Strengthening of WF Beam

[Surya77]

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)

 

[msquared48]

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

Mike McCann, PE, SE (WA)

 

[Surya77]

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

 

[msquared48]

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

Mike McCann, PE, SE (WA)

 

[KootK]

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.

 

[msquared48]

Done that with steel tube sections too...

Mike McCann, PE, SE (WA)

 

[KootK]

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.

 

[Surya77]

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

 

[jayrod12]

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.

 

[SlideRuleEra]

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.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[Surya77]

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.

 

[Surya77]

@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

 

[KootK]

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.

 

[SlideRuleEra]

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?

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[Surya77]

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

 

[Surya77]

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

 

[KootK]

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.

 

[Surya77]

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

 

[SlideRuleEra]

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.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[Surya77]

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

So I may have to resort to modifications.