RC Flanged Section

[abooos]

I am trying to calculate the nominal moment strength of the attached, but I keep getting the compression block, a = negative number. I am using
a = [(Asfy-A'sf's)/(0.85f'c)-(b-bw)hf+A's]/bw

Would you know what I might be doing wrong?

Thanks,

 

[Qshake]

I would suggest going over the formula in the text you have and make sure you're using all the correct formulas. Check for any math errors.

Such a problem is very fundamental and as a structural engineer or civil engineer you are expected to know how to check your work.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[BAretired]

I think there are several things you are doing wrong, but the most serious one is in failing to use common sense. If you will notice, the tension steel has an area less than the compression steel, so it is not surprising that the first term in your square brackets is less than zero and that when you subtract something else, it will be even less.

Next, you do not define the term 'hf'. What is that supposed to mean? Are you expecting us to be mind readers?

My advice to you is...do not simply apply formulas. Try to understand what they mean. Until you do that, your designs cannot be used in structures where human life is at stake.

BA

 

[TXStructural]

hf [h(sub)f] is the depth of the flange.

To start with, if the depth 1" at the top were correct, you do not have sufficient cover for corrosion protection or development of reinforcement. With 1" to the centroid of the longitudinal steel, and assuming 1/2" (#4) transverse bars at the top, your cover is 1/4", which is less than the tolerance for cover or bar placement. I have seen concrete work with approximately zero cover but it is never permitted by code. You probably have a code provision (ACI 318 or equivalent) which specifies 3/4" cover, which is to the surface of the reinforcement nearest the surface, not the centroid of longitudinal reinforcement.

For the example, where compression steel yield strength exceeds tension steel yield strength, it is conservative to consider the distance between the centroids of reinforcement as the moment arm. You have a tension-controlled section, and while the centroid of the theoretical compression block is located above the compression steel, engineering design is about practical design as much as it is about theoretical precision.

 

[csd72]

abooos,

for flanged beams I would normally ignore the compression reinforcement and treat it as singly reinforced. It really does not make that much difference.

It is fundamentally critical that you understand the theory behind these formulii as they are only approximations of reality. They are perfectly accurate 9 out of 10 times but for that tenth one you need to understand the shortcomings of the formula to ensure its safe application.

You are also recieving a bit of sharp feedback due to your lack of attempts to help yourself. A google will bring up a huge amount of worked examples that can be followed to ensure you have the correct methodology. Good luck.

 

[abooos]

Ok thanks for those you have replied and appreciate your comments - even those sharp ones. I guess my confusion was for what is the purpose of so much compression steel, but things are clearer now. Special thanks to csd72 and TXStructura as your comments were most helpful.