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# Section 1617 Earthquake Loads — Minimum Designlateral Force and Related Effects

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1617.1 Seismic Load Effect E and EM
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.
1617.2 Redundancy
The provisions given in Section 9.5.2.4 of ASCE 7 shall be used.

Exception: Structures designed using the simplified analysis procedure in Section 1617.5 shall use the redundancy provisions in Sections 1617.2.2.
1617.2.1 ASCE 7, Section 9.5.2.4.2
Modify Section 9.5.2.4.2 as follows:

9.5.2.4.2 Seismic Design Category D: For structures inSeismic Design Category D, ñ shall be taken as the largest of the values of x calculated at each story "x" of the structure in accordance with Equation 9.5.2.4.2-1 as follows:

ρ = 2 —20/ (rmaxxAx)

where:

rmaxx = The ratio of the design story shear resisted by the single element carrying the most shear force in the story to the total story shear, for a given direction of loading. For braced frames, the value of rmaxx is equal to the lateral force component in the most heavily loaded brace element divided by the story shear. For moment frames, rmaxx shall be taken as the maximum of the sum of the shears in any two adjacent columns in the plane of a moment frame divided by the story shear. For columns common to two bays with moment-resisting connections on opposite sides at the level under consideration, 70 percent of the shear in that column is permitted to be used in the column shear summation. For shear walls, rmaxx shall be taken equal to shear in the most heavily loaded wall or wall pier multiplied by 10/lw (the metric coefficient is3.3/lw), divided by the story shear, where lw is the wall or wall pier length in feet (m). The value of the ratio of 10/lw need not to be greater than 1.0 for buildings of light-framed construction. For dual systems, rmaxx shall be taken as the maximum value defined above, considering all lateral-load-resisting elements in the story. The lateral loads shall be distributed to elements based on relative rigidities considering the interaction of the dual system. For dual systems, the value of ñ need not exceed 80 percent of the value calculated above.

Ax = The floor area in square feet of the diaphragm level immediately above the story.

Calculation of rmaxx need not consider the effects of accidental torsion and any dynamic amplification of torsion required by Section 9.5.5.5.2.

For a story with a flexible diaphragm immediately above, rmaxx shall be permitted to be calculated from an analysis that assumes rigid diaphragm behavior and x, need not exceed 1.25.

The value of need not exceed 1.5, which is permitted used for any structure. The value of shall not be taken as less than 1.0.

Exception: For structures with seismic-force-resisting systems in any direction comprised solely of special moment frames, the seismic-force-resisting system shall be configured such that the value of calculated in accordance with this section does not exceed 1.25. The calculated value of is permitted to exceed this limit when the design story drift, Δ, as determined in Section 9.5.5.7, does not exceed Δa/ρfor any story where a is the allowable story drift from Table 9.5.2.8.

The metric equivalent of Equation 9.5.2.4.2-1 is:

ρx = 2 — 6.1/ (rmaxxAx)

Where: Ax is in square meters.

The value ρ shall be permitted to be taken equal to 1.0 in the following circumstances:

1. When calculating displacements for dynamic amplification of torsion in Section 9.5.5.5.2.
2. When calculating deflections, drifts and seismic shear forces related to Sections 9.5.5.7.1 and 9.5.5.7.2.
3. For design calculations required b y Section 9.5.2.6, 9.6 or 9.14.

For structures with vertical combinations of seismic-force-resisting systems, the value of ñ shall be determined independently for each seismic-force-resisting system. The redundancy coefficient of the lower portion shall not be less than the following:

ρL= RLρu/ Ru

where:

ρL = r of lower portion.

RL = R of lower portion.

Pu = r of upper portion.

Ru = R of upper portion.
1617.2.2 Redundancy (For Use in the Simplified Analysis Procedure of Section 1617.5)
A redundancy coefficient, ρ, shall be assigned to each structure designed using the simplified analysis procedure in Section 1617.5 in accordance with this section. Buildings shall not exceed the limitations of Section 1616.6.1.
1617.2.2.1 Seismic Design Category B or C
For structures assigned to Seismic Design Category B or C (see Section 1616), the value of the redundancy coefficient is 1.0.
1617.2.2.2 Seismic Design Category D
For structures in Seismic Design Category D (see Section 1616), the redundancy coefficient, shall be taken as the largest of the values of, ρi, calculated at each story "i" of the structure in accordance with Equation 16-54, as follows:

ρi = 2 — 20/ (rmaxiAi)                   (Equation 16-54)

where:

ρi = 2 — 6.1/ (rmaxiAi)

rmaxi = The ratio of the design story shear resisted by the most heavily loaded single element in the story to the total story shear, for a given direction of loading.

rmaxi = For braced frames, the value rmaxi, is equal to the horizontal force component in the most heavily loaded brace element divided by the story shear.

rmaxi = For moment frames, rmaxi shall be taken as the maximum of the sum of the shears in any two adjacent columns in a moment frame divided by the story shear. For columns common to two bays with moment-resisting connections on opposite sides at the level under consideration, it is permitted to use 70 percent of the shear in that column in the column shear summation.

rmaxi = For shear walls, rmaxi, shall be taken as the maximum value of the product of the shear in the wall or wall pier and 10/lw (3.3/lw for SI), divided by the story shear, where lw is the length of the wall or wall pier in feet (m). In light-framed construction, the value of the ratio of 10/lw need not be greater than 1.0.

rmaxi = For dual systems, rmaxi, shall be taken as the maximum value defined above, considering all lateral-load-resisting elements in the story. The lateral loads shall be distributed to elements based on relative rigidities considering the inter-action of the dual system. For dual systems, the value of need not exceed 80 percent of the value calculated above.

Ai = The floor area in square feet of the diaphragm level immediately above the story.

For a story with a flexible diaphragm immediately above, rmaxi shall be permitted to be calculated from an analysis that assumes rigid diaphragm behavior and need not exceed 1.25

The value, ρ, shall not be less than 1.0, and need not exceed 1.5.

Calculation of rmaxi need not consider the effects of accidental torsion and any dynamic amplification of torsion required by Section 9.5.5.5.2 of ASCE 7.

For structures with seismic-force-resisting systems in any direction comprised solely of special moment frames, the seismic-force-resisting system shall be configured such that the value of calculated in accordance with this section does not exceed 1.25 for structures assigned to Seismic Design Category D, and does not exceed 1.1 for structures assigned to Seismic Design Category E or F.

Exception: The calculated value of ρ is permitted to exceed these limits when the design story drift, Δ, as determined in Section 1617.5.4, does not exceed Δa/ρ for any story where a is the allowable story drift from Table 1617.3.1.

The value ρ shall be permitted to be taken equal to 1.0 in the following circumstances:

1. When calculating displacements for dynamic amplification of torsion in Section 9.5.5.5.2 of ASCE 7.
2. When calculating deflections, drifts and seismic shear forces related to Sections 9.5.5.7.1 and 9.5.5.7.2 of ASCE 7.
3. For design calculations required by Section 1620, 1621 or 1622.

For structures with vertical combinations of seismic-force-resisting systems, the value, ρ, shall be determined independently for each seismic-force-resisting system. The redundancy coefficient of the lower portion shall not be less than the following:

ρL = RLρu/ Ru

rL = r of lower portion.

RL = R of lower portion.

ru = r of upper portion.

Ru = R of upper portion.
1617.3 Deflection and Drift Limits
The provisions given in Section 9.5.2.8 of ASCE 7 shall be used.

Exception: Structures designed using the simplified analysis procedure in Section 1617.5 shall meet the provisions in Section 1617.3.1.
1617.3.1 Deflection and Drift Limits (For Use in the Simplified Analysis Procedure of Section 1617.5)
The design story drift Δ, as determined in Section 1617.5.4, shall not exceed the allowable story drift Δa, as obtained from Table 1617.3.1 for any story. All portions of the building shall be designed to act as an integral unit in resisting seismic forces unless separated structurally by a distance sufficient to avoid damaging contact under total deflection as determined in Section 1617.5.4. Buildings shall not exceed the limitations of Section 1616.6.1.

TABLE 1617.3.1
ALLOWABLE STORY DRIFT, Δaa
BUILDING SEISMIC USE GROUP
I II III
Buildings, other than masonry shear wall or masonry wall frame buildings, four stories or less in height with interior walls, partitions, ceilings and exterior wall systems that have been designed to accommodate the story drifts 0.025 h sxb 0.020 h sx
0.015 h sx
Masonry cantilever shear wall buildingsc 0.010 h sx 0.010 h sx 0.010 h sx
Other masonry shear wall buildings 0.007 h sx 0.007 h sx 0.007 h sx
Masonry wall frame buildings 0.013 h sx 0.013 h sx 0.010 h sx
All other buildings 0.020 h sx 0.015 h sx 0.010 h sx
For SI: 1 inch = 25.4 mm.
1. There shall be no drift limit for single-story buildings with interior walls, partitions, ceilings and exterior wall systems that have been designed to accommodate the story drifts.
2. h sx is the story height below Level x.
3. Buildings in which the basic structural system consists of masonry shear walls designed as vertical elements cantilevered from their base or foundation support which are so constructed that moment transfer between shear walls (coupling) is negligible.
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