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  • HORIZONTAL POST-TENSIONED CONNECTIONS FOR . PRECAST CONCRETE LOAD-BEARING SHEAR WALL PANELS

    R.L. Hutchinson ', S.H. Rizkalla', M. Lau3, and S. Heuvel4

    ABSTRACT

    Precast load-bearing shear wall panels are used extensively for high-rise construction because of the ease and speed of assembly, and the high quality of the precast panels. The connections between panels are extremely important since they affect both the speed of erection and the overall integrity of the structure. This paper presents the results of a research program conducted to investigate the behaviour and the capacity of post-tensioned horizontal connections typically used for precast load-bearing shear wall panels subjected to monotonic shear loading.

    Nine prototype specimens with three different connection configurations were tested. The first two wnfigurations modelled the connections which support the hollow-core floor slab. Post-tensioning was included in the second configuration. The third configuration consisted of post-tensioned connections without hollow-core slab. Two different levels of load normal to the connection were used to determine the effects of dead load.

    Rational mathematical models were developed to predict the shear capacity of the connections at the maximum and ultimate limit states. These models were found to be in good agreement with the experimental results.

    , Structural Engineer, Crosier Kilgour & Partners, Winnipeg, Manitoba 2 Professor, Civil Engineering Dept., University of Manitoba 3 Engineering Manager, Con-Force Structures, Winnipeg, Manitoba 4 Principal, ARC+CADD Architectural CAD Services and Structural Engineering, Winnipeg,

    Manitoba

  • HORIZONTAL POST-TENSIONED CONNECTIONS FOR I'RECAST CONCRETE LOAD-BEARING SHEAR WALL PANELS

    R.L. Hutchinson', S.H. Rizkalla', M.Lau3 and S. Heuvel4

    INTRODUCTION

    Precast load-bearing shear wall panels are used extensively for high-rise construction, due to the high quality control that can be achieved with off-site fabrication, the reduction of construction time and, consequently, reduction in cost. In addition, precast construction is seldom interrupted by adverse weather conditions which is an important factor for the Canadian climate.

    Large precast panel buildings consist of precast concrete load-bearing shear walls in both the longitudinal and transverse directions, designed to resist lateral loads, as shown in Figure l. The major difference between the longitudinal and transverse shear walls is mainly the type of the horizontal connection. Transverse shear walls are normally used to support the hollow-core slab of the noor system, whereas the longitudinal shear walls run parallel to the hollow-core slab and do not include the hollow-core slab at their connection, as shown in Figure 2.

    The connections between the panels are extremely important. A well designed simple connection normally minimizes the construction time, requires minimal falsework, and maintains the overall integrity of the structure.

    A recent innovation in horizon tal connections for load-bearing shear wall panels is the use of vertical post-tensioning tendons. The strands pass through galvanized ducts from the top panel to the base of the structure. While the panels are temporarily braced, the gap between panels which is necessary for alignment purposes, is packed with drypack grout. After the erection of several stories, the tendons are post-tensioned, the ducts are grouted, and the temporary braces are removed.

    , 3

    4

    Structural Engineer, Crosier Kilgour & Partners, Winnipeg, Manitoba Professor, Civil Engineering Dept., University of Manitoba Engineering Manager. Con-Force Structures. Winnipeg, Manitoba Principal. ARC+CADD Architectural CAD Services and Structural Engineering, Winnipeg, Manitoba

  • The post-tensioned horizontal connection, with or without hollow-core slab, has not been examined for a thorough unaerstanding of its shear behaviour under the various limit states. The available literature is not directly applicable to this type of connection .. Therefore, this study was undertaken to investigate the shear behaviour and capacity, under the various limit states. of post-tensioned horizontal connections typically used for precast concrete load-bearing shear wall panels.

    LITERATURE REVIEW

    Horizontal connections are typically reinforced with a combination of continuity bars and mechanical shear connectors. In a previous study conducted at the University of Manitoba [1]. it was found that the shear capacity of the horizontal connection may be predicted as the sum of the contributions from the shear resistance of the continuity bars, the shear resistance of the mechanical shear connectors. and the shear friction resistance of the concrete interfaces. To improve the shear capacity of this type of connection, some fabricators introduced the use of multipk shear keys along the horizontal portion of the joint surface of the panel. The presence of these shear keys was proven to enhance the shear capacity in comparison to the plain surface connection [2].

    Several tests have been conducted in Europe to determine the shear capacity of horizontal connections supporting hollow-core slab, however, the details of the European connections differ from those used in the North American practice, and their results are not directly applicable.

    Figure 3 shows a typical "North American type" horizontal connection supporting hollow-core slab. The majority of tests conducted on this type of connection were designed to investigate the vertical load carrying capacity of the connection. It was found that the total applied vertical stress. 0 , . across the connection is distributed to 0" and 0 ,2' to the hollow-core and the concrete fill respectively. as shown in Figure 4 [3) where:

    0" = 0, 3E,!(2E, + E2 )

    0 '2 = 0 , 3EJ (2E, + E.)

    Where E, and E2 are the elastic moduli of the hollow-core slab and the concrete fill, respectively.

    Harris and Abboud [4] tested sixteen 3/32 scale models of a prototype horizontal connection under reversed cyclic shear load with an applied load normal to the connection ranging from zero to 2 MPa. Hanson [5] conducted full-scale tests on the North American type horizontal connection under cyclic shear load with load normal to the connection varying from 3.5 to 21 MPa.

    No effort was made to separate the contributions of the various connection components to the total shear capacity of the connection. The shear friction coefficients determined from these two investigations were significantly varied. Harris and Abboud obtained shear friction coefticients ranging from 0.7 to 1.6. while the shear friction coefficients obtained by Hanson varied from 0.2 to 0.4.

  • EXPERIMENTAL PROGRAM

    A total of nine full scale specimens were tested in this experimental program. The first two groups of specimens were designed to simulate the transverse interior shear wall connections supporting the hollow-core slabs. The third group is designed to simulate the horizontal connection of the elevator shaft and stair well shear walls. For each category, the presence of post-tensioning prestressing was investigated and two levels of load normal to the connection were considered to simulate the effects of gravity.

    In this paper, the first digit of the specimen mark given in Table 1 represents the specimen number. The following two characters indicate the particular combination of parameters as follows:

    HD: hollow-core slab and drypack only; HP: hollow-core slab and post-tensioning; and PD: post-tensioned with drypack only

    The last digit represents the level of stress, 4 MPa and 8 MPa, normal to the connection.

    A typical specimen with hollow-core slab and post-tensioning is shown in Figure 5. The hollow-core slab rests on "Korolath" bearing pads on the bottom panel. The cores of the hollow-core slab and the gap between the ends of two slabs are filled with a flowable concrete fill. The gap between the hollow-core and the top panel is packed with drypack grout. Load was applied normal to the connection to simulate the effects of gravity, and the shear load was applied directly through the centre of the connection, as shown in Figure 5.

    To produce equivalent pressure on the bearing pad as produced by the long span hollow-core units of the actual structure, a loading apparatus consisting of threaded bars and load cells was used. Only a short portion of hollow-core was used to model the actual connections since only this portion of the cores of the hollow-core is typically filled with concrete.

    The strength of the concrete fill was comparable to that of the precast panels and the hollow-core slab, and is much stronger than the drypack.

    Seven days after drypacking, the strands were post-tensioned and the ducts were filled using an expansive grout. Twenty-eight days later, the specimen was moved to the testing machine where the horizontal connection was tested in a vertical orientation, as shown in Figure 6.

    The edges of the panels were post-tensioned with a system of dywidag bars to prevent premature cracking of the panels. The load normal to the connection was applied through a system of rollers to allow relative displacement of the two panels in the direction of the applied shear load.

    Vertical and horizontal relative displacements from panel to panel and across the various interfaces were measured as schematically shown in Figure 7. Electronic L VDT's were used on one surface of the panel. while mechanical dial gauges were used on the other.

  • TEST RESULTS

    The measured maximum and ultimate load carrying capacity of all the tested connections are given in Table 1. In general, the predominant mode of failure was due

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