Double-Deep Wide Flange Design

[EngiNinja]

Hello, I am doing a retrofit project on an old industrial building with a central girder (W18x40) supporting roof joists. Sharing the columns with the roof girder is also another W18x40 crane girder. The crane will be removed and the budget is tight, so I would like to use the extra 18x40 to reinforce the roof girder already in place by welding the crane girder to the bottom. Should I design this as a doubly-symmetric wide flange that is 36" deep with a continuous longitudinal stiffener? This seems a little conservative to me, although the bottom flange of the top beam and the top flange of the bottom beam are very close to the neutral axis and won't contribute much to moment capacity.

The next step is to do this same thing along the edge of the building but with a W10x12 top and W18x40 below, so it becomes singly symmetric.

Has anyone else dealt with welding wide flange shapes on top of one another? Thanks!

 

[KootK]

I agree with your approach. If the reinforcement will not be independently supported by the columns, take care that a tension tending to seperate the two beams near the ends will need to be considered.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[SAIL3]

the existing roof girders/bms may have already existing some dead load deflection ... so depending on the magnitude of that deflection will dictate wheather one would get a good fit btwn the two bms...also how would one weld a W18x40 to an W18x40 since the flanges are of same width....

 

[KootK]

also how would one weld a W18x40 to an W18x40 since the flanges are of same width....

I was thinking:

1) Weld an overhanging top flange plate to the reinforcing piece on the ground.

2) Top side fillet weld the flange plate to the existing beam bottom flange in the air.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[SlideRuleEra]

EngiNinga - The capacity of the roof supporting W18 can be roughly doubled (and deflection halved) if the former W18 crane beam is placed snugly below... without welding the beams together at all. The two stacked, independent beams will share the load more or less equally. SAIL3's comments about dead load deflection need to be heeded for this be accurate.

How much additional capacity or deflection reduction is needed for this project? There are other concerns, especially for the W10 / W18 combination you mention.

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[EngiNinja]

I'd like to get more than double the capacity if we can with the deeper configuration, but this is also about determining what our maximum new roof load could be. I like the connector plate idea - that sounds better than field welding a butt weld along the flanges, especially with any current DL Deflection.

 

[decepticon743]

Why cant you bolt the 2 sections together? easier to couple them tightly together with existing deflection?

you want higher capacity why not lace them together to create a mini truss and deeper overall section, shouldn't need too many lacings and cheap.