What grade of steel used in PEMB Girts and Purlins - circa 1974?

[structuresguy]

OK, so I am looking at a PEMB fire station where the original drawings are 1974. We have no info on the metal building itself, so I went out and measured everything. I am running the calcs now, but I need to know what grade of steel was commonly used in 1974 for cold formed C and Z girts/purlins. The oldest reference I have is a Star Buildings catalog from 1985 which lists 55 ksi as the yield stress for their girts/purlins.

Anyone know what was common in the mid-70's in the US? I don't have any contacts at any PEMB manufacturer, but will cold call a few if I need to. Thought I would ask here first.

Same question for roof and wall panels. We have pretty typical corrugated steel panels, 24 gage thickness.

The goal of the project is to determine what it will take to bring the building up to current Florida Building Code requirements. Right now, things are looking pretty good, amazingly. My main bents are only running at 75%, assuming Fy=36ksi. Roof purlins look pretty good (depending on Fy) for gravity, but need bottom flange bracing for wind uplift.

Thanks much.

Andrew

 

[JedClampett]

As far as the steel grades, for plates I'm sure they were A36 and the cold formed stuff was whatever they made cold formed out of in the day. It wans't really standardized until more recently.
As far as your main task, its been covered many times on this forum. It's very tough to get into recreating PEMB work at all, much less 40 years old. Now maybe since the computer power wasn't as prevalent tehn, the calculations were more conservative than they are now, but I wouldn't be surprised if you eventually felt like you were beating your head against a brick wall. The fact that you aren't getting frustrated right away is a good sign.
Good luck.

 

[structuresguy]

THanks Jed. THe main item I would like to develop a comfort level on is the strength of the cold formed steel. I am assuming 36 ksi for the structural plates.

Fortunately (or unfortunately, depending on your perspective), I've done evaluations of a few PEMB in the past, so I have a pretty good approach worked out now I think. Yes, I have had problems with the more recently built buildings. Mainly, the problems I encounter are almost always unbraced length issues due to lack of bottom (or inside) flange bracing.

For this building I am reviewing now, I already know I will be adding bottom flange bracing on the roof purlins for uplift. I will also be adding sag rods to the girts, which the metal building industry allow for out of plane bracing of the unsupported inside flange of the girt.

But so far, the stresses on the overall sections are looking very reasonable. I haven't yet started looking at the connections or foundations. But my experience has been if the overall members are pretty good, then the connections are usually pretty good as well. Fortunately, there is no seismic requirement here in Florida, so that simplifies things a bit.

 

[JedClampett]

I looked in my old steel textbook, from 1972, and they stated just what I said, that there was no standard for cold formed steel at that time. But I bet if your called a cold formed section manufacturer (Marino, AEC, etc.) they could give you a ball park number.
I've got some existing PEMB reroofing in Florida. But we got the client to not make us check the building. We just have to provide a standing seam roof that meets current code.

 

[hokie66]

Jed,
Was the original roofing standing seam? If not, your new standing seam roofing may not do the job of bracing the purlins.

 

[JedClampett]

Good point. I'll have to verify.

 

[audeuce02]

I am a PEMB Engineer, and I am suprised that everything is not failing for the current Code. Just losing the 1/3 stress increase for transient loads should be causing significant problems. In 74 the building was most likely designed as 12/20/20 (12 psf frame load, 20 psf roof secondary load, and 20 psf velocity pressure.) For some reason I am thinking the light gauge yield in that time frame should have been 42 ksi. I will dig around in the office tomorrow to see if I can verify this. Once again I am not 100% positive, but I would expect the 24 gauge corrugated to be 50 ksi yield.

 

[structuresguy]

Thanks very much. For the frames and purlins, I am using 10 psf DL and 20 psf live load. On top of that, I have 100mph wind loading per ASCE 7-05. The frames are only running at 75%, and the purlins are good for gravity loads as is, and need bottom flange bracing (I don't count inflection points) for uplift. The purlins have a bolted long splice over the frames, so I can analyze as continuous, but they work with 1/3 point bracing even simply supported.

 

[hokie66]

structuresguy,

Since your building seems to be working so well, perhaps it is not a PEMB after all, but rather one designed by a traditional consultant.

 

[structuresguy]

It's possible. But it is most definitely laid out LIKE a metal building, with cable bracing in the roof level, single angle vertical wall bracing, and very typical light gage girts and purlins. Also, the main structural columns and beams are fabricated shapes from welded plates. Maybe a "normal" engineer was working for a metal building builder at the time, who knows.

 

[hokie66]

I would agree that if welded shapes were used, it is a PEMB. Another possibility is that the loadings were conservatively specified by an architect or engineer, making provision for possible future use. At any rate, it sounds like yours is a relatively happy experience with this type of structure.

 

[structuresguy]

Yes, absolutely. I did find that the stiffened beams seats were a little boderline, where the main beam attaches to teh columns. So I will be adding some proper shear connector from the beam web to the column flange. But again, pretty minor retrofit considering the age and type of structure.

 

[Bobber1]

Are you checking for buckling via reduced section properties of the light gage members? Buckling stresses usually govern on these things.