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Project 2010-02

Project Title:

Transverse Reinforcement Requirements in Flexural Hinges of Large Beams of Special Moment Resisting Frames Subjected to Cyclic Loading

Institution:

University of California
Berkeley, California

Researchers:

Dr. Jack P. Moehle, Professor
Dr. Marios Panagiotou, Assistant Professor
Grigorios Antonellis and Tea Visnjic, Graduate Students

Award Period:

2010-2011 school year

Status:

Completed

Description:

Background – Recent developments in the construction of high-rise buildings in the seismically active West Coast have resulted in construction of reinforced concrete special moment frame beams larger than were typical in past practices. The current Building Code requirements for these beams were written around prevailing practices from many years ago and, when applied to these new buildings, can result in hoop spacing as large as 12 inches in the beam plastic hinge zone (see ACI 318-08, Section 21.5.3.2). Some engineers have questioned the performance capability of these beams and have recommended Building Code changes to reduce the maximum hoop spacing. An ongoing research project at the University of California, Berkeley is investigating the requirements through a laboratory test program.

Test Specimens – Two test beams have been designed, constructed, and tested. The beams cantilever from a common reaction block that is anchored to the laboratory floor, as shown in Figure 1. The beams are tested by imposing displacement cycles (upward and downward) to simulate the effects of deformation reversals that occur during a major earthquake. Beam 1 satisfies all provisions for special moment frame beams according to ACI 318-08. Beam 2 decreases hoop spacing as required in ACI 318- 11. Cross-sections of the beam specimens are illustrated in Figure 2. Concrete is normal weight, using pea gravel (not crushed) as the maximum size aggregate, with target compressive strength of 5000 psi. All reinforcement is Grade 60.

Moment-Deformation Relations – The response of the specimens is described in terms of relations between moment at the face of the support and beam drift ratio measured at the point of actuator attachment. The reported moment is the sum of moments due to actuator force and beam self weight.