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RE: Overturning check

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The intent of the building code is to use Allowable Stress load
combinations with a factor of safety of 1.0 when checking stability due to
overturning when subject to seismic loads.  Normal factors of safety are
then used when designing individual members.

The reason that this is acceptable is that unlike Wind loads earthquake
loads are not an estimate of the actual forces on a structure during an
earthquake.  In a major earthquake the forces in the structure will be
significantly larger than those estimated by the code imposed loads.  

You are not trying to insure that there will be no uplift.  Rather the goal
is to prevent the building from overturning.  This is consistent with the
codes objective of insuring like safety, which means that building damage
is acceptable as long as people can exit safely in the event of a major
earthquake. 

The use of normal factors of safety when designing individual members is
appropriate sine you want the building to have the integrity so that all of
the required uplift capacity can be engaged.

It is my belief that in many buildings  the factor of safety against
overturning is less than the factor of safety for the individual members. 
As a result we would expect these structures to disipate energy through a
rocking mechanism and as such the moment frames, shear walls, and braces
wouldn't  be subject to the inelastic demand that we designed them for.  It
is my contention that the building code should recognize rocking as an
acceptable mechanism for dissipating energy.


Mark Gilligan 



++++++++++++++++++++++++++++++++++++++++++++++++
Message text written by INTERNET:seaint(--nospam--at)seaint.org
>What does Section 1612 say?
 
FWIW, It's typically serviceability with it's own set of safety factors
(e.g. usually a factor of 1.5 to 2.0 against overturning).

-----Original Message-----
From: Javier Encinas [mailto:jencinas(--nospam--at)coqui.net]
Sent: Wednesday, February 02, 2000 9:44 AM
To: seaint(--nospam--at)seaint.org
Subject: Overturning check


Per UBC 1630.8, "every structure shall be designed to resist the
overturning
effects.... See Section 1612 for combining gravity and seismic forces".
 
Do you use the Strength Design combinations or the Allowable Stress Design
combinations? Is overturning dependent of the design method, or is it a
serviceability issue and therefore the service combinations should be used?
Regards,
 
Javier Encinas
ASDIP Structural Software

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