Improvement of bearing failure behaviour of T-shaped steel beam-reinforced concrete columns joints using perfbond plate connectors

To improve the bearing failure behavior of T – shaped S beam – RC column joints, joint details using perfobond plate connectors were proposed. Perfobond plate connectors were attached on the bottom flanges at right angles to the steel flange. The objective of this study is to clarify the effectiveness of proposed joint details experimentally and theoretically. Six specimens were tested. All specimens were T-shaped planar beam – column joints with 350mm square RC column and S beams with the width of 125mm and the depth of 300mm. The beams were all continuous through the column. Perfobond plate connectors were attached on the bottom flanges at right angles to the steel flange. Three holes were set up in the perfobond plate connectors. The diameter of the hole was 50mm. The experimental variable was the transverse reinforcement ratio of the joints. The transverse reinforcement ratio of the joints was 0.181% and 0.815%. For each transverse reinforcement ratio of the joints, specimen without the perfobond plate connectors, specimen with the perfobond plate connectors and specimen with the reinforcing bar inserted the hole of perfobond plate connectors were planned. For all specimens, the hysteresis loop showed the reversed S-shape. However, energy dissipation for specimens with perfobond plate connectors was larger than that of specimen without perfobond plate connectors. Bearing strength of specimens with perfobond plate connectors was larger than that of specimen without perfobond plate connectors. From the test results, shear strength of concrete connector a hole was 0.7 times compression strength of concrete. On the other hand, shear strength of inserted reinforcing bar was 1.25 times shear strength of reinforcing bar. Based on the stress transferring mechanism and resistance mechanism of joints proposed by authors, the design formulae of joints with perfobond plate connectors were proposed. The predictions were shown to be in good agreement with the test results.


Introduction
The composite structure (RCS) in which the columns are composed of reinforced concrete (here in after referred to as RC) and the beams are composed of steel (here in after referred to as S) is a structure utilizing the property of the members.Reinforced concrete is strong against axial force, steel is structure against bending and shear force.
For the joint composed of steel beams and reinforced concrete columns, shear failure and bearing failure are the key failure modes.The shear failure indicates stable hysteresis loop without the strength degradation.On the other hand, the bearing failure mode indicates large pinching and strength degradation after the attainment of the maximum load.
Accordingly, bearing failure in the joints should not be caused in RCS system.
To improve the bearing failure behavior of RCS joints, joints details using perfobond plate connectors were proposed.
The objective of this study is to clarity the effectiveness of proposed joints details experimentally for the T-shaped RCS joints.In addition, the objective of this study is to propose bearing design formulae taken account of the effect of perfobond plate connectors based on the stress tranferring mechanism and resistance mechanism proposed by author [1].

Proposed joint detail using perfobond plate connector
Fig. 1 shows the proposed joint detail.In the joint detail, perfobond plate connectors were attached on the bottom flanges at right angle to the steel flange.As shown in Fig. 2, the proposed detail suppresses the rotation of the steel flange by applying compression force or tensile force as the S beam rotates.

Experiment
To clarify the effectiveness of the proposed joint d etails, six specimens were tested.[2,3] The overall dimensions of the specimen, the cross se ctions and reinforcement details are shown in Fig. 3.All specimens were T -shaped beam -column joints with 350mm square columns and steel beam with the width of 125mm and depth of 300mm.The transverse reinforcement ratio of joints was 0.181% and 0.815%.Perfobond plate connector had three holes.The diameter of the hole was 50 mm.The inserted reinforcing bar was the deformed bar of the diameter of 13 mm.
The experimental variables were the transverse reinforcement ratio of joints and reinforcing bars inserted in the holes.The overall test program was shown in the Table 1.The mechanical properties of the materials are listed in Table 2.

Test results
Crack pattens after test are shown in Fig. 4. For all specimens, punching shear failure was observed on the flange of the embedded steel   beam.For specimens with Pw = 0.181%, peeling of concrete was remarkable.On the other and, for the specimens with Pw = 0.813%, no peeling of the concrete occurred.
The inner panel and outer panel were separated by torsion.
Load -displacement relationships are shown in Fig. 5.The vertical axis represents the applied load at the end of the column.The horizontal axis represents the sotry drift angle.For all specimens, the hysteresis loop shows the reversed S -shape.Fig. 6 shows the envelop curves for hysteresis loops.Bearing strength of specimens with perfobond plate connectors was larger than that  Vr2 Bearing strength of the specimens with reinforcing bars inserted in the hole was larger than that of the specimen without reinforcing bar.
From the test results, shear strength of concrete connector a hole was 0.7 times compression strength of concrete.On the other hand, shear strength of inserted reinforcing bar was 1.25 times shear strength of reinforcing bar.

Prediction of ultimate strength
The resistance mechanism of the T -shaped S beam -RC column joint was shown in Fig. 7.
The joint is assumed to be composed of the inner panel and the outer panel, and the ultimate strength of the joint is assumed to be estimated by superposing that of the inner panel and outer panel.
The ultimate strength of the inner panel is governed by punching shear strength of concrete on the top flanges of the embedded steel beam.On the other hand, the outer panel is assumed to be resisted by concrete compression strut (arch mechanism).The ultimate strength of the outer panel was effected by the strength of concrete compression strut and torsional strength between the inner panel and outer panel.
Based on these mechanisms, the ultimate strength pM was given as follows : Where, iM : strength of inner pannel  The comparison of the calculated values obtained by proposed formulae with the test results is shown in Fig. 8.In the proposed formulae, the specimens N, V and Vr evaluated the test result to the unsafe side.On the other hand, the specimens N2, V2 and Vr2 evaluated on the safe side.This is presumed to be because the punching shear strength of the inner panel is assumed to be 0.3 ･ Dc, regardless of the transverse reinforcement ratio.However the calculated values were shown to be in good agreement with the test results.

Conclusions
1 ) For all specimens, the hysteresis loops show the reversed S -shape.
2 ) The maximum load increases with perfobond plate connectors.In addition, the maximum strength of the specimen with reinforcing bars inserted in the hole increases more than that of the specimen without reinforcing bar.

Table . 2
. Mechanical properties of materials sc：Compressive strength, st：Tensile strength, Ec：Young's modulus of concrete sy：Yield strength, su：Maximum strength, Es：Young's modulus of reinforcing bar and Steel