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Assessment of Design Methods in Existing PBSD Standards Project

Summary:

The project involves critical problem-focused studies in Performance Based Seismic Engineering (PBSE) concerning the applicability and accuracy of adopting first-generation PBSE analysis methods used for evaluating existing buildings in the performance-based design of new buildings for both reinforced concrete and structural steel structures. The goal of this project is to evaluate the results of the studies and identify necessary changes to current model building codes and standards provisions that will accommodate more universal use of PBSE. The project will also result in an expanded archetype building model library for use in future NIST research projects. Projected completion of the structural steel portion of the study is the end of the fourth quarter of FY2012; the reinforced concrete study will be completed in the third quarter of FY 2013.

Description:

Objective: Reinforced concrete and structural steel frame buildings, designed using prescriptive ASCE 7[1] rules for new buildings, will be evaluated using ASCE 41[2] nonlinear response analysis rules for existing buildings that are being applied by practitioners who are engaged in PBSE, to identify inconsistencies in present PBSE methodology and to develop recommended modifications to current nonlinear response analysis provisions ASCE 7 and ASCE 41 by FY2014.

What is the new technical idea? With no specifically prescribed performance-based seismic engineering (PBSE) technique in ASCE 7, practitioners are now applying procedures for evaluating existing buildings that are detailed in ASCE 41 to analyze designs of new buildings. This approach is commonly seen as “PBSE.” This project seeks to validate this approach or provide recommendations to improve it. A number of structurally regular buildings[3] of varying numbers of stories are being designed in compliance with current prescriptive ASCE 7 design procedures for new buildings located in areas of high seismicity. These designs include structures with reinforced concrete moment frames, structural steel moment frames, structural steel concentric braced frames and structural steel eccentric braced frames. The designs are then being assessed using current ASCE 41 PBSE procedures devised for seismic evaluation of existing buildings. Seismic performance predicted using ASCE 41 ideally should confirm that new buildings designed using ASCE 7 will perform well in areas of high seismicity. The question is whether the ASCE 41 evaluation is consistent for designs produced conforming with the prescriptive procedures of ASCE 7. The project will identify inconsistencies between the two standards’ methodologies, with the results used to suggest improvements needed for ASCE 7 and/or for ASCE 41 to eliminate these differences. The project approach, methodologies and results will be reviewed by an extramural peer review team of leading practitioners and researchers, retained via NEHRP Consultants Joint Venture (NCJV) task order.

What is the research plan?

Structural steel portion of the project: Three sets of structural steel buildings containing combinations of concentric and eccentric braced frames with moment frames, have been designed, all in accordance with ASCE 7. Three different building heights (4, 8 and 16 stories) are included for each system, to capture the effects of higher response modes and P-Delta effects, and to study where plastic hinges develop. Each structural system has been designed for two target seismic hazard levels representing regions of high seismicity; modeled in detail, including material and geometric nonlinearities; and analyzed using nonlinear finite element analysis programs to determine dynamic response to a suite of selected earthquake ground motions.

Each completed building design is being evaluated using each of the four methods presented in ASCE 41, with particular emphasis given to nonlinear dynamice analysis, to judge compliance with predefined seismic performance objectives. Based on the ASCE 41 assessment and comparison with the ASCE 7 design results, the applicability of using ASCE 41 PBSE for the design of new structures is being quantified, indicating under what conditions the ASCE 41 methodology does not meet the design intent of the ASCE 7 standard. This assessment is the first of its kind, and will be disseminated through NIST publications and peer-reviewed journal articles.

An extramural Peer Review Committee (PRC) that includes leading practitioners and researchers has been formed. The PRC held an initial review meeting in February 2012, and recommended a number of additional issues to be studied, as well as several other project refinements. The additional work recommended by the PRC has significantly delayed previously stated 2012 project milestones and lengthened project work time. The PRC recommendations also impact the reinforced concrete part of the study and will be incorporated into that portion of the project as it proceeds.

The steel portion of the project will be completed in Q4 FY2012, a delay resulting from the PRC recommendation. The 2012 milestones do not appear in the list of milestones below, except for publication outputs.

Reinforced concrete portion of the project: Three sets of reinforced concrete (R/C) moment frame buildings have been designed in accordance with ASCE 7. Three different building heights (4, 8 and 16 stories) are included, as with the steel portion of the project. The designs have been completed for one of the two target seismic hazard levels; the second design level will be completed next.

Once the design work is completed, detailed finite element models will be developed and nonlinear dynamic analyses will be run to determine response to a suite of ground motions. Once this is complete, the ASCE 41 evaluation process will be performed as described earlier. The PRC will be engaged at the start of the nonlinear finite element work and at the conclusion of the ASCE 41 assessments. Informal contacts with the PRC chair also will be made at appropriate points during the completion of the work.

The R/C portion of the project will be completed in Q2 FY2013, with a technical report, publications and recommended standards provisions to follow in Q3 FY2013. PRC review of the R/C design assessments could impact these milestones, but delay is not anticipated.

Completing the reinforced concrete portion of the project was impacted by the June 2012 departure of co-PI Jeff Dragovich. Replacing the departing co-PI with a new research structural engineer (TBD) in a timely manner is essential to meet the schedule that is outlined. Moreover, originally a set of shear wall buildings was to be designed and evaluated, in addition to the moment frames. This work is not included for 2013 and will be undertaken once the new research structural engineer is on board. This scope of work will be scheduled under a later, separate project.

A significant project result will be the development of the NIST EL Building Models Library that will contain these peer-reviewed archetypical building designs for use by EL researchers in future projects. At the conclusion of this project, there will be approximately 35 steel and concrete building models that will be completed and peer reviewed. The intent is to merge other EL building models, notably those from progressive collapse studies, into a common EL Building Models Library. These buildings will then be a resource that can be utilized by EL researchers in future work.

 


[1] Minimum Design Loads for Buildings and Other Structures, ASCE 7-10, American Society of Civil Engineers, Reston, VA 2010

[2] Seismic Rehabilitation of Existing Buildings, ASCE 41-06, American Society of Civil Engineers, Reston, VA 2006

[3] Regular buildings are defined here as buildings whose seismic force-resisting systems (SFRS) do not contain any horizontal or vertical irregularities as defined in modern seismic code provisions (e.g., ASCE 7-10)

 

Major Accomplishments:

Recent Results:

Outputs:

  • Presentation of research with accompanying paper at 2011 SEAOC conference: Harris, J. L. III, Speicher, M. S. “Seismic Assessment of New Steel Buildings Designed per ASCE 7 with ASCE 41,” SEAOC Convention Proceedings, 2011.
  • Presentation of research with accompanying paper at the 2012 STESSA conference:  Speicher, M. S., Harris, J. L. III. “Seismic Assessment of a New Steel Moment Frame Designed Per ASCE 7 with ASCE 41,” STESSA Conference Proceedings, Santiago, Chile, 2012.

Standards and Codes:

The project results and conclusions will be used to develop proposed codes and  standards provisions for the next cycle of ASCE 41, ASCE 7, ACI 318, the ICC International Existing Building Code and the 2014 NEHRP Recommended Provisions for Seismic Regulations for New Buildings and Other Structures. It is anticipated that project findings will be used to refine Section 16.2, Nonlinear Response History Procedure, of ASCE 7, and Section 3, Analysis Procedures, in ASCE 41. Co-PI Harris is on the current ASCE 41-13 Steel Structures Subcommittee committee and the ASCE 7 Seismic Subcommittee (TC-2). PI McCabe is a former ACI 318 member representing NIST on the ACI 318-H Seismic Subcommittee and on the ACI 369 Seismic Repair and Rehabilitation Committee. In addition contact with the incoming chair of ASCE 41 has already been initiated.