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

• To: <seaint(--nospam--at)seaint.org>
• Subject: RE: QUESTION ON USING ACI'S MOMENT MAGNIFIERS FOR COMP. MEMBER DE SIGN
• From: "Rogers, Robert" <rogersr(--nospam--at)amkinney.com>
• Date: Fri, 5 Oct 2001 08:36:28 -0400

Scott,

Thanks for the feedback.  Your comments did start me thinking again. A
P-Delta can be done by I want to know what's going on here in the code.

If doing a first order or second order frame analysis using factored
loads the code recommends using the moments of inertia in 10.11.1
(unless a more exacting analysis is performed).  The frame analysis
would thus use load factors 1.4 for DL and 1.7 for LL.  When you go to
compute the Stability Index for the Story, it tells you to if you used
service loads for the frame analysis you can still calculate the
Stability Index by mulitplying the summation of the column axial service
loads by 1.2, using the service load story shear, and mulitplying the
relative deflection by 1.43.  If one conservatively assumes that all
factor), and that the story shear was due to wind or earthquake thus

1.7 x 0.75  = 1.275
1.7 x 1.1 x 0.75 = 1.40
1.3 x 1.1 = 1.43

and using the worst-case value from above in the denominator yields:

1.7
-------------- = 1.19  ~ approx 1.20
1.43

thus, I believe this explains the mulitiplication of the column loads by
1.2 and leaving the story shear at the service load level (i.e.,

However, I still am unclear about the multiplication of the relative
deflection by 1.43.  In the commentary to 10.11.1 it tells me to take
the reduced moment of inertias given in 10.11.1 and mulitply them by
1.43 to do a service load frame analysis (thus making the structure
stiffer).

Then, in the commenentary over in 10.11.4.2, it tells me to take the
deflection generated by a service load frame analysis (utilizing the
reduced moments of inertia termed "service load moments of inertia" in
10.11.1) and multiply the deflection by 1.43.  This I don't
understand....... if I used service loads and moments of inertia meant
for a "factored" analysis I have a mismatch.  I used lower loads than I
should have but I also used lower moments of inertia than I should have
(as evidenced by them telling me to mulitply the moments of inertia in
10.11.1 by 1.43 if doing a service load analysis).

When I go over to the stability index calculation, if I increase the
service loads to factored loads (by using the 1.2 multiplier in the
stability index equation) I have accounted somewhat for this mismatch.
However, by rights, I should have used stiffer moments of inertia as
well.  Whether this increase in loading, but also increase in moment of
inertia, leads to greater deflection (particularly 1.43) seems somewhat
ambigious to me in origon.  Where does the 1.43 x relative story
deflection come from ?

Perhaps I'm just missing the boat somewhere.... possible......I would
like someone from an ACI code committee to respond......

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: Thursday, October 04, 2001 1:36 PM
To: 'seaint(--nospam--at)seaint.org'
Subject: RE: QUESTION ON USING ACI'S MOMENT MAGNIFIERS FOR COMP. MEMBER
DE SIGN

One other thing on your question.

It says Q can be [(Pservice*1.2)*(Dservice*1.43)]/[Vservice*H].  1.7
gravity
factor /1.4 seismic factor =1.2 and .7Ig/.7=Ig so Dservice is a function
of
unfactored loads and an increased-reduced moment of inertia.  Where I
don't
see parity is Do is based on factored loads and is divided by a reduced
moment of inertia. So Do is not exactly the 1.43*Dservice, but there is
some
spare change if the gravity load factor is less than 1.7.

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

-----Original Message-----
From: Haan, Scott M. [mailto:HaanSM(--nospam--at)ci.anchorage.ak.us]
Sent: Thursday, October 04, 2001 9:09 AM
To: 'seaint(--nospam--at)seaint.org'
Subject: RE: QUESTION ON USING ACI'S MOMENT MAGNIFIERS FOR COMP. MEMBER
DE SIGN

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Your following message has been delivered to the list
seaint(--nospam--at)seaint.org at 10:16:25 on 4 Oct 2001.
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Robert:

I would take the easy way and use factored load combinations and reduced
moments of inertia for the strength check.  Do a P-Delta analysis and
you do
not have do worry about the moment magnification due to drift.  If you
have
to calculate the bulding period to figure seismic force use the 1.43
increased reduced moments of inertia [since the less flexible the
building,
the shorter the fundamental period, the higher the seismic force].  The
drift checks in the IBC and 1997 UBC are based on strength level loads
and
would not require additional messing around if you use the factored

You are right. It appears there is not exactly parity. I think the point
was
to give people using working stress loads some tools.

Respectfully,

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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