Need a book? Engineering books recommendations...

# Re: Dynamic drift calculations (1998 CBC/1997 UBC)

• To: seaint(--nospam--at)seaint.org
• Subject: Re: Dynamic drift calculations (1998 CBC/1997 UBC)
• From: Jake Watson <jwatson(--nospam--at)inconnect.com>
• Date: Fri, 15 Dec 2000 08:52:37 -0700

```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 I
> 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% depending
> on building type) of the static base shear (section 1630.2) to obtain delta
> 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 for
> drift calculations and Ev =0 if designing for working stress.  However, does
> 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 would
> make me think that the vertical component Ev is not to be included for drift
> calculations!
>
>
> Mark Harper PE, SE
> HDR INC.
> mharper(--nospam--at)hdrinc.com
> Phone: (626) 564-2820
> Fax: (626) 564-2873
>

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

```