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RE: Bridging for bar joist

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I agree with you, the problem i was having was the joist mfgr was not looking at the big picture.  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. 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.  
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.  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.
 
Thanks
 
Ed Jonson
 
 
> Date: Sun, 1 Nov 2009 10:35:55 -0400
> Subject: Re: Bridging for bar joist
> From: ad026(--nospam--at)hwcn.org
> To: seaint(--nospam--at)seaint.org
>
> Oooo, I love bracing discussions ... they tend to be so flexible. Throw in
> some stability and common structural systems that are not well understood
> beyond the specialty design office and things can get downright
> career-building!
>
> The joist manufacturer is right ... unless he structural consultant has
> designed anchors (or added bracing) to suit the building design expectations
> as well as the manufacturer's assumptions. Bare joists on their own have no
> (brace) load path continuity in the horizontal plane. Simply bridging joists
> together does not create a (stability) brace point at a vertical X-brace in
> the end bay or elsewhere.
>
> Let's define an anchor, for bracing/stabilizing purposes, as a point in a
> (brace) load path that just has to be stiff enough, with balanced strength,
> to prevent the braced element (OWSJ chord) from becoming unstable.
>
> Figure out what is required to be braced and how stiff/strong to make the
> anchor point. Then ensure that the anchor conditions are actually available.
>
> Simple span OWSJ have to be bridged during construction to stabilize the
> (top) compression chord due to self-weight and imposed weight of unfastened
> deck, iron-worker, supervisor, inspector, site engineer, etc. As Harold
> notes, the joist can "flip-over" under its own weight (the top chord buckles
> and the joist twists into weak axis bending). This condition will probably
> occur before you have any walls in place.
>
> The same joist in a completed roof application has to have the (bottom)
> chord braced for compression under uplift conditions. Maybe you can brace
> this into the wall with horizontal bridging if the stiffness and strength
> exist. When the top chord diaphragm is attached, the diaphragm should make a
> sufficiently stiff (shear) brace to permit vertical X-bracing to drag out
> the bottom chord stability forces - make sure the accumulation over multiple
> joists does not exceed the local deck fastening capacity.
>
> Most of this is Statics 101 - the rest they don't teach in undergrad - pity.
> An anchor is only an anchor if the structural consultant makes it an anchor.
> The specialty engineer, who designs the joists, will assume bridging/bracing
> locations for joist cost efficiency. It is the responsibility of the
> building structural consultant to ensure that those anchors are developed.
>
> Further comments below.
>
> > From: Harold Sprague <spraguehope(--nospam--at)hotmail.com>
>
> > By way of example: a deep long span bar joist can flip over just by its own
> > weight.
> >
> > The X bracing may provide an "anchor point" for the horizontal bridging. B=
> > ut an X brace WITH horizontal bridging is more positive and precludes the n=
> > eed for attaching the bridging to the wall.
>
> YES! But further, look for the brace load path to create the anchor!
>
> > From: Thomas Magnum <tm314052(--nospam--at)gmail.com>
>
> > 'Positive anchorage with X-bracing' doesn't mean anything. 'Positive
> > anchorage' with horizontal bracing doesn't mean anything either. Braces
> > stabilize, not anchor.
>
> Good! But "anchor" is relative to function. If a brace stabilizes, then it
> has achieved the role of an anchor - locally at the very least.
>
> > Welds and bolts anchor.
>
> Nope. Welds and bolts fasten. Anchor is a "functional" termination point in
> a brace load path.
>
> > X-bracing absolutely provides positive BRACING joist-to-joist and even
> > joist-to-wall.
>
> It provides bracing but does it provide a load path to an anchor?
>
> > From: Edward Jonson <edjonson(--nospam--at)msn.com>
>
> > So does X-bracing provide positive anchorage as required in SJI?
>
> Not unless you design the assembly to perform that function.
>
>
> > From: Thomas Magnum <tm314052(--nospam--at)gmail.com>
>
> > You do not use braces to provide positive anchorage. They stabilize, not
> > anchor.
>
> I'm ecstatic! But the brace has to terminate at a functional anchor in order
> to stabilize.
>
>
> > From: Kipp Martin <KMartin(--nospam--at)carollo.com>
>
> > But in answer to your question, I don't see how X-bracing does not provide
> > positive anchorage. I'd ask the joist manufacturer to explain.
>
> At the very least, the discussion between the building engineer and
> manufacturer's engineer may lead to a better mutual understanding of the
> design expectations and assumptions. This discussion should happen on EVERY
> project
>
>
> > From: Edward Jonson <edjonson(--nospam--at)msn.com>
>
> > Question about anchorage of bridging. I have project where X-bracing of en= d
> > bay of bridging is called out on plans rather than continuing bridging to=
> > concrete panels.
>
> Are you the structural consultant or are you involved with the construction?
>
> > Joist manufacture claims this X-bracing does not provide positive anchorage.
>
> I agree, but the word "positive" carries a lot of baggage that has no
> meaning in this discussion. For instance, replace "positive" with
> "sufficient" and we can avoid the possibility of "negative" anchorage. It is
> probable that the manufacturer really means to relate strength and stiffness
> to a "functional" brace termination point.
>
> > So the question is does the X-bracing provide positive anchorage?
>
> Not unless the structural consultant has made it so.
>
> > Any comments much appreciated.
>
> You're welcome.
>
> Further reading suggestions:
>
> Structural Stability Research Council (SSRC)
> http://stabilitycouncil.org/html/publications.htm
>
> Guide to Stability Design Criteria for Metal Structures (explains the
> fundamentals of the criteria in the design standards - AISC, CSA, Eurocode,
> etc.)
>
> Is your Structure Suitably Braced (contains presentations by a "Who's Who"
> list of deep thinkers on the subject)
>
> Attend the North American Steel Construction Conference (NASCC) in the
> spring, which includes the SSRC conference.
>
> Regards
> Paul
> --
> Paul Ransom, P.Eng.
> ph 905 639-9628
> fax 905 639-3866
> ad026(--nospam--at)hwcn.org
>
>
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