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RE: Seismic Provisions

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Jon,

The UBC code has changed markedly from 94 to 97.  Which code are you referring to?  I assume by your citing R instead of Rw that you are using the 97 code.  Why are your R values so low?

I will agree that the intermediate seismic zones do not receive the attention that the high seismic zones get, but the detailing requirements are prudent based on the research done to date.  I do wish that there was more relating to moderate seismicity.

Case 1:
You do not have to detail to the capacity of the members.  Generally it is more economical to multiply the forces by the omega sub 0 (3Rw/8 in the 94 code), especially in moderate seismic zones.

Case 2:
Please site the code sections that are driving you to the R=1 and 1.5.  An R of 1 would imply an elastic response.

Sorry that I could not be of more help.

Harold Sprague, PE
Krawinkler, Luth & Assoc.

-----Original Message-----
From:	terrence turner [SMTP:tnturner(--nospam--at)glen-net.ca]
Sent:	Sunday, September 13, 1998 9:43 AM
To:	seaint(--nospam--at)seaint.org
Subject:	Seismic Provisions

Hi:

	My question is whether some of the provisions with respect to detailing in
steel and concrete are too harsh for zones of moderate seismicity.

Case 1:  A R=2.0 steel structure using chevron or tension only bracing.
This type of structure is classified as nominally ductile.  Typically the
design of the bracing is governed by their slenderness ratio(kl/r <= 200
for compression(chevron bracing)..and kl/r <=300 for tension (tension only
bracing)).  How do we convince the steel detailers(who design the
connections) that the connections must be for the full nominal tensile
capacity of the member.  It is sometimes 5 or 6 times greater than the
compression or tension within the brace.  Of course we do not ever want to
have a failure at the connection before the failure of the strut...but is
there a limit to it being way over designed?


Case 2:  A R=2.0 concrete structure using shear walls.  This type of
structure is classified as nominally ductile.  In this case we must provide
shear reinforcement at the base (i.e plastic hinge region) for an increased
shear force using R=1.0...which typically leads to foot thick shear walls
for buildings of less than 3 stories.  How do we justify this?
Of course we do not want to have a premature shear failure before yielding
of the end vertical reinforcement.  Do you think it makes more sense to use
a value of R=1.5 to increase the shear force since a R value of 1.5 means
you do not provide any specialized detailing.

And finally Is it better to use a R value of 1.5 (which the code allows for
structures under 3 stories) and deal with the overturning problems...and
forget about all the specialized detailing?  


thanks in advance

Jon Turner

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