steel beam Strengthening

My question is about strengthening a W section steel beam with adding another W section on bottom flange. Adding cover plate or WT options are not sufficient because of large moment requirement .
I am planning to connect the flanges with weld, I can fillet weld the two flanges. Now my concern is the weld will be on the flange tips only and there will no connection in the space between the tips. It seems we may need some plug/slot weld as well. Or may be adding few bolts in longer spacing in addition to the fillet weld.What are other requirements other than providing the weld capacity for shear flow demand.
What is commonly done while attaching two W-sections. Your experience and thoughts will be appreciated.
thanks.

 

[Shu Jiang PEng SE]

treat the two W beams as on section and calculate the shear required at the interface between the two W sections. Design the connection strength per the derived shear. both bolt and weld connection should work.

 

[JStephen]

Check allowable stress of the combined section, may be different from the single beam. Check end connections for increased load. Weld distortion may occur.

 

[McGill10]

I agree about the design procedure.
For detailing side,I am worrying little about while welding.In my case the flange is about bf =10" wide.Fillet weld only is good or need to provide plug weld in combination.

 

[Shu Jiang PEng SE]

generally, the shear along the interface of the two W sections. you can comfort yourself doing the check. treating the flange as tension/compression member supported in the middle by the web, applying the shear to two tips of the flange.

 

[KootK]

Now my concern is the weld will be on the flange tips only and there will no connection in the space between the tips.

You may be on to something here. When the reinforcement possesses stiffness that is competitive with the main section, a substantial amount of vertical force must be transferred between sections in addition to VQ/It forces . In the middle of the beam, this can happen through direct bearing. At the ends of the beam, it will produce tension in the welds as the reinforcing attempts to hang from the main beam. It's a non-issue if the reinforcing makes it to a stiff bearing support but, often, that is not the case.

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.

 

[Shu Jiang PEng SE]

 

[dhengr]

McGill10:
You might be better off using a larger WT on the underside of the existing WF. Your flg. to flg. problem goes away, and also, you are not applying (wasting) so much steel area near the new N.A., as is the case when you add another WF shape. Now you weld right under the existing WF web to the WT web. Apply vert. end plates to the WT for new end bearings and to weld to the bot. flg. of the existing WF. This also protects the end of the WT web weld from unsipping. You should support (jack and shore) the existing WF to take most of the load off of it, then press the WT to its underside before starting the WT welding. This de-stresses the existing WF (and removes deflection) before applying the WT reinf’g., otherwise the existing bot. flg. will retain a significant tension, maybe start to yield, before the WT even comes into full play.

 

[SlideRuleEra]

Fillet weld only is good or need to provide plug weld in combination.

To be adding an entire new beam, instead of cover plates, the loads are obviously substantial. Not a time to design as if it is a brand new installation and minimum cost is the goal. Provide pairs of plug welds (one on each side of the web) even though fillet welds alone may be (theoretically) structurally adequate. I would not be overly concerned about spacing the plug welds. Depending on beam size (must be at least a W10 to have b[sub]f[/sub] = 10") space the pairs every couple of feet, or so. If possible, detail both the fillet welds and plug welds so that there is no overhead welding (drill the bottom flange of the existing beam for the plug welds). Agree with dhengr to take the load off the existing beam before starting work.

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