30 November 1998
The seismic design story shear, Vx (kN or kips) shall be distributed to the various vertical elements of the
seismic-force-resisting system in the story under consideration based on the relative lateral stiffness of
the vertical resisting elements and the diaphragm.
C10. STRUCTURAL OVERTURNING RESISTANCE. The structure shall be designed to resist
overturning effects caused by the seismic forces determined from Equation C-25. At any story, the
increment of overturning moment in the story under consideration shall be distributed to the various
vertical force resisting elements in the same proportion as the distribution of the horizontal shears to
those elements (FEMA 302, 5.3.6).
The overturning moments at Level x, Mx (kN-m or kip-ft), shall be determined from the following
Mx = ∑ Fi (h i = h x )
Fi = the portion of the seismic base shear, V, induced at Level i.
hi and hx = the height (m or ft) from the base to Level i or x.
Foundations shall be designed for the foundation overturning design moment, Mf (kN-m or kip-ft) at the
foundation-soil interface determined using Equation C-28 at the foundation level, multiplied by a
reduction factor of 0.75.
C11. STORY DRIFTS AND P-DELTA EFFECTS. The story drifts and, member forces and moments
due to P-delta effects shall be determined in accordance with the following guidance (FEMA 302, 5.37).
Story drifts shall be calculated based on the application of design seismic forces to a mathematical
model of the structure. The model shall include the stiffness and strength of all elements that are
significant to the distribution of forces and deformations in the structure and shall represent the spatial
distribution of the mass and stiffness of the structure. 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.
The deflections of level x, δ (mm or in) shall be determined according to the following equation:
C d δe
δe = the deflections determined by an elastic analysis (mm or in.) based on the forces defined in
For determining compliance with the story drift limitations in Table 3-2, the deflections of Level x, δ (mm
or in) shall be calculated as expressed in Equation C-29. For the purposes of this drift analysis only, the
computed fundamental period, T in seconds, of the structure may be used without the upper bound
limitations specified in Table C-4, when determining drift level seismic design forces.
The design story drift, ∆ (mm or in) shall be increased by the incremental factor relating to the P-delta
effects if required by the following guidance (FEMA 302, 188.8.131.52). The 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, θ as determined by the following equation
is equal to or less than 0.10: