Technical Questions

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Two-way slabs design, ACI 318 vs. FEM

Q. With the advent and popularity of finite element analysis software, engineers can create sophisticated models of two-way slabs supported by columns. To comply with ACI 318, should a designer check punching shear stress with the simultaneous biaxial moments reported by such a model?


A. The code equations were written at a time when structures were analyzed in independent directions. The approach of calculating shear stress independently has been proven to be conservative where frame lines are analyzed independently. Finite Element Modeling can provide a design that would be thinner than a traditional analysis. There is no rule of thumb that can identify when the design has become unconservative for this case. Some design engineers consider biaxial moments for two-way slabs and other elements such as corner columns when pushing the lower limits of the code. It would seem prudent that, if the designer is trying to get down to the half-inch, shear would be checked for the biaxial condition.


References: ACI 318-19

Topics in Concrete: 318 Building Code; Design; Slab

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