Technical Overview

ST-RISK is an earthquake risk management tool that can be used for individual buildings. In helping to manage risk, the program must first be able to quantify it.

In the course of quantifying a building's risk to earthquake damage, the hazard must first be determined. The hazard reflects the probability that various levels of ground shaking will occur. A specific site's susceptibility to ground shaking is due to the following factors:

1. Proximity of site to active faults
2. Recurrence rate of events on nearby faults
3. Size of events that can be generated on nearby faults
4. Local soil conditions

The hazard only reflects to the likelihood and intensity of ground shaking. To obtain risk measurements, the effects of the ground movement on the building must also be considered. As the ground underlying a building shakes, the building suffers damage, and damage results in economic loss. Higher levels of shaking cause higher levels of damage and higher levels of loss. Risk, therefore, reflects the likelihood that certain levels of economic loss will occur.

Historical loss data, organized engineering reconnaissance information, and instrumental ground shaking measurements are used by ST-RISK to correlate ground movement with economic loss. This correlation has been done for different levels of ground shaking, different building types and different building characteristics.

Important characteristics essential to seismic resilience have been identified by FEMA-310. ST-RISK provides a FEMA-310 worksheet that enables an engineer to properly characterize structural strengths and deficiencies.

Detailed Analysis

For the engineer performing a detailed analysis on specific building and soil characteristics, ST-RISK provides a worksheet for the identification of items important to seismic performance. This worksheet is a modification of the FEMA 310 procedure for evaluating seismic vulnerability of buildings. An example of this worksheet is shown below. This level of analysis must be completed by a licensed engineer.

Most of the worksheet items require TRUE/FALSE choices. Some require judgmental values within specified ranges. Typical values are supplied within the worksheet to help you with the evaluation process. The ST-RISK help system contains a substantial amount of applicable FEMA-310 documentation. In this way, you have immediate help on any questions regarding the worksheet input.

Degenkolb Fragilities

A wealth of information goes into constructing the ST-RISK fragility functions. Fragility functions indicate the variability of economic loss associated with different levels of ground shaking. In past earthquakes, observed damage has been recorded for wide ranges of ground shaking. If these observations are plotted for a particular level of shaking and fit to a curve, the result will be a graph that looks like the following:

The smooth curve representing the loss variability is known as a probability distribution density function (PDF). At each level of damage a probability of occurrence can be determined from the curve. You can also obtain a mean level of damage, a median, and a 90th percentile. The 90th percentile damage is also referred to as the Probable Maximum Loss (PML) for this particular level of ground shaking.

ST-RISK uniquely blends insurance loss data with post earthquake observed loss obtained as part of engineering reconnaissance information to generate damage functions that most accurately reflect reported losses from earthquakes.

The benefit of using both types of information is that paid insurance claims alone do not necessarily reflect potential losses to owners. Insurance-based losses pose significant obstacles if they are the sole basis of forming damage functions:

1. Insurance claims usually don't accurately represent losses that fall below the deductible. With relatively high deductibles, usually at least 10 percent, a significant number of structures are damaged below the deductible level. In most cases damage resulting in losses less than the deductible is not reported to the insurance company.
2. Insurance claims often reflect damage that includes pre-existing conditions. Although it might be appropriate to include these losses as part of paid, insured losses, they are not appropriate in estimating real losses to a property owner.
3. Often political pressure will cause insurers to be liberal with their payment of earthquake claims, particularly if they are covered by reinsurance. Any such overpayments would translate into an overestimation of loss for a property owner.

It is therefore preferable to rely on both sets of information deriving damage functions, as ST-RISK does. In this way ST-RISK is unique and more appropriate for use by anyone interested in understanding seismic risk for individual buildings.

HAZUS Damage Functions

ST-RISK Can Use HAZUS® Damage Functions

ST-RISK has been expanded to implement, as a user option, the HAZUS® building damage functions developed by FEMA and NIST (1997).  These damage functions use the Capacity Spectrum Method, which is an equivalent non-linear representation of building response during earthquake shaking.  It accounts for the lengthening of a building’s natural period of vibration as the building undergoes seismic damage, and for the increased vibrational damping associated with cracking, yielding, and other causes of non-linear response.

Buildings are represented by 15 structural systems, as follows:

• wood, light frame and commercial
• steel moment frame
• steel braced frame
• steel light frame
• steel frame, concrete shear walls
• steel frame, masonry walls
• concrete moment frame
• concrete shear wall
• concrete frame, masonry infill
• precast concrete tilt-up
• precast concrete, shear walls
• reinforced masonry, wood or metal deck diaphragms
• reinforced masonry, precast concrete diaphragms
• unreinforced masonry bearing walls
• mobile homes

In addition, the height of each building is represented as low, medium, and high-rise, resulting in 36 building types (some structural systems occur only for low-rise buildings). In addition to these characteristics, the occupancy and code design level are used to estimate the response of each building to seismic shaking. Building response is represented by the initial yield level and ultimate strength, which form the capacity spectrum for that building (see Figure 1). The capacity spectrum can be compared to any specific ground motion to estimate the nonlinear response incurred by the building during that ground motion (Figure 1). The nonlinear response allows us to estimate the probabilities of slight damage, moderate damage, extensive damage, and complete loss (see Figure 2). Each damage level is further characterized by the percent loss to structural systems, nonstructural systems, and contents. Uncertainty in the damage level sustained by the building during any given ground motion is also represented.

Figure 1. Capacity Spectrum

Figure 2. Fragility Curves

The implementation of HAZUS®-based loss estimates within ST-RISK has been verified with results available from published references, including Tobas and Lobo (2008) and Porter (2009, 2010). Thus we are confident that the estimates of structural response to ground motion, the damage estimates given structural response, and the loss estimates given damage of HAZUS® have been accurately implemented.

Figure 3. Risk Curve

ST-RISK integrates this loss methodology with its seismic hazard capability, accounting for hundreds of thousands of possible earthquakes and their relative frequencies of occurrence, to derive a risk analysis for the building. The risk analysis accounts for possible earthquakes, uncertainties in the associated ground motions, uncertainties in building response, and uncertainties in damage level, to give a composite risk curve (loss vs. return period). See Figure 3 for an example risk curve that shows Scenario Expected Loss (SEL), Scenario Upper Loss (SUL), and Probable Loss (PL) curves for a vulnerable building in California. Following the HAZUS® method, loss includes structural loss, non-structural loss, and loss to contents.

Advantages of ST-RISK over alternative programs

• Uses up-to-date earthquake hazard consistent with the US national seismic hazard maps
• Uses site-specific estimates of soil conditions and site response
• Estimates building structural loss, non-structural loss, and contents loss
• Accounts for uncertainties in all major components affecting seismic risk estimates
• Allows building-specific features of ST-RISK to be incorporated into loss estimates, which makes risk results unique and customized for each building.

Hazard Analysis

General

Probablistic seismic hazard analysis is the process of determining the probability of ground shaking intensity at a site. Many factors contribute to this process but they reduce to four important contributors; the proximity to faults, the size of the earthquakes that can be generated by these nearby faults (and how often they occur), the resulting base-level ground motion at the site, and the effects of local site conditions.

ST-RISK analyzes all "possible" events that can effect a site. Each event is assigned a probability of occurrence. If the effects of all events are aggregated, the hazard can be determined, that is, the probability that a ground motion intensity (A) will be exceeded. The hazard measures the likelihood that a building will encounter certain levels of ground shaking. The risk to the building then represents a probabilistic measurement of the hazard and the building's response to certain levels of shaking, or damage. Damage can then be translated into financial loss to the owner, casualty to occupants, operational loss to a business (including loss of market share), and other losses. ST-RISK considers financial losses from direct damage to the structure and from loss of business revenue.

Seismicity

ST-RISK bases its seismic hazard calculation on regional seismic models. In the USA, the calculations are based on the USGS 2008 National Seismic Hazard Map seismic model, and uses the same methodology and interpretations as used by California Division of Mines and Geology (CDMG) and the United States Geological Survey (USGS). In Canada, the calculations are based on the 2005 Geological Survey of Canada seismic model. In other areas, ST-RISK uses recent proprietary seismic models developed by Fugro USA Land, Inc.

ST-RISK uses fault sources, area seismic sources (also know as seismic zones), and variable seismicity background sources. The background source represents all events that can occur where faults are not known to exist. Using this methodology ST-RISK accounts for "unexpected" earthquakes.

To check the level of shaking that is possible you can take a look at the spectral response curve generated by the program. This curve indicates the ground shaking amplitudes that are predicted at user-specified return periods.

Site Conditions

ST-RISK can determine the NEHRP soil category for any site providing a reasonable estimate of the likely soil conditions at the site. In California, maps distributed by the CDMG are used. No regional database is error-free, however ST-RISK allows you to override the default soil condition if you have more accurate geotechnical information. Liquefaction maps for the San Francisco Bay area and Los Angeles are utilized by ST-RISK to determine the susceptibility to liquefaction.

Example of a Liquefaction Susceptibility Map

Example of a Soil Condition Map

Given the soil conditions, ST-RISK applies soil amplification measures based on the NEHRP recommendations. NEHRP provides amplification factors for only two periods, therefore ST-RISK contains supplemental factors for other periods based on our expertise and past experience. These amplification factors are vitally important to assessing the overall site-specific hazard, and having them resident within the program makes the calculation of site-specific spectra straightforward for the ST-RISK user.

ASTM Compliance

ST-RISK can be used for compliance with ASTM Standard E2026-16a, "Standard Guide for Seismic Risk Assessment of Buildings." The estimation of loss should be made by personnel with qualifications and experience in this field, as described in Section 3.4 of the Standard. Qualified personnel can use ST-RISK for ASTM E2026-16a compliance in the following areas:

• Seismic ground motion hazard assessment Levels G0 or G1
• Site stability assessment Levels SS0, SS1 and SS2.
• Building damageability assessment Levels BD0, BD1 and BD2.
• Business interruption assessment Levels B0 and B1.

ASTM E2557-16a is a standard-of-care. It does not require anything in regards to PML, SEL and SUL calculations that is not already proscribed in ASTM E2026-16a. ST-RISK is explicitly listed as an acceptable tool in ASTM E2026-16a Appendix X3 paragraph X3.4.

Any damageability assessment beyond the basic screening level assessment requires a site visit by qualified personnel, and ST-RISK can be an important tool that takes into account building-specific characteristics, as required by ASTM E2026-16a.