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Strengthening of WF Beam

Strengthening of WF Beam

Strengthening of WF Beam

(OP)
I have a 30 ft long girder which is overstressed by 35% (Combined Bending IR of 1.35). I cannot add any supports in between. Only cover plates are the option. But adding one at the bottom is not providing the req capacity. Any ideas will help.

By the way the portion that is deficit in capacity is 14 ft (7 ft on each direction from center)

RE: Strengthening of WF Beam

Add to the underside of each flange of the top flange?

Mike McCann, PE, SE (WA)


RE: Strengthening of WF Beam

(OP)
Do you mean add cover plates below the top and bottom flange

RE: Strengthening of WF Beam

Depending on your situation, why not? It is harder, granted, but may be doable.

Mike McCann, PE, SE (WA)


RE: Strengthening of WF Beam

Is there space to weld a WT to the underside to substantially increase Zx?

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.

RE: Strengthening of WF Beam

Done that with steel tube sections too...

Mike McCann, PE, SE (WA)


RE: Strengthening of WF Beam

External post tensioning? Kinda depends if the extra compression hurts you more than the balancing load helps. I've always wanted to try this but I can't ever seem to work it into anything.

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.

RE: Strengthening of WF Beam

(OP)
I tried TS underneath, the problem is I need big TS some thing like 6x6. The beam under consideration is W12x40.

I was thinking that may be odd retrofit. But looks like its been done

RE: Strengthening of WF Beam

What about welding angles to the insides of each Web to flange corner. That's my go to detail. 4 angles that create 4 hss members. It substantially increases the strength usually.

RE: Strengthening of WF Beam

Surya77 - I mentioned the span to depth ratio in another thread today, but I'll touch on it again. A W12 spanning 30' is really a stretch (30:1 span to depth ratio). This is just a hint that you can expect a "skinny" beam like this to have problems. What is the beam's unbraced length? If it is relatively "long" (but not necessary unacceptable), adding lateral braces to the compression flange to shorten the unbraced length will inherently increase the beam's moment capacity with having to do anything else. Under these conditions, this alone "MAY" solve the problem.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Strengthening of WF Beam

(OP)
I agree with you. There are lateral braces already present, BUT those braces are just connected to the web of W12x40 (The braces are also W12x40 with their flanges coped and connected to perpendicular running 30 ft W12x40 web to web connection with A325 bolts, more like a pin conn). I am thinking as the compression flange is not restrained I cannot consider this to reduce the un braced length. This is a existing structure and there are errors in the initial design which when corrected eats up all the margin for further modifications.

RE: Strengthening of WF Beam

(OP)
@jayrod12 (Structural)

I will talk to the field if they have access to do welds tot he web. They can access flanges from top and bottom. This is in a very tight space

I appreciate all your time and ideas. This is a great forum

RE: Strengthening of WF Beam

I would call that braced for LTB.

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.

RE: Strengthening of WF Beam

Concur with KootK... braced for LTB. However, if you don't agree, suppose that you do modified the existing brace connections to also brace the compression flange would this reduce the W12's unbraced length to the point that its moment capacity is adequate?

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Strengthening of WF Beam

(OP)
KootK

how can you say that is braced to LTB if it can rotate bout wen. Flanges have to be restrained to call it braced to LTB

I am be missing something here, please correct me

RE: Strengthening of WF Beam

(OP)
yes I have to modify the brace connection by adding another plate form bracing member flange to girder flange and then take adv of reduced un braced length

RE: Strengthening of WF Beam

Grabbing the flanges is good but not necessary. You just have to convincingly restrain section twist. I don't remember where it comes from but I believe that you can consider your beam LTB braced if you engage at least 60% of the beam depth with your roll beam connections. Alternately, AISC has provisions for what constitutes effective roll beam bracing. For the framing scenario that you've described, most engineers would call the infill beam spacing the LTB unbraced length. I wouldn't even bother to check it.

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.

RE: Strengthening of WF Beam

(OP)
I think that 60% requirement is in one of AISC papers. But I don't rem which one.

RE: Strengthening of WF Beam

Good, if accepting KootK's explanation OR modifying the existing connections will insure you that moment capacity is adequate, IMHO, that is preferred solution.

A lot of steel in industrial structures depends on framed beam connections to resist LTB. It works.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Strengthening of WF Beam

(OP)
I have to submit a reference document for my reviewer if I am using 60% concept.

So I may have to resort to modifications.

RE: Strengthening of WF Beam

I would also chime in that the intersecting beams adequately brace the beam.

The key is to prevent overall section twist. The amount of force and stiffness is provided in AISC's specification - appendix 6 I believe.
You can simply calculate the required strength (twist moment at each brace end) and see very quickly that the shear bolts would usually more than adequately provide the strength.

Yes the bolts may be bearing only but a little slip won't change the fact that the beam is ultimately resisted from twisting.

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RE: Strengthening of WF Beam

Why not weld two 12x2x1/2 (or x 3/8) tubesteel members on both sides of the WF12?

Without increasing depth of the WF12, you'd add 4x 12-inch verticals to the outside of the WF, and - you'd add the anti-twist resistance several others have commented on.

The width of the original member is increased, but the connecting beams can be attached to the sides of the TS12x2.

RE: Strengthening of WF Beam

(OP)
@raccokpe1978

Hey thanks for the reply

the issue we are facing is accessibility to do welds. It looks like they can access from top to weld plate to the bottom flange provided it is wider than the flange.

RE: Strengthening of WF Beam

A heavy tubesteel shape has substantial curves on every edge.

The ends of the two flanges on a WF shape are sharp, thus you have a "natural" V-shape "weld prep" for a full-penetration weld from the outside of the joint on the WF-TS connection that can be welded with no additional grinding other than clean up to remove mill scale, rust, and surface debris. A substantial stitch weld will be adequate, you will not need a full-length weld.

RE: Strengthening of WF Beam

I think he's saying if you used tube steel he wouldn't be able to weld to the bottom flange as the only access if from the top.

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