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

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I think there is an article available through AISC called “The Importance of Tension Chord Bracing”.  There may be other issues involved besides uplift.  I used to have a copy, but am still unpacking in our new office.

Joe

 

Joseph R. Grill, P.E. (Structural)

Shephard - Wesnitzer, Inc.

Civil Engineering and Surveying

P.O. Box 3924

Sedona, AZ  86340

PHONE (928) 282-1061

FAX (928) 282-2058

jgrill(--nospam--at)swiaz.com

 

-----Original Message-----
From: S. Gordin [mailto:mailbox(--nospam--at)sgeconsulting.com]
Sent: Friday, January 20, 2006 2:20 PM
To: Seaint@Seaint. Org
Subject: Re: Bottom Chord Bracing of Steel Joists in a Hybrid Roof

 

Bill,

 

1) You can look at similar details in the Vulcraft catalog.  Why not to use manufacturer's specs?

 

2) The uplift on the roof is always accompanied by the positive pressure on the wall, hence, the load in the brace may be higher.

 

Steve Gordin SE
Irvine CA

 

----- Original Message -----

From: Bill Allen

Sent: Friday, January 20, 2006 12:18 PM

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