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Re: Bridging for bar joist

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My comments below.

> From: Edward Jonson <edjonson(--nospam--at)msn.com>

> I agree with you the problem i was having was the joist mfgr was not looking
at the big picture.

The joist manufacturer is a manufacturer - period. They are not building
consultants. The fact that they happen to have engineers on staff to design
bits of steel is incidental. If you ask about E&O insurance you will get a
blank stare because they can only carry Product Liability insurance. You
bill for professional service hours and they bill by the ton.

The structural consultant is the ONLY entity with responsibility for the big
picture. They are the orchestra leader - the only person standing out in
front of the orchestra, who hears each instrument and blends it with the
others.

Too harsh?

> The joist mfgr completely ignored panel joint locations for attaching anchors
> to completely ignored areas where if anchors were installed they would have
> to core drill through reinforcing.  Also it seems the joist mfgr is not
> worried about joist deflection/camber compared to end walls that will not
> have any movement.

Did the specifications explicitly require them to consider these issues when
they were originally quoting? When they were designing? The structural
consultant should specify brace/bridge locations, if necessary, and
increasing stiffness or camber approaching hard walls or provide detailing
to suit.

Lots of "been doin' it like this forever" kinds of design standard details
have proven unreliable and, like Harold, some engineers have adjusted their
approach.

> BTW my experience has been if any of these problems due come to life later the
> EOR will be responsible because the joist mfgr will state they had approval
> from the EOR. 

Rightly so. That would become a logistical nightmare since they could
potentially be responsible for avoiding undefined, as-built conditions based
on design specifications such as, "24" c/c max.".

> Couple of questions i do have from your response i agree the diaphragm needs
> to be stiff enough and strong enough to anchor the X-bracing but the joist
> mfgr never provides these forces.

Ask for them. Alternately, worst case is 2% of chord axial force, cumulative
with each joist included by connected bridging. There are analytical means
to reduce the forces as large quantities of joists accumulate. This also
helps to explain why joists don't fail more often, despite apparent bracing
inconsistencies.

> Also i'm assuming in a steel building where there are no end walls for
> attaching horizontal bridging both temporary and permanent bridging must
> be anchored by x-bracing.

Anchored, yes.

I assume that you are referring to x-bracing in the vertical plane between
top and bottom chord. Where does the load go from there?

If the deck is fastened, this generally provides the shear path required
between bridging points along the joist length. This is direct for the top
chord and indirect (through the vertical x-bracing) to the bottom chords.

If the decking is not fastened, you need some other means to provide
horizontal equilibrium between joist bridging lines. This could be another
load path to the foundation (discrete anchors and the long way around the
issue) for each bridging line or temporary horizontal bracing between
bridging lines (discrete replacement for the diaphragm).

> Thanks
> 
> Ed Jonson

Regards
Paul
-- 
Paul Ransom, P.Eng.
ph 905 639-9628
fax 905 639-3866
ad026(--nospam--at)hwcn.org


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