Building Design and Construction Handbook - Merritt - Ventech!

STRUCTURAL THEORY 5.179
• 1.0 for the top 10 stories,
• 0.8 for the 20th story from the top and below, and
• value between 1.0 and 0.8 determined by a straight line interpolation for stories
between the 20th and 10th stories below the top.
Story Drift Determination. The design story drift � shall be computed as the
difference of the deflections at the center of mass at the top and bottom of the story
under consideration. Where allowable stress design is used. � shall be computed
using earthquake forces without dividing by 1.4. For structures assigned to Seismic
Design Category C, D, E, or F having plan irregularity types 1a or 1b of Table
5.12, the design story drift � shall be computed as the largest difference of the
deflections along any of the edges of the structure at the top and bottom of the
story under consideration.
The deflections of level x, � x, shall be determined in accordance with following
formula:
C �
d xe
�x � (5.324)
I
where C d � the deflection amplification factor in Table 5.9,
� xe � the deflections determined by an elastic analysis of the seismic force
resisting system, and
I � the Occupancy Importance Factor
For purposes of this drift analysis only, the upper bound limitation specified on
the computed fundamental period, T, in seconds, of the building, shall not apply.
The design story drift � shall be increased by the incremental factor relating to
the P-delta effects. When calculating drift, the redundancy coefficient � shall be
taken as 1.0.
P-Delta Effects. P-delta effects on story shears and moments the resulting member
forces and moments, and the story drifts induced by these effects are not required
to be considered when the stability coefficient �, as determined by the following
formula, is equal to or less than 0.10:
P �
x
� � (5.325)
Vh C
x sx d
where P x � the total unfactored vertical design load at and above level x; when
calculating the vertical design load for purposes of determining Pdelta,
the individual load factors need not exceed 1,0;
� � the design story drift occurring simultaneously within V x;
V x � the seismic shear force acting between level x and x � 1;
h sx � the story height below level x; and
C d � the deflection amplification factor in Table 5.9
The stability coefficient � shall not exceed � max determined as follows:
0.5
� � � 0.25 (5.326)
max
�C d
where � is the ratio of shear demand to shear capacity for the story between level
x and x � 1. Where the ratio � is not calculated, a value of � � 1.0 shall be used.