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WT Reinforcement on Existing W beam

WT Reinforcement on Existing W beam

(OP)
I am a CWI currently on a project where the only thing taking place is the installation of a WT section to the bottom flange of an existing beam. While I know this is a useful form of reinforcing an existing building I was curious about one thing. Could the heat input caused by welding the web of the WT to the bottom flange induce a positve camber into the beam due to the eventual shrinkage of the bottom flange? And if so, is this one of the reasons why it is performed or is it strictly to add more dimension to the cross section of the beam. The beam is being welded by 12" long fillets at each end and staggered fillets 4" on 6" throughout.

RE: WT Reinforcement on Existing W beam

Without knowing anything on your particular case, I would guess the section is added to increase the structural capacity of the beam.

The weld heat might improve the camber, but it will not increase the beam's capacity. And in any case my guess is that the effect will be negligible.

Is the beam termporarily propped or shored while strengthening? I worked on a retrofit job where that was the case, beams shored under constant load applied by hydraulic jacks at 1/3 points. The beams were temporarily supported and then the temporary supports removed once the additional steel was added. The behaviour and capacity of the final beam will depend on if that is done or not.

Another issue, welding heat actually weakens the section (yield strength reduces with temperature) before the additional steel has a chance to increase the capacity.

I am sure the Designer has already considered all those issues.

RE: WT Reinforcement on Existing W beam

"stitch" welding the WT would apply minimal heat and minimal weld shrinkage would occur.  

 

RE: WT Reinforcement on Existing W beam

I would do the welding symmetrically though, both side to side and longitudinally.

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: WT Reinforcement on Existing W beam

Agree with Mike.  If the contractor can install it "backwards", there is a good chance they will.  I also try to have the WT reinforcing terminate at equal distances from either end for the same rationale.

JWB

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