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Re: Cantilevered Wood Diaphragm

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Monty, you may have gotten yourself into a pickle.  The structural
system you have illustrated is not one that I would be comfortable
with.  I would have looked into putting vertical resisting elements
(shear walls or rigid frames) at or closer to the perimeter.  Perhaps
you did, and for some reason, weren't allowed.

My first sense is that if you check drift at the end of the diaphragm,
you will find it is in violation of the building code.  I would suggest
you look a little harder at what your plan checker is suggesting, that
is to show the transverse walls participating in longitudinal forces. 
If you do have a drift problem as I suspect, and you can't justify it
even with the participation of the transverse walls, I suggest this is
the time to consider additional vertical resisting elements closer or at
the perimeter.

You can probably point to the plan checker's comment or blame it on all
the changes in the 97 UBC to get your client to swallow the additional
design time required and construction cost.

Better to pay the price now so you can sleep easy later.  Good luck.

Jeff Coronado, S.E.
West Covina, CA


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 along 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.
> 
> 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 it's three shear walls) acting as a "three
> sided box".  The plan checker's written comment on this issue is as
> follows;
> 
>            "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 was 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.  I have been unable to locate
> any literature that addresses the design of a symmetrically cantilevered
> flexible diaphragm, although I know it is not that unusual in apartment
> buildings.  Any opinions or information, pro or con, would be greatly
> appreciated.
> 
> Monty Hart
> Associated Design Consultants, Inc.
> Anchorage, Alaska
> www.ADC-Engineers.com
>