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Re: z shape purlin at roof

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> From: Ahcene Djebli <adjebli(--nospam--at)>

> Hello all,
> Last month we had here in Colorado a huge snow fall. A lot of roofs 
> collapsed. I am doing an investigation for my own curiosity how the roofs
> collapsed. I am doing on my own a study on the z shape purlin. I need to
> know the maximum rotation of the z shape can have and according to which
> code? Also I need case studies. I will really appriciate if some body who
> have those info to e-mail them me or to guide me. Thank you in advance.

Rotation as in twisting about the length (e.g. torsion) or about the
section axis (e.g. stong axis bending)?

The analysis of Z-shaped members is covered in the AISI standard -
latest edition is the North American Specification for the Design of
Cold-formed Steel Structural Members.

There is no specific limit for what you are asking but there are
practical limits where the members will fail due to increased secondary
effects. This will vary per situation. Lateral movement should be
limited to L/360 by the bracing "system".

I would be interested in hearing any information that you might obtain
about the large quantity of roofs that collapsed in Colorado (e.g. type
of roof systems, mode of failure and if the load exceeded the presumed
design loads).

Paul Ransom, P. Eng.
Burlington, Ontario, Canada
<mailto:ad026(--nospam--at)> <>

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