Slab Trussed Rebars

[ugandabob]

I have a question concerning "trussed" or "bent" slab rebar. I want to determine the number of top bars I have at each column. I do not have the Typical Details drawing, so I do not know the lengths of the bar bends. The slab schedule indicates that the bent bars extend about 1/3 of the span past the column.

From your experience, do the bent bars typically bend close enough to the column that they provide top reinforcement to the adjacent span only, or are they bent to provide top steel to both the current span plus the adjacent span.

I hope the attached sketch clarifies the problem.

When I take the more conservative approach of top steel to the adjacent span only, the slab does NOT have the capacity to resist the full specified load. However, it does have the capacity to resist 72% of the full specified load, which I've learn on Eng-Tips was common practice back then (the slab is from the 1940's).

The slab is part of an industrial building. Specified Live Load is 250 psf. The new mechanical unit the client wants to add increases the load by about 3%. Since it is such a small increase in load, and the slab has stood for almost 70 years, it is hard to reject it. But calculating that the specified loads are too much for the slab, it is also hard for me to accept it.

It would be nice to know that I have the additional top steel provided by the bent bars.

Thank you,

 

[BAretired]

The trussed bars were considered reinforcement for both spans. The attached bending schedule is taken from ACI 318-63. It would likely have been the same in 1944.

BA

 

[BAretired]

Note that no bottom steel is continuous through the column, a requirement of today's codes.

BA

 

[ugandabob]

Thank you very much BA; this is very helpful.

 

[BAretired]

you're welcome, ugandabob.

BA