Brittle Punching Failure Research Articles

The effect of hybrid fiber and shear stud on the punching performance of flat-slab systems

In this paper, for the first time, the binary/ternary hybrid fiber and/or shear stud reinforcement as a measure was used to improve the punching performance of flat-slab systems by innovative self-compacting concrete (SCC). Because in these slab systems, sudden and brittle punching failure can be seen due to application and design errors, early removal of formwork, changes in the purpose of use of the building, earthquakes, etc. Besides, numerous studies investigated the punching performance of the single fiber and/or shear stud reinforced flat-slab systems, but research into the measures of the hybrid fiber or the combined use of hybrid fiber and shear stud reinforcement, which were the variable parameters of this study, was quite lacking. For this purpose, the half-scale slab-column connection elements were produced from SCC containing different punching measures (binary/ternary hybrid and/or shear stud) and tested to investigate the punching performance of flat-slab systems. In conclusion, it was found that hybrid fiber reinforcement was the best punching measure to improve the punching performance of slab-column connection elements with/out shear stud. Besides, 3D graphs were drawn so that designers and researchers could estimate the punching strength and energy absorption capacity for flat-slab systems with/out shear stud based on the parameters of micro fiber type and total volume fraction. On the other hand, empirical formulas were developed to predict the punching strength of binary/ternary hybrid fiber reinforced flat-slab systems with/out shear stud by compressive strength, fiber reinforcement index, the slab useful height, and the punching perimeter parameters.

Post-cracking punching shear behavior of concrete flat slabs partially reinforced with full-depth UHPC: Experiment and mechanical model

Concrete flat slab partially reinforced with full depth UHPC within the critical punching shear area was considered an advanced and economical form of slabs since the application of UHPC can significantly enhance the punching shear behavior after cracking due to its superior mechanical properties. An experimental study of six flat slabs was conducted to research the post-cracking punching shear behavior of NC-UHPC composite slabs, where the crack pattern, stiffness degradation, post-cracking deformability, and peak strengths under different UHPC dimensions were investigated. The test results revealed that the failure mode of slabs changed from brittle punching failure to ductile combined punching shear-flexural failure when the width of the UHPC coverage area reaches more than 1 times the slab thickness from the periphery of the loading plate face. Two new ductility indexes were proposed to evaluate the post-cracking deformability of NC-UHPC composite slabs, which was improved by about 65 % and 108 %, compared to that of NC slabs, respectively. The influence of different UHPC dimensions on the post-cracking punching shear behavior was analyzed. The optimal application of UHPC in NC-UHPC composite slabs was deemed to be the area enclosed by a perimeter located at about 1–2 times of slab thickness from the periphery of the loading plate face. Meanwhile, the peak strengths of NC-UHPC composite slabs were predicted by two mechanical models. It was concluded that the mechanical model proposed in this paper which consider both fibers’ contribution and the influence of different UHPC dimensions can predict the peak strength of NC-UHPC composite slabs accurately.

Investigation of the punching shear behaviors of reinforced concrete slab–steel reinforced concrete L-shaped column connections

To study the punching behaviors of reinforced concrete (RC) slab–steel reinforced concrete (SRC) L-shaped column connections, four slab–column connection tests were designed and completed, and a new punching shear resistance measure proposed. The test variables were column section shape, slab thickness and column cap. The test results show that the failure mode of L-shaped column was mainly two-way shear. Meanwhile, increasing slab thickness and installing column cap can effectively improve the ultimate bearing capacity of the slab–column connection. The punching angles formed by the punching cone and horizontal direction at the final failure of the connection were between 23.5° and 49.5°. In addition, a numerical simulation analysis was carried out using ABAQUS software, and the effects of various parameters on the performance of slab–column connections were studied. It was found that the change of column section shape will lead to more complex shear stress distribution and stress concentration phenomenon. However, the punching capacity of the special-shaped column was higher than that of the equivalent square column with the same section area. As a new punching shear resistance measure, inverted T-shaped steel can effectively improve the ultimate bearing capacity of the connections. In addition, when the ratio of slab thickness to web width of inverted T-shaped steel was less than 0.75, the failure mode of the connection changed from brittle punching failure to flexural failure.

Post-fire behaviour of continuous reinforced concrete slabs under different fire conditions

An experimental investigation of the performance of reinforced concrete continuous slabs is presented in this paper, following the exposure of the slabs to different compartment fires. The influence that several factors, including compartment fire scenarios, reinforcement ratio, and bar arrangement, have on the deflections, strains, crack patterns, and failure modes is analysed. Results that compared to the uniform fire case, localized or extended punching shear failure modes are more likely to occur in the fire-damaged slabs subjected to the traveling fire due to more cracks. The residual structural stiffness and ultimate loads are enhanced with the increasing reinforcing ratio, but the brittle punching failure readily appeared. Finally, the deflection failure criterion (l/50) and the ACI 318-08 punching shear theory are helpful in predicting the residual ultimate loads of the fire-damaged slabs subjected to any fire scenario.

Parametric studies on punching shear behavior of RC flat slabs without shear reinforcement

This paper proposed a numerical investigation based on finite elements analysis (FEA) in order to study the punching shear behavior of reinforced concrete (RC) flat slabs using ABAQUS and SAP2000 programs. Firstly, the concrete and the steel reinforcements were modeled by hexahedral 3D solid and linear elements respectively, and the nonlinearity of the used materials was considered. In order to validate this model, experimental results considered in literature were compared with the proposed FE model. After validation, a parametric study was performed. The parameters include the slab thickness, the flexure reinforcement ratios and the axial membrane loads. Then, to reduce the time of FEA, a simplified modelling using 3D layered shell element and shear hinge concept was also induced. The effect of the footings settlement was studied using the proposed simplified nonlinear model as a case study. Results of numerical models showed that increase of the slab thickness by 185.7% enhanced the ultimate load by 439.1%, accompanied with a brittle punching failure. The punching failure occurred in one of the tested specimens when the tensile reinforcement ratio increased more than 0.65% and the punching capacity improved with increasing the horizontal flexural reinforcement; it decreased by 30% with the settlement of the outer footings.

Tests of Slab-Column Connections Subjected to Reversed Cyclic Moments in Addition to Different Levels of Gravity Shear Load

Reinforced concrete flat slab-column structures are widely used because of their practicality. However, this type of structures can be subjected to punching-shear failure within the slab-column connections. Without shear reinforcement, the slab-column connection can undergo brittle punching failure, especially when the structure is subjected to lateral loading in seismic zones. This research is a part of an extensive investigation about the punching shear behaviour of interior RC slab-column connections under seismic loading. The main objective is to discuss the effect of the gravity shear level on the punching shear behaviour[1].The current paper represents only the results of the first four tested specimens without shear reinforcement. The first specimen was tested subjected to vertical gravity load only without cyclic loading while the other three specimens were tested under different vertical loads V which was kept constant during testing in addition to a reversed displacement controlled cyclic loading which was increased up to punching shear failure. The gravity load V was chosen as 0.4, 0.6 and 0.8 V0 respectively, where V0 is the vertical load causing punching shear failure according to ACI318-14[2]. All tested specimens have the same slab dimensions of 2000x2000mm, slab thickness 200mm, flexural reinforcement ratio of 1.62% and the same column dimensions 250mm x 250mm. Finally, the experimental results are analyzed and compared to international codes such as American Code ACI318-14 and Euro Code EC2-2004[3]. In light of these results, some preliminary conclusions are presented.

Punching CFRP-based strengthening solutions for reinforced concrete flat slabs

This paper presents an experimental research program on the punching strengthening of flat reinforced concrete (RC) slabs with carbon fibre reinforced polymer (CFRP) systems. The strengthening system consists of CFRP wet layup sheets that are installed into holes opened orthogonally to the RC slab, and bonded to the surrounding substract with epoxy adhesive. Two different techniques were adopted: one where the CFRP system fully wraps two consecutive holes, forming a close type stirrup; the second is formed by a type of flexible CFRP bar installed into each hole, working like a dowel.The experimental program is composed of 12 real scale prototypes representative of RC slab-column connection (2500 × 2500 mm plan dimensions and 180 mm thickness). One of the prototypes was not strengthened for constituting the reference specimen, while the others were strengthened by adopting different arrangements for the strengthening systems, namely the pattern of the holes (cross versus radial), number of strengthening perimeters, holes per perimeter and CFRP cross sectional area per perimeter. A maximum increase of punching capacity of 67% was obtained by using the proposed strengthening techniques, having been possible to convert brittle punching failure into a ductile flexure one with the most effective punching strengthening configurations.

STRENGTHENING OF INTERIOR SLAB COLUMN CONNECTIONS VIA POST INSTALLED STEEL SHEAR BOLTS

Flat slabs are widely used in construction operations because of their simplicity and ease. However, this type of structures can be subjected to brittle punching shear failure in the slab-column connections. Without shear reinforcement, the slab column connection can suffer brittle punching failure. In order to overcome the risk of brittle punching shear failure in the column vicinity, the use of post installed shear bolts as a transverse reinforcement is investigated in this paper. A set of six half scale reinforced concrete slab column connections were tested under monotonic concentric load. Three of the tested specimens were provided with steel shear bolts after the hardening of the concrete. Results indicated an enhanced punching strength for specimens provided with steel shear bolts. The increase in punching capacity was combined with a significant increase in both ductility and rotational capacity.

Punching shear failure in blast-loaded RC slabs and panels

Reinforced concrete (RC) slabs and panels are commonly encountered in critical infrastructure and industrial facilities with a high risk of close-range explosions due to accidents or terrorist attacks. Close-in detonations lead to high intensity concentrated loads which can cause a premature brittle punching failure of the member. The assessment of such type of failure mode is challenging since the loading source varies its magnitude in space and time. This paper proposes an analytical method by which the occurrence of punching (or otherwise) is assessed by comparing the dynamic shear demand and capacity (supply). An exponentially decaying distribution of reflected overpressures on the RC surface is presented for this analysis. The punching shear demand is estimated from the pressure and inertial forces acting in the free body diagram. The dynamic punching shear capacity is obtained using the Critical Shear Crack Theory with small slab deformations which are predicted from an equivalent single-degree-of freedom model. The proposed approach takes into account the impulsive behaviour of the member leading to a higher punching capacity and provides better predictions than using existing formulae for punching which are based on tests with quasi-static loading and deformations. The proposed analytical equations are further supported by numerical explicit finite element models providing useful information of crack development, dynamic reactions and deflections. The application of the proposed method has been illustrated and validated by comparison with various tests with scale distances from 0.2 to 1.5m/kg1/3. A practical example is presented to illustrate the applicability of the proposed method.

SEISMIC VULNERABILITY OF FLAT PLATE COLUMN JOINT WITH OUT SLAB SHEAR REINFORCEMENT

During an earthquake, a brittle punching failure can arise in flat plate-column connections due to poor transfer capacity of shearing forces and unbalanced moments. To increase the shear capacity of the slab, various types of shear reinforcement can be used in the slab around the connection. The aim of the project is to study the response of slab column connections containing without slab shear reinforcement when subjected to combined gravity and cyclic lateral loading. At first a calibration model was developed to simulate the tested flat plate-column joint using finite element analysis program MASA. This model was used to predict the load displacement behaviour. The predicted behaviour was compared with the observed behaviour as reported by James Lee & Ian Robertson (Ref.4). The comparison showed that the model predicts the load level excellently but significantly over estimates the stiffness of the joint compared to that observed by James Lee & Ian Robertson. Since the present study is to compare the relative behaviour of slab-column joints provided without slab shear reinforcement, the error in the estimation of joint stiffness will not alter the comparative conclusions drawn. Thus, the developed model was validated for application to various types of column slab connection behaviour.

Assessment of the effectiveness of steel fibre reinforcement for the punching resistance of flat slabs by experimental research and design approach

The present paper deals with the experimental assessment of the effectiveness of steel fibre reinforcement in terms of punching resistance of centrically loaded flat slabs, and to the development of an analytical model capable of predicting the punching behaviour of this type of structures. For this purpose, eight slabs of 2550 × 2550 × 150 mm3 dimensions were tested up to failure, by investigating the influence of the content of steel fibres (0, 60, 75 and 90 kg/m3) and concrete strength class (50 and 70 MPa). Two reference slabs without fibre reinforcement, one for each concrete strength class, and one slab for each fibre content and each strength class compose the experimental program. All slabs were flexurally reinforced with a grid of ribbed steel bars in a percentage to assure punching failure mode for the reference slabs. Hooked ends steel fibres provided the unique shear reinforcement. The results have revealed that steel fibres are very effective in converting brittle punching failure into ductile flexural failure, by increasing both the ultimate load and deflection, as long as adequate fibre reinforcement is assured. An analytical model was developed based on the most recent concepts proposed by the fib Mode Code 2010 for predicting the punching resistance of flat slabs and for the characterization of the behaviour of fibre reinforced concrete. The most refined version of this model was capable of predicting the punching resistance of the tested slabs with excellent accuracy and coefficient of variation of about 5%.

Carbon Fiber-Reinforced Polymer Punching Reinforcement and Strengthening of Concrete Flat Slabs

Three different carbon fiber-reinforced polymer (CFRP) systems were used to reinforce new or strengthen existing concrete slabs against punching. Untensioned CFRP plates or only very low prestressed CFRP straps could not prevent brittle punching failure. Strap prestressing of at least 15% of the tensile strength led to a ductile two-peak slab response although the CFRP material systems were brittle. The ultimate load of the unreinforced slabs could be increased by 77 to 118% via a redistribution of forces from the concrete to the strap system. An empirical model was established that is able to estimate the first peak load of all the applied CFRP systems. It takes into account the parameters CFRP plate or strap strength, CFRP geometry, CFRP prestressing level, and punching load of the unreinforced slab. A cantilever strut-and-tie model was established to estimate the second peak load. Both models provide acceptable accuracy.

Seismic design of concrete slabs for punching shear: a critical review of Canadian standard CSA A23.3-04

Unbalanced moments, transferred between concrete flat plates and columns due to earthquakes, produce shear stresses that increase the vulnerability to brittle punching failure. Appropriate design and detailing of the slab–column joints are essential to prevent such failure. Because of the high flexibility of flat plate structures, lateral-force-resisting systems, e.g., bracings, frames or shear walls, are necessary to limit the interstorey drift ratio to specified values. The slab–column joints must have the strength and the ductility to undergo, without punching failure, the lateral displacement of the structure. In the 2004 version of the Canadian standard, CSA A23.3-04, clause 21.12.3 is added, specifying the punching shear design of slab–column connections subjected to earthquakes. The provisions of the clause are critically reviewed using a design example. It is shown that the new clause permits absence of shear reinforcement in cases that lack strength and when it requires shear reinforcement, it specifies excessive amount. Changes to the standard are proposed.

Seismic behavior of slab-column connections

Brittle punching failure of flat plates can occur as a result of transfer of shearing forces and unbalanced moments between slabs and columns. During an earthquake, the unbalanced moments transferred between slabs and columns can produce significant shear stresses that increase the likelihood of brittle failure. This brittle punching failure mode must be avoided for seismic-resistant flat plate structures. The most common punching strengthening methods are provision of the slab-column connections with drop panels or shear reinforcement or use of high strength concrete in the slab at the vicinity of the connections. This paper compares the effect of these punching strengthening methods on the ductility of slab-column connections. The results of a part of an extensive experimental program conducted on edge slab-column connections, without and with shear reinforcement, are presented. The experiments show that provision of stud shear reinforcement results in seismic-resistant slab-column connections, in which brittle punching failure is avoided in severe earthquakes. The connections with stud shear reinforcement can undergo ductile deformations associated with up to 5% lateral interstorey drift ratio, without loss of resistance to punching due to gravity loads. Key words: concrete design, ductility, energy dissipation, flat slabs, lateral drift, moment transfer, punching, seismic, shear strength, slab-column connections, stiffness, stud shear reinforcement.