Original Message -----
Sent: Sunday, May 04, 2008
Subject: I am beginning to
think that those of you using the IBC are not as swift as I thought :)
Seriously, this is not an insult. I posted some questions a
few weeks ago and expected to get at least on if not more opinions on the
matter of Rho and basically received two that were more questions than advice. So
let me go through this once more as it is becoming more important in the design
of low-rise light-framed structures using flexible diaphragm assumptions.
After reading Breyer’s latest edition of “Wood Structural
Design” Chapter 16, the issue of plan irregularity seems much more important in
this code cycle than in pasts. There are a number of horizontal and vertical
plan irregularities that occur in most residential structures that trigger
penalties – even if the code exempts most penalties from the assumption that
2-story structures or less and in some cases 3-story or lower light framed wood
structures have exceptions to those irregularities that assume large torsional
irregularities. However, a simple vertical irregularity where the interior
shear walls that do not stack can force a penalty requiring the use of Eh +/-
Ev; where Eh = Rho*QE (12.4-3) and Ev=0.2SdsD (12.4-4). These are triggered
mostly in SDC of C, D, E & F.
This brings me to the same question related to Rho. In a one
story structure, Rho is always going to be 1.0, but in a multi-story single
family structure, the condition must be that each level carries more than 35%
of the base shear. Depending on the height of the levels it is possible but
many times unlikely that the percentage of shear in the diaphragm that is
transferred to Lateral (vertical) load resisting system is often less than 35%.
I need to know if my assumption that the percentage of
distributed shear is taken from the lateral distribution of forces typical to
each diaphragm Level based on the results of Fx – where wx*hx/Sum(Wi*hi)
represents the percentage of vertical shear distribution as indicated in
formula 12.8-11 and 12.8-12. In other words is it these lower levels where Rho
can increase to 1.3 while at the higher levels or in a one story building Rho
can be assumed 1.0?
Since no one has addressed this question directly other than
to offer their own interpretation, am I to assume that no one really knows or
that I am so inferior to others that it is simply a dumb question that does not
require a response J I fail to
believe that any of you would believe any question seeking knowledge is a dumb
question and therefore must assume that most do not know the answer?
The second question has to do with using the Overstrength
factor Omega when a vertical or horizontal irregularity occurs in structures
that use the Overstrength factor as a penalty over using a rigid or dynamic
analysis to solve the load distribution. This is triggered in section 126.96.36.199
and is relevant in almost any structure in which a vertical or horizontal
irregularity exists and flexible analysis is still allowed in lieu of a rigid
analysis and/or a dynamic analysis.
The horizontal Plan irregularities for structures with
reentrant corners triggers the above penalties if the offset divided by the
overall length of the side exceeds 15% (as it would in most “L” shaped or “U”
shaped structures. However, is the same penalty added if the irregular geometry
with reentrant corners is broken and analyzed in rectangular blocks where each
block is assumed to have two parallel shear walls at each exterior side (in
both orthogonal directions) even if the shear needs to be dragged into the
assumed shear wall though the collector?
The above penalties occur mostly in Seismic Design
Categories D through F and sometimes in B and C. My problem is that in
residential construction these common plan irregularities when considering the
entire geometry rather than the component blocks occurs most often than not. It
is rare to have all walls stack and because of this our analysis must take the
penalty into consideration.
Can anyone offer me some advice on this. I own the 2006 IBC,
ASCE 7-05 and Commentary as well as Breyer’s current edition of Wood Structural
Design (6th Edition) and none of these resources offer a clear
explanation of what to do. In fact, it appears that the Seismic Design manual’s
Examples 1a and 1b are also incorrect in the assumption of Fx and Vx when
irregularities exist. The only time that they do not seem to apply is when the
entire geometry is broken into blocks and joined at common lines of shear but I
don’t believe that this is what the authors of the code intended.
How about some help from all of you who have been using the
IBC since 2000 and seem to be so much more advanced than us lonely engineers
here in California J We really need
some straight answers.
S. Wish, PE
Dennis S. Wish, PE
California Professional Engineer
Structural Engineering Consultant
La Quinta, CA 92253
760.564.0884 (Phone, Fax and Answering Machine)