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Re: Unblocked Plywood Diaphragm Deflection Calculation

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Jefffery Seegert,

Thank you for your email posting on the SEAOSC List Server where you 
presented the equation for calculating the deflection of a wood structural 
panel sheathed (plywood) diaphragm.

As I understand this equation  it is the same equation that appears in the 
1994 UBC Section 23.222, page 3-725 and in FEMA 273, Equation (8-6), page 
8-27.  The portion of  both the equations,  "0188 L*en", relates to the nail 
slip contribution to the diphragm deflection. 

Let us examine the factor "en" in the 1994 UBC, which has values given in 
Table 23-2-K, page 3-739.  For "LOAD PER NAIL" of 180 lbs. for 8d nail, the 
value of en = .056 and for 200 lbs. for 10d nail,  the value of en = .047, 
where "Load per nail = maximum shear per foot divided by the number of nails 
per foot at interior panel edges.".

Now let us look at the values for "en" given  in FEMA 273 on page 8-27, under 
Equation (8-6):  "en = Nail deformation at yield load per nail based on 
maximum shear per foot vy divided by the number of nails per foot.  For 8d 
nails, en = .06.  For 10d nails, en = .04. " (Emphasis added.)

For the 1994 UBC, the "maximum shear per foot" would be calculated,  based on 
the diaphragm shears, resulting from the application of 1994 UBC, Equation 
(31-1),  which is based on Section 1628.2.1, " Design Base Shear",  Equation 
(28-1), 
 V = Z*I*C*W / Rw.  A typical value of  V  =  0.188 W is common for wood 
frame shear wall buildings. 

For FEMA 273, the "maximum shear per foot" would be calculated, based on 
diaphragm shears, resulting from the application of  FEMA 273, Equation 
(3-9), which is based on Section 3.3.1.3, "Pseudo Lateral Load", Equation 
(3-6),
V = C1*C2*C3*Sa*W.  A typical value for V =  greater than 1.0 W is common for 
wood frame shear walls buildings, about five times greater than 1994 UBC,  V 
= 0.188 W.

Do the values of "en" for FEMA 273,  at yield load per nail,  based on 
"Pseudo Lateral Load" Base Shear look correct when compared to "en" values 
for 1994 UBC, at maximum shear per foot?  The two values of "en" are about 
the same value, "en" = .06 for 8d nails and .047 for 10d nails.

In other words, if "en" relates to "nail slip", should not the values of "en" 
be greater for FEMA 273 at yield load per nail, based on "Pseudo Lateral 
Loads"  Base Shear than for the  1994 UBC "en" values of  based on " 
Allowable Stress" Base Shear.   Is it not correct to assume that the "nail 
slip" will be greater at large forces per nail?

Bottom Line:  1. Is FEMA 273, Equation (8-6) correct?  2.  Should not the 
"nail slip" contribution to the diaphragm deflection be greater for FEMA 273 
as compared to 1994 UBC?

The same concerns can be expressed concerning the correctness of  the "en" 
values for FEMA 273, Equation (8-2), page 8-19 as compared with the "en" 
values for 1994 UBC, Equation  in Section 23.223, page 3-725  for plywood 
shear walls.

I came across this possible problem area of the values of "en" for FEMA 273 
while conducting a "peer review" of one of  the Building Seismic Safety 
Council (BSSC)
Case Studies Project for a wood frame building.

I would welcome any responses to this email message.


Frank E. McClure      FEMCCLURE(--nospam--at)aol.com   May 19, 1999 

File: FEMA273diaphragmdeflections1.d0c