Use of GFRP Reinforcement for the Predetermination of Potential Plastic Hinges on Reinforced Concrete Frames

Fig. 1 – Hinge Detailing

When subjected to high forces, reinforced concrete elements experience inelastic deformations, followed by a redistribution of the internal forces. Occurrence of plastic hinges on beams, instead of columns, is highly desirable since beam-sway systems are characterized by a ductile response. Therefore, it is desirable to control the location of plastic hinges and their post-elastic response, as energy dissipative zones, in seismic design.

Predetermination of plastic hinges is not a trivial problem however, particularly for reinforced concrete elements. A new concept presented here involves the use of Glass Fiber Reinforced Polymers (GFRP) in combination with conventional steel reinforcement. GFRP reinforcement has a higher short term strength (>1200 MPa, >174ksi) and a lower modulus of elasticity (60 GPa, 8700ksi) than steel. These properties may be taken advantage of by using GFRP rebars where it is desirable to achieve high deformability without affecting the section strength. Such would be the case of the detailing of a potential plastic hinge.

Replacing the steel reinforcement by GFRP at the desired location would allow for higher rotational capacity at that point without hindering the flexural strength of the beam section. However, a GFRP reinforced section would deform even under normal service loading, resulting in cracking of the beam at that location, affecting the aesthetics of the structure. Since the higher deformability of the GFRP bars would only be needed when the structure is subjected to abnormally high loading, like in a seismic event, a combination of steel and GFRP rebars, as shown in Fig. 1, is expected to perform better for the following reasons:

  • The steel bars are designed such that under normal service loading they do not yield. They would however yield when the structure is subjected to extreme loading. While under service loading only, the GFRP will have a negligible effect on the structural response.
  • When subjected to abnormal loading the steel bars will yield and any additional stress will be attracted by the GFRP bars. The GFRP bars in this case will allow for high rotations at that point, while still providing the necessary strength. The typical properties of GFRP, high strength and low stiffness, make it quite efficient at this moment for the response of this section.

Nonlinear time-history analyses were carried out on frames with and without the hinge detailing, using the open-source finite element platform OpenSees. From these analyses it was concluded that:

  • GFRP reinforcement allowed for the creation of plastic hinges at the desired locations and this in turn resulted in the desired relative distribution of base shear forces across the vertical frame elements.

For more information, you may refer to a paper with the same title, published at the ACMBS 2016 Conference by D. Topuzi and M. A. Polak or contact Delta Innoveering.

By Dritan Topuzi, PhD, P.Eng., PMP, LEED AP  
Principal – Delta Innoveering