Punching Failure Research Articles

Comparative evaluation of concrete constitutive models in non-linear finite element simulations of slabs with different flexural reinforcement ratios

Two types of failures can be observed in reinforced concrete slabs supported on columns: flexural and punching. Flexural failure occurs due to the formation of a yield-line mechanism, while punching failure occurs when a punching shear crack with a truncated cone shape develops around the connection. However, flexure-driven punching shear failures due to the yielding of the slab’s flexural reinforcement can occur in the immediate vicinity of the column. These failures appear in a similar manner to “pure” punching shear failures with an additional ductility resulting from the flexural reinforcement yielding. Since these flexure-driven punching failures occur in reinforced concrete slabs with low reinforcement ratio, the paper investigates the punching shear response of reinforced concrete slab-column connections with various flexural reinforcement ratios to distinguish the different failure modes that can be observed: punching and flexural-punching. First, the study presents the calibration procedure needed in three-dimensional non-linear finite element analyses (FEA) to simulate the punching shear failure of reinforced concrete two-way flat slabs without transverse reinforcement under static concentric loading. The calibration approaches are developed with reference to two advanced constitutive concrete models available in two different FEA software. Two series of experimental tests are selected from literature: plain concrete specimens under uniaxial and biaxial loadings and slab-column connections under concentric punching. It is shown that the numerical solutions using both concrete models can capture the cracking and failure mode accurately. It is emphasized that the numerical models provide a valuable tool to characterise failure processes and behavioural mechanisms for the simulation of slabs with varied reinforcement ratios. Numerical simulations showed that the failure mode transition within low and high reinforced concrete slabs can be highlighted by flexural-punching and punching shear mechanisms. In addition, the new code predictions of ACI 318-19 for punching shear of slabs with low reinforcement ratios are presented and discussed in comparison to EC2-2004 predictions and the experimental results. Finally, a numerical assessment of slab-column connections with reinforcement ratios varying between 0.2 and 2.2% is conducted to thoroughly investigate the effect of the flexural reinforcement ratio on the punching behaviour.

Design of biaxially loaded external slab column connections

The paper investigates the influence of biaxial loading on punching resistance at square and elongated edge columns of flat slabs which is virtually neglected in the literature. In the absence of experimental data, the influence of biaxial loading is determined using nonlinear finite element analysis (NLFEA) with 3D solid elements. The resulting baseline punching resistances are compared with the predictions of various implementations of the Critical Shear Crack Theory (CSCT) including a Joint Shell Punching Model (JSPME) in which punching failure is simulated using nonlinear joint elements inserted between the nodes of nonlinear shell elements located around the punching control perimeter. The failure criterion of the JSPME, which is most suited for structural assessment, implicitly accounts for the effect of biaxial loading unlike the original (classic) and closed form versions of the CSCT. The classic CSCT indirectly accounts for loading eccentricity by reducing the punching control perimeter by a multiple ke which is determined in this paper using shear field analysis. Conversely, the closed form CSCT, which is adopted in the draft for the next generation of Eurocode2 (EC2), enhances the design shear force by a multiple β. The paper uses experimental data to determine an expression for β for edge column connections subject to inwards eccentricity normal to the slab edge. Subsequently, shear field analysis on representative flat slab to edge column connections is used to extend this expression for β to edge column connections subject to biaxial eccentricity. NLFEA simulations with 3D solid elements are used to validate the predictions of the JSPME, the shear field methodology used to determine ke in the classic CSCT and the proposed expression for β in the closed form CSCT. Reasonable agreement is achieved between all these analysis methods. The main advantage of the JSPME over NLFEA with 3D solid elements is its increased computational efficiency which makes it more suitable for the assessment of large structures.

Evaluation of Impact-Critical RC Beams Strengthened with a Bottom Layer of HPFRCC

One of the most hazardous consequences of impacts on concrete structures is the high tendency to develop shear or punching failures, with debris generation and associated flying fragments split out. In order to reduce such hazards, concrete structures can be strengthened with a thin bottom layer of high performance fiber-reinforced cement composites (HPFRCCs). As strengthening and anti-spalling layers, the most demanding application is for shear-critical elements, where extensive diagonal cracking leads to multiple fragments, besides concrete cover spalling typical of bending-critical members. An experimental campaign has been carried out on 21 beams to investigate their failure mode. Different configurations were studied for the HPFRCC to determine the influence of the fiber amount (2-4%) and the introduction of embedded steel bars in the strengthening layer. Specimens were subjected to drop weight tests and quasi-static reference tests. The tests have shown that a membrane-type resisting mechanism can be activated by the HPFRCC layer, which can retain potentially falling generated fragments and improve the carrying capacity. Embedding steel bars in the strengthening layers can avoid HPFRCC rupture. Moreover, members strengthened using tensile strain-hardening HPFRCC mixes have shown enhanced strength compared with unstrengthened members, while mixes without strain-hardening did not.

Test and analysis on axial performances of GFRP restraint connectors for sandwich insulation wall panels

The sandwich insulation wall panel (SIWP) is an essential component of most building insulation systems. To achieve the satisfactory insulation effect in certain areas, such as Sweden, the thickness of insulation layer in SIWPs can reach 300 mm. However, the shear and axial compressive performances of most existing connectors at this length are unsatisfactory. The authors have previously proposed a novel GFRP connector system suitable for 60–300 mm insulation thickness, and the study on its shear behaviors has been published. This paper focuses on axial loading behaviors of the system. Three types of glass-fiber reinforced plastic (GFRP) restraint connectors, named PlateA, PlateB and Cross, were proposed. Pull-out and push-off tests were conducted to investigate their tensile and compressive failure modes. Results show that anchorage zone concrete splitting is the predominate failure mode in all pull–out tests whereas concrete punching failure is the main failure mode in push-off tests. The 300-mm-length PlateA connector and the 300&150-mm-length PlateB connectors buckled before concrete failure. The concrete failure surface was determined, and formulas to calculate axial strength of connectors based on concrete failure modes were established by theoretical and finite element analysis.

New insights into soil arching behaviour in column-supported embankments

The observation of failure surfaces within column-supported embankments is critical to understanding how the embankment stresses are transferred towards the column heads. In this study, finite element simulations utilising a strain-softening constitutive model, nonlocal regularisation, and the Arbitrary Lagrangian–Eulerian formulation are used to examine these failure surfaces over various embankment geometries. This methodology offers insights into the nature of the failure mechanism, the development of a plane of equal settlement, and the influence of the subsoil settlement profile. Depending on the embankment geometry, the results indicate a punching failure, inverted general bearing failure, or a localised failure develops. The transition between punching and inverted general bearing failure is found to be closely related to the establishment of a plane of equal settlement within the embankment. The height of the plane of equal settlement and the range of failure mechanisms that develop were largely insensitive to the nature of the subsoil settlement profiles simulated. These findings have implications for the practical design of efficient embankments and the effective design of future experimental studies.

Modelling punching shear failure at edge slab-column connections by means of nonlinear joint elements

This paper is concerned with the modelling of punching shear failure at edge columns of flat slabs without shear reinforcement. Punching failure at edge columns is much less researched than at interior columns despite typical buildings having more edge than interior columns. The paper uses nonlinear finite element analysis (NLFEA) to study the influence on punching resistance of parameters including column aspect ratio and loading eccentricity. The NLFEA is carried out using both 3D solid elements and a Joint Shell Punching Model (JSPM) which combines nonlinear shell elements with nonlinear joint elements that incorporate the failure criterion of the Critical Shear Crack Theory (CSCT). Both constant and varying loading eccentricities are investigated since near failure eccentricity at edge column connections typically reduces below that calculated with elastic analysis due to moment being redistributed from the support to span. This reduction in eccentricity is beneficial since punching resistance is shown to depend on the final loading eccentricity. Consequently, designing for punching on the basis of elastic edge column moments is overly conservative.

Fracture prediction of circular steel tubular X-joints using a Lode parameter enhanced cyclic void growth model

The extremely low cycle fatigue failure of steel tubular joints under strong seismic action may lead to progressive collapse of the structures. The prediction models of extremely low cycle fatigue fracture focused on the tensile fracture models and often ignored the influence of shear effect. Within the scope of fracture model based on microscopic mechanism, a Lode parameter enhanced cyclic void growth model (LCVGM) was proposed by improving the cyclic void growth model (CVGM) in the study. The LCVGM can comprehensively consider stress triaxiality sensitivity and Lode parameter dependence in extremely low cycle fatigue fracture prediction. The monotonic tensile and extremely low cycle fatigue tests were conducted on the different types of base metal and weld metal specimens in circular steel tubular joints. The micro fracture morphology of the specimens was observed by scanning electron microscope, and the fracture properties of both materials were compared. Moreover, LCVGM parameters of base metal and weld metal were calibrated, and the model was validated against extremely low cycle fatigue tests results of plate specimens. The fracture failure tests of circular steel tubular X-joints under monotonic loading and extremely low cyclic loading were completed. The joints exhibit chord punching failure mode. Finally, the numerical simulations of fracture tests of the joints were performed by means of the LCVGM and CVGM. The analysis results demonstrate that the fatigue life predicted by LCVGM is consistent with the test results, while the predicted life obtained from CVGM is significantly longer than the test result due to neglecting the influence of shear effect.

Flexural performances and failure analyses of parallel bamboo strand lumber plates

Bending behaviors and fracture mechanics of parallel bamboo strand lumber (PBSL) plates are seldom investigated. In this research, orthotropic PBSL plates were designed and fabricated. Flexural experiments were performed to reveal the deformation, the failure process and the load carrying ability of the PBSL plate under concentrated load. Orthotropic mechanical behavior of the PBSL greatly influences the failure process of the PBSL plate. Progressive damage characterizes the failure process. Initial matrix tensile fracture, later fibre compression fracture and final punch failure take place successively. An orthotropic bamboo plate theory was suggested to predict the deformation, the internal forces, the stresses and the strains of the PBSL plate. A composite failure theory based on matrix fracture and fiber fracture was proposed to predict the cracking load and the ultimate load of the PBSL plate. All these attempts make great progress in the research of PBSL plates and could effectively promote their application in structural engineering.

Calculation Formula for the Pullout Force of Uniaxial Geogrid-Reinforced Expansive Soil

To obtain the action force between the reinforcing material and filler in reinforced soil, many scholars at home and abroad have conducted pullout tests, which are limited by the testing apparatus. Most of them have studied the pullout force between a biaxial geogrid and sand. The pullout force formula is generally believed to be composed of the surface friction and lateral resistance of transverse ribs, and uniaxial geogrids have rarely been researched experimentally on the basis of the pullout force between them and the expansive soil filler due to the large distance among transverse ribs. This study draws on existing methods to determine the pullout force, conducts a preliminary analysis, proposes a formula for calculating the maximum pullout force, uses a self-developed large-scale CNC pullout test system, and utilizes its large size, bidirectional airbag loading, and ability to eliminate side wall friction. The design of the test scheme is optimized to obtain two important calculation parameters in the formula: one is to cut grids of different lengths to provide lateral resistance and then implement them to obtain the coefficient of sliding friction between grid and fill; the other is to develop different numbers of horizontal ribs in the pullout test of a grille, and the lateral resistance of the end bearing of a single transverse rib is measured. The results of the lateral resistance of the end bearing based on the analysis of the three mechanisms of shear, punching, and Prandtl failures are compared. The lateral resistance of the lateral rib end bearing measured from the test is proven reasonable. Two types of expansive soils reinforced by different types and sizes of grids are analyzed with the calculation formula of the maximum pullout force, and the coefficient of pullout friction is determined to be rational. The research results can be used to guide the design and construction of uniaxial geogrid-reinforced expansive soil and the modification and upgrading of products by grid manufacturers.

Punching Analysis of Underlying Karst Rock Foundation Considering Self-Weight of Rock Mass

The 3D equation of the ultimate bearing capacity Pb of the karst cave foundation under self-weight γ is established using the associated flow law in the upper limit method of limit analysis on the basis of the Hoek-Brown strength criterion and the punching failure mode of the karst cave foundation. In combination with the variational principle, the equation of the punching failure line and the ultimate bearing capacity expression of the karst cave foundation are derived. The ultimate bearing capacity Pb and critical hole diameter of the karst cave foundation under self-weight are calculated by programing. The theoretical rationality is verified by indoor model tests. The research shows that: (1) The influence of the rock RMR value and uniaxial compressive strength σc on the ultimate bearing capacity of the karst cave foundation has nothing to do with the consideration of the self-weight γ factor. Both factors increase with the increase in uniaxial compressive strength σc and RMR value, and vice versa. These factors decrease with the increase in self-weight γ; however, the change is obscured; (2) The ultimate bearing capacity of the karst cave foundation Pb basically linearly increases with the increase in the thickness of the roof h of the cave; when considering the self-weight σ and thickness of the roof h, the change of the ultimate bearing capacity Pb of the karst cave foundation mainly depends on the difference between the increment of the internal dissipative and gravity power; (3) When RMR 60, self-weight has a small influence on the critical span D of the cave; (4) When RMR ≤ 40, the influence of self-weight γ on the ultimate bearing capacity Pb must be considered; when RMR ≥ 70, self-weight γ must be considered when calculating the ultimate bearing capacity Pb of the karst cave foundation because the error is generally within 3%.

Numerical Study of the Behaviour of a Circular Footing on a Layered Granular Soil Under Vertical and Inclined Loading

This paper aims to study the behaviour of a circular footing resting on two granular layers, i.e., a dense sand layer resting on loose sand strata, subjected to a vertical and an inclined loading (α=0°, 10°, 20°, 30°) using the finite element (FE) software PLAXIS-3D. The Mohr-Coulomb criterion is employed for the analysis of the model, in which two parameters are considered to vary significantly; (1) thickness of the top layer (dense layer) and (2) friction angle (ф) of both the layers. In the circular footing, the bearing capacity on the layered soil profile is assessed using the mechanism of punching shear failure following the desired area approach. The punching shear failure mechanism formed in dense sand has a parabolic shape at the ultimate load when the maximum mobilization of shear force through the failure surface is taken into account, otherwise, the punching failure is the actual failure while punching in the lower layer continues to a greater extent, depending on the interface load. Bearing pressure decreases as the inclination increases with respect to the vertical, along with bearing pressure increasing as the thickness of the dense sand layer increases. The software results compare well with data available from the literature.

Punching Shear Strength Improvement of Flat Slabs using Polypropylene Fibers

The punching shear failure at the region around the column is considered the most dangerous type of failure in the flat slab system, causing an instantaneous collapse of the buildings. It is a brittle failure mode that happens suddenly without any warning. Recently many techniques were introduced by researchers to avoid shear punching failure and enhance shear punching resistance of flat slab. Improving the mechanical properties of concrete is a powerful approach line using polypropylene fibers (PF) in concrete in which details are presented in this paper. Twelve slabs were made with dimensions of (1000*1000) mm, and 100mm in thickness. The flexural reinforcement was the same for all models; for the upper layer was (6ϕ6), while the lower layer was (10ϕ12) without shear reinforcement. The factors studied in this investigation involve the polypropylene fibers quantity, compressive strength value. The results showed a noticeable improvement in punching shear strength when adding (PF%=1%) by cement weight. However, the impact of polypropylene fibers was negligible when adding a ratio greater than 2%, and it was also observed that the failure changed from a brittle failure of non-polypropylene fibers models to a ductile failure in slabs containing polypropylene fibers. The failure cracks in a slab with polypropylene fibers are tiny and less in number than the NC slab. This is because fibers increase the internal bonding of the concrete material component. Also, increasing the ratio of polypropylene fibers reduced the perimeter and area of the punching shear region while the failure angle slightly increased.

Experimental investigation on the blast resistance of framed PVB-laminated glass

PVB-laminated glass has been widely adopted for building windows as safety glass due to its significant post-crack load bearing capacity. To achieve a better understanding of the mechanical behaviour of framed PVB-laminated glass under explosion, a series of field blast tests were carefully designed and conducted aiming to study the dynamic responses and failure modes of framed PVB-laminated glass subjected to various blast loadings. The reflected overpressure and displacement time histories were measured in the tests. In particular, DIC technology was adopted to record the 3-D deflection field of the glass panel, which is essential for analyzing the dynamic response feature of laminated glass panels under blast loading. Three typical failure modes, i.e. global flexural failure, local punching failure and mixed failure, were identified and corresponding failure mechanisms were analyzed. Finally, the test results were used to check the effectiveness of different design approaches. The results from this study can provide reference for further blast resistant analysis and design for framed PVB-laminated glass.

Reinforcement of floating stone columns with spaced reinforcing rings

The major failure mode of long floating stone columns (FSCs) is lateral bulging. Geosynthetic encasement may be a solution to provide additional confinement to stone column materials and reduce their lateral bulging. However, encasing FSCs may lead to a punching failure mode with no significant additional bearing capacity achieved. In this paper, a new reinforcing method is proposed for increased enhancement of the load–settlement behaviour of FSCs. In this method, a set of ring-type reinforcement is used around the periphery of the FSC, called spaced reinforcing rings (SRRs) method. These rings are located along the whole or a part of the FSC length at specified spacing intervals. Laboratory experiments and numerical analyses were performed to investigate the proposed method and contributing parameters on the performance of SRR-reinforced FSCs (SRR-FSCs). The results showed that the use of SRRs is more beneficial than full or partial encasement of FSCs in most situations. This finding, together with the use of lower amounts of reinforcement materials may be of practical interest. In addition, the best configuration for such discrete reinforcement and guidelines for practice are presented.

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.

Eccentric punching strength of continuous flat slabs—Analysis of different experimental setups

AbstractThis paper analyses the performance of the experimental setups to assess the punching strength of slab‐column connections in continuous flat slabs under vertical and horizontal loading. In the last years, several experimental campaigns have been performed to investigate the punching strength of slab‐column connections, but most of the experimental tests concerned isolated slab‐column connections. Among the few setups aimed at reproducing the eccentric punching failure in continuous flat slabs, the setup developed at the NOVA School of Science and Technology in Lisbon is considered in this paper. The performance of the Lisbon setup is assessed through nonlinear finite element analyses, calibrated on experimental data, by comparison with numerical results of a theoretical continuous setup. Then, the performance of the isolated setups, used in many researches and at the base of some international codes, is also evaluated through the same finite element model. Numerical analyses highlight that the setup developed in Lisbon could provide reliable ultimate rotations of continuous flat slab connections, but it underestimates the punching strength. Despite isolated setups lead to similar results when compared with the Lisbon setup, the latter seems to provide a better representation of a continuous slab‐column connection. The numerical analyses presented in this paper have been performed assuming monotonic lateral loading.

Strength and ductility of lightweight reinforced concrete slabs under punching shear

Preventing a sudden punching failure and achieving an economic design, require reliable mechanical models that make physical sense. In addition, Lightweight (LW) concrete is gaining a lot of attention due to its economic advantage. Moreover, all design codes offer punching shear provisions for LW concrete as a viable alternative for the conventional Normal weight (NW) concrete. However, these punching shear provisions were developed for NW concrete slabs. The purpose of this study is to develop a mechanical model for the punching shear behavior of LW concrete slabs, that make physical sense. Inspired with the well-established critical shear crack theory, a punching shear mechanical model for LW concrete slabs was developed, validated, and proposed. In addition, using the proposed model, a parametric study was conducted in order to investigate the effect of concrete density, slab depth, flexure reinforcement ratio on the strength and ductility of LW concrete slabs under punching loads. The proposed model accounted for the effect of concrete type using both the aggregate size and the concrete density. Thus, resulted in a more consistent and more accurate strength and rotation predictions compared to existing models and design codes with respect to the experimental results of LW concrete slabs. The results of the parametric study showed that increasing the concrete density, decreasing the slab depth leads to an increase in the failure strength, the failure rotation, and the ductility. In addition, increasing the flexure reinforcement ratio increase the strength but decreases the failure rotation and ductility.

Punching of RC slabs without transverse reinforcement supported on elongated columns

The paper investigates the influence of support elongation on punching resistance at internal slab column connections without shear reinforcement. Nonlinear finite element analysis (NLFEA) with 3-D solid elements is used to study the influence of column elongation on stress and strain in the slab around the column. Punching failure is shown to be triggered by localised peaks in shear stress around the corners of the support. Significantly, one-way shear is shown to increase the shear resistance of slabs supported on columns with cross sectional dimensions greater than around six times the slab effective depth (d). The common laboratory practice of supporting slabs in punching tests on elongated plates rather than columns is investigated numerically and is found to be reasonable despite uplift occurring in the central region of elongated plates. NLFEA with solid elements gives useful insights into punching failure but nonlinear shell elements are better suited to the practical assessment of slabs in building structures. The disadvantage of conventional nonlinear shell elements is that shear failure can only be detected through post processing of results. To circumvent this, the authors have previously developed a novel modelling approach in which 3-D joint elements are used to connect shell elements located to either side of a punching control perimeter positioned at 0.5d from the column face. The joint shear resistance is calculated using the Critical Shear Crack Theory (CSCT) in which punching resistance is related to slab rotation. Based on insights gained using the solid element modelling, this paper extends the use of the joint model to the modelling of punching failure at elongated supports by including one-way joints to model linear shear.

Numerical Analysis of Reinforced Concrete Slab with Subsoil

Abstract The article deals with the interaction of a reinforced concrete slab with subsoil. The paper contains a non-linear analysis based on an experiment of reinforced concrete slabs with dimensions 2000 x 2000 mm and thickness 150 mm. A steel mesh with a diameter of 8/100 mm was used as reinforcement. The calculations and analysis are complemented by a comparison with EC2 design approaches. The research area combines the design of concrete structure and geotechnical tasks. The real behavior of the concrete structure with subsoil is considered for the analysis for advanced design. The selected computational approach of nonlinear analysis allows to capture the change of stiffness after the creation of cracks and modelling the shear punch failure of the slab - the collapse of the structure. The paper focuses on comparing the experiment with the numerical model in select loaded states for various input parameters of subsoil. Based on the experiment and numerical analysis the failure mechanism was determined. It was the punching of the slab. The calculations and the experiment verified that the critically controlled perimeter is at a distance of 1d. The effect of the modulus of elasticity on the slab punch mechanism was verified. In case of low modulus of subsoil, the load-bearing capacity of the slab is significantly reduced. The punching mechanism is influenced not only by the mechanical properties of the concrete but also by the properties of the subsoil. The performed parametric study also verified the influence of the size the nominal cover reinforcement depending on the modulus of elasticity of the subsoil. The deformation variant finite element method and a 3D computational model were used for numerical modelling. Nonlinear analysis was based on the Newton-Rapson method.

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.