Lecture # 09

The Design of Composite Steel-Concrete Stringers

Composite Members.

Loading a concrete slab that is simply resting upon a steel beam will produce slippage between their inter-face. If both are tied together, for example with steel studs, then the composite action of both results in the concrete slab carrying the compressive stresses and the steel carrying the tensile stresses. The new single unit has both materials working in their ideal state and a larger section modulus. The I-shaped stringer has now been replaced with a T-shaped slab-stringer member.

The new unit (1) can provide longer spans, (2) stiffer cross-sections, (3) a reduction of live load deflec-tions, (4) an increase in overload capacity, (6) enhanced resistance to lateral loads, and (7) may decrease the stringer size or weight to provide the same load capacity.

The effective flange width is different for an interior stringer than for an exterior stringer.

For the interior stringer, the effective flange width beff, is the smallest of (1) ¼ of the span length, or (2) the center-to-center distance between stringers, or (3) 12 times the minimum thickness of the slab.

For the exterior stringer beff, is the smallest of (1) 1/12 of the span length, or (2) half the distance between stringers, or (3) 6 times the minimum thickness of the slab.

Steel studs (¾” diameter and 5” long are welded to these curved stringers to integrate the deck concrete slab to the steel stringers to form a composite action.

View from underneath the deck showing the steel lost-forms to cast the concrete deck (Palmetto amd NW 103 Street overpass and connector).