Cover plate beam where shoring is not feasible 2

[Dennis59]

Hello,
I have a situation where I need to install bottom cover plates on steel beams over an area where it would be extremely difficult/expensive to install shoring under the beams. The undersides of the ends of the beam are accessible, but not the main 30-ft span.
My understanding is that it is best practice to shore a beam before coverplating to relieve the dead load stresses in the existing beam.
I am thinking of calling for a W8 section for the "cover plate", and requiring that the W8 be cambered upward. One end of the W8 would be placed tight up to the bottom flange of existing beam. Then the other end would be jacked up until it also comes in contact with the existing beam. The upward camber in the new W8 cover plate would serve as "shoring" under the existing beam, thus relieving at least most of the dead load stress before the new W8 is welded on.
Does this sound conceptually feasible? Have any of you done something like this successfully?
Thank you.

 

[canwesteng]

Shoring without jacking the beam up won't remove the dead load anyway. The first thing to check is if the stresses in the beam under dead load are significant. Would load increase so much that the existing section would yield under service loads?

 

[canwesteng]

A relevant steelwise

https://www.aisc.org/globalassets/modern-steel/steelwise/steelwise-january-2014.pdf

And refer to slide 43 here

https://www.aisc.org/globalassets/p...ngthening-for-existing-steel-handout_2per.pdf

 

[KootK]

This does make sense to me conceptually but I worry about the practical application of it. There will be lots of uncertainties to account for. Another option might be to install the low beam with a gap between it and the existing beam and then jack between the beams at the third points. Finally, devise a detail that allows you to make the final, composite shear connection across the gap

 

[BAretired]

Something like this? Wouldn't the W8 be a bit flexible to relieve much load over a 30'-0" span? I think the concept would work, and it would relieve some load from the main beam. Would you provide a good strong weld each end, then stitch weld from each end toward the center?

 

[KootK]

I'd imagined the cover plate beam being oriented strong axis.

 

[Dennis59]

Thank you for your thoughts so far, I really appreciate it.
I am attaching a sketch since I didn't describe the situation very well.

 

[Dennis59]

 

[canwesteng]

You still haven't sold me on this being required at all, and it's going to be very costly endeavor. You should sharpen the pencil to see if there will be some yielding that is of concern, or otherwise demonstrate that the bottom flange might rupture before yielding the cover plate before going this route. And even if either of these are a concern, you are likely better served with more reinforcing steel than this jacking proposal.

 

[271828]

Neat idea. What limit state is causing the need for the reinforcement? Strength? Deflection?

 

[Dennis59]

Thank you for additional replies.
Canwesteng, I am sharpening my pencil.
271828, the problem is bending stress moreso than deflection.

 

[BAretired]

So to prevent lateral torsional buckling of the new beam, you are relying on friction between its top flange and bottom flange of beam to be reinforced, right?

 

[Dennis59]

Thank you BAretired. The top flange of the existing W16 is braced laterally at 5' on center by W8 beams that frame in from both sides.
The bottom W8 (new "cover plate") would be stitch welded on both sides to the bottom of the existing W16.
Does that answer your question?

Thank you.

 

[BAretired]

Not really. You plan to camber the W8 upward, then jack each end until it meets the W16. That puts the top flange in compression with nothing to prevent LTB other than friction between flanges. LTB in the W8 could occur before any welding is performed.

 

[KootK]

So to prevent lateral torsional buckling of the new beam, you are relying on friction between its top flange and bottom flange of beam to be reinforced, right?

I'd be positively bracing the joint between beams somehow. Maybe kickers back to adjacent framing etc.

I'll add that I agree completely with canwesteng about the hierarchy of solutions that ought to be considered. Start with no shoring and rule out that possibility first.

My understanding is that it is best practice to shore a beam before coverplating to relieve the dead load stresses in the existing beam.

I would rephrase that as follows:

It is best practice to consider the existing stresses in the beam being reinforced to determine if jacking is required.

In my experience, many more beams get reinforced without jacking than with. You've likely noticed that many of the steel shapes that we use to reinforce beams possess relatively small Ix in their own right. That is, in part, to limit total depth. Is is also, however, because reinforcing members with small Ix are easier to bend to conform to the deflected shape of the beam being strengthened.

 

[BAretired]

Pre-stressing the bottom flange of the W16 might be worth considering.

 

[BAretired]

As a matter of fact it is well worth considering!!!

 

[Smoulder]

First point of contact at midspan. Clip there for lateral restraint. Jack both ends.

 

[mfrad]

Dennis59

I have used Steel WT's instead of WF’s successfully. Easier to weld intermittently both sides. Also make 2x6 template of existing beam since it has deflected some in order to cut wt’s web to fit.

 

[BAretired]

Here's another idea. Rods could be stressed by turn of nut method and direct strain measurement; or rods could be Diwidag bars with couplers and fittings as required. Dimension 'a' can be zero to about 4'-0", at a point where welding can be performed without concern about weakening the W16.