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Re: Collectors & Drag Struts

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     I haven't done any seismic design but have a question regarding the subject.  When you use a rebar as a collector as in over a large wall opening, are the seismic ties the lateral stabilizing elements when the top of the wall is in compression?  I can see the concern for double top plate in tension and splice connections but compression is also a factor there.  Does tension control?

Greg
-----Original Message-----
From: Swingle, Mark <Mark.Swingle(--nospam--at)dcp.state.ma.us>
To: 'seaint(--nospam--at)seaint.org' <seaint(--nospam--at)seaint.org>
Cc: 'mswingle(--nospam--at)earthlink.net' <mswingle(--nospam--at)earthlink.net>
Date: Tuesday, August 01, 2000 8:19 AM
Subject: RE: Collectors & Drag Struts


>Dear Mark Deardorff and Michael Bryson,
>
>If the building has a double top plate instead of a beam (such as where
>there is only blocking parallel to the shear wall but no rim joist), then
>the double top plate would be the collector.  If the building had BOTH a
>beam and double top plate in the same line, then either could be used.  The
>designer, in that case, can choose the force path for design.  S/he may
>choose to use the double top plate since the splice connections are usually
>designed already, but beam connections (for collector forces) may be extra.
>
>Fortunately, buildings braced mainly with light-frame shear walls (or
>language sim to that) are exempt from the omega-times-collector forces
>provision.  Have you ever heard of an engineered wood-framed building whose
>double top plates OR splice connection failed in an earthquake?  Have you
>ever heard of a CONVENTIONAL CONSTRUCTION wood-framed building whose double
>top plates or splice connection failed in an earthquake?
>
>I believe that the collectors AND their connections are included in the
>omega requirement (I don't have the UBC in front of me).  This would include
>connections FROM a collector TO a diaphragm AND connections FROM a collector
>TO a vertical LFRS.  Connections from the diaphragm DIRECTLY to the vertical
>LFRS need not be designed for omega-times-force.
>
>The intent, I believe, is that collectors and their connections should
>remain elastic under earthquake forces, and that yielding should occur ONLY
>in the vertical LFRS.  It is difficult to make most collector connections
>yield in a ductile manner.
>
>I am not sure what the code philosophy is regarding diaphragms.  I think the
>basic point is that failure of diaphragms subject to past earthquakes has
>been rare.  Requiring omega times the diaphragm force must have been seen as
>overkill.  Ductile detailing is not required, however, so my suspicion is
>that our typical estimation of diaphragm capacities is far too conservative
>(that is, we underestimate the capacity).
>
>Mark Swingle
>
>------------------------
>
>on 31 July 00 Mark Deardorff wrote:
>
>>Subject: RE: Collectors & Drag Struts
>
>>The parallel beams at either end of the frame are >collectors. Not the top
>>beam. The beams connecting to the ends of the plywood >shearwall are
>>collectors. Not the top plate.
>
>>> -----Original Message-----
>>> From: Michael Bryson [mailto:bryson(--nospam--at)civil.ubc.ca]
>>> Sent: Monday, July 31, 2000 9:28 AM
>>> To: seaint(--nospam--at)seaint.org
>>> Subject: Re: Collectors & Drag Struts
>>
>>
>>
>>> Thanks for your reply. So you would NOT consider
>>> the top
>>> plate of a shear
>>> wall to be a collector? Nor the top beam in a
>>> frame? It seems
>>> to me there
>>> is no physical difference. I am trying to apply the >> Omega
>>> factor as per
>>> UBC97 1633.2.6 and the definitions are not clear to >> me. Also,
>>> should the
>>> connection of the diaphragm to the collectors be
>>> designed for
>>> the Omega
>>> factor?
>>
>> On Mon, 31 Jul 2000, Nels Roselund wrote:
>>
>> > Michael,
>> >
>> > I don't believe I've seen 'drag strut' used in a formal
>> discussion or in a
>> > Building Code.  'Collector' is the more formal term.  Like
>> Mark, I use the
>> > terms 'collector' and 'drag strut' interchangeably.
>> >
>> >
>> > They are used to collect shear forces that are distributed
>> to a diaphragm
>> > edge into tension or compression forces that they deliver
>> to shear walls or
>> > other bracing elements.  An edge of a rectangular diaphragm that is
>> > uniformly braced along its edge does not need a drag strut;
>> if the bracing
>> > elements are concentrated at discrete locations, such at a
>> shear wall at one
>> > end, or at a short length in the middle of a long edge, a
>> drag strut is
>> > needed.
>> >
>> > Nels Roselund
>> > Structural Engineer
>> >
>
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