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RE: UBC'97 and AISC - ASD

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See comment in text below.
Bill Cain, SE
Oakland, CA

	-----Original Message-----
	From:	Harold Sprague [SMTP:harold.sprague(--nospam--at)neenan.com]
	Sent:	Thursday, February 18, 1999 8:05 AM
	To:	'seaint(--nospam--at)seaint.org'
	Subject:	RE: UBC'97 and AISC - ASD 

	Giammarco,

	In this country and in the UBC, we are in the process of
converting from an
	allowable stress design methodology to a strength (limit state)
methodology.
	It is intended that in the 1997 UBC and future codes, seismic
pseudo static
	loads will be entered into equations unfactored.  This will help
dispel the
	belief (held by some) of elastic performance during the design
seismic
	event.

	The 1.4 number is simply the factor that converts the limit
state design
	seismic pseudo static forces to the allowable stress design
seismic pseudo
	static forces.  By the time you get down to a base shear the R
factors (as
	opposed to the 1997 Rw factors) will get you to about the same
base shear
	magnitudes as 1994 UBC. [Bill Cain:  Except in near source areas
where 
	the load can be substantially larger (larger by a factor of up
to 2.0) due 
	to the near source factors, Na (1.5 max.) and Nv (2.0 max.)]

	If the intent is to use the seismic load derivations of the 1997
UBC, you
	need to divide by 1.4 to apply the ASD steel design provisions
contained in
	Division V 1997 UBC.

	I would strongly caution against using the AISC Manual of Steel
Construction
	Seismic Design Provisions mixed with the UBC.  It is confusing.
The AISC
	uses the 1994 NEHRP as its basis with the Av & Aa and different
R factors.
	Avoid the problems and stay within the 1997 UBC.  

	If your intent is to use allowable stress design:
	1.  Derive the forces in the 1997 UBC Ch 16
	2.  Design the steel using 1997 UBC Ch 22 Div 5

	Hopefully, soon the AISC Seismic Design Provisions and 2000 IBC
will be in
	total harmony.

	Regards,
	Harold Sprague


	-----Original Message-----
	From: Studio Negrini [mailto:stn(--nospam--at)staff.it]
	Sent: Wednesday, February 17, 1999 8:29 AM
	To: seaoc(--nospam--at)seaoc.org
	Subject: UBC'97 and AISC - ASD 
	Importance: High


	In the edition 97 of UBC ( Section 1612.3 ), if I consider 
	Earthquake, I have the following combination :

	D + L + S + E /1.4    
	and one-third increase shall be permitted in allowable stresses.

	In the Manual of Steel Construction - LRFD Vol.1 - Second
Edition
	Part 2,  pag 2-5  I read 

	The ASD method ....  (D+ L' + E) x0.75 
	that is the same to increase one-third the allowable stresses.

	Now I have two questions :

	1 - What is the value 1.4 ?
	     If I consider  E(ubc97)/1.4  the result is lower than the
value
	     stated in accordance with the past UBC edition.

	2 - If my customer requires to perform calculations in
accordance with
	     AISC - ASD  what value of E  I must assume ?   
	     E   or E/1.4 as stated in UBC ?
	     
	Many thanks in advance for Your answers.


	     


	Giammarco Negrini
	Studio Negrini sas di G.Negrini e C.
	Lainate ( MI ) - Italia
	e-mail  : stn(--nospam--at)staff.it