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Re: Timber Frame on Concrete Boxes

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Thor:

First, please forgive my memory...it is a little bit fuzzy since it has
been a LONG time since I looked at the SEAOC Bluebook.  However...

The SEAOC Bluebook did (and maybe does others might offer some feedback
on that) have an appendix that addressed how to deal with soemthing like a
"setback" at a particular height of a building.  This had the same net
effect as your change in stiffness.  I don't recall if the appendix
directly dealt with change of stiffnees in the vertical lateral system or
not or if it was just a indirect effect.  Anyway the appendix gives some
"guidance" on how to handle such situations when using the code
"approaches" (i.e. you are not doing a full 3D dynamic analysis etc).  I
had used it on a building that was an existing structure with concrete
shearwalls where we were adding two parallel sets of floor additions with
an attrium between them.  In effect, this was a concrete shearwall
building (about 3 stories if memory serves me correctly) with two new 2-3
story steel framed buidlings with steel concentric braces sitting on top
of the concrete building.  We needed to figure out lateral load
distribution both vertically and to the braced bays/shearwalls.  I was
able to make use of the SEAOC Bluebooks' appendix for this.  I will note
that this was for a relatively low seismic area (Michigan), but it got
"bumped" up some since it was higher importance building, so it ended up a
Seismic Permance Catergory C building (under old NEHRP/BOCA nomenclature).

The above has the potential to handle questions a & b.  For c, I would say
that it is largely a function of relative stiffness.  If you are attaching
a timber frame (assume for the moment that the timber frame is going to
take the lateral load by way of knee braces or some other method...note,
however, that we generally don't rely on timber frames for lateral
resistence, especially for seismic...wood moment connections are "iffy" as
best leaving only knee braces or some other type of brace as a "viable"
option...in my opinion at least) to the SIDE of similar height concrete
structure, then I would have to say that in reality the concrete structure
is going to "suck" all the load to it.  If you use wood (or some other
material for that matter) shearwalls (either SIP or stick-framed or
proprietary) for the lateral resistance of the timber frame, then it has a
"sporting chance" to keep some decent part of the lateral load from being
sucked into the concrete portion, but it would largely be a function of
relative stiffness.  The other thing out there is if there is a code
requirement for some minimum load (percentage) to be taken to one part of
the combined system (an example is that moment frames in combination with
stiffer systems such as shearwalls must typically still be designed for
25% of the lateral load even if relative stiffness says that all or most
of the load would be going to the shearwall system).

As to question d, I am not aware of anything specific to you situation
(i.e. timber frame on a concrete box), but it certainly possible.  I would
say that there certainly is information on the general issue (i.e.
different stiffness lateral systems over the height of a building).

HTH,

Scott
Adrian, MI


On Wed, 22 Feb 2006, Avicpeng wrote:

> Subject: Timber Frame on Concrete Boxes
>
>
> At a 2005 NBCC seminar over the weekend, the issue of how to deal with timber-framed structures on elevated concrete supports (walls, columns, whatever) raised its head.  Listening to the responses from the floor and the podium, it seemed to me that the issue was perhaps not clear (enough) cut.  Discussing this with local peers, we concurred that the concrete structure could/should (?) be regarded as very stiff compared with the timber structure, so that the timber frame thinks it's on the ground.
>
> Fearing an, "oh s**t!" moment ...
>
> a)    What do you do?
>
> b)    Do you calculate the (relative) stiffnesses?
>
> c)    How do you distribute the base shears up a timber structure horizontally tied (i.e. attached to the side of said box) to such an elevated concrete structure?  Are they the same as an isolated structure?
>
> d)    Has anyone done any usable investigations of this/ese issues?
>
> Thor Tandy P.Eng MIStructE
> Victoria, BC
> Canada
> vicpeng(--nospam--at)telus.net

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