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Lateral Force-Resisting Structural Elements and Systems Project

Summary:

This project provides experimentally validated structural behavior models for refining key seismic design provisions in U.S. model building codes and standards that are used for designing lateral force-resisting elements. There are three parallel tasks:

  • Develop accurate behavioral models for deep, slender wide-flange steel beam-columns that are subjected to large earthquake-induced displacements. This is the initial implementation of the long-term research plan outlined in NIST GCR 11-917-13[1] that was developed by leading practitioners and researchers in the structural steel field.
  • Develop accurate structural response models for base plate connections for wide-flange steel beam-columns that are subjected to earthquake ground motions. This research is also outlined in NIST GCR 11-917-13, and is identified as a highest priority study in NIST GCR 13-917-23[2], the new "roadmap" of practitioner and researcher recommendations for high priority NIST research.
  • Develop accurate behavioral models and corresponding new design provisions for reinforced concrete walls that address a range of wall design parameters, including wall slenderness and ground motion-induced uplift forces.

 


[1] Research Plan for the Study of Seismic Behavior and Design of Deep, Slender Wide-Flange Steel Beam-Column Members, NIST GCR 11-917-13, 2011.

[2] Development of NIST Measurements Science R&D Roadmap: Earthquake Risk Reduction in Buildings, NIST GCR 13-917-23, National Institute of Standards and Technology, Gaithersburg, MD, 2013.

 

Description:

Objective: This project provides recommended refinements in key seismic provisions in U.S. model building codes and standards, including:

  • By 2016, develop accurate behavioral models for deep, slender wide-flange steel beam-columns, assisting designers in characterizing earthquake behavior, as required for nonlinear Performance Based Seismic Engineering (PBSE) (ASCE 41[3]) analysis.
  • By 2017, develop accurate behavioral models and corresponding design rules for base plate connections for steel beam-columns in earthquake-resistant construction, providing important guidance to practitioners by augmenting or replacing current approaches that are not robust.
  • By 2015, develop new experimental data, accurate behavioral models and corresponding new design provisions for reinforced concrete structural walls that addresses issues found in wall behavior during the 2010 Maule, Chile, earthquake.

What is the new technical idea?

Task (1), Steel beam-column models: In earthquake-resistant steel frame building design, lateral force resistance is often concentrated in a few frames or bays of frames. Beam-columns in these systems are subject both to significant axial force, particularly in lower stories, and to bending. In new construction, wide-flange beam-column sections are often selected to be deeper and more slender, to increase in-plane stiffness, which reduces earthquake motion-induced drift, but can lead to member instabilities as displacements increase. The American Institute of Steel Construction (AISC) identified improved characterization of this behavior as a critical research need[4]. For such characterization, the behavior of plastic hinges that develop in these sections is particularly critical. Available test data on slender beam-columns loaded in this manner are limited. This lack of data was identified in the long-term research plan outlined in NIST GCR 11-917-13[5] for steel beam-columns that was developed by leading practitioners and researchers in the structural steel field. In response to this high priority need, experimental testing to provide such data will be performed by the NEHRP Consultants Joint Venture (NCJV) for NIST via an FY 2012-funded task order. This project will use the results of these laboratory tests to develop validated nonlinear models of beam-column elements.

Task (2), Base plate connection models for steel frame columns: Current methods for designing column base plates and their foundation anchorage for steel moment frames and braced frames are not rigorous. Failures of plates, plate-to-column connections, or plate anchorages can compromise the development of assumed yield mechanisms in these structures. The NIST BSSC R&D Roadmap[6] has identified improving base plate design as a high priority research need. Baseplate behavior characterization has also been independently identified by AISC as a critical research need[7]. Base plate behavior research was also outlined in the long-term research plan for steel beam-columns outlined in NIST GCR 11-917-13[8]. Available test data on base plate connections subjected to cyclic-type loadings are limited. Expanding the data set supports enhancing the accuracy of building codes and design standards, developing improved design and assessment provisions, and assisting designers in identifying behavior as required for nonlinear Performance Based Seismic Engineering (PBSE) (ASCE 41) analysis. Experimental testing to provide data on steel beam-columns, including base plates, will be performed by the NEHRP Consultants Joint Venture (NCJV) for NIST via an FY 2012-funded task order. This project will use the results of the laboratory tests to develop validated nonlinear analytical models of base plates associated with these beam-columns and in turn develop rational rules and code provisions for base plate design.

Task (3), Reinforced concrete (R/C) structural wall performance: Improved R/C wall performance was ranked as an urgent technical need in the NIST BSSC Measurement Science R&D Roadmap[9]. The 2010 Maule (Chile) earthquake highlighted possible shortcomings in some design configurations of R/C structural walls. Many walls in low-rise and mid-rise buildings that were designed according to Chilean building codes, which generally comply with customary U.S. practice, except for reinforcement confinement detailing, exhibited poor performance during strong shaking. Post-earthquake analysis in NEHRP Consultant Joint Venture (NCJV) work on two separate NIST task orders addressed Chile earthquake implications on wall design. The first task produced a baseline report comparing Chilean and U.S. model building code provisions for such walls[10]. The second task is nearing completion, with draft NIST GCR 13-917-25[11] in review.

The consensus of structural wall experts involved in the NCJV study is that the poor wall behavior observed in Chile has potential negative implications for U.S. design standards. Major focus areas in the NCJV work include wall confinement and detailing, overall demands on structural walls in terms of axial loads combined with shear forces and moments, and wall thickness and slenderness characteristics. Current U.S. model building codes do not limit absolute wall thickness or slenderness, although the Uniform Building Code[12] once had limits that were lost when the International Building Code[13] (IBC) came into existence. Structural wall performance is a complex issue and the opinions of national experts are divided on the role of wall demands, detailing, and wall thickness/slenderness on performance, thus pointing to the need for a comprehensive research program into this question (Birely, et al[14]). The project will assimilate the results of NIST-funded laboratory testing of R/C wall sections at the U.S. Army Engineer Research and Development Center (ERDC) Construction Engineering Research Laboratory (CERL) with a thorough program of nonlinear analytical modeling that will produce new wall modeling techniques for use by practitioners and suggest wall detailing improvements that can be incorporated in model building codes and standards (ACI-318[15]).

What is the research plan?

Task (1), Steel beam-column models: This research involves a coordinated experimental and analytical study that is outlined in the Beam-Column Research Plan, NIST GCR 11-917-13. An FY 2012 NCJV task order supports a comprehensive experimental research program to characterize the behavior of approximately 20 deep, slender beam-column members that are considered stability-sensitive at large deformations. NCJV will subcontract with one or more well-qualified large-scale structural testing laboratories (e.g., member sites of the NSF-funded Network for Earthquake Engineering Simulation) to perform the tests. The loading protocol for testing the beam-column elements will ensure that plastic hinges form, enabling analysis of their inelastic behavior under different combinations of axial loads and bending moments and providing interaction information in the inelastic regime, data which are currently unavailable. This project will complement those experiments with analytical modeling to develop validated design relationships, including axial load-bending moment interaction modeling equations for model building code provisions, as well as to develop modeling guidelines for beam-column analysis. The numerical modeling will employ advanced nonlinear finite element analysis, to analytically evaluate changes in boundary conditions from the idealized ones employed in the experiments, and assess the larger question of inelastic stability of simple frames. Appropriate recommendations for future research to expand and extend the validity of the design relationships and the data available will also be identified. A detailed research plan, including the names of all project participants for the FY 2012 task order, will be developed prior to NCJV's initiation of the experimental testing program. The 2012 project description for that work will be revised at that time. Subcontractor award for the FY 2102-funded testing is anticipated in July 2013.

Task (2), Base plate connection models for steel frame columns: This project will synthesize and consolidate the limited existing research found in the literature with results from experimental and analytical work being conducted as part of the study of steel beam-column behavior outlined above. Base plates will be designed in accordance with current provisions for the beam-column test specimens. Advanced nonlinear finite element analysis will be used to identify how the base plate configuration interacts with demands from the tested beam-columns, and to determine problematic areas with current design provisions and demands on connection anchorage. Test data will be evaluated and analyzed to identify parametric relationships between base plates and columns that control performance. The analytical work will also be used to expand the range of applicability of design guidance beyond the limited data obtained by tests. Once completed, the observations from analysis and testing will be utilized to provide new, robust design rules that better characterize base plate design for implementation in model building codes.

Task (3), Reinforced concrete (R/C) structural wall performance: The research goals will be achieved through a combined numerical simulation and experimental research program. The experimental program, which was funded via an FY 2012 Interagency Agreement with ERDC, will be conducted at the ERDC /CERL. The experimental research will design, construct, instrument, and test approximately 20 R/C structural wall specimens that will be designed to fail in flexure, but with a varied range of selected major design parameters, including: wall slenderness (wall height-to-thickness ratio), absolute wall thickness, axial load stress ratio (axial stress divided by f'c), wall confinement and detailing, boundary element detailing and shear demand. Secondary parameters to be investigated include geometric aspect ratio (wall length to wall thickness) and wall core thickness to wall thickness ratio. Walls will have absolute wall thicknesses of 8-12 inches, representing walls on the "thinner side" of typical U.S. design practice. Available data on structural walls do not include significant research results in this region of wall thickness, which is larger than the very thin walls seen to do poorly in Chile. The intent is to establish the point at which walls designed in accordance with U.S. design rules become too thin. In addition, a special loading protocol will be developed to investigate the effect of significant early wall tensile loading (from uplift forces) during an earthquake, and its impact on wall performance and behavior. The portion of the research supported by this project includes project oversight by NIST of the testing program, including review of design and construction work, witnessing of tests, and review of data reduction and nonlinear analysis results. An extramural project management (oversight) committee composed of key R/C researchers and practitioners, experienced in the design of structural walls, has been formed to assist with the experimental program formulation and to provide technical feedback. Upon completion of the research program, building code recommendations for the ACI 318 Building Code and corresponding IBC provisions will be developed with an ultimate impact on U.S. building code requirements (IBC, ACI) and design practices.

 


[3] Seismic Rehabilitation of Existing Buildings, ASCE/SEI 41-06, American Society of Civil Engineers, 2006.

[4] Seismic Provisions for Structural Steel Buildings, ANSI/AISC 341, American Institute of Steel Construction, Chicago, IL, 2010.

[5] See reference [1].

[6] See reference [2].

[7] Specification for Structural Steel Buildings, ANSI/AISC 360, American Institute of Steel Construction, Chicago, IL, 2010.

[8] See reference [1].

[9] See reference [2].

[10] Comparison of US and Chilean Building Code Requirements and Seismic Design Practice 1985-2010, NIST GCR 12-917-18, 2012.

[11] Recommendations for Seismic Design of Reinforced Concrete Wall Buildings Based on Studies of the 2010 Chile Earthquake, NIST GCR 13-917-25, 90% Draft Report, 2013.

[12] Uniform Building Code, International Congress of Building Officials, Whittier CA 1997.

[13] International Building Code, International Code Council, Washington, D.C. 2009.

[14] Investigation of Performance of Slender RC Structural Walls, Anna C. Birely, Laura N. Lowes, and Dawn E. Lehman, Department of Civil and Environmental Engineering, University of Washington, 2011.

[15] Building Code Requirements for Structural Concrete, ACI 318-08, American Concrete Institute, Framingham, MI, 2008.

 

Major Accomplishments:

Outcomes: There has been limited progress to date on two of the three 2013 tasks in this project, since much of the work envisioned for the project is starting in late 2013. Start of work on steel beam-column models has been delayed because of continuing work on Assessment of Design Methods in Existing PBSD Standards by the Project Leader – the volume of work required for that effort significantly exceeded the anticipated workload. Start of CFS shear panel design guidance work at ERDC/CERL has been delayed by slow progress in processing of the Interagency Agreement with ERDC/CERL; this work will be starting in Q4 FY2013. The work on the R/C structural wall study is well underway, with testing to commence in Q4 FY2014.