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RE: ASCE 7-05 Diaphragm Design

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I tried to submit this last night and as of this afternoon I had not seen it posted. Please excuse the redundancy, (Rho J) but I wanted to post it to the list and have made a few clarifications.



Actually, Rho is not usually applied to horizontal diaphragms ( it is not applicable to inertial elements such as diaphragms) as it applies to the redundancy of vertical shear transfer objects such as wood structural panels with rated plywood or light-gage steel. The tests for Rho are applied to the transfer load resisting members (i.e., shearwalls or frames) and is not always considered equal to One in SDC B and C. While I agree that in the lowest category subject to seismic activity SDC ‘A’ rho defaults to one and in one story buildings where 100% of the shear is applied at one level Rho will default to 1.0, there are other tests that must be applied to determine if Rho is applicable to unity or needs to be increased to 1.3.


I too thought that Rho should be applied in the Base Shear analysis when designing Multi-Lat™, but after discussions with others came to see the wisdom in comparing base shear wind and seismic loads without consideration to Rho because of the possibility that Rho would be greater than one in certain conditions even in SDC B-F as I mention below.


If your are specifically referencing Lateral resisting elements other than Wood structural shear panels that are rated, then you may have a point – however, for multi-story light framed structures with Wood Shearwalls and/or mixed lateral resisting force systems (LRFS) then you need to return to section and to check the tests applied to the plan irregularity type that occurs (including “L” shaped structures where the offset is greater than 15% of the matching length).


The overstrength factor is also applied to the vertical lateral load resisting members or transfer members based on their compliance to plan irregularity as noted in IBC Section 1605.4 and ASCE 7-05 Sections 12.4.3 for Em. Omega is applied to the vertical element to penalize it for the irregularity by boosting the load applied that will effect overturning of the wall. Omega is specified in the increased seismic force for overturning in the formula;


Em = W0Qe±0.2SDSD where W0  is determined from ASCE 7 Table 12.2-1

The increase is intended to be used in the vertical wall design as well as an increase for chord and collector strength but W0  is not necessarily applied to light-framed wood structures with wood shear walls as noted in the ASCE code.




From: Paul Feather [mailto:PFeather(--nospam--at)]
Sent: Tuesday, April 22, 2008 3:27 PM
To: seaint(--nospam--at)
Subject: RE: ASCE 7-05 Diaphragm Design


Rho would apply because it is applied to the base shear.  Omega would not apply to the diaphragm.  Rho is always one for categories B or C.


Having a full height frame and then some less than full height frames is not a discontinuity in my opinion, anymore than using less bays of bracing in a frame as you go up in elevation.  Each frame is still continuous from start to foundation.  A discontinuity exists when the five story brace stops at level two and needs to transfer to another line.


Yes, force transfer can occur when you introduce an additional element in the lower parts of the building, but be careful in the analysis because computers will show large force jumps that are not realistic.  But this is not a discontinuous system.


From: Gerard Madden, SE [mailto:gmse4603(--nospam--at)]
Sent: Tuesday, April 22, 2008 1:57 PM
To: seaint(--nospam--at)
Subject: ASCE 7-05 Diaphragm Design


The way I read section, it means that diaphragms shall only be design for the RHO factor when they are transfer diaphragms in Seismic Design Categories D, E, & F.

This seems logical (and OMEGA would kick in too due to the discontinuity).

This can be difficult to track if another frame line is taking load out of adjacent frame (even one that is not discontinuous) through the diaphragm (like unzipping it). This can happen say when you have say:

A 5 story braced frame from foundation to roof and then near bye a frame from foundation to 2nd floor. At the second floor diaphragm, force transfer can occur even though the 5 story frame is continuous.

Anyone beg to differ?

I guess the moral of the story is have RHO = 1.0 and get 2 frames per side...