Equivalent Frame Method Simplified for Beam and Slab Construction


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Title: Equivalent Frame Method Simplified for Beam and Slab Construction

Author(s): D. J. Fraser

Publication: Concrete International

Volume: 4

Issue: 4

Appears on pages(s): 66-73

Keywords: beams (supports); bending moments; con-crete slabs; elastic analysis; finite element method; flexural strength; floors; frames; reinforced concrete; stiffness; structural analysis; torsion.

Date: 4/1/1982

Evaluation of stiffnesses of equivalent beams, calculation of initial joint moments, and the transverse distribution of moments after frame analysis are the three most important steps in the Equivalent Frame Method. Current rules in ACI 318 that deal with these items have been criticized as too complex and ambiguous. Current procedures also include the artificial device of an equivalent column. Given that the mathematical model is far from perfect, there is little justification for a complex, seemingly accurate procedure. A revised set of rules for beam and slab construction, that should allay previous criticisms, are presented. The proposed rules, derived from finite element analyses of beam-slab floors, involve direct evaluation of the first two dominant items from relatively simple formulae. The transverse distribution of moments, both for interior and edge beams, is ob-tained from two tables of coefficients. The need for equivalent columns is eliminated because the effect of twisting the spandrel beam is included in the second formulae and tables of coefficients. A numerical example illustrates the application of both sets of rules to a beam-slab structure. These results and the design effort are compared.