33410Connection of Bottom Chord to Supports

In the traditional use of stub girders, the girder is supported as a simple beam and the bottom chord end connections need to be able to transfer vertical reactions to the supports. The supports maybe columns or the girder may rest on corbels or other types of supports that are part of the concrete core of the building. For both of these cases the reactions that are to be carried to the adjacent structure are given by the analysis.

Any shear-type beam connections may be used to connect the bottom chord to a column or a corbel or similar bracket. It is important to ascertain that the chord web shear capacity is sufficient, including block shear.

Some designers prefer to use slotted holes for the connections and to delay the final tightening of the bolts until after the shoring has been removed. This is done on the premise that the procedure will leave the slab essentially stress free from the construction loads, leading to less cracking in the slab during service. Other designers specify additional slab reinforcement to take care of any cracking problem. Experience has shown that both methods are suitable.

The slab maybe supported on an edge beam or similar element at the exterior side of the floor system. There is no force transfer ability required of this support. In the interior of the building the slab will be continuously cast across other girders and around columns; this will almost always lead to some cracking, both in the vicinity of the columns as well as along beams and girders. With suitable placement of floor slab joints, this can be minimized, and appropriate transverse reinforcement for the slab will reduce, if not eliminate, the longitudinal cracks.

Data on the effects of various types of cracks in composite floor systems are scarce. Current opinion appears to be that the strength is not significantly influenced. In any case, the mechanics of the short-and long-term service response of composite beams is not well understood. Studies have developed models for the cracking mechanism and the crack propagation [15]; the correlation with a wide variety of laboratory tests is good. However, a comprehensive study of concrete cracking and its implications for structural service and strength needs to be undertaken.

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