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weld for wide flange shape with cover plate
when adding a cover plate to a wide flange section flange, to increase section modulus and moment of inertia, is it necessary to provide a continuous weld vs. a weld with spaces.
if the plate is a36 steel and the wf is a992, how is the new section modulus figured.

you can use a stitch weld or you can use a continuous weld - it depends on strength and whether you have a fatigue condition like for a crane beam (stitch welds are bad for fatigue).
the stitch weld would preferrably be at least a 1 1/2" [38 mm] long weld each side and spaced perhaps between 6 and 12 inches [150 to 300 mm] on center - maybe closer depending on strength.
aisc has a nice section at the end of chapter b that outlines special requirements for the end of the cover plate where you need a length of solid weld beyond the point where the cover plate is no longer required.
section modulus does not depend on fy - at least as long as you are in the elastic range of stress.
recommend welds connecting cover plate to a flange be continuous. the continous weld seals the plate flaying surface, is cheaper to install, has less distortion and, less stress concentrations. the weld should be capable of transmitting the horizontal shear at any point. if you attach a plate only on one side, the beam will camber.
the new section moment of inertia is calculated using composite section parallel-axis theorem.
i agree with jae.
i may add that the section modulus does not depend on fy, however you will need to check stesses in the plate as well as in the wide flange section, and compare them to the allowable stresses.
the allowable stresses in the plate and the wide flange section will be different based on fy.
good luck.
blodgett's design of welded structures has very good examples of cover plate and built-up girder designs and addresses the issues previously discussed.

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