Building Design and Construction Handbook - Merritt - Ventech!

5.180 SECTION FIVE
When the stability coefficient � is greater than 0.10 but less than or equal to
� max,
interstory drifts and element forces shall be computed including P-delta ef-
fects. To obtain the story drift for including the P-delta effect, the design story drift
shall be multiplied by 1.0/(1 � �). Where � is greater than �max the structure is
potentially unstable and shall be redesigned.
Seismic Load Effect. Where the effects of gravity and seismic loads are additive,
seismic load E shall be defined as:
E � �Q � 0.2S D (5.327)
E DS
and where the effects of gravity counteract the seismic load, seismic load E shall
be defined as
E � �Q � 0.2S D (5.328)
E DS
where E � the combined effect of horizontal and vertical earthquake-induced
forces,
� � a reliability factor based on system redundancy,
Q E � the effect of horizontal seismic forces,
S DS � the design spectral response acceleration at short periods,
D � the effect of dead load
Where seismic forces and dead loads are additive,
Where seismic forces and dead loads counteract,
E � � Q � 0.2S D (5.329)
m o E DS
E � � Q � 0.2S D (5.330)
m o E DS
Where E, Q E, S DS and D are as defined above and � o is the system overstrength
factor as given in Table 5.9. The terms � oQ E need not exceed the maximum force
that can be transferred to the element by the other elements of the lateral forceresisting
system.
Where allowable stress design methodologies are used with the special load
combinations with E m design strengths are permitted to be determined using an
allowable stress increase of 1.7 and a resistance factor �, of 1.0.
Redundancy. A redundancy coefficient, �, shall be assigned to all structures in
accordance with this section, based on the extent of structural redundancy inherent
in the lateral forces resisting system.
For structure assigned to Seismic Design Category A, B, or C, the value of the
redundancy coefficient � is 1.0. For structures in Seismic Design Categories D, E,
and F, the redundancy coefficient � shall be taken as the largest of the values of � i,
calculated at each story i of the structure as follows:
20
� � 2 � (5.331)
i
r �A
max i
i