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RE: Dynamic drift calculations (1998 CBC/1997 UBC)

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Drift calculations are not all at "Strength" level.  Errata to section
1630.9.1 says to use load combinations of section 1612.3 where Allowable
Stress Design is used (the 1.4 factor is taken out).  See the following

Chris Roper

-----Original Message-----
From: Jake Watson [mailto:jwatson(--nospam--at)]
Sent: Friday, December 15, 2000 7:53 AM
To: seaint(--nospam--at)
Subject: Re: Dynamic drift calculations (1998 CBC/1997 UBC)

1. All drift calculations are at "Strength" level.  You must leave the
1.4 factor in.

2. My response is an opinion here - no code section to back it up.  I
would reduce the load to the appropriate level (80%,90%,100% etc.)
before deflection.  My interpretation is that we design for "design
level" forces.  So calculate Delta S with design level forces, then
magnify by 0.7R to get delta M.  Then divide by 1.4 to get to WSD.  If
you are doing dynamic design note two other things: first, you must
design for forces in both the X and Y axis simultaneously.  You can use
the SRSS method or 100% in one direction and (I think?) 40% in the
other.  Secondly,  if you are using concrete shearwalls you are only
allowed to assume 50% of Ig for stiffness (see concrete shearwall
section) when calculating deflection.  This can have a huge impact on
secondary moments.

3. Opinion again here - I would say include and move on.  It's not
likely to make a difference on most typical buildings for deflection and
no one will question you.  If it makes a difference it might be worth a
call to one of the code Gods at ICBO to find out.

Hope this helps.

Jake Watson, E.I.T.
Salt Lake City, UT

"Harper, Mark" wrote:
> Questions: (Reference 1998 CBC VOL 2, based on the 1997 UBC)
>         1.      Section 1630.9.1 refers to section 1612.2 for load
> combinations to be used with Allowable Stress Design.  Does this mean that
> take the dynamic base shear and scale it as required to the base shear
> obtained in section 1630.2 without dividing by 1.4 for the drift
> computation?
>         2.      When calculating the drift using a dynamic analysis, and
> working stress design do I scale down the load to (80, 90, or 100%
> on building type) of the static base shear (section 1630.2) to obtain
> S  then multiply that value by .7R to obtain delta M.
>         That is Delta M = Delta S*.7*R (reference 1630.9.2)
>                 Then when calculating the forces in the members scale down
> the dynamic forces by an additional 1.4 to obtain working stress forces.
>         3.      When calculating "E" (section 1630) ROW is equal to 1.0
> drift calculations and Ev =0 if designing for working stress.  However,
> Ev have to be taken into account for drift design?
>                 Example; Dynamic base shear gets scaled down to E = Eh or
> gets scaled to E = Eh+Ev, where Eh = the base shear obtained in section
> 1630.2?  Because the drift calculation "Delta S" appears to use ultimate
> design loads I would think that Ev must be included in the design base
> shear. However section 1630.9.1 refers only to section 1630.2.1 which
> make me think that the vertical component Ev is not to be included for
> calculations!
> Thanks for your help!
> Mark Harper PE, SE
> mharper(--nospam--at)
> Phone: (626) 564-2820
> Fax: (626) 564-2873

Jake Watson, E.I.T.
Salt Lake City, UT