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Re: Cantilevered Wood Diaphragm -Reply

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Just thoughts:

For lateral forces acting in the long direction:

If I had a beam 56-feet long, and supported only by two supports 6-feet apart 
at the center of the beam, I would be worried, very worried.

No, there is nothing in the code that would prevent me from doing it, but 
common sense tells me that it is not the prudent thing to do, no matter what 
the arithmetic says!

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Monty Hart wrote:

>>I recently designed a wood framed, two story, apartment building for
lateral loads.  In the longitudinal direction, there are no exterior
shear walls, however, there are longitudinal shear walls along a 6' wide
corridor at the center of the building.  The floor and roof diaphragms
are cantilevered out about 25' on each side of the corridor walls
(building is about 56' wide x 184' long).  There are transverse
shear walls at about 46' oc between living units.  The shear wall plan
looks something like this;

    46'          46'         46'       46'
|          |          |          |          |  25'
|          |          |          |          |
_____________________________________________   6'
|          |          |          |          |
|          |          |          |          |  25'

Shear walls on the first and second floors stack, seismic governs(Zone
4), diaphragms are assumed flexible.  I am analyzing the diaphragm as a
continuous 184' deep horizontal beam symmetrically cantilevered out 25'
on each side of the building.  I maintain that because the seismic
loading and the ("H shaped") geometry are symmetrical, there is no
rotation that would distribute forces to the transverse shearwalls from
longitudinal seismic loading.

The municipal plan checker, who is a licensed engineer, has stated that
the shear walls must be designed for diaphragm rotation, with each of
the living units (bounded by three shear walls) acting as a "three
sided box".  The plan checker's written comment on this issue is as

"Engineers assertion that the diaphragm will not rotate because the
center of mass is at the
geometric center is not correct.  Flexible diaphragms distribute forces
to shear walls based on location and not the rigidity of the elements. 
Only rigid diaphragms act in that way.  Also, the shear walls break the
diaphragms into smaller pieces, and these pieces are not H shaped as
assumed.  Each small diaphragm acts in rotation, although the forces on
the center elements may be canceling each other out- that is subject to
analysis.   Please check all three-sided elements, and provide design
for the diaphragm, the chord forces, the nail slip, and the added shear
and overturning of the back walls
as a result of rotation forces.  Show all elements on the plans."

If this were a simple "three sided box" structure, I would certainly
agree with the plan checker about designing for rotation.  However, I do
not believe the condition is comparable and I have been unable to locate
any literature that addresses the design of a symmetrically cantilevered
flexible diaphragm (although I know the condition is not that unusual in
apartment buildings).  Although I'm generally not that crazy about
cantilevered diaphragms, I am in no position to require the architect to
redesign and sheath the exterior walls at this time. Any opinions or
information, pro or con, would be greatly appreciated.

Monty Hart<<