The Deformation Capacity of Lap Splices in RC Walls: From Experimental Testing to Numerical Modelling

Field observations as well as experimental tests have shown that both the strength and displacement capacity of reinforced concrete (RC) members might be significantly reduced by the presence of lap splices. This degradation applies in particular if the longitudinal reinforcement is spliced in regions where inelastic deformations concentrate, which is frequently the case for RC building walls or bridge piers. In fact, these members often feature lap splices above the foundation level, where seismic demands are largest and damage is likely to occur. In performance-based earthquake engineering (PBEE), which now sets the standards for seismic assessment, deformation rather than force capacities are compared to the demand; however, past experimental studies on spliced members have focused on the characterization of the strength rather than the deformation capacity of lap splices. Further-more, these tests were primarily performed on spliced RC beam and column specimens, typically subjected to mono-tonic loading. Experimental investigations on the deformation capacity of members with lap splices under cyclic loading are underrepresented. This applies in particular to walls, despite the fact that splicing of longitudinal reinforcement in their plastic hinge regions is common construction practice. In line with the available experimental work, most of the developed empirical and analytical expressions aim solely at quantifying the force capacity of lap splices. The preceding observations have motivated the following objectives of the present work: (i) investigate the displacement capacity of spliced RC walls through experimental tests; (ii) propose expressions characterizing the deformation capacity of lap splices subjected to monotonic and cyclic loading; and (iii) develop numerical and mechanical models suitable for practicing engineers to simulate the behaviour of RC members with lap splices. Existing experimental programmes on spliced RC walls, including the cyclic test of two units recently carried out at the structural laboratory of EPFL, are first reviewed and collected in a database. The review of the experimental data shows that the failure of the outermost lap splices, located in the boundary element, typically triggers the failure of the RC wall. Moreover, the main parameters influencing the deformation capacity of lap splices are identified. Building on these findings, an experimental programme on spliced RC wall boundary elements is designed using lapsplice length, confining reinforcement, and loading history as variable parameters. From the obtained results, an empirical expression for the lap-splice strain capacity is derived. A 2D shell element model is first developed to simulate the global force-displacement response of the walls in the aforementioned database. The lap splice response is considered through a new equivalent uniaxial steel stress-strain law. Secondly, an axially equilibrated displacement-based beam element model is proposed in which the lap splice response can be included by using the derived strain limit expression. Finally, a novel mechanical model describing the behaviour of spliced RC wall boundary elements is presented. It extends the tension chord model by accounting for anchorage slip and the presence of lap splices.

Structural Response of R-UHPFRC - RC Composite Members Subjected to Combined Bending and Shear

Talayeh Noshiravani

The addition of a thin overlay of Ultra-High Performance Fibre Reinforced Concrete (UHPFRC) to Reinforced Concrete (RC) members is an emerging technique to strengthen and protect existing structures and to design durable new structures. Combining UHPFRC with closely spaced, small-diameter steel rebars in Reinforced UHPFRC (R-UHPFRC) layers improves the UHPFRC's strain hardening behaviour. For reasons of practicality, R-UHPFRC layers are cast or glued (in the case of prefabricated elements) on top of RC members, thus changing the latter into R-UHPFRC - RC composite members. The high strength and deformation capacity of R-UHPFRC elements make them a suitable external flexural reinforcement for RC members over intermediate supports, e.g., bridge decks and slabs or beams in buildings. Over reinforcement of RC beams and slabs with tensile flexural reinforcement can result in their shear failure at either a lower resistance or deformation than the associated values for member failure in flexure. A comprehensive experimental program was conducted to study the flexure-shear behaviour of R-UHPFRC - RC composite beams. The program comprises two test series on cantilever beams and continuous beams. The test parameters include shear span-depth ratio (a/d), the amount of transverse reinforcement ( ρν), the amount of longitudinal reinforcement, and the strength and bond condition of the R-UHPFRC rebars. The experimental results reveal the different failure modes of R-UHPFRC - RC composite members and the contribution of the R-UHPFRC elements to the member resistance, ductility and capacity to redistribute the internal stress. It was shown that in R-UHPFRC - RC beams with ribbed rebars and a shear span to depth ratio greater than 2.5 the stresses are carried by beam action. Depending on the degree of longitudinal reinforcement, all but two of the beams with 3.0≤a/d≤3.4 and ρν≤0.17 had a flexure-shear failure; the rest failed in flexure. The flexure-shear failure of the composite beams was at an approximately equal rotation level as their RC reference beam but at a resistance 2.3 times that of the RC beam. This is due to (1) the debonding interface zone between the elements that allows the R-UHPFRC - RC beams to rotate more freely and (2) the out-of-plane resistance of the R-UHPFRC element that contributes to the shear resistance. The internal flow of forces and the structural response of composite members strongly depend on the bond condition between the R-UHPFRC and RC, the UHPFRC and its rebars, as well as the concrete and its rebars. Cracking of the concrete along the interface zone causes bond reduction, i.e., softening of the shear connection, between the two elements. In presence of high shear stresses and diagonal flexure-shear cracks, interface zone softening is observed between the elements prior to the maximum resistance, while UHPFRC is strain hardening. The cause of this softening behaviour is the prying action due to the relative rotational movement of the RC rigid bodies separated by the flexure-shear cracks. Static and kinematic solutions of the theory of plasticity for RC beams are extended to predict the collapse load of R-UHPFRC - RC composite beams at the ultimate limit state. A mechanical model for predicting the structural response of composite beams is proposed. In combination with truss models, the concept of an R-UHPFRC - RC plastic hinge is introduced to calculate the force-displacement response of composite beams. The failure criterion based on the collapse mechanisms (kinematic solutions) sets the limit of the force-displacement response. The model is corroborated by the experimental results. This model provides a tool for analysis of RC members reinforced with an added tensile R-UHPFRC element.

EPFL2012

Seismic Behaviour and Design of Mixed RC-URM Wall Structures

Alessandro Paparo

In several countries of central Europe, many modern residential buildings are braced by reinforced concrete (RC) and unreinforced masonry (URM) walls coupled with RC slabs. Such mixed constructions, in fact, behave better under seismic loading than buildings with URM walls alone. Similarly, the insertion of RC walls is a technique used to retrofit existing modern URM constructions that feature RC slabs, since both strength and displacement capacities of the repaired structure increase with respect to the un-retrofitted configuration. Although modern mixed RC-URM wall buildings are rather common, very little is known about their seismic behaviour and codes do not provide adequate design and assessment guidelines. Hence, the thesis focuses on four objectives: (1) to provide experimental data on the seismic behaviour of mixed RC-URM wall structures; (2) to formulate recommendations for setting up numerical models for structures with both RC and URM walls; (3) to develop a mechanical model that represents the interaction between RC and URM walls; (4) to propose a displacement-based seismic approach for the design of new mixed RC-URM wall structures and the retrofitting of URM wall buildings by adding RC walls or replacing URM walls with RC ones. An experimental investigation of two mixed RC-URM wall substructures leads to new findings with respect to the actual distribution of the reaction forces between RC and URM walls and gives insight into the displacement profile of mixed RC-URM wall structures. The results are used to validate two numerical strategies and recommendations for setting up models for structures with both RC and URM walls are formulated. A simple mechanical model, which takes into account the most important parameters influencing the seismic behaviour of mixed RC-URM wall structures, is proposed and validated. The model is based on the shear-flexure interaction as URM and RC walls display dominant shear and flexural deformations, respectively. In the last part of the thesis a displacement-based design approach for mixed RC-URM wall structures is developed. The design method is verified through inelastic time-history analyses (ITHA) of three-to-five-storey case study buildings. Comparison between the design values and the results from ITHA suggests that the design methodology controls the horizontal deflection of the structures, being almost linear over their height, and avoids concentrations of deformations in the bottom storey, a typical feature of URM wall structures. On the other hand, for the three-storey configurations, it was observed that the approach overestimates the maximum horizontal displacement.

EPFL2015

Seismic performance of slender RC U-shaped walls with a single-layer of reinforcement

Katrin Beyer, Ryan Hoult, João Saraiva Esteves Pacheco de Almeida

Reinforced concrete walls are commonly used to resist the lateral loading induced by wind and earthquake actions. While most walls include two vertical reinforcement layers, some regions of the world construct slender, non-rectangular concrete walls with a single vertical layer of reinforcement. The seismic performance of such elements is largely unknown given the paucity of experimental research. This paper presents the results of two slender reinforced concrete U-shaped walls tested at the Earthquake Engineering and Structural Dynamics Laboratory (EESD Lab), École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland. Both wall specimens, designed similar to construction practice in Colombia, were tested using quasi-static cyclic loading to observe if out-of-plane instability would develop when deformations were limited to prevent the flange boundary ends crushing. Initial failure of both wall specimens corresponded with local out-of-plane buckling in the boundary ends of the flanges occurring on load reversal. The buckling lengths were approximately 700–800 mm, which corresponded to 44–50 bar diameters. The crack patterns were observed to be steepest in the web of the walls, demonstrating the increased shear demand in comparison to that of a rectangular wall. Both wall specimens reached ultimate drifts larger than 2.5–3.0% before global failure occurring in the web-flange intersection due to crushing. A small twist was subjected to one of the walls when centered and loaded diagonally, which showed that the decay in torsional stiffness is proportional to the decay in translational stiffness.