Q.: In ACI 117-90, "Standard Specifications for Tolerances for Concrete Construction and Materials," what are the tolerances for deflections of concrete slabs placed on metal decking supported by structural steel frames? My client has a problem with excessive deflection.

A.: ACI 117 doesn't cover deflections of composite deck slabs. Section covers top-of-slab level alignment for cast-in-place concrete for buildings, allowing a tolerance of 3/4 inch on elevation of top surfaces of formed slabs before removal of supporting shores. But it wouldn't be appropriate to apply this tolerance to composite construction.

You may have encountered a problem involving excessive deflections in structures built with recently developed higher-yield steels, designed by Load Factor Resistance Design, or a combination of both. We covered this subject in an article by J. Thomas Ryan, "Controlling Deflection of Composite Deck Slabs" (Concrete Construction, September 1997, pp. 734-739). Generally, higher-yield steels and LFRD produce shallower structural-steel sections that safely carry the applied loads but can deflect excessively before the concrete hardens and composite action occurs. The phenomenon is discussed in detail in Ryan's article.