4.5.
Overstrength.
Paragraph 4-4
A system overstrength factor, So, provided in Table
QE = the effect of horizontal seismic forces
7-1 is intended to quantify the actual force that can be
delivered to sensitive individual brittle elements, the
failure of which could result in the loss of a complete
sec
lateral-force-resisting system or in instability or
collapse. The factor is similar in application to the
D = the effect of dead load.
3Rw/8 factor prescribed in the UBC, and represents an
estimate of the combined design, material, and
b.
Special Combination of Loads.
When
system overstrengths that could effect a brittle or
specifically required by FEMA 302, or when in the
force-controlled element.
judgement of the designer the effects of structural
overstrength need to be considered, the design
seismic
force
on
brittle
or
force-controlled
4-6.
Combination of Load Effects.
components shall be defined by the following
equations:
a.
combinations from ASCE 7 are:
E = S oQE + 0.2SDS D
(4-6)
1.2D + 1.0E + 0.5L + 0.2S
(4-2)
E = S oQE - 0.2SDS D
(4-7)
or 0.9D + 1.0E
(4-3)
Where E, QE, SDS and D are as defined in Paragraph a
above, and S o is the system overstrength factor
where: D,E,L, and S, are respectively, dead,
defined in Paragraph 4-5 and listed in Table 7-1.
earthquake, live, and snow loads.
Example applications for Equations 4-6 and 4-7
include the design of columns under discontinuous
The effect of the earthquake load, E, is defined by:
shear walls or braced frames, and the design of frame
members in braced frames effected by overstrength in
E = DQE + 0.2SDSD
(4-4)
the bracing.
or E = DQE - 0.2SDSD
(4-5)
4-7.
Performance Levels.
Where:
Three structural performance levels, as described in
E = the effect of horizontal and vertical
Table 4-3, are considered to be acceptable by this
earthquake-induced forces.
document. Performance Level 1 (Life Safety) is the
minimum performance level required of all Seismic
4-7
Previous Page
