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# 1617.1 Seismic Load Effect E and EM

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The seismic load effect,E, for use in the basic load combinations of Sections 1605.2 and 1605.3 shall be determined from Section 9.5.2.7 of ASCE 7. The maximum seismic load effect,Em, for use in the special seismic load combination of Section 1605.4 shall be the special seismic load determined from Section 9.5.2.7.1 of ASCE 7.

Exception: For structures designed using the simplified analysis procedure in Section 1617.5, the seismic load effects,E and Em, shall be determined from Section 1617.1.1.
1617.1.1 Seismic Load Effects, E and EM (For Use in the Simplified Analysis Procedure of Section 1617.5)
Seismic load effects, E and Em, for use in the load combinations of Section 1605 for structures designed using the simplified analysis procedure in Section 1617.5 shall be determined as follows.
Where the effects of gravity and the seismic ground motion are additive, seismic load, E, for use in Equations 16-5, 16-10 and 16-17, shall be defined by Equation 16-50:

E = ρQE + 0.2SDSD                 (Equation 16-50)

where:

 D = The effect of dead load. E = The combined effect of horizontal and vertical earthquake-induced forces. r = A redundancy coefficient obtained in accordance with Section 1617.2. QE = The effect of horizontal seismic forces. SDS = The design spectral response acceleration at short periods obtained from Section 1615.1.3 or 1615.2.5.

Where the effects of gravity and seismic ground motion counteract, the seismic load, E, for usein Equations 16-6, 16-12 and 16-18 shall be defined by Equation 16-51.

E = ρQE - 0.2SDSD                 (Equation 16-51)

Design shall use the load combinations prescribed in Section 1605.2 for strength or load and resistance factor design methodologies or Section 1605.3 for allowable stress design methods.
1617.1.1.2 Maximum Seismic Load Effect, EM
The maximum seismic load effect, Em, shall be used in the special seismic load combinations in Section 1605.4.

Where the effects of the seismic ground motion and gravity loads are additive, seismic load, Em, for use in Equation 16-19, shall be defined by Equation 16-52.

Em= ΩQE + 0.2SDSD              (Equation 16-52)

Where the effects of the seismic ground and gravity loads counteract, seismic load, Em, for use in Equation 16-20, shall be defined by Equation 16-53.

Em= ΩQE - 0.2SDSD              (Equation 16-53)

where

E, QE, SDS are as defined above and Ω0 is the system over strength factor as given in Table 1617.6.2.

The term 0QE need not exceed the maximum force that can be transferred to the element by the other elements of the lateral-force-resisting system.

Where allowable stress design methodologies are used with the special load combinations of Section 1605.4, design strengths are permitted to be determined using an allowable stress increase of 1.7 and a resistance factor, f, of 1.0. This increase shall not be combined with increases in allowable stresses or load combination reductions otherwise permitted by this code or the material reference standard except that combination with the duration of load increases in Chapter 23 is permitted.
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