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Fwd: Period Calculation for One Story Building with Flexible Diaphragm, FEMA 310
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- Subject: Fwd: Period Calculation for One Story Building with Flexible Diaphragm, FEMA 310
- From: <FEMCCLURE(--nospam--at)aol.com>
- Date: Thu, 11 Jun 1998 07:26:35 EDT
In a message dated 98-06-11 07:20:54 EDT, FEMCCLURE writes: << com >>
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- To: seaoc(--nospam--at)seaoc.com
- Subject: Period Calculation for One Story Building with Flexible Diaphragm, FEMA 310
- From: FEMCCLURE(--nospam--at)aol.com
- Date: Thu, 11 Jun 1998 07:20:54 EDT
Section 4.2.2.1.1 Pseudo Lateral Force, page 4-3, FEMA 310 presents the following: "For a one story building with a single span flexible diaphragm, in accordance with Equation (4-1) T = (0.1 x in-plane wall displacement + 0.078 x diaphragm displacement)**0.5, (4-1), where the in-plane wall and diaphragm displacements in inches due to a lateral force equal to the weight tributary to diaphragm in the direction under consideration, or......" Has anyone used the above equaton? Consider a one story tilt-up concrete building, 80 feet x 120 feet with 19 feet high 5 3/4" tilt-up reinforced concrete walls without any openings except a few small doors. The roof diaphragm is 3/4" plywood with 10d @ 4" o.c. nailing and the roof weight is 20 lbs. per sq. ft. What is the fundamental period, T, for this building? Remember the determination of the period, T, is to be used in a base shear equation. The 1994 Uniform Building Code, Section 23.22, has an equation for the calculation of a plywood diaphragm deflection, which includes a factor "A" for the area of the diaphragm chord. In an existing tilt-up building, what is the diaphragm chord? Is it the continuous steel 4" x 4" x 1/4 " angle which supports the roof construction at the face of the 5 3/4 inch reinforced concrete tilt-up wall? Is it some portion of the 5 3/4 inch reinforced concrete tilt-up wall, say 5 3/4 " x 24", like the flange of a reinforced concrete tee beam? Again, this problem is not a design problem where one can assign, say two #6 reinforcing bars that are welded at their splices in the 5 3/4 inch reinforced concrete walls at the 3/4 plywoood diaphragm level to be considered as the diaphragm chords. We are trying to determine the period, T, to determine the base shear which will be used to evaluate the building for seismic forces and design a seismic retrofit. It makes a big difference in the period, T, depending on how the diaphragm chord is defined. This is not an "academic" problem. I am reviewing a real existing building for seismic evaluation following FEMA 310. It makes a big difference in the base shear depending on how the period, T, is determined. If we can not agree on how to calculate the fundamental period of a one story building with a single span flexible diaphragm, then it will be difficult for different engineers to evaluate and design the retrofit of an existing building in a consistent manner, using equation (4-1) in FEMA 310. I would appreciate any assistance you can provide. Frank E. McClure FEMCCLURe(--nospam--at)aol.com
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