Effective Width of PT Beams

[mijowe]

I am running ADAPT PT and have compared two strips with the same geometry. I designated one as a PT Beam, the other as a two way slab with a continuous drop panel. When I run the PT Beam version stresses are calculated using the ACI 318 effective flange width, which in my case varies per span but is always being governed by the span/4 provision (ACI 318 8.10.2). When I run the strip as a two way slab there are no such restrictions on the effective section the software uses to calculate stresses. As a result I am getting significantly larger PT forces in the PT Beam option.

The ADAPT manual touches on this subject. It states that the ACI rules are for conventionally reinforced beams, and that ACI does not specifically address PT construction. It goes further by saying that effective flanges can be as high as the stem width +24X (ACI is 16X) the slab thickness for interior beams, and half that for L-beams. This recommendation does not even consider beam span.

Any input on effective flange widths would be appreciated. I am fine using the ACI recommendations, but don’t think I should be penalized by calling it a beam as opposed to a two way slab.

 

[rapt]

mijowe,

It is not a flat slab and should not be design as one. This is completely inappropriate (though it is how Adapt were incorrectly telling people to design a couple of years ago).

The beam rules should be used, though I do disagree with the 25% maximum rule in ACI318. It is not in other major codes.