Seismic and Geotechnical Updates

2013 California Building Code

Zia Zafir, PhD, PE, GE

Senior Principal Engineer

San Francisco Geo-Institute Workshop, October 16, 2014

Overview

Introduction

Background

Chapters 16 & 16 A (Soil lateral and eq. loads)

Chapters 18 & 18 A (Soils and Foundations)

Summary

Introduction

2013 CBC is based on 2012 IBC, and ASCE 7-10

Will discuss changes relevant to seismic and geotechnical engineering

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

Estimation of Ground Motions

Ground Motions

Code-BasedChapter 1613, 1613A

Chapter 11 of ASCE 7-10

USGS Web Application

For DSA & OSHPD

SDC = E & F

Ground Motion

Hazard AnalysisPer Chapter 21 of ASCE 7-10

As Modified by Section 1803A.6

Site-Specific Ground

Motions Hazard and Site

Response AnalysisPer Chapter 21 of ASCE 7-10

No Site Class F (exceptions) Yes

NoYes

Major Changes

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

Major ChangesTable 1604.5: Occupancy Category ⇒⇒⇒⇒ to Risk Category

MCE is defined in two waysGeometric Mean (MCEG)

Risk-Targeted (MCER)

PGA is associated with MCE and ≠ SDS/2.5

New table for factor FPGA to estimate site modified PGA consistent with MCE

Long-period transition period (TL) 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

Table 1604.5

Table 1604A.5 has

some additional language

Maximum Considered Earthquake

MCE is now defined in two ways

Maximum Considered Earthquake Geometric Mean (MCEG) Peak Ground

Acceleration: Geometric mean peak ground acceleration and without adjustment for

targeted risk. The MCEG peak ground acceleration adjusted for site effects (PGAM) is

used for evaluation of liquefaction, lateral spreading, seismic settlements, and other

soil related issues. General procedures for determining PGAM are provided in Section

11.8.3; site-specific procedures are provided in Section 21.5.

Risk-targeted Maximum Considered Earthquake (MCER) 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 MCER Ground Motion values

are provided in Section 11.4.3; site-specific procedures are provided in Sections 21.1

and 21.2.

Risk Targeted – MCER

Mapped values of SS and S1 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

Fa and Fv values are same as before

SMS, SM1, SDS, and SD1 equations are same as

before

Background

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 50th percentile value of a set of geometric means computed from the as-recorded orthogonal horizontal motions rotated through all possible period-independent non-redundant rotation angles (Boore et al., 2006).

Geometric Mean vs. Maximum

Rotated Motions

OSHPD and DSA adopted maximum rotated motion concept for site-specific ground motion hazard analysis in 2010 CBC

Duzce Earthquake – Bolu Station

Loma Prieta Earthquake – LGPC

Landers Earthquake – Joshua Tree

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

Maximum Rotated MotionsPeriod (sec) ln(SAmaxRot/SAGMRot) SAmaxRot/SAGMRot

0.01 0.184 1.202

0.05 0.181 1.198

0.10 0.178 1.195

0.15 0.187 1.206

0.20 0.196 1.217

0.25 0.204 1.226

0.30 0.212 1.236

0.40 0.219 1.245

0.50 0.225 1.252

0.75 0.225 1.252

1.00 0.237 1.267

1.50 0.237 1.267

2.00 0.240 1.271

2.50 0.244 1.276

3.00 0.247 1.280

4.00 0.256 1.292

Maximum Rotated MotionsASCE 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.

Comparison - Probabilistic

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Period (sec)

Sp

ec

tra

l A

cc

ele

rati

on

(g

)

NGA-max rotated NGA

Probabilistic Spectra for 2% in 50 Years

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 22-17 and 22-18)

Risk Targeted – Design Spectrum

SS

Figure 22-1

S1

Figure 22-2

TL

Figure 22-12

Chapter 16Seismic Design Category

Chapter 16ASeismic Design Category

Site Class E Issues

Be Careful for the Site Class E

Sometimes the SM1 value exceeds the SMS value

Example:Site in Rialto –Latitude: 34.083N Longitude: 117.400WSS = 1.628 S1 = 0.707Site Class E:Fa = 0.9 Fv = 2.4SMS = 1.465 SM1 = 1.697 ≤ SMS

To = 0.232 Ts = 1.16

Code-Based – Site Class E

Site Class E Issues

Site Class E

ASCE 7-10 using USGS Web Application My Calculations

USGS-Report Sections 11.4.5 and 11.4.6 (Page 3-4 of the printout)

Site No. Latitude Longitude SS S1 Fa Fv SMS SM1 Ts T0 SMS SM1 Ts T0

1 32.7651 -117.2258 1.219 0.468 0.9 2.4 1.097 1.123 1.024 0.205 1.081 1.097 1.015 0.203

2 33.7444 -118.2776 1.699 0.660 0.9 2.4 1.529 1.583 1.035 0.207 1.498 1.529 1.021 0.204

3 33.7606 -118.2359 1.722 0.663 0.9 2.4 1.550 1.592 1.027 0.205 1.527 1.550 1.015 0.203

4 33.969 -118.446 1.528 0.638 0.9 2.4 1.375 1.530 1.113 0.223 1.290 1.375 1.066 0.213

5 37.4098 -122.0203 1.500 0.600 0.9 2.4 1.350 1.440 1.067 0.213 1.301 1.350 1.038 0.208

6 37.5835 -122.3191 1.777 0.823 0.9 2.4 1.600 1.976 1.235 0.247 1.417 1.600 1.129 0.226

7 37.7672 -122.39 1.500 0.600 0.9 2.4 1.350 1.441 1.067 0.213 1.301 1.350 1.038 0.208

8 37.8129 -122.3128 1.500 0.600 0.9 2.4 1.350 1.440 1.067 0.213 1.301 1.350 1.038 0.208

9 37.954 -122.491 1.500 0.600 0.9 2.4 1.350 1.440 1.067 0.213 1.301 1.350 1.038 0.208

10 38.2285 -122.615 1.677 0.662 0.9 2.4 1.510 1.588 1.052 0.210 1.467 1.510 1.029 0.206

Geometric Mean MCEG and PGAM

PGAM is used for liquefaction, lateral spreading, and seismic settlement

PGAM is associated with MCE and not 2/3 MCE

Can be estimated either using site-specific methods of Chapter 21 or the following equation

PGAM = FPGA PGA

where PGA is the mapped value

FPGA is the site coefficient (same as Fa)

MCEG - PGA

Figure 22-7

Geometric Mean MCEG and PGAM

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

Impacts – Liquefaction

2010CBC 2013CBC

Impacts on Seismic Design

Parameter

Depending on the location, SDS and SD1 may

increase or decrease

For low to moderate seismicity areas, SDS

and SD1 will be probably higher

Impacts – Ground Motion

2010 2013 % Change

SDS 0.459 0.534 16.3

SD1 0.285 0.318 11.6

PGA 0.184 0.302 64.1

2010 2013 % Change

SDS 1.000 1.000 0

SD1 0.630 0.601 -4.6

PGA 0.400 0.529 32.3

2010 2013 % Change

SDS 0.524 0.568 8.4

SD1 0.311 0.355 14.1

PGA 0.210 0.309 47.1

2010 2013 % Change

SDS 0.918 1.000 8.9

SD1 0.499 0.553 10.8

PGA 0.367 0.500 36.2

USGS Web Applicationhttp://earthquake.usgs.gov/hazards/designmaps/usdesign.php

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)

Section 1616.10.2 & 1616A.1.3

(New)

Modify ASCE 7 Section 11.4.7 by adding the

following

For buildings assigned to Seismic Design Category Eand F, or when required by the buildings official, aground motion hazard analysis shall be performed inaccordance with ASCE 7 Chapter 21, as modified bySection 1803A.6 of this code.

Section 1616.10.11 &

1616A.1.12 (New)

Replace ASCE 7 Section 12.8.1.3 by the following

12.8.1.3 Maximum SS value in determination of CS.For regular structures five stories or less above thebase, as defined in Section 11.2 and with a period, T, of0.5 s or less, CS is permitted to be calculated using thelarger of either SS = 1.5 or 80 percent of the value of SS

determined per Section 11.4.1 or 11.4.7

Major ChangesChapters18 & 18A

Section 1803.5.11 & 1803A.5.11

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.

2010 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. Total and differential

settlement.

4. Surface displacement due

to faulting or seismically

induced lateral spreading or

lateral flow

Section 1803.5.12 & 1803A.5.12

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 SDS/2.5, where SDS

is determined in accordance with

Section 1613.5.4.

2010 CBC 2013 CBC

1. 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 11.8.3 of

ASCE 7.

Section 1804.3.1 (New)2013 CBC

[HCD 1] Construction plans. Construction plans shallindicate how the site grading or drainage system willmanage all surface water flows to keep water fromentering buildings in accordance with the CaliforniaGreen Building Standards Code (CALgreen), Chapter 4,Division 4.1

Section 1803A.1

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.

2010 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.

Section 1803A.2

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.

2010 CBC 2013 CBC

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.2.

2. 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.

Section 1803A.6

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.

2010 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) 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.

Section 1803A.8 (New)2013 CBC

Geotechnical peer review. [DSA-SS and DSA-SS/CC]When alternate foundations designs or groundimprovements are employed or where slope stabilizationis required, a qualified peer review by a California-licensed geotechnical engineer, in accordance withSection 3422, may be required by the enforcementagency. In Section 3422, where reference is made tostructural or seismic-resisting system, it shall bereplaced with geotechnical, foundations, or groundimprovement, as appropriate.

Section 1807A.2.2

Design lateral soil loads. Retaining

walls shall be designed for the lateral

soils loads determined by a

geotechnical investigation in

accordance with Section 1803A.

2010 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 and

shall not be less than eighty percent of

the lateral soil loads determined in

accordance with Section 1610A.

Section 1809A.15 (New)2013 CBC

Grade beams: [DSA-SS, DSA-SS/CC] For structuresassigned to Seismic Design Category D, E or F, gradebeams in shallow foundations shall comply with Section1810A.3.12

ASCE 7-10 Overview

Introduction

Chapter 21

Chapter 16

Summary

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

Chapter 21 – Site-Specific Ground Motion Procedure for Seismic Design

Analysis is needed if required by Section 11.4.7 of ASCE 7-10

ASCE 7-10 Section 11.4.7

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 20.3.1 is

applicable. For seismically isolated structures and for structures with damping

systems on sites with S1 greater than or equal to 0.6, a ground motion hazard

analysis shall be performed in accordance with Section 21.2.

Exception in Section 20.3.1

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 11.4-1 and 11.4-2.

Chapter 21 – Site-Specific Ground Motion Procedure for Seismic Design

2013 CBC Section 1616.10.2 and 1616A.1.3

Modify ASCE 7 Section 11.4.7 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.

2013 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.

The requirements of Section 21.1 shall be satisfied

where site response analysis is performed or required by

Section 11.4.7. The analysis shall be documented in a

report.

Section 21.1 – Site Response Analysiso Base ground motion for MCER for bedrock or equivalent using

GMHAo Site model. For deep soil sites, at least to Site Class Do Site response using nonlinear or equivalent-linear methodso At least five recorded or simulated scaled time historieso Ratios of surface ground motions to input ground motionso Recommended MCER spectrum shall not be less than MCER

spectrum of the base motion multiplied by average surface-to-base ratio

Section 21.1 – Site Response Analysis

The requirements of Section 21.2 shall be satisfied

where a ground motion hazard analysis is performed or

required by Section 11.4.7.

Section 21.2 – Ground Motion Hazard Analysis

MCER is based on both probabilistic and deterministic

Based on the direction of maximum horizontal motion

Probabilistic = 2% probability in 50 years

Deterministic = 84th 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 MCER = lesser of probabilistic and deterministic

Section 21.2 – Risk-Targeted Maximum Considered Earthquake (MCER) Ground Motion Hazard Analysis

Deterministic Lower Limit

Site Specific – Risk-Targeted Methods

Method 1: At each spectral period MCER is taken as

MCER = Probabilistic MCE x CR

where CR is the risk coefficient and shall be obtained using the values of CRS and CR1 from Figures 22-17 and 22-18

CR = CRS for periods ≤ 0.2 s

CR = CR1 for periods ≥ 1.0 s

CR = 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

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 22-17 and 22-18)

Caution ASCE 7-10 wrongly mentions that maps are Figures 22-2 and 22-3

Risk Coefficient Maps

Risk Coefficient Maps

Design spectral acceleration at any period shall be taken

as

Sa = 2/3 SaM

Where SaM is spectral response acceleration obtained

from Section 21.1 or 21.2.

Sa shall not be less than 80% of Sa determined from

Section 11.4.5

For Site Class F requiring site response analysis, Sa

shall not be less than 80% of Sa for Site Class E

determined from Section 11.4.5

Section 21.3 – Design Response Spectrum

SDS 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.

SD1 shall be taken as larger of the value at 1 s or two

times the value at 2 s.

SMS and SM1 shall be taken as 1.5 times SDS and SD1

SMS and SM1 shall not be less than 80% of values

obtained from 11.4.3

SDS and SD1 shall not be less than 80% of values

obtained from 11.4.4

Section 21.4 – Design Acceleration Parameters

Section 21.5 - MCEG PGAM

Site specific MCEG PGAM = lesser of probabilistic and

deterministic

Probabilistic = geometric mean 2% probability of

exceedance in 50 years

Deterministic = geometric mean 84th percentile motions

from the controlling fault

Deterministic should not be less than 0.5FPGA where

FPGA is estimated using Table 11.8-1 with value of PGA

is taken as 0.5g

Site specific PGAM should not be less than 80% of the

value using equation 11.8-1

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

Chapter 16 – Seismic Response History

ProcedureFor 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 fault-

parallel directions of the causative fault and shall be scaled so that the

average of the fault-normal components is not less than the MCER response

spectrum for the period range from 0.2T to 1.5T.

SummaryGround motion parameters are based on Risk-targetedMaximum Considered Earthquake (MCER)

Spectral accelerations are in the direction of maximumhorizontal ground motions

PGA is associated with the maximum consideredearthquake with geometric mean motions and are muchhigher than before

Site Class shall be established using Chapter 20 ofASCE 7-10

Ground motion parameters have increased or decreasedbased on the location

Dynamic earth pressures are required for retaining wallswith backfill height of 6 feet or more

Note changes in ASCE 7-10 especially in Chapter 21

ASCE 7-16

New tables for Fa, Fv, and Fpga

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 SM1 > SMS

Procedure to develop vertical spectrum in

Chapter 12

Questions?