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RE: Story Drift: 1994 UBC vs. 1997 UBC

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Good question, Bruce.

If you believe the code (Eqn. 30-17) and it is O.K. with you to let the
structure deflect more than twice that in the 1994 UBC, here is the
comparison I have for a project I am currently working on:

Strength requirements: Bending moments based on 1997 UBC is about 20% higher
than 1994 UBC.
Deflection requirements: Required moment of inertia is roughly half that
required by 1994 UBC.
If your experience is anything like mine, deflection controlled in the past
so you would see cantilevered column sections getting smaller, but possibly
not much lighter.

In my particular case, I have:

Trib weight = 21,572 lbs.
h=21.63 ft. (yes.)

1994 UBC:

V=0.40*1.0*2.75/3 *W = 0.367 * W = 7,916 lbs.
M=7,916*21.63 = 171.223 ft.-k.
Lu=21.63 ft.
1/3 increase in allowable stresses
Ds<0.005*h = 0.005 *21.63 *12 = 1.32"

I required = 7,916 * (21.63)^3*1728/(3*29E6*1.32)=1205 in^4.
USE W21x68

1997 UBC:
V=2.5*0.44*1.3/(1.4*2.2) * W = 0.464 * W = 10,010 lbs.
M = 10,010 * 21.63 = 216.5 ft.-k.
Lu=21.63 ft.
1/3 increase in allowable stresses????

Ds<0.025 *h /(0.7*R*2.2*1.4)=3.06" (= h/86 !!)

I required = 10,016 * (21.63)^3*1728/(3*29E6*3.06)=655 in^4
USE W16x67

If the same exercise were run with h=8 ft.
1994 UBC: W12x40
1997 UBC: W10x35

Time to throw darts.

Bill Allen, S.E.
ALLEN DESIGNS
Laguna Niguel, CA


> -----Original Message-----
> From: Parkerres(--nospam--at)aol.com [mailto:Parkerres(--nospam--at)aol.com]
> Sent: Thursday, August 12, 1999 2:31 PM
> To: seaint(--nospam--at)seaint.org
> Subject: Re: Story Drift: 1994 UBC vs. 1997 UBC
>
>
> Bottom Line Question:
>
> For the same building in the same seismic zone with the same
> cantilevered
> columns at the front of the building, do I get larger columns
> using the 1994
> UBC or the 1997 UBC?  If they are smaller, is this good?  If
> they are larger,
> how much larger?  If they are the same, why are we talking
> about this?
>
> Bruce Resnick, SE
> Parker Resnick Str. Eng.
>
>
>