INVESTIGATION ON THE LOAD TRANSFER MECHANISM FOR THE JOINT OF REINFORCED CONCRETE COLUMN AND STEEL BEAM

Transcription

1 INVESTIGATION ON THE LOAD TRANSFER MECHANISM FOR THE JOINT OF REINFORCED CONCRETE COLUMN AND STEEL BEAM Seng Hn KIM 1, Sang Whan HAN 2 And Li Hng LEE SUMMARY The composite framed strctres, which consist of reinforced concrete colmns and steel beams, have become more poplar in Korea becase of their efficienc and qalit. However, the load transfer mechanisms between the colmn and beam ma be ver complicated since the material properties of colmns and beams are different. This std investigates the load transfer mechanisms for the composite joint and develops the tpe of joint which cold improve its strength, deformation, and energ dissipation capacities compared to existing composite joints. The tpe of joint developed in this std is called "colmn penetration joint" where the web of steel beam does not penetrate into the colmn and which is confined b sqare tbe and exterior diaphragm. So it can improve the compactness of concrete in the joint. The sqare tbe and exterior diaphragm resist moment that is transferred from beam, and the crciform stiffening plates confine the end of sqare tbe. This paper describes an experimental investigation on the parameters sch as the sqare tbe, exterior diaphragm, and crciform stiffening plates that can inflence the load transfer mechanism for the joint. Two tpes of specimens are tested in this std. One is for the simple tension test of the joint to investigate the stress distribtion and load transfer mechanism in the joint area. The other is for the cclic test on the beam-colmn joint. This test is for investigating the hsteretic behavior of the joint. From this test its strength, stiffness, and dctilit capacit are discssed. INTRODUCTION The demands on the development of strctral sstem to economicall satisf different reqirements on strength, dctilit, and serviceabilit are increasing in Korea. One of the strctral sstems satisfing the demands is the composite framed strctre in which both steel and concrete materials are effectivel combined to maximize the advantages of each material. Figre 1 shows beam-colmn joints of varios composite framed strctres. Figre 1 : Tpes of Composite Framed Strctre. The composite framed strctre which consists of Reinforced Concrete Colmns and Steel Beams(RCCSB) has become more poplar in Korea becase of their efficienc and qalit. RCCSB is the sstem which increases strength and econom as sing reinforced concrete, strong to compression, in colmn and sing steel, strong to 1 2 Department of Architectre, Hanang Universit, Seol, Korea 1-71 Department of Architectre, Hanang Universit, Seol, Korea 1-71 Department of Architectre, Hanang Universit, Seol, Korea 1-71

2 flexre and shear, in beam. In addition, it has man advantages sch as easiness of the constrction of a joint, econom from reinforced concrete constrction, constrction speed, and low self-weight of strctral steel constrction. However, since members of different materials are connected at the joints, the internal stresstransfer mechanisms are more complicated and not clearl nderstood than those in either reinforced concrete or steel strctre. Ths man stdies on the assessment of the internal stress-transfer mechanisms and the reinforcing details have been carried ot. RCCSB can be classified as two tpes as the beam-colmn joint details. One is a beam-penetration tpe where the steel beam penetrates into the colmn, and the other is a colmn-penetration tpe where the steel beam does not penetrate into the colmn as shown in Figre 2. (a) Colmn-Penetration Tpe (b) Beam-Penetration Tpe Figre 2 : Tpes of RCCSB The objective of this research is to develop the joint details, which transfer the stress effectivel, and evalate the load transfer mechanism at the joint. In this std, RCCSB of the colmn-penetration tpe is developed. Then two tpes of experiment are condcted to evalate the load transfer mechanism for the joint. One is the simple tension test of the joint to investigate the stress distribtion and load transfer mechanism in the joint area. The other is the cclic test on the beam-colmn joint. This test is for investigating the hsteretic behavior of the joint. DEVELOP THE JOINT DETAILS OF RCCSB A RCCSB of the colmn-penetration tpe has a problem on the load transfer mechanism at the joint, becase beam is not continos at the joint. To solve this problem, a rectanglar steel tbe was sed at the joint to increase the strength of the joint and an exterior diaphragm is sed to resist the tension at the beam flange. And a Crciform Stiffening Plate(CSP) was welded at the pper and lower ends of a rectanglar steel tbe so as to resist the shear force, transferred from the beam, and to restrain the rectanglar steel tbe at the joint. The new joint of RCCSB developed is shown in Figre. CSP CSP Exterior Diaphragm Rectanglar Steel Tbe Exterior Diaphragm Beam Flange Steel Beam Rectanglar Steel Tbe tc ts ac 18 Reinforced Concrete Colmn t c 6t c 6t c t c Figre : The New Joint of RCCSB Figre 4 : Specimen for the Simple Tension Test 2

3 SIMPLE TENSION TEST Test Specimens To investigate the transfer mechanism of the tension force prodced b bending moment acting on the steel beam, the specimens which have the joint connected onl the pper flange of steel beam as shown in Figre 4 were designed for the simple tension test. Test parameters are (1)the existence of joint elements which are the sqare tbe, exterior diaphragm, and CSP, (2)the failre modes which are joint failre mode and beam flange failre mode, ()the strength b the size variation of joint elements, (4)the vertical stiffness for the exterior joint, and (5)the asmmetric steel tbe. Table 1 presents the list of the specimens. Figre 5 shows details of specimens. Table 1 : List of Specimens Specimen B b B c ac t c t d t b t s Comment 1 D-C1-B No exterior diaphragm 2 D1-C-B No steel tbe D1-C1-B No CPS 4 D1-C1-B Prototpe specimen 5 D1-C1-B1a Asmmetric specimen 6 D1-C1-B Variation of a CSP thickness 7 D2-C1-B Variation of a diaphragm weak section 8 D1-C2-B Variation of t c, t s D1a-C1-B Exterior specimen (nit: mm) B c : width of a rectanglar steel tbe B b : width of a beam flange ac : length of a diaphragm weak section t c : thickness of a rectanglar steel tbe t b : thickness of a beam flange t d : thickness of a diaphragm t s : thickness of a CSP ac 25.5 t c t s (a) D1-C1-B1, D1-C1-B2, D2-C1-B, D1-C2-B5 t c 6t c 6t c t c (b) D1-C-B 25.5 LVDT (c) D-C1-B1 (d) D1-C1-B CSP(5x4x) (e) D1-C1-B1a (f) D1a-C1-B1 Figre 5 : Details of Specimen Figre 6 : Loading Sstem

4 A prototpe specimen is the D1-C1-B1 which have the rectanglar steel tbe, the exterior diaphragm and the CSP. The size of the rectanglar steel tbe is mm mm sqare, while the width and the thickness of the steel beam flange is 18 mm and mm, respectivel. The thickness of the exterior diaphragm is also mm. The variable of D-C1-B1, D1-C-B and D1-C1-B specimens is the existence of joint elements sch as a rectanglar steel tbe, a exterior diaphragm and a CSP. D1-C1-B2, D2-C1-B and D1-C2-B5 specimens are designed for the assessment of strength b the size variation of joint elements sch as the thickness of the CPS, the length of weak cross-section of an exterior diaphragm and the thickness of a rectanglar steel tbe. D1-C1- B1a specimen, of which the pper and lower length of the steel tbe from the beam flange are asmmetric like a real joint, is bilt for comparison with the strength of a smmetric specimen, D1-C1-B1. D1a-C1-B1 specimen that has a vertical stiffener instead of a diaphragm of the prototpe specimen is designed for an exterior joint of a strctre. Beam flanges and exterior diaphragms were ct in V-shape and welded on the rectanglar steel tbe, while the corners of the rectanglar steel tbe which the exterior diaphragm meet are not welded. The face where the exterior diaphragm meets the beam flange is crved to prevent the concentration of stress. Material Properties The design strength of the concrete is 21 kg/cm 2. The mean ltimate compressive strength of concrete at 28 das b clinder test was 22.5 kg/cm 2. The material properties of a steel tbe and a steel plate sed to specimens are presented in Table 2. Table 2 : Material Properties of a Steel Tbe and a Steel Plate Tpe σ (t/cm 2 ) σ (t/cm 2 ) / Steel tbe mm mm mm Steel plate mm mm mm σ : The ield tensile strength, σ : The ltimate tensile strength, Elo : Elongation Ratio Loading Sstem and Data Acqisition σ σ (t/cm 2 ) Elo (%) The loading sstem is shown in Figre 6. The tensile force, monotonicall increased b sing a Universal Testing Machine(UTM), was applied to the specimen ntil the beam flange or the weak section of a joint failed. The longitdinal displacement was measred b two Linear Variable Differential Transformers(LVDTs). Wire strain gages were attached to the important region of the exterior diaphragm, the CSP, the rectanglar steel tbe and the beam flange to evalate the stress distribtion. Experimental Reslts The prototpe specimen, D1-C1-B1, failed b the fractre of the weak cross-section of the exterior diaphragm and simltaneosl the fractre of the pper and lower CPS, and then b the rip of the rectanglar steel tbe. The ielding and ltimate strength of the D1-C1-B specimen withot a CPS was abot % less than that of the prototpe specimen. This shows that the CPS improves mch the strength of the joint b constraining the deformation of pper and lower ends of the rectanglar steel tbe. D-C1-B1 specimen failed b the fractre of the CSP. D2-C1-B and D1-C2-B5 specimens failed at the beam flange. The failre mode and the strength of the asmmetric D1-C1-B1a specimen were ver similar to those of the prototpe specimen. This shows that the strength of most specimens, which are smmetric in pper and lower, will be the same to the strength of a real joint. The strength of D1a-C1-B1 specimen with a vertical stiffener was similar to that of the prototpe specimen. However, the dctilit was less than 16 %. Table presents experimental reslts on the strength and the displacement and Figre 7 shows the load and displacement relationship. 4

5 Specimen Table : Experimental Reslts on the Strength and the Displacement Yield Strength (ton) Yield Displacement (mm) Ultimate Strength (ton) Ultimate Displacement (mm) Dctilit Ratio (µ) 1 D-C1-B D1-C-B D1-C1-B D1-C1-B D1-C1-B1a D1-C1-B D2-C1-B D1-C2-B D1a-C1-B Load(ton) D-C1-B1 D1-C-B D1-C1-B D1-C1-B Displacement(mm) Load(ton) D1-C1-B2 D1-C1-B Displacement(mm) Load(ton) (a) D-C1-B1, D1-C-B, D1-C1-B, D1-C1-B1 (c) D1-C2-B5, D2-C1-B, D1-C1-B1 D2-C1-B D1-C2-B5 D1-C1-B Displacement(mm) Load(ton) (b) D1-C1-B2, D1-C1-B1 (d) D1a-C1-B1, D1-C1-B1a, D1-C1-B1 D1a-C1-B1 D1-C1-B1a D1-C1-B Displacement(mm) Figre 7 : Load-Displacement Diagrams CYCLIC TEST ON THE BEAM-COLUMN JOINT Test Specimens Total three specimens are designed to evalate the strctral behavior of a colmn-penetrated RC colmn and steel beam joint where lateral loads sch as seismic and wind loads are applied. Experimental variables are the length of weak cross section in diaphragm and the thickness of diaphragm in the beam-colmn joint. Table 4 presents the list of the specimens and the configration of the prototpe specimen is shown in Figre 8. The 5

6 rectanglar RC colmn section is mm. The prototpe specimen is C1-D1-A1 specimen. C1-D1-A2 specimen has twice weak section area than the prototpe specimen throgh the weak cross section length of diaphragm is increased. In the C1-D2-A1 specimen, the weak section area is smaller than the prototpe specimen throgh the weak cross section length of diaphragm and the thickness of diaphragm is decreased. These specimens are designed to fail in weak cross section of the diaphragm and the rectanglar steel tbe. Table 4 : List of Specimens Specimen ac t c T d B b T s Comment C1-D1-A Standard specimen C1-D1-A C1-D2-A Variation of weak cross-section length Variation of the weak cross section length of diaphragm and the thickness of diaphragm x45x 81 H-5x15x6x Figre 8 : Configration of the Prototpe Specimen (nit: mm) Material Properties The design strength of the concrete is 5 kg/cm 2. The mean ltimate compressive strength of concrete at 28 das b clinder test was kg/cm 2. The material properties of a steel tbe and a steel plate sed to specimens for the cclic test are presented in Table 5. Table 5 : Material Properties of a Steel Tbe and a Steel Plate Tpe σ (t/cm 2 ) σ (t/cm 2 ) / Steel tbe D D mm mm Steel plate mm mm mm mm σ : The ield tensile strength, σ : The ltimate tensile strength, Elo : Elongation Ratio σ σ (t/cm 2 ) Elo (%) 6

7 Loading Sstem and Data Acqisition Reverse cclic loads were applied at the end of both steel beams b 25 ton actator, and simltaneosl a constant axial load, 7. ton, was applied at the RC colmn ends b the 1 ton oil jack. The ends of colmn and beam were connected with actators, oil jack, and strong frame in the pin. Test set-p is shown in Figre. The cclic loading was controlled b drift angle calclated b dividing the vertical displacement of loading point of beam end b the distance from loading point to the colmn mid-point. Two ccles were applied to each drift angle increased as 1/4, 2/4, 4/4, 6/4, 8/4, 16/4, 24/4(Rad) as shown in Figre 1. Loads are applied qite slowl so that there is little effect of loading rate on the strctral response. Figre : Test Set-p Figre 1 : Loading Histor Instrmentation for displacement and rotation measrements of the joint was planned considering the characteristics of the joint deformation. 5mm and 1mm LVDTs were attached to the panel zone of joint, the top and bottom concrete colmn, and the ends of the steel beam. Wire strain gages were attached to the rectanglar steel tbe, diaphragm and CSP to analze the stress flow. Experimental Reslts In all specimens, the horizontal cracks initiating from the corners of the concrete colmn at the top and bottom of the joint were observed at the 7 ccle. The seemed to be cased b bearing stresses of the rectanglar steel tbe in the connected zone of the rectanglar steel tbe and concrete. In the prototpe specimen, at ccle, the cracks occrred in the weak cross section of diaphragm. At 1ccle, the weak cross section of diaphragm and the top and bottom of CSP were ct when the maximm load is 1.72 ton, and then failed finall. C1-D1-A2 specimen was similar to the prototpe specimen. It s maximm strength is ton, p to 4% than the prototpe specimen. In the C1-D2-A1 specimen, at ccle, the CSP was ct, and then the weak cross section of the diaphragm and the rectanglar steel tbe were torn. It s maximm load is 1.62 ton, which was decreased to 2% than the prototpe specimen. Initial stiffness is.86 ton/mm. These experimental reslts on the ield and ltimate strength are shown in Table 6. Figre 11 shows the load and drift angle relationship. Accmlated absorption energ of specimens was compared in Figre. Figre shows that accmlated absorption energ increases in proportion to the area of the weak cross section in diaphragm Specimen Table 6 : List of test reslt Yield strength, P Maximm strength, P (ton) (ton) P / P C1-D1-A C1-D1-A C1-D2-A CONCLUSIONS RCCSB sstem of the colmn-penetration tpe, which has the steel tbe, the exterior diaphragm, and the CSP, was developed. Two tpes of experiment, the simple tension test and the cclic test, are condcted to evalate the load transfer mechanism for the joint. Conclsions from these tests are as follows. 7

8 (1) In the simple tension test, most of specimens failed b the fractre of the weak cross-section of the exterior diaphragm and simltaneosl the fractre of the pper and lower CPS, and then b the rip of the rectanglar steel tbe. (2) The CPS improves mch the strength of the joint b constraining the deformation of pper and lower ends of the rectanglar steel tbe. () The specimen that has a vertical stiffener instead of a diaphragm can be designed for an exterior joint of a strctre becase the strength of the specimen with a vertical stiffener is similar to that of the specimen with diaphragm. (4) In the cclic test, a load - drift ratio relationship crve has a spindle shape. (5) As the area of the weak cross section in diaphragm is lager, the strength and the accmlated absorption energ is increased more. Load (ton) Load (ton) (c) C1-D2-A1 Figre 11 : Load-Drift Angle Diagrams Drift Ratio (mm) Drift Ratio (mm) Load (ton) Drift Ratio (mm) (a) C1-D1-A1 (b) C1-D1-A2 Accmlated Absorption Energ (ton cm) 6 C1-D1-A1 5 C1-D1-A2 C1-D2-A CYCLE Figre : Accmlated Absorption Energ REFFERENCES ASCE Committee on Design Criteria for Composite Strctres in Steel and Concrete (14). "Gideline for Design of Joints Between of Joints Between Steel Beams and Reinforced Concrete Colmns", Jornal of Strctral Engineering, Vol., No.8 Ag. 14 Deierlein, D.D.(188) "Design of Moment Connections for Composite Framed Strctres", Ph.D. Dissertation, Department of Civil Engineering, The Universit of Texas at Astin, p25, Ma. 188 Roichi Kanno (1). "Strength, Deformation, and Seismic Resistance of Joints between Steel Beams and Reinforced Concrete Colmns", Ph.D. Dissertation, Cornell Universit, Agst. 1 Sheikh, T. M.(18). "Beam-colmn Moment Connections for Composite Frames : Part1. Part2", J. Strct. Eng. ASCE, Vol.115, pp , November. 18 8