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Re: Lateral bracing of longspan steel joists

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My comments embedded.   

Phil Hodge
phil(--nospam--at)joistdesign.com

DOLORES (DOLLY) TRIVIZ wrote:
> 
> Dear Sirs:
> 
Some knowledgable members of this list are Madams.  Some may be Others,
for that matter.

> I am working on a project in which I have designed lonspan steel joists (70'
> span), two configurations--scissors joists 48" deep, and a pitched top chord
> joist with a maximum top to bottom chord depth of about 12' at midspan. 

Why so deep?  70' long joists are probably most economical at about 72"
deep.

 I
> have specified full depth bridging (diagonal bridging running from top chord
> on one joist to bottom chord of adjacent joist) throughout.  The pitched top
> chord joist occurs in the middle of the building and supports a moveable
> folding door on the bottom chord, and the scissors joists occur on each side
> of this joist at about a maximum 13'-6" spacing.

Watch deflection of the joist and local bending of the bottom chord with
the folding wall at each of its' possible locations.
> 
> The architect has tried to convince me that we can effectively bridge these
> joists with 30" deep bridging throughout, with the bridging being connected
> to a "stiff" (how stiff??) member that will  connect the top to the bottom
> chords of the joist where the bridging member intersects the joist.  I have
> not given my approval because I am not convinced that this constitutes an
> effective bridging system that will prevent lateral movement of the top and
> bottom chords of the joists.  The roof diafram will consist of plywood.
> 
> What are anyone' thoughts to this type of bridging system suggested by the
> architect?

I don't like it.  It can be done, but then again ANYTHING can be done if
you through enough steel and $$ at it.  The bottom chord is laterally
braced only by the cantilevered web sticking out from the bottom of the
bridging, 18 to 114" away.  The web would end up being as stiff as the
chords.  Why not put the bridging at the bottom?