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Re: seaint Digest for 12 Nov 2002

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Why would you speak of a finite element analysis? I was just using SFrame on
a 4-story building to handle the beam loads with non-continuous shear walls.
I has occurred to me, to just model the whole thing, with shear walls, in
SFrame. I am finding that it is not that time consuming.

We are using old design method shortcuts that are too tedious with the new
code demands. I see two approaches. Temporarily, it would be nice to show
that the actual base shears are lower if one applies a high period to the
equations. Long term would be to change the code back to something easier to
handle in designs. We have come a long way with codes to reflect new
discoveries. Tools to counter the new wood demands in today's code are to
find dynamic periods and ductility, and to request a reduced seismic
return-period expectation. We could end up with something simple again.

Wood construction is rigid due to the non-structural finishes. Shearwalls
rarely receive a design load until the finishes are broken free. Non-shear
partitions, may be stronger than and fail the supporting joists prior to
developing the shear in an adjacent shear wall. I doubt that any conclusion
can be made about shear wall stiffness by a spinning washer or by jumping up
and down in protest.

So... break up the finishes and deflect the shearwalls. For eight foot tall
walls the minimum deflection is a little less than 0.5 inches. This
increases if uplift occurs. That deflection results in a high period.

Here is a single-mass and spring model. There is a way to greatly simplify
finding a worthy period that is within the bounds of possibilities. This
method needs no Sframe program. The resulting period is less, but still
significant. Consider the adjacent story (above or below) is considered to
be infinitely rigid and consider not allowing uplift at all levels. Assume a
constant for the design level deflection. Remember, with finishes unbroken,
it is unlikely that the shear walls at that level have yet to receive design
loads and are not deflected.

The single-mass and spring model lets code equations be reduced to a single
base-shear weight factor. This is a factor, independent of weight and
geometry. This exercise would only need to be done once for most wood
designs. That is a single factor that could always apply for each case of
minimum defection, and each case of seismic zoning. I have found some soil
conditions needing a higher deflection to conclude a lowered base shear.

David Merrick, SE
> 16                               Message:0016                           16
> --------------------------------------------------------------------------
> From: "Haan Scott M DPW CIVIL ENGR(n)" <scott.haan(--nospam--at)>
> To: seaint(--nospam--at)
> Subject: RE: Lateral Analysis Software for Wood Structures?
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> Why doesn't the code just ... I think the codes should
> take a step back and specifically allow the old wood building
> conventional assumptions of stiffness proportional to length, flexible
> diaphragms, and drift checks not required for wood buildings.  Handle
> with the "simplified method" and 2:1 aspect ratios.
> I think that the market is unwilling ... I think...The extra design time
is probably a waste
> ...>
> Also high period for wood buildings?  Does your house resonate violently
> when your dryer has a wet comforter in it or when you walk down the hall?
> Although it is contrary to the popular world view, most residential wood
> buildings have  very short periods because they are light and stiff.   The
> periods are probably so short that they would land in the ramp area of a
> response spectrum and have less input, but the code does not allow you to
> that.  Maybe the code could allow the response spectrum "ramp" to be used
> for wood buildings.
> Scott Haan.

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