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RE: ASCE 7-05 Diaphragm Design
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- Subject: RE: ASCE 7-05 Diaphragm Design
- From: "Dennis Wish" <dennis.wish(--nospam--at)verizon.net>
- Date: Tue, 22 Apr 2008 15:37:17 -0700
Gerard (and others), First, Section 12.10.1.1 states that Rho is checked in SDC D, E
and F only in Transfer elements, which are vertical elements such as shear
walls that move shear from diaphragms to foundations. They are not calculated
in inertial elements and Eq. 12.10-1 is indicated which is the diaphragm design
force that “distributes” shear through each diaphragm to the
vertical components. With that said, you still cannot assume Rho is 1.0 until you
evaluate the structure using 2 possible methods (according to Breyer’s “Design
of Wood Structures”). In Seismic Zones A-C. Zone A can be assumed Rho to
be set to 1.0 but B and C must be verified to meet the first to checks in ASCE
7-05 Section 12.3.4.1. The first is whether or not you are in SDC B or C but
the second is a requirement to check drift calculations and P-delta effects.
There are no exceptions noted in section 12.3.4.1 for this. In Seismic Design Category D, E and F, Breyer points to two
methods that must be verified; “In the first method
(ASCE 7 Section 12.3.4.2, Item a), the user is asked to remove vertical
resisting elements on at a time, and check to see if (1) the story strength is
reduced by more than 33 percent OR (2) if an extreme torsional irregularity is
created with the element removed. If either of these conditions exist, a Rho of
1.3 must be used; if it does not, Rho may be taken as 1.0. In shearwall
structures, it is only shearwalls with a length shorter than the wall height
that need be investigated. If all shearwalls have a length at least equal to
their height, the structure will automatically qualify for Rho=1.0. Where
removal of shearwall elements must be investigated, a rigid diaphragm analysis will
be required, resulting in a significant analysis effort. The Second method (ASCE 7
Sec. 12.3.4.2, Item b) applies only to buildings that are regular in plan at
all levels (i.e., no irregularities are triggered). It requires that there be
two qualifying shearwalls in the building perimeter at all sides in each
evaluated story. For wood-frame shearwall buildings, the length of each
shearwall is to be not less than one-half the story height. For other wall
types, the length of each wall is required to be not less than the height of
the story. If the required perimeter shearwalls are provided, the structure
will qualify for Rho of 1.0; otherwise Rho will need to be taken as 1.3. This
approach is much easier to apply in most simple buildings.” “Design of Wood Structures – ASD/LRFD” by;
Breyer, Fridley, Cobeen and Pollock – Sixth Edition, McGraw Hill
Publishing Chapter 2 Page 2.48-2.49 Section: Redundancy/reliability factor. I still have an issue with this in multi-story structures. The
distribution of shear to each inertial element using Equation 12.10-1 is a
distribution of the base shear to each level in proportion to the total base
shear (the sum of the distributed story shears is equal to the base shear). In
this case, Eq. 12.10-1 represents the percentage of shear distributed at each
level Fx. According to Section 12.3.4.2 Rho in SDC D, E and F is equal to 1.3
unless the tests indicated in 12.3.4.2 (a) and (b) are satisfied. In; “ (a) “Each story
resisting more than 35 percent of the base shear in the direction of interest
shall comply with Table 12.3-3 and (b) Structures that are regular in plan at
all levels provided that the seismic force-resisting systems consist of at
least to bays of seismic force-resisting perimeter framing on each side of the
structure in each orthogonal direction at each story resisting more than 35
percent of the base shear. The number of bays for a shear wall shall be
calculated as the length of shear wall divided by the story height or two times
the length of shear wall divided by the story height for light-framed
construction” This being the case, most multi-story structures below the roof
will distribute through Fx or the diaphragm inertial shear, less than 35% of
the base shear and thus qualify for Rho equal to 1.3. I think we need to be cautious as to the intent of the code
writers on this issue and look at the numbers. In most cases, light-framed
structures will not develop large torsional distributions (where the distance
from the center of mass to the center of rigidity is large), but the percentage
of base shear distributed to lower levels (Fx and not Sum Fx which is the
Vertical Element transfer) will be less than 35% in at least one of the levels. Dennis Dennis 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) dennis.wish(--nospam--at)verizon.net http://structuralist.wordpress.com http://www.structuralist.net From: Gerard Madden, SE
[mailto:gmse4603(--nospam--at)gmail.com] The way I read section 12.10.1.1,
it means that diaphragms shall only be design for the RHO factor when they are
transfer diaphragms in Seismic Design Categories D, E, & F. |
- References:
- ASCE 7-05 Diaphragm Design
- From: Gerard Madden, SE
- ASCE 7-05 Diaphragm Design
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