Capacity of diaphragm strengthened with FRP: comparison between ACI 440.2R and in-situ tests

Abstract

Seismic codes are frequently revised and updated, especially after large earthquakes. These updates entail that existing structures must be assessed to meet the new requirements and, very often, retrofitted. In the specific case of concrete slabs, the assessment often concludes that strengthening is required to increase the tension capacity of the ‘tie’ members of the diaphragm. Recently, Fibre-Reinforced Polymer materials (FRP) have been extensively used to strengthen existing buildings due to its light weight, fast installation and less invasive approach in comparison with more conventional materials. Design codes have been developed specifically for strengthening buildings using FRP, with ACI440.2R probably being the most widely used standard for the design of FRP strengthening of concrete buildings. However, equations for strengthening concrete diaphragms are not provided in ACI440.2R and, therefore, the equations given for flexural or shear strengthening of other structural members have to be adopted for concrete diaphragms. An example of a FRP strengthening of a hollowcore diaphragm of an existing supermarket building located in Wellington is presented in this study. The Equivalent Static Method (ESM) was used to obtain the seismic demand acting on the diaphragm and the analysis of the diaphragm was carried out using a grillage model. Due to the lack of guidance in ACI440.2R for strengthening of floor diaphragms, the equations to determine the effective stress of FRP for flexural and shear strengthening were used, and the calculated results were compared with the results from in-situ tests of FRP specimens installed on the actual diaphragm. The results show that the equations provided in ACI440.2R significantly underestimate the effective stress of FRP (up to 50%) applied to concrete diaphragms.