Question about reinforcing existing beams

[jay156]

I have a question. My structural software is giving me answers that don't seem to make sense. I'm trying to add a W section to the bottom of some existing W16x40's to reinforce them for a mill that's in the room above.

Now I first tried adding a WT, and a little WT6x13 worked. But the contractor wants to use a W instead, so I'm trying some. One with an equal flange width and thickness is a W6x20, so I tried that but it failed, by a lot. I've been trying bigger and bigger sections, but they're not working. What is so fundamentally different about having the W section's extra flange's worth of steel in there that would make it fail like that when the WT worked?

 

[Lion06]

Yes, RAM looks at it as a singular member and there is a design report that gives the equations that it uses.

 

[ToadJones]

maybe he could post the output?
I'd be curious to compare it to STAAD...I have never used STAADS double member facility.

 

[ToadJones]

I think the lordly Tomanovich has a spreadsheet for reinforcing beams for both W's and WT's.
Check SteelTools.org
It would be check if nothing else ( I hate using spreadsheets I didn't write or check)

 

[wannabeSE]

For a reality check, I would use elastic section properties and calculate the bending stress in each built-up section.

I am going to guess that there is an error with the input into RAM Elements. Your sketch says the W16x40 + WT6x13 has a D/C = 0.26 (demand/capacity). While the W16x40 + W6x20 has a D/C = 1.5. That would indicate that the built up section with the WT has almost 6 times the capacity! For another reality check, what is the D/C for the W16x40 alone?

RAM Elements has nice output. It will give you the section properties of the sections. It will also provide the beam's capacity for each limit state checked with a reference to the specification section.

 

[Lion06]

I missed those ratios. Something is definitely wrong. I think it's the WT reinforcing that's wrong. If you have a D/C ratio of 0.26 with the WT then the existing beam probably works by itself, in which case you wouldn't be reinforcing it.

 

[jay156]

Sorry, I misremembered the ratios. 1.5 was the ratio of the 16x40 alone. With the W6x20, the ratio was about 1.01. With the WT6x13, the ratio was about 0.34. It still seems like a huge spread to me, but maybe I'm underestimating the effect of the neutral axis position.

Thanks PA for all your help. The beam isn't laterally braced at all, bottom or top. According to the contractor, the deck apparently just sits on top of the beam, which is a shame, because the composite section probably would have passed.

I've attached the report from RAM on the 16x40 alone if you'd like to look at it. I'll post the reinforced-with-WT and the W reports in the next 2 posts.

 

[jay156]

Here is the next report for the beam reinforced with the WT6x13.

 

[jay156]

And here is the final report for the beam reinforced with a W6x20.

As it turns out, they can't use the reinforcing I designed anyway, because of utilities that they don't want to move, which are 4" below the beam. So I tried another section, A C10x20 welded to the bottom flange, and by some reason that's just as perplexing as these other sections, it comes out passing with a ratio of 0.26.

I really need to figure this out. These problems come up all the time.

 

[Lion06]

There's a problem with the last link. Please re-post.

 

[ToadJones]

You'll likely also have a hybrid section of A36, Fy=36ksi and A992, FY=50 ksi

 

[Lion06]

J of the WF+WT is much higher than for the WF alone. This is driving up the Fcr for WF+WT, significantly. I bet you'll see that with the WF+WF the J is much lower.

That doesn't quite make sense to me and I would do a hand calc of J for each case to see if it matches what RAM is spitting out.

 

[jay156]

Here is a repost of the last pdf. I had an ampersand in the file name.

 

[Lion06]

J is the problem. J for the WF+WF is only 1.2, and it's over 6 for the WF+WT. I would look into why that is the case. I'm not sure I believe that to be true.

 

[jay156]

Well, now the program really doesn't make any sense. Isn't J supposed to be the sum of Ix and Iy?

For the 16x40 alone, Ix=518, Iy=28.9, and Jz=0.794?

What's wrong here?

 

[Lion06]

J is only Ix+Iy for a closed section, not for WF sections.

 

[jay156]

Oh right. Okay, so yeah, using J=[Σ](b'*t^3/3), I come up with a rough number of 1.169 in^4 for the beam reinforced with the WT6x13 (Ignoring the fillets). Way off from 6.22 in^4 that RAM gave me.

But the W16x40 alone I come up with 0.743, which isn't so far wrong from RAM's number of 0.794, and is likely right considering I ignored the fillets.

What the heck is going on here? Is my program hosed?

If anyone has the updated RAM Elements, could you check if your numbers are right for J? Because I'm still running RAM Advanse. Maybe it's time to update.

 

[Lion06]

I would call RAM and ask how they are computing it and why it's so far off from what you would expect to see.

They're usually pretty good about fixing problems and pointing you in the right direction if there is a problem in their software. 1-800-778-4277, prompt 7, then prompt 2.

 

[wannabeSE]

For the built-up beam with the WT, the wrong depth is used for the W16x40. Edited the properties so it is 16" instead of 21.62"

 

[jberg]

For what it is worth, I ran both situations through ShapeBuilder from IES (which I have had good luck with in the past), and adding the WT6 results in a J = 1.23 and with the W6, J = 2.42. So it appears something may be hinky with how RAM calculates J in this case.

JWB

 

[Lion06]

That's the problem. The WF in the WF+WT is called a W16x40, but the section properties for a W21x93 are input. J for aW21x93 is 6.03. There's your problem.