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

RE: Collectors & Drag Struts

[Subject Prev][Subject Next][Thread Prev][Thread Next]
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
> >

******* ****** ******* ******** ******* ******* ******* *** 
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp 
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) to the list, send email to 
*   admin(--nospam--at)seaint.org and in the body of the message type 
*   "join seaint" (no quotes). To Unsubscribe, send email 
*   to admin(--nospam--at)seaint.org and in the body of the message 
*   type "leave seaint" (no quotes). For questions, send 
*   email to seaint-ad(--nospam--at)seaint.org. Remember, any email you 
*   send to the list is public domain and may be re-posted 
*   without your permission. Make sure you visit our web 
*   site at: http://www.seaint.org 
******* ****** ****** ****** ******* ****** ****** ********