Moment redistribution in continuous plated RC flexural members. Part 1: neutral axis depth approach and tests

It is now common practice to retrofit or rehabilitate existing reinforced concrete beams and slabs by adhesively bonding fibre reinforced polymer (FRP) or metal plates to their surfaces. Advanced design rules are available for quantifying the various plate debonding mechanisms and consequently the shear and flexural capacities of the plated sections. These design rules show that even though the required increase in strength can be obtained by plating, plate debonding can severely reduce the ductility of a flexural member to such an extent that plating guidelines often exclude moment redistribution. This exclusion may reduce the application of plating, in particular to retrofitting buildings where ductility is often a requirement, or it may require the development of a radically different approach to design that does not rely implicitly on ductility. In this paper, it is shown that the commonly used neutral axis depth approach for moment redistribution in RC flexural members cannot be used for most plated structures because plate debonding often occurs before the concrete crushes. Tests on plated flexural members are also reported which show that moment redistribution can occur. In Part 2 of this paper, a moment redistribution analysis procedure is developed that can cope with plate debonding of externally bonded plates.