Influence of Lap Splices on the Deformation Capacity of RC Walls. I: Database Assembly, Recent Experimental Data, and Findings for Model Development

Recent postearthquake missions have shown that reinforced concrete (RC) wall buildings can experience critical damage owing to lap splices, which led to a recent surge in experimental tests of walls with such constructional details. Most of the 16 wall tests described in the literature thus far were carried out in the last six years. This paper presents a database with these wall tests, including the description of a new test on a wall with lap splices and a corresponding reference wall with continuous reinforcement. They complement the existing tests by investigating a spliced member with a shear span ratio smaller than two, which is the smallest among them. The objective of this database is to collect information not just on the force capacity but mainly on the deformation capacity of lap splices in reinforced concrete walls. It is shown that (1) well-confined lap splices relocate the plastic hinge above the lap splice, (2) lap splices with adequate lengths but insufficiently confined attain the peak force but their deformation capacity is significantly reduced, and (3) short and not well-confined lap splices fail before reaching the strength capacity. The analysis of the test results, which are used in the companion paper for the finite element analysis of walls with lap splices, indicates in particular that the confining reinforcement ratio and the ratio of shear span to lap splice length influence the lap splice strain capacity.

Seismic behaviour and analysis of U-shaped RC walls

Raluca-Tereza Constantin

In many countries of moderate to high seismicity, reinforced concrete (RC) core walls are used as lateral bracing systems in mid- to high-rise buildings where they typically accommodate lift shafts or stair cases. Unlike planar walls which provide horizontal strength and stiffness mainly in the in-plane direction of the wall, core walls provide bidirectional strength and stiffness. This feature complicates their inelastic behaviour which is not yet fully understood as experimental and numerical studies are still rather scarce when compared to planar walls. Therefore, current design codes are based on findings from planar walls while specific guidelines for the design and analysis of core walls are still lacking. In addition, simple analysis tools widely used by design engineers such as the plastic hinge model, have been derived and calibrated for columns, beams or planar walls and their suitability for core walls has been only marginally verified. For these reasons the current study focuses on: (1) improving the knowledge on the inelastic behaviour of U-shaped walls under bidirectional loading and (2) on extending easily applicable engineering type models, such as the plastic hinge model, to the analysis of U-shaped walls. The scope of the thesis is limited to U-shaped walls, which is the simplest type of core wall that still retains the key characteristics of such walls. The first part of the thesis focuses on understanding the behaviour of U-shaped walls under loading along the geometric diagonal of the section. This loading direction is not typically considered in the design process but it was found to determine the shear design of the flanges while the displacement capacity for this direction might be the lowest of all the loading directions. In order to address the behaviour under diagonal loading, two large-scale quasi-static cyclic tests on U-shaped walls were carried out. Failure mechanisms specific to diagonal loading and possible critical design aspects related to these failure modes were identified from the experimental results. In addition, the influence of the longitudinal reinforcement distribution on the wall behaviour was investigated. The second part of the thesis focuses on adapting equations used in plastic hinge model for the analysis of U-shaped walls. Plane section analyses and a shell element model validated against the experimental data were used to perform parametric studies on U-shaped walls. From the results of the parametric studies a new equation for the yield curvature for any direction of horizontal loading was proposed as well as a modified yield displacement equation that accounts for the partially cracked wall height at yield. With this equation, predictions of yield displacements were improved especially for very slender walls. Quantities that rely on the yield displacement, such as the effective stiffness of the wall were also better predicted when the cracked height was accounted for in the prediction. And finally, plastic hinge length equations were also modified to account for the variation with the different loading directions.

EPFL2016

Micro-mechanical finite element modeling for in-plane behavior of historical masonry

Shenghan Zhang

Seismic assessment of unreinforced masonry structures remains a challenge. This dissertation investigates the in-plane behavior of historical masonry elements and proposes an advanced simulation method based on the cohesive zone model (CZM) in a finite element framework. Cohesive elements are often used to simulate dynamic fracture. To consider contact and friction, a new node-to-node algorithm is implemented in the open-source code Akantu, which offers a parallel computing model. Based on the proposed framework, two types of tests are investigated: the shear test, which is a basic material test to determine parameters such as cohesion and friction coefficient, and the diagonal compression test, which is a standard masonry material test to determine the nominal diagonal tensile strength and shear modulus of masonry. The simulation results correspond well with experimental results. One potential advantage of the detailed micro-modeling method is the ability to represent and study the influence of the micro-structure of the stone masonry on the force-displacement behavior. However, previous studies focused only on one specific block layout. This constraint was caused by the absence of a stone masonry typology generator. Based on related research in computer vision and geology, a versatile typology generator is developed. To quantify the micro-structure of a stone masonry typology, the line of minimum trace (LMT) is often used. Determining the LMT manually is time consuming and subjective. To facilitate the calculation process, an algorithm to calculate LMT automatically from pictures of masonry patterns is developed. Based on the proposed typology generator, the influence of the masonry typology on the masonry shear strength under shear-compression boundary condition is then systematically investigated. Starting with regular brick masonry patterns, the roundness and the arrangement of the units are altered gradually. The commonly used shear failure criterion by Mann-Müller is investigated in detail and it is shown that the assumed stress distributions are good approximations of the stress state in the linear elastic range but deviate significantly from the stress distribution at peak shear strength. The numerical results also highlight that the effect of the interlocking between stones on the shear strength is overestimated by the Mann-Müller model. For irregular stone masonry, the correlation between LMT and shear resistance is studied. While CZM has been widely used to simulate tensile damage dominated fracture and inter-facial failure, continuum modeling method, i.e., concrete damage plasticity (CDP), is more commonly used for simulating the compressive behavior of quasi-brittle materials, e.g., mortar in masonry and concrete. While existing studies already established the equivalence between the continuum modeling method and the CZM under tension dominated fracture process, the behavior of CZM under compression has not been investigated. To consider a wider range of loading conditions, a new traction-separation law based on the plasticity theory was proposed and therefore expanded the usage of CZM.

EPFL2018

Modelling shear–flexure interaction in equivalent frame models of slender reinforced concrete walls

Katrin Beyer, Panagiotis Mergos

Quasi-static cyclic tests on reinforced concrete (RC) walls have shown that shear deformations can constitute a significant ratio of the total deformations when the wall is loaded beyond the elastic regime. For slender RC walls that form a stable flexural mechanism the ratio of shear to flexural deformations remains approximately constant over the entire range of imposed displacement ductilities. This paper proposes a method for incorporating shear-flexure interaction effects in equivalent frame models of slender RC walls by coupling the shear force-shear strain relationship to the curvature and axial strain in the member. The suggested methodology is incorporated in a finite element consisting of two interacting spread inelasticity sub-elements representing flexural and shear response, respectively. The element is implemented in the general finite element code IDARC and validated against experimental results of RC cantilever walls. In a second step, it is applied in inelastic static and dynamic analyses of tall wall and wall-frame systems. It is shown that ignoring shear-flexure interaction may lead to erroneous predictions in particular of local ductility and storey drift demands.