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Re: Braced Steel Frame System Question

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First of all , thanks for your input. They are most valuable.

I though only Mr. Dennis Wish was the only one who responded to my mail..
I am in the process of checking the building. While I was checking the proposed
erection plans, there were some moment frames at second floor level but the
frames below are not even braced. I have to admit that I may not have the luxury
of performing a 3D Respones Spectrum Analysis so I'll be doing a static analysis
and a manual computation and distribution of loads.  The system does not have
any shear walls but have a lot of braced frames that are discontinuous. The
floor system is a composite metal deck. Due to the number of studs welded to the
top flanges the addition of temp. bars at the slab, the composite deck would
perform as a rigid diaphragm. I noted that the original designers distributed
the seismic forces based on tributary area. So I still must check the effect of
distributing the forces through the frame's stifness. With regards to the
gravity loads, the system was designed in a manner that the stress ratio is
around 0.95-0.99. I suspect as early as now that seismic forces will further
increase the stress ratio to more than one.
There are still a lot for me to do. The hardest part is convincing the original
designers that they may have overlooked some important considerations and
provisions. But that is still a result which I would have to find out and I hope
would not be the case.
Again, thanks for all the replies. Your suggestions are very well noted.
Haan, Scott M. wrote:

>         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
> > >
> >
>