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# RE: More questions about rigid plywood Diaphragms

• To: <seaint(--nospam--at)seaint.org>
• Subject: RE: More questions about rigid plywood Diaphragms
• Date: Mon, 5 Jul 1999 14:43:52 +0100

```Dennis, I was, I think, just refering to the the defination for flexible,
stif and rigid diaprhagm and the formulas 8-2, 8-6 and 8-7 in FEMA273 and
ASCEE/FEMA273 1 drawt Prestandard for a unglued blocked diaprhagm. I was
just emphasising the defination for stif diaprhagm not being rigid, that is
not being able to transfer rotation in it's plan. When I say, it is very
likly that the diaphragms shall be calculated asuming flexible diaphragms, I
meant calculated using the simplified flexible methode or the tribute area
methode.  I maybe wrong but this assumption is based on my caculations. I
was expecting you to comment on this subject earlier the FEMA273 and
ASCEE/FEMA273 1 drawt Prestandard.
I know things are complicated, openings, aspect ratios, elastic design,
strenght design etc., but I think there is a need for simplified methodes in
the Codes for the design of small houses because of small fees, specialy for
wood houses. The computer calculations may all be fine, but the end results
are never better than the assumptions and the data that we use in the
calculations.  So I think there some room for simplifications.
Dennis I will e-mail you privatly my calculations and an example of the
danish methode for small houses for review and mayby something extra if your
bandwith can take it ( just a joke ).

My 10 cent's

-----Original Message-----
From: Seaintonln(--nospam--at)aol.com [mailto:Seaintonln(--nospam--at)aol.com]
Sent: 3. júlí 1999 19:14
To: seaint(--nospam--at)seaint.org
Subject: Re: More questions about rigid plywood Diaphragms

I'm a bit confused by your explanation. The rigidity of a diaprhagm is
calculated by UBC formula for a blocked diaprhagm. Since most residential
diaphragms are not blocked, the American Plywood Association has recommended
multipying the rigid diaphragm results by a factor of 2.5 to determine the
deflection for an unblocked diaphragm. This result is compared against two
times the story drift which is calculated as 0.005h where h is the height of
the story from plate to plate.
The only time a residential diaprhagm will be considered flexible is if the
aspect ratio (distance between shear walls) compared to the depth of the
diaprhgam is large (say approaching 3:1 or more). The closer the shearwalls
occur, the stiffer the diaphragm is and the less deflection is expected.

I would guess the rationale comes from the design of retail stores which
historically used very little shear restraint in the open front. As the
depth
of the diaphragm increased, the rigidity of the diaphagm became more
appearent. Historically, these were designed to calculate the rotation and
design the three "closed" sides to resist the open side.
The problem was that after earthquakes like Loma Prieta and Whittier it
became quickly noted that the front of the building collapsed like an open
front. If my memory serves me correctly, Los Angles City B&S stopped
allowing
design of openfront structures by rotation.
After Northridge, Los Angeles and nearby cities created Open Front
ordinances
to stiffen the open front of any commercial structure and residential
structure (such as a garage or parking area) with a living unit above.

The '97 code takes a slightly different approach. It makes you determine the
worst resulting shear in each line based on Rigid and Flexible analysis (and
Wind). Therefore, even though the diaphragm works out to be rigid, It is my
opinion from the design examples, that you can not simply design the whole
structure as rigid if the resulting shears are controled by the results
obtained from a flexible analysis.

Dennis S. Wish PE

In a message dated 7/2/99 4:25:15 PM Pacific Daylight Time, byggtak(--nospam--at)itn.is
writes:

<< The defination for rigid is one
that has less than half the overage story drift. Both stif and flexible
diaphragms shall be calculated asuming flexible diaphragms. The result of
this for wood Structural Panel Sheathed diaphragms and shear walls is that,
if allowable shear is used in full for the nailing of the panels, then it
is
very likly that the diaphragms shall be calculated asuming flexible
diaphragms. >>

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It may be late, but I just like to mention that FEMA273 and ASCEE/FEMA273
Prestandard define diaphragms as being rigid, stif or flexible. The
defination for flexible is the same as in UBC1994 and is one that has more
than two times the overage story drift.  The defination for stif is one that
has more than half the overage story drift. The defination for rigid is one
that has less than half the overage story drift. Both stif and flexible
diaphragms shall be calculated asuming flexible diaphragms. The result of
this for wood Structural Panel Sheathed diaphragms and shear walls is that,
if allowable shear is used in full for the nailing of the panels, then it is
very likly that the diaphragms shall be calculated asuming flexible
diaphragms. Most part of the deflection is because of nail slip according to
the formulas 8-2, 8-6 and 8-7.
I would like to see simplified calculations for small houses in the Codes
asuming flexible diaphragms. One methode is to divide the house in three
parts with shear walls in every part. The Danish Building Research Institute
http://www.sbi.dk has a book describing this methode. It is in danish,
sorry, but it is a simple methode.