| Summary: | 博士 === 國立中央大學 === 土木工程研究所 === 88 === In 1958, the American Concrete Institute (ACI) Committee 438 on Torsion was recreated to study the torsion problem and to promote research in this area. As a result of the efforts of the committee, a new set of torsion design criteria was incorporated into the 1971 ACI Building Code. It was continuously adopted in the 1977, 1983 and 1989 code. The torsion provisions are based on the skew-bending theory.
In the 1995 ACI Building Code, the design for torsion is based on a thin-walled tube, space truss analogy. In the thin-walled tube analogy the resistance is assumed to be provided by the outer skin of the cross section roughly centered on the closed stirrups. The concrete outside these stirrups is relatively ineffective. For this reason, the area enclosed by the shear flow path around the perimeter of the tube is defined as the area enclosed by the centerline of the outermost closed hoops. However, because the spalling of concrete cover will influence the failure behavior mode of beams under pure torsion, the range of concrete cover suitable for being used with the torsion provisions should be studied and defined.
In the beginning, this paper describes tests on the torsional behavior of high strength concrete beams(fc’＞700 kgf/cm2). At the second stage, this paper is to study the failure behavior mode of reinforced concrete beams subjected to pure torsion by observing the spalling of concrete cover. The phenomenon of concrete cover spalling is observed and confirmed by the pink surface between core and cover concrete.
This study has conclusions as follows：
(1)The reinforced concrete beams under pure torsion basically have two failure modes, skew-bending and space truss mode. The skew-bending type has only one weak failure plane. The space truss type has diagonal cracks uniformly distributed along the surface of beams.
(2)The spacing of the uniformly distributed diagonal cracks on the surface of beams increase with the spacing of stirrups and the concrete cover thickness. The increasing spacing between diagonal cracks will change the failure behavior of beams and decreases the ductility of beams.
(3)Beams with moderate concrete cover thickness would have the problem of spalling. The softened truss model is suitable for estimating the torsional strength of beams with concrete cover not spalling. If the concrete cover spalls, the non-sofened truss model should be used with concrete cover cross area neglected.
(4)Procedures to estimate the torsional strength of beams are suggested. A space area is defined by three parameters, (Ac－Aoh)/Aoh,rl1,rt1. As one beam with parameters located within the area mentioned above, it’s torsional strength should be estimated by non-softened spalling truss model.
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