Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

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

[Subject Prev][Subject Next][Thread Prev][Thread Next]
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


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


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 loads.

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



-----Original Message-----
From: Rogers, Robert [mailto:rogersr(--nospam--at)amkinney.com]
Sent: Thursday, October 04, 2001 4:07 AM
To: seaint(--nospam--at)seaint.org
Subject: QUESTION ON USING ACI'S MOMENT MAGNIFIERS FOR COMP. MEMBER
DESIGN


I am looking at ACI 318-95 and have found, what I believe, to be
sections of the code commentary related to frame/compression member
design that are contradicting each other.  In the commentary to Section
10.11.1, on page 116, it states:

*	"Sections 10.11 through 10.13 provide requirements for strength
and assume frame analyses will be carried out using factored loads"

and further down in the same paragraph it states:

*	"The moments of inertia of the structural members in the service
load analyses should, therefore, be representative of the degree of
cracking at the various service load levels investigated.  Unless a more
accurate estimate of the degree of cracking at design service load level
is available, it is satisfactory to use 1/0.70 = 1.43 times the moments
of inertia give in 10.11.1 for service load analysis."

This implies that when doing a frame analysis the values for the moments
of inertia provided in 10.11.1 should be multiplied by 1.43 when using
service level loads (i.e., unfactored loads).  However, when reading the
commentary to Section 10.11.4.2 (computing stability index) 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 permissible to compute Q in Eq. (10-7) using 1.2 times
the sum of the service gravity loads, the service load story shear, and
1.43 times the first order service load story deflections"

What I believe is contradictory is the portion of the above sentence
that says "service load moments of inertia given in 10.11.1" which seems
to imply that the moment of inertia given in 10.11.1 are for service
load frame analysis, but in the commentary prior to, it seems to
indicate that those moments of inertia needs multiplied by 1.43 for
service load frame analysis.  Am I confused here ?   Anybody agree or
disagree with my perceived contradiction ?  Can anybody shed some light
here ?

Thanks

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


******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
* 
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you 
*   send to the list is public domain and may be re-posted 
*   without your permission. Make sure you visit our web 
*   site at: http://www.seaint.org 
******* ****** ****** ****** ******* ****** ****** ******** 

******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
* 
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you 
*   send to the list is public domain and may be re-posted 
*   without your permission. Make sure you visit our web 
*   site at: http://www.seaint.org 
******* ****** ****** ****** ******* ****** ****** ******** 

******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
* 
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you 
*   send to the list is public domain and may be re-posted 
*   without your permission. Make sure you visit our web 
*   site at: http://www.seaint.org 
******* ****** ****** ****** ******* ****** ****** ********