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RE: Wood Moment Frame in historical building - retrofit

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Is a knee-brace really a moment frame?  

1997 UBC 1627 defines a moment resisting frame as "...a frame in which
members and joints are capable of resisting forces primarily through
flexure."  A knee braced frame behaves like a moment frame and deflects
through flexure of the members.  

The difference is that knee braced frames have braces even though the braces
are offset from the beam-column joints. The brace axial load induces moments
in the beams and columns but the brace connections do not resist forces
through flexure. 

I would define knee-braced heavy timber frames as ordinary braced frame
bearing wall systems by the definition in the UBC Chapter 16. I would also
say knee-braced frames are eccentrically braced frames with braces are
offset from the column bottoms. They may be a bad idea though because they
could cause hinges at column brace connections that would form collapse
mechanisms, but there is not a prohibition in UBC against connecting wood
braces to wood column centers for heavy timber ordinary braced frames.   

However, UBC Chapter 22 does not allow steel braces to be connected to the
middle of steel columns [K-braced frames] in higher seismic zones

Scott M Haan P.E.
Plan Review Engineer
Building Safety Division 
Development Services Department
Municipality of Anchorage
http://www.muni.org/building
phone:907-343-8183  
fax:907-249-7399
mailto:haansm(--nospam--at)ci.anchorage.ak.us



-----Original Message-----
From: jian zhou [mailto:jiz290(--nospam--at)hotmail.com]
Sent: Thursday, October 04, 2001 7:56 PM
To: seaint(--nospam--at)seaint.org
Subject: Wood Moment Frame in historical building - retrofit


We are currently working on a project for retrofiting a historical building
The buliding is wood-framed structure with bow string truss (120' span).
The building is 120'x200'x50' high with knee bracings at both ends.
The trusses are supported by large timber section columns (3-6x12),
There is no shear wall existed along the truss direction and wood bracing
is provided as lateral support in the perpendicular direction.

Based on UBC, wood moment frame system is not allowed
(even though the column is very heavy and the building looks very good along
truss direction?). Could somebody having similar experience give some input 
on how to retrofit moment-resisting connection instead of adding shearwall 
or using other options, Right now it is not possible to add shear wall or 
bracing along the truss direction.

Thanks very much.

JZ




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