Chapter 34 and Metal Buildings?

[SteelPE]

I have been given the opportunity to do some work on an existing metal building (at least bid on the work). The contract would be done through a contractor. The building was constructed around 1984 and I believe was manufactured by VP. I just so happens that I know someone who worked on for VP during that time and have consulted them about the project.

During our conversation my fried alerted me to a problem with the roof purlins. He said that not much was know about LTB at the time and it was assumed that the standing seam deck was assumed to brace the top flange of the purlin. He highly recommended going in and adding bridging to all of the purlins in the building…. Even in areas outside of the proposed construction.

I kind of know what I need to do but this brings up a very interesting question. Chapter 34 of the IBC basically allows you to make modifications to a structure as long as your stresses in any member are not greater than 105% of what they are if the member was unaltered. In this instance we would not be changing the loading on any of the existing girts but we would still be required to reinforce them. What would others do in this instance?

 

[MiketheEngineer]

Analyze it and advise customer of your concerns.

 

[msquared48]

At first you mention the purlins regarding LTB, which, yes, I would be concerned about with the roof loading. Then you mention the girts that I would not be so concerned with in LTB. If it was a major concern, I would update the purlins with the bridging, but not the girts.

Which, if not both, were you concerned with?

For others, "VP" is "Varco-Pruden".

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[SteelPE]

msquared

I'm sorry, I keep thinking of these things as girts. The members in questions are roof purlins.

 

[ajh1]

The change mentioned occurred with the 1996 AISI Specification. Prior to that point there was no explicit restriction towards using a standing seam panel as lateral resistance for a purlin design. After that date the designer has two possibilities.
1) Design the purlin with discrete bracing and only assume lateral resistance at the braces.
2) Perform what is known as a "Base Test" where the actual resistance of the panel is evaluated in a full scale test. The result of the test is an "R Factor" which indicates exactly what percentage of full bending capacity the system has available. The test is to be run with a specific purlin/panel combination as appropriate. Typical R factors run from around 0.65 to 0.80 of full capacity.
Pay particular attention to end bay locations. Interior bay locations of continuous purlins tend to be controlled by combined bending and shear at the end of the lap. This limit state is unaffected by the lateral support of the panel or lack thereof. Talk to a VP engineer and they should be able to give you some guidance regarding what capacity the purlins would have today.

From a different perspective, keep in mind this building would appear to have performed adequately for the last 18 years, so something must be working correctly, whether intentionally or by default. Adding braces may be the best solution, but be clear what you will gain by it.

 

[JedClampett]

If you know of a design issue you need to escalate it to the owner. I see two approaches:
1) Analyze the problem and define the extents. Maybe it won't be as bad as predicted. Then notify the owner.
2) Put your blinders on and sing "lalala" loudly as you do the project. Don't do any analysis of the roof purlins beyond your scope. After all, if you don't know there's a problem, there's nothing to tell the owner about.
I'm not sure that it's required to dig into existing designs for adherence with code changes. Maybe the old code was more conservative in some areas (like wind or live loads), but not conservative in others and it balances out.

 

[SteelPE]

We are still at engineering bidding phase of the project so I don't even know if I will get the project. I probably won't once this information comes out.

I was just interested in the fact that if I looked at the structure w/o talking to the engineer I would have used chapter 34. If the loads I was adding were less than 5% then I wouldn't have even considered looking at the design of the system. From my standpoint, I would have followed the code (like anyone else would have done) and if there was a problem how would I have known?

 

[dhengr]

SteelPE:

Is your statement correct, “If the loads I was adding were less than 5% then I wouldn't have even considered looking at the design of the system.” ? Or is the correct statement something like this ‘if my new loads don’t change the member stress by more than 5%.... I don’t have to make major changes to the member’? I think there was a fairly long thread on this very topic a few weeks ago. I can imagine a 5% change in ‘whatever load?’ being applied differently and causing quite a change in the serviceability of a member. Study the intent... a few percent change in a member stress shouldn’t necessitate the redesign of the whole bldg.

Secondly, I think you have to examine your conscience long and hard, once you know about a potential problem, then you probably should bring it to the attention of your client. You know,
moral responsibility and P.E. code of ethics, public safety and well being, and all that good stuff. Or, lalala, real loud as someone suggested, but you are on public record already, having brought the issue here, so keep that in mind.

 

[SteelPE]

dhengr

I agree with your points, especially the points about my conscience.

With regards to the 5%, if I don't increase the loads to the structure or increase the stress in any way then I would be code compliant. This issues comes up due to a code change that ahj pointed out. If you designed the building toady you would need bridging. This is something the other engineer pointed out to me and highly recommended I do whether or not I increase load or stress.

 

[hokie66]

I wouldn't regard the requirement to brace purlins supporting standing seam deck as a code change. It is just a correction of an error, an egregious one at that.

 

[audeuce02]

There is a little test data out there that shows a "friction diaphragm" that will occur between the purlin and roof panel when insulation is compressed. I would not rely on this, but I could probably drag out some documentation if you need it. The other consideration would be actual loading. Is the building in an area with potential snow loading?

 

[SteelPE]

Spoke with the contractor and it went much better than expected. They didn't resist the idea of adding the bridging. So I guess that is the way that we are going to go for now.

Thanks for the help.