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Re: Bridging for bar joist

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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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