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RE: QUESTION ON USING ACI'S MOMENT MAGNIFIERS FOR COMP. MEMBER DE SIGN

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Scott,

The moment of inertia is really based on the degree of cracking
present..... it appears that the increase in moment of inertia assumes
less cracking at service load levels (versus factored load levels) which
is a logical progression of thought.......however, this may not be
necessarily true if load levels are low enough or if service load levels
are high enough..... the commentary of 10.11.1 cites a 1977 reference
for justification of the moments of inertia presented in 10.11.1.....

And yes, I agree that in principle, multiplying the service load
deflection by 1.43 is the same as multiplying the moments of inertia by
1.43 in principle.....thus everything in the equation for the stability
index for a story goes from being service load level oriented to
factored load level oriented...... which makes perfect sense to me
now........ however, the 10.11.4 commentary would then be incorrect when
it states:

*	"If the lateral load deflections of the frame have been computed
using service loads and the service load moments of inertia given in
10.11.1, it is......"

Because the moment of inertia given in 10.11.1 are "factored moments of
inertia".  Only after they are multiplied by the 1.43 are they service
load moments of inertia.

It should say:

*	"If the lateral load deflecitons of the frame have been computed
using service loads and the factored load moments of inertia given in
10.11.1, which have been appropriately altered by the 1.43 factor as
discussed in the 10.11.1 commentary for service load analysis, it
is......"

I think I'll submit it for consideration in the code change process.
Would you agree ?

Robert C. Rogers, PE
Senior Structural Engineer
A.M. Kinney, Inc. 
Consulting Engineers / Architects
150 East Fourth Street, 6th Floor
Cincinnati, Ohio 45202
PH: 513-421-2265, ext. 125
FX: 513-345-1318
rogersr(--nospam--at)amkinney.com 
www.amkinney.com



-----Original Message-----
From: Haan, Scott M. [mailto:HaanSM(--nospam--at)ci.anchorage.ak.us]
Sent: Friday, October 05, 2001 10:49 AM
To: 'seaint(--nospam--at)seaint.org'
Subject: RE: QUESTION ON USING ACI'S MOMENT MAGNIFIERS FOR COMP. MEMBER
DE SIGN


The 10.11.1 commentary  says to increase the 10.11.1 moments of inertia
by
1.43=1/.7 for service load deflection [this assumes most of the
deflection
is from bending and rotating of the beams, columns and walls].  When you
multiply the service load deflection by 1.43=1/.7 it is the same as
using
10.11.1 moments of inertia. The difference in the 1.43*Dservice and Do
deflections then is that one uses factored loads and the other doesn't.

Scott M Haan P.E.
Plan Review Engineer
Building Safety Division 
Development Services Department
Municipality of Anchorage
http://www.muni.org/building
phone:907-343-8183  
fax:907-249-7399
mailto:haansm(--nospam--at)ci.anchorage.ak.us




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