Where *S*DS is as defined in Paragraph 3-2b, and *R*

level of the structure when vibrating in its *m*th mode.

and *T*m, are as defined above.

The modal seismic response coefficient, *C*sm, shall be

2.

When the general design response

determined in accordance with the following

spectrum of Paragraph 3-2c(2)(a) is used for

structures where any modal period of vibration, *T*m,

equation:

exceeds 4.0 seconds, the modal seismic design

coefficient, *C*sm, for that mode is permitted to be

(3-16)

determined by the following equation:

4*S * D1

where:

(3-18)

2

(*R*)*T*m

period *T*m determined from either the general design

Where *R*, and *T*m are as defined above, and *S*D1 is the

response spectrum of Paragraph 3-2c (2)(a), or a

design spectral response acceleration at a period of 1

site-specific response spectrum per Paragraph 3-5,

second as determined in Paragraph 3-2b.

the response modification factor

(c)

Modal forces, deflections, and drifts.

determined from Table 7-1, and

The modal force, *F*xm, at each level shall be

determined by the following equations:

the modal period of vibration (in

seconds) of the mth mode of the structure.

(3-19)

and

1. When

the general design response

(3-20)

spectrum of Paragraph 3-2c (2)(a) is used for

∑ wf

structures on Site Class D, E, or F soils, the modal

seismic design coefficient, *C*sm, for modes other than

the fundamental mode that have periods less than

where:

0.3 seconds is permitted to be determined by the

following equation:

0.4*S * DS

(1.0 + 5.0*T*m )

(3-17)

3 - 10

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