UBC97 Code Seismic Force Equation

Now I have same question as Mr. Giammarco Negrini for applying a static seismic g-factor.
I generally have used UBC94 Section 1630 , equation(30-1) for the piping component. However UBC97 changed this equation as UBC Section 1632, equation(32-2). Also, I am very confused since new Code seismic force is jumped to twice than old one.
The major differences between UBC97 and UBC94 are as follows ;

-Static equivalent g-factor is approximately twice than old Code for same seismic zone with hx/hr=1.
-UBC 97 add the Load combinations using allowable stress design.
-The load combinations using strength design is changed with less seismic load multiply factor, for instance ; UBC97 Section 1612.2.1, equation(12-6).

I think the piping stress engineer should use UBC97, equation(32-2) with individual piping support location on structure. Also, I suspect that new Code seismic load factor is suddenly twice than before is due to some different conception in load combination.
Please let me know the basis why Mr.Richard Ay recommend that UBC Section 1634.5 can be applied for piping component as seismic force calculation. Also, I would like to confirm which equation is applicable for piping components.

Soon Ryang, WEE


Posted by Giammarco Negrini on November 16, 1999 at 17:29:58:

:I need to know what section of UBC'97 concerns piping.
:I suppose Section 1632, but I need a confirmation.

:Many thanks in advance.


Posted by Richard Ay on November 16, 1999 at 17:48:37:

:In Reply to: UBC code section for piping analysis posted by Giammarco Negrini on
November 16, 1999 at 17:29:58:

:I would use 1634.5 for piping systems.

:Regards,
:Richard Ay (COADE, Inc.)


Posted by Giammarco Negrini

: I need to know what section of UBC'97 should be used
: to define seismic forces acting on piping system.
: I generally refer to section 1632 formula (32-2)
: assuming (hx/hr)=1.

: Many thank for Your comments.

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[This message has been edited by Soon Ryang, WEE (edited December 21, 1999).]

[This message has been edited by Soon Ryang, WEE (edited December 21, 1999).]

[This message has been edited by Soon Ryang, WEE (edited December 21, 1999).]

Dear Sir,

You are indeed correct that UBC has changed the methodology and equations in the newer 1997 edition. The changes do make computations more difficult than in previous issues of the code.

In my opinion, most of the building Codes are setup for calculations of vertical structures and do not lend themselves nicely to piping networks and other "non uniform" structures. In static seismic analysis of piping systems, the CAESAR II input prompts for "G loading" values which are a fraction of gravity in a given direction, X, Y or Z. The program then uses these values to compute a force on the end of each element which is a product of the element's mass and the G loading value in that direction. The value of the G load ranges from 0 (no load) to about 0.4 g's (severe earthquake regions). Designers have used these values with success for quite a number of years and this method traditionally produces conservative results when compared to the more complex time history or response spectrum earthquake evaluation methods. I believe that it would be unnecessary and overly conservative to stray from G loading values that are beyond those mentioned above.

I readily acknowledge that I did not directly answer you question, but perhaps some light has been shed on how the program operates and uses these G loading factors.

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Scott Mayeux
COADE Inc.

Gentlemen,

The following is my opinion and is in part based upon informal conversations with outside resources, and information gained in discussions with knowledgeable qualified structural engineers.

The code changes which you comiserate about have been as a result of an increase in depth of knowledge and understanding of the mechanics involved in seismic design. Some of these newer sources have been the KOBE Japan earthquake, Turkeys problems etc.

These factors along with a desire of FEMA to produce less disaster prone U.S. buildings have stimulated to a certain extent some of these changes.

As for what remains the biggest threat to piping integrity? My opinion is that SAM or Seismic Anchor Motions are as big or a bigger threat to integrity! SAM effects are not covered per se under the UBC for piping but rather are dealt with in one of the piping code sections.

I suggest you look over some of the writings at www.fema.gov
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In summation loads have increase due to increased knowledge and concern.

To design for lateral loads and to not consider SAM when it is a factor will probably lead to piping failure during a major seismic event!

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Best Regards,

John C. Luf

I do have a hard time believing in the validity of applying a Building Code to the seismic analysis of piping. The building code approach is really aimed at determining base shear reactions and anchorage forces for structural design associated with piping or other mechanical attachments. This approach is not intended to predict associated bending stresses in piping.

However, having said that, I do use criteria in the NBC (Canadian equivalent of UBC) for seismic evaluation of piping using uniform load factors. Calculation of these load factors requires a certain amount of interpretation of the code (i.e., how flexible a system is) and this interpretation may not be uniform for all users.

As a final note, ASME B31.3 does make reference to ASCE-7. Chapter nine of this reference provides quite a thorough treatment of pipe seismic reactions but probably in the same vain as the UBC, which I've only partially glimpsed.

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B. Harrington
brian.harrington@veco.com
Burnaby, BC
Phone: (604) 659-3335
Fax: (604) 659-3345