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Re: Diaphragm

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Ashraf,
It is all a matter of economics: joists versus purlins. If your joists are spanning 74 ft, then they are long span steel joists and it is probably cheaper to use light beams or cold-rolled members for your purlins.
Gary

Ashraf, Mohammed (Qatar) wrote:
Gary:
I looked to Bemo Manual and they also say 5 feet spacing. So I will try
to reduce spacing of joists and keep it within limits as you mentioned.
But how can I defend it to the client since it has large cost
implications?. How about providing purlins at a spacing of 5' and
keeping joists at a spacing of 10'. Does it help to keep joist spacing
at 10'?.
The minimum design load is 25 psf. Thanks for your help and regards
Ashraf

-----Original Message-----
From: Gary L. Hodgson and Assoc. [mailto:ghodgson(--nospam--at)bellnet.ca] Sent: Tuesday, May 30, 2006 3:05 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: Diaphragm

Ashraf,
I had the same feelings as Daryl when I saw a spacing of 10 ft between joists. Of course, we are used to snow loads, (here in Niagara Falls 39 psf on a flat roof), but I would think you should be designing for a minimum of 20 psf (that is code here in Ontario). This probably covers construction loads, etc. My Butler Manual recommends a spacing of 5 ft for their MR24 standing seam roof, but they don't say why or what are the ramifications of larger spacings. I have
walked on a ss roof with 5'-6" spacing and it was like walking on mush.

The contractor told me to stay over the purlins.
In your case, the flat bar bracing depends on local conditions and abilities of the work forces. Bridging is not bracing; rather it is individual bracing of the joists to stabilize them during construction, as individual joists have a tendency to roll over easily.

The Can. Institute of Steel Const emphasizes that bridging is not
bracing.
Gary


Ashraf, Mohammed (Qatar) wrote:
Gary:
I've joists spaced at 10 foot and span of joists is about 74 foot.
Joists are supported on steel girders and girders are supported on
concrete columns spaced at 30 foot spacing. Hence I think flat bar
spacing will be a better choice. What do you say?. Is this called as
bridging?. Thanks and regards
Ashraf


-----Original Message-----
From: Gary L. Hodgson and Assoc. [mailto:ghodgson(--nospam--at)bellnet.ca] Sent: Monday, May 29, 2006 3:05 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: Diaphragm

Ashraf,
You can install X-bracing angles under the steel joists from col to column; or you can install flat bar bracing on top of
the joists, under the deck.  This pretty well has to be field welded
and
needs to be pulled tight--details to work out
with the erector. The angle braces are usually bolted with connection plates at the columns. Count on some of the angles not being correct and they will need to be field-welded--this depends on the quality of the fabricator and his
detailer.
Gary

Ashraf, Mohammed (Qatar) wrote:
Gary,
Thanks. You are right and I am not comfortable with the diaphragm
action
of metal roofs. I am planning to use open web joists to support the
standing seam metal roof as shown in the client documents. But if I
want
to put X-bracings in the roof, how can I provide it?.
Regards
Ashraf
-----Original Message-----
From: Gary L. Hodgson and Assoc. [mailto:ghodgson(--nospam--at)bellnet.ca] Sent: Monday, May 29, 2006 2:39 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: Diaphragm

Ashraf,
For a building that size, I would put in X-bracing in the roof. The CISC has stated that for large buildings, diaphragm action is not reliable. However some eminent engineer (Galambos?) has disagreed with that.
Gary

Ashraf, Mohammed (Qatar) wrote:
Thanks GM. I am using IBC 2003 code.
A brief description of building is given below for your info.
The building is of 107x60 m in plan size and maximum 17m height with
roofs at different levels.
The columns are of concrete and most of the beams are of steel. Roof
is
of steel deck type. I have about 7 braced frame in the x-direction
(107m
is in x-direction). The span of frames vary from 10 to 22m. In the
y-direction there are 7 columns @ a spacing of 10m. Only 3 bays are
braced. The height of braced bay is 8m and span is 10m.
The maximum size of diaphragm comes around 60x22m. What can I do to make this system more rigid?. Since I am using Open
Web
Joists to support metal deck, does bridging help me to make the
system
more rigid?.
Thanks and regards
Ashraf

-----Original Message-----
From: Gerard Madden, SE [mailto:gmadden(--nospam--at)maddengine.com] Sent: Saturday, May 27, 2006 9:43 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: Diaphragm

Most likely not. I'm assuming the walls are concrete or steel braced
frame.
Unless the metal deck has concrete fill, it will be considered
flexible
diaphragm. See the UBC/IBC for the definition of Rigid Diaphragms
and
Flexible
Diaphragms. The deflection of the diaphragm would need to be less
2.5
times
the deflection of the vertical resisting elements of the lateral
system
to consider it rigid and at best your would have a partially rigid
diaphragm
but with those plan dimensions, you would need several interior
lines
of
lateral resistance and you must also be careful to make sure that
your
diaphragm
span ratios are not too great to violate whatever code your
structure
falls
under.

-gm
-- Original Message --
Subject: Diaphragm
Date: Sat, 27 May 2006 09:14:22 +0300
From: "Ashraf, Mohammed (Qatar)"
<mohammed.ashraf(--nospam--at)worleyparsons.com>
To: <seaint-ad(--nospam--at)seaint.org>
Cc: <seaint(--nospam--at)seaint.org>
Reply-To: <seaint(--nospam--at)seaint.org>



"This email is to be read subject to the disclaimer at the end of
this
message
or attached to this message."

Hello all,

I have building of 107mx60m in plan size and maximum height of 17m.

The roof is of seamless metal deck from Bemo USA or other
manufacture.
The metal deck is supported by open web joists. The maximum span of
joists is 22m.

Can I take this deck as rigid diaphtragm?.

Thanks in advance,

Ashraf




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