drasticxxxx
Civil/Environmental
- Aug 4, 2015
- 74
As Written in ETABS manual
3.4.2.2 Determine Moment Magnification Factors
The moment magnification factors are calculated separately for sway (overall
stability effect), δs, and for non-sway (individual column stability effect), δns.
Also, the moment magnification factors in the major and minor directions are, in
general, different (ACI R10.10.5, R10.10.5).
The moment obtained from analysis is separated into two components: the sway
Ms and the non-sway Mns components. The sway components are identified by
the “s” subscript. The sway moments are primarily caused by lateral loads and
are related to the cause of sidesway. The non-sway components, which are
identified by “ns” subscripts, are primarily caused by gravity load.
For individual columns or column-members, the magnified moments about two
axes at any station of a column can be obtained as
M = Mns +δs Ms (ACI 10.10.7, Eqn. 10-18, 10-19)
The factor δs is the moment magnification factor for moments causing sidesway.
The program takes this factor to be 1 because the component moments Ms and
Mns are assumed to be obtained from a second order elastic ( P-Δ ) analysis (ACI
R10.10, 10.10.4, R10.10.4). For more information about P-Δ analysis, refer to
Appendix A.
For the P-Δ analysis, the analysis combination should correspond to a load of1.2
(dead load) + 1.6 (live load) (ACI 9.2.1). See also White and Hajjar (1991). The
user should use reduction factors for the moments of inertia in the program as
specified in ACI 10.10.4.1. The moment of inertia reduction for sustained lateral
load involves a factor βds (ACI 10.10.4.2). This βds for sway frames in second-
order analysis is different from the one that is defined later for nonsway
moment magnification (ACI 2.1, R10.10.4.2, 10.10.6.2). The default moment of
inertia factor in this program is 1.
The computed moments are further amplified for individual column stability
effect (ACI 10.10.6, 10.10) by the nonsway moment magnification factor, δns, as
follows:
Mc = δnsM (ACI Eqn. 10-11)
Mc is the factored moment to be used in design.
So why in earth he consider Mns twice in the calculation, since in ACI-2011 he consider the nonsway or sway calculations,but here after considering the sway calculation he reconsider the non sway calculation again(doesn't make scene?
Thank you in advance
3.4.2.2 Determine Moment Magnification Factors
The moment magnification factors are calculated separately for sway (overall
stability effect), δs, and for non-sway (individual column stability effect), δns.
Also, the moment magnification factors in the major and minor directions are, in
general, different (ACI R10.10.5, R10.10.5).
The moment obtained from analysis is separated into two components: the sway
Ms and the non-sway Mns components. The sway components are identified by
the “s” subscript. The sway moments are primarily caused by lateral loads and
are related to the cause of sidesway. The non-sway components, which are
identified by “ns” subscripts, are primarily caused by gravity load.
For individual columns or column-members, the magnified moments about two
axes at any station of a column can be obtained as
M = Mns +δs Ms (ACI 10.10.7, Eqn. 10-18, 10-19)
The factor δs is the moment magnification factor for moments causing sidesway.
The program takes this factor to be 1 because the component moments Ms and
Mns are assumed to be obtained from a second order elastic ( P-Δ ) analysis (ACI
R10.10, 10.10.4, R10.10.4). For more information about P-Δ analysis, refer to
Appendix A.
For the P-Δ analysis, the analysis combination should correspond to a load of1.2
(dead load) + 1.6 (live load) (ACI 9.2.1). See also White and Hajjar (1991). The
user should use reduction factors for the moments of inertia in the program as
specified in ACI 10.10.4.1. The moment of inertia reduction for sustained lateral
load involves a factor βds (ACI 10.10.4.2). This βds for sway frames in second-
order analysis is different from the one that is defined later for nonsway
moment magnification (ACI 2.1, R10.10.4.2, 10.10.6.2). The default moment of
inertia factor in this program is 1.
The computed moments are further amplified for individual column stability
effect (ACI 10.10.6, 10.10) by the nonsway moment magnification factor, δns, as
follows:
Mc = δnsM (ACI Eqn. 10-11)
Mc is the factored moment to be used in design.
So why in earth he consider Mns twice in the calculation, since in ACI-2011 he consider the nonsway or sway calculations,but here after considering the sway calculation he reconsider the non sway calculation again(doesn't make scene?
Thank you in advance
