Letter Report to Alexander Avenue Overhead (Bridge No. 27C-0150) Retrofit Project, City of Larkspur, Marin County, California 1.

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1 Parsons Brinckerhoff 303 Second Street Suite 700 North San Francisco, CA Fax: July 06, 2011 PB Project No A PARSONS BRINCKERHOFF 2329 Gateway Oaks Drive, Suite 200 Sacramento, CA Attention: Subject: Mr. Benjamin Consolacion Letter Report to Alexander Avenue Overhead (Bridge No. 27C-0150) Retrofit Project, City of Larkspur, Marin County, California 1.0 INTRODUCTION This letter report is to provide updated seismic design data based on the Caltrans Seismic Design Criteria dated November 2010, Version 1.6. Our services are provided in accordance with our overall scope and proposal for this work. 1.1 PROJECT DESCRIPTION The existing Alexander Avenue overhead bridge is located in the City of Larkspur (Plate 1) and is about 29 ft wide and 163 ft long with straight alignment. The superstructure is a 7 spancontinuous concrete slab bridge. The abutments are seat type. Bents 2, 3, and 4 are two column bents while bents 5, 6, and 7 are three column bents. The existing foundations consist of spread footings. The proposed project includes seismic retrofit of Alexander Avenue OH, primarily by constructing seismic approach structure, infill shear wall, retrofitting existing columns and bent caps. 1.2 SCOPE OF SERVICES Our scope of services includes: Review of available existing field exploration and laboratory data provided. Prepare this letter to update the seismic design data of the reference project based on the most updated Caltrans Seismic Design Criteria dated November 2010, Version REVIEW OF EXISTING DATA 2.1 EXPLORATION DATA The field exploration program at Alexander Avenue OH bridge, which was conducted by Parikh Consultants, Inc. (PCI) on August 9, 1997, consisted of two borings. Boring B-1 was drilled at Over a Century of Engineering Excellence 1

2 Seismic Retrofit of Alexander Ave OH Bridge City of Larkspur, California. June 29, 2011 the west side and boring B-2 was drilled on the east side of the bridge. No new borings were drilled for this letter report. The approximate locations of the borings are shown on LOTB Plan, Plate SUBSURFACE DATA Based on the field exploration and laboratory test results, the soil material generally consisted of very stiff to hard clay down to elevation 75 ft underlain by highly weathered sandstone to the maximum depth of 42 ft (elev. 58 ft). Groundwater was encountered in boring B-1 at depth of 20 ft (elev. 80 ft) during drilling and fell to elevation 62 ft at the end of drilling. No groundwater was encountered in boring B-2. Groundwater is subject to fluctuation due to run off water, high and low tide in the bay, and other factors that may affect groundwater level. 3.0 FINDINGS AND RECOMMENDATIONS 3.1 SEISMIC DESIGN The project site is located in the City of Larkspur, Marine County, California. The project site is located in a seismically active area of San Francisco Bay Area where earthquakes along various fault lines closer to the project site can produce significant ground shaking. The Alexander Avenue OH bridge should be designed in accordance with the Caltrans Seismic Design Criteria (SDC) dated November 2010, Version 1.6, to resist the lateral forces generated by earthquake ground shaking. Effective September 30, 2009, all projects within Caltrans right-of-way should comply with the 2007 Caltrans Fault Data Set and the Caltrans Seismic Design Criteria (SDC). The development of the new guidelines were in an effort to update the Caltrans Seismic Hazard Map (SHM). The Caltrans fault database had undergone numerous updates since the SHM was last produced in It was recognized that the SHM update presented an opportunity to adopt recently developed Next Generation Attenuation (NGA) ground motion prediction equations (GMPE s) for the map. In addition to the deterministic approach, the new guidelines include probabilistic criteria in the specification of the design spectrum. This eliminates any doubt of the past practice which was solely relying on a deterministic PGA contour map and standard spectral curves. The newly adopted probabilistic criterion captures the additional complexity associated with implementing the NGA ground motion prediction models. The Caltrans ARS Online developed by California Department of Transportation calculates both deterministic and probabilistic response spectrum for any location in California. Over a Century of Engineering Excellence 2

3 Seismic Retrofit of Alexander Ave OH Bridge City of Larkspur, California. June 29, DESIGN RESPONSE SPECTRUM DEVELOPMENT According to the latest version of Caltrans Seismic Design Criteria, November 2010 Version 1.6, the design response spectrum for any Caltrans standard bridge is based on the envelope of a deterministic and probabilistic spectrum. Both the deterministic and probabilistic spectra can be obtained from web based tool Caltrans ARS Online at This tool generates curves based on criteria provided in Appendix B of Caltrans Seismic Design Criteria. Upon entering the Latitude. Longitude and Average shear wave velocity for the top 30m (V s30 ) of any particular site, Caltrans ARS online generates both the spectrums. The deterministic spectrum is based on the average of medium spectra calculated using the Campbell-Bozorgnia (2008) and Chiou-Youngs (2008) ground motion prediction equations (GMPE). The web based tool determines the magnitude (Mmax), type, distance (R), top and bottom distances (Z) of the rupture plane of the governing fault and also applies any adjusting factors. The probabilistic spectrum is obtained from the (2008) USGS Seismic Hazard Map (Petersen, 2008) for the 975 year return period (5% in 50 year probability of exceedance). For Alexander Avenue OH bridge, both the deterministic and probabilistic curves were generated and the envelope of both spectrum values was used as the design response spectrum (Plate 5.1 ). The deterministic data from Caltrans ARS Online is compared with CY-CB spreadsheet results that were generated from the Caltrans Deterministic Response Spectrum Spreadsheet. The comparision curves and data were provided in Plates 3.1 to 3.4. The analysis show a maximum difference of 1% between the two curves. The probabilistic data from Caltrans ARS Online is compared with USGS Deaggregation Hazard (Beta) with near field and basic factor adjustments using Caltrans Probabilistic Response Spectrum Spreadsheet. The comparision curves and data were provided in Plates 4.1 to 4.2. The analysis show a maximum difference of 3.6% between the two curves. As per the Caltrans Geotechnical Services Design Manual Section , the design spectrum is the upper envelope of the spectrum values. The ARS Online Deterministic, Probabilistic and USGS Interactive Deaggregation Spectrum (2008) data were plotted and the upper envelope which was USGS Interactive Deaggregation Spectrum is considered as the Design Spectrum and the plot is provided in Plate 5.1 and 5.2. Over a Century of Engineering Excellence 3

4 Seismic Retrofit of Alexander Ave OH Bridge City of Larkspur, California. June 29, 2011 The following table shows the seismic data for Alexander Avenue Overhead Bridge. TABLE 1 SEISMIC DATA FOR ALEXANDER AVENUE OH BRIDGE Site Data Latitude Longitude Shear Wave Velocity (V s30 ) 360 m/sec Based on Fig B.12 of App B, SDC Soil Profile Type C Very dense soil and soft rock based on Geotechnical Report by PCI San Andreas Fault Zone (North Coast Section) Data Fault ID Max Magnitude (MMax) Fault Type RLSS Right Lateral Strike Slip Fault Dip: 90 Deg - Dip Direction V - Top of Rupture Plane (Ztor) 0 km - Bottom of Rupture Plane: 11 km - Rrup/Rjb/Rx 11.7km Depth to Vs=1.0km/s (Z 1.0 ) 264m Basin Factor Depth to Vs=2.5km/s (Z 2.5 ) 2.00km Basin Factor Over a Century of Engineering Excellence 4

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6 4.0 REFERENCES Geotechnical Services Design Manual, Version 1.0, August Caltrans Division of Engineering Services, Geotechnical Engineering. Development of Caltrans Deterministic PGA Map and Caltrans ARS Online, July Tom Shantz, Division of Research and Innovation, Martha Merriam, Geotechnical Services. Caltrans Seismic Design Criteria, Appendix B, Design Spectrum Development, November 2010, Version 1.6. Geotechnical Engineering Investigation Report, Seismic Retrofit, Alexander Avenue OH (Br. No. 27C-0150), City of Larkspur, California, prepared by Parikh Consultants, Inc., dated August 19, USGS 2008 Interactive Deaggregations (Beta), Geologic Hazards Science Ceter. Over a Century of Engineering Excellence 6

7 PLATES SITE PLAN PLATE 1 LOTB PLAN FROM PCI GEOTECHNICAL REPORT PLATE 2 DETERMINISTIC COMPARISON OF ARS CURVES PLATES 3.1 TO 3.4 PROBABILISTIC COMPARISON OF ARS CURVES PLATES 4.1 TO 4.2 DESIGN RESPONSE SPECTRUM PLATE 5.1 TO 5.2 Over a Century of Engineering Excellence 7

8 PARSONS BRINCKERHOFF Tel: Second Street, Suite 700 North Fax: San Francisco, CA Approximate Project Site SOURCE: Google Maps Scale: SITE PLAN PB Project # 12399A Alexander Avenue Overhead (Bridge No. 27C-0150) Retrofit Project, City of Larkspur, Marin County, California

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10 Analysis of CY-CB Attenuation Prediction Equation vs ARS Online Results Comparison of ARS Curves (unlock sheet with "shmi") Model Inputs Fault Magnitude 7.9 (5 to 8.5) F RV 0 (input 1 = Rev) F NM 0 (input 1 = Normal) Dip (degree) 90 ( 0 to 90) Z TOR (km) 0 Distance R RUP (km) 11.7 R JB (km) 11.7 R x (km) 11.7 Hanging Wall? Near-Field Factor? Yes? FALSE TRUE Yes? Site V S30 (m/sec) 360 (270 to 1500 m/s) Z 1.0 (m) 264 (0 - No Basin) Z 2.5 (km) 2 (0 - No Basin) No. Cal. Basin? So. Cal. Basin? Analysis Yes? TRUE FALSE Yes? ARS Online vs CY-CB Spreadsheet Results MAX. % Diff. = 1% (Check only for sites located within a Basin) Spectral Acceleration, Sa (g) Deterministic ARS (5% Damping) Comparison of Spreadsheet vs ARS Online Period (sec) CY-CB Spreadsheet ARS Online Min. Spectrum for CA Min Sprectrum for ECSZ Deterministic ARS Curve Comparison_Alexander Ave OH.xls 6/23/2011 5:46 PM

11 Analysis of CY-CB Attenuation Prediction Equation vs ARS Online Results CY-CB Spreadsheet Results Place ARS Online Deterministic Data Here "Paste" Basin Factor Near Fault Factor Final Adj. S(a) Min. Spectrum for CA Min Sprectrum for ECSZ Diff. (%) T (sec) S (a) T (sec) S (a) T (sec) CB-CY S(a) T (sec) Base S(a) % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % For Comparsion Plots of Min. Sprectra, Paste Special into Cells Deterministic ARS Curve Comparison_Alexander Ave OH.xls 6/23/2011 5:42 PM

12 Analysis of CY-CB Attenuation Prediction Equation vs ARS Online Results CY-CB Spreadsheet Results T (sec) CB-CY S(a) T (sec) Base S(a) Place ARS Online Deterministic Data Here "Paste" Basin Factor Near Fault Factor Final Adj. S(a) For Comparsion Plots of Min. Sprectra, Paste Special into Cells Min. Spectrum for CA Min Sprectrum for ECSZ Diff. (%) T (sec) S (a) T (sec) S (a) % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % Deterministic ARS Curve Comparison_Alexander Ave OH.xls 6/23/2011 5:42 PM

13 Analysis of CY-CB Attenuation Prediction Equation vs ARS Online Results CY-CB Spreadsheet Results T (sec) CB-CY S(a) T (sec) Base S(a) Place ARS Online Deterministic Data Here "Paste" Basin Factor Near Fault Factor Final Adj. S(a) For Comparsion Plots of Min. Sprectra, Paste Special into Cells Min. Spectrum for CA Min Sprectrum for ECSZ Diff. (%) T (sec) S (a) T (sec) S (a) % % % % % % % % % % % % % % % % % % % % % % % Deterministic ARS Curve Comparison_Alexander Ave OH.xls 6/23/2011 5:42 PM

14 Comparison spreadsheet of the 2008 USGS Probabilistic Seismic Hazard Data and ARS Online Probabilistic Data Spectral Accelerations Points from USGS Website at (unlock spreadsheet "shmi") * Note: This spreadsheet uses the given latitude and longitude data provided by the user to estimate spectral acceleration values with a probability of exceedence 5% in 50 yrs (or 975 yr return period). The four spectral acceleration data points plotted on the graph are from the USGS website and are based on a 0.05 degree grid. Basic interpolation is used to estimate intermediate values inside each grid. Raw Data points are provided in the tabs of this spreadsheet. Corner grid spectral acceleration data are shown in the "calculation" tab. Input Site Information Latitude Longitude V S30 (m/s) = 360 Near Fault Factor, Derived from USGS Deagg. Dist (km) = 11.7 Z 1.0 (m) = 264 Z 2.5 (km) = 2 Spectral Acceleration, Sa (g) Probabilistic ARS (5% Damping) Comparison of USGS Data & ARS Online 2008 USGS Deag. Hazard (Rock Adj. by CT) ARS Online 2008 USGS Deag. Hazard (Beta) Period (sec) Probabilistic ARS Curve Comparison_Alexander Ave OH.xls 6/23/2011 5:53 PM

15 Place ARS Online Probabilistic Data Here "Paste" T (sec) Base Spectrum S(a) Basin Factor Near Fault Factor Final Adj. Spectrum S(a) Analysis of ARS Online Results vs USGS Deaggregation Hazard (Adj. By CT) USGS Interpolated Adj. for Adj. For Final Adj. ARS Online % Difference Period Spectral Near Fault Adj. for Soil Basin USGS Final Adj. (bet. USGS & (sec) Accel. Effect Amplification Effect Spec Accel Spect. Accel. ARS Online) % % % % Max % Difference = 3.6% USGS Deaggregation Hazard (Beta) with Near Field and Basin Factors % % % % % % % % % % Max % Difference = 3.4% Period (sec) INPUT USGS Deagg. Spec Accel Adj. for Near Fault Effect Adj. For Basin Effect Final Adj. USGS Deagg Spec Accel ARS Online Final Adj. Spect. Accel. % Difference (bet. USGS & ARS Online) Probabilistic ARS Curve Comparison_Alexander Ave OH.xls 6/23/2011 5:51 PM

16 Design Response Spectrum (5% damping) Spectral Acceleration, Sa (g) Period (sec) USGS Interactive Deaggregation Spectrum (DESIGN) ARS Online Deterministic Response Spectrum ARS Online Probablistic Response Spectrum ALEXANDER AVENUE OVERHEAD BRIDGE CITY OF LARKSPUR JOB NO.: - PLATE NO.:

17 Caltrans ARS Online USGS 2008 Inter. Deaggre. T Sa (g) T (Sec.) Sa (g) Sec. Determ. Probalist