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© 2016 HDR, Inc., all rights reserved.

Lila Leon, PE, PhD

THE IMPORTANCE OF SITE CLASS DETERMINATION IN SEISMIC DESIGN

TOPIC OVERVIEW Motivation Site Class Concept Seismic Code Practices Practices Introducing Errors Practices Allowing Engineering Judgment Summary

Motivation Site Class Concept Seismic Code Practices Practices Introducing Errors Practices Allowing Engineering Judgment Summary

TOPIC OVERVIEW

Motivation Common practices introducing erroneous

site class determination - uncertainties in site response and soil amplification

Practices in modern seismic codes allowing for engineering judgement – modification of site class

TOPIC OVERVIEW Motivation Site Class Concept Seismic Code Practices Practices Introducing Errors Practices Allowing Engineering Judgment Summary

AmplificationSite Class is used to determine the appropriate site

amplification factors to transform the earthquake ground motion at the bedrock level to the ground

motion at the ground surface.

Site Class Concept

Parameters that control the response at the surface of soil deposits

Parameters that affect the amplification factors

(frequency content and amplitude of ground motion at bedrock, geometry and material properties of the soil layers

above the bedrock.)

Spectral Frequency/Period-dependent amplification factors

Site Class Concept

(From Dobry et al. 2000)

SAsoil = f(T)*SArock

Complex relationship between rock outcrop and soil surface motions depends ono fundamental period of the soil profile,o degradation of soil stiffness and damping with stiffness,o the characteristics of the rock outcrop motion.

Site parameters-proxies that provide information on the stiffness and thickness of the soil column include:o Depth to bedrock,o Average Vs over bedrock depth,o Average Vs over 100 feet,o Vs of bedrock,o Velocity contrast (Vsrock/Vssoil)

Site Class Concept

Impedance contrast between bedrock velocity and sediment velocity most satisfactorily reproduces the characteristics of the site amplification. However, it is expensive to measure, especially for thick soil deposits

Preferred to measure the soil stiffness in the top 100 feet, i.e. Vs,100.

Site Class Concept

0 10 20 30 40-600

-400

-200

0

200

400

600

Time (sec)

Acce

lera

tion

(cm

/sec

2 )

Surface Motion

0 10 20 30 40-600

-400

-200

0

200

400

600

Time (sec)

Acce

lera

tion

(cm

/sec

2 )

Input Motion

0.01 0.1 1 10 1000.4

1

2

3

Frequency (Hz)

Site

Res

pons

e

Transfer Function

TOPIC OVERVIEW Motivation Site Class Concept Seismic Code Practices Practices Introducing Errors Practices Allowing Engineering Judgment Summary

Seismic Code Practices

Soil Stiffness within the top 100 feet of the subsurface profile

NEHRP Recommended

Seismic Provisions for New Buildings and

other Structures

ASCE 7 Standard

International Building Code

(IBC)

Seismic Code Practices

Site Classification Criteria(Table 20.3-1 of ASCE 7-05, ASCE 7-10 and ASCE 7-16)

or Vs,30

Automatic Class ESites with more than 10 feet of soft soils mustbe automatically classified as Site Class E.

Generic ApproachClass D assigned by default…due to insufficient site characterization.

Site Class FSites with problematic soils are assigned a SiteClass F and a site response may be required.

Site Classes A through DIf profile is not classified as either E or F, itis evaluated for Site Classes A through D.

Seismic Code Practices

Calcu

late S

tiffne

ss w

ithin

the to

p 100

fee

t of th

e sub

surfa

ce pr

ofile method

(Method A in AASHTO LRFD)

method (Method B in AASHTO LRFD)

and ̅ method(Method C in AASHTO LRFD)

∑

∑

∑

∑

∑̅

∑

0 100 100

100

100 ) 5

100 )

Seismic Code Practices

SMS = Fa Ss SM1 = Fv S1

Earthquake Demands

Spec

tral R

espo

nse

Acce

lerati

on, S

a(g

)

Period, T (sec)23

23

https://asce7hazardtool.online/Seismic Code Practices

Practices Introducing Errors

ASCE 7-16 C20.3 “encourages” theuse of the Vs method indicating thatthere is a better correlation betweenshear wave velocity and generic sitefactors. Site factors tend to besmaller if the Vs method is used.

MethodPreferred method for seismic site classdetermination is with shear wavevelocity measurements.

Ignoring the uncertainty in thecorrelations of soil/rock stiffness withSPT-N blows introduces can introducesignificant error in the site classdetermination and therefore site effects.

∑

∑

…approach in SCDOT GDM v2.0Site amplification factors are determineddirectly from the shear wave velocity averagewithin the depth of interest H (V*s,h) of thesoil/rock profile. The requirement for a siteclass determination no longer exists.

Practices Introducing Errors

ExampleMethod: SPT-N or Vs

Practices Introducing Errors

∑

∑10 ,

∑

∑1,335 /

ExampleMethod: SPT-N or Vs

Practices Introducing ErrorsExample (continue)

Different Methods

Different Answers

Practices Introducing ErrorsExample (continue)

Different Methods

Different Answers

Changing the classification from Site Class D to a Site Class E constitutes an 85% increase in site amplification factor for structural periods exceeding 0.5 sec resulting in 85% higher design load for most typical bridge structures.

85% higher

Practices Introducing Errors Influence of Rock Depth – Impedance Effects

, / , , / ,

correlates better with amplification factors

(Pitilakis, 2004)

Recent research has highlighted that the current practice of averaging the SWV of the top 100 feet layered profile does not correlate well with amplification factors.

vs.

When there is a sharp impedance contrast (change in soils stiffness or Vs) in the column the computed Vs,100 is not representative of the site response.

Practices Introducing Errors Influence of Rock Depth – Shallow Rock Sites

Depending on the depth to rock and impedance contrast, the aggravation of site effects can be different.

Apply and average boring dataIf the 100 ft. of cumulative thickness include bedrock,methods B and C should not include values for rockhigher than the maximum values for SPT blow countsand undrained shear strength specified for soil.

Apply and average Vs dataIf the depth of soil/rock interface is not considered in the 100-ft, Vs-based site classification may lead to stiffer site classes, underestimating the site effects.

100 5

Other factors to consider…Site amplification schemes originated from the high seismicity regions of the Western United States with bedrock not evident within or near 100 feet depth. Code-based Fa factors reflect the western US geology and ground motions.

Adopting a 100-ft based site classification in shallowbedrock sites may result in erroneous siteclassification and seismic design response spectraparameters

Practices Introducing Errors

SCDOT GDM v2.0SCDOT no longer requires a site class as it is basing the site amplification factors directly on the shear wave velocities as determined at the site. GEOR evaluates the measured Vs profile for high variations that could potentially overestimate the V*s,h. SCDOT uses V*s,h to calculate site amplification factors.Methodology defines amplification factors as continuous functions of shear wave velocity which is more consistent with the concept of soil profile stiffness representation.

Influence of Rock Depth – Other Approaches

Practices Introducing Errors Influence of Rock Depth – Other Approaches

NYCDOT SCGBDZ 2016The 2016 New York City Department of Transportation Seismic Design Guidelines for Bridges in Downstate Region differentiates the sites in “soil” (H>10 ft) or “rock”, accounts for the thickness and stiffness of the soil layers up to rock (VHR, A, B), and provides generic response spectra for design of critical and non-critical bridges in the region.Methodology proposes a new classification scheme considering thickness and average stiffness of the soil layers rather than averaging shear wave velocities of top 100 ftlayers.

Practices Introducing Errors

V 720fps → 2,200fps

ExampleShallow Bedrock

Practices Introducing Errors

V 720fps → 2,200fps

ExampleShallow Bedrock

,∑

∑964 /

,∑

∑2,682 /

Practices Introducing Errors

ExampleShallow Bedrock

Underestimation of site effects

Practices Introducing Errors

ExampleShallow Bedrock

Underestimation of site effects

120% higher

Changing the classification from Site Class B to a Site Class D constitutes an 120% increase in site amplification factor for structural periods exceeding 0.5 sec resulting in 120% higher design load for most typical bridge structures.

10

, 1,335

Underestimation or overestimation of

site effects

Practices Introducing Errors

Effect of Liquefaction

An erroneous estimation of liquefaction hazard can occur

due to an inaccurate site classification

Practices Allowing Engineering Judgement

Modification of Site Class

The structural cost can substantially increase as the site class increases (A → E).The two approaches for reducing the seismic loads and lowering cost are:

Site-Specific Seismic Response AnalysisThis approach for reducing the seismic loads on a structure is established in the ASCE 7 codes. However, the maximum reduction in design ground motions cannot exceed 20% of those determined following the code-based amplification factors. Is the SSRA worth performing?

Is the Site-Specific Response Analysis worth performing?

Performing a site-specific study could result inhigher seismic loads than those determined usingthe generic code based amplification factors. Doesthe Code enforce the higher seismic loads overthe ones from the generic approach?

Practices Allowing Engineering Judgement

Modification of Site Class

Ground ModificationCan be used to increase SPT-N, Su, Vs such as the resulting Site Class is reduced. Routinely performed on sites classified as Site Class F to mitigate liquefaction. Testing for performance verification shall be used to measure the properties of the improved site.

Ground modification as a measure toreduce the site class and improve thesite is not acknowledged in ASCE 7codes. Therefore, engineering judgmentis needed for determining the lateralextents of the ground improvement.

The structural cost can substantially increase as the site class increases (A → E).The two approaches for reducing the seismic loads and lowering cost are:

Summary

Essential part of site classification. Implications of erroneous determination of site

effects. Common practices and parameters not captured by current code-based factors.

Consider the worth of an added cost to the subsurface exploration program for a more favorable site classification.

Consult with structural engineers to discuss implications on earthquake demands/design, construction practices, and cost.

QUESTIONS?