practical implementation of asce-41 and nlrha procedures ......oriented ub and lb analysis at mce....

2017 SEAOC CONVENTION PROCEEDINGS

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Practical Implementation of ASCE-41 and NLRHA Proceduresfor the Design of the LLUMC Replacement Hospital

Gregory Nielsen PE, Simon Rees SE, Branden Dong PE, Kermin Chok SE,Eaman Fatemi, Atila Zekioglu SE

ArupLos Angeles, CA

Abstract

The Loma Linda University Medical Center CampusTransformation Project (LLUMC CTP) is a new 17 story base-isolated 1,000,000 square foot replacement acute care hospitallocated less than 1 km from the San Jacinto Fault. The seismicdesign and analysis of the structure used LS-DYNA toefficiently perform nonlinear response history analysis(NLRHA) with 110 individual ground motion analysesincorporating DE, MCE, upper bound, lower bound, andvarying ground motion direction. Implementation of triplependulum isolators, fluid viscous dampers, buckling restrainedbraces, and SidePlate moment frames in LS-DYNA will bedescribed. As required for OSHPD-1 facilities, the NLRHAresults demonstrated Immediate Occupancy performance atDE and Life Safety performance at MCE using elementbackbone curves and acceptance criteria from ASCE 41 asamended by the California Building Code. Inconsistencies inelement acceptance criteria for combined lateral systems andother code implementation challenges will be discussed. Acloud computing and database framework, using Penguin-on-Demand and Amazon Web Services, was developed to managethe 8 terabytes of data generated from each set of 110 groundmotion analyses performed on each design iteration.Automated processes enabled the team to reduce the timebetween design iterations to 2 weeks for the complete suite ofNLRHA, post-processing, report generation, and designoptimization. The team’s approach to analysis datamanagement, design optimization procedures based onNLRHA results, automated post-processing, and automatedreport generation will be detailed.

Introduction

The Loma Linda University Medical Center CampusTransformation Project (LLUMC CTP) is a new base isolated1,000,000 square foot replacement hospital in Loma Linda,CA. The project will bring the LLUMC campus in compliancewith the California Hospital Seismic Safety Act and is anOSHPD-1 acute care facility providing Level 3 Traumaservices to San Bernardino County. An image of the structuralmodel is shown in Figure 1. This paper describes Arup’s

performance-based seismic design process for this project,including management of large data sets, nonlinear responsehistory driven design, and navigating the practicalimplementation of ASCE 41-06 criteria and CBC 2013OSHPD amendments.

Figure 1: Structural Revit model illustrating the LLUMC CTPbuilding.

Basis of Design

The hospital building is located at a highly seismic site only 1km from the San Jacinto Fault and 5 km from the San AndreasFault. A site-specific seismic hazard assessment wasperformed by GeoPentech and the resulting DesignEarthquake and Maximum Considered Earthquake spectra areshown in Figure 2. Due to the close proximity of the site to thecausative faults, the design ground motions developed for theproject include near-fault source effects and pulsecharacteristics. In addition, a vertical response spectrum wasalso produced for the site which far exceeded the code


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minimum vertical seismic component (0.2SDS) as shown inFigure 2. Arup’s approach to mitigating this severe verticalseismic component through the use of an innovative verticalisolation system (VIS) will be detailed in a future paper. Asuite of 11 tri-directional ground motions was used to beconsistent with the provisions of ASCE 7-16 and forms thebasis for the nonlinear response history analysis (NLRHA)procedure used for the structural design. These were originallydeveloped at the MCE level and scaled by two-thirds for theDE level. Ultimately 110 individual ground motions wererequired for each design iteration: 11 ground motions, 2orthogonal directions, upper bound (UB) and lower bound(LB) properties, DE and MCE, and an additional 45 degreeoriented UB and LB analysis at MCE.

Figure 2: Horizontal and Vertical MCEr spectra for LLUMCSite

Arup considered a wide variety of structural steel lateralsystem configurations before concluding that the optimalstructural system was a combination of base isolation withBRB’s and SidePlate moment frames above. The choice oflateral system was driven by the high seismicity and pulse-likenature of the site ground motions at periods greater than 1second (SD1 equals 1.54g and is greater than SDS) and theoverall final height and relative slenderness of the hospitaltowers. At 17 stories and 230 feet tall the LLUMC CTP willbe the tallest hospital in California. Initial evaluations into afixed-base biaxial moment frame design proved impossible toachieve the required 1.25% interstory drift even when usingevery column line in the building as a frame and the strongestand stiffest SidePlate moment frame beam-columncombinations permissible by AISC 358-10. Similarly, a fixed

base BRBF solution was rejected due to incompatibilitybetween the required number of brace lines and the functionalprogram of the hospital.

The final selected structural design uses a base isolationsystem comprised of 126 triple friction pendulum bearingswith +/-42” displacement capacity manufactured byEarthquake Protection Systems and 104 fluid viscous damperswith 800 kip MCE capacity manufactured by Taylor Devices.The pendulum isolators have an effective period of 4.5 secondsand the dampers have a velocity exponent of 0.7. The totalequivalent system damping coefficient is 50% of criticaldamping. High damping using supplemental dampers wasselected to control the overall building displacements andreduce reliance on the friction pendulum system for systemdamping, which is affected by the changing vertical load dueto the high site-specific vertical ground motion component.Controlling the isolator displacements to 42” instead of 84”without supplemental damping resulted in an optimal costsolution by controlling the isolator and damper componentcosts, the costs associated with stability framing above andbelow the isolators, and the costs associated with expansionjoint covers and flexible service connections.

Figure 3: Representative BRBF elevation

Stabilizing the top of the isolators is a moment connected gridof 60” deep steel plate girders designed for ImmediateOccupancy performance at MCE. The majority of these


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moment connections are bolted connections to control weldingdistortion in the field during steel erection. A SidePlate specialmoment frame system is used in the East-West (tower longaxis) direction and a BRBF system is used in the North-South(tower short axis) direction. The frames are designed to widenat the base in order to reduce the overall uplift demands on thebase isolation system to manageable levels. A representativebraced frame elevation is shown in Figure 3. The design usesASTM A913 Grade 65 steel sections for columns and selectedBRBF beams in order to achieve the required IO performanceat MCE for these “overstrength” category elements. A full listof the performance requirements of various elements of thestructure are listed in Table 1. While the average result wasrequired to meet the criteria of Table 1, no ground motion wasallowed to result in elements exceeding the CollapsePrevention (CP) limit.

Table 1: Performance requirements for NLRHAElement Performance at

DEPerformance atMCE

FPTIsolator

LRFD Design Expected StrengthDesign

FV Damper LRFD Design Expected StrengthDesign with 1.5 FOS

MatFoundation

LRFD Design,Settlement < 1.5”

Expected StrengthDesign, Settlement <6”

Level AIsolatorFraming

IO, 1 y IO, 1 y

SidePlateColumns

IO, 0.25 y IO, 0.25 y

SidePlateBeams

IO, 0.02 radians LS, 0.03 radians

BRBFColumns

IO, 0.25 y IO, 0.25 y

BRBFBeams

IO, 0.25 y IO, 0.25 y

BRBFBraces

IO, 3 y LS, 10 y

DragConnections

LRFD Design Expected StrengthDesign

Diaphragms LRFD Design Expected StrengthDesign

NLRHA Workflow

The major analytical challenge facing the design team wascreating an analysis framework that could handle the large setsof data generated by the NLRHA models which were largerthan any Arup had previously had to deal with. Arup’s priorwork on the base isolated San Francisco General Hospitalutilized an essentially elastic design on top of isolation with

substantially less nonlinearity in the model and hencesubstantially less analysis time and data generation. ForLLUMC the seismic demands were much higher andoptimization of the NLRHA performance was required toreduce the overall structural tonnage. NLRHA optimizationcould not be practically done without first creating anautomated workflow which could generate, analyze, and post-process all of the required analyses into a reduced formsuitable for design while also minimizing the feedback time tofit within the aggressive project schedule. A single designiteration of 110 ground motions resulted in over 6 TB of datagenerated and over 10 design iterations were conductedthrough the course of the project from Design Development tofinal Permit. From the outset, the goal was to turn the timeintensive NLRHA into a practical tool in the designer’stoolbox, similar to response spectrum analysis, so that from adesigner’s view NLRHA was not used as a final performanceverification but rather an integral design process.

The team chose to use LS-DYNA as the analysis engine for theproject. This choice was for the following reasons:

• Excellent model stability under 3d ground motions

• Reduced analysis time using explicit time domainsolver versus other software

• Staff familiarity

• Ability to use with cloud computing services

There was a considerable learning curve in introducing LS-DYNA to OSHPD plan review staff. The design utilized theCollaborative Plan Review (CPR) process which allowed formonthly meetings between the design team and the reviewteam during the 18 month review duration which was essentialin helping OSHPD become confident in the nonstandardanalysis tools. A bounding analysis study was performed inorder to select the most demanding set of bounding parametersfor the soil behavior, isolator behavior, damping behavior, andBRB strengths. Figure 4 shows the result of the boundinganalysis on a 2-dimensional frame which indicated that theleast favorable sets of bounding values corresponded to LowerBound of all parameters and the Upper Bound of allparameters. The Lower Bound analysis controlled the isolatordisplacements and damper velocities while the Upper Boundanalysis controlled the superstructure drifts, frame demands,and floor accelerations. Selecting a single Lower Bound and asingle Upper Bound analysis reduced the possiblepermutations considerably and was essential in keeping theanalysis set to a manageable 110 ground motions.


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Figure 4: Results of 2d frame bounding analysis (run 1 iswith all upper bound properties, run 16 is with all lowerbound properties)

In addition, a number of verification analyses were performedprior to analyzing the full 3d model to ensure that the modelingmethodology used in LS-DYNA appropriately captured theessential physical behavior and met OSHPD’s reviewrequirements. These elements and their LS-DYNA materialsare tabulated in Table 2.

Table 2: List of LS-DYNA materials and applicationElement LS-DYNA material model and calibrationFPTIsolator

Three single pendulum*MAT_SEISMIC_ISOLATOR elements inseries. An additional*MAT_GENERAL_NONLINEAR_1DOF_DISCRETE_BEAM element was added inparallel to model the failure behavior of theisolator rim at maximum displacement. Theperformance of the combined element wascompared to MCEER Technical Report # 08-0018 for 3d movement and uplift behavior.

FVDDamper

*MAT_NONLINEAR_VISCOUS_DAMPER element in parallel with a*MAT_SPRING_GENERAL_NONLINEARelement to model the lock-up strength andstiffness of the viscous damper andconnections upon exceeding the strokecapacity of the damper. This stiffness wascalibrated to physical test measurements andthe strength was capped as 1.5x the MCEdamper capacity.

SidePlateBeamElements

*MAT_HYSTERETIC_BEAM elementscalibrated to experimental SidePlate momentframe cyclic test results.

OtherMomentFrameElements

*MAT_HYSTERETIC_BEAM elementscalibrated to ASCE 41-06 backbone curves.

BRBbraces

*MAT_HYSTERETIC_BEAM elementscalibrated to experimental cyclic test resultsof Nippon Steel BRBs.

SoilSprings

*MAT_SPRING_GENERAL_NONLINEARelements calibrated to ASCE 41-06 backbonecurves.

Figure 5: Illustration of the NLRHA Workflow

The NLRHA workflow is illustrated in Figure 5 including thevarious time durations for each activity. The framework forpre-processing and visualization utilizes a MySQL databasewith a graphical front end using Rhino and Grasshopper. Theanalysis uses cloud analysis nodes from Penguin-On-Demand(POD) and a small number of post-processing operations suchas floor drifts are performed on the analysis nodes. After theanalyses are complete, analysts can view the results remotelyto check for any bugs or spurious results. After this qualitycontrol process, the resulting data is then transferred toAmazon Web Services (AWS) Redshift for the remaining bulkof the post-processing and images of the visualized data arecaptured in report-ready format. The report is then compiledback on the local server where the design team can assess theperformance of the model and make the necessary changes tofurther optimize the behavior. The overall time from theimplementation of a design change to having the full suite ofcompiled results and engineering demand parameters (EDP’s)was reduced to 1 week. This short feedback time allowed thestructural design to be finely tuned over multiple iterations andwithin the framework for design and review.

The process begins with the structural Revit model whichdefines the basic frame geometry and section sizes. These arepulled into the database and the structural analysis geometry isgenerated based on the various element types. Both the lateral


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and vertical force resisting elements are imported, includingthe floor slabs. Rigid end offsets, element discretization,element offsets, and particular material assignments are addedusing a set of Grasshopper components ensuring the generatedanalysis geometry model is fully connected and aligns with allof the essential parameters validated in the LS-DYNAvalidation package signed off by OSHPD. As part of thisprocess the floor slabs are generated as cracked elastic 2d shellelements with the required seismic mass and all gravity beammembers are included as elastic beam elements. While thebaseline output is that all elements and nodal displacements arewritten by LS-DYNA at 20 Hz sampling rate, selected nodesand elements, such as drift nodes, building separation nodes,and floor acceleration nodes, must be selected prior to analysisto output at a higher 200 Hz sampling rate. A 200 Hz samplingrate is not used on all elements as this would generate moredata than is physically necessary to capture the relevant EDPsand slow the entire process. Once all of these assignments arecomplete, the Grasshopper components write the necessarykeyword cards for the complete LS-DYNA analysis models inall 110 variants (ground motion, bounding properties,directionality, and severity). This model preparation process isfully automated. An image of the LS-DYNA model is shownin Figure 6.

Figure 6: Image of the LS-DYNA analysis model.

The analysis models are uploaded to the Penguin-On-Demand(POD) cloud computing cluster which has sufficient scale toallow all 110 models to be run in parallel. The longest groundmotion was able to run on a single compute node in 8 hours sothe full suite of analyses could be sent off at the end of theworkday all runs would be finished by the following morning.The models are then checked for spurious results or earlytermination errors, which could be corrected during theworkday, and then some minor post-processing wasperformed. Post-processing on the more expensive compute

nodes was limited to those processes which primarily servedto reduce the data sets, for instance reducing a set ofdisplacement histories into drift histories or converting damperrelative displacements from the three axes of globaldisplacement to a single rotating along-damper axis. Theessential data was then transferred to AWS Redshift, a clouddatabase warehousing service, where it could be stored,queried, and processed at a cheaper cost per core. Due to ITinfrastructure limits in Arup’s LA office, the transfer fromPOD to AWS was much faster than transferring the data fromPOD to Arup for internal processing. The AWS platform alsoallows for scalability and automatic backups for the large datasets. At the time that the LLUMC project was in analysisproduction, two other projects of similar scale and datageneration were being run out of the same office. Having acentral off-site repository allowed the teams to focus ondelivering the projects using a common set of tools rather thanspending valuable time maintaining internal IT infrastructure.

Finally, the data was post-processed on AWS Redshift usingstandard SQL queries to turn the 110 individual records intosets of EDP’s representing the average of the maximumresponse from each ground motion over the ground motionsuites. This final database of EDP’s was visualized in Rhinoon the same geometry wireframe model used to generate theanalysis models at the start. These final visualizations, alongwith a hard drive with all of the post-processed data tables,analysis run files, and keyword files, were submitted toOSHPD for review. Intermediate iterations were used tooptimize the performance of the structure primarily usingconservation of energy approaches for resizing deformation-controlled elements with target inelastic deformation limits.The EDPs tracked are tabulated in Table 3. A sample of thefigures generated from the process is shown in Figure 7.

Table 3: List of EDPs tracked for NLRHAElement Metadata OutputForce-Controlledelements

Forces and inelasticdeformations

Deformation-Controlledelements

Forces and inelasticdeformations

FPT Isolators Forces, displacements, and riminelastic displacements

FV Dampers Forces, stroke displacement,stroke velocity, lock upelement force and deformation

Gravity momentconnected elements

Forces

Drag, collector, anddiaphragm trusselements

Forces

Soil Springs Forces and inelasticdeformations

Building Separations Relative displacements


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Interstory Drift Relative drifts at key points inbuilding

Floor Accelerations Accelerations at key points inbuilding

Lateral Frames Story force time histories fromcross-sections

Diaphragms Moment and shear envelopesderived from concurrent storyforce time histories

Figure 7: BRBF Frame elevation showing plastic strains fromDE suite

Practical Implementation Issues

The design and plan review process of the LLUMC CTPproject illustrated a number of possible differences inreasonable interpretation of code requirements as well aspotential unintended consequences of certain code provisions.A number of these issues are summarized below and worthfurther investigation by other researchers as part of future in-depth studies.

Vertical Load Factor for NLRHA of FrictionPendulum Isolated buildings

A unique feature of friction pendulum isolated buildings is thatthe shear force in the bearings is directly proportional to theaxial load applied to the bearing. This can lead to, and did onthe LLUMC project, a difference in interpretation regardingthe appropriate load combination to be applied to the overallbuilding when performing NLRHA. This issue has previouslybeen avoided in the designs for elastic structures on top ofisolation in which case the NLRHA was performed using anunfactored 1.0 D combination roughly equivalent to theseismic mass and then factoring the additional dead loads onthe superstructure elements using linear superposition.However this is not possible for a combined NLRHA modelwith nonlinearity in both the isolation system and thesuperstructure lateral frame. A number of interpretations couldbe considered ranging from 1.0 D + 0.25 L (ASCE 7-10Section 16.2.3) to as much as 1.2 D + (0.2SMS W) + 0.5L or aslittle as 0.9 D – (0.2SMS W) (ASCE 7-10 Section 17.8.2.5). OnLLUMC this maximum case could require the building to beanalyzed as though it was under 0.48g or 1.62g, meaning thatthe same seismic force could act on the building but with 50%or 160% of the lateral resistance in the isolators. A series ofanalyses were performed to illustrate that the effect of the timevarying vertical ground motion on the lateral shear of theisolation system was on the order of 5% variation in shear and2% for isolator displacement. This result is primarily due to thelarge amount of damping present in the isolation systemdespite the very high vertical ground motions. The final loadcombinations for the NLRHA lateral analysis were selected asa compromise between the two extremes:

• Design Earthquake Cases: 1.0D + 0.2SDSD + 0.25L and0.9D – 0.2SDSD

• Maximum Considered Earthquake Cases: 1.0D + 0.25L

The individual member design for force controlled elementsused additional load factors on the vertical loads per theapplicable sections of the code. It should be noted that the loadcombination of 1.0D + 0.25L does not correspond to theseismic weight of the building and therefore still results inconservatism in lateral resistance in friction pendulum isolatedbuildings.

Gravity Column Deformation Compatibility for SteelFrames

ASCE 7-10 Section 12.12.5 requires deformationcompatibility to be assessed for non-seismic force-resistingsystem elements due to the displacements at the design storydrift. For steel buildings designed in accordance with AISC341-10, Section D3 applies. However since only the User Note


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of Section D3 specifies that “flexible shear connections thatallow member end rotations per Section J1.2 of theSpecification should be considered to meet theserequirements.” The commentary further illustrates that onlydifferential drift between floors leads to additional columnmoment demands. Since this is not written into the body of theSeismic Provisions additional analysis was required toillustrate that the columns did not have inelastic rotationdemands when the drift histories of the building were applied.To prove this a typical gravity column stack was modeled inLS-DYNA with ASCE 41-06 plastic hinge properties modeledand FEMA 355 gravity shear tab inelastic hinges where thecolumns connected to gravity beams. The full suite of DE andMCE ground motions were then applied in two orthogonaldirections and the plastic rotation was monitored. Only 1ground motion at MCE resulted in hinging in the column. Thesizing design for the columns considered a 0.3% differentialstory drift as an added moment in the LRFD column designand appears to be a reasonable design factor for this buildingwhich was designed to 1.5% drift at DE. While the projectrequired that this be proven through rigorous methods it is alogical result considering that the NLRHA did not indicatehinging in force controlled columns which are part of thelateral system and, in the case of the moment frames, are muchstiffer than the gravity columns and hence will yield at a lowerrotation than the gravity columns. The use of Grade 65 steelfor the column sections also delays the onset of plastic hingingand is recommended.

BRBF Beam Plastic Rotation Limits Incompatiblewith BRB Strain Limits

An unintended consequence was discovered by virtue ofhaving modeled all of the lateral frame elements with plastichinges. ASCE 41-06 requires that beams of buckling restrainedbraced frames be treated similarly to the columns of the sameframes and in some cases are force controlled. However, theselow plastic rotation thresholds appear to be incompatible withthe BRB strain limits of 3 y and 10 y for IO and LSperformance, respectively. The imposed deformation patternof the frame in the NLRHA that is required to yield the braces,particularly for the W18 or W21 beam sections which are usedin typical designs, often results in inelastic behavior of thebeams which cannot be controlled except by increasing thestiffness and strength of the braces. This in turn increase thecolumn and brace forces required for capacity design. Due tothese reasons, the typical BRBF beam on the LLUMC projectis a stocky W14 column section since its shallow depth resultsin delayed onset of a flexural hinge. Further the majority of theBRBF beams were proportioned to have less than 0.5 P/Pcl soas to keep them in the deformation controlled category ofASCE 41-06. Nonetheless this still required a plastic rotationof 0.25 y in accordance with ASCE 41-06. In a few selectinstances the beams were even increased to Grade 65 to further

delay onset of the flexural hinge. It should be noted that themagnitude of the forces in the BRBF beams was too great tomake a true pin connection viable. This appears to be anunintended consequence of the code since it does not appearlogical to design a BRBF based on the performance of themoment connected frame rather than rely on the energydissipating capacity of the BRB itself. It is unlikely that priorprojects utilizing deeper beam section and allowed to driftbeyond 1.5% perform as required by this strict interpretationof the code provisions.

Floor Accelerations – Expected versus Actual

Seismic isolation is often used to reduce in-floor accelerationsfor sensitive equipment. However, as Table 4 shows, the flooracceleration results from LLUMC were in some cases higherthan the code prescriptive floor accelerations for nonstructuralcomponent design in a fixed base building at the same site. Therelatively tall and flexible structure on top of the isolationplane likely leads to higher accelerations than may beoriginally expected. Further, the design team found that LS-DYNA tends to predict higher accelerations (by up to 50%)than SAP2000. This will be investigated in the future andcompared to actual experimental shake table tests. It may alsobe the case that the prescriptive code floor accelerations fornonstructural components in fixed base buildings may belower than they should be to ensure reliable performance. Thisshould also be investigated further and compared to actualinstrumented buildings and experimental tests.

Force-Controlled Frame Columns

A final lesson learned relates to force-controlled columns andtheir treatment in the NLRHA model. Per the requirements ofASCE 41-06 these force-controlled columns should bemodeled as elastic elements and the average of the maximumforces developed in these elements from the suite of groundmotions should be compared to the capacity of the column.However this could underestimate the response of the buildingif these elements were to exceed their capacity in any oneground motion. Due to this fact OSHPD requested that forcecontrolled elements be modeled with axial load dependentinelastic hinges. This then created the problem ofinterpretation of results from the average of the suite ofanalyses. If any one of the motions resulted in plastic hinging,no matter how slight, the average of the suite would indicate anonzero plastic rotation. This would then no longer meet therequirement that there should be no yielding in force-controlled elements. The only way to satisfy the requirementwould be to design the column such that it did not hinge in anyof the ground motions, which clearly represents overdesignrelative to the averaging methodology used throughout ASCE41-06. This issue was finally concluded by allowing a plasticrotation of up to 0.05 y in force-controlled elements. This also


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highlights the need to remain consistent with the procedures ofa single standard without modification.

Conclusion

The LLUMC CTP project represents one of the first designsby Arup utilizing NLRHA as an integral part of the designdevelopment and optimization process. Key to unlocking thispotential was the development of an automated workflow forassembling, analyzing, and post-processing complex and verylarge structural analysis models in a short enough timeduration to respond to design changes. This required aconsiderable investment in analysis, database, andvisualization tools but reduced the engineer time required byan order of magnitude while increasing the throughput ofanalysis models. The project generated over 50 TB of dataover 1000 individual analyses during the design andoptimization process. This optimization process was essentialin eliminating every excess ton of structural steel on theproject and mitigating the substantial budget impacts of anextremely challenging site. This case study illustrates thatdesign and optimization using NLRHA is feasible and theframework is set to apply this methodology to future projectsto increase performance reliability and reduce cost.

practical implementation of asce-41 and nlrha procedures ......oriented ub and lb analysis at mce....

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