Realistic modelling of irregular slabs under extreme loading

This paper presents a new triangular flat shell element for reinforced concrete slabs of complex planar configuration subjected to extreme loading. The element is developed within a co-rotational framework, and it incorporates the effects of geometric as well as material non-linearities. To improve the approximation of the solution, additional hierarchic parameters are introduced within the local system of the element. The element formulation allows for composite action between different layers under the assumption of perfect bond between the slab concrete material, the reinforcement layers and the steel deck for composite slabs. To account for floor slabs of irregular geometric configurations, due allowance is made for uniaxial reinforcement to be oriented arbitrarily within the slab plane. The paper briefly describes the element formulation followed by several numerical verification examples. The applicability of the element to modelling concrete slabs is demonstrated using several validation studies against existing experimental results. The versatility of the element is further exemplified with a realistic large-scale floor slab model subjected to extreme loading scenarios. It is shown that the developed element provides a good balance between accuracy and efficiency in the modelling of irregular floor slabs subject to extreme loading conditions

Multiscale model calibration by inverse analysis for nonlinear simulation of masonry structures under earthquake loading

The prediction of the structural response of masonry structures under extreme loading conditions, including earthquakes,requiresthe use of advanced material descriptionsto represent the nonlinear behaviour of masonry. In general, micro-and mesoscale approaches are very computationally demanding, thus at present they are used mainly for detailed analysis of small masonry components. Conversely macroscale models, where masonry is assumed as a homogeneous material, aremore efficient and suitable for nonlinear analysis of realistic masonry structures. However, the calibration of the material parameters for such models, which is generally basedon physical testing of entire masonry components, remains an open issue. In this paper, a multiscale approach is proposed, in which an accuratemesoscale modelaccounting for the specific masonry bond is utilised invirtual tests for the calibration of a more efficient macroscale representation assumingenergy equivalence between the two scales. Since the calibration is performed offlineat the beginning of the analysis, the method is computationally attractive compared to alternativehomogenisation techniques. The proposed methodologyis applied to a case study consideringthe results obtained in previous experimental testson masonry components subjected to cyclic loading, and on a masonry building under pseudo-dynamic conditions representingearthquake loading.The results confirmthepotential of the proposedapproach and highlight somecritical issues, such asthe importance of selecting appropriatevirtual tests for model calibration,which can significantlyinfluence accuracy and robustness

Identification of critical mechanical parameters for advanced analysis of masonry arch bridges

The response up to collapse of masonry arch bridges is very complex and affected by many uncertainties. In general, accurate response predictions can be achieved using sophisticated numerical descriptions, requiring a significant number of parameters that need to be properly characterised. This study focuses on the sensitivity of the behaviour of masonry arch bridges with respect to a wide range of mechanical parameters considered within a detailed modelling approach. The aim is to investigate the effect of constitutive parameters variations on the stiffness and ultimate load capacity under vertical loading. First, advanced numerical models of masonry arches and of a masonry arch bridge are developed, where a mesoscale approach describes the actual texture of masonry. Subsequently, a surrogate kriging metamodel is constructed to replace the accurate but computationally expensive numerical descriptions, and global sensitivity analysis is performed to identify the mechanical parameters affecting the most the stiffness and load capacity. Uncertainty propagation is then performed on the surrogate models to estimate the probabilistic distribution of the response parameters of interest. The results provide useful information for risk assessment and management purposes, and shed light on the parameters that control the bridge behaviour and require an accurate characterisation in terms of uncertainty

Mesoscale modelling of a masonry building subjected to earthquake loading

Masonry structures constitute an important part of the built environment and architectural heritage in seismic areas. A large number of these old structures showed inadequate performance and suffered substantial damage under past earthquakes. Realistic numerical models are required for accurate response predictions and for addressing the implementation of effective strengthening solutions. A comprehensive mesoscale modeling strategy explicitly allowing for masonry bond is presented in this paper. It is based on advanced nonlinear material models for interface elements simulating cracks in mortar joints and brick/block units under cyclic loading. Moreover, domain decomposition and mesh tying techniques are used to enhance computational efficiency in detailed nonlinear simulations. The potential of this approach is shown with reference to a case study of a full-scale unreinforced masonry building previously tested in laboratory under pseudodynamic loading. The results obtained confirm that the proposed modeling strategy for brick/block-masonry structures leads to accurate representations of the seismic response of three-dimensional (3D) building structures, both at the local and global levels. The numerical-experimental comparisons show that this detailed modeling approach enables remarkably accurate predictions of the actual dynamic characteristics, along with the main resisting mechanisms and crack patterns

Assessment of progressive collapse in multi-storey buildings

Accepted versio

Modelling of RC beam-column sub-assemblages under sudden column loss scenario

The design of structures against disproportionate collapse is commonly achieved through structural assessment under a sudden column loss scenario. In this regard, a novel framework has been previously developed at Imperial College London, aiming to improve the accuracy of robustness assessment whilst maintaining a certain level of practicality. To predict the dynamic behaviour under sudden column loss, the framework transforms the nonlinear static response of a structure into a pseudo-static response through principles of energy balance. Furthermore, the response of an entire structure can be obtained based on the assemblage of its individual components. As such, the structural assessment under sudden column loss can be performed, for example, by simply providing the nonlinear static response of a beam-column sub-assemblage (BCSA) extracted from the structure. Complement to the above framework, this paper aims to propose efficient numerical models to predict the nonlinear static response of RC BCSAs under column loss scenarios. The BCSAs are modelled in the nonlinear finite element analysis program ADAPTIC using one-dimensional fibre-beam elements and a combination of joint/link elements. To reproduce the fracture of reinforcement, a uniaxial steel constitutive model is developed, in which a tensile softening branch follows an exponential degrading function. Two approaches are employed to assess the relevance of considering bond-slip. The first approach, which uses conventional fibre-beam elements, is more simplistic, with the inherent assumptions of linear cross-sectional strain distribution and full-bond between reinforcement and concrete. The second approach is more sophisticated since it relies on two separate elements for the modelling of concrete and reinforcement/bond, respectively. Lastly, parametric studies are performed to assess the sensitivity of the predicted nonlinear static response to various model parameters, including the tensile softening rate of reinforcement and the bond strength

Ultimate behavior of idealized composite floor elements at ambient and elevated temperature

This paper is concerned with the ultimate behavior of composite floor slabs under extreme loading situations resembling those occurring during severe building fires. The study focuses on the failure state associated with rupture of the reinforcement in idealized slab elements, which become lightly reinforced in a fire situation due to the early loss of the steel deck. The paper describes a fundamental approach for assessing the failure limit associated with reinforcement fracture in lightly reinforced beams, representing idealized slab strips. A description of the ambient-temperature tests on isolated restrained elements, carried out to assess the influence of key material parameters on the failure conditions, is firstly presented. The results of a series of material tests, undertaken mainly to examine the effect of elevated temperature on ductility, are also described. A simplified analytical model is employed, in conjunction with the experimental findings, to assess the salient material parameters and their implications on the ultimate response at both ambient and elevated temperature. © 2009 Springer Science+Business Media, LLC

Efficient 3D modelling of punching shear failure at slab-column connections by means of nonlinear joint elements

Failures of isolated slab-column connections can be classified as either flexural or punching. Flexural failure is typically preceded by large deformation, owing to flexural reinforcement yield, unlike punching failure which occurs suddenly with little if any warning. This paper proposes a novel numerical strategy for modelling punching failure in which nonlinear joint elements are combined with nonlinear reinforced concrete (RC) shell elements. The joint elements are employed to model punching failure which limits force transfer from slabs to supporting columns. The shear resistance of individual joint elements is calculated using the critical shear crack theory (CSCT) which relates shear resistance to slab rotation. Unlike other similar models reported in the literature, the joint strength is continually updated throughout the analysis as the slab rotation changes. The approach is presented for slabs without shear reinforcement but could be easily extended to include shear reinforcement. The adequacy of the proposed methodology is verified using experimental test data from isolated internal RC slab-column connections tested to failure under various loading arrangements and slab edge boundary conditions. Comparisons are also made with the predictions of nonlinear finite element analysis using 3-D solid elements, where the proposed methodology is shown to compare favourably whilst requiring significantly less computation time. Additionally, the proposed methodology enables simple calculation of the relative contributions of flexure, torsion and eccentric shear to moment transfer between slab and column. This information is pertinent to the development of improved codified design methods for calculating the critical design shear stress at eccentrically loaded columns

Failure of Lightly Reinforced Concrete Floor Slabs with Planar Edge Restraints under Fire

Accepted versio

3D macroelement approach for nonlinear FE analysis of URM components subjected to in-plane and out-of-plane cyclic loading

The paper presents a novel 3D macroelement approach for efficient and accurate nonlinear analysis of unreinforced masonry components subjected to in-plane and out-of-plane cyclic loading. A macroscopic description for masonry is employed, where macroelements, consisting of deformable blocks interacting through cohesive interfaces, are used to represent large portions of masonry walls, enhancing computational efficiency. Enriched kinematic characteristics are adopted for the homogeneous blocks, where in-plane shear and out-of-plane bending modes are described by two independent Lagrangian parameters. Moreover, a detailed material model for the nonlinear interfaces connecting adjacent elements enables an accurate representation of complex failure modes and cracking patterns in masonry walls. As a result, the proposed FE strategy can be employed for accurate response predictions of large masonry structures subjected to cyclic loading conditions. The accuracy of the macroelement approach is validated through comparisons against results of experimental tests of solid and perforated masonry walls under in-plane and out-of-plane loading