Beefing Up Composite Beams

[jakeh76]

I am trying to reinforce an existing composite beam. The beam is a W12X14, 20' long, with a 5" concrete deck. There are 21 uniformly spaced shear studs across its length.

Are their any references that cover something like this?

 

[NigelB]

Think about the stability of the section during modification, and the effect of heating the section when welding. You probably want to shore the existing member, and even jack it to relieve existing stresses if possible.

If you approach the design based on limit states principles, you should be able to add to the steel section and design based on the full combined section. Remember that LRFD is based on the plastic section of steel, provided taht you meet the appropriate compactness criteria. Therefore the analysis assumes very large strains. You would still have to check deflections at service loads taking into account any pre-existing stresses, in case the section yields before reaching service loads.

Nigel

 

[CSEllc]

There are several ways to reinforce the beam.

1. Check the beam deign using LRFD. Chances are the existing composite beam was designed using ASD method. By checking it using LRFD, you might be able to get a few more PSF out of the beam.

2. You can try to add more studs. If you access to the space below the beam you might be able to locate the existing studs on the beam by "burn" marks on the undfeside of the top flange. With this information you can map out the location and add additional studs in between the existing. To install the additioanl studs, the existing slab will need to be cored, a 5 to 6 inch core hole is usually all you need. The additioanl studs would be welded to the beam (same as if no slab were present). After the additioanl studs are inspected, if required, the hole should be filled in with a non-shrink grout.

3. You can cover plate the beam. The cover plate size would be based on the additional load stress. You should make the plate wider, I repeat, WIDER than the beam flange. The reason is you DO NOT what to weld across the beam flange while it is under load. It is possible that the beam could fail on you since you are welding the plate. (As you are welding the plate, the steel is heated to about 1600 degrees and "liquifing" at the point.)

In cases #2 and #3, the beam does not need to be shored or jacked up. If you do shore or jack the beam up, the additional studs and/or coverplate would need to be designed for the full load...existing and additional.