AISI Material Properties

[jay156]

I'm looking for some data on yield strength for Z purlins in this manufactured building the client wants to hang stuff from. It was built around the 1970's, and I don't know what kind of steel they were typically using for purlins at the time. Does anyone know offhand or know where I can find out? Of course the client doesn't have the original drawings of the building, or know who the manufacturer was.

Thanks,

Jay

 

[ajh1]

I would expect the purlin material is either 50 ksi or 55 ksi yield. Hard to guess beyond that if you don't know the manufacturer. If the purlins are relatively thin (in the 0.060" range), the differential between the two yields is going to be minimal due to local buckling and effective section rules. For thicker purlins (in the 0.120" range) the two yields will produce different capacities.
Keep in mind that the AISI spec has changed over the years (what spec hasn't?) and capacities today for the same cross-section are probably going to show as lower than the original design capacities.

 

[weab]

I'll start by saying that Z-purlins are not my specialty. However, I'd be very, very surprised to find out that the yield is 50 ksi or greater back in 1970. I would have supposed that 36-42 ksi would be correct.

I hope that ajh1 is correct.

 

[jay156]

Okay, thanks. Now my next question is this. The bending stress of these things is way over the code. I don't know if they had lighter snow loads back then or if whoever designed it messed up or just didn't care, but how does bolting a C-purlin to the bottom flange of these things sound as a way to fix it? The flanges are 3 inches wide. Will I be causing other problems by bolting? Would welding be better?

 

[hokie66]

I would think adding C purlins web to web with the existing would be the idea. If by bolting to the bottom flange you mean placing a new C horizontal and trying to make the two composite, that would require a lot of bolts. In situ welding of light gauge material would not be a consideration for me.

 

[ajh1]

A couple of responses:
1) It appears my company moved to 55 ksi around 1971, although I find prints dated 1960 on some parts that show 55 ksi back then. Light gage material such as cold-formed purlins tends to run at higher yields than equivalent hot-rolled structural shapes and plates. Easier to push the strength levels on the thinner coils.
2) Purlins are generally controlled by lateral torsional buckling of the compression flange and/or compression lip. For gravity load, that would be the top flange, not the bottom. It is unlikely that bolting a C flat on the bottom is going to give you a lot of extra net capacity. I would agree with hokie66 that adding a vertical C attached to the web would work better. Keep it an inch or so shorter than the existing purlin and align the bottom edges. That keeps the top flange out of the way of things like projecting fasteners coming through the existing top flange. Analysis on a non-symmetric composite section like that is a bit of a pain, but programs like CFS do a good job generating the properties.
As I noted in my earlier response, the AISI spec has gotten stricter since the time in question. I would anticipate at least a 7-10% reduction in allowables from then to now.

 

[jay156]

Thanks, that's what I'll do. I have another question now. Does the C purlin that they're bolting to the web of the existing Z have to be continuous, or could it somehow be spliced in the middle? The issue is there's a 25' span, and the existing purlin is overstressed for almost the middle 19 feet of it. There are things hanging all over the place, and getting a 19' length of steel up there is going to be difficult. Could two 9.5' C purlins be used since it's going to be bolted the entire length? Maybe if the flanges are also bolted together with plates?

 

[ajh1]

Ultimately it would need to function as a continuous piece, at least in it's flanges. If you are going to put it up in pieces I would recommend not having a splice right at the center since that is where the largest moment would be. Working out some sort of splice on the flanges would be fine although a bit of a pain.
Basically you need to decide how much of the loading remains in the existing member keeping in mind that any load present at the time of the reinforcement installation is going to remain in the current member. Design each of the two pieces for the desired percentage (plus a little). Design connections between the two to properly transfer the new member's load. Extend the new member a foot or two at each end beyond the minimum length necessary to insure a good transfer of forces near the ends. Keep in mind that the forces that are in the new piece will need to transfer back into the old piece for transfer to the frame supports unless the new piece spans the full length (undesirable but you are almost there anyway).