Библиографический список
Neville A. M. ( 1997), Properties of Concrete, Pitman publishing, London.
Hanson J. A. (1963), “Optimum steam curing procedure in pre-casting plants”, ACI J. Proc. 60, 75-100.
ACI 517.2 R-87 (1992), “Accelerated Curing of Concrete at Atmospheric Pressure- State of the Art”, ACI Manual of Concrete, Revised.
Торопова М.В. «Влияние тепловлажностной обработки на структурообразование и эксплуатационные свойства бетона». Дис. … канд. техн. наук: 05.23.05 / Ивановская государственная архитектурно-строительная академия. – Иваново, 2002. – 130 с.
УДК 691.328
студент Тришин С.А.
Сибирская государственная автомобильно-дорожная академия (СибАДИ), Омск
Abstract. Open stirrups are provided principally to resist shear forces in concrete beams and they are applied in locations in which the effect of torsion is insignificant. U-shaped stirrups are placed in the tension side of concrete beams in which shear cracks would occur. However, when concrete beams are designed to resist a substantial amount of torsion, closed stirrups should be used instead.
Аннотация: В статье рассматриваются различные виды арматурных хомутов: открытый арматурный хомут, замкнутый арматурный хомут, U-образный арматурный хомут. Обсуждаются особенности их применения.
Keywords: open stirrups, closed stirrups, concrete beams, flexural failure, shear failure.
Ключевые слова: открытый арматурный хомут; замкнутый арматурный хомут; бетонная балка; разрушение при изгибе; разрушение при сдвиге
APPLICATION OF OPEN STIRRUPS AND CLOSED STIRRUPS IN CONCRETE BEAMS
Historically, beam stirrups had been used sparingly in residential construction. However, in recent years sizes of concrete beam have gotten shallower and spans have increased. That has been the result of architectural design and building occupant requirements.
Beams sustain two basic types of failures, namely:
Flexural (or Bending) Failure: As the beam sags under increased loading, it can fail in two possible ways. If relatively more steel is present on the tension face, concrete crushes in compression; this is a brittle failure and is therefore undesirable. And if relatively less steel is present on the tension face, the steelvyields first and redistribution occurs in the beam until eventually the concrete crushes in compression;
Shear Failure: A beam can also fail due to shearing action. A shear crack is inclined at 45° to the horizontal; it develops at mid-depth near the support and grows towards the top and bottom faces. The stirrups are provided to avoid such shearing action. Shear damage occurs when the area of these stirrups is insufficient [1].
Fig.1 Two types of damage in a beam
The stirrups, as well known, perform in two ways:
to repair damages caused by shear, or occasionally by torque, offering several benefits such as:
suppressing flexural tensile stress in the cantilever blocks by means of the diagonal compression force.
limiting the opening of diagonal cracks within the elastic range, thus enhancing and preserving shear transfer by aggregate interlock
improving the contribution of the dowel action.
providing confinement to the longitudinal compression reinforcement and to concrete
Obviously, all these tasks can be performed successfully only if the stirrup is efficient, which means that it must not open up to the breaking point load.
Therefore, it is evident that the geometry of the stirrups is depending on the manufacturing technology of the beam and on the geometry of its cross section. Perhaps for this reason all the codes give few information regarding manufacturing details of the stirrups, leaving to the designer the responsibility of their efficiency [2].
Types of stirrups
The stirrups in “A” are defined as “closed stirrups” both in the American Standards and in the Eurocodes. In option “B” (which allows an easier assembly of the reinforcement cage on site) the rebar shown in black in figure 4 does not confine the compression reinforcement, but is utilized only to hold together the longitudinal rebars during concrete casting. However, if the bending direction changes along the beam axis (as it happens in a continuous beam), the 135-degree hooks of the “U” shaped stirrups will be in a tensile zone with the risk of slipping-off [3].
Fig.4 Types of stirrups
Concrete beams vary in depth. The deeper the beam, the more shear capacity. When the depth is not adequate, steel stirrups must be added to increase the shear capacity of the beam. These stirrups are usually one piece of steel that is bent into a rectangular shape. Often small diameter steel is used. The stirrup is typically wrapped around the bottom and top bars of the beams.
Open stirrups are provided principally to resist shear forces in concrete beams and they are applied in locations in which the effect of torsion is insignificant. U-shaped stirrups are placed in the tension side of concrete beams in which shear cracks would occur. However, when concrete beams are designed to resist a substantial amount of torsion, closed stirrups should be used instead.
If external loads act far away from the vertical plane of bending, the beam is subjected to twisting about its longitudinal axis, known as torsion, in addition to the shearing force and bending moment.
Fig. 2 Loads act away from the vertical plane of
bending
In Figures 2 several examples of beams subjected to torsion are shown. In these figures, torsion results from either supporting a slab or a beam on one side only, or supporting loads that act far away transverse to the longitudinal axis of the beam. Shear stresses due to torsion create diagonal tension stresses that produce diagonal cracking. If the member is not adequately reinforced for torsion, a sudden brittle failure can occur.
Principal Stresses Due to Pure Torsion.
When the beam shown in Figure 3.a. is subjected to pure torsion, shearing stresses develop in the four faces as shown by the elements. The principal stresses on these elements are shown in Figure 3.b.
The principal tensile strength is equal to the principal compressive stress and both are equal to the shearing stress. Ultimately, when the principal tensile strength exceeds the maximum tensile strength of the beam, cracking will occur spiraling around the outside surface of the beam as shown in Figure 3.c.
In a reinforced concrete member, such a crack would cause brittle failure unless torsional reinforcement is provided to limit the growth of this crack. Closed stirrups and longitudinal bars in the corners of the section are usually used as torsional reinforcement.
Fig.3 Principal stresses and cracking due to pure torsion:
(a) shear stresses; (b) principal stresses; (c) crack
Experimentally proved: It was found that the closed stirrup configuration shows smaller shear crack width in comparison to the open stirrup configuration at the same stirrup strain as result of the slip of stirrup end which affect the slip at the end of vertical leg of stirrup. The open stirrup configuration is with less efficient hook than that of the closed stirrup configuration. Hence, larger slip between stirrup and concrete occurs, resulting in larger shear crack width at the same stirrup strain [4].
Increasing the longitudinal reinforcement amount can better control shear crack opening in the vicinity of longitudinal reinforcement, implying that the longitudinal reinforcement ratio has a significant effect on shear cracking mechanism. The larger amounts of longitudinal reinforcement cause smaller spacings between shear cracks, and thus result in smaller shear crack openings. This can be explained due to the increase in bond effect between the longitudinal reinforcement and the surrounding concrete, thereby enhancing crack control characteristics and reducing the crack spacing. Also, larger amounts of longitudinal reinforcement effectively restrict the widening of flexural cracks and their development into flexure-shear cracks is considered a possible reason [4].
Научный руководитель – М.А. Ращупкина, канд. тех. наук, СибАДИ; Т.Г. Галкина, канд. филоогичл. наук, доцент кафедры «Иностранные языки» СибАДИ, Омск..