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Project 2009-01

 Project Title:

The Use of Ultrahigh-Strength Reinforcement in Columns of Frames to Resist Seismic Loads

 Institution:

Purdue University
West Lafayette, Indiana

 Researchers:

Dr. Santiago Pujol, Assistant Professor of Civil Engineering
Jeffrey M. Rautenberg, Research Assistant (Wheaton, Illinois)

 Award Period:

2009-2010 school year

 Status:

Completed

Description:

For many years, the design of reinforced concrete structures in the U.S. has been dominated by the use of steel reinforcement with a specified yield stress, fy, of 60 ksi (410 MPa). Although higher values of fy are allowed, fy has been limited to 80 ksi (550 MPa) since the 1971 edition of ACI 318. The last building code published by The American Concrete Institute (ACI 318-08) permits the use of reinforcement with fy limited to the stress that corresponds to a strain of 0.35% but not to exceed 80 ksi (550 MPa), except for transverse reinforcement, for which a nominal yield stress of 100 ksi (690 MPa) is allowed. This exception applies to requirements for confinement in compression members and not to requirements for shear, torsion, flexure, and axial strength.

Steel with a yield stress in excess of 80 ksi (550 MPa) is currently available in the U.S. and is often referred to as Ultrahigh-Strength Steel (UHSS). In this proposal, the term UHSS is used to designate steel having a yield stress in excess of 80 ksi (550 MPa) and elongation at rupture exceeding 5%. This designation is adopted regardless of the alloy elements used to strengthen the steel.

After the approval of ASTM A1035, there has been growing interest in UHSS bars. But their use remains limited for three main reasons:

  1. There is paucity of test data on the behavior of concrete members reinforced with UHSS bars,
  2. The use of UHSS steel as longitudinal reinforcement in beams requires special considerations about crack widths, especially in design using conventional load and strength-reduction factors, &
  3. The use of UHSS steel as compression reinforcement requires special considerations about the deformation capacity of the surrounding concrete.

This project is aimed at addressing the first and third reasons listed. The second reason is of little relevance for columns, shear walls, and beams in which strength requirements are not controlled by gravity loads; the investigation is focused on columns of frames to resist seismic loads. In these columns, not only is crack-width control not an issue, but also, design provisions for confinement and shear strength require the use of relatively large quantities of transverse reinforcement. Transverse reinforcement causes an increase in the deformability of concrete, allowing for effective use of UHSS steel as compression reinforcement.

Using UHSS introduces several benefits to the construction industry. By using higher-strength bars, required member cross sections and reinforcement quantities may be reduced leading to savings in material, shipping, and placing costs. Also, the reduced number of bars prevents congestion problems leading to better construction quality. UHSS will make reinforced concrete a more efficient material. The more efficient reinforced concrete becomes, the more it will be used, which is in line with the mission of CRSI.