Dr. Praveen K. Malhotra, P.E. Ground Motion Histories for Seismic Design of Kingdom Tower in Saudi Arabia Contact Information: Praveen.Malhotra@StrongMotions.com www.StrongMotions.com 781-363-3003 Dr. Praveen K. Malhotra, P.E. SEAOA Convention, Tucson, Arizona June 18-20, 2015

Outline Engineering Perspective of Probabilistic Seismic Hazard Analysis (PSHA) Ground Motion Histories for Seismic Design of Kingdom Tower in Saudi Arabia

Overview Structural Geotechnical Seismological Geological

Where Will Future Earthquakes Occur?

Where Will Future Earthquakes Occur?

How Big Will They be and When Will They Occur? Mw 6 @ 794 years MRI Mw 6.1 @ 977 years MRI Mw 6.2 @ 1202 years MRI Mw 6.3 @ 1479 years MRI Mw 6.4 @ 1820 years MRI Mw 6.5 @ 2240 years MRI

How Strong Will the Ground Shake During a Magnitude M 7 How Strong Will the Ground Shake During a Magnitude M 7.5 Earthquake at 20 km?

How Strong Will the Ground Shake During a Magnitude M 7 How Strong Will the Ground Shake During a Magnitude M 7.5 Earthquake at 20 km?

Probabilistic Seismic Hazard Analysis Geological and seismological models are so uncertain that only probabilistic predictions can be made regarding ground shaking

Peak Ground Acceleration Hazard Curve for a Site in Los Angeles

Maximum Acceleration in 50 Years µ = 0.208 g = 0.211 g COV = /µ = 1.01

Geographical Variation of Uncertainty City µ (g)  (g) COV = /µ Los Angeles 0.208 0.21 1.01 San Francisco 0.189 0.9 Memphis 0.083 0.177 2.15 Boston 0.023 0.07 3.11

Why Does Uncertainty Matter?

Expected and Design Acceleration µPGA = 0.208 g DesignPGA = 0.536 g

Expected and Design Accelerations for Different Cities City Expected (g) Design (g) Design/Expected Los Angeles, CA 0.208 0.536 2.6 San Francisco, CA 0.21 0.514 2.4 Memphis, TN 0.0669 0.345 5.2 Boston, MA 0.0226 0.0946 4.2 Uncertainty increases the cost of mitigation

Uncertainty increases the cost of insurance Return Period of Collapse for Same Ratio Between Expected and Design Accelerations City MRP of Collapse Los Angeles, CA 4250 years San Francisco, CA 5000 years Memphis, TN 1500 years Boston, MA 2200 years Uncertainty increases the cost of insurance

Design Versus Expected Annual Acceleration City Expected (g) Design (g) Design/Expected Los Angeles, CA 0.0113 0.536 47 San Francisco, CA 0.0108 0.514 48 Memphis, TN 0.00212 0.345 163 Boston, MA 0.00053 0.0946 178 Uncertainty makes mitigation less attractive than insurance

Effects of Uncertainty Increased cost of mitigation Increased cost of insurance Voluntary mitigation becomes less attractive compared to insurance, but insurance cannot reverse: Loss of life Loss of business Damage to environment Insurance without mitigation is costly

Historic Variation of Uncertainty 50-year Maximum PGA COV = /µ 1996 0.25 0.188 0.75 2002 0.258 0.2 0.78 2008 0.234 0.221 0.95 2014 0.208 0.21 1.01 Geoscientists are less certain now than in 1996

Reasons for Increased Uncertainty As computers became faster and more powerful, it became possible for the USGS to consider more sources of uncertainty in the PSHA The number of opinions regarding the geological and seismological models increased over the years due to a greater number of researchers investigating the seismic hazard Seismic hazard analysis is in its early stages and geoscientists are still discovering more and more that they do not know

Significance of Change Between the Results of Successive PSHA Effect size, d = (µ1 - µ2)/ d > 0.8 is large change d ~ 0.5 is moderate change d < 0.2 is small change Cohen, J. (1988). Statistical Power Analysis for the Behavioral Sciences (second ed.), New Jersey, Lawrence Erlbaum Associates.

Significance of Change Between the Results of Successive PSHA Interval Effect-Size of Change 1996-2002 0.04 2002-2008 0.11 2008-2014 0.12 Changes in ground motion predictions were statistically insignificant

Suggestions to Avoid Misuse of PSHA PSHA should not be considered as an excuse for using models with high uncertainty. Models with high uncertainty increase the costs of mitigation and insurance. There is a common misconception that uncertainty does not matter as long as it can be modeled in the PSHA

Suggestions to Avoid Misuse of PSHA The focus in seismic hazard analysis should shift from considering uncertainty to reducing uncertainty. In the seismic hazard community, there are more discussions on considering uncertainty, and practically no discussions on reducing uncertainty. The real progress in the science of ground motion prediction can only be made by reducing uncertainty

Suggestions to Avoid Misuse of PSHA Uncertainty in geological and seismological models should be reduced by gradually replacing empirical models with physics-based models and by limiting the number of opinions considered in the PSHA. Empirical models, by their very nature, are uncertain and there are several opinions regarding empirical models. For example, at present time, there are about 300 ground motion prediction models in the published literature.

Suggestions to Avoid Misuse of PSHA The design ground motions should not be revised unless a statistically significant change occurs in the predicted ground motions. The uncertainty in ground motion prediction is so high that a slight changes in ground motion estimates do not justify a change in the design ground motions

Suggestions to Avoid Misuse of PSHA Site-specific PSHA should not be used for the sole purpose of reducing design ground motions. Since there are many opinions regarding geological and seismological models, a combination of opinions can be easily found which provides a lower estimate of the design ground motion. This practice should be stopped, because it provides a biased assessment of the risk

Suggestions to Avoid Misuse of PSHA Same predictions of ground motions should be used for seismic design and insurance of all types of structures. There is no convincing argument for separate PSHA for buildings, bridges, dams and nuclear power plants. Design ground motions should be different for different types of structures, based on the consequence of damage; but the underlying assessment of the hazard should be the same

Suggestions to Avoid Misuse of PSHA Seismic hazard models should not be made more complex unless they reduce uncertainty. It does not make sense for ground motion prediction models to become more complex and less certain at the same time. Complexity without certainty is self-serving

Ground Motion Histories for Seismic Design of Kingdom Tower

Seismic Map of Region According to USGS Kingdom Tower

Propagation of High- and Low-Frequencies

2475-year MRP Response Spectrum from PSHA by Langan International

Earthquake Scenarios Controlling the Seismic Hazard Natural Period, T (s) Magnitude, Mw Distance (km) 0.3 5.9 70  4 7.1 1000

Long-Period Transition for Different Magnitude Earthquakes (NEHRP 2003) Magnitude Mw TL (s) 6.0 – 6.5 4 6.5 – 7.0 6 7.0 – 7.5 8 7.5 – 8.0 12 8.0 – 8.5 16 8.5 – 9.0+ 20

2475-year MRP Response Spectrum

Target Response Spectrum for Closer-Smaller Earthquakes

Target Response Spectrum for Distant-Larger Earthquakes

Vertical Response Spectra for Closer and Distant Earthquakes

‘Seed’ Ground Motion for Closer Earthquake Scenario

Response Spectra of ‘Seed’ Ground Motion for Closer Earthquake Scenario

Response Spectra of Modified Horizontal Components

Response Spectra of Modified Horizontal and Vertical Components

Comments High-rise buildings respond to ground displacements rather than ground accelerations Site-specific free-surface ground motions do not depend on the structure being analyzed; they depend only on the geology and seismology of the region All three components of a ground motion need to be simultaneously modified to make it site-specific