Need a book? Engineering books recommendations...

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

Seismic Provisions

[Subject Prev][Subject Next][Thread Prev][Thread Next]

	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