Seismic and Geotechnical Updates 2013 California Building Code. Zia Zafir, PhD, PE, GE Senior Principal Engineer

Transcription

1 Seismic and Geotechnical Updates 2013 California Building Code Zia Zafir, PhD, PE, GE Senior Principal Engineer San Francisco Geo-Institute Workshop, October 16, 2014

2 Overview Introduction Background Chapters 16 & 16 A (Soil lateral and eq. loads) Chapters 18 & 18 A (Soils and Foundations) Summary

3 Introduction 2013 CBC is based on 2012 IBC, and ASCE 7-10 Will discuss changes relevant to seismic and geotechnical engineering

4 Chapters 16 and 16A (Earthquake Loads) Scope: buildings, structures and community college buildings regulated by DSA-Structural Safety/Community Colleges (DSA-SS/CC) and OSHPD

5 Estimation of Ground Motions Ground Motions No Site Class F (exceptions) Yes Yes For DSA & OSHPD SDC = E & F No Site-Specific Ground Motions Hazard and Site Response Analysis Per Chapter 21 of ASCE 7-10 Ground Motion Hazard Analysis Per Chapter 21 of ASCE 7-10 As Modified by Section 1803A.6 Code-Based Chapter 1613, 1613A Chapter 11 of ASCE 7-10 USGS Web Application

6 Major Changes Chapters16 & 16A (ASCE 7-10 Chapter 11)

7 Major Changes Table : Occupancy Category to Risk Category MCE is defined in two ways Geometric Mean (MCE G ) Risk-Targeted (MCE R ) PGA is associated with MCE and S DS /2.5 New table for factor F PGA to estimate site modified PGA consistent with MCE Long-period transition period (T L ) maps Risk coefficients maps Site-specific ground motion analysis is required for Seismic Design Category E &F for OSHPD and DSA-SS/CC Dynamic earth pressures for retaining walls supporting more than 6 feet of fill Engineering Geologic Reports Geohazard Reports

8 Table Table 1604A.5 has some additional language

9 Maximum Considered Earthquake MCE is now defined in two ways Maximum Considered Earthquake Geometric Mean (MCE G ) Peak Ground Acceleration: Geometric mean peak ground acceleration and without adjustment for targeted risk. The MCE G peak ground acceleration adjusted for site effects (PGA M ) is used for evaluation of liquefaction, lateral spreading, seismic settlements, and other soil related issues. General procedures for determining PGA M are provided in Section ; site-specific procedures are provided in Section Risk-targeted Maximum Considered Earthquake (MCE R ) Ground Motion Response Acceleration: Ground motions for the orientation that results in the largest maximum response to horizontal ground motions and with adjustment for targeted risk. General procedures for determining the MCE R Ground Motion values are provided in Section ; site-specific procedures are provided in Sections 21.1 and 21.2.

10 Risk Targeted MCE R Mapped values of S S and S 1 are for maximum rotated motions with adjustment for targeted risk The geometric mean spectral acceleration values from USGS have been factored by 1.1 for 0.2s or 1.3 for 1.0s to convert (approximately) to max direction. Site Class definitions use Chapter 20 of ASCE 7-10 F a and F v values are same as before S MS, S M1, S DS, and S D1 equations are same as before

11 Background

12 Geometric Mean vs. Maximum Rotated Motions All GMPEs are based on geometric mean (GM) values of 2 orthogonal horizontal motions GM = (h1 x h2) NGA is based on GMRotI50 GMRotI50 is defined as the 50 th percentile value of a set of geometric means computed from the as-recorded orthogonal horizontal motions rotated through all possible periodindependent non-redundant rotation angles (Boore et al., 2006).

13 Geometric Mean vs. Maximum Rotated Motions OSHPD and DSA adopted maximum rotated motion concept for site-specific ground motion hazard analysis in 2010 CBC

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16 Duzce Earthquake Bolu Station

17 Loma Prieta Earthquake LGPC

18 Landers Earthquake Joshua Tree

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20 Maximum Rotated Motions NGAs are based on GMRotI50 For true maximum rotated motions, new relationships are needed using the MaxRotI50 Alternatively, there are ratios of MaxRot to GMRotI Beyer & Bommer (2006) Watson-Lamprey & Boore (2007) Huang, Whittaker, & Luco (2009) USGS used 1.1 and 1.3 ratios

21 Maximum Rotated Motions Period (sec) ln(sa maxrot /SA GMRot ) SA maxrot /SA GMRot

22 Maximum Rotated Motions ASCE 7-10 Supplement No. 1 If the spectral response accelerations predicted by the attenuation relations do not represent the maximum response in the horizontal plane, then the response spectral accelerations computed from the hazard analysis shall be scaled by factors to increase the motions to the maximum response. If the attenuation relations predict the geometric mean or similar metric of the two horizontal components, then the scale factors shall be: 1.1 for periods less than or equal to 0.2 sec; 1.3 for a period of 1.0 sec., and, 1.5 for periods greater than or equal to 5.0 sec., unless it can be shown that other scale factors more closely represent the maximum response, in the horizontal plane, to the geometric mean of the horizontal components. Scale factors between these periods shall be obtained by linear interpolation.

23 Comparison - Probabilistic 1.8 Spectral Acceleration (g) Probabilistic Spectra for 2% in 50 Years Period (sec) NGA-max rotated NGA

24 Risk Target Adjustment Probabilistic ground motions are adjusted for targeted risk in the following manner Risk-Targeted GM = 2% in 50 years UHS x risk coefficient Risk coefficient maps are provided (Figures and 22-18)

25 Risk Targeted Design Spectrum

26 S S Figure 22-1

27 S 1 Figure 22-2

28 T L Figure 22-12

29 Chapter 16 Seismic Design Category

30 Chapter 16A Seismic Design Category

31 Site Class E Issues Be Careful for the Site Class E Sometimes the S M1 value exceeds the S MS value Example: Site in Rialto Latitude: N Longitude: W S S = S 1 = Site Class E: F a = 0.9 F v = 2.4 S MS = S M1 = S MS To = Ts = 1.16

32 Code-Based Site Class E

33 Site Class E Issues Site Class E ASCE 7-10 using USGS Web Application My Calculations USGS-Report Sections and (Page 3-4 of the printout) Site No. Latitude Longitude S S S 1 F a F v S MS S M1 T s T 0 S MS S M1 T s T

34 Geometric Mean MCE G and PGA M PGA M is used for liquefaction, lateral spreading, and seismic settlement PGA M is associated with MCE and not 2/3 MCE Can be estimated either using site-specific methods of Chapter 21 or the following equation PGA M = F PGA PGA where PGA is the mapped value F PGA is the site coefficient (same as F a )

35 MCE G - PGA Figure 22-7

36 Geometric Mean MCEG and PGAM

37 Impacts on Liquefaction PGA is about 40 70% higher Results for high seismicity areas will not be impacted significantly For low to moderate seismicity areas, the results in terms of liquefaction-induced settlements could be 20-50% higher

38 Impacts Liquefaction 2010CBC 2013CBC

39 Impacts on Seismic Design Parameter Depending on the location, S DS and S D1 may increase or decrease For low to moderate seismicity areas, S DS and S D1 will be probably higher

40 Impacts Ground Motion % Change S DS S D PGA % Change S DS S D PGA % Change S DS S D PGA % Change S DS S D PGA

41 USGS Web Application

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43 Chapter 16 & 16A Section 1614 and 1614A have been changed to 1615 and 1615A Section 1615 and 1615A have been changed to 1616 (Additional requirements for DSA- SS/CC) & 1616A (Modifications to ASCE 7)

44 Section & 1616A.1.3 (New) Modify ASCE 7 Section by adding the following For buildings assigned to Seismic Design Category E and F, or when required by the buildings official, a ground motion hazard analysis shall be performed in accordance with ASCE 7 Chapter 21, as modified by Section 1803A.6 of this code.

45 Section & 1616A.1.12 (New) Replace ASCE 7 Section by the following Maximum S S value in determination of C S. For regular structures five stories or less above the base, as defined in Section 11.2 and with a period, T, of 0.5 s or less, C S is permitted to be calculated using the larger of either S S = 1.5 or 80 percent of the value of S S determined per Section or

46 Major Changes Chapters18 & 18A

47 Section & 1803A CBC 2013 CBC For structures assigned to Seismic Design Category C, D, E or F in accordance with Section 1613, a geotechnical investigation shall be conducted, and shall include an evaluation of all of the following potential geologic and seismic hazards: 1. Slope instability. 2. Liquefaction. 3. Differential settlement. 4. Surface displacement due to faulting or lateral spreading. For structures assigned to Seismic Design Category C, D, E or F in accordance with Section 1613, a geotechnical investigation shall be conducted, and shall include an evaluation of all of the following potential geologic and seismic hazards: 1. Slope instability. 2. Liquefaction. 3. Total and differential settlement. 4. Surface displacement due to faulting or seismically induced lateral spreading or lateral flow

48 Section & 1803A CBC 2013 CBC 1. The determination of lateral pressures on foundation walls and retaining walls due to earthquake motions. 2.The potential for liquefaction and soil strength loss evaluated for site peak ground accelerations, magnitudes and source characteristics consistent with the design earthquake ground motions. Peak ground acceleration shall be permitted to be determined based on a site-specific study taking into account soil amplification effects, as specified in Chapter 21 of ASCE 7, or, in the absence of such a study, peak ground accelerations shall be assumed equal to S DS /2.5, where S DS is determined in accordance with Section The determination of lateral pressures on foundation walls and retaining walls supporting more than 6 feet (1.83 m) of backfill height due to design earthquake ground motions. 2.The potential for liquefaction and soil strength loss evaluated for site peak ground accelerations, earthquake magnitudes and source characteristics consistent with the maximum considered earthquake ground motions. Peak ground acceleration shall be determined based on: 2.1 A site-specific study in accordance with Section 21.5 of ASCE 7; or 2.2 In accordance with Section of ASCE 7.

49 Section (New) 2013 CBC [HCD 1] Construction plans. Construction plans shall indicate how the site grading or drainage system will manage all surface water flows to keep water from entering buildings in accordance with the California Green Building Standards Code (CALgreen), Chapter 4, Division 4.1

50 General. Geotechnical investigations shall be conducted in accordance with Section 1803A.2 and reported in > I I accordance with Section 1803A. 7. The classification and investigation of the soil shall be made under the responsible charge of a California registered geotechnical engineer. All recommendations contained in geotechnical and engineering geology reports shall be subject to the approval of the enforcement agency. All reports shall be prepared and signed by a registered geotechnical engineer and an engineering geologist where applicable. Section 1803A CBC 2013 CBC General. Geotechnical investigations shall be conducted in accordance with Section 1803A.2 and reported in > I I accordance with Section 1803A. 7. The classification and investigation of the soil shall be made under the responsible charge of a California registered geotechnical engineer. All recommendations contained in geotechnical and engineering geology geohazard reports shall be subject to the approval of the enforcement agency. All reports shall be prepared and signed by a registered geotechnical engineer, a certified engineering geologist, and a registered geophysicist, where applicable.

51 Section 1803A CBC 2013 CBC Exception: Geotechnical reports are not required for one-story, wood-frame and light-steel-frame buildings of Type II or Type V construction and 4,000 square feet (371 m2) or less in floor area, not located within Earthquake Fault Zones or Seismic Hazard Zones as shown in the most recently published maps from the California Geological Survey (CGS). Allowable foundation and lateral soil pressure values may be determined from Table 1806A.2. Exception: 1. Geotechnical reports are not required for one-story, wood-frame and light-steel-frame buildings of Type II or Type V construction and 4,000 square feet (371 m2) or less in floor area, not located within Earthquake Fault Zones or Seismic Hazard Zones as shown in the most recently published maps from the California Geological Survey (CGS). Allowable foundation and lateral soil pressure values may be determined from Table 1806A A previous report for a specific site may be resubmitted, provided that a reevaluation is made and the report is found to be currently appropriate.

52 Section 1803A CBC 2013 CBC Exceptions: Reports are not required for one-story, woodframe and light-steel-frame buildings of Type II or Type V construction and 4,000 square feet (371 m2) or less infloor area, not located within Earthquake Fault Zones or Seismic Hazard Zones as shown in the most recently published maps from the California Geological Survey (CGS); nonstructural, associated structural or voluntary structural alterations and incidental structural additions or alterations, and structural repairs for other than earthquake damage. Exceptions: Reports are not required for one-story, woodframe and light-steel-frame buildings of Type II or Type V construction and 4,000 square feet (371 m2) or less infloor area, not located within Earthquake Fault Zones or Seismic Hazard Zones as shown in the most recently published maps from the California Geological Survey (CGS) or in a seismic hazard zones as defined in the Safety Element of the local General Plan; nonstructural, associated structural or voluntary structural alterations and incidental structural additions or alterations, and structural repairs for other than earthquake damage.

53 Section 1803A.8 (New) 2013 CBC Geotechnical peer review. [DSA-SS and DSA-SS/CC] When alternate foundations designs or ground improvements are employed or where slope stabilization is required, a qualified peer review by a Californialicensed geotechnical engineer, in accordance with Section 3422, may be required by the enforcement agency. In Section 3422, where reference is made to structural or seismic-resisting system, it shall be replaced with geotechnical, foundations, or ground improvement, as appropriate.

54 Section 1807A CBC 2013 CBC Design lateral soil loads. Retaining walls shall be designed for the lateral soils loads determined by a geotechnical investigation in accordance with Section 1803A. Design lateral soil loads. Retaining walls shall be designed for the lateral soils loads determined by a geotechnical investigation in accordance with Section 1803A and shall not be less than eighty percent of the lateral soil loads determined in accordance with Section 1610A.

55 Section 1809A.15 (New) 2013 CBC Grade beams: [DSA-SS, DSA-SS/CC] For structures assigned to Seismic Design Category D, E or F, grade beams in shallow foundations shall comply with Section 1810A.3.12

56 ASCE 7-10 Overview Introduction Chapter 21 Chapter 16 Summary

57 Introduction ASCE 7-10 is the basis for IBC 2012 and CBC 2013 CBC 2010 already adopted some of the ASCE 7-10 provisions for DSA and OSHPD

58 Chapter 21 Site-Specific Ground Motion Procedure for Seismic Design Analysis is needed if required by Section of ASCE 7-10 ASCE 7-10 Section A site response analysis shall be performed in accordance with Section 21.1 for structures on Site Class F sites, unless the exception to Section is applicable. For seismically isolated structures and for structures with damping systems on sites with S 1 greater than or equal to 0.6, a ground motion hazard analysis shall be performed in accordance with Section Exception in Section For structures having fundamental periods of vibration equal to or less than 0.5 s, site response analysis is not required to determine spectral accelerations for liquefiable soils. Rather, a site class is permitted to be determined in accordance with Section 20.3 and the corresponding values of Fa and Fv determined from Tables and

59 Chapter 21 Site-Specific Ground Motion Procedure for Seismic Design 2013 CBC Section and 1616A.1.3 Modify ASCE 7 Section by adding the following For buildings assigned to Seismic Design Category E and F, or when required by the buildings official, a ground motion hazard analysis shall be performed in accordance with ASCE 7 Chapter 21, as modified by Section 1803A.6 of this code CBC Section 1803A.6 Modification The three Next Generation Attenuation (NGA) relations used for the 2008 USGS seismic hazard maps for Western United States (WUS) shall be utilized to determine the site-specific ground motion. When supported by data and analysis, other NGA relations, that were not used for the 2008 USGS maps shall be permitted as additions or substitutions. No fewer than three NGA relations shall be utilized.

60 Section 21.1 Site Response Analysis The requirements of Section 21.1 shall be satisfied where site response analysis is performed or required by Section The analysis shall be documented in a report. Section 21.1 Site Response Analysis o Base ground motion for MCE R for bedrock or equivalent using GMHA o Site model. For deep soil sites, at least to Site Class D o Site response using nonlinear or equivalent-linear methods o At least five recorded or simulated scaled time histories o Ratios of surface ground motions to input ground motions o Recommended MCE R spectrum shall not be less than MCE R spectrum of the base motion multiplied by average surface-tobase ratio

61 Section 21.2 Risk-Targeted Maximum Considered Earthquake (MCE R ) Ground Motion Hazard Analysis The requirements of Section 21.2 shall be satisfied where a ground motion hazard analysis is performed or required by Section Section 21.2 Ground Motion Hazard Analysis MCE R is based on both probabilistic and deterministic Based on the direction of maximum horizontal motion Probabilistic = 2% probability in 50 years Deterministic = 84 th percentile from the controlling fault and should not be less than Deterministic Lower Limit Adjustment to probabilistic for targeted risk using one of the two methods Site Specific MCE R = lesser of probabilistic and deterministic

62 Deterministic Lower Limit

63 Site Specific Risk-Targeted Methods Method 1: At each spectral period MCE R is taken as MCE R = Probabilistic MCE x C R where C R is the risk coefficient and shall be obtained using the values of C RS and C R1 from Figures and C R = C RS for periods 0.2 s C R = C R1 for periods 1.0 s C R = linearly interpolated for 0.2 s < period < 1.0 s Method 2: Based on iterative integration of a site-specific hazard curve with a lognormal probability density function representing the collapse fragility

64 Risk Target Adjustment Probabilistic ground motions are adjusted for targeted risk in the following manner Risk-Targeted GM = 2% in 50 years UHS x risk coefficient Risk coefficient maps are provided (Figures and 22-18) Caution ASCE 7-10 wrongly mentions that maps are Figures 22-2 and 22-3

65 Risk Coefficient Maps

66 Risk Coefficient Maps

67 Section 21.3 Design Response Spectrum Design spectral acceleration at any period shall be taken as S a = 2/3 S am Where S am is spectral response acceleration obtained from Section 21.1 or S a shall not be less than 80% of S a determined from Section For Site Class F requiring site response analysis, S a shall not be less than 80% of S a for Site Class E determined from Section

68 Section 21.4 Design Acceleration Parameters S DS shall be taken as the value at 0.2 s but should not be less than 90% of spectral acceleration at any period larger than 0.2 s. S D1 shall be taken as larger of the value at 1 s or two times the value at 2 s. S MS and S M1 shall be taken as 1.5 times S DS and S D1 S MS and S M1 shall not be less than 80% of values obtained from S DS and S D1 shall not be less than 80% of values obtained from

69 Section MCE G PGA M Site specific MCE G PGA M = lesser of probabilistic and deterministic Probabilistic = geometric mean 2% probability of exceedance in 50 years Deterministic = geometric mean 84 th percentile motions from the controlling fault Deterministic should not be less than 0.5F PGA where F PGA is estimated using Table with value of PGA is taken as 0.5g Site specific PGA M should not be less than 80% of the value using equation

70 Chapter 16 Seismic Response History Procedure Not less than 3 suites of motions When 7 motions are used, take the average response When less than 7 motions are used, take the maximum response For 2D analysis, motions should be scaled such that the average value of the response spectra of scaled motions shall not be less than the design response spectrum of the site for periods between 0.2T and 1.5T where T is fundamental period of the structure

71 Chapter 16 Seismic Response History Procedure For 3D analysis, the ground motions shall consist of pairs of horizontal motions. For each pair of horizontal ground motion components, a square root of the sum of the squares (SRSS) spectrum shall be constructed by taking the SRSS of the 5 percent-damped response spectra for the scaled components (where an identical scale factor is applied to both components of a pair). Each pair of motions shall be scaled such that in the period range from 0.2T to 1.5T, the average of the SRSS spectra from all horizontal component pairs does not fall below the corresponding ordinate of the response spectrum used in the design At sites within 3 miles (5 km) of the active fault that controls the hazard, each pair of components shall be rotated to the fault-normal and faultparallel directions of the causative fault and shall be scaled so that the average of the fault-normal components is not less than the MCE R response spectrum for the period range from 0.2T to 1.5T.

72 Summary Ground motion parameters are based on Risk-targeted Maximum Considered Earthquake (MCE R ) Spectral accelerations are in the direction of maximum horizontal ground motions PGA is associated with the maximum considered earthquake with geometric mean motions and are much higher than before Site Class shall be established using Chapter 20 of ASCE 7-10 Ground motion parameters have increased or decreased based on the location Dynamic earth pressures are required for retaining walls with backfill height of 6 feet or more Note changes in ASCE 7-10 especially in Chapter 21

73 ASCE 7-16 New tables for F a, F v, and F pga Additional exceptions for Site Class F in Chapter 20 Complete overhaul of Chapter 16 which allows spectra matching and conditional mean spectrum and additional requirements for near-fault sites Will take care of the issue where S M1 > S MS Procedure to develop vertical spectrum in Chapter 12

74 Questions?