Department of Architecture and Civil Engineering

Investigation of moment redistribution in FRP strengthened continuous RC beams

At a glance

Principal investigator: Tim Ibell
Co-investigator: Antony Darby, Mark Evernden
Researcher: Abbas Tajaddini
Dates: 2012-2015


Fibre-Reinforced-Polymer (FRP) can improve the performance of Reinforced Concrete (RC) members in sustaining increased loads applied to them. However, the problem is that FRP-strengthened RC members show a decrease in ductility compared to the original member before strengthening.

“Ductility” of a beam indicates how large plastic (permanent) deformations (or strains) can be under loading and is a fundamental implicit assumption in all structural design. If it exists in a statically indeterminate RC beam, moments can be redistributed in a new pattern different from elastic moment distribution. This ability allows moments to be changed from higher to lower values in some parts and inversely in other parts to balance strengthening materials and improve the economy. In addition, consideration of ductile behaviour of structural members in design and strengthening is crucial, because it can absorb energy and gives prior warning of collapse to the occupants, especially in overload (or dynamic) conditions such as earthquake, blast, etc.

Brittle Structural behaviour of FRPs can significantly decrease the expected ultimate load capacity and endanger the ductility of strengthened members. Thus, the process of strengthening is often done by using Elastic theory (not elasto-plastic), resulting in higher cost for strengthening or even worse, a potentially unsafe structure. It is even more critical when the member has been originally designed assuming moment redistribution.

There are few studies on the subject and the goal of the research is to explore the behaviour of FRP-strengthened RC beams under applied loading when entering into the plastic stage. It involves quantifying the moment redistribution in critical sections as well as studying curvature capacity of the sections and evaluating of the ductility demand and ultimate capacity. The philosophy and fundamental concept of forming plastic hinges in FRP-strengthened indeterminate beams needs to be examined as well.

The proposed research is based on analytical study, theoretical analyses and modelling to better understand of the behaviour and problem, supported by suitable small tests to be compared with analytical models and to aid in understanding behaviour of these structures.