Which section modulus should I be using? Sb or St?
Which section modulus should I be using? Sb or St?
(OP)
Hi all. Am working on analyzing an old concrete slab/T-beam using ACE 318-63. Calculating the section modulus of the t-beam/slab, I of course get two section moduli - one based on the distance from the neutral axis to the top of the slab St, the other from the neutral axis to the bottom of the t-beam Sb. A big difference between the two.
Am checking the flexural capacity and wasn't sure which one St or Sb I should be using. I had figured since I was checking the allowable compressive stress which is at the top of the slab/t-beam, I'd be using St since that is where the compression block is. It's a simple span so there is no negative moment.
Thanks.
Am checking the flexural capacity and wasn't sure which one St or Sb I should be using. I had figured since I was checking the allowable compressive stress which is at the top of the slab/t-beam, I'd be using St since that is where the compression block is. It's a simple span so there is no negative moment.
Thanks.






RE: Which section modulus should I be using? Sb or St?
Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin
RE: Which section modulus should I be using? Sb or St?
RE: Which section modulus should I be using? Sb or St?
RE: Which section modulus should I be using? Sb or St?
If you want to use WSD, the transformed section is used for calculating properties of the T-beam. As you surmised, you must use St when calculating compressive stresses in the top fibers of the beam.
Using Sb based on the distance from the N.A. to the bottom of beam makes no sense at all since the transformed section is presumed to have no concrete below the N.A., i.e. the concrete in that area is presumed to be cracked.
Using USD seems to me to be more consistent with current practice. Is there some reason why you selected WSD?
BA
RE: Which section modulus should I be using? Sb or St?
RE: Which section modulus should I be using? Sb or St?
RE: Which section modulus should I be using? Sb or St?