To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
Subject: RE: Seismic evaluation of an old concrete building
From: "Sprague, Harold O." <SpragueHO(--nospam--at)bv.com>
Date: Fri, 1 Feb 2002 12:19:00 -0600
Ah yes. I remember my first time.
You need to get the following FEMA documents as a starting point. I'll just
give you the numbers. FEMA will know the titles.
154, 155, 172, 175, 178, 273, 274, 310, 343
273 may no longer be available. It was replaced by 310, but the commentary
in 274 is still applicable.
FEMA 310 and 274 are the most critical documents. I would also refer to the
FEMA 368 and 369.
Your questions don't lend themselves to a straight forward answer. After
you gain a familiarity with FEMA 310 and 274, you will be able to understand
the whole process a bit more. The short answer to your question is yes you
can install concentric braced frames.
You need to discuss with your client what level of performance is expected
for a given event probability.
As a minimum you will design for a DBE that is 2/3 of the MCE. I would get
a seismologist to define the MCE for your site. I would also get a geotech
to determine the soil profile. Then you can develop the MCE (2500 year
event) and the DBE. The USGS determined the MCE for all of the US. They
only developed a process that can be used for other countries. E.V.
Leindecker of the USGS is THE guru on developing the site seismicity. He is
a very knowledgeable structural engineer and a seismologist in his spare
The more nonductile you design the seismic force resisting system the higher
the base shear. The more ductile systems will result in lower base shears,
but the nonlinear deformations may compromise the structural performance,
and will require evaluation. There is a movement that is promulgating the
concept of using low ductility systems with very low R values and using the
MCE. The concept is that if you design for an R of 1 for a 2500 year event,
you will get linear performance for the rare catastrophic event (very
conservative). The down side is that the base shears are BIG, and the
bracing can get expensive.
Now that should have totally confused everyone. It sure confused me. Read
the 310, 274, 368, and 369 to get much better guidance. After you read
these documents, you will be better able to gauge the degree of your
confusion. Seriously, the documents should help. Good luck.
Harold O. Sprague
> Kevin Below wrote:
> > Talking about flow charts...
> > I would like to work out a method for approaching the problem of
> > evaluating the seismic 'capacity' of a reinforced concrete building
> > built around 1950 in Quebec, Canada.
> > That is a high risk zone (for Canada) Za = 4, Zv = 3.
> > Actually, the context is that this building has been designed and used
> > as a school, and now the City wants to know if it can be recycled as a
> > library. This means a higher floor live load, so I need to evaluate the
> > cost of the required reinforcement, in order to help make the decision
> > to demolish or reinforce. Once we make major modifications like
> > reinforcing, we would also need to make it satisfy the current code with
> > regards to seismic resistance.
> > Of course, I have no drawings showing the existing steel reinforcement.
> > This is the first time I have had to do a seismic evaluation, so I have
> > been trying to work out how to go about it.
> > The building is a 2-story frame, no shear walls, floor slabs are
> > one-way, and there are beams on all column lines in both directions.
> > There is also a line of beams in the longitudinal direction mid-way
> > between column lines.
> > Step 1. Evaluate the base and story shears according to code, assuming
> > a non-ductile concrete frame.
> > Step 2. If I knew the reinforcement, I could check the columns and
> > beams for adequate strength. But of course, I don't, so...
> > Step 3. No choice but to add some sort of bracing (ductile ?) to resist
> > the seismic loads, assuming no contribution from the existing members.
> > Is that right ?
> > What sort of choices do I have for bracing systems in this case? I have
> > searched on Intenet, as well as in my concrete books. Nada.
> > If I understand the code correctly, I do not really need to provide
> > ductility for this building if I calculate it as a non-ductile frame.
> > So would diagonal steel bracing in the appropriate places be sufficient?
> > Any comments or guidance would be appreciated.
> > Kevin Below
> > Génécor inc., Québec
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