Improvements to the Development of Acceleration Design Response Spectra Nicholas E. Harman, M.S., P.E., SCDOT
Thanks Clemson University Dr. Ron Andrus Co-Principal Investigator Dr. Nadarajah Ravichandran Co-Principal Investigator Shimelies Aboye PhD Candidate Ariful Bhuiyan PhD Student
Background Kramer (1996) indicates that the effect of local site conditions has been previously noted. MacMurdo (1824) 1819 Cutch, India EQ Mallet (1863) 1857 Neapolitian EQ 2 EQ events emphasized the effect of local site conditions on Site Amplification 1985 Michoacán, Mexico (aka Mexico City) EQ 1989 Loma Prieta, California EQ
Michoacán, Mexico (aka Mexico City) EQ September 19, 1985 M w = 8.1 R = 217 miles from Mexico City Duration = 3 to 4 minutes Fault displaced approximately 10 feet Most of the damage was caused by resonance between the soils, the earthquake and the structures
Loma Prieta, California EQ October 17, 1989 M w = 6.9 R = 61 miles from San Francisco Duration = 10 to 15 seconds Maximum fault displacement 7.5 feet Numerous bridges damaged
Results Based on the destruction for both EQ events Rock Sites are stiffer Soil Sites tend to amplify the EQ motion Especially for soft sites To account for these different responses based on site stiffness NEHRP recommendations were developed
Each site is classified based on its stiffness (aka Site Class) Stiffness can be measured using V s N Undrained shear strength, S u Developed Site Amplification Factors (F) F factors based on Site stiffness Acceleration NEHRP
Site Class Source: AASHTO Guide Specifications for LRFD Seismic Bridge Design, 2 nd Edition, 2014 Interim Revisions, AASHTO
SC Practice SITE CLASS SOIL PROFILE NAME AVERAGE PROPERTIES IN TOP 100 FT (30 M) Below Z DTM SITE STIFFNESS V s A Hard Rock V > 5,000 ft/sec ( V >1500 m/sec) s s B Rock 2,500 V s 5,000 ft/sec (760 V s 1500 m/sec) C Very Dense Soil and Soft Rock 1,200 V s 2,500 ft/sec (360 V s 760 m/sec) D Stiff Soil 600 V 1,200 ft/sec (180 V 360 m/sec) s s E F Soft Soil Soils Requiring Site Specific Response Evaluation V s < 600 ft/sec ( < 180 m/sec) Any profile with more than 10 ft (3m) of soft clay defined as: PI > 20; w > 40%; and s u < 500 psf (25 kpa) Any soil profile containing one or more of the following characteristics: 1. Peats and/or highly organic clays (H>10 ft [3 m] of peat and/or highly organic clay where H = thickness of soil) 2. Very high plasticity clays (H>25 ft [8 m] with PI > 75) 3. Very thick soft/medium stiff clays (H> 120 ft [36 m]) Definitions: PI = Plasticity Index (AASHTO T89, T90 or ASTM D 4318) w = Moisture Content (AASHTO T265 or ASTM D 2216) = Average shear wave velocity for the upper 100 ft (30 m) below Z DTM. (ft/sec or m/sec) = Average undrained shear strength ( ) for cohesive soils in the upper 100 ft (30 m) below Z DTM. (psf or kpa) (AASHTO T208 or T296 or ASTM D2166 or D2850) Z DTM = Depth-to-motion is the location where the ground motion transmits the ground shaking energy to the structure. Notes: (1) The shear wave velocity for rock, Site Class B, shall be either measured on site or estimated by a geotechnical engineer or engineering geologist/seismologist for competent rock with moderate fracturing and weathering. Softer and more highly fractured and weathered rock shall either be measured on site for shear wave velocity or classified as Site Class C. (2) The hard rock, Site Class A, category shall be supported by shear wave velocity measurements either on site or on profiles of the same rock type in the same formation with an equal or greater degree of weathering and fracturing. Where hard rock conditions are known to be continuous to a depth of 100 feet (30m) below Z DTM, surficial shear wave velocity measurements may be extrapolated to assess shear wave velocities. (3) Site Classes A and B should not be used when there is more than 10 feet (3m) of soil between the rock surface and the depth-to-motion, Z DTM. When rock is encountered within the 100 feet (30m) below the depth-to-motion, Z DTM, and the soil layer is more than 10 feet (3m) use the Site Class pertaining to the soil above the rock. (4) A Site Class F is not required if a determination is made that the presence of such soils will not result in a significantly higher response of a bridge. Consideration of the effects of depth-to-motion, Z DTM, shall be taken into account when making this determination. Such a determination must be approved by the PCS/GDS.
Spectral Accelerations Once Site Class determined Determine Spectral Accelerations (PSA) for PGA S S S 1 Spectral Accelerations developed at B-C Boundary or Hard Rock surface V sb-c > 2,500 ft/sec V shr >11,500 ft/sec
Site Amplification Factors F factors are determined using F = S site S outcrop S site Spectral Acceleration at a selected period at the ground surface S outcrop Spectral Acceleration at a selected period at the soft-rock outcrop F factors for the short (0.2 sec) (F a ) and long (1.0 sec) (F v ) periods were developed
F a and F v were developed empirically using strong motion records from Treasure and Yerba Buena Islands in San Francisco Bay PGA ~0.1g For strong levels of shaking one-dimensional equivalent linear and non-linear site response analyses were performed. These are the factors currently in use F a was developed for a spectral period ranging from 0.1 to 0.5s F v was developed for a spectral period ranging from 0.4 to 2.0s F PGA is equal to F a
Site Amplification Factors Determine F using PSA and Site Class Source: AASHTO Guide Specifications for LRFD Seismic Bridge Design, 2 nd Edition, 2014 Interim Revisions, AASHTO
3-Point ADRS Develop the 3-point ADRS Curve using the above information to determine PGA = A s = F PGA PGA B C S DS = F a S 0.2B C = F a S sb C S D1 = F v S 1B C T s = S D1 S DS and T o = 0.2 T S
Concerns The existing F factors were developed for Relatively thin soil profiles (~200 feet) Soft rock (V s = 2,500 to 5,000 ft/sec) Relatively low accelerations (PGA ~0.1g) South Carolina has Soil profiles in excess of 1,000 ft Hard rock (V s = 8,200 to 11,000 ft/sec) Accelerations at the B-C Boundary in excess of 0.6g
Concerns confirmed in a paper by Kimball and Costantino (2000) F factors at SRS lower than used by NEHRP SCDOT has enough site-specific seismic response analyses showing dramatic decreases in ADRS curves to show F factors may be conservative Decreases in some case on the order of 50% SCDOT contracted with Clemson University to study SC specific F factors
SC General Geology
Piedmont Coastal Plain
Clemson s Approach Investigate and recommend F factors for the Coastal Plain and the Piedmont Investigate period shifting observed by SCDOT in site-specific seismic response analysis Look at the applicability of continuing the use of Site Class determination using only the upper 100 feet of a site instead of something deeper
Investigate the existing Site Class system as applied to SC Provide procedures so engineers can understand the repercussions of the new ADRS curves on design Develop spreadsheet to determine F factors
F Factor Determination One-dimensional equivalent linear analysis (SHAKE ~36,000 runs) One-dimensional total stress non-linear analysis (D-MOD ~12,000 runs) Earthquake histories were developed using SCENARIO_PC (2006)
NEHRP F a and F v based on spectral period ranges of 0.1 0.5s and 0.4 2.0s, respectively (Borchedt (1994)) Narrower period ranges allow of better spectral acceleration prediction Therefore period ranges of: 0.01 PGA 0.01 0.4 T 0.2 0.41 0.8 T 0.6 0.81-1.2 T 1.0 1.21 2.0 T 1.6 2.01 4.0 T 3.0
F is controlled by: V s100 Shear wave velocity for top 100 feet T m Mean predominant period used to present period (frequency) of ground motion T 330 Proxy variable for site fundamental period K H Dimensionless adjustment coefficient to account for shallow soft rock (B-C Boundary) S outcrop Spectral acceleration at outcrop at a specific spectral period
T m Increases with increasing distance from the epicenter (R) and depth to hard rock (Z HR ) R comes from USGS Deaggregation Charts Z HR Scenario_PC T m = 0.102 Z HR + 0.301 R + 0.233 1000 1000 T 330 Site period to a depth of 330 feet T 330 = 4 330 V s330 = 1320 V s330
Research Results V s100 F data pairs have 3 general features Increasing trend in F as V s100 increases from a very low value A zone of peak F values depending on S outcrop Decreasing trend in F as V s100 increases beyond the zone of peak F values
Procedure Obtain from field testing or existing geologic information V s100 V s330 or T 330 Z HR Z B-C Obtain from ground motions S outcrop or PSA outcrop R T m
Peak Site Factor Determination F P = x 1 exp x 2S outcrop g T m T 330 x 3 + 1 KH1 V s100p = x 4 S outcrop g x 5 T m s x 6 x 1, x 2, x 3, x 4, x 5, x 6 Regression coefficients KH2 Once F P and V s100p are determined then the median V s100 - F relationship can be determined
For V s100 < V s100p and all T F = F P V s100p V s100 For V s100 V s100p T < 0.2s F = F P 1 2500 V s100 + 1 2500 V s100p T 0.2s F = a + be cv s30
a regression coefficient b = 1 a e760c unitless c = ln 1 a F P a 760 V s30p units of s/m V s30p = V s100p and V 3.28 s30 = V s100 3.28
Results The F factors determined using this new procedure result in F factors more similar to those developed using site response analyses F factors are far more dependent on V s then indicated using NEHRP/AASHTO Anticipated differences between site response analyses and 3 point method should be greatly reduced
Example
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