Punching Strength Research Articles

PUNCHING SHEAR BEHAVIOR OF THREE DIMENSION TEXTILES REINFORCED CEMENTITIOUS COMPOSITE PLATES

Self compacting mortars (SCMs) plate specimens with dimension of (500×500×40) mm were cast with three-dimension (3D) textile glass fiber having three diverse thicknesses 6, 10, and 15mm to measure their punching strength. Plates with one and two layers of chicken wires, as well as micro steel fiber of 0.75 % volume fraction were tested under punching for comparison. Punching shear tests have been carried out by applying concentrated load with steel cylinder of 50mm diameter and 10 mm height. The mechanical behavior of SCMs plate was discussed in terms of observed behavior, ultimate load, load - deflection curves, and crack pattern. The results indicated an enhancement in the ultimate load at (28 and 90) day ages by about (7.82% and 24%), respectively. The maximum ultimate load was increased by about (58.4 and 54.1) % for plates reinforced by micro steel fiber at 28 and 90 days, respectively as compared with reference. The maximum deflection at the center of the Self-compact mortars plates for all tested plates was improved.

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 strength of conventional slab-column specimens

This paper presents an improved rational method for predicting the punching strength of conventional reinforced concrete slab-column specimens extending to the nominal line of contraflexure in a flat slab structure. The proposed method of analysis is for square and circular, isotropically reinforced, conventional slab-column specimens, concentrically loaded using square or circular columns. The method is based on an earlier two-phase approach, in which the punching strength was predicted as the lesser of the flexural punching strength and the shear punching strength. The earlier approach had previously been shown to be more reliable than other methods, including the major building code methods, and the proposed method represents a further significant improvement. The improvement in the proposed method is due to the incorporation of slab depth factors for both the flexural and shear modes of punching failure and refinements to the effects of concrete strength, reinforcement percentage and reinforcement yield strength, for the shear mode of punching failure. Comprehensive test data is presented for 217 tests on conventional slab-column specimens reported in the literature in the sixty year period 1956–2016. Analysis of these results by the proposed method resulted in significantly improved correlation over that of the authors’ previous two-phase approach. The method is also shown to be significantly more accurate and consistent than the current Eurocode 2 (2004) method, the ACI 318-14 (2014) method and the fib Model Code (2010) method for predicting the punching strength of conventional reinforced concrete slab-column specimens.

Improved Shear Reinforcement for Footings—Maximum Punching Strength

Improved Shear Reinforcement for Footings—Maximum Punching Strength

Two-parameter kinematic theory for punching shear in reinforced concrete slabs without shear reinforcement

Measurements taken from recent test series with varying slenderness reveal strong differences between fracture kinematics of slender and compact slabs failing in punching. In accordance with the assumptions of the existing kinematic punching shear resistance models, the deformation behavior of slender slabs is governed by flexural deformations. However, in very compact slabs only small flexural deformations occur and the deformation behavior is dominated by translational deformations. In the transition region between slender and very compact slabs, the deformation behavior is influenced by both deformation components. As a consequence, the general application of the existing kinematic models to both slender and compact slabs might yield unexpected results.In this paper, a two-parameter kinematic theory for punching shear in reinforced concrete slabs without shear reinforcement is developed taking into account the aforementioned observations. In the theory, it is assumed that shear forces are transmitted along the failure crack by four shear contributions, namely the contributions of compression ring, aggregate interlock, residual tensile stresses, and dowel action. The magnitude of shear contributions is estimated based on the deformed slab accounting for two degrees of freedom (DOFs). While the first DOF accounts for flexural deformations, the second DOF considers translational deformations. Subsequently, the punching strength is calculated by summation of the contributions. The evaluation of the proposed theory by means of systematic test series and databanks yields good agreement between predictions and experimental results. Especially, the differences between flat slabs and column bases can be explained in a consistent manner by the theory.

Non-linear finite element analysis of reinforced concrete flat plates with opening adjacent to column under eccentric punching loads

Nonlinear finite element analyses were performed on interior slab-column connections to investigate the effects on the punching strength of flat slabs. A total of twenty-seven three-dimensional finite element models were constructed to represent large-scale flat plate slabs (2000 × 2000 × 155 mm) subjected to concentric and eccentric punching loads. The main parameters in this study were the eccentricity of the load, the opening size, the location of the opening relative to the direction of the transfer moment, and the configuration of the reinforcement adjacent to the opening that is used to compensate the cutting bars. The dimensions and reinforcement of the slab and column were kept constant. Predictions of the numerical models were in good agreement with experimental results published in the literature for slab column connections subjected to concentric loading. The numerical investigation showed that the presence of an opening adjacent to a column in a specimen subject to concentric loads could cause a significant reduction in the ultimate shear capacity which increases with the increase in the size of the opening. Specimens with 250 × 250-mm size openings indicated a reduction of 35% in their ultimate shear strength capacity while specimens with 250 × 450-mm size openings indicated a reduction of 45%. Specimens without openings subjected to load eccentricities of 112.5 and 225 mm resulted in a reduction in the load capacity of 18 and 28%, respectively. It was found that providing continuous bars adjacent to the opening to compensate the cutting areas of reinforcement could result in a considerable improvement in the serviceability of the slab represented by an increase the stiffness of the slab-column connection and no significant effect on the punching capacity for the case of concentric loads. Finite element analyses performed are in good agreement with provisions in the building code (e.g., ACI-318) for the case when the opening is located parallel to the direction of the principal moment and provide conservative values when the opening is in the upward side of the bending moment.

Fracture kinematics of reinforced concrete slabs failing in punching

Over the past decades, a large number of punching shear resistance models with different backgrounds have been developed. Among them, punching shear resistance models based on kinematic failure mechanisms have been found to be in good agreement with punching tests on slender slabs. The existing kinematic models generally determine the punching strength based on suitable failure criteria relating punching failure to a certain slab rotation. Hence, slab deformations are assumed to occur as a result of flexural deformations only. Yet, measurements taken from recent punching tests with varying slenderness reveal differences between fracture kinematics of slender slabs (e.g. flat slabs) and compact slabs (e.g. column bases). In this context, the deformation behavior of compact slabs is rather governed by translational deformations. Consequently, a general application of the existing models to both slender and compact slabs might yield inconsistent results.In this paper, the punching shear behavior of reinforced concrete flat slabs and column bases is investigated in detail. Based on measurements from tests and theoretical investigations, the fracture kinematics of slabs failing in punching are analyzed. The investigations verify that the total deformation of compact slabs at punching failure is significantly underestimated by considering the slab rotation as single degree of freedom (DOF). A more general description of the deformation behavior of both slender and compact slabs is possible by introducing a second DOF considering translational deformations. Based on the aforementioned observations, a general kinematic model is introduced to describe the fracture kinematics of reinforced concrete slabs by means of two DOFs. The proposed model is verified by means of measurements taken from punching tests.

Evaluation of punching shear strength of flat slabs supported on rectangular columns

The article presents the methodology and results of an analytical study of structural parameters influence on the value of punching force for the joint of columns and flat reinforced concrete slab. This design solution is typical for monolithic reinforced concrete girderless frames, which have a wide application in the construction of high-rise buildings. As the results of earlier studies show the punching shear strength of slabs at rectangular columns can be lower than at square columns with a similar length of the control perimeter. The influence of two structural parameters on the punching strength of the plate is investigated - the ratio of the side of the column cross-section to the effective depth of slab C/d and the ratio of the sides of the rectangular column Cmax/Cmin. According to the results of the study, graphs of reduction the control perimeter depending on the structural parameters are presented for columns square and rectangular cross-sections. Comparison of results obtained by proposed approach and MC2010 simplified method are shown, that proposed approach gives a more conservative estimate of the influence of the structural parameters. A significant influence of the considered structural parameters on punching shear strength of reinforced concrete slabs is confirmed by the results of experimental studies. The results of the study confirm the necessity of taking into account the considered structural parameters when calculating the punching shear strength of flat reinforced concrete slabs and further development of code design methods.

Punching-shear behaviors of RC-column footings with various reinforcement and strengthening details

In this study, various reinforcement and strengthening details for the improvement of the punching-shear behavior of concrete footings were developed. Eight test specimens of reinforced-concrete (RC) footings were constructed and tested to investigate the punching-shear behavior of the concrete footings with the developed details. The test parameters include the additional placement of a longitudinal reinforcement around the column-footing connections, the inclination of the shear reinforcement, an additional cast of high-strength concrete, and an additional concrete cast with amorphous metallic fibers (AMFs). The test specimens were supported on a bed of steel car springs that simulate the elastic behavior of soil. The test results show that the punching strength was significantly enhanced by the additional placement of the longitudinal reinforcement around the column-footing connections and by the addition of the retrofit materials (high-strength concrete and AMF-reinforced concrete). Contrarily, the shear reinforcement that was placed around the connections did not significantly affect the punching-shear strength of the footings. The test results were also compared with strength predictions for which the current design codes and the existing theoretical models were used.

Prediction of Punching Shear Capacity of Two-Ways FRP Reinforced Concrete Slabs

An innovative solution to the corrosion problem is the use of fiber-reinforced polymer (FRP) as an alternative reinforcing material in concrete structures. In addition to the non corrodible nature of FRP materials, they also have a high strength-to-weight ratio that makes them attractive as reinforcement for concrete structures. Extensive research programs have been carried out to investigate the flexural behavior of concrete members reinforced with FRP reinforcement. On the other hand, the shear behavior of concrete members, especially punching shear of two-way slabs, reinforced with FRP bars has not yet been fully explored. The existing provisions for punching of slabs in most international design standards for reinforced concrete are based on tests of steel reinforced slabs. The elastic stiffness and bonding characteristics of FRP reinforcement are sufficiently different from those of steel to affect punching strength. In the present study, the equations of existing design standards for shear capacity of FRP reinforced concrete beams have been evaluated using the large database collected. The experimental punching shear strengths were compared with the available theoretical predictions, including the CSA S806 (CSA 2012), ACI-440.1R-15 (ACI 2015), BS 8110 (BSI 1997), JSCE (1997) a number of models proposed by some researchers in the literature. The existing design methods for FRP reinforced concrete slabs give conservative predictions for the specimens in the database. This paper also presents a simple yet improved model to calculate the punching shear capacity of FRPreinforced concrete slabs. The proposed model provides the accurate results in calculating the punching shear strengths of FRP-reinforced concrete slender slabs.

Non-structural lightweight concrete with volcanic scoria aggregates for lightweight fill in building’s floors

This paper aims to characterize the mechanical and physical behavior of non-structural lightweight concrete (NSLWC) produced with volcanic scoria, regarding its application in building’s floors. The density, thermal conductivity, capillary absorption, compressive, tensile and punching strength, elasticity modulus, shrinkage, abrasion resistance and behavior at high temperatures of different NSLWC fill solutions were analyzed. For comparison purposes, conventional NSLWC with expanded clay and expanded polystyrene aggregates were also studied. Despite its greater density and slightly lower abrasion resistance and thermal conductivity, NSLWC with scoria showed similar mechanical strength, less shrinkage, higher punching strength and better behavior at high temperatures than conventional NSLWC.

Punching shear tests on compact footings with uniform soil pressure

AbstractPunching shear is usually the governing failure criterion when selecting the depth of reinforced concrete footings. Despite the fact that large experimental programmes aimed at the punching strength of slender flat slabs have been performed in the past, only a few experimental campaigns on full‐scale compact reinforced concrete footings can be found in the literature. This paper presents the results of an experimental programme including eight reinforced concrete footings with a nominal thickness of 550 mm. These experiments investigated the influence of column size, member slenderness and the presence of compression and shear reinforcement. The tests were performed using an innovative test setup to ensure a uniform soil pressure. The experimental results show that slenderness influences the punching shear strength as well as the effectiveness of the shear reinforcement. The experiments also show that an important interaction occurs between bending and shear for high levels of shear force near the column (the typical case of compact footings or members with large amounts of shear reinforcement). Different continuous measurements recorded during the experimental tests allow a complete description of the kinematics and strains at failure. On that basis, experimental evidence is obtained showing that crushing of the concrete struts near the column is the phenomenon that triggers the punching failure of compact footings.

Compressive Membrane Action Effects on Punching Strength of Flat RC Slabs

The design of reinforced concrete flat slabs in practice can be governed at failure by punching shear close to concentrated loads or columns. Punching shear resistance formulations provided by codes are calibrated on the basis of experimental tests on isolated slabs supported on columns in axisymmetric conditions. Nevertheless, the behavior of flat slabs can be different than isolated specimens due to the potentially beneficial contributions of moment redistributions and compressive membrane actions. Accounting for the significance of these effects, nonlinear finite element analyses are performed with the crack model PARC_CL implemented in Abaqus. This paper aims to investigate a series of punching shear tests on slabs with and without shear reinforcement, different reinforcement ratios and loading conditions accounting for the potential contribution to the enhancement of the punching strength due to compressive membrane action (CMA). The numerical results with a multi – layered shell modeling are then post – processed adopting the failure criterion of the Critical Shear Crack Theory (CSCT). The results pointed out the significant outcomes and differences between standard specimens and actual members showing how the current codes of practice may underestimate the punching capacity.

­­Performance of Punching Shear Reinforcement under Gravity Loading: Influence of Type and Detailing

The performance of 11 different shear reinforcement systems against punching of inner slab-column connections under gravity loading was compared on the basis of experiments on 12 full-scale specimens, 8 of them newly reported. The slab geometry and flexural reinforcement ratio (1.5%) were kept constant. The shear reinforcement systems included different layouts of double-headed studs, individual links, bent-up bars and bonded post-installed reinforcement. All the systems were found to increase both the strength and the deformation capacity of the members but exhibited varying performances. The factors influencing the maximum punching strength of different systems, such as the layout and the anchorage conditions of the transverse reinforcement units, are described and analyzed. The mechanical model of the Critical Shear Crack Theory is used to explain the observed differences and provide design guidance. Comparisons to the codes of practice (ACI 318, Eurocode 2 and Model Code 2010) are also presented.

Theoretical approach to the evaluation of the load-carrying capacity of the tie rod anchor system in a masonry wall

The behaviour of tie rod anchor plates is at present not well established, and the criteria suggested do not seem to have consistent theoretical basis.This paper suggests a theoretical approach to determine the load-carrying capacity of stiff anchor plates acting on a masonry wall. According to this approach the punching strength is the lower of two values. The first value follows from the hypothesis that collapse occurs because of the slippage of the units on the mortar bed, whereas the second comes from the alternative hypothesis that punching creates a solid consisting of a truncated cone in which the outer face is the surface of the anchor plate (units break whereas the mortar does not crumble). When doing so it is supposed that the anchor plate is so stiff that pressure it applies on the masonry wall is almost constant. The issue of setting an adequate stiffness is solved for the most common types of anchor plate.Some numerical examples show the capabilities of the suggested approach.

하이브리드 해상풍력발전 지지구조물의 콘크리트 베이스-슬리브 연결부에 대한 실험 연구

In this paper, concrete base to sleeve connections of hybrid substructures for offshore wind turbines were suggested and investigated experimentally. Punching shear strength tests with well-instrumented three connections under different reinforcement ratios and loading conditions were conducted to investigate the punching shear strength and the behavior of the concrete base to a sleeve connection. The test results showed that the punching strength and stiffness of the connections are affected mainly by the reinforcement ratios. The loading conditions with an axial load and proportional moment cannot affect the stiffness but affect the strength of the connections because of the axial load-moment interaction. The punching shear failure and critical section of the each test specimen are also discussed.

Study on Influence of Column Size and Slab Slenderness on Punching Strength

This paper presents the results of a systematic experimental campaign consisting of 13 symmetric punching tests on interior slab-column connections. The study focuses on the influence of varying the size of the supported area and the slenderness of the slab. Other investigated parameters are the flexural reinforcement ratio and the presence of shear reinforcement. The results of the present campaign and of previous tests are compared to the predictions of different codes of practice and to the critical shear crack theory (CSCT). The comparison shows that the CSCT and fib Model Code 2010 give the most consistent predictions, whereas the results of Eurocode 2 for small support sizes have a large scatter. The predictions of ACI 318 are observed as overall conservative in the investigated range. This study shows that slenderness has an important influence on the punching strength of slabs with shear reinforcement, despite the fact that it is neglected in many codes of practice.

Influence of transverse reinforcement on perforation resistance of reinforced concrete slabs under hard missile impact

This paper describes the work conducted by the Canadian Nuclear Safety Commission (CNSC) related to the influence of transverse reinforcement on perforation capacity of reinforced concrete (RC) slabs under “hard” missile impact (impact with negligible missile deformations). The paper presents the results of three tests on reinforced concrete slabs conducted at VTT Technical Research Centre (Finland), along with the numerical simulations as well as a discussion of the current code provisions related to impactive loading.Transverse reinforcement is widely used for improving the shear and punching strength of concrete structures. However, the effect of this reinforcement on the perforation resistance under localized missile impact is still unclear. The goal of this paper is to fill the gap in the current literature related to this topic.Based on similar tests designed by the authors with missile velocity below perforation velocity, it was expected that transverse reinforcement would improve the perforation resistance. Three slabs were tested under almost identical conditions with the only difference being the transverse reinforcement. One slab was designed without transverse reinforcement, the second one with the transverse reinforcement in form of conventional stirrups with hooks and the third one with the transverse reinforcement in form of T-headed bars. Although the transverse reinforcement reduced the overall damage of the slabs (the rear face scabbing), the conclusion from the tests is that the transverse reinforcement does not have important influence on perforation capacity of concrete slabs under rigid missile impact. The slab with T-headed bars presented a slight improvement compared to the baseline specimen without transverse reinforcement. The slab with conventional stirrups presented slightly lower perforation capacity (higher residual missile velocity) than the slab without transverse reinforcement. In conclusion, the performed tests show slightly better performance of slabs with transverse reinforcement in form of T-headed bars compared to the slabs with conventional stirrups with hooks with regards to perforation capacity under hard missile impact.Non-linear dynamic behavior of reinforced concrete slabs under impact loading by rigid missile was analyzed using the commercial Finite Element (FE) code LS-DYNA. FE blind predictions based on Winfrith concrete material model were compared to the tests on slabs with and without transverse reinforcement. The FE predictions obtained were in general agreement with tests. Two different types of transverse reinforcement were examined (stirrups and T-headed bars) using simplified models. Similar to the tests, the FE predictions show that transverse reinforcement localizes damage induced by missile impact but does not increase the perforation resistance of the concrete slab. FE predictions also showed that T-headed bars perform better than stirrups, providing approximately the same perforation resistance and smaller damaged area comparing with a slab with longitudinal reinforcement only. Additionally, FE modeling was conducted for two different slab thicknesses to assess the effect of the thickness.

Experimental study of punching failure in LWAC slabs with different strengths

The constant demand of LWAC applications in structural engineering increases the need of performing studies focused on the behavior of reinforced members produced with LWAC. Mainly, by combining reduced weight and good mechanical performance, LWAC is an efficient solution for flat slabs. The design methods of punching shear strength of LWAC slabs are commonly based on experimental studies of NWC. However, both the stress–strain relation and the distribution of internal stresses of LWAC are quite different from those of NWC, due to the stiffness compatibility between the binding matrix and the LWA, and due to the enhanced performance of interfacial transition zone LWA-matrix. This behavior influences the distribution of internal stresses of LWAC, when compared with NWC, and results in a linear stress–strain relation until around 90 % of maximum stress and a brittle failure after peak when unconfined (Costa in, Lightweight aggregate structural concrete: precast and strengthening of existing structures, 2012). This difference is ignored by the main structural concrete codes or the design expressions of NWC are modified by a corrective coefficient for LWAC, depending on its density. This paper presents an experimental study focused on punching capacity of LWAC slabs. Six slabs were produced with equal longitudinal reinforcement ratio and without shear reinforcement, varying the compressive strength of LWAC from 29 to 54 MPa. Based on the recorded data during the tests, the cracking and maximum loads, the displacements, rotations and stiffness, the failure modes and cracking patterns are presented and analyzed. Experimental results were compared with design predictions of main codes, namely, EC2, MC2010 and ACI318. The results revealed that the variation of LWAC strength influences the punching strength, but has no significant effect on the stiffness and on the angle of the main crack of punching cone. The evaluation of punching shear strength achieved by design methods is higher than the experimental results and, in the case of MC2010 with level I of approximation, is more than double. Excepting for EC2, the ratio between the maximum experimental punching strength and the corresponding code prediction decreases with the increase of LWAC strength.

Punching behaviour of concrete slabs incorporating coarse recycled concrete aggregates

This paper presents experimental, numerical and analytical investigations about the effects of incorporating coarse recycled concrete aggregates (CRCA) on the punching behaviour of reinforced concrete (RC) slabs. For this purpose, four concrete mixes were designed with various substitution ratios of natural coarse aggregates (NCA) by CRCA: 0% (reference mix), 20%, 50% and 100%. Subsequently, eight 1100×1100×90mm reinforced concrete slabs (two per concrete type) were cast and subjected to punching tests. The experimental results indicate that the incorporation of CRCA affects the mechanical behaviour of RC slabs according to the following trends: (i) the punching resistance of slabs made of concrete with CRCA (CRCAC) is similar to that of the reference mix; (ii) the stiffness decreases, particularly in the uncracked state; and (iii) the cracking load slightly decreases. The numerical study comprised the development of three-dimensional non-linear finite element models of the slabs. The numerical models were able to trace the experimental responses of the slabs tested, highlighting the influence of the concrete fracture energy on the numerical results. The best predictions of punching strength were obtained when similar fracture energy was adopted for all concrete mixes. Finally, the accuracy of some of the most relevant design codes in predicting the punching strength of CRCAC slabs is assessed. The most accurate estimates were obtained using Model Code 2010 (MC 2010) with levels of approximation II to IV. Eurocode 2, ACI 318 and MC 2010 with level of approximation I provided less accurate and relatively conservative punching strength predictions.