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A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenThu, 30 Nov 2023 18:09:40 +0000Modeling Steel Frame Buildings in Three Dimensions. I: Panel Zone and Plastic Hinge Beam Elements
https://resolver.caltech.edu/CaltechAUTHORS:20130311-091205741
Authors: Krishnan, Swaminathan; Hall, John F.
Year: 2006
DOI: 10.1061/(ASCE)0733-9399(2006)132:4(345)
A procedure for efficient three-dimensional nonlinear time-history analysis of steel framed buildings is derived. It incorporates two types of nonlinear beam elements—the plastic hinge type and the elastofiber type—and nonlinear panel zone elements to model yielding and strain-hardening in moment-frames. Floors and roofs of buildings are modeled using 4-node elastic diaphragm elements. The procedure utilizes an iteration strategy applied to an implicit time-integration scheme to solve the nonlinear equations of motion at each time step. Geometric nonlinearity is included. An overview of the procedure and the theories for the panel zone and the plastic hinge elements are presented in this paper. The theory for the elastofiber element along with illustrative examples are presented in a companion paper. The plastic hinge beam element consists of two nodes at which biaxial flexural yielding is permitted, leading to the formation of plastic hinges. Elastic rotational springs are connected across the plastic hinge locations to model strain-hardening. Axial yielding is also permitted. The panel zone element consists of two orthogonal panels forming a cruciform section. Each panel may yield and strain-harden in shear.https://authors.library.caltech.edu/records/yh1at-tfx37Modeling Steel Frame Buildings in Three Dimensions. I: Panel Zone and Plastic Hinge Beam Elements
https://resolver.caltech.edu/CaltechAUTHORS:20130311-091205741
Authors: Krishnan, Swaminathan; Hall, John F.
Year: 2006
DOI: 10.1061/(ASCE)0733-9399(2006)132:4(345)
A procedure for efficient three-dimensional nonlinear time-history analysis of steel framed buildings is derived. It incorporates two types of nonlinear beam elements—the plastic hinge type and the elastofiber type—and nonlinear panel zone elements to model yielding and strain-hardening in moment-frames. Floors and roofs of buildings are modeled using 4-node elastic diaphragm elements. The procedure utilizes an iteration strategy applied to an implicit time-integration scheme to solve the nonlinear equations of motion at each time step. Geometric nonlinearity is included. An overview of the procedure and the theories for the panel zone and the plastic hinge elements are presented in this paper. The theory for the elastofiber element along with illustrative examples are presented in a companion paper. The plastic hinge beam element consists of two nodes at which biaxial flexural yielding is permitted, leading to the formation of plastic hinges. Elastic rotational springs are connected across the plastic hinge locations to model strain-hardening. Axial yielding is also permitted. The panel zone element consists of two orthogonal panels forming a cruciform section. Each panel may yield and strain-harden in shear.https://authors.library.caltech.edu/records/0z74g-f6c59Case Studies of Damage to Tall Steel Moment-Frame Buildings in Southern California during Large San Andreas Earthquakes
https://resolver.caltech.edu/CaltechAUTHORS:20130305-081257267
Authors: Krishnan, Swaminathan; Ji, Chen; Komatitsch, Dimitri; Tromp, Jeroen
Year: 2006
DOI: 10.1785/0120050145
On 9 January 1857, a large earthquake of magnitude 7.9 occurred on the San Andreas fault, with rupture initiating at Parkfield in central California and propagating in a southeasterly direction over a distance of more than 360 km. Such a unilateral rupture produces significant directivity toward the San Fernando and Los Angeles basins. Indeed, newspaper reports of sloshing observed in the Los Angeles river point to long-duration (1–2 min) and long-period (2–8 sec) shaking. If such an earthquake were to happen today, it could impose significant seismic demand on present-day tall buildings. Using state-of-the-art computational tools in seismology and structural engineering, validated using data from the 17 January 1994, magnitude 6.7 Northridge earthquake, we determine the damage to an existing and a new 18- story steel moment-frame building in southern California due to ground motion from two hypothetical magnitude 7.9 earthquakes on the San Andreas fault. Our study indicates that serious damage occurs in these buildings at many locations in the region in one of the two scenarios. For a north-to-south rupture scenario, the peak velocity is of the order of 1 m•sec^(−1) in the Los Angeles basin, including downtown Los Angeles, and 2 m•sec^(−1) in the San Fernando valley, while the peak displacements are of the order of 1 m and 2 m in the Los Angeles basin and San Fernando valley, respectively. For a south-to-north rupture scenario the peak velocities and displacements are reduced by a factor of roughly 2.https://authors.library.caltech.edu/records/7vf1x-b6q68Performance of Two 18-Story Steel Moment-Frame Buildings in Southern California During Two Large Simulated San Andreas Earthquakes
https://resolver.caltech.edu/CaltechAUTHORS:KRIes06
Authors: Krishnan, Swaminathan; Komatitsch, Dimitri; Tromp, Jeroen
Year: 2006
DOI: 10.1193/1.2360698
Using state-of-the-art computational tools in seismology and structural engineering, validated using data from the Mw=6.7 January 1994 Northridge earthquake, we determine the damage to two 18-story steel moment-frame buildings, one existing and one new, located in southern California due to ground motions from two hypothetical magnitude 7.9 earthquakes on the San Andreas Fault. The new building has the same configuration as the existing building but has been redesigned to current building code standards. Two cases are considered: rupture initiating at Parkfield and propagating from north to south, and rupture propagating from south to north and terminating at Parkfield. Severe damage occurs in these buildings at many locations in the region in the north-to-south rupture scenario. Peak velocities of 1 m.s−1 and 2 m.s−1 occur in the Los Angeles Basin and San Fernando Valley, respectively, while the corresponding peak displacements are about 1 m and 2 m, respectively. Peak interstory drifts in the two buildings exceed 0.10 and 0.06 in many areas of the San Fernando Valley and the Los Angeles Basin, respectively. The redesigned building performs significantly better than the existing building; however, its improved design based on the 1997 Uniform Building Code is still not adequate to prevent serious damage. The results from the south-to-north scenario are not as alarming, although damage is serious enough to cause significant business interruption and compromise life safety.https://authors.library.caltech.edu/records/mv9s9-0p816Case studies of damage to 19-storey irregular steel moment-frame buildings under near-source ground motion
https://resolver.caltech.edu/CaltechAUTHORS:20130305-075249536
Authors: Krishnan, Swaminathan
Year: 2007
DOI: 10.1002/eqe.657
This paper describes the three-dimensional nonlinear analysis of six 19-storey steel moment-frame buildings, designed per the 1997 Uniform Building Code, under strong ground motion records from near-source earthquakes with magnitudes in the range of 6.7–7.3. Three of these buildings possess a reentrant corner irregularity, while the remaining three possess a torsional plan irregularity. The records create drift demands of the order of 0.05 and plastic rotation demands of the order of 4–5% of a radian in the buildings with reentrant corners. These values point to performance at or near 'Collapse Prevention'. Twisting in the torsionally sensitive buildings causes the plastic rotations on the moment frame on one face of the building (4–5% of a radian) to be as high as twice of that on the opposite face (2–3% of a radian). The asymmetric yield pattern implies a lower redundancy in the lateral force-resisting system as the failure of the heavily loaded frame could result in a total loss of resistance to torsion.https://authors.library.caltech.edu/records/17mtr-4f432Modified Elastofiber Element for Steel Slender Column and Brace Modeling
https://resolver.caltech.edu/CaltechAUTHORS:20101108-142429209
Authors: Krishnan, Swaminathan
Year: 2010
DOI: 10.1061/(ASCE)ST.1943-541X.0000238
An efficient beam element, the modified elastofiber (MEF) element, has been developed to capture the overall features of the elastic and inelastic responses of slender columns and braces under axial cyclic loading without unduly heavy discretization. It consists of three fiber segments, two at the member ends and one at midspan, with two elastic segments sandwiched in between. The segments are demarcated by two exterior nodes and four interior nodes. The fiber segments are divided into 20 fibers in the cross section that run the length of the segment. The fibers exhibit nonlinear axial stress-strain behavior akin to that observed in a standard tension test of a rod in the laboratory, with a linear elastic portion, a yield plateau, and a strain-hardening portion consisting of a segment of an ellipse. All the control points on the stress-strain law are user defined. The elastic buckling of a member is tracked by updating both exterior and interior nodal coordinates at each iteration of a time step and checking force equilibrium in the updated configuration. Inelastic postbuckling response is captured by fiber yielding, fracturing, and/or rupturing in the nonlinear segments. The key features of the element include the ability to model each member using a single element, easy incorporation of geometric imperfection, partial fixity support conditions, member susceptibility to fracture defined in a probabilistic manner, and fiber rupture leading to complete severing of the member. The element is calibrated to accurately predict the Euler critical buckling load of box and I sections with a wide range of slenderness ratios (L/r=40, 80, 120, 160, and 200) and support conditions (pinned-pinned, pinned-fixed, and fixed-fixed). Elastic postbuckling of the Koiter-Roorda L frame (tubes and I sections) with various member slenderness ratios (L/r=40, 80, 120, 160, and 200) is simulated and shown to compare well against second-order analytical approximations to the solution even when using a single-MEF element to model each leg of the frame. The inelastic behavior of struts under cyclic loading observed in the experiments of Black et al., Fell et al., and Tremblay et al. is accurately captured by single-MEF-element models. A FRAME3D model (using MEF elements for braces) of a full-scale six-story braced frame structure that was pseudodynamically tested at the Building Research Institute of Japan subjected to the 1978 Miyagi-Ken-Oki earthquake record is analyzed and shown to closely mimic the experimentally observed behavior.https://authors.library.caltech.edu/records/s7410-6pt81Modified Elastofiber Element for Steel Slender Column and Brace Modeling
https://resolver.caltech.edu/CaltechAUTHORS:20101108-142429209
Authors: Krishnan, Swaminathan
Year: 2010
DOI: 10.1061/(ASCE)ST.1943-541X.0000238
An efficient beam element, the modified elastofiber (MEF) element, has been developed to capture the overall features of the elastic and inelastic responses of slender columns and braces under axial cyclic loading without unduly heavy discretization. It consists of three fiber segments, two at the member ends and one at midspan, with two elastic segments sandwiched in between. The segments are demarcated by two exterior nodes and four interior nodes. The fiber segments are divided into 20 fibers in the cross section that run the length of the segment. The fibers exhibit nonlinear axial stress-strain behavior akin to that observed in a standard tension test of a rod in the laboratory, with a linear elastic portion, a yield plateau, and a strain-hardening portion consisting of a segment of an ellipse. All the control points on the stress-strain law are user defined. The elastic buckling of a member is tracked by updating both exterior and interior nodal coordinates at each iteration of a time step and checking force equilibrium in the updated configuration. Inelastic postbuckling response is captured by fiber yielding, fracturing, and/or rupturing in the nonlinear segments. The key features of the element include the ability to model each member using a single element, easy incorporation of geometric imperfection, partial fixity support conditions, member susceptibility to fracture defined in a probabilistic manner, and fiber rupture leading to complete severing of the member. The element is calibrated to accurately predict the Euler critical buckling load of box and I sections with a wide range of slenderness ratios (L/r=40, 80, 120, 160, and 200) and support conditions (pinned-pinned, pinned-fixed, and fixed-fixed). Elastic postbuckling of the Koiter-Roorda L frame (tubes and I sections) with various member slenderness ratios (L/r=40, 80, 120, 160, and 200) is simulated and shown to compare well against second-order analytical approximations to the solution even when using a single-MEF element to model each leg of the frame. The inelastic behavior of struts under cyclic loading observed in the experiments of Black et al., Fell et al., and Tremblay et al. is accurately captured by single-MEF-element models. A FRAME3D model (using MEF elements for braces) of a full-scale six-story braced frame structure that was pseudodynamically tested at the Building Research Institute of Japan subjected to the 1978 Miyagi-Ken-Oki earthquake record is analyzed and shown to closely mimic the experimentally observed behavior.https://authors.library.caltech.edu/records/2yfwv-bqm08Hope for the Best, Prepare for the Worst: Response of Tall Steel Buildings to the ShakeOut Scenario Earthquake
https://resolver.caltech.edu/CaltechAUTHORS:20110922-134535249
Authors: Muto, Matthew M.; Krishnan, Swaminathan
Year: 2011
DOI: 10.1193/1.3563621
This work represents an effort to develop one plausible realization of the effects of the scenario event on tall steel moment-frame buildings. We have used the simulated ground motions with three-dimensional nonlinear finite element models of three buildings in the 20-story class to simulate structural responses at 784 analysis sites spaced at approximately 4 km throughout the San Fernando Valley, the San Gabriel Valley, and the Los Angeles Basin. Based on the simulation results and available information on the number and distribution of steel buildings, the recommended damage scenario for the ShakeOut drill was 5% of the estimated 150 steel moment-frame structures in the 10–30 story range collapsing, 10% red-tagged, 15% with damage serious enough to cause loss of life, and 20% with visible damage requiring building closure.https://authors.library.caltech.edu/records/6yjbp-rgd42Rupture-to-Rafters Simulations: Unifying Science and Engineering for Earthquake Hazard Mitigation
https://resolver.caltech.edu/CaltechAUTHORS:20110712-113612355
Authors: Krishnan, Swaminathan; Muto, Matthew M.; Mourhatch, Ramses; Bjornsson, Arnan Bjorn; Siriki, Hemanth
Year: 2011
DOI: 10.1109/MCSE.2011.23
High-performance computing has brought about a renaissance in computational seismology and earthquake engineering. Researchers in both fields are using advanced numeric tools and high-fidelity numerical models synergistically to create rupture-to-rafters simulations. This end-to-end approach promises to significantly advance earthquake
damage prediction, preparation, mitigation, and disaster response.https://authors.library.caltech.edu/records/33pkz-gw934Mechanism of Collapse of Tall Steel Moment-Frame Buildings under Earthquake Excitation
https://resolver.caltech.edu/CaltechAUTHORS:20130125-100014925
Authors: Krishnan, Swaminathan; Muto, Matthew M.
Year: 2012
DOI: 10.1061/(ASCE)ST.1943-541X.0000573
The mechanism of collapse of tall steel moment-frame buildings is explored through three-dimensional nonlinear analyses of two 18-story steel moment-frame buildings under earthquake excitation. Both fracture-susceptible and perfect-connection conditions are investigated. Classical energy-balance analysis shows that only long-period excitation imparts energy to tall buildings large enough to cause collapse. Under such long-period motion, the shear-beam analogy alludes to the existence of a characteristic mechanism of collapse or a few preferred
mechanisms of collapse for these buildings. Numerical evidence from parametric analyses of the buildings under a suite of idealized sawtooth-like ground-motion time histories, with varying period (T), amplitude (peak ground velocity, PGV), and duration (number of cycles, N), is
presented to support this hypothesis. Damage localizes to form a quasi-shear band over a few stories. When the band is destabilized, sidesway collapse is initiated, and gravity takes over. Only one to five collapse mechanisms occur out of a possible 153 mechanisms in either principal
direction of the buildings considered. Where two or more preferred mechanisms do exist, they have significant story-overlap, typically separated by just 1 story. It is shown that a simple work-energy relation applied to all possible quasi-shear bands combined with plastic analysis principles
can systematically identify all the preferred collapse mechanisms.https://authors.library.caltech.edu/records/01j76-a2y52Mechanism of Collapse of Tall Steel Moment-Frame Buildings under Earthquake Excitation
https://resolver.caltech.edu/CaltechAUTHORS:20130125-100014925
Authors: Krishnan, Swaminathan; Muto, Matthew M.
Year: 2012
DOI: 10.1061/(ASCE)ST.1943-541X.0000573
The mechanism of collapse of tall steel moment-frame buildings is explored through three-dimensional nonlinear analyses of two 18-story steel moment-frame buildings under earthquake excitation. Both fracture-susceptible and perfect-connection conditions are investigated. Classical energy-balance analysis shows that only long-period excitation imparts energy to tall buildings large enough to cause collapse. Under such long-period motion, the shear-beam analogy alludes to the existence of a characteristic mechanism of collapse or a few preferred
mechanisms of collapse for these buildings. Numerical evidence from parametric analyses of the buildings under a suite of idealized sawtooth-like ground-motion time histories, with varying period (T), amplitude (peak ground velocity, PGV), and duration (number of cycles, N), is
presented to support this hypothesis. Damage localizes to form a quasi-shear band over a few stories. When the band is destabilized, sidesway collapse is initiated, and gravity takes over. Only one to five collapse mechanisms occur out of a possible 153 mechanisms in either principal
direction of the buildings considered. Where two or more preferred mechanisms do exist, they have significant story-overlap, typically separated by just 1 story. It is shown that a simple work-energy relation applied to all possible quasi-shear bands combined with plastic analysis principles
can systematically identify all the preferred collapse mechanisms.https://authors.library.caltech.edu/records/wwp3a-v7224Rapid Estimation of Damage to Tall Buildings Using Near Real‐Time Earthquake and Archived Structural Simulations
https://resolver.caltech.edu/CaltechAUTHORS:20130103-134600173
Authors: Krishnan, Swaminathan; Casarotti, Emanuele; Goltz, Jim; Ji, Chen; Komatitsch, Dimitri; Mourhatch, Ramses; Muto, Matthew M.; Shaw, John H.; Tape, Carl; Tromp, Jeroen
Year: 2012
DOI: 10.1785/0120110339
This article outlines a new approach to rapidly estimate the damage to tall buildings immediately following a large earthquake. The preevent groundwork involves the creation of a database of structural responses to a suite of idealized ground‐motion waveforms. The postevent action involves (1) rapid generation of an earthquake source model, (2) near real‐time simulation of the earthquake using a regional spectral‐element model of the earth and computing synthetic seismograms at tall building sites, and (3) estimation of tall building response (and damage) by determining the best‐fitting idealized waveforms to the synthetically generated ground motion at the site and directly extracting structural response metrics from the database. Here, ground‐velocity waveforms are parameterized using sawtoothlike wave trains with a characteristic period (T), amplitude (peak ground velocity, PGV), and duration (number of cycles, N). The proof‐of‐concept is established using the case study of one tall building model. Nonlinear analyses are performed on the model subjected to the idealized wave trains, with T varying from 0.5 s to 6.0 s, PGV varying from 0.125 m/s, and N varying from 1 to 5. Databases of peak transient and residual interstory drift ratios (IDR), and permanent roof drift are created. We demonstrate the effectiveness of the rapid response approach by applying it to synthetic waveforms from a simulated 1857‐like magnitude 7.9 San Andreas earthquake. The peak IDR, a key measure of structural performance, is predicted well enough for emergency response decision making.https://authors.library.caltech.edu/records/xk1ep-8jn36Sensitivity of the Earthquake Response of Tall Steel Moment Frame Buildings to Ground Motion Features
https://resolver.caltech.edu/CaltechAUTHORS:20130307-144505436
Authors: Krishnan, Swaminathan; Muto, Matthew
Year: 2013
DOI: 10.1080/13632469.2013.771587
The seismic response of two tall steel moment frame buildings and their variants is explored through parametric nonlinear analysis using idealized sawtooth-like ground velocity waveforms, with a characteristic period (T), amplitude (peak ground velocity, PGV), and duration (number of cycles, N). Collapse-level response is induced only by long-period, moderate to large PGV ground excitation. This agrees well with a simple energy balance analysis. The collapse initiation regime expands to lower ground motion periods and amplitudes with increasing number of ground motion cycles.https://authors.library.caltech.edu/records/fecv9-n0g97Low-Complexity Candidate for Benchmarking Collapse Prediction of Steel Braced Structures
https://resolver.caltech.edu/CaltechAUTHORS:20141106-105505763
Authors: Bjornsson, Arnan B.; Krishnan, Swaminathan
Year: 2014
DOI: 10.1061/(ASCE)ST.1943-541X.0000938
To aid in the evaluation of the collapse-prediction capability of competing methodologies, a case study of a water tank subjected to the Takatori near-source record from the 1995 Kobe earthquake, scaled down by a factor of 0.32, is presented. The water tank, supported by a five-segment steel lattice tower, is so configured as to have a characteristic collapse mechanism that is triggered due to catastrophic column and brace buckling at the bottommost segment of the lattice under all forms of ground motion. A FRAME3D model of the tank reveals severe buckling in the bottom megacolumns and one of the two braces on the west face of the tower when the structure is impacted by the Takatori near-source pulse, resulting a tilt in the structure. This is followed by sequential compression buckling of braces on the south and north faces leading to P−Δ instability and complete collapse of the tank. In order to verify the predictions of the FRAME3D model, a comparable PERFORM-3D model of the tank, using fiber elements and constitutive material models that are suitably calibrated against experimental data, is developed. The response of this model to the scaled Takatori ground motion compares very well against that of the FRAME3D model; the smallest scaling factor needed to collapse the PERFORM-3D model is 0.323, whereas the corresponding factor needed to collapse the FRAME3D model is 0.315. The sequence of column- and brace-buckling failures and the collapse mechanisms are quite similar in the two models.https://authors.library.caltech.edu/records/pzwjp-8q133Low-Complexity Candidate for Benchmarking Collapse Prediction of Steel Braced Structures
https://resolver.caltech.edu/CaltechAUTHORS:20141106-105505763
Authors: Bjornsson, Arnan B.; Krishnan, Swaminathan
Year: 2014
DOI: 10.1061/(ASCE)ST.1943-541X.0000938
To aid in the evaluation of the collapse-prediction capability of competing methodologies, a case study of a water tank subjected to the Takatori near-source record from the 1995 Kobe earthquake, scaled down by a factor of 0.32, is presented. The water tank, supported by a five-segment steel lattice tower, is so configured as to have a characteristic collapse mechanism that is triggered due to catastrophic column and brace buckling at the bottommost segment of the lattice under all forms of ground motion. A FRAME3D model of the tank reveals severe buckling in the bottom megacolumns and one of the two braces on the west face of the tower when the structure is impacted by the Takatori near-source pulse, resulting a tilt in the structure. This is followed by sequential compression buckling of braces on the south and north faces leading to P−Δ instability and complete collapse of the tank. In order to verify the predictions of the FRAME3D model, a comparable PERFORM-3D model of the tank, using fiber elements and constitutive material models that are suitably calibrated against experimental data, is developed. The response of this model to the scaled Takatori ground motion compares very well against that of the FRAME3D model; the smallest scaling factor needed to collapse the PERFORM-3D model is 0.323, whereas the corresponding factor needed to collapse the FRAME3D model is 0.315. The sequence of column- and brace-buckling failures and the collapse mechanisms are quite similar in the two models.https://authors.library.caltech.edu/records/qfgsj-g0d75A Laboratory Earthquake‐Based Stochastic Seismic Source Generation Algorithm for Strike‐Slip Faults and its Application to the Southern San Andreas Fault
https://resolver.caltech.edu/CaltechAUTHORS:20150827-103413623
Authors: Siriki, Hemanth; Bhat, Harsha S.; Lu, Xiao; Krishnan, Swaminathan
Year: 2015
DOI: 10.1785/0120140110
There is a sparse number of credible source models available from large‐magnitude past earthquakes. A stochastic source‐model‐generation algorithm thus becomes necessary for robust risk quantification using scenario earthquakes. We present an algorithm that combines the physics of fault ruptures as imaged in laboratory earthquakes with stress estimates on the fault constrained by field observations to generate stochastic source models for large‐magnitude (M_w 6.0–8.0) strike‐slip earthquakes. The algorithm is validated through a statistical comparison of synthetic ground‐motion histories from a stochastically generated source model for a magnitude 7.90 earthquake and a kinematic finite‐source inversion of an equivalent magnitude past earthquake on a geometrically similar fault. The synthetic dataset comprises three‐component ground‐motion waveforms, computed at 636 sites in southern California, for 10 hypothetical rupture scenarios (five hypocenters, each with two rupture directions) on the southern San Andreas fault. A similar validation exercise is conducted for a magnitude 6.0 earthquake, the lower magnitude limit for the algorithm. Additionally, ground motions from the M_w 7.9 earthquake simulations are compared against predictions by the Campbell–Bozorgnia Next Generation Attenuation relation, as well as the ShakeOut scenario earthquake. The algorithm is then applied to generate 50 source models for a hypothetical magnitude 7.9 earthquake originating at Parkfield, California, with rupture propagating from north to south (toward Wrightwood), similar to the 1857 Fort Tejon earthquake. Using the spectral element method, three‐component ground‐motion waveforms are computed in the Los Angeles basin for each scenario earthquake and the sensitivity of ground‐shaking intensity to seismic source parameters (such as the percentage of asperity area relative to the fault area, rupture speed, and rise time) is studied.https://authors.library.caltech.edu/records/7cq3c-ff368Toppling Analysis of the Echo Cliffs Precariously Balanced Rock
https://resolver.caltech.edu/CaltechAUTHORS:20161213-141035303
Authors: Veeraraghavan, Swetha; Hudnut, Kenneth W.; Krishnan, Swaminathan
Year: 2017
DOI: 10.1785/0120160169
Toppling analysis of a precariously balanced rock (PBR) can provide insight into the nature of ground motion that has not occurred at that location in the past and, by extension, can constrain peak ground motions for use in engineering design. Earlier approaches have targeted 2D models of the rock or modeled the rock–pedestal contact using spring‐damper assemblies that require recalibration for each rock. Here, a method to model PBRs in 3D is presented through a case study of the Echo Cliffs PBR. The 3D model is created from a point cloud of the rock, the pedestal, and their interface, obtained using terrestrial laser scanning. The dynamic response of the model under earthquake excitation is simulated using a rigid‐body dynamics algorithm. The veracity of this approach is demonstrated through comparisons against data from shake‐table experiments. Fragility maps for toppling probability of the Echo Cliffs PBR as a function of various ground‐motion parameters, rock–pedestal interface friction coefficient, and excitation direction are presented. These fragility maps indicate that the toppling probability of this rock is low (less than 0.2) for peak ground acceleration (PGA) and peak ground velocity (PGV) lower than 3 m/s^2 and 0.75 m/s, respectively, suggesting that the ground‐motion intensities at this location from earthquakes on nearby faults have most probably not exceeded the above‐mentioned PGA and PGV during the age of the PBR. Additionally, the fragility maps generated from this methodology can also be directly coupled with existing probabilistic frameworks to obtain direct constraints on unexceeded ground motion at a PBR's location.https://authors.library.caltech.edu/records/na4ma-37d69Probabilistic Estimates of Ground Motion in the Los Angeles Basin from Scenario Earthquakes on the San Andreas Fault
https://resolver.caltech.edu/CaltechAUTHORS:20180427-151741871
Authors: Mourhatch, Ramses; Krishnan, Swaminathan
Year: 2018
DOI: 10.3390/geosciences8040126
Kinematic source inversions of past earthquakes in the magnitude range of 6–8 are used to simulate 60 scenario earthquakes on the San Andreas fault. The unilateral rupture scenario earthquakes are hypothetically located at 6 locations spread out uniformly along the southern section of the fault, each associated with two hypocenters and rupture directions. Probabilities of occurrence over the next 30 years are assigned to each of these earthquakes by mapping the probabilities of 10,445 plausible earthquakes postulated for this section of the fault by the Uniform California Earthquake Rupture Forecast. Three-component broadband ground motion histories are computed at 636 sites in the greater Los Angeles metropolitan area by superposing short-period (0.2–2.0 s) empirical Green's function synthetics on top of long-period (>2.0 s) spectral element synthetics. The earthquake probabilities and the computed ground motions are combined to develop probabilistic estimates of ground shaking in the region from San Andreas fault earthquakes over the next 30 years. The results could be useful in city planning, emergency management, and building code enhancement.https://authors.library.caltech.edu/records/yf7az-03g27Lower Bounds on Ground Motion at Point Reyes during the 1906 San Francisco Earthquake from Train Toppling Analysis
https://resolver.caltech.edu/CaltechAUTHORS:20190201-085310780
Authors: Veeraraghavan, Swetha; Heaton, Thomas H.; Krishnan, Swaminathan
Year: 2019
DOI: 10.1785/0220180327
Independent constraints on the ground motions experienced at Point Reyes station during the 1906 San Francisco earthquake are obtained by analyzing the dynamic response of a train that overturned during the earthquake. The train is modeled as a rigid rectangular block for this study. From this analysis, we conclude that the peak ground acceleration (PGA) and peak ground velocity (PGV) at Point Reyes station would have been at least 4 m/s^2 and 0.5 m/s, respectively. This lower bound is then used to perform simple checks on the synthetic ground‐motion simulations of the 1906 San Francisco earthquake. It is also shown that the hypocenter of the earthquake should be located to the south of Point Reyes station for the overturning of the train to match an eyewitness description of the event.https://authors.library.caltech.edu/records/v9k4v-p4a45Modeling the Rocking and Sliding of Free-Standing Objects Using Rigid Body Dynamics
https://resolver.caltech.edu/CaltechAUTHORS:20200514-144614293
Authors: Veeraraghavan, Swetha; Hall, John F.; Krishnan, Swaminathan
Year: 2020
DOI: 10.1061/(asce)em.1943-7889.0001739
A rigid body dynamics algorithm is presented in this paper to simulate the interaction between two rigid bodies, a free-standing rigid object, and a pedestal that has infinite mass, in the presence of static and kinetic friction forces. Earlier algorithms led to different solutions for the contact forces when parameters external to problem description, such as the ordering of contact points, are changed. This paper addresses the issue of selecting an appropriate solution for the contact forces and impulses from the infinite set of solutions by picking the solution that is closest to the previous state of the rigid body. The capability of this algorithm in simulating pure rocking, pure sliding, and coupled rocking-sliding response modes of a rectangular block is validated using analytical/semianalytical results. This validated algorithm is later used to identify the various response modes of a rectangular block, which is given an initial tilt and then released.https://authors.library.caltech.edu/records/0sb59-t4y92