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# Fwd: Period Calculation for One Story Building with Flexible Diaphragm, FEMA 310

• To: seaoc(--nospam--at)seaoc.org
• 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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