From: Roger Turk <73527.1356(--nospam--at)compuserve.com>
Date: Tue, 29 Jan 2002 12:50:12 -0500
Have you looked at ATC-19, "Structural Response Modification Factors?"
Chapter 2: "History of Response Modification Factors" (all of 5 pages long!)
Chapter 3: "Use of Response Modification Factors" (also 5 pages long!)
Basically, as I understand it, the response factors started so that they
would be equivalent with what was then in the code, and proceeded exactly
from there. As far as rationality, there doesn't seem to be any, except
that the "old code buildings" performed well for the most part. They start
with a "box" building, i.e., a shear wall building, aka a 1.33 building,
assigned an R = 6, and end with a moment frame building, aka a 0.67
building, assigned an R = 12, with other types of structural systems assigned
R values in between those buildings. Then various industry reps had the R
values modified to more "precisely" represent their materials.
A. Roger Turk, P.E.(Structural)
Tom Higgins wrote:
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Thanks for your response. This seems not to be a hot topic, as yours was the
only one! I have been aware of the work of Park and Paulay for a number of
years, and once had regular access to their textbook on reinforced concrete
design, which I found very insightful. I suppose the New Zealanders are
probably far out ahead of US practice in reinforced concrete design for
high-seismic areas. As I recall, we owe the concept of X-type reinforcement
for shear wall linking beams to them, among other things.
I likewise do not suppose that the R factors and height limitations are
completely rational, but how would an engineer use research results or
analysis to make a persausive case for a) modifying the R factor, or b)
ignoring the height limitation?
You may want to see my later post, "Dual system combined concrete shear
wall/concrete SMRF," to have a better understanding of the concept I'm driving
at - and trying to shoot holes through. I appreciate your interest. Perhaps
we can get a Kiwi to comment.
>>> "Terangue E. Remengesau Gillham" <teric(--nospam--at)gk2guam.com> 01/17 2:41 PM >>>
I've had the opportunity to read some articles on US code rationale wrt
reduction factors (Rw) and height limitations, and from what I recall much
of the basis for these determinations is empirical/historical in nature.
This has led to inconsistencies among the world's codes, case in point being
the fact that the New Zealand code does not have these limitations, while
the UBC does. IMO the Kiwis are pretty darn good at seismic design of R/C
elements, and hence I would defer to them rather than the UBC when it comes
to this particular issue.
That said, I don't have enough time to pull out the actual references that I
have seen over the years regarding the design of ductile structural walls
for seismic action, save Paulay and Priestley's "Seismic Design of
Reinforced Concrete and Masonry Buildings" which I still hold to be the best
book I have read on the subject. If you are looking for research in this
area, I would search for the names Paulay, Priestley or Park in an article
database (EERC, EERI, MCEER). I would reach back a number of years, perhaps
to 1970 or so, since the research in this area on the NZ end has been going
on for a long time.
Based on my experience with the subject, and actual use on projects, the
improvements come from confinement of the critical areas of the wall
section, namely the ends, based on more rigorous analyses of the expected
compressive strains during the actual inelastic action on the part of the
wall. This requires the use of moment curvature analyses and inelastic
rotation analyses, which is touched on in UBC97 but in a simplified (IMO)
manner. It also requires the results of these analyses, and some additional
factors, in arriving at a reasonable estimate of the expected maximum shear,
based on inelastic action and overstrength, not just a 1.25*Fy factor.
Finally, the foundation must be designed for the same expected MAXIMUM axial
force, moments and shears, in order to force the yielding into the
appropriately designed wall section.
Although it turned out for me to be a LOT more work than what the UBC
requires (as a minimum, mind you), it was much much more satisfying in that
it gave me some good insight into the actual expected action on the part of
the ductile wall in the Big One.
Hope that you find what you are looking for.
T. Eric Gillham PE
From: Tom Higgins [mailto:thiggins(--nospam--at)grpmack.com]
Sent: Thursday, January 17, 2002 8:32 AM
Subject: Ductile Concrete Shear Walls
Is anyone aware of design methodology, backed by persuasive research
results, for unusually ductile reinforced concrete shear walls such as would
justify building above the 160 foot limit for a non-dual system in Zones 3
and 4, and/or liberalization in R beyond the 1997 UBC level (4.5 for Bearing
Wall System/Shear Walls/Concrete)?
I speculate that real improvements in ductility could come from use
composite steel/concrete elements, and use of fiber-reinforced concrete. Is
anyone working actively in this area? Who defines the state of the art in
Thomas B. Higgins, P.E., S.E.
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