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RE: Bottom Chord Bracing of Steel Joists in a Hybrid Roof

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Consider the situation where wind may cause uplift. This could induce compression in the bottom chord in which case the braces really are doing something and the uplift also reduces the resisting load.

 

Mark E. Deardorff, Structural Engineer
Burkett & Wong
San Diego, CA


From: Bill Allen [mailto:T.W.Allen(--nospam--at)cox.net]
Sent: Friday, January 20, 2006 12:18 PM
To: seaint(--nospam--at)seaint.org
Subject: Bottom Chord Bracing of Steel Joists in a Hybrid Roof

 

I’m reviewing some shop drawings and I’m having some angst with a bracing detail.

 

Roof joists are steel joists (Vulcraft)

 

Roof framing is a panelized wood roof with 2x subpurlins attached to nailers on the steel joists with F hangers.

 

The detail shows a single angle running from the top of the double angle bottom chord and attaching to a sub-purlin (slope of brace is 1:1, depth of joists vary between 24” and 40”) with (4)-10d face nails.

 

I originally rejected this connection since the sub-purlin is attached to the nailer with an F hanger which has no uplift capacity.

 

The contractor has come back and said that this is a standard connection and they do this all the time, yadda, yadda.

 

In the interest of having an open mind, I did verify with Simpson that the F hangers do not have an uplift capacity. I looked at the joists, took the maximum moment and divided that by the minimum joist depth and considered 2% of the chord force as the brace force. I admit, the load is dinky; something like 65 pounds. Considering 8 PSF dead load, the minimum reaction is still positive (no net uplift) and there is no negative bending (unbraced length) in the sub purlin. Based on this, it seems like it would be O.K. to install the brace to the sub-purlin.

 

Am I missing something?

 

T. William (Bill) Allen, S.E.

ALLEN DESIGNS

Consulting Structural Engineers