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RE: UBC'97 and AISC - ASD

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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.

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