edge column design 3

[RLC32681]

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.

 

[BAretired]

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

 

[rowingengineer]

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

 

[RLC32681]

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

 

[hokie66]

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.

 

[rapt]

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.

 

[seny]

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.

 

[BAretired]

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

 

[RLC32681]

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.

 

[hokie66]

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

 

[canstructural]

Provide a square column at corner.

 

[phuduhudu]

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.

 

[dhengr]

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.

 

[BAretired]

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

 

[rowingengineer]

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

 

[RLC32681]

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.

 

[BAretired]

RLC32681,

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

BA

 

[etbetz]

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.

 

[Robbiee]

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