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[SEAOC] ATC 19, "Structural Response Modification Factors"

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According to a recent Applied Technology Council Report, "ATC 19, Structural 
Response Modification Factors", 1995, on page 9, are the following conclusions:

"Recent studies by researchers (e.g. Bertero, 1986) and design professionals, 
including Project ATC-34, have identified MAJOR SHORTCOMINGS in the values and 
formulation of the response modification factors used in seismic codes in the 
United States.

"1. A single value of R for all buildings of a given framing type, irrespective 
of building height, plan geometry, and framing layout, CANNOT BE JUSTIFIED.

"2. The use of the values assigned to R for some framing systems WILL LIKELY NOT 
 produce the desired performance in the design earthquake.

"3. The response modification factor is intended, in part, to account for the 
ductility of the framing system.  Recognizing that a constant ductility ratio
CANNOT be used to uniformly reduce elastic spectral demands to design 
(inelastic) spectral demands (measured typically as base shear), R must be 
period-dependent.  This dependence is recognized in the Eurocode and the Mexican 
Code (see Chapter 3 for further discussion).

"4. The reserve strength (strength in excess of the design strength) of 
buildings designed in different seismic regions will LIKE VARY SUBSTANTIALLY.  
Given the reserve strength is a key component of R (see Chapter 4), R should be 
dependent on either the seismic zone or some ratio of gravity to seismic loads.

"5. Seismic design using the response modification factors listed in seismic 
codes and guidelines in the United States will most probably NOT RESULT in a 
uniform level of risk for all seismic framing systems."  (Emphasis added).

In other words, according to ATC-19, the fundamental basis (the use of uniform 
values of structural response modification factors that vary with the building 
framing types regardless of building period) for the 1994 NEHRP Recommended 
Provisions for Seismic Regulations for New Buildings and the 1994 Uniform 
Building Code Provisions seismic provisions is "fatally flawed." 

For additional information concerning how to obtain ATC-19, call (415) 595-1542 
or FAX (415) 593-2320.  Let's hear some discussion about these conclusions in 

Frank McClure    fmcclure(--nospam--at)       File ATC191.txt