Can Rebar area size of columns vary for different floors? 2

[Mechanicslearner]

Hello,

I have designed 2 storey building but rebar size varies for columns at 4 corners.. bottom storey rebar area size is 700 mm2 and top storey rebar area size is around 1464 mm2. So am I doing anything wrong or results are acceptable?

 

[hokie66]

It is impossible with the information you have given to determine whether you know what you are doing. But it is not uncommon for analysis to show a greater bending requirement in the top floor columms.

 

[dik]

Often your upper floor column reinforcing is greater because of the higher moment anticipated at the beam/slab column junction.

Dik

 

[Mechanicslearner]

two storey building with flat slab and column size is 250 x 350 mm and only in corners and in top columns the rebar area is more.
dik, So shall I use same rebar area as bottom columns?

 

[hokie66]

I hope this is not the same design as your "flat slab" spanning 12 metres. Those are tiny columns. How are you dealing with punching shear? For a flat plate or flat slab, the controlling criteria for thickness is typically punching shear, especially at the edges and corners. Maybe you know that, and have some perimeter beams?

 

[Mechanicslearner]

hokie66, This is different design .. span is 5 m x 4.5 m you mean column thickness should be increased only for 4 corners or even in the columns in the middle?

 

[hokie66]

All I am saying is that you need to design against punching shear, which is a brittle mode of failure. Thickness is your friend. In addition to direct shear, design must account for unbalanced moments which magnify the shear stresses, and that is especially the case at edges and corners. Codes, textbooks, etc. prescribe methods and acceptance criteria for punching shear design.

 

[Mechanicslearner]

is it true that when punching ratio is above 1 means it exceeded capacity? and thus have to incresae thicknees

 

[dik]

"dik, So shall I use same rebar area as bottom columns?"

You have to design them... but this could be unsafe if you require greater vertical reinforcing for the columns above. Safer to use the reinforcing required for the columns above, but, you have to design for the loading... axial load and moment.

Dik

 

[Mechanicslearner]

solved it by increasing column and slab thickness.. Now punching shear ratio is less than 1.. Thank you all for helping, you guys are awesome.

 

[hokie66]

That would be nice if I thought you knew how to design a floor, not just to input numbers into a program and accept results. You haven't given me that confidence.

 

[cliff234]

Please do not take offense, however I am getting increasingly concerned about our profession when engineers punch their designs in to a computer and don't understand how to manually validate the results - or even understand the reasons for seemingly unusual results.

But to his/her credit the original poster is at least questioning the results.

I suspect that the problem lies in how we are educating today's engineers. I suspect that they are being taught how to model and not how to design. They are learning how to manipulate software and they are not learning how structural framing systems work. I don't think anyone should be using structural engineering software who does not know how to manually design the structures that they are modeling. Structural engineering software should be used as a tool to improve productivity, not as a crutch to bypass the need to have a thorough understanding of the codes and design standards and the need to have a fundamental understanding of how structures work.

I apologize for sounding like an old curmudgeon (which I suppose I am).

 

[MotorCity]

I suggest using the larger bar size for the entire column. The amount of material you save for changing the bar size for a single story is negligible. In addition, unless you vary the bar sizes by 2 or more, you run the risk of the wrong bars being installed in the field.

 

[cliff234]

MotorCity - A possible problem with using larger bars is that at the upper level the flat plate is cranking moment into the column and if the column bars are too big, they won't be able to easily hook into the slab, and even if they fit in the slab there won't be enough development length for them to develop. (Hopefully the vertical bars at the top of the uppermost column are hooking into the slab.) I've seen engineers specify #9 hooked column bars into an 8" slab - which doesn't work. There were a few times (decades ago) when I got phone calls from the field asking if they could cut hooks off the top of column vertical reinforcing steel. I always went out to the field to look at the situation - and every time I went out, the hooks were already cut off. I learned the hard way. So I tell young engineers in our office that if they ever get that call, they can rest assured that the hooks were already cut off. (But hopefully they won't get that call, because we strive to prevent this from happening.)

 

[JAE]

I would agree with MotorCity but alter their statement to "larger bar area" instead of "larger bar size".


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