Wood Unblocked Diaphragm Deflections - Wind Design 1

[Nulukkizdin]

I'm looking at deflections for an unblocked wood sheathing diaphragm under wind loading (seismic is much lower in my case). Heading to the special design provisions for wood and seismic (AWC), I found myself looking at section 4.2.2 for deflection. The deflection calculation has (3) components, accounting for bending deflection, shear deflection, and fastener deformation.

The bending deflection and fastener deformation terms seem to be fairly straightforward, but I'm a bit curious regarding the shear deflections. The concept itself seems fine, but when I go to look at the Ga term, I'm slightly unsure of its use for wind. All of the 4.2 tables which list these values seem to place a value for only SEISMIC loading. The table has no Ga term for wind loading.

Jumping back to the commentary seems to take this one step further. C4.2.2 has an equation defining Ga, but calculates it based on the shear capacity for seismic. Furthermore, in the example, we again are looking at a seismic case to determine the value of Ga.

The conservative approach would be to use the same Ga factor for wind design as you would use for seismic design. While this may be conservative, I am still curious if it is the correct approach if deflections were needed in a wind controlled diaphragm design. With this in mind, it also seems odd that for a fairly uniform distribution of load (as is the case for wind), you would get a large contribution of shear deformation at the mid-span of your "beam"?

Let me know what you think!

 

[medeek]

I've actually had this same question when I looked at the diaphragm section in Breyer's book and then referenced the SDPWS-2008. I just assumed that the Ga term is the same for wind as for seismic and the reason they don't print it twice is to simply save space on the page/table. Any enlightenment on this issue would be greatly appreciated from this end as well.

 

[mike20793]

I've always used the same values. The deformation equation references column A in the table, which is the seismic portion. I think its like medeek says, they wanted to save space and not reprint. Maybe they should have it in a separate table then to avoid confusion.

 

[Nulukkizdin]

It's interesting that in their calculation of Ga (in the commentary), they specifically note that the "v" to use is the seismic design value. If this is a coefficient for both, wouldn't it be whichever shear you were applying? And if their point was to note the increase of 1.4 for seismic, I feel like that should be noted as for seismic only?

 

[mike20793]

Very good point Nulu. Has anyone tried contacting AWC about this? It might be useful to know the intent of the code instead of all of us just assuming it's the same value.

 

[woodman88]

Per the ASCE 7-10

"12.12.1 Story Drift Limit
The design story drift (Δ) as determined in
Sections 12.8.6, 12.9.2, or 16.1, shall not exceed the
allowable story drift (Δa) as obtained from Table
12.12-1 for any story."

"C.1.2 Drift of Walls and Frames
Lateral deflection or drift of structures and
deformation of horizontal diaphragms and bracing
systems due to wind effects shall not impair the
serviceability of the structure."

As such, the code requires a calculation for seismic loads to insure the drift limit is met.
Wind loads has no such requirements, in the code. So is not addressed.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[mike20793]

Wind serviceability definitely needs to be considered, ASCE 7 just doesn't set limits for wind drift (the second quote verifies this).

"C.1.2 Drift of Walls and Frames
Lateral deflection or drift of structures and
deformation of horizontal diaphragms and bracing
systems due to wind effects shall not impair the
serviceability of the structure."

 

[Nulukkizdin]

I emailed AWC about it and received the following response. Looks to be similar to several of the points raised above.

Thank you for the detailed question. The Ga term is calculated at the 1.4xASD seismic unit shear value as you noted. The main reason for emphasis on seismic in the SDPWS tabular format of Ga as well as commentary discussion is due to presence of required code based drift checks for seismic design (at strength level seismic forces) while there are no code based requirements that we are aware of that are explicitly require drift checks for wind loads. The format is not intended to suggest that Ga is not applicable for wind design. Note that the 1.4xASD seismic unit shear value coincides with the ASD wind unit shear value for the same diaphragm configuration so that the tabulated Ga is equally applicable for determining deflection of diaphragms. I believe use of this Ga is applicable and the correct approach for ASD wind design. With regard to the large contribution of shear deformation component to overall deflection, this is typical for diaphragms of permitted aspect ratios in SDPWS which have behavior like short deep beams.

 

[mike20793]

Thanks, Nulu. That's a good bit of info.