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Advancing the Seismic Implementation of High Strength Reinforcement Bars in Reinforced Concrete Structural Wall Systems

Summary

Grade 60 reinforcement, with a nominal yield strength of 60 ksi (420 MPa), has been the primary type of reinforcement used in concrete construction in the US during the past several decades. However, in recent years, engineers and contractors have expressed strong interest in using high-strength reinforcing bars (HSRB), generally defined as reinforcement with yield strength of 72 ksi (504 MPa) or greater (NIST, 2014), to eliminate reinforcement congestion, especially in structures constructed in areas of high seismicity. Steel mills in the United States are currently producing ductile Grade 80, Grade 100, and even Grade 120 reinforcement. However, the current standard for reinforced concrete design, ACI 318 (2014) Building Code Requirements for Structural Concrete, limits the specified strength of reinforcement to 60 ksi (420 MPa) in special moment frame and special structural wall systems due to a lack of test data demonstrating acceptable seismic performance of structural elements with HSRB. Grade 80 reinforcement is already included in ASTM A706, which is the specification for reinforcing bars in earthquake-resistant structures. Based on current research, and because a Grade 80 ASTM A706 specification already exists, the adoption of Grade 80 HSRB for special seismic systems is gaining traction. Nevertheless, there is a demand for Grade 100, and potentially Grade 120, HSRB in special moment frame and special structural wall systems. Implementation of Grade 100 and 120 HSRB in design is challenging because high-strength reinforcing bars generally have smaller elongation properties than the minimum requirements of ASTM A706. Also, the shape of the stress-strain curve for HSRB can differ from that of Grade 60 steel, depending on how the added strength is obtained.  These factors can result in smaller deformation capacities for components constructed with HSRB and larger lateral drift demands for buildings constructed with HSRB, both of which may have serious implications concerning the collapse risk of such buildings. Furthermore, seismic design provisions have been developed based on a 60 ksi nominal yield. Increasing design yield strength to 100 ksi or 120 ksi requires a review of the detailing and bond requirements to ensure they are still valid.

NIST (2014) completed a study to evaluate the feasibility of using HSRB in earthquake lateral force-resisting systems, concluding that research in this area is warranted because high-strength reinforcement has the potential to significantly reduce the costs and improve the efficiency of reinforced concrete structures in general, and earthquake-resistant construction in particular. The use of HSRB will reduce the number of reinforcing bars required to meet a given required load capacity. Fewer bars mean construction times could be significantly reduced given the application. In areas of high seismicity, the reduction of the number of bars and alleviation of reinforcing bar congestion in beam-column joints, walls and columns can result in faster construction and reduced labor costs. The NIST report confirmed the feasibility of using Grade 80, Grade 100, and Grade 120 HSRB for seismic applications and identified a number of research needs to improve understanding of material-related issues for HSRB and to develop design limits for structural components utilizing HSRB. A separate study conducted by the Applied Technology Council (ATC, 2014), funded by the Charles Pankow foundation, confirmed these findings and introduced a strategic research plan to support the adoption of HSRB for special moment frame and special structural wall systems. The ATC study identified the research and engineering studies necessary to incorporate HSRB into ACI 318, which, in turn, will bring the ACI code and the state of design and practice into the 21st century. Incorporating HSRB for special seismic systems in ACI 318 would be the first comprehensive update related to reinforcement strengths since Grade 60 steel was first permitted in the 1971 version of ACI. The effort described here will likely have a huge impact on the construction and design of reinforced concrete buildings.

Adoption of high-strength reinforcement in special moment frame and special structural wall systems necessitates demonstrating the capability of such systems to provide an adequately low probability of collapse, consistent with the intent of ASCE 7-16 (ASCE 2016). Current modeling approaches, including those in ASCE 41 (ASCE, 2017), are not intended for HSRB, making it difficult to predict the collapse risk of structures constructed with HSRB and, thus, challenging to directly compare to the collapse risk of buildings utilizing Grade 60 reinforcement. This study aims to fill in gaps in the available data through a series of carefully planned tests on special structural wall system components. 

The original intent of this project was to study the seismic performance of special moment frame beam and column components constructed with Grade 60, Grade 80, and Grade 100 reinforcement. Based on a review of current research including recently completed work, HSRB production capabilities in the United States, and current construction trends, it was determined that research on special structural wall systems constructed with Grade 100 HSRB should be prioritized. Cyclic tests will be conducted on isolated walls and on coupling beams. Individual walls are often joined together by coupling beams, resulting in a coupled wall system that resists lateral earthquake and wind forces somewhat like a frame constructed of beams and columns. Experimental data will be generated for Grade 60 (ASTM A706) and Grade 100 walls and coupling beams. The experimental data will be used to develop simplified modeling parameters (backbone curves) for special structural wall and coupling beam components. The newly developed component models  will be employed to develop nonlinear finite element models to study the collapse risk of special structural wall buildings constructed with high-strength reinforcement. Achieving these goals will be feasible through a multi-year project that will be conducted in multiple phases that include laboratory testing of wall and coupling beam components. 

Description

Objective -  

  1. To conduct a series of tests on wall and coupling beam components reinforced with Grade 60 A706 reinforcement and high-strength Grade 100  reinforcement. Cyclic tests will be conducted on isolated walls and on coupling beams. Individual walls are often joined together by coupling beams, resulting in a coupled wall system that resists lateral earthquake forces similar to a frame constructed of beams and columns. This testing program aims to  study the component level response of walls reinforced through the use of various HSRBs, and to evaluate material characteristics preliminarily deemed acceptable for high-strength reinforcement but that do not meet current ASTM A706 requirements for reinforcement in earthquake-resistant structures. The tests will evaluate the observed response to determine preliminary bounds for characterizing the uncertainty. 
  2. To perform numerical analyses comparing seismic response parameters for buildings designed with ASTM A706 Grade 60 reinforcement to similar buildings redesigned with Grade 100 reinforcement. Component modeling parameters will be developed based on the wall and coupling beam component tests. This analytical program will investigate variability in building-level response due to differences in component-level behavior as impacted by reinforcement design strength and production method, specifically for HSRB. 
  3. To conduct a second series of tests on wall and coupling beam components investigating the impact of various design parameters such as axial stress demand, shear force demand, the degree of coupling action between wall piers in coupled wall systems, shear span-to-depth ratio, and the quantity and arrangement of reinforcement. Specimens reinforced with Grade 120 reinforcement may also be included in the second series of tests. Experimental data will be used to further develop and validate modeling criteria developed from the initial tests.
  4. To conduct a comprehensive analytical study to quantify the collapse potential of structural wall buildings designed and constructed with HSRB.

This four-year project will address items 1 and 2, while items 3 and 4 will be addressed in a follow-on study.  

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 structural wall 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 to quantify component strengths and to develop nonlinear component models for special structural walls and coupling beams. The newly developed component models will be employed in a series of nonlinear response history analyses on an archetype building to investigate the impact of material-level behavior on building-level response of structural wall buildings constructed with HSRB. 

The project proposal was originally focused on the performance of special moment frame systems with HSRB. The project team has determined that it should instead focus its attention on special structural wall systems with HSRB for  several reasons. First, a comprehensive research review indicates that recently completed and ongoing research programs have made substantial progress to characterize the seismic performance of special moment frame beams and columns constructed with Grade 100 reinforcement produced in the United States (e.g., To and Moehle, 2017; Ibarra and Bishaw, 2017; Sokoli and Ghannoum, 2016; Sokoli et al., 2017), fulfilling many of the research recommendations made by NIST (2014) and ATC 115 (2014), and a portion of the original intent of this project. On the other hand, only one research program has been identified that has studied the seismic performance of special structural walls constructed with Grade 100 reinforcement produced in the United States (Huq et al., 2017). Structural wall systems are widely used in mid-rise and high-rise  buildings, while special moment frames have become less common in the United States, especially in areas of high seismicity. Walls designed in practice often have complex geometries and configurations (C-shape, T-shape, core walls, etc.). Research has demonstrated that the behavior of walls varies considerably with geometry, necessitating specific research to assess the impact of using HSRB for a variety of wall system designs. This project will focus primarily on core wall and coupled core wall systems which are common due to their architectural efficiency and strength and stiffness characteristics.

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., ASTM A706). For example, Grade 100 and Grade 120 reinforcing bars can demonstrate a lower tensile-to-yield strength ratio (T/Y) and smaller rupture strain capacity than currently required by ASTM A706 for Grade 60 reinforcement. Exploratory tests on beams (To and Moehle, 2017; Ibarra and Bishaw, 2017), columns (Sokoli and Ghannoum, 2016; Sokoli et al., 2017), and walls (Huq et al., 2017) have studied the impact of these material parameters and have helped to identify some preliminary material requirements for seismic-grade high-strength reinforcement. Additional experimental research is needed to assess the performance of wall and coupling beam components utilizing steel that satisfies these preliminary criteria. Additionally, detailed analytical studies are necessary to compare the collapse potential of special structural wall buildings constructed with reinforcement satisfying initial seismic-grade HSRB requirements to those constructed with ASTM A706 Grade 60 reinforcement. The experimental research and analytical studies will help to inform the development of new design and detailing guidelines to achieve an acceptably low level of earthquake collapse risk for HSRB buildings. 
The experimental phase of this project will focus on quantifying strength and developing nonlinear component models for special structural walls and coupling beams over a range of design parameters. A total of fourteen test specimens, including eight isolated walls and six coupling beams, will be designed based on prototype Grade 60 walls and coupling beams. The first series of tests will consist of 5 walls and 4 coupling beams. A preliminary test matrix for the series one specimens is provided in Table 1. One wall specimen and one coupling beam specimen will be constructed with ASTM A706 Grade 60 reinforcement and will serve as the reference test specimens. Initial plans are to test two Grade 100 companion wall specimens and two Grade 100 companion coupling beam specimens constructed with high-strength steel produced by two different production methods, making it possible to investigate the potential variability in component response due to differences in HSRB material properties that may result from the different production methods used obtain higher strength. Uncertainty in experimental measurements will be addressed by conducting redundant measurements and repeated tests of identical specimens.  A fourth wall specimen and fourth coupling beam specimen will each be identical to one of the Grade 100 specimens to investigate uncertainty in experimental measurements.  Regular strength concrete (f’c=5 ksi [35 MPa]) will be used for the series one tests.

Table 1 - Preliminary Series One Test Matrix

#

Component Type

Primary variable

fy

f'c

1

Wall

Reference specimen

60 ksi

5 ksi

2

Wall

fy: HSRB production method 1

100 ksi

5 ksi

3

Wall

fy: HSRB production method 2

100 ksi

5 ksi

4

Wall

Experimental uncertainty: specimen identical to specimen #2

100 ksi

5 ksi

5

Wall

Wall geometry: cross-section shape

100 ksi

5 ksi

6

Coupling beam

Reference specimen

60 ksi

5 ksi

7

Coupling beam

fy: HSRB production method 1

100 ksi

5 ksi

8

Coupling beam

fy: HSRB production method 2

100 ksi

5 ksi

9

Coupling beam

Experimental uncertainty: specimen identical to specimen #7

100 ksi

5 ksi

 

Following the series one laboratory tests, an initial analytical study will be conducted to study the impact of reinforcement design strength and production method, specifically for HSRB, on building-level response parameters. Observations from the first series of tests and this initial analytical study will influence the designs of the second series of tests which will likely investigate the impact of shear stress demand, the degree of coupling action between walls in coupled wall systems, shear span-to-depth ratio, axial load demand, and the quantity and arrangement of longitudinal and transverse reinforcement. It is envisioned that Grade 120 reinforcement will be included in the series two tests, depending on outcomes of the series one tests. If Grade 120 reinforcement is used, it may be necessary for the project team to work directly with steel producers to develop new production processes to meet project material specifications for the Grade 120 bars. 

The second series of experiments are planned for a follow-on three-year project. In this follow-on project, the experimental data and component backbone models developed from both series of tests in this project will be used to conduct a comprehensive analytical study to quantify the collapse risk of special structural wall buildings utilizing Grade 100 reinforcement, and potentially Grade 120 reinforcement. Results from the analyses will be used to determine whether special structural wall buildings designed with high-strength reinforcement provide a probability of collapse that is consistent with the intent of ASCE 7-16. 
 

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, column, wall, and coupling beam 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. This task was completed in Q2 of FY 2018.

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 walls and coupling beams which will be redesigned for equivalent flexural strengths assuming Grade 100 steel. During this planning phase, a committee of engineering practitioners will be established, from which common special structural wall and coupling beam details (i.e., dimensions and reinforcement arrangements) will be identified and used to design prototype specimens. This task will be completed in Q3 of FY 2019.

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. Five wall specimens and four 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. Specimen designs will be finalized in Q4 of FY 2019. 

Any necessary loading beam assemblies and miscellaneous instrumentation will be purchased once specimen designs have been finalized. Loading assemblies may require some outside fabrication. The Statement of Work to purchase any necessary loading assemblies and miscellaneous instrumentation will be completed in Q2 of FY 2020. 

The test specimens will be cast in the laboratory and prepared for testing based on Structures Laboratory availability; coordination with other projects such as the ASR project is required. Casting of specimens will be completed in Q4 of FY 2020, depending on lab scheduling.
Experimental testing will be scheduled in accordance with the availability of space and access to the laboratory MTS systems in the Structures Laboratory. Testing will also depend upon the timeframe required to acquire new concrete reaction blocks in the laboratory, which is planned for procurement at the end of FY 2018. Test specimens will be carefully instrumented and the series one tests will be conducted in coordination with other lab projects and procurement of lab reaction blocks. Archetype buildings, to be used for the analytical portion of the project, will be designed in parallel with the first series of experimental tests. Nonlinear two-dimensional analytical models will be developed in OpenSees (McKenna et al., 2000) for the archetype buildings, and initial analyses will be conducted to compare the responses of the archetype Grade 60 and Grade 100 buildings. The series one experiments and analyses of the archetype buildings will be completed in Q4 of FY 2021.

Research results from the experimental and analytical investigations will be used to design the second series of component tests which will investigate critical design parameters identified by the experimental and analytical investigations. A set of nonlinear modeling parameters will be developed for structural wall and coupling beam components with high-strength reinforcement using the test data inform this study as well as existing data from literature. The nonlinear modeling parameters will be used to conduct a detailed analytical study to assess and compare the collapse risk of Grade 60 and Grade 100 archetype buildings. Results from the collapse analyses will be used to introduce any new design provisions that may be necessary to achieve a low collapse risk, consistent with the intent of ASCE 7, for Grade 100 buildings. Nonlinear modeling parameters will also be introduced for inclusion in performance-based design documents such as ASCE 41, the Pacific Earthquake Engineering Research (PEER) Center’s Tall Building’s Initiative (TBI) guidelines (PEER, 2017), and the Los Angeles Tall Buildings Structural Design Council’s guidelines (LATBSDC, 2017).

Created January 2, 2018, Updated October 17, 2019