Performance Criteria Development

Summary

Experimental and analytical data are vital for advancing seismic design standards (e.g., ASCE 7, ASCE 41, AISC 341, ACI 318) and are essential for developing and validating analytical modeling tools used to evaluate the seismic performance of buildings. This project consists of four research tasks to develop new data in the following emerging and/or underexplored topic areas: (1) high-strength reinforcement in earthquake-resistant concrete buildings; (2) dependence of structural component behavior on seismic loading protocol; (3) energy-based approaches for seismic performance assessments; and (4) seismic behavior of pre-Northridge steel moment frame weak panel zones and column splices. The project consists of targeted experimental tests on structural components and comprehensive analytical investigations to generate new knowledge that will be applied in the development and advancement of seismic design standards.

Description

Objective:

To advance component-level seismic performance criteria in current design standards by conducting focused experimental tests and analytical studies.

What is the Problem?

Performance-based seismic engineering methodologies require a thorough and accurate understanding of component-level performance.  For well-established structural systems there may be a wealth of experimental data to inform the performance-based design methodology. However, for structural systems lacking experimental data, either because new materials are used (e.g., high-strength concrete and reinforcing bars) or because available laboratory tests do not account for important detailing issues (e.g., partial joint penetration welds), new test data are vital to assuring a strong scientific-basis for building codes and standards. Furthermore, current performance-based methodologies in ASCE 41 use component acceptance criteria that are based on a maximum deformation. Research has shown that this approach can result in excessively conservative performance assessments because component behavior can be highly dependent on the loading history. Though there has been a wealth of large-scale structural testing conducted on key components, there is still a need for more information in order to expand the state-of-knowledge and improve upon performance-based assessment approaches.

What is the Technical Idea?

The project consists of three research tasks (RT). In the following, the technical idea for each of the RTs is presented separately:

RT1: High-Strength Reinforcement in Earthquake-Resistant Structures

Reinforced concrete (RC) design standards (e.g., ACI 318 (ACI 2019)) have historically limited the design yield strength (f_y) of reinforcing bars used for earthquake-resistance to 60 ksi (420 MPa). Using high-strength reinforcing bars (HSRB), defined herein as reinforcement with f_y greater than or equal to 72 ksi (500 MPa), can decrease construction cost and time and improve construction quality (NIST 2014). Steel mills in the United States are now capable of producing Grade 80 (f_y=80 ksi [550 MPa]), Grade 100 (f_y=100 ksi [690 MPa]), and Grade 120 (f_y=120 ksi [830 MPa]) HSRB that may be suitable for seismic design applications. However, the strength and ductility properties of HSRB generally do not satisfy ASTM A706 (ASTM 2016) requirements for earthquake-resistant reinforcing bars. In comparison to ASTM A706 Grade 60 reinforcement, HSRB generally has smaller strain capacity and can have a different stress-strain response, depending on the manufacturing process. These factors can result in smaller deformation capacities for components with HSRB and larger lateral drift demands for buildings constructed with HSRB, both of which may have serious implications on the collapse risk of such buildings.

This research task consists of experimental and analytical research to investigate the seismic behavior of coupled RC structural wall (shear wall) systems constructed with HSRB. Coupled shear walls are formed as the result of openings (i.e., doorways and windows) that create separate wall piers connected together by coupling beams above the openings. They are widely used for earthquake resistance in mid- and high-rise buildings due to their exceptional lateral strength and stiffness, as well as their architectural efficiency (e.g., forming elevator shafts). The coupling beams are designed as structural fuses that dissipate energy during strong ground shaking. Coupling beams are often connected to floor slabs in addition to the surrounding wall piers. The connection to walls and floor slabs restrains the coupling beams from axial growth that can occur as reinforcement yields and can lead to the development of higher forces in the coupling beam and wall piers than would occur if no axial restraint was present. The development of these forces may affect the system-level response, potentially introducing an unintended failure mechanism different from that assumed for design. A shortcoming of many coupling beam laboratory tests is that the specimens are unrestrained at one end and are, thus, allowed to elongate. The potential magnitude of coupling beam restraining forces is not well understood, and it is not clear if restraint conditions will have the same impact for components constructed with Grade 60 reinforcement and HSRB because the stress-strain behavior of HSRB differs from that of Grade 60 reinforcement.

The experimental phase of this task focuses on quantifying the strength and deformation capacity of coupling beams with different reinforcing steel design strengths (Grade 60 and Grade 100) and tested under different axial restraint conditions (i.e., unrestrained, fully restrained). Based on experimental findings, a follow-on analytical investigation may be conducted to assess and compare the collapse risk of archetype buildings constructed with high-strength (Grade 100) reinforcing bars to that of similar archetypes with Grade 60 reinforcement.

RT2: Assessment Criteria for Structural Steel Components Considering Loading History

Performance-based seismic design has gained traction in the U.S. building industry as an alternative way to design new buildings to resist seismic effects because it gives engineers more design freedom  which can result in cost savings and a clearer understanding of building performance. The current standardized performance-based design methodology is contained within ASCE/SEI 41 – Seismic Evaluation and Retrofit for Existing Buildings (ASCE 2017).

ASCE/SEI 41 takes a component-level approach in assessing the seismic performance. The challenge is that current permissible deformation limits used in the assessment criteria are largely based on fully-reversed cyclic tests, which is often the only loading protocol explored given the cost of experimental testing. The permissible deformation limits do not explicitly account for the influence of a component capacity’s dependence on deformation history, even though it is well-known that a typical component loaded monotonically will often have a much different capacity than a component loaded cyclically (Krawinkler 1983, Krawinkler 1996). Recent research has highlighted that some component level actions do not normally experience fully-reversed cyclic deformation demands, especially when rare, collapse-level events are investigated (Maison 2016). This suggests that a new paradigm needs to be established for performance-based assessment procedures in ASCE/SEI 41. It is proposed that components should not be assessed for maximum values, but rather cumulative-based values which directly capture deformation history. To complement this component-level cumulative approach, new limits should also be established for acceptable global system performance to ensure reasonable behavior. 

To achieve these goals, an experimental testing program will be conducted on eccentrically braced frame link beams as described in the research plan. An eccentrically braced frame is a lateral force-resisting system in which the brace centerlines do not intersect at the beam level, resulting in a shear “link beam” that is expected to be a structural “fuse” under strong ground shaking. The advantage of such a system is that it provides the stiffness typical of a braced frame and the ductility typical of a moment frame. Moreover, damage is intended to be focused in the fuse which can be replaced following strong shaking. Researchers have investigated the effects of loading protocols on link beams (Okazaki 2007, Richards 2006), but their focus was mainly on design-level rather than collapse-level earthquakes, which resulted in mostly fully-reversed cyclic response being investigated.

RT3: Energy-Based Evaluation

This task focuses on extending the work on cumulative assessment approaches to capture a component capacity’s dependence on loading history (i.e., RT2) to address the problem of evaluating structural performance and identifying structural collapse based on an energy approach. Energy is computed by multiplying force and deformation. The advantages of using the amount of energy dissipation as the performance and collapse indicator include: (1) being the product of force and deformation, energy can be used to assess the conditions of force-controlled elements as well as deformation-controlled components; (2) being cumulative over time, energy can be used to measure the amount of work associated with monotonic loadings, cyclic loadings, or the combination thereof and the progression of damage with repeated loading; (3) structural components have already been tested to failure, and therefore the amount of energy dissipation associated with structural failure can be quantified at the component level without testing the entire structure to collapse; and (4) energy is positive definite (i.e., always positive) making it a stable, useful parameter. In view of this, the proposed energy-based approach to evaluate structural performance is independent of the loading protocol. As a result, it addresses the lack of knowledge whether the loading is monotonic, fully-reversed cyclic, or somewhere in between. Quantification of structural damage and collapse can be performed by comparing energy demand from earthquake shaking to the corresponding energy capacity of the structural element obtained in the laboratory.

The use of seismic energy in damage assessment is not new. Energy approaches such as those based on cyclic responses for assessing structural damage dates back to the 1980s (Park and Ang 1985; Tembulkar and Nau 1987; McCabe and Hall 1989). Currently, energy is calculated using empirical formulas, where the stored hysteretic energy is calculated by the area underneath the force-displacement curve and the input energy is calculated by integrating the square of ground acceleration time history (Fajfar 1994). One study recently published on correlating energy demand with structural collapse has been proposed (Deniz et al. 2017), but it is also based on empirical formulas. The major problem of using empirical formulas is that the equations used to quantify such energy are not based on engineering mechanics, and therefore the law of energy conservation may be violated. An analytical method for calculating the energy demand of linear structures was also developed (Uang and Bertero 1990), yet only limited analytical research has been conducted since then, especially on nonlinear structures. Most of the research on dissipation of earthquake energy today is based on empirical formulas for quantifying response spectra of simple structures, which is impractical for evaluating damage in complex nonlinear structures. 

What is the Research Plan?

In the following, the research plan for each of the three RTs is presented separately:

RT1: High-Strength Reinforcement in Earthquake-Resistant Structures

The study will begin with the collection of existing data and assembly of a detailed database of previous beam, column, wall, and coupling beam specimens constructed and tested with high-strength reinforcement. The database will be parameterized to study the influence of: (1) reinforcement yield strength, tensile strength, and elongation properties; (2) concrete compressive strength and tensile strength (if reported); (3) reinforcement ratio; (4) arrangement of transverse reinforcement; and (5) flexural, shear, and axial demands.

The project team will work with engineering practitioners to identify typical dimensions and reinforcement arrangements for special structural walls and coupling beams. This information will be used to design prototype Grade 60 coupling beams which will be redesigned for equivalent flexural strengths assuming Grade 100 steel. Six coupling beam specimens will be designed (i.e., cross-sectional dimension and reinforcement will be determined) based on the prototypes developed during the planning phase. The objective of the coupling beam tests will be to: (1) quantify the deformation capacity of coupling beams with Grade 100 reinforcement; (2) better understand the role of axial restraint on strength and deformation capacity; and (3) generate new data to improve prescriptive and performance-based seismic design standards.

In order to conduct the planned tests, a steel reaction frame will be designed and fabricated. The test specimens will be cast in the laboratory and prepared for testing based on PERFORM Laboratory availability and coordination with other projects. Experimental testing will be scheduled in accordance with the availability of space and access to the laboratory MTS systems in the PERFORM Laboratory.

Research results from the experimental investigation will be used to assess whether additional tests are needed as part of a follow-on study. If additional tests are warranted, the second series of tests will be designed. The primary focus of a follow-on study, if one is conducted, will be to assess and compare the collapse risk of Grade 60 and Grade 100 archetype buildings. To do so, an archetype building will be developed, and an analytical model of the archetype will be developed. The analytical model will be validated using data from the experiments completed under this research task, as well as data from other applicable laboratory tests.

RT2: Assessment Criteria for Structural Steel Components Considering Loading History

This research subtask will rationally address the above technical challenges by undertaking the following: (a) conduct a series of experimental tests on eccentrically braced frame link beams using multiple loading protocols, (b) conduct complementary analytical simulations using nonlinear finite element software to extend the results of the testing scheme to configurations not tested, (c) using the results gleaned from (a), (b), and existing literature, formulate an improved set of assessment criteria for eccentrically braced frame link beams that explicitly captures loading history.

A test-setup will be designed and fabricated to facilitate flexibility in specimen size (both length and cross-section). A total of 24 tests (4 specimen sizes, 6 tests each) are initially anticipated to be performed in the EL PERFORM Laboratory, contingent on availability of funds to procure specimens. Each specimen will be subjected to a set of loading protocols, including fully-reversed cyclic (often considered the “default” testing approach), one-sided cyclic, and monotonic protocols. One protocol will be repeated three times for the initial specimen size to investigate the variation in behavior. Repeat tests for the remaining specimen sizes will be conducted based on the findings. Low cycle fatigue behavior will be investigated by using the protocol suggested in FEMA 461 (FEMA 2007). Pending availability of laboratory space and equipment, the test setup may also be able to apply axial load, which has been shown to affect the rotation capacity of a link beam (the testing program will be expanded if this capability is realized).  

This research will have an auxiliary goal of addressing the uncertainty of full-scale experimental measurements. The experimental tests will be devised in such a way to produce redundant measurements and repeated tests (as described above) of identical specimens to further glean uncertainty information. The steel mill certificates will be reviewed to quantify the range and uncertainty of material properties. This information will be fed into a concurrent project studying the propagation of uncertainty in earthquake engineering analysis.

To extend the experimental results from task (a), task (b) will involve constructing nonlinear finite element models to simulate the response of the link beams. The models will provide a valuable pre-test prediction of the response and then will be calibrated to match the actual test results. The results will be compared to other test results available in the literature and then used to establish trends of link beam capacity by extending the experimental results obtained in this project. Furthermore, collaboration with Dr. Hussam Mahmoud of Colorado State, a member of the NIST CoE, will be pursued to expand the impact of this work. Dr. Mahmoud has developed analytical results for buildings with eccentrically braced frames using link beam properties currently found in the literature. The results of this work will be employed by Dr. Mahmoud to ascertain the impact of this new link beam data and acceptance criteria using his analytical approach.

The planned outcome of tasks (a) and (b) is an improved set of acceptance criteria for link beams, which will be formalized in subtask (1c). The criteria will directly account for loading history by using the components cumulative plastic deformation capacity as an input parameter. By doing this, the new criteria will provide a more fundamentally-sound basis for assessing a component’s performance while still maintaining simplicity of the current ASCE/SEI 41 approach. Further, a secondary outcome of this project will be a new framework for deriving experimentally validated cumulative-based assessment criteria. This framework will have the potential of being applied to additional components and systems.

RT3: Energy-Based Evaluation

The goal of this task is to extend the knowledge of quantifying energy dissipation of linearly-responding structures to the nonlinear domain, and at the same time seeking an improved method of defining collapse for performance-based seismic design. To limit the scope, the focus is placed on investigating the structural performance of steel frames. Structural models will include the use of NIST-designed 4-story and 8-story frames (Harris and Speicher 2015a, 2015b). These will include special moment frames and eccentrically braced frames for each height building, comprising a total of four different structures. Research will be carried out for each frame type as follows: (1) derive the energy balance equation and develop a computational tool to calculate all forms of seismic energies with validation; (2) quantify the energy demand and energy capacity considering both geometric and material nonlinearities; (3) address the consistency in energy dissipation for various loading conditions, including monotonic, fully reversed cyclic, or partially reversed cyclic response; (4) perform damage assessment and correlate energy with structural collapse; and (5) investigate the practical use of energy as a metric for resilience.

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References:

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AISI (2015), “North American Standard for Seismic Design of Cold-Formed Steel Structural Systems.  AISI S400. American Iron and Steel Institute.  Washington, D.C. 

ASTM A706/A706M (2016). Standard Specification for Deformed and Plain Low-Alloy Steel Bars for Concrete Reinforcement. American Society for Testing and Materials (ASTM) International. West Conshohocken, USA.

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