Objective - To conduct a series of tests on beam and column components, and potentially beam-column joints, reinforced with high-strength reinforcement, which will be used to develop modeling parameters of reinforced concrete beam-columns utilizing high-strength reinforcement. This testing program aims to identify the bound (uncertainty) in the component level response due to the use of various HSRBs, and to evaluate material characteristics preliminarily deemed acceptable for high-strength reinforcement and that do not meet current requirements for Grade 60 reinforcement.
What is the technical idea? The technical idea for this project consists of experimental and analytical research to determine the suitability of using high-strength reinforcement in special moment frame systems designed according to current ACI 318 provisions, which were developed for Grade 60 reinforcement. Experimental tests will be conducted to close the gap in data in this area, and will be used to quantify component strengths and develop nonlinear component models for special moment frame beams and columns. The newly developed component models will be employed in a series of nonlinear response history analyses on an archetype building to assess the collapse risk of special moment frame structures utilizing HSRB.
In the last few years, research has been conducted to investigate the mechanical properties of high-strength reinforcement currently being produced in the United States (Slavin and Ghannoum, 2015; Zhao and Ghannoum, 2016). The studies indicate high-strength reinforcing bars now available in the market typically do not satisfy the material specifications developed for earthquake-resistant design (i.e., Grade 60 ASTM A706) because the means to acquire the higher strength causes the stress-strain characteristics to be different than those of Grade 60 steel. For example, high-strength reinforcement generally demonstrates a lower tensile-to-yield strength (T/Y) ratio than currently required by ASTM A706 for Grade 60 reinforcement. Exploratory tests on beams (To and Moehle, 2016) studied the impact of this parameter, concluding that relatively low T/Y values hinder the spread of inelastic curvature and, hence, limit plastic rotation capacity. Exploratory column tests (Sokoli and Ghannoum, 2016; Sokoli and Ghannoum, 2017) have demonstrated encouraging performance in a few cases; however, the researchers reported issues related to bond between normal-strength concrete and high-strength reinforcing bars, indicating that changes in detailing requirements would likely be needed. The scope of these exploratory beam and column tests was primarily limited to identifying acceptable material properties for seismic-grade high-strength reinforcement. Other research projects such as WJE (2015) has developed recommendations for the acceptable properties of HSRB based on preliminary tests. However, experimental research is still needed to assess the performance of components utilizing steel that satisfies these criteria.
The experimental phase of this project will focus on quantifying strength and developing nonlinear component models for special moment frame beams and columns over a range of design parameters. A total of twelve test specimens, including six columns and six beams, will be designed based on prototype beams and columns. The first series of tests will consist of 3 beams and 2 columns designed as follows: one reference beam specimen and one reference column specimen with Grade 60 reinforcement and normal-strength concrete (f’c = 5 ksi (35 MPa)); one Grade 100 companion beam specimen (f’c = 5 ksi (420 MPa)) and one Grade 100 companion column specimen (f’c= 5 ksi (35 MPa)); and one Grade 100 beam specimen with high-strength concrete (f’c= 10 ksi (70 MPa)). High-strength concrete is included as a variable in the series one tests, in part, because it may help to offset the stiffness reductions associated with high-strength reinforcement. Also, using high-strength concrete in combination with high-strength steel may improve bond characteristics between the concrete and steel reinforcement, which was identified as a potential issue in exploratory column tests. Experimental observations for the first five specimens will influence the designs of the second series of tests, which will investigate, among other things, the impact of shear stress demand, axial load demand, and the quantity and arrangement of longitudinal and transverse reinforcement. The second phase of tests will address the uncertainty of full-scale experimental measurements performing redundant measurements and repeated tests of identical specimens to quantify the uncertainty in the measurement as well as ensure the repeatability of the test results. It is envisioned that Grade 120 reinforcement will be included in the series two tests; although, that decision will be made following the series one tests.
The experimental data and component backbone models developed in this study will be employed in a follow-on three-year project, that will be launched after the completion of this project, to quantify the collapse risk of special moment frame buildings utilizing Grade 80, 100, and 120 reinforcement, as well as normal strength and high-strength concrete. Results from the analyses will be used to determine whether special moment frame buildings designed with high-strength reinforcement provide an adequately low probability of collapse, consistent with the intent of ASCE 7-10. The follow-on project will also conduct a cost assessment study to quantify the economic benefits associated with the use of high-strength reinforcement.
What is the research plan? This project consists of experimental and analytical research to close the data gap that currently exists for high-strength reinforcement in earthquake-resistant structures. The study will begin with a planning phase in which a comprehensive literature review will be conducted and a detailed database of previous beam and column tests utilizing high-strength reinforcement will be assembled. 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. Additionally, the team will work with engineering practitioners to identify typical dimensions and reinforcement arrangements for special moment frame beams and columns. This information will be used to design prototype Grade 60 beams and columns which will be redesigned for equivalent flexural strengths assuming Grade 80, 100, and 120 steel. During this planning phase, a committee of engineering practitioners will be established, from which common special moment frame beam and column details (i.e., dimensions and reinforcement arrangements) will be identified and used to design prototype specimens. This task will be completed in Q2 of FY 2018.
Because experimental data is quite limited in this area, the first series of experiments will be planned and conducted prior to performing the detailed analytical study. Three beam specimens and two column specimens will be designed based on the prototypes developed during the planning phase. It is likely that a specially fabricated loading beam will be needed to conduct the experiments. This will likely require a relatively long lead time on the order of three to six months. Also, acquisition of specialized instrumentation, such as reinforcing bar strain gages, will also require several months. Once tests specimen geometries have been determined, the test setup will be designed and an outside fabricator will fabricate the loading components necessary for testing. Supporting instrumentation will be acquired to carry out the full experimental program. This task will be completed in Q4 of FY 2018.
Experimental testing will be scheduled in accordance with the availability of space and access to the laboratory MTS systems. The test specimens will be cast in the laboratory and prepared for testing based on laboratory availability. This task will be completed in Q2 of FY 2019.
Test specimens will be carefully instrumented and the series one tests will be conducted on an aggressive schedule. One archetype building, to be used for the analytical portion of the project, will be designed in parallel with the first series of experimental tests. The design of the archetype building will be conducted externally by practicing engineers. This task will be completed in Q4 of FY 2019.
The nonlinear two-dimensional analytical model will be developed in OpenSees for the archetype building, and initial analyses will be conducted to compare the responses of archetype Grade 60 and Grade 100 buildings. Research results from the experimental and analytical investigations will be used to design the second, and final, series of component tests which will investigate critical design parameters identified by the experimental and analytical investigations. Series two test specimens will be designed and cast in the laboratory. A set of nonlinear modeling parameters will be developed for beam-column elements with high-strength reinforcement using the test data inform this study as well as existing data from literature. These tasks will be completed by Q4 of FY 2020.