Increasing Beam Capacity
Increasing Beam Capacity
(OP)
At work we are looking at changing the loading of a room. The dead load on the floor will be dramatically increased. I have checked and the primary steel beams supporting the load will fail in flexure. However, the beams are sufficent in block fracture, shear, etc. An initial quick check of the secondary steel beams WILL support the added load.
I'm looking for a good method to increase the flexural capacity of the primary steel beams.
What do you guys recommend??
There is a bit of room below the slab to increase the depth of the beams. I'm thinking about welding on a plate (perpindicular) to increase the depth (web). Then weld a plate at the bottom to create another flange. Is this a viable option?? Would the flange plate be necessary, use a tee instead?? What possible problems would this option bring on??
Using the above example, what if the additional plates had to be added in sections due to space limitations (say four 5' sections to make up the 20' span)?? What additional concerns would this bring up??
P.S. Removing the existing beams and installing new ones are not an option in this situation.
Any advice appreciated!!
I'm looking for a good method to increase the flexural capacity of the primary steel beams.
What do you guys recommend??
There is a bit of room below the slab to increase the depth of the beams. I'm thinking about welding on a plate (perpindicular) to increase the depth (web). Then weld a plate at the bottom to create another flange. Is this a viable option?? Would the flange plate be necessary, use a tee instead?? What possible problems would this option bring on??
Using the above example, what if the additional plates had to be added in sections due to space limitations (say four 5' sections to make up the 20' span)?? What additional concerns would this bring up??
P.S. Removing the existing beams and installing new ones are not an option in this situation.
Any advice appreciated!!
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Andrew






RE: Increasing Beam Capacity
Welds between should take the q = VQ/I horizontal shear. With 5' pieces, I would provide spliced connections capable of resisting the full tensile force developed in the added steel.
It sounds like you have static loading. If it is dynamic/repetitive you need to check fatigue on all these connections.
RE: Increasing Beam Capacity
Has this calculation been based on the full moment capacity of the section, or a reduced moment capacity due to effective length?
It may be possible to reduce the effective length of the section, hence increasing the moment capacity of the section, by adding additional restraints to the primary beams.
Just a thought.
RE: Increasing Beam Capacity
Good Point!! That is something that I completely overlooked. I was being very conservative and assuming the whole span as being unrestrained.
Looking again, the beam (W16x31) may already be "restrained" by the 6" concrete slab that it is supporting. The problem is that I'm unsure if the metal decking is welded to the beam (and if so at what spacing). If the decking is attached, is it proper to assume the slab provides adequate lateral restaint??
If not, what is the most common way to provide lateral support?? I need lateral support at 4' spacings to get the allowable moment I need.
Sorry, I don't work with steel very often.
TIA!!
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Andrew
RE: Increasing Beam Capacity
RE: Increasing Beam Capacity
If not, another gentleman in our office did the same thing JAE suggested and used a WT to extend the depth. I never heard from the contractor whether or not it was easy to do. I would suspect welding a WT to the bottom flange of an existing beam would be rather difficult. Make sure welder is AWS certified.
RE: Increasing Beam Capacity
Agree that the deck should be adequate for lateral bracing (Lb = 0).
RE: Increasing Beam Capacity
Secondly, since you have overlooked the role of the metal decking as lateral restraint, you might forget to design the beam as a composite beam as well. Composite beam design method is definitely gonna increase the beams flexural capacities, both moment and stiffness.
RE: Increasing Beam Capacity
DMWWengr,
By me solutions for this problem are:
1. increasing the depth.
2. incresing the with of bottom flange.
Why not trying to ad a wide plate at the bottom flange.
this will increase the Moment resistance of the beam and also it's lateral stability.
For the good welder with corect equipment it will not be a problem, as the weld will be added downward.
Here where I am from it is not restricted by Codes.
It will help if you have minor problem with moment capacity.
In other case Jae and Pylko-THE best!
RE: Increasing Beam Capacity
Try to use longest pieces possible...otherwise, since your addition will be in direct tension, the splice weld becomes a bit more critical.
Good comments..all. Are these forums great or what!!?
RE: Increasing Beam Capacity
To weld on a sufficiently large plate to the bottom flange(or alternately, two smaller plates on the top of the bottom flange, both sides) is my next choice, but weight and handlling difficulties come into play. TS is not a good choice from the welding standpoint, although it would probably be the lightest option(If the TS section used exceeds the width of the flange, welding becomes much easier). What ever option you choose, make certain that you unload the beam to be welded on sufficiently (falsework or such added temp supports, watch the camber!!!) Never weld to a loaded beam like this!
For safety sake always use qualified union Ironworkers whenever possible. When in doubt, ere to the 'too strong' side.
Rod
RE: Increasing Beam Capacity
1. Ricker, David T. "Field Welding to Existing Structures." Engineering Journal, 1st Qtr 1988.
2. Tide, R.H.R. "Reinforcing Steel Members and the Effects of Welding." Engineering Journal, 4th Qtr 1990."
I purchased the Ricker paper from AISC. Excellent paper. The paper covers weld selection/design, reinforcing, heat input, weldability, and shoring and stress relieving among other things. Ricker mentions that adding a plate to the bottom flange of an existing overloaded beam and forcing camber in, may force the flange into compression. It doesn't sound like the beam that's being discussed is overloaded yet. Ricker goes on to say that welding a cover plate to the bottom flange of an unloaded beam tends to make the beam arch upward because of the heating and cooling from welding. This is one reason, he says, to keep welding as symmetrical as possible.
Briansch
RE: Increasing Beam Capacity
Another option, if feasible, it to weld a plate at mid-span with with holes 6" or 8" down from the lower flange, and plates at the ends of the beams with holes close to the lower flange. Connect the plates with tie rods with turnbuckle thus creating a truss. When you tighten the turnbuckles you will put a compressive force in the lower flange and a tension force in the upper flange, thus gaining moment capacity for additional load.
RE: Increasing Beam Capacity
It is the practice in my office to attempt to balance the added reinforcement by also adding rods (not rebar) to the underside of the top flange. Thes are attached with intermittent, staggered flare-bevel welds. We have had very good success with this, and we have never bothered to relieve the loaded beam by jacking.
RE: Increasing Beam Capacity
1. Longitudinally placed on the flange edges (I agree that it is very unwise to weld ACROSS a tension flange as this would create an instantaneous plastic hinge).
2. The welds are usually stitch welds (such as 2" at 12" o.c. or similar)
Be sure to note the AISC LRFD (and ASD) Section B10 where it discusses requirements for connection plates. Usually the end sections of a cover plate are welded continuous for some distance with the staggered welds between.
RE: Increasing Beam Capacity
This time I was just barely off the moment capacity that I needed. There are no shear studs into the concrete so I cannot look at the beam as a composite section. I've went ahead with JAE's orginal suggestion and I'm going to "tube" the beam in with plates on each side. This will increase the capacity as well as provide some additonal limitation to deflection.
Since I've never worked with this type of beam, is there anything that would change dramatically with this beam that would need to be rechecked (i.e. shear flow, etc...) Any hints as to the best way to "orient" the plate sections for the easiest/safest welding??
Thanks again everyone.
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Andrew
RE: Increasing Beam Capacity
i would re-read JAE's post/s - when he says "tube", i think he means a rectangular tube welded to bottom flange NOT boxing it ("tubing") with vertical side plates which is what i understand you are referring to in your last post.
HTH
RE: Increasing Beam Capacity
Capacity-wise it works fine I'm just concerned about overlooking/changing one of the buckling aspects??
TIA!!
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Andrew
RE: Increasing Beam Capacity
You state in your earlier post that the section nearly works, if considered as fully restrained. (The situation that you describe would almost certainly achieve full restraint.)
I assume that you have discounted other solutions, such as the addition of columns beneath the primary beams, to halve the span.
Another trick that I have used in the past, where the beam section is nearly adequate, is to add knee braces off the support columns, inclined at 45 degrees. This effectively reduces the span of the beam, and may get the result you're after. It does however encroach on your headroom local to the column positions. The fabrication and erection is however easier than adding a fabrication to the bottom flange of the beam. It also results in horizontal forces transferred into the columns, which you will need to check for.
Of course, it is possible that other criteria, such as deflection, governs your design.
RE: Increasing Beam Capacity
My deflection is just below an inch for this beam. Is that an accepatable amount for a beam supporting a concrete slab?? I have no feel for deflection....yet :)
What is the proper way to calculate Zx (plastic modulus)?? Is this shown in the LRFD manual somehwere and I'm not seeing it?? My EM books don't show it either. I know how to do it on a symmetric member but not on an unsymmetric member. I need to do this before I can calculate adding something to the bottom of the beam.
The plate idea will probably work fine for this situation but I'll need to know how to calculate (unsym) Zx in the future.
Also, still looking for any "ease-of-construction" suggestions on welding the plates??
Any comments appreciated.
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Andrew
RE: Increasing Beam Capacity
Adding plates to create a box-type beam can work as well...although probably not as efficiently. It will not affect your shear connections at the end of the beam as they are the "stiff" to which the shear will flow at the ends of the beam. The plates can be simply fillet welded to the flange tips - but this again is adding cost as now you have two large plates and four welds which all work less efficiently than the tube at the bottom with two welds (one each bottom flange tip).
RE: Increasing Beam Capacity
How do I calculate the Zx if I place a box beam at the bottom?? I would like to check this but I'm not sure how to calculate Zx in unsymmetrical sections!!
Can someone post the method??
TIA!!
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Andrew
RE: Increasing Beam Capacity
As for finding your Z-
Go to the Properties of Geometric Sections in your LRFD manual, and break your new shape up into parts, and figure out Z for each appropriate part and add them together.
RE: Increasing Beam Capacity
That doesn't work....at least when I try it. You have to account for the shift in the neutral axis. I can do that with a symmetric shape but the same method doesn't seem to work with an unsymmetrical shape.
I keep trying to find the Z of angles and I can't seem to get them to come out right!!
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Andrew
RE: Increasing Beam Capacity
RE: Increasing Beam Capacity
pylko...I only suggested adding the tube to the bottom flange.
RE: Increasing Beam Capacity
You also probably do not need to add this new section through the full length of the beam, however, you do need a certain length past that from which your moment capacity without the addition would be adequate. This extra is for development of the welds for your new built up section. I think ASD 9th (maybe 7th)edition has that procedure.
What angles are you trying to find the Z of anyway?
RE: Increasing Beam Capacity
I still like the WT solution, I have used it before for ease of construction, but it has stability problems while being welded, sooo , on second thought, I like JAE's tube steel solution as long as it is sufficiently wide enough to allow simple stitch welds to the flange from above. Narrow tube sections can be a real bear to weld overhead.
To redhead's solution with the barstock welded at the top flange web juncture (or for that matter the bottom also) this is the solution the GM engineers dearly loved, for the life of me I don't know why. Implimentation of this solution in the field is a MONSTROUS headache. Getting it
welded sufficiently to pass inspection is outlandishly time consuming. ie. round section bar to flat plate.(square bar?)
Briansch--- It's hard to teach an old dog new tricks. With all your research it's hard to disagree, but none the less I do. Heavy welding on loaded beams or trusses has always been a no no. You may get by now and again, but it only takes one time--- This is a fairly small beam and doesn't appear to be overloaded, but it still would be a good idea to ck shear values at both ends and perhaps to use the same jacks you would use to install the bottom plate or tube to perhaps take a bit of the load off the beam until you get everything tacked up. Then perhaps a bit of reinforcement with a couple of seat lugs or such?
DMWWEngr---Whatever solution you choose (all will probably work), keep in mind ---'It's all about time and money, my time and your money!'.
Rod
RE: Increasing Beam Capacity
And finding Z of angles is quite tricky. Why not try to find Z of a shape thats symmetrical at least in one axis and then compare to the value in the book to see if you got it right? Once you get the hang for that, try finding Z of a W shape with a C. Find your answer, then compare with LRFD. If you get the hang of that, finding Z of a W section with a HSS on the bottom should be a piece of cake.
RE: Increasing Beam Capacity
The tube will stick out a ~1/4" on each side. Since I'm not very familiar with welding practices...Is the 6" tube going to be wide enough to allow for relatively simple welding?? Would it be better to use a slightly larger tube...6-1/2", 7" etc???
What is a good resource for designing welds?? LRFD has very limited information.
Thanks again for all the great information recieved,
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Andrew
RE: Increasing Beam Capacity
If the beam is an old shape, make sure that the weld type and procedure are compatible.
I have used the WT method before with very good results.
-RKC
RE: Increasing Beam Capacity
The tube extending only 1/4" on either side sounds tight to me. For sweep variations in the field, you may have difficulty getting the weld installed properly. An 8" tube would probably work the best.
RE: Increasing Beam Capacity
RE: Increasing Beam Capacity
I have never had to reinforce the entire span. Determine the required extents of reinforcing and perhaps you don't have to splice. I avoid splices in these critical elements if at all possible.
In extreme cases where we may need to reinforce the top flange we have used angles on each side, sort of forming tubes with the flange and web. The welds are fillets along the toes.
Historically, we have always specified unloading the beam (shoring) while welding.
I like the Blogett reference also. The minimum connections are spelled out in the AISC ASD Section B and J among others. The end welds are based on F=MQ/I and the intermediate connections are based on f=VQ/I. There are also rules about proportions. The existing connections should also be verified with the new reactions.
RE: Increasing Beam Capacity
Thanks everyone for their responses. It is much appreciated.
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Andrew
RE: Increasing Beam Capacity
RE: Increasing Beam Capacity
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Andrew