Existing PEMB Roof Analysis 1

[toddf02]

I have an existing PEMB (built in the 1980’s) that I am analyzing because the existing exhaust fans need to be replaced. Structurally, the building has many elements that seem to be common practice such as the roof consisting of metal panel fastened to cold formed z-purlins which are lapped at each tapered beam support. Each z-purlin also has a 1” dia. vertical pipe brace at each end and is braced at third-points with struts. I understand that these types of structures are typically designed to be efficient. I am only looking at the z-purlins underneath each fan in Risa with a roof dead load, 20 psf roof live load, the weight of the each fan, and a 5 psf collateral load. I have also analyzed the same system in CFS. However, I am getting much different results between these two programs. In Risa, my model fails in combined bending and compression with a unity of over 4 while the results in CFS show that the z-purlins are failing at about 1.1 unity. Has anyone analyzed this type of structure using Risa or CFS and had a similar experience? These results vary greatly and I’m not sure what differences between Risa and CFS would cause such a variation in results.

Thanks,

Todd

 

[DaveAtkins]

Have you included the bracing in RISA? If not, the purlin will be considered unbraced, which could result in the large overstress.

Also, where is the compression coming from? Isn't this a bending member?

DaveAtkins

 

[toddf02]

Dave,

Thank you for the response. I have included the vertical bracing and struts in Risa and in CFS. The roof panel is fastened every 1'-4" to the top of z-purlins, so I have also included this as the Lcomp top in Risa. As for the compression, there is an axial force of about 0.3 kips in the z-purlins that I assume is a result of the deflection. This compression force is not from an applied load. According to Risa, the AISI S100-16 governing equation is H1.2-1 which is for combined compressive axial load and bending.

 

[WARose]

I don't know what "CFS" is....but I have checked a number of PEMB before and what you are finding isn't unusual: there isn't a lot of gravy in those systems. As DaveAtkins said you may want to check your unbraced lengths.

In the past, I've had to do (what I call) a "load swap". In other words, where I add something like a roof fan or something.....take advantage of the fact there can't be a live load there anymore because of that equipment's presence. That can squeak it by sometimes.

 

[Lomarandil]

Just to be sure, have you actually told RISA to consider braced lengths between the struts, or have you only included the bracing members in the model? Two different things.

Yes, RISA will report a combined limit state regardless of whether an engineer would throw out the 300lbf and say flexure controls.


----
just call me Lo.

 

[toddf02]

Thank you for the response WARose; I have considered doing something along those lines to see if it helps me out.

Lomarandil: Yes, I have taken into account the struts and vertical braces for my unbraced lengths (LByy & LBzz).

The issue seems to be that Risa is calculating a much smaller moment capacity in comparison to CFS Software. I haven't been able to determine why that would be.

 

[WeezerMike1]

Hi Todd,

I don't have any experience designing PEMBs, but I have a good amount of experience working with them. I was almost never able to get anything in the building to calc out to my satisfaction even for the original applied loads. They're usually designed with the absolute most efficient analysis possible to show code compliance, so if you aren't doing that exact same analysis, it won't calc out. If you want to try those specific design processes, you might want a PEMB design guide. User EDB9 recently recommended the book by Alexander Newman called Metal Building Systems: Design and Specifications.

Whenever I did work on a PEMB I always started with the basic assumption that there is zero extra capacity in anything. I would skip analysis of the existing elements, and either justify my scope-of-work by showing all member/connections forces wouldn't increase by more than 5%, or I would add at least as much strength as the loads I was adding.

For what it's worth, I eventually stopped doing any work on PEMB because I didn't have confidence in the adequacy of the existing work. It's not that they don't meet code, but they're just the cheapest structures possible without any redundancy or resiliency. I did earthquake response work in El Centro after the Mexi-Cali earthquake, and a 120 foot PEMB bent frame collapsed because one of the L2x2 column flange braces was bent by a forklift impact years prior. If that's all it takes to bring a structure down, then I don't want my name or license anywhere near it.