To: HaanSM(--nospam--at)ci.anchorage.ak.us, seaint(--nospam--at)seaint.org
Subject: Re: Braced Steel Frame System Question
From: Efren Allan Yango <engreay(--nospam--at)pacific.net.ph>
Date: Tue, 13 Mar 2001 22:10:04 +0800
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
> > third floor). No braces
> > at the first floor were located. I believe that this
> > 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
> > 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
> > 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
> > meantime. I hope engineers with experience in this type of
> > 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
> > -----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
> > >