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This paper focuses on the development of a theoretical model for predicting the behavior of laterally restrained, fiber-reinforced concrete bridge decks. Based on observations of the failure modes in experimental testing and a review of models used for the punching failure of reinforced concrete slabs, a rational model is developed. The assumptions and equations used in the model development are presented along with the solution algorithm. The theoretical model is checked through comparison with several half-scale and full-scale tests on laboratory bridge decks. The results of the experimental programs are recorded elsewhere and listed in the references.

Flexural and punching failure experiments were performed on flat slabs by controlling the vertical displacement under the column. This allows the monitoring of the descending branch of the load-displacement curve and establishes that flexural failure is characterized by a smooth decrease of the load-carrying capacity, whereas punching failure exhibits a sudden reduction of this capacity. It is shown both experimentally and with force equilibrium that one condition for punching failure that occurs is that the internal strength of the punching cone is larger than the punching strength itself. The punching load was reached for a punching crack inclination of approximately 30 deg (0.5 rad) to the middle plane of the slab. By increasing this inclination, the flexural failure load was approached and the sudden decrease of the load-carrying capacity was reduced. A smooth transition between punching and flexural failure, which is controlled by the punching crack inclination, is revealed, and an interaction formula is proposed.

Slab-column connections were tested under combinations of gravity and lateral loads to investigate the effect of using high-strength concrete slab on the structural behavior of the slab-column connections. The variables selected for this study are the strength of concrete slab, the flexural steel reinforcement ratio, and the moment to shear ratio. As the concrete slab strength increases from 35 MPa to 75 MPa, the shear strength increases by 7% and 15% for loading cases of zero and high moment-shear ratio, respectively. However, the current design codes provisions for interior connections have greatly reduced the factor of safety regarding the punching shear strength of high-strength concrete slabs. The use of high-strength slab has a significant effect on the load-deflection characteristics for specimens subjected to high-moment. The ultimate deflection increases and the failure mode becomes less sudden and more gradual, if high-strength concrete slab is used. The first yielding for specimens constructed with high-strength concrete slab occurs at loads considerably lower than those for specimens constructed with normal-strength concrete. The radius of yielding significantly increases for specimens constructed with high-strength concrete slab; therefore, the steel reinforcement is utilized better and a much more desirable steel stress distribution is produced in the area around the column by the use of high-strength slab. In the meantime, connection displacement and rotation ductility increases by 75% for specimens with high-strength concrete slabs under high moment. Therefore, the use of high-strength concrete slabs can be justified economically when good ductility and higher levels of absorbed energy of slab-column connections are required under high moment-shear ratios. On the other hand, the use of high-strength concrete slabs for slab-column connection subjected to gravity loads only is not recommended because of the small increase of the punching shear capacity of the connection compared with the high cost of using high-strength concrete.

The paper presents the results of an investigation about the behavior of reinforced concrete flat slabs made with high strength concrete, steel fibers and ordinary shear reinforcement. We tested twelve flat slabs made with different combinations of high strength concrete, ordinary strength concrete, shear reinforcement and steel fiber volume fraction (0%, 0.75% and 1.5%). The square-shaped slabs were designed to represent the slab-column connection in an interior column. A significant increase in the punching shear strength was observed, due to the use of high strength concrete and the addition of steel fibers. The combination of high strength concrete with 1.5% fiber volume fraction and shear reinforcement provided twice the punching shear resistance of an ordinary strength concrete slab without shear reinforcement and without fibers. Fiber addition is believed to be responsible for an increase of approximately 50% in the resistance and also for an increase in ductility. Comparisons with theoretical code provisions, including CEB/90, ACI-318/89 and Eurocode 2, are presented.

An experimental investigation was carried out on shear enhancement of high-strength concrete plates for six high-strength concrete slabs to study punching shear capacity and shear enhancement. Five types of different shear reinforcement, namely single bend, U-stirrup, double bend, shear-stud, and T-headed shear reinforcement, were tested and evaluated. The emphasis is placed on the evaluation of their contribution to punching shear capacity of high-strength concrete plates. Structural behaviors were evaluated in terms of overall load-deflection response, ultimate loading capacity, postultimate loading capacity, ductility and energy absorption. Failure patterns and strain distribution were also discussed.

An analytical expression developed to compute the punching strength of reinforced concrete is derived from results obtained with numerical simulation of the failure phenomenon. This simulation reveals that punching strength can be computed by integrating the vertical tensile stresses (assumed constant) around the punching crack (assumed to be a truncated cone in shape). Punching strength is also expressed in terms of the tensile strength of concrete, percentageof flexural reinforcement, slab thickness, and radius of punching crack initiation. The effects of reinforcement are taken into account in a unified approach by adding the vertical tensile resistance of each type of reinforcement. Punching strength computed with the developed analytical expression enables prediction of the experimental punching loads of slabs with different sizes, concrete strengths, and percentages of reinforcement, reinforced with ring reinforcement, orthogonal reinforcing bars, stirrups, shearhead, and prestressing tendons.

The primary objective of this experimental study was to determine whether addition of steel fibers to the concrete mix could significantly increase the punching shear strength of reinforced concrete slabs. Thirteen slab specimens and their companion cylinder specimens were tested. Fiber concentrations in the slabs ranged from 0 to 6.4 percent by weight of concrete. Test results of this study indicate that the addition of steel fibers to the concrete mix does significantly enhance the punching shear strength of slabs. A modification of the standard code design equation for shear punching strength of slabs is suggested to reflect the strength increase produced by the fibers. The tests also provided information on the shape and inclination of the fracture surface, and indicated that only a small, shallow region of the slab in the vicinity of the slab-column intersection is highly stressed in tension before rupture. This means that punching shear failure occurs in a progressive manner.

The transfer of shearing forces and moments between concrete flat slabs and columns can produce brittle punching failure. Slab-column connections must satisfy adequate strength against punching failure. In seismic zones, the connections are expected to undergo deformations into the inelastic range and, hence, it is necesary to design connections with adequate strength and ductility. In addition, the connections must be able to undergo a specified limit of interstory drift without punching failure. The ductility and drift requirements that must be adhered to in design are discussed. Tests reported in the literature show that the strength under cyclic moment transfer is less than the strength under monotonic loading. For an earthquake-resistant structure, the design must be based on the strength under cyclic loading. Test results are reviewed and discussed. As an application, a hypothetical structure is designed according to American Concrete Institute (ACI) Building Code (ACI 318-89). The structure is subjected to the 1940 Elcentro ground motion, and time-history dynamic analysis is performed. The results show that to obtain adequate strength, drift capacity, and ductile behavior of a slab-column connection without shear reinforcement, it is necessary in most cases to increase substantially the slab thickness in the connection region beyond the minimum thickness required to control deflections by providing shear capitals. The disadvantages of shear captials can be avoided by using shear reinforcement.

The effect of superimposed slab loading on the behavior of exterior slab-column connections was studied by testing 2-bay slab-column subassemblies. Each subassembly consisted of an interior and two exterior connections. Three identical slab-column subassemblies were subjected to the same cyclic lateral-displacement history while each supported a different superimposed slab load. The tests show that increased slab load substantially reduces the lateral drift capacity of the connections. A limiting shear stress is suggested to insure adequate drift capacity for exterior slab-connectiions. A rational method to calculate the moment capacity of exterior connections is developed. The test results are compared with present ACI 318-83 Building Code provisions and ACI Committee 352's recommendations for design of slab-column connections.

Punching tests on 12 restrained reinforced concrete slabs are re­ ported in this paper. The slab panels were supported and restrained on all jour sides by edge beams. The influence of the degree of edge restraint, percentage of steel reinforcement, and span-depth ratio of the slabs on the structural behavior and punching shear capacity of the slabs was investigated. The punching shear strengths observ.ed were much higher than those predicted by Johnson's yield-line the­ ory, BS 8110, and ACI 318. The enhanced punching shear capacity was a result of compressive membrane action caused by restraining action at the slab boundaries.

Seventeen reinforced concrete slabs were tested to investigate the deformation and strength characteristics of punching shear failure of high-strength concrete slabs. The tested specimens had different slab depths and reinforcement ratios varying between 0.49 and 2.33 percent. Test results revealed that high-strength concrete slabs exhibit a more brittle failure than normal strength concrete. Experimental results indicated that as the level of reinforcement is increased the punching strength of the slabs also in increased. It was found that using the cubic root of the concrete compressive strength to predict the punching resistances of the concrete slabs generally yields better results than the square root expression used in North American codes.

The paper presents a design method to predict the punching strength and deflection of flat plates at interior columns. Failure is assumed to occur when the compression zone of the slab in the vicin ...

Extensive tests presented earlier have establidhed the effectiveness of stud-shear reinforcement in increasing the punching-shear strength and ductility of slab-column connections transfering axial force V and bending moment M. The shear reinforcement is composed of vertical rods mechanically anchored at their top and bottom ends. This paper presents rules to design and detail stud-shear reinforcement in accordance with the 1989 ACI Building Code (ACI 318-89). Three numerical examples illustrate the design of shear studs and their distribution in slabs in the vicinity of interior, edge, and corner columns transferring V and M. Because of effective anchorage, design rules that reduce the amount of shear reinforcement are suggested and applied. Rules for the design according to the Canadian standard CAN3-A23.3-M84 are also given.

The results of an experimental investigation of the punching shesr strength of reinforced concrete slabs with varying span-depth ratios are summarized. Ten axisymmetric slabs were tested. The slab thickness was kept constant while the support diameter was varied to produce different span-depth ratios. The punching shear strength was found to significantly increase as the span-depth ratios decreased below 6. Maximum test strengths were nearly 5 times greater than the 1983 ACI Building Code predicted strengths. The higher strengths were a result of smaller span-depth ratios, in-plane compressive forces caused by restraining action at the supports, and excellent anchorage provided for the shear reinforcement.

The background theory and the punching shear design provisions contained in the New Australian Standard for Concrete Strucures, AS 3600-1988 are described. These design provisions are not only simpler to use than those in ACI 318-83 but also cover situations that are not readily handled by the ACI Building Code. The Australian design method is illustrated by an example. The correlation of the design equations with test data is also presented. It is believed that the Australian method could serve as a useful alternative to the ACI Building Code provisions.

Tests on 4 specimens of a flat-plate floor in the vicinity of an edge column are reported. The purpose of the tests was to study the influence of a spandrel beam and closed ties on the failure strength of the slab in the vicinity of an edge column. The effect of concentration of slab steel in the region near the column is also examined. The 3 specimens with no spandrel beam failed due to the combined effect of moment and shear transferred by the slab to the column. The presence of the spandrel beam effectively prevented punching shear failure of the other specimen. The test results are compared with the provision of the ACI and the Australian codes.

This paper reviews recent research and code provisions on punching shear capacity and its relation to concrete strength. It presents the results of an experimental investigation relating punching shear to concrete strength and steel ratio. The paper concludes that shear capacity is proportional to the cube root of concrete strength and steel ratio and that the ACI-CSA code provisions should be reviewed.

Findings are reported of punching tests conducted on the deck slabs of model composite steel-concrete bridges. These tests formed part of an investigation to determine more economical use of reinforcement in such systems. The study has indicated that orthotropic decks reinforced in accordance with existing AASHTO specifications have extremely high safety factors against failure in punching. It is suggested that adequate safety factors can be achieved by using isotropic reinforcement equal to the amount necessary to limit shrinkage and temperature cracking. This gives a reduction of over 60 percent in the reinforcement for a conventional 7 in. (17.8 cm) deck slab.