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edge column design
3

edge column design

edge column design

(OP)
Hi guys..

I am presently designing a two storey with basement sub-station made of reinforced concrete frames. i have a problem in the edge columns in the upper floor because columns are only provided in the perimeter and support a 20m beam. the beam size is limited to 500x1200mm. Based on my analysis, the moment in the beam support that transfer to the column is very large, thus giving a steel requirement of 50-25MM dia. for the column. the column size is limited to 500x800mm.

Your opinion on how to reduce the column moment is highly appreciated. I would appreciate also if you can share any alternative to design this frame.  

RE: edge column design

Depends on what is causing the moment and what direction it's in.  If it's dead load and live load perpendicular to the edge, you've got a problem.  If it's seismic and wind parallel to the edge, you can add bracing or a shear wall.

RE: edge column design

At these sizes you should be able even to produce very specific details to deal with the problem, this is the high range of spans within buildings.

This meaning that you should be able to modify the general scheme (path of forces, lateral forces resisting system) to allow to deal effectively with your problem with some alternatives, the simple of which to place some hinge there, and if unwanted, then have bigger columns, something even bigger from where to hang or from some nucleus with some cables, very variegated ideas might appear.

I have already said of one case where I dismissed a idea for precast members L T etc that required 60 MPa and awesome sections, well, 10 years later someone has seen opportunity in it and someone has put money enough to keep functioning a glistening precast plant just on these principles. Most of us that do not deal with monster structures are not enough acquainted with the monstrosities of piercing the bottom of the seas with giant pipes, or make shear walls of steel over a foot thick. Some get opportunity of such possibilities and resolve such problems maybe in their case not otherwise more easily resolved.

RE: edge column design

2
I assume your beam is 500 wide x 1200 deep, and the column is 500 wide x 800 deep.  Correct?  What limits the width of your beam and column?  What is the spacing of the columns in the other direction and how thick is the slab between the beams?  With better understanding of the whole structure, we can possibly give some ideas to help.

RE: edge column design

One idea is to post tension the beam to balance some or all of the dead load.

A second is to create a hinge at the top of each edge column and design the beam as a simple span.

A third is to reinforce the beam as a restrained but not fully continuous span and use a reduced amount of reinforcement in the column, ensuring that the sum of the positive and negative moments in the beam satisfy statics.  Some cracking may be expected in the column.

BA

RE: edge column design

I agree with BA. I have done this in the past. The bigger you make your column the more moment it will attract. Hence, you are better off using the third option as suggested by BA. Make sure you provide closely spaced shear reinforcing at the top of the column and provide additional bottom reinforcing in the beam.

In your structural model, you can model this by reducing the moment of inertia of the column by using crack factors.

RE: edge column design

Good points, BA and slickdeals.  But we still need more information from the OP to give good advice.  He needs to answer Jed's query and mine.  The other thing which should be considered is to add an additional column.  20 metres is a long span, and 10 metre spacings can be accommodated in most buildings.

RE: edge column design

Good point, hokie.  If a column at midspan can be accommodated, we don't have a problem.

BA

RE: edge column design

(OP)
Thank you BA and slickdeals, however i consider already that option and requires me a big rebar diameter in order to meet the moment requirement.

BA,

Can you explain how to introduce a hinge at the top of the column. How should it be detail? Sorry for the ignorance.

Jed,

the load that governs is the combination of dead load and live load. it is parallel with the span of the 20m beam.

Hokie,

Intermediate column is not possible is not advisable because of its occupancy. for the beam, 500mm is the width and 1200mm is the depth. you are also correct with the column, 500mm wide and 800mm depth. The distance between columns in the other direction is 6.2m and the slab thickness is 150mm.

we can adjust the width of the column and beam but i think it would increase the dead load and one thing more the moment is acting perpendicular to the width of the column. Please correct me if i'm wrong.


 

RE: edge column design

RLC32681,

I don't believe a hinge at each column is your best option because the beam is too shallow for a simple span of 20m unless it is prestressed.

You said

Quote:

however i consider already that option and requires me a big rebar diameter in order to meet the moment requirement.

You do not have a specific moment requirement to meet.  Ignore the elastic analysis and consider:  

Simple span moment = wL2/8.

Mneg = wL2/24 (minimum recommended).

Mpos = wL2/12.

I would be inclined to boot that up to Mpos = wL2/10 for good measure.

In the circumstances, that is a better solution than a hinge at each column, but you must expect some cracking in the column.
 


 

BA

RE: edge column design

You can always add compression steel in your beam which will also help in deflection control.

RE: edge column design

Try turning the column 90 degrees so the 500 dimension is parallel to the span.  As a general rule for constructability, it is not good practice to use the same width beam as column, as the reinforcement clashes.  And I agree with BA, the beam should be post-tensioned.  Balance all the dead load with the prestress, and use bars for the live load.

If you have other means of coping with the horizontal forces, you could design the beam to column connection with a bearing, similar to bridge construction.  This would eliminate bending from the connection.

RE: edge column design

hokie66,

You cannot ignore all of the dead load in that case. Factors still need to be applied so

1.2 * SW + 1.2 * SDL + 1.5 * LL + 1.0 * PT (factors vary depending on code)

So, if you balance SW, there is still
.2 * SW + 1.2 * SDL + 1.5 * LL

if you balance all DL, there is still
.2 * SW + 2 * SDL + 1.5 * LL

remaining.

Also, I would question a slab depth of 150mm spanning 6.2m. You might be able to make it work with PT, definitely no as RC.

RE: edge column design

I agree with rapt.  The slab depth is not adequate for the span.  It seems to me that the OP is trying to stretch permissible spans far beyond where they ought to be.  It would be prudent to take a look at the building code in your area.

BA

RE: edge column design

rapt,

Yes, I agree.  My suggestion was too simplified.  And I also agree that the slab is too thin.

RE: edge column design

Is it possible to space the columns on the other direction to 3.1metre instead of 6.2 metres.More over 150 mm thick slab for 6.2 metres span will not be sufficient and by reducing the spacing there will be much reduction in loads/moments

francis

RE: edge column design

(OP)
Post tension is a very good solution for this project; however, the contractor which is our client demands to use reinforced concrete. I have made a further analysis and incorporated your opinion guys, I came up with an idea of extending the rebars coming from the beam to the column to satisfy the required moment. FYI, huge moment in the column occurs only at a distance 2m below the beam. May I know your comments guys with my solution. I would appreciate if you can add some recommendations.

BA, Francis, Hookie

Thanks for reminding me about the slab thickness, I will check the deflection if it is okay otherwise increase it to 180mm.

RE: edge column design

RLC32681,

Is there a reason you cannot turn the column 90 degrees as I suggested?  That would reduce the column moment considerably, at the same time increasing the beam moment.

It will increase the beam deflection also, and 20/1.2=16.7 is already pushing it for a simple span reinforced floor beam.  Some compression reinforcement will help, but the beam really should be tensioned.

RE: edge column design

I think you need to discuss these things with your supervising engineer.

Extending the beam bars is possible but the details depend on the code you are using. And the bars will have to develop past the point where they are needed using full tension development. And the builder will hate you.

RTe the concrete thicknesses, we are not suggesting the slab thickness might not work, we are saying it WILL NOT work. If you design shows it will, change design tools!

I hope you client does not mind deflections!  

RE: edge column design

I can assure you, you will find it very difficult to detail the beam column connection. Your max moment is on the outside top edge of the column/top end of the beam so you will end up having to take your column bars round the corner into the beam. The internal radii of these bends need checking for concrete bearing strength. If you have more than 1 layer of reinf then you have additional problems. And if you have a perpendicular beam coming in as well then you have another problem. For this type of detail it is the detailing that can determine the member sizes rather than plain member section size calcs. The beam span just does not seem practical.

RE: edge column design

(OP)
hokie,

I try rotating the column by 90 degrees but im sorry to tell you that column moments doesn't  decrease, rather it increases.

Rapt,

Im using ACI code, it seems that extending the bars is more economical option since huge moment is only near the roof beam. i try to check the deflection but it does exceed the req't of the code. Regarding slab thickness, can you please discuss further why it will not work. Sorry if i did not get your point.

phuduhudu,

You are correct that detailing is the problem but i'm planning to increase the size of the beam and column at the point where the huge moment occurs by tapering it.

Thanks guys for your opinions, additional comments and recommendations would still help. I appreciate if you can share engineering solution for this project.
 

RE: edge column design

The column stiffness of a 500 x 800 column is least when the 500 dimension is parallel to the beam.  You should find that this results in the smallest moment.  If not, you are doing something wrong.

BA

RE: edge column design

Perhaps the column was originally orientated to be 500 deep and rotating it 90deg increased its stiffness, hence the moment increased.

Regarding the slab thickness;  a 150 thick slab spanning 6.2 m will have L/d of about 50; deflection will be excessive.

RE: edge column design

RLC32681,

If you do not have the experience to know, as apsix has pointed out, that an L/d of 50 for an RC slab will not work, then you should not be sizing this structure without doing the calculations to justify it. One way RC slabs definitely will not work at this L/d. An experienced design engineer would know this. An inexperienced design engineer should be talking to his supervising engineer about it, not us!

In fact, PT slabs will not work either. They will fail in vibrations, probably ductility and be very expensive to make work in deflections. For an RC slab, you have no chance in deflections either!

If you have changed your column to 500 in the direction of the beam span and 800 wide, the moments must reduce in the end column and increase in the beam!

RE: edge column design

The ACI has minimum requirements for thickness of members  for a slab take a look at TABLE 9.5(a)

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

RE: edge column design

rowingengineer,

I think you should point this out to RLC32681, not me. But I think these numbers back up our arguement with minimal calculation required. For a 6200 end span, 6200 / 24 = 260mm thick slab!

Now the beam is in trouble due to extra load, and the columnn moments just got a lot bigger!

I do not base design on L/D ratios, I believe in calculating estimates of real deflection (not kcs fudges either).

RE: edge column design

Rapt, my comment was aimed at RLC32681, as he had stated earlier that he was and is using ACI code.  

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

RE: edge column design

(OP)
I think ACI equations 9-12 or 9-13 must be use in the computation of minimum slab thickness since it is two way slab.

RE: edge column design

9-12 or 9-13 for minimum thickness - yes, but it's not final required thickness check for particular case. after you need to do strength & deflection calculation to prove it (or to go w/ thicker)          

RE: edge column design

RLC32681,      

You said:

Quote:

I think ACI equations 9-12 or 9-13 must be use in the computation of minimum slab thickness since it is two way slab.

I don't have the ACI code to check the equations you cited but I understood this to be a one way slab.

BA

RE: edge column design

I was of the opinion this is a one way slab as well.  

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

RE: edge column design

We were told of beams spanning 20m at 6.2m centres, with a 150mm slab.

We should have insisted on a sketch at the onset.

RE: edge column design

apsix,

A case of garbage in, garbage out!

But then there must be secondary beams! If not, it is  still a 1way slab.

Even so, 150 thick will not span 6200mm as a 2way slab!

And I would like to see the calculated deflections for 1200 deep 500 wide beams spanning 20m as an end span!

And interestingly, hokie66, when I run it with the columns about their weak axis, the column moments drop a lot! Who would have thougt that!

RE: edge column design

RLC32681,

I strongly feel the 20m span beam would have a maximum positive moment & somwehat reduced moment at the column location as this is an end span & a very long end span!

FYI I have used 150 thick slab for 7m x6.5m two way slab using AS3600 with compression reo for an office block which is still rock solid! If your slab is oneway, i would be thinking around the 180-210 thk mark using 40mPa rc concrete.

Would like to see a plan sketch.

RE: edge column design

what moments did you ahve to get compression reo in a 150mm slab? What % of the compression reo did you manage to engage? I have never heard of compression reo in a 150mm slab before.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

RE: edge column design

I agree that a sketch is in order.  The OP has not stated that this is an end span.  I thought it was a single span with an edge column each end.

BA

RE: edge column design

(OP)
sorry if I did not mention erlier that I have an intermediate beams. As requested I attached here the framing plan for further reference.

seny,

As stated in the ACI code, the minimum thickness of two way slab is computed using either equations  9-12 or 9-13 depending upon the value of alpha. Slab thickness less than the minimum required by 9.5.3.1, 9.5.3.2, and 9.5.3.3 shall be permitted where computed deflections do not exceed the
limits of Table 9.5(b).

RE: edge column design

Aha!
nice sketch, the 150thk slab sounds right to me based on that sketch. i also note that its a roof slab. Would there be a BBQ plate up there with some VB beer? I would be calling & rechecking for a 4kPa live load if it is. Just a thought.
 

________________________________
Use RAPT for Slender RC Columns

RE: edge column design

80smetalfm,
I would like to here how you got compression reo to work in a 150mm slab when you have the time.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

RE: edge column design

80smetalfm,

Actually, I would like to know how he got it into compression at all.

A lot of designers assume that reinforcement at the top at mid span is "compression reo". Unfortunately it is "compression face" reinforcement only. It is not in compression and cannot be used to reduce kcs in the deflection calculations in AS3600. You have to PROVE that the reinforicement is significantly in compression in the first place, and you would not be able to do that in this case.

Would be interesting to see the deflections, and the sketch!

In my opinion, another case or garbage i, garbage out, this time into code formulae.

80smetalfm
I would not be boasting about this to anyone. I cannot believe that the deflections are acceptable.

RLC32681
Now that the slab spans are 2way and about 4m, yes they should work at 150, depending on loading. But I would still be worried about the beams. I noticed on your plan they are now 1450 deep, instead of 1200!  

RE: edge column design

500 x 1450 beams with a clear span of 18 600.  It is starting to look pretty good.  Check your deflections, though.  They could be fairly large.

BA

RE: edge column design

RLC32681,
your slab looks better than it sounded 1450 it s positive step. and 150slabs looks good.

sorry for the off shoot,
Unfortunately rapt you are correct, i know of two people with senior engineer on there business cards that up until last week had the wrong opinion about steel in the top face. Luckily they are mainly focused on steel with a few mezzanine floors the only concrete design they do.

This is why I want the Kcs clause removed from the code but if they must keep it should be updated to be based on Fc' (or fcm or cement %, or something relevant) and restrict members of 250mm or more in depth.

80smetalfm,
I am still keen to hear what you have to say, what is your interpretation of the code?

 

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

RE: edge column design

(OP)
The beam dimension is 500x1200 instead of 500x1450. Im sorry if i did not update the drawing. The depth was limited to 1200mm. The slab carries materials for waterproofing and minimum live load. So far, based on analysis the deflection did not exceed the allowable deflection provided by the code.

My problem here is the placement of the rebars in the columns since it requires about 6%. As i mention earlier, I am thinking of extending some rebars from the beam to the column (2m below beam) to satisfy the requirement.

I would like to know your comments/ opinions about this. Suggestions are very helpful also.

Thanks

RE: edge column design

Until you understand the principle that stiffness attracts moment, thus a 500 deep x 800 wide column will have less moment than a 800 deep x 500 wide column, I am afraid I won't be able to help any further.

RE: edge column design

RLC32681

I think I will say what hokie66 is thinking and suggest you change the structure.

I do not think you can detail this column/beam connection to work properly. Draw if up to a large scale with bars drawn actual width to see how it fits together.

I do not think you should use the top beam bars cogging into the top of the column as the column bars.

The beam itself works if you are happy to accept a deflection of between 60 and 100mm depending on a lot of details we do not have. I would not be happy to accept it.

And if your software/analysis is still saying the moment into the column is larger if the column is rotated to bend about the weaker axis, listen to hokie66 and get new software or check it again.

RE: edge column design

seems slab is not a guy of this layout. as per sk 150 is ok, as far as load is in border of regular for roof. could consider thicker slab to implement T-beam conditions, but even so deflection is in question.         

RE: edge column design

RLC32681

Have you considered using precast double or single tees spanning 20m onto either cast-in-place or precast edge beams?  That would eliminate all of the interior beams and would seem to me to be more economical than the cast-in-place scheme you have selected.

Also with a building length of 50m, it might be prudent to consider a contraction joint for a cast-in-place roof.

BA

RE: edge column design

(OP)
Actually in the first place that i saw the plan, the first thing that comes in my mind is to use double tee precast concrete but the client opposed my suggestion and demand to use cast-in-place RC since that is the usual thing they do. I have no choice but to used reinforced concrete.

I am planning also to make haunce in the beam support to eliminate rebar placement problem. Is it possible? I appreciate your opinion on this option.

RE: edge column design

A haunch is possible.  It will attract even more moment to the column.

RE: edge column design

Provide a square column at corner.

RE: edge column design

I have recently done this detail and have made the column edge bars extend over and into the beam. I had to draw the whole detail up in 3D to satisfy myself that it worked as it is very difficult especially if you have another perpendicular beam framing into the column to figure out the clashes with the bends. Your rebar at the corner needs to have an adequate bend radius. I think in ACI you use a minimum of 8d so there is no problem but with the BS the standard bend is 3d so there is a real danger of crushing at the interior of a fully stressed bend.

As others have said, the haunch will definitely help. It will also attract more moment though that won't be easy to analyse. I would suggest analysis using your column member section to the underside of the beam (or the haunch) and the beam section to the edge of the column (or haunch) and then using a rigid link (or a very stiff element) in between. That will give you a more realistic model to figure out how much moment the column attracts.

RE: edge column design

This design by committee, with so few of the real facts and details in hand, seems incredibly dangerous.  Who undersized these beams and slabs during the prelim. design phase and who did the plan we finally saw?  They should be answering his questions.  And, he should be man enough to ask them how to do the impossible, when he isn't ever sure how to do the possible and isn't absorbing your suggestions.  Since no one else has asked yet, (with the exception of hokie66 on 24SEPT, in slightly different words) I will ask; are we being helpful here or just encouraging malpractice?  I'm all for helping young engineers become better engineers, but they've got to have a good grasp of the basic subject concepts to progress.  And, we shouldn't be encouraging non-engineers who try to practice real structural engineering, on significant structures, just because they have a structural analysis program that they don't fully understand.

RLC32681:  When you ask a question you have to give enough info. so you stand a half a chance of getting some intelligent advice which pertains to the problem at hand.  You got some good gut feeling and rule of thumb advice given what the others had to go on, but it seemed to go right over your head for the most part.  You need and engineering education, and if you have that already, then you certainly need a real mentor, not our encouragement.  I don't know if you are working in a consulting engineer's office, or what, but knowing a couple guys who have structural engineer printed on their business cards isn't what you need.  You need a real practicing engineer,  who knows the local codes and construction industry, and who signs and seals construction documents and is licenced.  He should be looking over your shoulder and answering your questions in real time and giving you some direction.  You should be able to look at the plans together so he can point out what he is talking about, and/or draw you a thumb-nail sketch of what he means and wants.  He can point you to appropriate text books and other reading, etc.  Without this kind of guidance, you should not be doing the conc. design engineering on this job, you could end up behind the eight-ball on a project this size.  Don't bite off more than you can chew, without a good mentor, that won't help your development.  A good mentor, and/or working under a qualified engineer will give you the guidance and experience you need to advance properly.

That's how I see this thread, irrespective if the final column size or slab depth.  We are not furthering the engineering profession by participating in a charade like this.

RE: edge column design

dhengr,

RLC32681 has provided a drawing of his Roof Framing Plan. I understand that the main beams, shown 500 x 1450 should be 500 x 1200.  

I do not believe the client has selected the most cost effective solution to framing this roof, but if that is his choice, then we have to proceed from there.

I have occasionally encountered problems of a similar sort.  In this case, the column attracts too much moment using elastic analysis.  As I stated quite early in this thread, I don't have a problem ignoring elastic analysis and relying on the fact that moments will redistribute.  

My approach would be to place as much negative steel in the beam as can be accommodated by the column within the 4% rule and to make up for the deficit by adding positive reinforcement as required.  I would expect some cracking in the column as it cannot be avoided given the design restrictions imposed by the client.

I would advise the client of the expected magnitude of beam deflections.  I would urge him to consider alternative framing systems, but if he stubbornly refuses to listen to my arguments, I would be faced with the decision to continue or withdraw from the project.   

BA

RE: edge column design

dhengr,
In a perfect world we would all get to decide what jobs we work on, whom we work for and what our clients impose on us. But last time i checked this isn't a perfect world, and if your a junior it is very unlikely that your really going to have a say in what you work on, or who you work for, and how much they help you. mostly I think you will just be happy that you have a job.

While  RLC32681 may be struggling with this problem with his knowledge level, as least he is trying to understand and get a better feel, who knows he might just be trying to build up his confidence to talk with the senior engineer. Some time us senior engineers think the problem is simple when it isn't, because we didn't spend the time looking at the problem as closely as we should. and a junior will try to stick to our instructions by trying to get it to work just because we said it would.  

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

RE: edge column design

(OP)
rowingengineer,

thank you for your understanding.

dhengr,

don't judge the book by its cover.

BA,

i think as a structural engr we dont need to withdraw from the project instead we take the problem as a challenge to our profession.  

RE: edge column design

RLC32681,

Kindly tell me that again after you have completed the examination for discovery.

BA

RE: edge column design

Have you considered moment redistribution of the negative beam moments ath the column face? If you reduce the negative reinforcement by some percentage, a plastic hinge forms and redistributes the moment in to the positive moment region. You will have to use trial and error to see how much stiffness release you will need at the column-beam connection to increase the positive moment and reduce the negative moments enough to suit your geometry.

This better utilizes the concrete at the middle of the beam, and releases much of the load and reinforcement otherwise needed in both the beam ends AND the columns. Very old technique used for decades and accepted wintin the ACI code.  

RE: edge column design

RLC32681,
Did you consider the T action of the beam and cambering to counteract DL?

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