Author(s): Abdulqader S Najmi
Punching shear failure at column-slab connection occurs in a brittle manner, which involves the formation of a truncated cone or a truncated pyramid. A set-up of swimmer bars system takes the shape of truncated steel pyramids to counteract the potential truncated pyramids of cracks. Each counteract truncated steel pyramid is formed by swimmer bars will generate four inclined planes intercepting approximately perpendicularly the four inclined planes of the failure truncated pyramid. A reference to a testing program of four square slabs with a concentric column will be used; these were constructed with overall dimensions of 2000 mm by 2000 mm, 150 mm thickness and 140 by 140 mm column in the middle. The results obtained from testing has proved that the efficiency of using swimmer bars system in slab-column connections has increased their punching shear capacity to more than 250% and has decreased their deflection by increasing the stiffness of the slab in the vicinity of the concentrated loads, moreover, the nature of the punching shear failure becomes ductile instead of brittle and obviates sudden failure. The need for drop panels to resist the punching shearing forces becomes unnecessary. The ultimate strength of punching shear is limited by the compression shear failure, which was never measured for the case of punching shear. The gain in ductility can reach levels matching those in flexural behavior.
Punching shear failure takes the form of a truncated pyramid or truncated cone. A counteract steel cage truncated pyramid using swimmer bars will generate four inclined planes intercepting at perpendicular angles approximately the four inclined planes of the failure. The swimmer bars themselves are a new type of shear reinforcement; these are short inclined bars welded to the steel squares (rectangles for rectangular columns) forming the base and the top of the truncated steel cage pyramid. The flat plate is an extremely simple structure in construction, consisting of a slab of uniform thickness supported directly on columns. Reinforced concrete flat slabs are extensively used in buildings and parking garages. When no punching shear reinforcement is provided, failure develops in a brittle manner. Punching shear failure occurs with almost no warning signs, because deflections are small and cracks at the top side of the slab is usually invisible. The critical shear section in ACI Code is taken at d/2 from periphery of the column face [1].
Figure 1: Pyramid shows its upper base and its lower base welded to 8 swimmer bars. Figures 1 and 2 show the set-up of the first shear reinforcement pyramid, counteracting the failure pyramid where 8 swimmer bars are welded to the landing bases of the pyramid. Both bases are welded to the flexural reinforcement of the slab.
Figure 2: First reinforcement pyramid
Najmi who supervised a number of M.Sc. students to verify the concept experimentally started the topic of swimmer bars at the University of Jordan.
Investigated the use of a single plane crack interceptor for shear in beams. The concept developed into the use of swimmer bars as, investigated the use of swimmer bars in reinforced concrete beams. The experimental test results showed substantial improvement in the shear performance of the reinforced concrete beams in comparison with the traditional stirrup system. The impact of using swimmer bars left a remarkable enhancement on the deflection of the beams. It was also noted that the width of the cracks in the beams that had swimmer bars were smaller by far than the beams reinforced with the traditional stirrup system [2-3].
Studied the effective use of space swimmer bars in reinforced concrete flat slabs as punching shear reinforcement [4]. The test results showed that the pyramid form of swimmer bars provided four plane crack interceptors, which increased the punching shear strength of the tested slabs. The use of swimmer bars as part of the pyramid steel cage could give the designer the option of reducing the slab thickness and consequently, reducing the cost. The deflection was also reduced and the tested slabs showed an increase in their flexural rigidity.
Figure 3: Punching shear failure
The slab that is under the effect of a punching load , will experience the formation of a failure bounded by four planes of a pyramid when the load is delivered through a square or a rectangular column. The volume of concrete between the column face and the four planes of failure is subjected to confinement. However, this confinement is limited, and a failure at the face of the column occurred when the shear resistance of the prismatic shape of the column itself did not withstand the punching shear, and the column dipped through the slab as observed by [5]. Figure 4 shows the column stub dipping the slab underneath. In order to prevent dipping of the column into the slab, the first pyramid should have its swimmer bars intersection the four faces of the column, the area needed is obtained from:
Figure 4: Column Stub dipping the slab Underneath
The example given later will comprehensively provide all the details needed. Most important here, is guarantee that the nominal shear strength at the face of the column plus the needed reinforcement is larger than the nominal applied shear. The confinement available at the face of the column, could be considered by considering the design of the first pyramid will cover dipping as well.
Figure 5: Welded Swimmer Bars Deformed with Flexural Reinforcement
Being welded to the flexural reinforcement, the swimmer bars will deform at their ends the same deformation of the flexural bars. Figure 5 shows two cases, of swimmer bars, one inclined at an angle of 45 degrees. Simple analysis shows a very important result due to this welding. The swimmer bars at a flexural strain equals 0.1% top and bottom, will develop a compressive force in the swimmer bars, and the stress is of the order of 100MPa.
The welding of swimmer bars to both bottom and top steel in the slab, will enable them to carry stresses once the loads are applied, this is very different behavior from all other shear reinforcement. The welding is essential and cannot be replaced by anchorage length. The welding has the purpose of integrating these swimmer bars to the flexural reinforcement to form the “skeleton reinforcement” instead of the “cage reinforcement”. The strain in the swimmer bar is equal
In spite of the large compressive forces sustained by the swimmer bars, their buckling length is so small and is compared to the crack width of the surrounding concrete, hence buckling of swimmer bars is unlikely. Reported that the cracks were still invisible in specimens provided with swimmer bars shear reinforcement, compared to the reference specimen without shear reinforcement. Proved experimentally, that the use of swimmer bars system has increased the capacity of slabs against shear failure by about 272% compared to the traditional slabs [6-7]. It was also proved that the use of swimmer bars system has reduced the deflection up to 24% compared to traditional slabs.
1. The Ultimate capacity is characterised by the compression shear failure, and as in the case of over- reinforced beams is associated with brittle failure, and in order to avoid such a failure, designers limit the tensile reinforcement using different approaches.
2. The ultimate capacity in punching shear is attainable, when the maximum tensile reinforcement is provided to prevent all modes of diagonal shear failure. Once this reinforcement area is calculated, it becomes possible to choose the area of swimmer bars. This area is distributed among a number of swimmer bars. Each pyramid can have 8, 12, 16, 20, 24, bars each.
In conventional design, shear reinforcement is isolated, and not connected to the flexural reinforcement. The swimmer bars are essentially welded to the flexural reinforcement, and in flat plates it is welded to both layers of reinforcement through the landing bars (pyramid bases). Without shear reinforcement, the diagonal shear failure occurs at a load equals 1/5 the load required for diagonal compression failure. As given in ACI-14 Eqn. 22.5.1.2
Figure 6: Dimensions related to Punching Shear
The thickness of the plate should meet the requirements to resisting punching shear, flexural strength and deflection. If only the concrete strength is considered in resisting punching shear, the failure of the punching shear takes the form of a pyramid (diagonal tension failure on four sides). Diagonal tension failure equals 1/5 of the shearing force for diagonal compression failure in accordance with Eqn. 22.5.1.2 – ACI-14. If the slab was strengthened by steel reinforcement (swimmer bars), the diagonal failure will not occur, and further shearing load will be accompanied by a transformation in the failure of the slab itself, the failure begins to behave in a ductile manner.
The maximum punching shear capacity of any area around the column distanced (x) from the column face to resist diagonal compression failure is:
Figure shows the use of swimmer bars inclined 450, the first pyramid shown in Figure will have its position or location from inside the column, to prevent dipping of the column, as reported by. This is presented here by assuming that the critical perimeter is at a distance
The behaviour of any slab becomes dependent on the amount of shear reinforcement installed.
1.For no reinforcement, the punching shear takes place at the vicinity of the column, and designers choose to increase the slab depth, use better concrete grade to resist the punching shear failure. This failure is brittle, and dangerous.
2.The added shear reinforcement in terms of using steel pyramids, will promote the ductile behaviour.
In order to build a procedure for the design of swimmer bars, an example taken from Darwin’s textbook [8].
A flat plate has thickness mm and is supported by 450 mm square columns spaced 6m on centers each way. The floor will carry a total factored load of 14.0 kN/m2. Check the adequacy of the slab in resisting punching shear at a typical interior column, and provide shear reinforcement, if needed, using swimmer bars. An average effective depth mm may
The author wishes to thank the Deanship of Academic Research at the University of Jordan for providing financial support of this research during the sabbatical year.
