Effects of bolt-plate arrangements on steel plate strengthened reinforced concrete beamsR.K.L. Su1, W.H. Siu2, S.T. Smith3Department of Civil Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, Chinaabstract：A structure that can behave in a ductile fashion under extreme events is of paramount importance due to safety considerations. Because of such importance, ductility performance of bolted side-plated (BSP) concrete beams under different bolt-plate arrangements is investigated experimentally through four large-scale tests on BSP beams. It is observed that bolt-plate arrangements have a dominant effect on the ductility performance of beams in terms of both the post-elastic strength enhancement (defined as the ratio between the peak strength and the yield strength of the beam), and the displacement ductility(defined as the ratio between displacements at the ultimate and notional yield stages). In order to ensure ductile beam behaviour, the concept of maximum allowable plate-force demand is introduced, of which the strength of additional plates should be kept below the balanced failure point, while sufficient shear strength of the bolt connections should be provided so that the strengthened beam will possess both sufficient strength enhancement and ductility.Keywords: Bolts Bi-linear Concrete beams Curvature Ductility Plate Strengthening1.IntroductionDue to deterioration of materials and demand for additional strength, retrofitting of existing beam structures is often needed. There are basically two methods for strengthening or stiffening existed reinforced concrete (RC) beams. The first method is to attach advanced composites, such as carbon fibre reinforced polymers or glass fibre reinforced polymers 1, onto the tension surface or the side faces of the members. These composites are generally capable of increasing the ultimate load resistance but are prone to peeling and edge debonding failure 2. The second method for strengthening or stiffening existed RC beams is to attach steel plates to the external surfaces of the structural components by means of bolting. This method can enhance both the flexural and shear performances of RC beams. A number of studies were carried out to investigate the flexural and shear behavior of bolted side-plated(BSP) beams 3-8 and coupling beams 9 as well as the behavior of the connecting bolt groups 10,11. However, as the allowable plateforce demand in the bolt-plate system has not been specified in these studies, the designed BSP beams may be over-reinforced when thick plates and strong bolts are used. It is well recognized that ductility is an important structural property which provides noticeable warning at the beginning of failure of the structure to users for evacuation. Although substantial research has shown that BSP beams are effective in providing additional strength, over-reinforced BSP beams could lead to un-desirable non-ductile failure 4. It is therefore imperative to develop retrofitting techniques that can enhance flexural strength and maintain sufficient ductility. With this consideration, four-point bending tests of five simply-supported RC and BSP specimens were conducted. All these specimens have the same RC geometries, but are strengthened with different bolt-plate arrangements. The bolt-plate arrangements of specimens were properly selected so that the present experimental study would cover both over-reinforced and under-reinforced BSP beams. The experimental data obtained was analyzed and the most appropriate strengthening system that can enhance flexural strength and maintain sufficient ductility has been identified. The proposed bolt plate arrangement can help designers to establish a ductile retrofitting scheme quickly and conveniently. It is noted that the investigation of the effect of bolt-plate arrangements on the partial interaction behavior of BSP beams is beyond the scope of this paper. The related experimental study can be found in the companion paper 12.Table 1 Summary of boll-plate arrangements of BSP specimens.Specimen SBSPSpecimen WBSPSpecimen WBWPSpecimen SBWPNo. of bolts on shear span83 35Strength of bolts on shear span Pb(kN)608228228380Plate size(mm *mm)6* 150 dp6 * 150 dp 6 * 75 dp6 * 75 dpPlate force at ultimate state in full interaction analysis Fp.fi (kN)605605302302Degree of shear connection (Pb Fp.fi) 1.000.380.751.26Fig.1. Test setup2. Experimental program2.1. Test setupA four-point bending test setup was adopted, as shown in Fig. 1.The beam specimens were simply-supported with a clear span of3600 mm. Load was applied by a 1000 kN hydraulic jack in the vertical direction, and divided into two equal point loads and exerted symmetrically onto the specimen through a transfer beam. With this arrangement, a pure bending zone with constant moment was created so that the flexural behavior under pure bending co