Return to index: [Subject] [Thread] [Date] [Author]

RE: Skewed (45 degree) shearwalls

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Debbie,
I completed one home where the shape had a long diagonal wall - a glass wall
overlooking the golf course. There were sufficient orthogonal walls to
control the diaphragm and, essentially, no additional shear was needed in
the glass wall.
Still, I did not feel comfortable - if only because a force occurring in
line with this wall will have some effect and I believed it important to
provide strength in this direction to accommodate the possibility.
Rather than try to revise the analysis to consider this wall (which was
essentially post and beam), I calculated the loads to each of the columns
and designed cantilevered restraints equivalent to 46% of the dead load to
the worst case column so that each would be equally stiff. The 46% of dead
load is what the lateral load would be using an R of 2.2 for cantilevered
columns.
Will this affect my rigid analysis - most likely yes, but I don't believe
that we should be looking at the rigid diaphragm design as a row of domino's
waiting to fall if one stiffer member fails. I consider it as a means to
account for wall stiffness so as to account for soft story conditions such
as this.

While the debate continues over Rigid and Flexible design, I seriously
suggest that you interpret your results with a grain of caution. In most
cases, the results of my spreadsheet (which, by the way David Merrick
contributed considerably in the rigid design by creating the ability for it
to design skewed walls - credit deserves to be given to David for his
work)are going to be conservative - especially if you follow the envelope
solution and do not try to differentiate between wind and seismic load
controls. It is the only way to design that won't require a tremendous
amount of time "tweaking" out something that you have not control over
during construction.

The final point is to consider any wall that you believe to be weak in the
event the structure is loaded parallel to the wall. This is especially true
of any exterior wall which, by nature of construction, is positively
connected to the structural diaphragm.

One final note of clarification. I am pleased to take credit for most of the
UBC 97 Spreadsheet, however, I believe it is important to note that people
like James Lord, who wrote the original spreadsheet for Rigid Diaphragm
design and the great work by
David Merrick who refined the Rigid analysis and added the ability to
consider skewed walls is what makes this spreadsheet as great a tool as it
is. The Rigid Analysis is exceptionally strong in the program and I have
taken great pains to check the accuracy as have a few others. I think that
most will find it an extremely useful tool and have people like James and
David to thank for their willingness to create and donate their work to the
profession.

Regards,
Dennis S. Wish, PE
SEConsultant(--nospam--at)Earthlink.net <mailto:SEConsultant(--nospam--at)Earthlink.net>
(208) 361-5447 Efax


> -----Original Message-----
> From: merrick group [mailto:merrickgroup(--nospam--at)compuserve.com]
> Sent: Thursday, January 06, 2000 10:14 AM
> To: Seaint
> Subject: RE: Skewed (45 degree) shearwalls
>
>
> RE: Skewed (45 degree) shearwalls
>
> No straps around corners.
> It may not hurt and the cost is low so go ahead.
> The top plates must overlap.  Or use metal plate on top of topplate.
> Detail a wedged cross section for one or both of the facing or
> meeting corner studs.
>
> If the corner has the weight of a stone fire place, I would rather
> rely on additional straps that would attach directly to the body of
> the fire place. It is conceivable that there may be a short top plate
> of no use between the corner and the edge of fire place. I would
> consider straps on diagonal blocking directing the pulling-out
> tension directly into the body of the diaphragm.
>
> The diaphragm shear may need more nailing to match a shear in
> the diagonal wall. A triangular section of diaphragm at the
> diagonal wall with interior edges perpendicular to framing could
> be considered. The triangle body having heavier diaphragm nailing
> and having the interior edges of the triangle collecting the
> diaphragm load or capacity further inside the building.
>
> If corners are not shear walls, then the triangular element is not
> needed.
>
> If a rigid diaphragm is considered.
>
> If the corners are of shear walls. The principle axis of the whole
> diaphragm may be skewed. The maximum shears will be with a
> force skewed to the direction of framing.  The principle axis is not
> rotated, when the skewed shear walls have equal and opposite
> skewed shear walls, balancing the affects of rotating the principle
> axis. Get the Dennis Wish Program.
>
> For flexible diaphragms.
>
> If the diagonal shear wall is larger and stiffer than the walls
> perpendicular to the shear wall line being considered, then as the
> load is added the skewed wall will have an additional tilt. This
> makes the diagonal wall very flexible and not rigid enough to
> collect its tributary shear due to its other wise assumed relative
> stiffness. I am not talking about the already reduced stiffness based
> only on the 45 degree component.
>
> With out correctly knowing the true stiffness it is best to design the
> diagonal wall with out considering other inline walls, and design
> the other inline walls with out sharing the load with the diagonal
> wall.
>
> David Merrick, SE
> MerrickGroup(--nospam--at)compuserve.com
>
>