D bars



RE: 12' Retaining Wall Question

Because Nilsson's tests have been brought up, and the initial design was an 8" stem matching the ~200mm used in lab tests, I'll give a pointer to the following interesting articles on opening corners by N. Jackson (possibly Neil Jackson). Jackson revisited Nilsson's tests and those of another researcher (Noor), supplemented with his own tests. Only overlapping U-bars are considered as that was the ACI recommended detail at the time, but there should be some relevance beyond this. U-bars would be my recommendation for the wall under discussion here.

Jackson's hypothesis is that the ratio of available bond length within the corner to the bar size determines whether full strength and ductility are achieved in the opening corner joint - similar to the usual calculation of bar development. The images below are from the first article (without diagonal reinforcement), with my summary in red. Note that researchers tend to use smaller bars than real structures so Nilsson's raw results would be optimistic for 5/8" bars.










The diagonal bar article gives recommendations on diagonal quantity for a given crack width at service loads. As a bonus, the diagonals increase the section capacity within the joint because they pass through the tensile zone at the critical section, which reduces the stress in the main U-bars, but this is not considered in the ultimate design.

Corners without diagonal bars:
https://www.istructe.org/journal/volumes/volume-73...


Corners with diagonal bars:
https://www.istructe.org/journal/volumes/volume-75...

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