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Re: Wood Shear Walls at Skewed lines[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Re: Wood Shear Walls at Skewed lines
- From: David Merrick <MRKGP(--nospam--at)winfirst.com>
- Date: Wed, 17 May 2006 10:30:20 -0700
I suspect this is coming from the curved wall discussion. A
curved wall is bunch of little skewed walls.
Dr. Munoz said: “The IBC and the IBC Codes do not address the skewed braced or shear walls.”
Here it is in the codes
UBC 1633.1 last paragraph “…orthogonal affects…plus 30%...perpendicular direction…”
Check the statics, the worst-case orthogonal-wall example cannot add more than the UBC prescribed 30% in the perpendicular to the 100% load direction.
IBC 1620.1.10 “Direction of seismic load. …most critical load affect in each component. …separately independently…two orthogonal directions.”
By analysis, the 30% rule still applies for the IBC. Analysis numerical results are not intended to be part of the code.
The skewed wall is a basic static problem. To restrain a skewed wall, walls perpendicular to the applied force will have a force but their sum total will not exceed 30% of the applied lateral load.
It is a grave mistake that the code wording for skewed walls only applies to seismic. It is irrational to ignore basic statics on skewed walls when the force is wind. Both codes demand a rational analysis and therefore skewed walls are required to be considered.
The code should not over prescribe numerical results of analysis. A skewed wall is rare to most engineers who perform simple lateral analysis. Skewed walls should be included in the “General Design Requirements” not just for seismic!! The IBC really should add the 30% rule for the confused one-time use of a skewed wall.
Dr. Munoz said: “there are no prescriptive provisions for continuity of loads at corners”
Outside corners should not be designed for horizontal loads unless the diaphragm has a skewed angle or there is a skewed wall within the system. Vary interior joist axial forces to balance the rotational element. See previous discussions on this web site for the gabled roof wall and other skewed shear wall subjects. Isolate the triangle diaphragm that is formed by the skewed corner making the other two sides square with the balance of walls. Extend collectors into the diaphragm from the interior intersection of the two perpendicular lines of the triangle.
To balance the statics of a triangular plywood panel edge a
uniformly varying tension/compression is needed across the line’s face.
That must be transferred through the joists. If the joists are parallel
to that line then all of the rotation will be taken out by the
perpendicular edge where joists cross over the line. This way there is
no corner tension/compression. The loads are so small that there seems
to be no special detail required. It is a good academic exercise to
determine when it could be significant.
I have had a conversation with an engineer who has always just
split the skewed wall into its two stiffness components and did not
apply the 30% rule for wind nor seismic. I believe the practice is
common and needs to be changed.
David Merrick, SE
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