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Two span steel beam design

Two span steel beam design

Two span steel beam design

(OP)
I am in the process of designing a steel beam for a new 21' long x 8' high opening in an existing 2 wythe unreinforced masonry building (common brick). I'm going to use a column at the center of the span to reduce the beam size to reduce costs and beam weight. If the plate on top of the column is stiff and doesn't flex, it will inhibit rotation of the beam, so I'm planning to use a 1/2" plate on top of the TS columns. As far as the column design goes, it's strictly an axial load. I'm going to attach the columns to the masonry with bent rebar welded to the columns.

As far as bracing goes, you can't account for the center support laterally bracing the beam, as it's essentially a flagpole. In this case you have the full 21' span unbraced. How is this accounted for in a two span design? What I was thinking is using the table 3-10 in AISC, and design the beam for the negative (max) moment, but for the full 21' length. Would this account for the full unbraced length?

There will also be torsion as a result of the eccentric brick loading, but it will be minimal because of the center support. The height of the building is 25' with a flat roof. The second floor and roof joists run parallel to the front wall where the storefront opening will be installed.

RE: Two span steel beam design

I would use the full 21 ft length. Then calculate the applicable CB factor based on the moments across that 21 ft span. The presence of the center column doesn’t affect how you calculate CB.

Be careful to deal with the wind forces perpendicular to the wall as the beam will take not only the vertical forces but also the lateral forces and these will flow to the end of the beam.

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