Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

RE: Dynamic seismic design

[Subject Prev][Subject Next][Thread Prev][Thread Next]
See Below for comments.

Bill Cain, SE
Oakland, CA


	-----Original Message-----
	From:	Stan Mulder [SMTP:seatown(--nospam--at)icon.co.za]
	Sent:	Thursday, February 18, 1999 12:22 PM
	To:	seaint(--nospam--at)seaint.org
	Subject:	Dynamic seismic design

	Reading through the CBC 1998 and UBC 1994 prompted me to query
the following (and comment) :
	 
	1.     What factor is used to multiply the custom spectrum
ordinates in UBC figure 16.3,  to get the model spectral accelerations
?  I find that accelerations (and units) seem to become obscure in
earthquake formulas.
	 [Bill Cain:  The ordinates are in fractions of the acceleration
of gravity.  The units after multiplying by the appropriate value of g
would be dependent on the units you used for g.]
	2.    I understand that the dynamic base shear reactions are to
be compared with the statically calculated reactions and brought in
line. [Bill Cain: per section 1631.5.4 ('97 UBC) the factor ranges from
80% to 100% of the statically determined base shear depending on
regularity of the building and whether you use a site specific spectrum
or not.  For a regular building with a site specific spectrum the
allowable value per code is 80%, for an irregular building it is 100%
MINIMUM.  Remember that you are trying to make a building perform in an
acceptable manner.  Doing a dynamic analysis allows you to better
understand your structure.  Obtaining more site specific ground motion
data also allows you to better understand your structure.  Although the
code ALLOWS you to reduce your forces for the prescribed 10% exceedance
in 50 year motion to the static base shear or below, why might you want
to?  If you  1) know the code specifies a 10/50 event and 2) you know
that the forces under that event would be higher a good lawyer could
have a field day with you should the building fail.  However, if the
dynamically determined forces are smaller than the static base shear,
you would be on a much stronger base in reducing the force since you
have documented the expected effect of more specific information.  I
guess what I'm really trying to say is to remember that the code is a
MINIMUM set of requirements.  If you know that the forces(remember that
forces are really a surrogate for deformations in the '97 code) are
larger for your specific case, you are doing no one a favor in
arbitrarily reducing the values. ]All member forces are then similarly
factored up or down.   
	 
	This suggest that all we are achieving with a dynamic analysis
is distributing the effects of the earthquake realistically over the
structure and using the static calculated shear force as a base value
for design forces. [Bill Cain: No. You are developing a better
understanding of your structure and how it is likely to behave.  This
will allow a more realistic design] The period used in the static method
must therefore be very important. For combination of forces further
reductions (1.4) are allowed. 
	 
	The ductility of the steelwork is being heavily relied upon
[Bill Cain: Definitely!] ? Are these adjustments implying that the
theory is not reliable [Bill Cain:  If the dynamic analysis is being
done elastically, it certainly becomes less reliable when the structure
goes inelastic.  Remember that a model is not the actual structure but
only a mathematical representation of it.  It guides your thought
process but must be tempered with reality from the observations from
actual earthquakes (but remember that these observations are also a
model since you don't know all the factors that produced a particular
result.)  Although we like to think our work is entirely in the realm of
a "science" there is much "art" and judgement involved.  

	I think Jim Amrhein of the Masonry Institute of America has said
it best in a humorous quote that I keep on my wall to remind myself to
not to place too much reliance on "models" and to strive for
understanding of structural behavior in my designs.  He said:
"Structural Engineering is the art of molding materials we don't wholly
understand, into shapes we can't fully analyze, so as to withstand
forces we can't really assess, in such a way that the community at large
has no reason to suspect the extent of our ignorance." ]?
	 
	Correct me if I am wrong and help some of us understand 
earthquake design.
	 
	Thanks,
	Stan