Z-purlin 4

[Triangled]

A client is considering replacing the HVAC on his metal building roof framed with Z purlins similar to the attached sketch.
I would sure appreciate some advice for this, specifically:
A) direction to literature for typical section properties
B) insight as to typical steel grade(s)
c) typical reinforcement details

 

[jimstructures]

Butler Manufacturing Company has 8 1/2" & 10" deep sections in the following thicknesses: 0.060", 0.068", 0.073", 0.079", 0.088", 0.098" & 0.113" in a G30 galvanized material. They used to have a 0.122" thickness also in a Galvanized material but I don't remember what the galvanized rating was. I am not familiar with a 0.156" thickness material, but it might exist. Any of the manufacturers can provide a set of section properties for the various thicknesses. AISI and LGSEI can also provide depths, section properties and thicknesses available. Butler also has legacy sections available in 9 1/2" depth.

AISI has design standards for "Z" members. They fairly complicated and require iteration to come to a solution.

Jim,

 

[KootK]

Here's an article that deals with z-purlin reinforcing:Link. Options are a bit more limited than with hot rolled members.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[ajh1]

You don't mention the construction date of the building. Purlin depths change from time to time even within the same company. For example, Butler used to sell 8" and 9.5" purlins, although we never had 0.156" thickness. Also you should verify whether the lips on the edges of the flanges are at 90 degrees or approx. 45 degrees. Most metal building companies use the 45 degree lips because they nest better on continuous purlins. The majority of purlins currently in use are Fy = 55 ksi depending upon the age of the structure.

 

[SteelPE]

I will also add that I do some work with an engineer who used to design these types of buildings and now routinely reinforces them. I only glanced at KootK's link and it appears to have similar reinforcing details to what he has recommended. He also has details where he adds new purlins in-between existing ones. The detail involves using a structural angle at the support that cantilevers out beyond the support by a few inches. You can then screw the new purlin to this angle with some tek5 screws. I'm sure others wouldn't like the detail but he has used is successfully for many years.

One other thing to note. He has told me before that you need to be careful about LTB with these purlins. He also stated that many building manufacturers never properly accounted for LTB for quite some time (I'm not sure if this has something to do with standing seam roofing or not). This has led to a few problems. One easy way he has corrected the problem when required is by installing 4"-6" steel stud as bridging in-between the existing purlins. Quick easy detail to correct a problem.

 

[Triangled]

@jimstructures, @KootK, @ajh1, @SteelPE: each of your posts has gone straight to the heart of solid data and methods. Thank You!!
I'll likely have some follow up queries after I've had a chance to digest this excellent information.