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Required Number of Ground Motions
Usually, ground motion records consist of pairs of orthogonal horizontal ground motions components. Ground motions may also include a vertical ground motion component that can be used when required in the analysis. The selection of ground motions should generally satisfy the following:
•Ground motion records should be selected from events within the same general tectonic regime
•Consistent magnitudes and fault distances as those controlling the target spectrum
•Have similar spectral shape to the target spectrum
Different codes have slightly different requirements for the required number of ground motion records and the method of scaling those records to the target spectrum. The following tables illustrates the requirements of various US Codes.
Table: Required number of ground motions by Code
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Code
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Section
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Number of GM Pairs
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GM Sa Measure
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GM Level
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Scaling Instructions
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ASCE/SEI 7-16 Far-Field Provisions [2]
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16.2.2
(Regular Structures)
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11
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Max Direction
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MCER
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MaxRot > 0.9x Target Spectrum
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ASCE/SEI 7-16 Near-Field Provisions [2]
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16.2.2
(Regular Structures)
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11
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Max Direction
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MCER
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Rotate to FN/FP directions
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ASCE/SEI 7-16
Far-Field Provisions [2]
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18.2.2.2a (Damping Systems)
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7
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SRSS
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MCER
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SRSS > Target Spectrum
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ASCE/SEI 7-16 Near-Field Provisions [2]
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18.2.2.2a (Damping Systems)
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7
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SRSS
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MCER
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Average of FN > Target Spectrum & Average FP > 50% Target Spectrum
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ASCE/SEI 7-10 Far-Field Provisions [3]
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16.1.3 (Regular Structures)
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7 (Avg) or
3 (Max)
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Max Direction
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2/3 MCER
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SRSS > Target Spectrum
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ASCE/SEI 7-10
Near-Field Provisions [3]
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16.1.3 (Regular Structures)
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7 (Avg) or
3 (Max)
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Max Direction
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2/3 MCER
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Average of FN > Target Spectrum
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ASCE/SEI 7-05 [4]
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16.1.3 (Regular Structures)
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7 (Avg) or
3 (Max)
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Geomean
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2/3 MCE
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SRSS > 1.17x Target Spectrum
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ASCE/SEI 41-17 [5]
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14.2.2.1 (Isolation Systems)
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7
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Max Direction
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BSE-2N,BSE-1N, BSE-2E, BSE-1E
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MaxRot > 0.9x Target Spectrum
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ASCE/SEI 41-13
Far-Field Provisions [6]
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2.4.2.2b (Regular Structures)
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10 (Avg) or
3 (Max)
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Max Direction
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BSE-2N, BSE-1N
BSE-2E, BSE-1E
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SRSS > Target Spectrum
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ASCE/SEI 41-13
Near-Field Provisions [6]
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2.4.2.2b (Regular Structures)
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7x2 (Avg) or
3x2 (Max)
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Max Direction
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BSE-2N, BSE-1N
BSE-2E, BSE-1E
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Average of FN > Target Spectrum
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ASCE/SEI 41-06 [7]
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1.6.2.2 (Regular Structures)
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7 (Avg) or
3 (Max)
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Geomean
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BSE-2E, BSE-1E
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SRSS > 1.3x Target Spectrum
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a According to section ASCE 7-16 section 18.2.2.2, design earthquake and MCER ground motion suites may each consist of at least seven pairs of horizontal acceleration components selected and scaled from individual recorded events that have magnitudes, fault distance, and source mechanisms that are consistent with those that control the design earthquake and MCER events.
For each pair of horizontal ground motion components, the square root of the sum of the squares (SRSS) spectrum may be constructed by taking the SRSS of the 5% damped response spectra for the scaled components (when amplitude scaling is used, an identical scale factor is applied to both components of a pair).
b ASCE/SEI 41-13 imposed different criteria for the required number of ground motions for nonlinear dynamic analysis. This criteria included distance condition, method of computing the results and the performance objective. If the desired site is classified as a far-field (distance from fault > 5 km or 3 miles), then the generated records may not be rotated. However, if the site is classified as near-fault site, then the generated recorded ground motion pairs may be rotated.
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Period Range
Codes require the average of the suite spectra to match the target over a specified period range, which is generally a function of the structure's first period of vibration.
Table: Period range according to different codes
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Code
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Section
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Lower Bound Period
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Upper Bound Period
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ASCE/SEI 7-16 [2]
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16.2.3.1a (Regular Structures)
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0.2Tc
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1.5T
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ASCE/SEI 7-16 [2]
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18.2.2.2b (Damping Structures)
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0.2T1Dd
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1.25T1Me
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ASCE/SEI 7-10 [3]
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16.1.3.1 & 16.1.3.2 (Regular Structures)
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0.2T
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1.5T
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ASCE/SEI 41-17 [5]
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2.4.3 (Regular Structures)
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0.2 Tminf
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1.5 Tmaxf
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ASCE/SEI 41-17 [5]
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14.2.2.1 (Isolated Structures)
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0.75Txg
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1.25Tx
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ASCE/SEI 41-13 [6]
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2.4.2.2b (Regular Structures)
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0.2T
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1.5T
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a If the vertical component is considered, the lower bound period used for modification of vertical components of ground motion should be the maximum of :
•0.1 seconds
•Lowest period at which significant vertical mass participation occurs
b The average of the SRSS spectra from all horizontal component pairs does not fall below the corresponding ordinate of the response spectra used in the design
c The 0.2T requirement is now supplemented with an additional requirement that the lower bound also should capture the periods needed for 90% mass participation in both directions of the building
d T1D = Effective period, in seconds, of the fundamental mode of vibration of the structure at the design displacement in the direction under consideration
e T1M = Effective period, in seconds, of the fundamental mode of vibration of the structure at the MCER displacement in the direction under consideration
f Tmin and Tmax are the smallest and largest first-mode period for the two principal horizontal directions of response, respectively. Note that the upper-bound period may not be taken as less than 1 second.
g Tx = Effective period of the seismically isolated structure in seconds at the displacement Dx in the direction under consideration
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