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RE: Braced Steel Frame System Question

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	Assuming the 1997 UBC and seismic zone 4.

	Question 1: >     1)    Can the braced frame be located at the 2nd
floor to 3rd floor
	> only? (One end of the digonal at the 2nd floor and the other at
the
	> third floor). No braces
	>             at the first floor were located. I believe that this
would
	> create a soft storey. In terms of member forces, this would pass
but I
	> have doubt on this type of
	>             design. That's like terminating your shearwall at the
second
> floor instead at the foundation level.


Response 1: Upper level braces can be offset from lower level braced bays
but beams and columns and connections below need to be designed for
OMEGAo*Eh and there are special detailing requirements outlined in 1997 UBC
1630.8.2.  Weak stories are not allowed in buildings more than 2 stories
tall or 30 feet tall per 1997 UBC 1629.9.1. Soft story buildings require
response spectrum analysis per UBC 1629.8.4.  Do they have an in-plane or
out-of-plane offset from a braced elevator core? You can mix bracing systems
floor-to-floor using the lowest R per UBC 1630.4.2 but you would have to use
modal analysis for a soft story.


	Question 2:>     2)    Can flat bars (designed as tension-only
members) be used as a
	> bracing system? I believe that tension-only design is discouraged
by the
	> governing codes.
	>             How about for low-rise structures? Is the slenderness
limit
> of 720/sqrt(fy) strictly followed?

Response 2:  Tension only bracing is only allowed for ordinary braced frame
buildings less than 2 stories per 97 UBC 2213.8.5 and 97 UBC 2211.4 (9.5).
Bracing not meeting slenderness ratios or width-to-thickness ratios is only
allowed for ordinary braced frame buildings less than 2 stories. This
bracing needs to be designed for OMEGAo*Eh.   I can't speak for other plan
checkers but slenderness ratios and width-to-thickness ratios are among my
favorite comments.

	Comment 3:> For now, I don't want to conclude that the building has
been improperly
	> designed. I would like to know first if I'm missing out on
something. I
	> have other questions but I would like to limit them to these 2 for
the
	> meantime. I hope engineers with experience in this type of
structures
	> could help.
> Thanks

Response 3:  I would call designer and ask how exactly they are bracing the
lower level and whether they used a response spectrum analysis for the
seismic loading.


Scott M Haan  P.E.
Plan Review Engineer
Building Safety Division http://muni.org/building, 
Development Services Department,
Municipality of Anchorage
phone: 907-343-8183   fax: 907-249-7399
mailto:haansm(--nospam--at)ci.anchorage.ak.us

> -----Original Message-----
> From:	h.d.richardson(--nospam--at)home.com [SMTP:h.d.richardson(--nospam--at)home.com]
> Sent:	Monday, March 12, 2001 7:07 AM
> To:	seaint(--nospam--at)seaint.org
> Subject:	Re: Braced Steel Frame System Question
> 
> To Efren Allan Yango,
> 
> 	I have used flat bars as bracing with mixed results.  If you do
> decide
> to use them keep the following in mind.
> 
> 1.)	They do not meet the requirement for L/r ratios < 300.  This is a
> code technicality; some European codes (I'm told) don't even have this
> requirement.
> 
> 2.)	They will only work if they're tightly installed and this is the
> most critical part.  Rod bracing overcomes this by being adjustable.
> 
> 3.)	If you're going to use them in spite of the code they work best when
> you provide some form of intermediate support such as connecting them to
> metal studs within a wall.
> 
> 4.)	If practical, and architectural considerations permit, you are
> better off using something else other than flat bars as bracing.
> 
> 	I hope this is helpful.
> 
> 				Regards,
> 
> 				H. Daryl Richardson
> 
> Efren Allan Yango wrote:
> > 
> > To Engineers with braced frame experience :
> > 
> > I am doing a design check on a mall-type structure with some portions
> > 3-storey high and others 2-storey high. All beams and girders were
> > assumed simply supported by the original designers (no moment frame
> > construction). The columns were designed for simple axial load and
> > minimal moment usually resulting from the beam connection eccentricity.
> > The lateral force resisting system adopted was an X-braced frame system.
> > The size of the whole structure is around 15,000 sq. m with beam spans
> > as long as 12m.
> > 
> > I have checked the path for the gravity loads and it met all the
> > criteria of our codes here. My questions are now related to the braced
> > frames.
> > 
> >     1)    Can the braced frame be located at the 2nd floor to 3rd floor
> > only? (One end of the digonal at the 2nd floor and the other at the
> > third floor). No braces
> >             at the first floor were located. I believe that this would
> > create a soft storey. In terms of member forces, this would pass but I
> > have doubt on this type of
> >             design. That's like terminating your shearwall at the second
> > floor instead at the foundation level.
> >     2)    Can flat bars (designed as tension-only members) be used as a
> > bracing system? I believe that tension-only design is discouraged by the
> > governing codes.
> >             How about for low-rise structures? Is the slenderness limit
> > of 720/sqrt(fy) strictly followed?
> > 
> > For now, I don't want to conclude that the building has been improperly
> > designed. I would like to know first if I'm missing out on something. I
> > have other questions but I would like to limit them to these 2 for the
> > meantime. I hope engineers with experience in this type of structures
> > could help.
> > Thanks
> > 
> > AllanYango
> > 
>