Geotechnical Aspects of the Seismic Update to the ODOT Bridge Design Manual. Stuart Edwards, P.E Geotechnical Consultant Workshop

Transcription

1 Geotechnical Aspects of the Seismic Update to the ODOT Bridge Design Manual Stuart Edwards, P.E Geotechnical Consultant Workshop

2 Changes Role of Geotechnical Engineer Background Methodology Worked Examples 2

3 OUR BASIC OBJECTIVE AASHTO Bridges shall be designed to have a low probability of collapse, when subject to earthquake ground motions that have a 7% probability in 75 yrs (about 1 in 1000 yr return period). 3

4 BDM SEISMIC DESIGN Earthquakes arise from the movement of underlying bedrock. Ground motion resulting from the movement of underlying bedrock can be amplified or dampened by the overlying soil profile. Designers shall analyze soil borings to identify overlying soil profiles that can amplify ground Motion Propagating from underlying rock according to LRFD Bridges located in Class D, E and F may require additional design considerations as noted in BDM Sections

5 BDM SEISMIC DESIGN Earthquakes arise from the movement of underlying bedrock. Ground motion resulting from the movement of underlying bedrock can be amplified or dampened by the overlying soil profile. Designers shall analyze soil borings to identify overlying soil profiles that can amplify ground Motion propagating from underlying rock according to LRFD Bridges located in Class D, E and F may require additional design considerations as noted in BDM Sections

6 BDM SEISMIC PERFORMANCE ZONE 1 All bridges in the State of Ohio are located within Seismic Performance Zone 1. Bridges designed according to the Strength and Service Limit States of the AASHTO LRFD Bridge Design Specifications are assumed to have sufficient capacity to resist Seismic Performance Zone 1 design loads applied at the Extreme Limit State. Seismic analysis is not required except as noted in BDM Sections a and b. 6

7 BDM SEISMIC PERFORMANCE ZONE 1 All bridges in the State of Ohio are located within Seismic Performance Zone 1. Bridges designed according to the Strength and Service Limit States of the AASHTO LRFD Bridge Design Specifications are assumed to have sufficient capacity to resist Seismic Performance Zone 1 design loads applied at the Extreme Limit State. Seismic analysis is not required except as noted in BDM Sections a and b. 7

8 BDM SEISMIC PERFORMANCE ZONE 1 (continued) For bridges located in Site Class D, E and F, Designers shall determine the acceleration coefficient, S D1, according to LFRD Eq with F v = 2.4. For Ohio only areas where S 1 >= and Site Class D, E and F exist, will 0.10 <=S D1 <0.15 (See Figure ). 8

9 BDM SEISMIC PERFORMANCE ZONE 1 (continued) For bridges located in Site Class D, E and F, Designers shall determine the acceleration coefficient, S D1, according to LFRD Eq with F v = 2.4. For Ohio only ares where S 1 >= and Site Class D, E and F exist, will 0.10 <=S D1 <0.15 (See Figure ). 9

10 BDM SEISMIC PERFORMANCE ZONE 1 (continued) For bridges founded at locations with 0.10<=S D1 <0.15, the transverse reinforcement requirements at the top and bottom of columns shall be specified in LRFD d and e. If sufficient geotechnical information is not available to determine Site Class, and the project is located in areas where S 1 >=0.042, then Designers shall assume 0.10<=S D1 <0.15. Otherwise, Designers shall assume S D1 <

11 BDM SEISMIC PERFORMANCE ZONE 1 (continued) Designers may use the Seismic maps, LRFD Figure or the USGS US Seismic Design Maps web application to determine S 1 and S D a Minimum Support Length Requirements b Substructure. 11

12 BDM Existing Structures Seismic vulnerability of a structure shall be considered for rehabilitation projects requiring complete deck or superstructure replacements. New substructure units shall be designed in accordance with LRFD , and If sufficient geotechnical information is not available, Designers may assume: A. A s > 0.05 B. S D <

13 BDM Existing Structures Seismic vulnerability of a structure shall be considered for rehabilitation projects requiring complete deck or superstructure replacements. New substructure units shall be designed in accordance with LRFD , and If sufficient geotechnical information is not available, Designers may assume: A. A s > 0.05 B. S D <

14 BDM S General Procedure When required by BDM Section to determine the seismologic data for a project location, Designers may use the Seismic maps, LRFD Figure or the USGS US Seismic Design Maps web application using latitude and longitude. BDM S Site Specific Procedure This procedure is not required for Ohio. 14

15 BDM S General Procedure When required by BDM Section to determine the seismologic data for a project location, Designers may use the Seismic maps, LRFD Figure or the USGS US Seismic Design Maps web application using latitude and longitude. BDM S Site Specific Procedure This procedure is not required for Ohio. 15

16 BDM S Site Class Definitions In the absence of sufficient geotechnical information, Designers shall assume Site Class D for the project soil profile. Designer shall use blow counts corrected to an equivalent rod energy ration of 60%, N 60 as defined in the ODOT Specifications for Geotechnical Explorations for the average SPT blow count. 16

17 BDM S Site Class Definitions In the absence of sufficient geotechnical information, Designers shall assume Site Class D for the project soil profile. Designer shall use blow counts corrected to an equivalent rod energy ration of 60%, N 60 as defined in the ODOT Specifications for Geotechnical Explorations for the average SPT blow count. 17

18 BDM S Seismic Performance Zones All bridges in the state of Ohio are located in within Seismic Performance Zone 1. 18

19 Table Seismic Zones Acceleration Seismic Zone Coefficient S D1 S D1 < < S D1 < < S D1 < < S D1 4 ODOT BDM S All bridges in Ohio are located within Seismic Performance Zone 1. 19

20 What information should be provided by the Geotechnical Engineer? In an ideal world.. (actually, WA State) The Geotechnical Designer is responsible for providing geotechnical/seismic parameters to the Structural Engineers for their use in structural design. Specific elements include: Design ground motion parameters Site Response Input for evaluation of soil-structure interaction (foundation response to seismic loading), earthquake induced earth pressures on retaining walls 20

21 In the real world Designer: What s the Site Class? Geotechnical Engineer: C Designer: Thank-You 21

22 BACKGROUND We need to understand how seismically induced ground motion is transmitted to structural components. Washington Cathedral Virginia Event

23 Epicenter Distance Seismic Performance Zone (1-4) Design Response Spectrum Structure of Interest Fault Rupture (Epicenter) Energy Propagation In Soil Site Class A-E Earthquake Magnitude M Energy Propagation In Rock as ground motion USGS derives MCE Maximum Considered Earthquake M7.7 NMSZ 2% Probability in 50 yrs ( 1 in 2500 yrs) PGA Peak Ground Acceleration And Response Spectrum 7% Probability in 75 yrs (1in 1000 yrs) 23

24 Seismic Hazard How do we define or describe a seismic hazard? Earthquake magnitude? Earthquakes are always classified on a logarithmic scale, usually 1 10, so that 7.0 is 100 times stronger than 5.0 and 8.0 is 1000 times stronger than

25 Distance to Epicenter (500 miles is better for your structure than 5 miles) 25

26 Site Conditions Can have a significant local effect by amplifying ground motion Frequency of Occurrence -Infrequent is best! 26

27 Ohio is on the fringe of the New Madrid Seismic Zone. We get the left-overs from any event that occurs on that system. Leftovers can be a problem - Cincinnati experienced some damage in the 1811/12 event. 27

28 Home Grown Ohio Earthquakes New Madrid Seismic Zone 28

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31 Anna Area 31

32 Note a correlation with the period of peak hydrocarbon (gas and crude oil) extraction in this area. Northwest Ohio

33 Lake and Ashtabula counties Note the correlation with operations of a deep well injection system from mid-1980 s to early 2000 s in this area. 33

34 Here s the Youngstown Area Note correlation with recent fracking and deep well disposal operations post

35 Which is more critical? A magnitude 8.0 earthquake on the New Madrid Fault 500 miles away or a magnitude 5.4 earthquake on an unknown fault in northeast Ohio 3 miles from your new bridge? 35

36 Cornell's Method (1979) Ln A p = * M l * ln (R +25) A p = Peak Ground Acceleration (cm/s 2 ) M l = Magnitude R = distance (km) Note: 289 GMPE equations published for PGA. 36

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38 acceleration factor (xg) METHODOLOGY Spectral Acceleration What is it? Raw Accelerograph Record t (seconds) Convert this to something that can be useful for design Develop a Design Response Spectrum Displays Maximum Acceleration for a range of ground motion periods 38

39 Construction of the Design Spectrum Input: S s PGA Peak Ground Acceleration S s short period Spectral Acceleration S 1 long-period Spectral Acceleration Apply Site Specific Factors PGA S 1 39

40 Site Class A B C D E Soil Profile Name Soil Shear Wave Velocity Average Properties in Top 100 ft Standard Penetration Resistance Soil Undrained Shear Strength E may, together with F, be a special case very weak soils. 40

41 Site Class Soil Profile Name Soil Shear Wave Velocity, v s, (ft/s) A Hard rock v s > 5,000 B Rock 2,500 < v s < 5,000 C Very dense soil and soft rock 1,200 < v s < 2,500 D Stiff Soil Profile 600 < v s < 1,200 E Soft Soil Profile v s < 600 Difficult to measure Can use seismic CPT, seismic refraction or published values, with caution. 41

42 Soil/Rock Type P Wave ft/s S Wave ft/s Site Class Scree, vegetal soil D/E Dry sands C/D/E Wet sands C Saturated shales and clays B/C/D Marls B Saturated shale and sand sections Porous and saturated sandstones B/C A/B Limestones A Dolomite A Coal B Classification Criteria and below 600 E D C B 5000 and above A 42

43 Site Class Soil Profile Name Standard Penetration penetration Resistance, N N A Hard rock NA B Rock NA C Very dense soil and soft rock N > 50 D Stiff Soil Profile 15 < N < 50 E Soft Soil Profile N < 15 43

44 Site Class Soil Profile Name Soil Undrained Shear Strength, s u, (psf) A Hard rock NA B Rock NA C Very dense soil and soft rock s u > 2,000 D Stiff Soil Profile 1,000 < s u < 2,000 E Soft Soil Profile s u < 1,000 44

45 Site Class Soil Profile Name Soil Shear Wave Velocity Standard Penetration Resistance Soil Undrained Shear Strength Any profile with more than 10 ft of soil having the following characteristics: E - 1. Plasticity index PI > 20, 2. Moisture Content w > 40%, and 3. Undrained shear strength s u < 500 psf 45

46 Site Class Soil Profile Name Soil Shear Wave Velocity Standard Penetration Resistance Soil Undrained Shear Strength Any profile containing soils having one or more of the following characteristics: F - 1. Soils vulnerable to potential failure or collapse under seismic loading such as liquefiable soils, quick and highly sensitive clays, collapsible weakly cemented soils. 2. Peats and/or highly organic clays (H > 10 ft of peat and/or highly organic clay where H = thickness of soil) 3. Very high plasticity clays (H > 25 ft with plasticity index PI > Very thick soft/medium stiff clays (H > 120 ft) 46

47 Site Class PGA 1 < >0.50 S 1 s < >1.25 A B C D E F Use straight-line interpolation for intermediate values. Site-specific geotechnical investigation and dynamic site response analysis should be performed for all sites in Site Class F. Tables and

48 Site Class Spectral Acceleration Coefficient At Period 1.0 sec (S 1 ) 1 < A B C D E F Use straight-line interpolation for intermediate values. Site-specific geotechnical investigation and dynamic site response analysis should be performed for all sites in Site Class F. 48

49 (1) For the purpose of determining the Seismic Hazard Level for Site Class E Soils (Article ) the value of F v and F a need not be taken larger than 2.4 and 1.6, respectively, when S 1 is less than or equal to 0.10 and S s is less than

50 (2) For the purposes of determining the Seismic Hazard Level for Site Class F Soils (Article ) F v and F a values for Site Class E soils may be used with the adjustment described in Note 1 above. 50

51 Site PGA 1 < >0.50 Class S 1 s < >1.25 A B C D E F Use straight-line interpolation for intermediate values. ODOT BDM use site factors for Site Class D for Site Class E and F (S ). 51

52 Spectral Acceleration Coefficient Site At Period 1.0 sec (S 1 ) 1 Class < A B C D E F Use straight-line interpolation for intermediate values. ODOT BDM use site factors for Site Class D for Site Class E and F (S ). 52

53 METHODOLOGY Construction of Design Response Spectrum Using LRFD Figures through -21 PGA = 0.3s S s = 0.5s S 1 = 0.2s Site Class = C 53

54 Elastic Seismic Coefficient (C sm ) (g) A s S DS 1. A s = F pga * PGA e.g. 1.1 * 0.3 = 0.33 PGA - peak ground acceleration - rock - Site Class B F pga - Site factor at zero period 2. S DS = F a * S s e.g. 1.2 * 0.5 = 0.60 F a - Site Factor - short period range S s - Short period acceleration (0.2 sec) 3. S D1 = F v * S 1 e.g. 1.6 * 0.2 = 0.32 F v - Site Factor - long period range S 1 - Short period acceleration (1.0 sec) S D1 4. T s = S D1 /S DS e.g / 0.60 = 0.53 T s = corner period 5. T o = 0.2*T s e.g. 0.2 * 0.53= T o = reference period Construction of Design Response Spectrum T o Period, T m (sec) T s 54

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57 Ground Motion (PGA, S s, S 1 ) + Site Classification (A-F) + Site Factors (F pga, F s, F v ) Design Response Spectrum Seismic Zone Determination (1-4) 57

58 BDM Fig S D1 = S 1 * F V if S D1 > 0.10 and F v = 2.4 S 1 > 0.10/2.4 = (4.2%g) 58

59 All bridges in Ohio are in Seismic Performance Zone 1 S D1 <= 0.15 Determine Site Class designer shall analyze soil borings.. LRFD Is Site Class A, B or C? NO YES Consider only BDM a BDM b YES Is Site Class F? Consider BDM a, b LRFD d, e NO Is S 1 >=0.042? (Fig ) YES NO BDM a YES OR UNKNOWN Is 0.1<S D1 <0.15? (use F v =2.4) NO 59

60 WORKED EXAMPLES 60

61 EXAMPLE 1 Site Classification Lucas Bridge Depth N 60 B-093 B

62 Summary by Layer i N i from to d i d i /N i Σ d i /N i 9.89 Σ d i 100 Class Σ d i /Σd i /Ave N i 10 E Note: N i should be based on the N60 not raw N values (BDM S ). Lucas Bridge

63 Lucas Bridge Depth s u B-093 B

64 Summary by Layer i su i from to d i d i /su i Σ d i /su i Σ d i 100 Class Σ d i /Σd i /su i Ave su i 1.22 D Note: For Cohesive soils, S u is more reliable than N. Lucas Bridge

65 Lucas Bridge Design Response Spectrum 65

66 EXAMPLE 2 Fairfield County, Ohio 66

67 Forward Abutment Rock (Sandstone) 2600 < V s < 5900 ft/s B S D1 = Rear Abutment 50 ft embankment + overburden 50 ft rock (sandstone) i V si from to d i d i /su i Σd i 100 Class Σd i /Σd i /su i S D1 = Ave su i 833 D Σd i /su i 0.12 Fairfield County, Ohio 67

68 EXAMPLE 3 Mill Creek Crossing Abutment Rock (Shale) V s = 1700 ft/s Soil Class C Piers soft clay Soil Class E (AASHTO) Soil Class D (ODOT) Soil Class S D1 C (<0.1) D (>0.1) (SPZ 1-BDM) E (SPZ 2 AASHTO) HAM-Western Hills Viaduct 68