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RE: Wood Moment Frame?

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Dan,
In my opinion, you can not prevent rotation in wood unless you satisfy a
few conditions:
1. Embed the wood member into concrete
2. Embed the wood member into a steel sleeve and restrain rotation
through a welded connection and preventing the wood from resisting
rotation through a bolted or nailed connection. An example of this might
be to create a joint using tube steel that is welded together. Slide the
wood into the TS sleeves so that bending occurs around the post or wood
member and is not reliant on a bolted or nailed connection penetrating
the wood member.

The Vertical or Horizontal diaphragm deflection calculations take this
into account by calculating hold-down elongation, fastener bending. A
wood shear wall is required to have an aspect ratio of 2:1 in Seismic
zone 4 and I believe you can reduce this to 3.5:1 for zone 3. However,
the engineer's assumption that he has prevented rotation below the
extended header is incorrect as the panel will "Rock" which allows
rotation to occur. Rocking is controlled by the strap restraints as well
as the deflection in the panel caused by bending. The narrower the
aspect ratio the greater the deflection or drift due to bending.

In the case you describe, I would conclude that the engineer does not
have the aspect ratio the code requires (I am assuming 10-feet to the
bottom of the header because if this distance is 7-feet then with a
3.5:1 ratio, a 2'-0" panel would be appropriate). I would, however,
judge the height of the panel by the braced height which is between slab
or floor and bottom of header. I believe his argument as to where the
inflection point occurs is incorrect for wood shearwalls.

There are alternative braced panels such as the Simpson Strongwall, and
Simplified Structural System Hardy Frame (that I understand was recently
purchased by Mitek). These systems are tested for deflection and
stiffness based on their empirical values and can be used even though
they may not comply with the code related H/b ratio.

Please feel free to follow up on this discussion on the
Structuralist.net Professional Forum as it is of interest to subscribers
there as well.

By-the-way, I don't think that enough information is discussed on the
topics of shearwall rocking except in masonry (or Unreinforced Masonry).
I think that the topic is relevant to wood but contingent upon the
capacity of the holddowns and straps. 

Dennis S. Wish, PE
California Professional Engineer
Administrator - The Structuralist.Net
Website: 
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"It occurs once in a minute, twice in a moment and not at all in ten
thousand years?"


-----Original Message-----
From: Dan Goodrich [mailto:dang(--nospam--at)karren.com] 
Sent: Friday, November 30, 2001 9:38 AM
To: seaint(--nospam--at)seaint.org
Subject: Wood Moment Frame?

I'm looking at a fairly simple garage.  Wood trussed roof,
wood shear walls, etc.  The walls are 11 ft. high.  The 
front of the garage has a 16 ft. x 10 ft. door.  The panels
on either side of the garage are only 2 ft. wide.  Total 
seismic shear on this wall line is 1200 lbs.  Seismic zone 3.

The engineer of record shows the LVL header over the 
garage door extending to the end of the 2 ft. wide panels on
either side of the garage.  He has holdown straps from the
foundation to the holdown posts.  He also placed straps from
the holdown posts to the LVL header at the top.  Something
like a moment frame?  

He claims that since he is restraining the rotation of the shear
wall at the top, then the inflection point is at mid-height of the
panel.  Therefore, the panel height is only 5-1/2 ft., which 
doesn't violate the 3-1/2 to 1 ratio for shear walls required
by the UBC.

I don't agree with this, and told him so.  I think the panel height
should be from the bottom of the header to the top of the
foundation wall.

What do you think?  Any kind of documentation on something
like this?

TIA,
Dan Goodrich, P.E.


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