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1、Shear Strength of Stud Connectors in Lightweight and Normal-Weight Concrete JORGEN G. OLLGAARD, ROGER G. SLUTTER AND JOHN W. FISHER STEEL-CONCRETE composite construction using normal- weight concrete has been used since early in the 1920s. Substantial use of composite construction began mainly for b
2、ridge structures in the 1950s as a result of the work done by Viest.16-18 Its primary growth in building construction during the last decade was a result of the simplified design provisions introduced into the 1961 AISC Specification. The development of these provisions were based on studies reporte
3、d by Slutter and Driscoll.5,11 The type of shear connectors has changed substantially during the past 20 years. Bridge construction made extensive use of spiral connectors in the early 50s. These were replaced by the flexible channel and stud connectors. Today, headed studs are used extensively for
4、both bridge and building construction. The first studies on stud shear connectors were undertaken by Viest, who tested full scale pushout specimens with various sizes and spacings of the studs.16 Later studies on bent and headed studs were initiated at Lehigh University by Thurlimann.15 A series of
5、beam and pushout tests were reported by Slutter and Driscoll, who developed a functional relationship between the shear connector strength and the concrete compressive strength.5,11 The mathematical model was comparable to the useful capacity proposed earlier by Viest.17 Since 1961, several investig
6、ations of composite beams using lightweight concretes have been made. Studies at the University of Colorado3,14 and at Lehigh University6,12,13 evaluated the strength of stud connectors in a number of different types of lightweight aggregate concretes using pushout specimens. Investigators Jorgen G.
7、 Ollgaard is Design Engineer, Hellerup, Denmark; formerly, Research Assistant, Fritz Engineering Laboratory, Lehigh University, Bethlehem, Pa. Roger G. Slutter is Assoc. Professor of Civil Engineering, Fritz Engineering Laboratory, Lehigh University, Bethlehem, Pa. John W. Fisher is Professor of Civ
8、il Engineering, Fritz Engineering Laboratory, Lehigh University, Bethlehem, Pa. at University of Missouri1,2,4 examined various sizes of stud shear connectors, the effect of haunches, and the behavior of beams. These studies showed that the strength of a shear connector embedded in lightweight concr
9、ete was 5 to 40% lower than the strength of connectors embedded in normal- weight concrete. Considerable variation was apparent in the pushout data because of variation in specimen geometry, slab reinforcement, and experimental techniques. Also, the tensile strength of the stud connectors varied (fr
10、om 62 to 82 ksi) and in many instances was unknown. Because of these variations and the limited data, it was not possible to provide rational design recommendations. The purpose of this investigation was to determine the strength and behavior of connectors embedded in both normal-weight and lightwei
11、ght concretes so that design recommendations could be made. A series of pushout specimens were constructed and tested to assist with the evaluation. The tests with normal-weight concrete provided directly comparable data under the same controlled conditions. The ultimate loads found from tests of pu
12、shout specimens provide a lower bound to the strength of connectors in beams.5 A companion study on the behavior of composite beams with lightweight concrete slabs was undertaken at the University of Missouri.8 TEST SPECIMENS, PROGRAM, AND PROCEDURES The test program was developed after the controll
13、ed variables were selected. The variables considered included the basic material characteristics as determined by standard control tests (i.e., concrete compressive strength fc, split tensile strength fsp, modulus of elasticity Ec, and density w), the stud diameter, type of aggregate, and number of
14、connectors per slab. The stud connector tensile strength, slab reinforcement, and geometry were considered in the experiment design as one-level factors. 55 APRIL/1971 2003 by American Institute of Steel Construction, Inc. All rights reserved. This publication or any part thereof must not be reprodu
15、ced in any form without the written permission of the publisher. Table 1. Pushout Results and Average Concrete Properties Individual Specimen Average Connector Average Concrete Properties AggregateUltimate Load, kipsCompressive Strength Tensile StrengthDensity Concrete Modulus Spec. No. 1Spec. No. 2
16、Spec. No. 3fc(ksi)fsp(ksi)w(pcf)Ec(ksi) A29.332.530.65.080.51148.13740 LA*24.526.524.73.640.43147.63510 SA*19.520.819.94.010.43147.43580 B27.425.425.44.780.47140.53180 LB*18.318.117.32.670.32138.62190 SB*18.216.918.84.030.46142.63170 2B26.125.525.04.780.47140.53180 C-19.921.321.04.690.2489.11510 C21
17、.621.522.24.280.35108.22060 D-24.123.022.74.720.3299.22430 D21.623.324.44.920.36113.42530 E-19.619.217.83.600.3097.71840 E23.122.521.64.300.37111.12190 LE*18.719.519.73.220.32111.41880 SE*15.715.717.04.000.33112.32060 2E21.223.122.74.400.39111.12210 * L indicates series with lower compressive streng
18、th. * S indicates series with 5/8-in. connectors; all other tests on -in. connectors. 2 indicates series with 2 connectors per slab. Specimens with lightweight aggregate and fines. Description of Specimens Most of the specimens had four connectors embedded in each slab, as illustrated in Fig. 1. How
19、ever, several specimens with a single row of two studs, located at mid-height of the slabs, were also tested. All specimens had the same slab reinforcement. The specimens were cast with the beam vertical and in an inverted position, to assure that voids would not form under the studs on their bearin
20、g side. A common form was fabricated so that three specimens could be cast simultaneously. Test Program Forty-eight pushout specimens were tested during this investigation. The program consisted of groups of two slab specimens with three specimens in each group (see Table 1), to provide replication
21、and permit the variability to be evaluated. The normal-weight concrete was manufactured from two types of coarse aggregate. Type A was a crushed limestone and Type B was a natural river gravel. Three different types of lightweight aggregates were used (Types C, D, and E). Each type of lightweight ag
22、gregate was combined with either lightweight fine aggregate or with natural sand. A description of the lightweight coarse aggregate is given in Table 2. The experiment design considered the stud diameter, number of stud connectors per slab, type of concrete, and the concrete properties. The stud ten
23、sile strength and type specimen were considered as one-level factors. This permitted the direct evaluation of the various types of aggregates and concrete properties on the connector shear strength. Table 2. Description of Coarse Lightweight Aggregates MaterialExpanded Shale (C) Expanded Shale (D) E
24、xpanded Slate (E) ColorBrownGray to Black Gray to Black Max. Size-in.-in.-in. ShapeRoundedCubical to irregular Cubical to irregular Production Meth.Rotary kilnRotary kilnRotary kiln Loose Unit Wt. (Approx.) 35 pcf47 pcf45 pcf Control TestsThe characteristics of the concrete slab in which the connect
25、ors were embedded were determined by control tests. Standard 6 in. 12 in. control cylinders were cast along with the pushout specimens to assist in determining the characteristics of the concrete slabs. Sixteen cylinders were cast for each group of specimens. The cylinders were moist cured for 5 to
26、7 days, along with the pushout specimens. They were then stripped and air cured until the day of testing, along with the pushout specimens. The modulus of elasticity was obtained during the compression test of the cylinders. An averaging compressometer with a 6-in. gage length was mounted on the cyl
27、inder. The dial gage was read at each 10 kip load increment. The modulus of elasticity was calculated from the difference in readings at 10 and 50 kips. Often the modulus of elasticity is taken as the tangent modulus at zero load. Obviously, this would result in slightly higher values than the secan
28、t modulus determined 56 AISC ENGINEERING JOURNAL 2003 by American Institute of Steel Construction, Inc. All rights reserved. This publication or any part thereof must not be reproduced in any form without the written permission of the publisher. Fig. 1. Details of pushout specimen from the deformati
29、ons at 10 and 50 kips. The concrete tensile strength was obtained from split cylinder tests, and the density of the concrete was determined from the weight and volume of the cylinders. All stud shear connectors were provided from the same lot. The physical properties of the connectors were determine
30、d from standard tension tests. The average ultimate strength was 70.9 ksi for the -in. studs and 70.2 ksi for the 5/8-in. studs. Pushout TestsThe pushout specimens were tested in a 300-kip capacity hydraulic testing machine. The specimens were placed on sheets of 0.5-in. homosote in order to obtain
31、a uniform load distribution on the bearing surface of the slabs. Testing was usually conducted on the 28th day after casting. Loads were in 10-kip increments, maintained constant at each load level while the vertical slips between the slab and beam were measured. One specimen from each group was loa
32、ded to ultimate load without unloading. The remaining two pushout specimens were loaded to approximately the working load level for the connectors, then unloaded, and reloaded to their ultimate load. TEST RESULTS The average properties of the cylinders that correspond to the pushout specimen are lis
33、ted in Table 1. This includes the concrete compressive strength, fc, the split tensile strength, fsp, the modulus of elasticity, Ec, and the concrete density, w. All lightweight concrete mixes, except C, satisfied the requirements of ASTM C330. The C-mix was composed of lightweight coarse and fine a
34、ggregates and did not yield a satisfactory level of split tensile strength as proportioned and used. ASTM C330 requires an average split tensile strength of 290 psi for structural lightweight concrete. The C-concrete provided a strength of 244 psi. Typical load-slip curves for a normal weight and a
35、lightweight concrete specimen with two slabs are shown in Fig. 2a. Both types of concrete exhibited substantial inelastic deformation before failure. At ultimate load, there was no sudden failure evident. After further deformation accompanied by a decrease in load, failure was evidenced by a shearin
36、g off of the stud connectors or by failure in the concrete slab. The average load-slip curves for a group of three specimens are compared in Fig. 2b for normal-weight and lightweight concrete pushout tests. It is apparent that the average curves are nearly the same for each specimen group. Two speci
37、mens from each group were unloaded after reaching an average load of 10 kips per connector. Subsequent reloading did not change the shape of the overall load-slip relationship (Fig. 2b). Fig. 2. Typical Load-Slip curves. 57 APL/1971 2003 by American Institute of Steel Construction, Inc. All rights r
38、eserved. This publication or any part thereof must not be reproduced in any form without the written permission of the publisher. The ultimate load per shear connector for each pushout specimen is listed in Table 1. The ultimate loads did not vary much between the replicate specimens of a test group
39、. Very seldom did the standard deviation exceed 1 kip. It is apparent that the connector strengths were decreased significantly (from 15 to 25%) when the connectors were embedded in lightweight concrete. The sanded lightweight concretes provided slightly higher shear strengths than did the all light
40、weight concrete mixes. In this study the tensile strengths of all the 5/8-in. and - in. connectors were the same (approximately 70.7 ksi). Hence, the results of the tests on different diameter connectors provided direct information on the influence of connector diameter. Stud connectors of both size
41、s were embedded in the two normal-weight concretes and one lightweight concrete. The results show that the connector shear strength is nearly proportional to the cross-sectional area of the stud. Failure Modes Most specimens were subjected to additional loading and deformation after the ultimate loa
42、d was reached. Often, slab cracks were visible just after ultimate load was reached. The loading was normally continued until one or both slabs separated from the steel beam. This occurred at large slips. There were basically two separation modes observed. In one, the studs were sheared off the stee
43、l beam and remained embedded in the slab after unloading occurred. In the other, the concrete failed in the region of the shear connectors. In many tests both types of failures were observed in the same specimen. Specimen A2, which had normal-weight concrete slabs, (a) Studs sheared off. exhibited t
44、he typical stud shear failure. Figure 3a shows the four studs that were embedded in one slab which sheared off. The other slab was still connected to the steel beam. The photograph also indicates that the studs did not shear off at the same slip levels since the gaps between the studs and the slab a
45、re not the same size indicating that different amounts of plastic deformation occurred. A typical specimen which exhibited concrete failure is shown in Fig. 3b. The connectors were pulled out of the slab together with a wedge of concrete. Both normal-weight and lightweight concrete slabs had wedges
46、of similar shape pulled out of the slab. The cracks in the slabs were more numerous and larger in lightweight concrete than in the normal-weight concrete specimens. The pushout specimens with only one pair of connectors in each slab all failed by shearing off the studs. One reason for this observati
47、on could be that the distance from the studs to the end of the slab was greater and the slab force smaller. Also, since the reinforcement in the slab was identical to that used in the other specimens, more reinforcement would be available per connector. However, the ultimate shear strength per conne
48、ctor did not increase for this type of specimen. The observed mode of failure after slab separation was not applicable to the ultimate load. In order to evaluate the failure mode and determine the state of deformation and type of failure, two specimens were sawed longitudinally through the slab and
49、connectors. One specimen had a normal-weight concrete slab and the second had a lightweight concrete slab. Loading was discontinued just after the ultimate loads were reached in these two specimens and unloading started to occur. (b) Studs and concrete failure. Fig. 3. Typical failure views after slab separation. 58 AISC ENGINEERING JOURNAL 2003 by American Institute of Steel Construction, Inc. All rights reserved. This publication or any part thereof must not be reproduced in any form without the written permission of the publisher. The slabs were cu
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