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RE: Unblocked Plywood Diaphragm Deflection Calculation
[Subject Prev][Subject Next][Thread Prev][Thread Next]- To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
- Subject: RE: Unblocked Plywood Diaphragm Deflection Calculation
- From: "Jeffery Seegert (x 485)" <jbseegert(--nospam--at)matrixti.com>
- Date: Fri, 21 May 1999 08:04:42 -0400
Just a question. If the diaphragm is correctly engineered to begin with (assuming loads not to exceed the allowables), why would one need to worry about nail slippage past the elastic capacity. I can understand if analysis is being performed on an existing structure. Also, I agree with the negligible nail slip and chord slippage in the majority of all relatively small diaphragms. But to every generalization there are exceptions. Some of the roofs that I have designed in the industrial and warehousing areas were so large that nail slippage did become a sizeable concern when looking at story drift. -----Original Message----- From: Niaz A. Nazir [mailto:NAN(--nospam--at)eqe.com] Sent: Thursday, May 20, 1999 1:56 PM To: seaint(--nospam--at)seaint.org Subject: Re: Unblocked Plywood Diaphragm Deflection Calculation ---------------------- Forwarded by Niaz A. Nazir/EQE on 05/20/99 11:00 AM --------------------------- Niaz A. Nazir 05/19/99 05:12 PM To: FEMCCLURE(--nospam--at)aol.com cc: seaint(--nospam--at)seaint.org, David L. McCormick/EQE@EQE Subject: Re: Unblocked Plywood Diaphragm Deflection Calculation (Document link not converted) I agree to your argument. I was looking thru ABK report and came across the hysteresis behavior of the tested diaphragms. The diaphragm deflection is certainly not linear as the load increases and follows a curve defined by: F(e) = F(subscript: 2)e/(F(subscript: 2)/K(subscript: 1) +abs(e)) Definitions of various terms may be found in ABK report. The slippage of nails increases as the force increases beyond initial elastic capacity. Furthermore, before the elastic capacity (defined by the Code allowable capacity) is reached, the nail slippage will be very small, if any and could be ignored for all practical computation purposes. This will lead to a equation without the nail slippage and chord slippage contribution. Regards FEMCCLURE(--nospam--at)aol.com on 05/19/99 03:55:55 PM To: seaint(--nospam--at)seaint.org cc: FEMCCLURE(--nospam--at)aol.com, David L. McCormick/EQE, Niaz A. Nazir/EQE Subject: Re: Unblocked Plywood Diaphragm Deflection Calculation 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
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