Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Re: FEMA 273

[Subject Prev][Subject Next][Thread Prev][Thread Next]

The SEAOSC's Nonductile Concrete Subcommittee of the Existing Building
Committee  has found some inconsistencies/confusing statements in the documents
which seem to greatly influence the type of analysis you perform on your
structure and the results obtained.  You may want to contact the subcommittee
chair, Mr John Kariotis, at Kariotis and Associates (626) 355-4184.  

If you would like to do a comparison analysis of your structure using a
seperate and established document, you may want to consider the City of Los
Angeles' Division 95, "Voluntary-Earthquake Hazard Reduction in Existing
Reinforced Concrete Buildings and Concrete Frame Buildings with Masonry
Infills"  The procedure is based on stress/strain analysis of the material and
it lends itself to a more understandable procedure than the many factors and
procedural checks of the FEMA 273.  Currently, the committee is considering a
revision proposal to Div. 95 to add a second level of performance for essential
building by increasing the spectrum level to mean plus one sigma.  If you give
me some time, I could fax you a copy.  Else, you may want to contact the City
of Los Angeles Dept of Building & Safety, for a copy. Try 213\977-6216.  I
think SEAOSC also has copies since they distrib. a copy to members also at one
of the seminars entitled, "Considerations of Bldg. Conservations Toward the
21st Century".  SEAOSC's Number is of course 562\908-6131.

Ed Gonzalez
>>> Lynn <lhoward(--nospam--at)> 08/05/98 07:58AM >>>
I would like some input on those using FEMA 273 guidelines for seismic
analysis and retrofit of existing concrete structures.

I am searching for a guideline that is accurate in predicting  D/C
ratios, and guidelines as to what D/
C ratios are acceptable for minimal life-safety performance for
individual structural elements.

Could the following scenario happen using the FEMA 273 guidelines:

Consider an 8 story concrete shearwall building, with columns and beams
that are not a part of the shear resisting elements.  Design was done
using Static lateral analysis procedures in the mid 1980's using the
1982 UBC.
A Dynamic analysis on the concrete structure is done using FEMA 273
guidelines.  The results tell you that the building has a potential risk
for failure.

I guess the problem is that I really do not alwyas believe what the FEMA
273 guidelines are telling me.  The building is a shear wall building.
The building's primary mode period is small (around .3) and overall
deflections are small.  When you sit back and study the overall
building's structural system, you come to the conclusion that it will
suffer some damage in an earthquake, but should not fail..

The Owner is only concerned about the safety of the people in the
building during an earthquake.  The Owner is not concerned at all about
the possibility of the building being a complete loss after an
earthquake.  A retrofit could cost millions of dollars.  FEMA 273 tells
me we should recommend a retrofit.  But as I look at the building, I
just do not see it failing.  This kind of building does not have a
history of failing during earthquakes.  It is after all a shear wall
building.  It was designed in accordance with the 1982 UBC.  Sure, it
does not meet all of the current 1997 UBC requirements, but I just do
not believe it will fail to a point where the occupants will be in for
anything more than a "wild ride" in an earthquake.

Are there any others out there that have run across this situation?  If
you have, what have you done?  When faced with a FEMA standard that
tells you the building is a danger, do you dare stand up and take the
liability of saying that you think the FEMA 273 standards are in some
circumstances too conservative and do not reflect how the building will
actually perform in an earthquake?  By stating such a position, your
liability could be tremendous.  Or do you just play is safe and have the
Owner spend millions on a retrofit, even though in your heart you do not
believe it is necessary.

This is my moral dilemma of the day.  Any insight would be appreciated.