Which section modulus should I be using? Sb or St? 2

[btomcik]

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.

 

[paddingtongreen]

Have you calculated this as though the T-beam was an homogenous material?

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[btomcik]

Yes.

 

[JAE]

Why are you using an old code?

 

[BAretired]

I assume you meant to say ACI 318-63 (not ACE 318-63). Article 901 permitted either of two methods of design, working stress design (WSD) or ultimate strength design (USD).

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

 

[btomcik]

Due to the age of the loft, it would have been designed with ACI 318-63 originally at the time.

 

[btomcik]

BA, we believe that at the time the slab was originally designed and constructed, it was done using WSD so we're trying to check the slab based on those design principles matching what was done back in those days.

 

[hokie66]

ACI318-63 was the first to introduce ultimate strength design, side by side with working stress design. Most of us who started with that code used USD, most of the time, because the universities mostly converted to teaching USD as soon as the code was introduced.