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How is the response-spectrum scale specified?

Answer: The value for each force-related design parameter of interest, including story drifts, support forces, and

individual member forces for each mode of response, shall be computed using the properties of each mode and the

response spectra defined in either ASCE -!" Section ##$%$" or $& divided by the 'uantity R / I $ The value for

displacement and drift 'uantities shall be multiplied by the 'uantity C d   / I  (ASCE -!", Section #&$)$&*$

Therefore, the response-spectrum scale factor is I g / R , where g  is acceleration due to gravity (+$% in.sec& for /ip-in

and )$# m.sec& for /0-m*$ After analysis, users should review the base shear due to all modes, reported in the

1esponse Spectrum 2ase 1eaction Table$ 3f the dynamic base shear reported is more than "4 of the static base

shear, no further action is re'uired$ However, if dynamic base shear is less than "4 of the static base shear, then

the scale factor should be ad5usted such that the response-spectrum base shear matches "4 of the static base

shear$ 3n this case, the new scale factor would be (I g / R * 6 (!$" 6 static base shear . response-spectrum base

shear*$ Analysis should then be rerun with this scale factor specified in the response-spectrum case$

12.9 MODAL RESPONSE SPECTRUM ANALYSIS

12.9.1 Number of Modes. An analysis shall be conducted todetermine the natural modes of vibration for the structure. Theanalysis shall include a sufficient number of modes to obtain a

combined modal mass participation of at least 90 percent of the

actual mass in each of the orthogonal horizontal directions of response considered by the model.

12.9.2 Modal Response Parameters. The value for each forcerelateddesign parameter of interest, including story drifts, support

forces, and individual member forces for each mode of responseshall be computed using the properties of each mode and the

response spectra defined in either Section 11.4. or !1.! divided

 by the "uantityf $ The value for displacement and drift "uantities

shall be multiplied by the "uantity y.12.9.3 Combined Response Parameters. The value for each

 parameter of interest calculated for the various modes shall becombined using either the s"uare root of the sum of the s"uares

method #S$SS% or the complete "uadratic combination method#&'&%, in accordance (ith AS&) 4. The &'& method shall beused for each of the modal values or (here closely spaced modes

that have significant cross*correlation of translational and torsional

response.

12.9.4 Scaling Design alues of Combined Response. A baseshear #+% shall be calculated in each of the t(o orthogonal horizontaldirections using the calculated fundamental period of the

structure T in each direction and the procedures of Section 1!.,

e-cept (here the calculated fundamental period e-ceeds #&,%#Ta%,

then #&,%#Ta% shall be used in lieu of T in that direction. herethe combined response for the modal base shear #+t% is less than

percent of the calculated base shear #+% using the e"uivalent

lateral force procedure, the forces, but not the drifts, shall be multiplied by 0./

(here+ 7 the e"uivalent lateral force procedure base shear, calculated

in accordance (ith this section and Section 1!.

+t 7 the base shear from the re"uired modal combination

12.9.! "ori#ontal S$ear Distribution. The distribution of horizontalshear shall be in accordance (ith the re"uirements of Section 1!..4 e-cept that amplification of torsion per Section

1!..4. is not re"uired (here accidental torsional effects are included

in the dynamic analysis model.

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12.9.% P&Delta 'ffects. The 2*delta effects shall be determinedin accordance (ith Section 1!..3. The base shear used to determine

the story shears and the story drifts shall be determined inaccordance (ith Section 1!../.

12.9.( Soil Structure )nteraction Reduction. A soil structureinteraction reduction is permitted (here determined using &hapter 

19 or other generally accepted procedures approved by the

authority having urisdiction.

. 5or periods greater than TLY a ,n d less than or e"ual to TL,t he

design spectral response acceleration, Sa, shall be ta6en asgiven by )". 11.4*/

4. 5or periods greater than TL, Sa shall be ta6en as given by

)". 11.4*3

(here

Sds 7 the design spectral response acceleration parameter atshort periods

Sdl 7 the design spectral response acceleration parameter at

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  1-s  period

T 7 the fundamental period of the structure, s

TL 7 long*period transition period #s% sho(n in 5ig. !!*1 #&onterminous

7nited States%, 5ig. !!*1/ #$egion l%, 5ig. !!*13

#Alas6a%, 5ig. !!*1 #8a(aii%, 5ig. !!*19 #2uerto $ico,&ulebra, +ie"ues, St. Thomas, St. ohn, and St. &roi-%,

and 5ig. !!*!0 #:uam and Tutuila%.

11.4.% MC' Response Spectrum. here a ;&) responsespectrum is re"uired, it shall be determined by multiplying the

design response spectrum by 1..

11.4.( Site&Specific *round Motion Procedures. The sitespecificground motion procedures set forth in &hapter !1 are

 permitted to be used to determine ground motions for any structure.

A site response analysis shall be performed in accordance(ith Section !1.1 for structures on Site &lass 5 sites, unless the

e-ception to Section !0..1 is applicable. 5or seismically isolatedstructures and for structures (ith damping systems on sites (ith

S1 greater than or e"ual to 0./, a ground motion hazard analysisshall be performed in accordance (ith Section !1.!.