Reinforcing existing beam under stress

[mfstructural]

I'm evaluating an existing framing system (steel framing with 8" concrete deck) and have found there is no margin to add additional loading. The client is asking for a fix to reinforce the existing beams. However they don't want to remove any existing heavy machinery prior to reinforcing. My question is what is the difference in designing reinforcing for the beams if the beam is at full stress and under no stress?

 

[JAE]

Don't weld across the bottom flange all at once.

Any welding should be carefully planned so as to minimize any negative effects on the section under load.

 

[mfstructural]

Thanks. We typically here don't weld across the bottom of beams. Someone here in the office suggested a larger larger section would be needed for the reinforcement if the beam is under load. I kind of don't understand that. Whether or not it's under load, the maximum stresses will be the same. Am I missing something here?

 

[msquared48]

Whether or not it's under load, the maximum stresses will be the same. Am I missing something here?

Yes. The load it is seeing now will be reduced if the machinery is removed during the reinforcing, and hopefully less than what it sees now when the machinery is reinstalled.

Mike McCann
MMC Engineering

 

[mfstructural]

Ah...yes of course because the section modulus will be larger. big oversight on my part

 

[paddingtongreen]

If the beam is at full stress and then you simply reinforce it, it will still be at maximum stress prior to you adding more load.

One solution is to calculate the exact load when all is removed that can beremoved, or is allowed to be removed, and see if you have any spare capacity to use with the reinforcing.

Can you insert more beams in between the existing ones.

There have been cases where hot reinforcing plates were added so that they would, on cooling, compress the tension flange, stretch the compression flange and be placed in tension themselves. They would work like prestressing.

Except for adding hot plates to the bottom flange these do not address the compression flange, can you increase the allowable load by increasing lateral stability?

I'm done.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[LarryAndrews314]

How about shooting a mid span elevation, and then jacking it up from below until a certain amount of deflection is removed? That ought to temporarily remove the stresses while you reinforce the beam.

 

[hokie66]

I don't get too excited about residual stress, or stress existing in a beam before it is reinforced. The existing steel will still carry load, and the reinforcement on the bottom flange will drop the neutral axis, so as long as that solves your stress issues, I would just do it. If there are deflection issues, that is a different story.

 

[efsinc]

"My question is what is the difference in designing reinforcing for the beams if the beam is at full stress and under no stress?"
Interesting question.
Supposing a beam in bending, I think of the difference having to do with the amount of strain in the existing beam. If the extreme fiber in bending in the existing is strained corresponding to a stress of 60% Fy, and reinforcement added with a strain corresponding to a stress of 0% Fy, won't subsequent loading to the reinforced system strain the existing and the new proportionately, creating stresses beyond 60% and 0% respectively?
There is still a long way to go to 100% Fy, which I would think of as the theoretical point at which the existing steel ceases to contribute proportionately to additional load carrying capacity of the reinforced system. But it suggests to me that the proportion of increased load carrying capacity sought and the proportion of existing capacity remaining would want to be considered.

"they don't want to remove any existing heavy machinery prior to reinforcing." I might not be too concerned to add beam reinforcements to increase the load capacity by 10%. But if, say, "they" who "don't want to remove any existing heavy machinery prior to reinforcing" want their engineer to just add on more steel so they can add 50% more "heavy machinery", I'd take a deeper look.

 

[mfstructural]

I can't jack it up...there is very little room in the area as it is. It is a nuclear plant and there are pipes everywhere.
I'm not so sure the hot plate would be accepted by the client as everything has to adhere to the plant's specs.

I'd say the additional load required is approximately 20%. The existing calculations show that stress is at 24 ksi (Fy=36ksi) so is it in order to design just the reinforcement for the additional 20%? But the new Sx is based on the total section properties, and the stresses are already there. So how do you go about differentiating what's there before and after reinforcing? I hope what I'm asking is clear. Thanks

 

[structSU10]

http://www.eng-tips.com/viewthread.cfm?qid=243663

That thread has some discussion by Lufti, with various resources. I tend to agree with the idea that the current stress in the section remains, then you do an analysis of the new cross section under only the additional super imposed loads, and superimpose the stress with the initial section.

If i have issues reinforcing with just something on the bottom, due to overstress on the compression flange due to shifted neutral axis, then I try to put rebar welded to the steel in the fillet areas so the section remains balanced.