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Updates to Ground Motion and Response Spectra for Bridge Structures

Description:

Background

Seismic design maps for bridge structures were last updated over 10 years ago. Since that time, several developments have taken place that should be considered when updating the seismic design values used for bridge design. Among the issues to be considered as part of this RNS are: (1) Updated mapped values; (2) Alternative definitions for site coefficients; (3) Direct definition of displacement spectra (including corner point period models); (4) and Response spectra definition (i.e. value of NN for RotDNN).

Updated Mapped Values

The National Seismic Hazard Maps (NSHM) provide expected levels of ground shaking at specified probabilities based on the best available science; which refers to findings validated and accepted by the scientific community (Petersen et al., 2014; 2015). Advances in the assessment of seismic hazards in the US are considered by periodic updates to the NSHM conducted by the United States Geological Survey (e.g., Algermissen and Perkins 1976; Frankel et al. 1996, 2000, 2002; Petersen et al. 1996, 2008, 2012, 2014). Such advances may include, but are not limited to updated databases and earthquake catalogs, new source and ground motion models, and new ground motion weighting information for epistemic uncertainty characterization. These maps are instrumental to the development of seismic-design regulation for buildings and critical infrastructure such as bridges and nuclear power plants.

The development of NSHM requires the implementation of appropriate ground motion models (Bommer et al., 2010; Cotton et al., 2006; Petersen et al., 2014). In 2014, the next generation attenuation relationships research program (i.e. NGA-West2) led by the Pacific Earthquake Research Center (PEER), provided a new set of ground motion prediction equations (GMPEs) for shallow crustal earthquakes in active tectonic regions (Bozorgnia et al., 2014). The intensity measure (IM) provided by these GMPEs is SaRotD50, which was originally proposed by Boore (2010), and refers to the median spectral acceleration of the rotated orientation-independent, period-dependent combined horizontal components (Shahi and Baker, 2012; Bozorgnia et al., 2014). Values of SaRotD50 can be converted to maximum spectral acceleration observed in any orientation of two-component horizontal ground motion (i.e. SaRotD100) by implementing the model proposed by Shahi and Baker (2013, 2014). Such conversion is desirable given that SaRotD100 is the IM adopted by the US building code (Bozorgnia et al., 2014; Stewart et al., 2011). A discussion on potential implications of using SaRotD100 versus SaRotD50 for seismic design is provided in later sections of this research needs statement.

Another important feature of the 2014 updated NSHM (and the 2008 version) is that they were generated for a reference site condition corresponding to a uniform firm rock with a time-averaged shear wave velocity over the top 30 m (Vs30) of 760 m/s (i.e., B/C site class boundary; Petersen et al., 2014). Typically, site-amplification factors provided by the National Earthquake Hazards Reduction Program (NEHRP) Provisions (e.g., BSSC 2009) are used in engineering practice to accommodate the influence of local geologic conditions on the expected ground shaking initially estimated for a reference rock site (Stewart and Seyhan, 2013). For low intensity ground motions, site factors in current NEHRP Provisions are derived empirically; while for stronger ground shaking, they rely on numerical simulations of soil nonlinear behavior (Stewart and Seyhan, 2013). The Provisions Update Committee of the Building Seismic Safety Council has approved revised site factors, which appear in the 2015 version of the NEHRP Provisions (Stewart and Seyhan, 2013). Such revisions followed the identification of discrepancies between the NEHRP site factors and site amplifications obtained from the NGA-West models (Stewart and Seyhan, 2013; Seyhan and Stewart, 2014). The seismic design maps for bridge structures should promptly adopt at least the aforementioned fundamental updates or consider using the maps to be generated for various site classes which will be included in the 2020 NEHRP Provisions. Moreover, practitioners and the scientific community would benefit from further exploring the predictive power of other site response parameters (Rodriguez-Marek et al., 1999; Stewart et al., 2013), for example the fundamental period of the profile, depth to bedrock, etc. Additional insights on this topic are provided in the following section.

The current NSHM considered information from external projects such as the NGA-East and a new Electric Power Research Institute (EPRI) study on Central and Eastern US (CEUS) ground motions as additional guidance to define weights for the characterization of epistemic uncertainty in ground motion models applicable to stable tectonic regions (Rezaeian et al., 2015). In future updates of the maps, NGA-East advances and proposed ground motion models must be explicitly considered. Likewise, site factors specific for CEUS should be developed to account for unique effects of local geologic conditions on ground shaking in that region. Other relevant aspects to contemplate in the development of future NSHM versions include updated ground motion models for subduction zones (i.e. NGA-subduction project) as well as induced seismicity (Petersen et al, 2014). The latter has proven to be rather challenging to address given the unique characteristics of earthquakes caused by fluid injection as compared to natural earthquakes (Petersen et al., 2014). Recent research efforts aim to better characterize these events and have provided preliminary insights on how to assess the suitability of current GMPEs to estimate motions from induced earthquakes (e.g., Atkinson and Assatourians, 2017).

Finally, multi-period design response spectra were suggested as part of the report on issues and research needs from the 2015 NEHRP provisions, prepared by the Provisions Update Committee of the National Institute of Building Sciences Building Seismic Safety Council (Gosh et al., 2015). The development of multi-period spectral parameter data and spectra also constitutes one of the main objectives of Project 17. The idea is to cover a broad natural period range, while accounting for relevant site conditions such as deep basin effects and the nonlinear behavior of different types of soils.

Alternative Definitions for Site Coefficients

Local soil conditions can significantly affect the intensity of ground shaking as observed in past earthquakes, such as the 1985 Mexico City (Seed et al., 1987) and 1989 Loma Prieta (Seed et al., 1991) earthquakes. Ground motion recordings from the aforementioned seismic events have particularly led to improvements in the assessment of site effects within seismic design recommended procedures. As described in the previous section, site coefficients are used in practice to adjust the ground shaking intensity estimated for reference rock conditions in NHSM, in accordance with the characteristics of near surface deposits.

One of the main areas that could benefit from further research is the assessment of site amplification in the context of probabilistic seismic hazard assessment. The first attempt to consider the influence of local geologic conditions in GMPEs included a binary classification; rock versus soil. Nowadays, GMPEs and building codes rely on values of Vs30 as a proxy for site response. For instance, the proposed models in the NGA-West2 project include site terms which are a function of Vs30 (Bozorgnia et al., 2014).

The effect of site conditions on the characteristics of ground motions is a complex function of multiple factors, such as the depth to bedrock (or the thickness of the soil deposit), the stiffness of the materials, and damping. Due to soils’ nonlinear behavior, seismic site response is also dependent on the intensity of ground shaking. This nonlinear stress-strain behavior also varies with soil type (e.g., Vucetic and Dobry, 1991; Darendeli, 2001; Menq, 2003). Cementation and geologic age may also play a role on soils’ nonlinearity, but their effects have not been rigorously considered yet (Rodriguez-Marek et al., 1999).

Multiple parameters have been considered in the literature as site response predictors, mostly based on their ability to capture the aforementioned key characteristics of the site. Values of Vs30 provide a measure of the stiffness of near surface materials, while the natural period of the site combines the effect of Vs and the soil thickness, site-specific κ values (i.e., κ0) as proposed by Anderson and Hough (1984) can reflect near-surface attenuation characteristics at a site (Ktenidou et al., 2013; 2015). The 1976 version of the seismic code contemplated the natural period of the site as a site response proxy (Rodriguez-Marek et al., 1999), but recent updates have chosen Vs30 instead. Most of the debate around such decision focuses on the ability of Vs30 to properly characterize site response because it is based only on information from near-surface deposits.

Malekmohammadi and Pezeshk (2013) analyzed nonlinear site amplification factors using deep soil profiles from the Mississippi Embayment and found that “the amplification (or de-amplification) at various frequencies implied by the depth of sediment was greater than that implied just by site classification based on Vs30 values”. Rodriguez-Marek et al. (1999) also noted that site coefficients and/or site classification based solely on Vs30 overlook the potential influence of the depth to bedrock. However, Stewart et al., (2013) did not found any other site response proxy that could perform as well as Vs30 for GMPEs applications. Other researchers have investigated the predictive capabilities of other parameters such as average velocities over different depth ranges, depth to specific Vs horizons, maximum impedance ratio (e.g., Kaklamanos et al., 2013), and even the topographic slope (Wald and Allen, 2007). More recently, Rezaeian et al. (2015) identified κ (Anderson and Hough, 1984) as a relevant parameter in ground motion modeling, especially for stable tectonic regions such as CEUS. It would be worth exploring whether it is the combination of different site proxies what could provide the answers to some of the unresolved issues in the prediction of the effects of local geology on ground motions.

Direct Definition of displacement spectra

In order to facilitate the implementation of design approaches such as Direct-Displacement Based Design, direct definition of the displacement spectral ordinates from mapped values would be useful. Currently, engineers wishing to apply such an approach convert values of pseudo-spectral to displacement response spectra by dividing by the circular frequency squared. Direct definition of mapped values that are consistent with ground motion models would eliminate the need to do such calculations. However, obtaining such displacement values directly from ground motions has proven to be a rather challenging task (e.g., Facciolli et al., 2004; Douglas 2006; Akkar and Bommer 2007, Campbell and Bozorgnia, 2008). For instance, the NGA-West2 project opted to exclude peak ground displacement (PGD) as part of their model outcomes because PGD values are more sensitive to frequency-filtering parameters and record processing (Campbell and Bozorgnia, 2008).

Furthermore, as part of the definition of displacement spectra, additional investigation into corner point period models should be conducted. Corner point period models are absent from the LRFD and guide spec, although maps are available in ASCE-7. However, the source of the mapped values in ASCE-7 is not well understood, and the values possibly too high (up to 16 seconds). Currently, the EuroCode uses a constant 2 second period, which is likely too low. Individual researchers (Faccioli et al, 2004) have proposed magnitude dependent corner point period models that fall in between those two extremes. These three approaches are compared in Fig. 1 below (Priestley et al. 2007). Clearly, additional work is needed in this area such that consensus may be reached.

Fig. 1 Comparison of Corner Point Period Models (Priestley et al. 2007)

Response Spectra Definition

Over the last decade, a debate has been underway in the earthquake engineering community with regards to the appropriate definition for design response spectra (Stewart et al., 2011). The essence of the argument relates to the representation of bi-directional motion via response spectra.

In both the AASHTO LRFD Bridge Design Specifications as well as the _AASHTO Guide Specifications for LRFD Seismic Bridge Design _(SGS), response spectra are established by defining spectral ordinates at 2 or 3 different periods from design maps developed by USGS for a return period of 1000 years. The resulting spectra is then adjusted for local site conditions, resulting in the final design spectra.

In establishing the design maps for parameters such as Ss and S1, USGS has traditionally relied upon probabilistic seismic hazard analysis which utilizes ground motion prediction equations (GMPEs) defined by the geometric mean of the two principle directions of recorded motion. In 2006, Boore introduced a new rotation independent geometric mean definition termed GMRotI50 (Boore, 2006). In 2010, Boore developed a new definition that does not rely upon the geometric mean termed RotD50 spectra which can be generically expressed as RotDNN spectra where NN represents the percentile (i.e. 50 is consistent with the median, 0 the minimum, and 100 the maximum). The NGA West2 project GMPEs utilized RotD50 spectra for the ground motion models, however, the 2009 NEHRP provisions adopted RotD100 as the spectra for the design maps (Stewart et al., 2011).

In order to appreciate the impact of these choices, a brief discussion of RotDNN spectra is warranted. As described in Boore (2010), for a given recording station, the two as-recorded orthogonal-component time series are combined into a single time series corresponding to different rotation angles, as shown in Equation 1:

< (1)

where

<

Figure 2 Combination of Time Series to Generate Rotation Dependent Spectra

The process is repeated for a range of azimuths from 0° to one rotation-angle increment less than 180°. If the rotation-angle increment is θ, then there will be < single time series, as well as < corresponding response spectra. For example, if θ=30°, then 6 single time series will be generated, as well as 6 response spectra, as shown in Figure 3.

<

Figure 3 Example rotations for theta = 30

Once the response spectra for all rotation angles are obtained, then the nnth percentile of the spectral amplitude over all rotation angles for each period is computed (e.g., RotD50 is the median value and RotD100 is the largest value for all rotation angles). For example, at a given period of say 1 second, the response spectra values for all rotation angles are compared, and the RotD100 value would be the largest value from all rotation angles, while RotD50 would be the median. Figure 4 shows an example of the displacement spectra for RotD50 and RotD100 for a recorded ground motion.

<

Figure 4 Sample spectra for a recorded ground motion pair

As can be seen in the example spectra of Figure 4, the RotD100 spectra represents a substantial increase in demand when compared to the RotD50 spectra. The main question facing the bridge community going forward is the appropriate selection of response spectra definition. This can only be answered by conducting designs to both the RotD50 and RotD100 spectra which would then be evaluated via non-linear time history analysis. Such a study will require multiple bridge configurations and multiple ground motions.

A recent study by Palma and Kowalsky (2019) has aimed to investigate the impact of response spectra definition on design of RC bridge columns. As of the writing of this RNS, the study has focused only on SDOF systems, however, clear trends have been observed which demonstrated that displacements, on average, are 20% higher for structures designed to ROTD50 vs those designed to ROTD100. A complete study on MDOF systems is essential before recommendations on appropriate response spectra definition can be offered.

Objective:

The objective of this research needs statement is to develop new seismic hazard maps considering (1) Response spectra definition, (2) Alternative site coefficients, and (3) Displacement spectra. Each of these issues is discussed in detail in prior sections of this RNS.

The objective of this research is consistent with the AASHTO Highway Subcommittee on Bridges and Structures Strategic Plan, which calls for addressing the grand challenges of optimizing structural systems (Grand Challenge 2) and advancing the AASHTO specifications (Grand Challenge 4).

Benefits:

The current AASHTO seismic hazard maps are based on the 2002 U.S. Geological Survey (USGS) National Seismic Hazard Model (NSHM). In 2008 and 2014, the USGS updated its NSHM. Significant improvements include: (i) ground motion characterization from the Next Generation Attenuation (NGA) projects of the Pacific Earthquake Engineering Research Center (PEER; e.g., http://peer.berkeley.edu/ngawest2/final-products/); (ii) earthquake source characterizations from Version 3 of the Uniform California Earthquake Ruptures Forecast (UCERF3; http://pubs.usgs.gov/of/2013/1165/); and (iii) the Central and Eastern U.S. Seismic Source Characterization (CEUS-SSC; http://www.ceus-ssc.com/).

The current AASHTO site coefficients are based on the 2003 (and 2009) NEHRP Recommended Seismic Provisions for Building and Other Structures. In 2015, the Building Seismic Safety Council (BSSC) Provisions Update Committee (PUC) updated those site coefficients, based on research from the PEER NGA-West2 Project. The updated site coefficients have also been adopted for the 2016 edition American Society of Civil Engineers (ASCE) 7 Standard. For the next edition of the NEHRP Recommended Seismic Provisions, the BSSC PUC is considering site coefficients that are directly based on hazard maps for different site classes.

The seismic hazard maps presented in the AASHTO LRFD Bridge Design Specifications and AASHTO Guide Specifications for LRFD Seismic Bridge Design are over ten years old. Newer hazard maps and site coefficients have been developed by the USGS and others. In order to capture the best current hazards, the AASHTO specifications must be updated with the new information.

Because seismologists are always working on improved seismic predictions based on an increased knowledge of fault locations, attenuation relationships and earthquake recordings, it would be advantageous to update the seismic hazard maps as soon as possible.

Related Research:

The written summary in this section refers to response spectra definitions and its impact on seismic response. Boore et al. (2006) and Boore (2010) introduced orientation-independent measures of seismic intensity from two horizontal ground motions. Boore et al. (2006) proposed two measures of the geometric mean of the seismic intensity, which are independent of the in situ orientations of the sensors. One measure uses period-dependent rotation angles to quantify the spectral intensity, denoted GMRotDnn. The other measure is the GMRotInn, where I stands for period-independent. The ground-motion prediction equations of Abrahamson and Silva (1997), Boore et al. (1997), Campbell and Bozorgnia (2003), and Sadigh et al. (1997) have been updated using GMRotI50 as the dependent variable.

Since more users expressed the desire to use the maximum spectral response over all the rotation angles without geometric means, Boore (2010) introduced the measures of ground shaking intensity irrespective of the sensor orientation. The measures are RotDnn and RotInn, whose computation is similar to GMRotDnn and GMRotInn without computing the geometric means.

With regards to impact on seismic response the opinion paper by Stewart et al. (2011), and the work by Mackie et al. (2011) on impact of incidence angle on bridge response are relevant. Specifically, Stewart et al. (2011) noted the importance of computational analysis of structures (that had not been done as of 2011) in proposing appropriate spectra definitions.

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Palma, A.L., and Kowalsky, M.J. (2019). Impact of response spectra definitions on the seismic design of RC bridge columns. In preparation.

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Implementation:

Bridge design engineers will use the results of this research to more accurately predict seismic displacement demands.

Updated seismic hazard maps would replace the current maps in the AASHTO LRFD Bridge Design Specifications and Guide Specifications for LRFD Seismic Bridge Design. Additional code and commentary language would be provided to address the development of the updated seismic hazard and to describe the application and limitations of the spectral values.

This RNS was selected as the top priority by the TRB AKB50 Seismic Design and Performance of Bridges committee.

Relevance:

Problem Statement Author(s): Mervyn Kowalsky, PhD, P.E. Member of AFF50 Professor - Structural Engineering Department of Civil, Construction, and Environmental Engineering North Carolina State University 919-515-7261 kowalsky@ncsu.edu

Ashly Cabas Assistant Professor – Geotechnical Engineering Department of Civil, Construction, and Environmental Engineering North Carolina State University amcabasm@ncsu.edu

Nico Luco Research Structural Engineer United States Geological Survey (USGS), Golden, CO nluco@usgs.gov

Sanaz Rezaeian Research Structural Engineer United States Geological Survey (USGS), Golden, CO srezaeian@usgs.gov

Others Supporting the Problem Statement: Elmer E. Marx, P.E., S.E. Senior Bridge Design Engineer / Chair TRB AKB50 seismic committee State of Alaska DOT&PF - Bridge Section elmer.marx@alaska.gov

Derek Soden, P.E., S.E. Senior Structural Engineer Structures Team Federal Highway Administration Resource Center derek.soden@dot.gov

Person Submitting the Problem Statement: Richard A. Pratt. PE Chief Bridge Engineer / Chair AASHTO T-3 Seismic Committee State of Alaska DOT&PF Richard.pratt@alaska.gov

Elmer E. Marx, P.E., S.E. Senior Bridge Design Engineer / Chair TRB AKB50 seismic committee State of Alaska DOT&PF - Bridge Section Work: (907) 465-6941 elmer.marx@alaska.gov

Sponsoring Committee:AKB50, Seismic Design and Performance of Bridges
Research Period:Longer than 36 months
Research Priority:High
RNS Developer:Mervyn Kowalsky, PhD, P.E.; Ashly Cabas; Nico Luco; Sanaz Rezaeian
Source Info:Potential Panel Members:
Tom Shantz, Caltrans
Don Anderson, Jacobs
Carl Puzey, Illinois DOT
Nick Murray, Alaska DOT&PF
Rick Ellis, Arkansas DOT
Albert Nako, Oregon DOT
Dennis Heckman, Missouri DOT
Bijan Khaleghi, Washington DOT
Derek Soden FHWA resource Center
Lee Marsh, WSP
Ian Buckle, UNR
Stephanie Brandenberger, Montana DOT
Date Posted:11/03/2021
Date Modified:11/16/2021
Index Terms:Seismicity, Earthquake resistant design, Earthquake resistant structures, Bridge design, Highway bridges,
Cosponsoring Committees: 
Subjects    
Highways
Design
Bridges and other structures

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