Seismic Behavior of Deep Steel Beam-Columns with Plastic Hinges

This project will characterize the behavior of plastic hinge(s) that develop in deep, slender wide-flange steel beam-columns at large deformations generated by strong ground motion from earthquakes. This is the first project in implementing the long-term research plan developed by the NIST-funded ATC-90[1] project. Both analytical and experimental research will be conducted, with a goal of providing needed test data for this class of structural element that enables refinement of model building code provisions and nonlinear analysis procedures. [1] NEHRP Consultants Joint Venture (2011). Seismic Behavior and Design of Deep, Slender Wide-Flange Steel Beam-Columns – ATC–90, Produced by the NEHRP Consultants Joint Venture, a partnership of the Applied Technology Council and the Consortium of Universities for Research in Earthquake Engineering, for the National Institute of Standards and Technology, Gaithersburg, MD, NIST GCR 11-917-X, in progress.

Objective: The NIST-funded ATC-90 [1] project is producing a long-term research plan to characterize the seismic behavior of deep, slender wide-flange steel beam-columns used in moment frames. The research will investigate seismic behavior at the member-level, concentrating primarily on the behavior of plastic hinges[2] after they have formed during ground motion-induced strong shaking. The project will be four years in duration and completed by 2015 [3].

What is the new technical idea? This project will characterize the behavior of the plastic hinge(s) that develop in deep, slender wide-flange steel beam-columns at large-deformations generated by strong ground motions from earthquakes. Results from this project will:

·        Develop force-dependent (i.e., axial force–moment interaction) phenomenological hysteretic models that account for cyclic and in-cycle degradation, for use in nonlinear dynamic analysis. Behavior to be characterized includes axial-moment-rotation, axial-moment-curvature, and axial-displacement. Data of this kind are very limited at present and are needed for developing improved design and assessment provisions, and for assisting designers in identifying behavior as required for nonlinear (ASCE 41) analysis.

·        Develop force-dependent equations to determine plastic hinge length and location(s) within the span of beam-column members so that nonlinear analysis can accurately assess seismic performance.

·        Develop practitioner guidelines for modeling beam-column members in nonlinear analysis, including axial shortening and curvature shortening of the member caused by the formation of plastic hinge(s).

·        Develop fragility curves based on critical evaluation of member inelastic stability results from full-scale tests, for use in the FEMA-funded ATC-58[4] project that is developing the next generation of Performance-Based Seismic Engineering (PBSE) procedures.

What is the research plan? The proposed research initiates a program of combined experimental and analytical study that is outlined in the plan that is being provided by the ATC-90 project (see footnote 1). The experimental research will characterize via laboratory testing the behavior of 30 beam-column members that are considered stability-sensitive at large deformations. Loading/displacement will be conducted to failure to determine the behavior of these specimens under combined axial loads and applied bending moments. Different combinations of axial load and bending moments will be employed to obtain interaction information in the inelastic regime, which is currently unavailable. Boundary conditions of the test specimens will be idealized as "fixed-fixed" and "fixed-pinned" (using a true pin) so that only pertinent information of the plastic hinges will be extracted. Advanced nonlinear finite element analysis will be coupled with the full-scale experiments, to provide statistically confident data points and to validate design and modeling recommendations for beam-columns. Future projects will assess variations of the developed rules and develop appropriate recommendations that recognize changes in boundary conditions and inelastic stability failures.

This project will be conducted as a joint effort between NIST/EL and several contracted non-government testing laboratories specializing in the topic area. Advanced nonlinear analysis techniques will be utilized in-house for modeling behavior and developing analytical assessments of observed behavior, as well as specimen design. This NIST/EL work will extend over the entire duration of the project. Testing will be conducted in one or more full-scale testing laboratories that have strong track records in conducting large scale structural testing of this kind (e.g., member sites of the NSF-funded Network for Earthquake Engineering Simulation - NEES). The incorporation of the analytical work together with the experimental results will be performed at NIST/EL leading to the development of design relationships, including axial load-bending moment interaction equations, for model building code provisions.

A detailed research plan, including the names of all project participants, will be developed during the experimental testing contract proposal development process. This project description will be revised following testing contract award to reflect the additional detail that will be provided by the research plan.

[2] A plastic hinge is a portion of the element where inelastic strains are concentrated

[3] This project is proposed for 4 years to accommodate experimental testing and subsequent analysis.

[4] Applied Technology Council (2010). Development of Next Generation Performance-Based Seismic Design Procedures for New and Existing Buildings (75% draft) – ATC-58, Produced by the Applied Technology Council for the Federal Emergency Management Agency, Washington, DC, in progress.

Recent Results: None – this is a new project.

Standards and Codes: The expected project outcome will be a clear and succinct document providing guidance to structural engineering practitioners as to how to model deep, slender wide-flange steel beam-columns for nonlinear analysis. The document will be compatible with existing and developing model building code provisions, principally AISC 360[1] Chapter H and AISC 341[2] Chapter D and E. Future projects will directly impact the above noted standards with recommended provisions for adoption.

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

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