Rebar slippage in reinforced concrete (RC) elements results in concrete expansion, large cracks, and consequently, early deterioration of strength as well as premature stiffness degradation, ...particularly in the inelastic energy dissipating zones. Although design standards prescribe different minimum concrete compressive strength, seismic evaluation and retrofit standards, and guidelines permit the use of provisions regarding bond strength and bar slippage issues regardless of the minimum specified concrete strength postulated in design standards. To better understand the seismic behavior of special moment-resisting (SMR) beams exhibiting fixed-end rotation resulting from the rebars inelastic elongation and slip, quasi-static cyclic tests were performed on eight full-scale SMR beams. The chosen beams have longitudinal reinforcement ratios of 0.84% (Type-1) and 1.26% (Type-2) with a shear-span to depth ratio of 6.14 and detailed following the provisions of ACI-318-19. Two specimens were prepared for each reinforcement ratio using concrete with compressive strengths equal to 2000 psi (14 MPa, M14) and 3000 psi (21 MPa, M21). The specimens were tested under cyclic displacement protocols, exhibiting flexure yielding that was followed by diagonal shear cracking and, ultimately, bond failure at the beam–block interface. It is even though the beams fulfill the requirements of ACI 318-19 for steel bars embedment and end hooks for anchorage. Force–displacement hysteretic response curves were obtained revealing pinching behavior in the cyclic response. Both types of beams deformed up to maximum chord rotations of 5.22% and 5.73% in case of beams with M14 and M21 concrete, respectively, and experienced cover concrete crushing at the compressed toe. Representative numerical models were assembled implementing fiber-section force-based inelastic beam elements. Additionally, lumped inelastic rotational springs were added to the model for fixed-end rotation. A tri-linear moment-rotation hysteretic response curve has pinching behavior was used to simulate the reduction in re-loading stiffness. This was verified with the measured response of tested beams; excellently simulates the hysteretic response. Moreover, to examine the seismic response of a total structural system regarding these findings, several response history analyses were performed on capacity-designed five-story frames to demonstrate the importance of modeling beam element fixed-end rotation for predicting the story drift demands subjected to different earthquake ground motions. It was found that despite the bar-slip phenomenon the beams developed their yield capacities; however, the response of the frame was subjective depending on the characteristics of input motions, particularly the valleys and hills of the spectral shape.
This paper assesses the influence of slab continuity on the punching resistance of a realistically proportioned flat slab floor plate without shear reinforcement. The edge column punching resistance ...of a symmetric flat slab extending bays in each direction was assessed by means of NLFEA with TNO DIANA, MC2010 levels II, III, IV, Eurocode 2 and NBR 6118. Both Eurocode 2 and NBR 6118 are seen to give similar predictions for punching resistance, while MC2010, which is based on the Critical Shear Crack Theory and depends on how rotations are calculated and FE modelling assumptions, varies significantly with its levels of approximation with Level IV agreeing reasonably well with predictions from NLFEA. Direction for the critical rotations is shown to vary and can also be influenced by the reinforcement over the span. For EC2, NBR 6118 and MC2010 LoA II and III punching shear design are independent of span, unlike the results obtained with MC2010 LoA IV.
resumo: Este artigo avalia a influência da continuidade na resistência à punção de uma laje lisa de concreto armado com dimensões realísticas, sem armadura de cisalhamento. O estudo da resistência à punção teve enfoque em um pilar de borda avaliado por meio de análise não linear em elementos finitos com o TNO DIANA, MC2010 níveis II, III, IV, Eurocode 2 e NBR 6118. O Eurocode 2 e a NBR 6118 apresentaram resultados similares para a resistência à punção, enquanto o MC2010, baseado na Teoria da Fissura Crítica de Cisalhamento, que depende do método de cálculo das rotações e das premissas adotadas de modelagem em elementos finitos, apresentou variações em função dos níveis de aproximação, com o nível IV concordando razoavelmente bem com os resultados provenientes da análise não linear.A direção das rotações críticas também apresentou alterações, possivelmente influenciada pela taxa de armadura ao longo do vão. Para o EC2, NBR 6118 e MC2010 LoA II e III, o dimensionamento à punção independe da continuidade da laje, contrariamente aos resultados obtidos com o nível IV do MC2010.
Als eine sehr effektive Durchstanzbewehrung haben sich seit Jahrzehnten Doppelkopfanker etabliert, da bei diesen für Querkraft‐ und Durchstanzbeanspruchungen optimierten Bewehrungselementen eine ...besonders hohe Verankerungsleistung mit einer praktikablen Einbaubarkeit kombiniert werden kann. In diesem Beitrag wird ein Überblick über aktuelle Verwendbarkeits‐ und Anwendbarkeitsnachweise für Doppelkopfanker in Deutschland gegeben. Danach werden die aktuell geltenden Bemessungs‐ und Konstruktionsregeln für die Verwendung der Doppelkopfanker als Querkraft‐ sowie als Durchstanzbewehrung zusammengestellt. Abschließend wird auf die nächste Eurocode‐2‐Generation 2023+geblickt, wobei auszugsweise einige wesentliche Neuerungen bei der zukünftigen Einbindung der Doppelkopfanker in die Querkraft‐ und Durchstanzregelungen vorgestellt werden.
Double‐headed studs as shear and punching reinforcement – Current status and outlook
Double‐headed studs have been established for decades as a very effective punching shear reinforcement because these reinforcement elements, which are optimized for shear and punching loads, combine a particularly high anchorage performance with a practicable installation on site. In this paper, an overview of current usability and applicability verifications for double‐headed studs in Germany is given. Afterwards, the currently valid design and construction rules for the use of double‐headed studs as shear reinforcement as well as punching reinforcement are summarized. Finally, the next Eurocode 2 generation 2023+is looked at, with excerpts presenting some significant innovations in the future integration of double‐headed studs into the shear and punching rules.
Experimental evidence have proved that punching shear capacity of flat slabs deteriorate with the presence of openings located within the critical perimeter around columns. It is understood that this ...deterioration varies inversely with the distance of openings from column’s face. However, effect of the shape of openings on punching shear capacity is not well known. This study presents experimental results of 14 flat specimens to investigate the effects of the number (2 and 4), shape (circular, square, and rectangular), and location (1 and 4 times of slab’s thickness from column’s face) of openings on punching shear strength. It was found that circular openings had least influence on punching capacity followed by square and rectangular openings, respectively. Further, placing openings at a distance of four times the slab’s thickness from column’s face had minimal impact on punching capacity. Further, increasing the number of openings from 2 to 4 substantially reduced the punching capacity. An effort was made to predict the punching capacities of all specimens using the descriptive equations of ACI 318-19 and Eurocode 2. Mean of the ratio of experimental to analytical results and standard deviation of ACI equations were found to be more accurate than those of Eurocode 2 predictions.
•Major test series with beams from an ASR-damaged bridge was conducted.•ASR expansion induced significant tensile strains in the reinforcement.•The shear strength is not reduced as much as the ...compressive strength of drilled cores.•Generally, calculated shear strengths are lower than measured shear strengths.
Although the residual shear strength of ASR-damaged slab bridges without shear reinforcement has been discussed for several decades, the amount of published research on this topic is very limited. This paper presents the results of a test series on 18 reinforced beams sawn from a severely ASR-damaged flat slab bridge. Flexural failure as well as shear failure were observed. The test results indicate that the ASR cracks in the beams strongly influenced the propagation of load-induced cracks. Generally, the calculated moment capacity of beams tested in three-point bending was lower than the maximum moment carried by the beams. For the beams tested in asymmetric four-point bending, an increase in the shear span-to-effective depth ratio resulted in a decrease in the measured shear strength. The measured shear strengths were compared with calculated shear strengths using the Eurocode 2. Calculations based on the compressive strength of drilled cores were rather conservative at low shear span-to-effective depth ratios. However, the conservatism of the Eurocode 2 decreased with increasing shear span-to-effective depth ratios. With the inclusion of ASR-induced pre-stress effect, the calculated shear strengths correlated better with the measured shear strengths. The test results indicated that the ASR-induced pre-stress effect can, to some extent, compensate for the significant loss in material properties.
The level of maximum punching strength is of major importance to design highly stressed slab‐column connections and to choose the type of punching shear reinforcement. Since a couple of years, there ...is consensus that the maximum punching strength of shear‐reinforced concrete slabs can be defined as a multiple of the reference punching strength without shear reinforcement. The upper bound of this load‐increasing effect depends mainly on the anchorage and arrangement of shear reinforcement elements. Despite numerous research studies and database evaluations, there are ongoing discussions regarding the final effectiveness of stirrups and double headed studs installed in flat slabs and column bases.
In this paper, a strictly filtered punching database is used to highlight the significant impact of underlying reference strength comparing the maximum load increase according to current Eurocode 2 and the draft for the next generation of Eurocode 2 (prEC2). Based on this unifying database, the origin of current discrepancies is discussed and a combination of constant lower bound coefficients with a refined formula as specified in prEC2 is proposed. Additionally, the effect of adapting current European Technical Approvals for double‐headed studs and punching shear optimized lattice girders to prEC2 is shortly analyzed. A further database evaluation also allows for setting the basis for optimized detailing provisions regarding the maximum permissible diameter of shear reinforcement.
In the recent years, bridges, as an important part of the national and international infrastructure, had to comply with stricter requirements due to increased heavy load traffic. Many of these bridge ...structures built in the 1960s and 1970s often contain less web reinforcement than the modern required minimum web reinforcement. In this context, the shear resistance under cyclic loading is of special interest. For this reason, experimental tests were conducted on prestressed concrete beams with and without shear reinforcement at the Institute of Structural Concrete of RWTH Aachen University to investigate the shear fatigue strength. This paper describes the recent tests on ten Tshaped prestressed beams with web reinforcement. The specimens were able to resist more load cycles than predicted by the approaches implemented in the Eurocodes for bridges. Based on the test results, design models for shear under cyclic loading should be reviewed and improved, especially regarding the assessment of existing structures.
Für die Ermittlung der Mindestbewehrung zur Rissbreitenbegrenzung unter Zwang wird im aktuellen Eurocode 2 ein risskraftbasierter Ansatz verwendet. Infolge der Änderung der anzusetzenden ...Betonzugfestigkeit im nationalen Anhang ist eine wirtschaftliche Bemessung weitestgehend nicht mehr möglich. Die Ursache der Überdimensionierung der Mindestbewehrung liegt unter anderem darin, dass der Bemessungsansatz auf unvollständigen empirischen Daten und konservativen Annahmen basiert. Zur Untersuchung der Einflüsse auf die Rissentstehung und ‐bildung unter Zwang sind genau definierte Randbedingungen nötig. Dadurch werden die Vergleichbarkeit und Reproduzierbarkeit der experimentellen Untersuchungen ermöglicht. Der verwendete Versuchsaufbau erlaubt eine vollständige Zwängung unter konstanten klimatischen Randbedingungen der Versuchskörper. Die systematische Auswertung ausgewählter Versuche und Analyse der Rissbildung bildet die Grundlage für eine zukünftige Identifizierung maßgeblicher Phänomene.
Experimental Investigations on crack formation in reinforced concrete due to restraint stress
In the current Eurocode 2, a crack‐force‐based approach is used to determine the minimum reinforcement in restrained concrete members. Due to the tightening of the requirement by a change of the German national annex in Eurocode 2, an economic design is no longer possible. One of the reasons for the overdimensioning of the minimum reinforcement is that the design approach is based on incomplete empirical data and conservative assumptions. To investigate the influences on crack initiation and the effects of crack formation on restrained concrete members, precisely defined boundary conditions are necessary. These allow not only the comparability of the experimental investigations but also their reproducibility. The test setup used allows full restraint under constant climatic conditions of the test specimens. By systematic evaluation of selected tests, the crack formation of restrained plates is analyzed, allowing interpretations and the identification of basic phenomena.
Punching tests on 13 specimens under uniform soil pressure were conducted to evaluate the punching shear behaviour of footings with practical dimensions. The test series included square footings with ...and without punching shear reinforcement. The dimensions of the footings varied between 1.20 × 1.20 m and 2.70 × 2.70 m and the slab thickness varied between 0.45 and 0.65 m, resulting in shear span‐depth ratios aλ/d between approx. 1.25 and 2.00.
In addition to the measured steel strains in the flexural reinforcement and the stirrups, the increase in the slab thickness as well as the saw‐cuts were examined to investigate the internal cracking and failure characteristic. In combination with previous tests conducted at RWTH Aachen University, this test series permits a description of the effect of the main parameters on the punching shear strength of footings. These parameters are the size effect of the effective depth, the concrete compressive strength, the flexural reinforcement ratio and the punching shear reinforcement.
AbstractMany models to determine the shear capacity of shear reinforced beams are based on the truss analogy. Various proposals have been formulated in recent years, all of which differ with regard ...to the limits of strut inclination. Remarkably, those limits do not depend on the type of concrete, which could be expected to be critical for the shear friction capacity of the cracks, which is supposed to be a major influencing factor with regard to the limit of strut rotation. Tests on beams with I-shaped cross sections have been carried out on beams made of normal, lightweight, and high-strength concrete. The experimental results are compared, in Part I of this paper, with those obtained by analytical models and, in Part II, with those obtained by nonlinear finite element programs tailored to this specific application. The result is that the type of concrete does not lead to significant changes in strut rotation capacity, so that the strut rotation limit values have general validity. The level of approximation approach, as presented in the Fédération Internationale du Béton/International Federation for Structural Concrete (fib) Model Code of Concrete Structures 2010 is justified: more sophisticated calculation models lead indeed to more accuracy in the determination of the shear capacity.
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