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

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

Accepted versio

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

EULERIAN FORMULATION FOR LARGE-DISPLACEMENT ANALYSIS OF SPACE FRAMES

Accepted versio

Failure assessment of lightly reinforced floor slabs. I: Experimental investigation

This paper is concerned with the ultimate behavior of lightly reinforced concrete floor slabs under extreme loading conditions. Particular emphasis is given to examining the failure conditions of idealized composite slabs which become lightly reinforced in a fire situation as a result of the early loss of the steel deck. An experimental study is described which focuses on the response of two-way spanning floor slabs with various materials and geometric configurations. The tests enable direct assessment of the influence of a number of key parameters such as the reinforcement type, properties, and ratio on the ultimate response. The results also permit the development of simplified expressions that capture the influence of salient factors such as bond characteristics and reinforcement properties for predicting the ductility of lightly reinforced floor slabs. The companion paper complements the experimental observations with detailed numerical assessments of the ultimate response and proposes analytical models that predict failure of slab members by either reinforcement fracture or compressive crushing of concrete. © 2011 American Society of Civil Engineers

Nonlinear simulation of masonry vaults under earthquake loading

Masonry vaults are present in a large number of historical structures and often used as floor-ing and roofing systems in monumental palaces and religious buildings, typically incorporat-ing no backfill. Many of these structures are located in seismic regions and have been shownto be particularly vulnerable during recent earthquakes, with a need for accurate modelling to avoid future losses. Masonry vaults are often analysed using limit analysis procedures un-der the hypotheses of no-tension material and absence of sliding along the masonry joints.However, this method can be inaccurate for barrel vaults found in buildings, which are typi-cally slender with no backfill. In this case, the masonry tensile strength and the progressive damage propagation play an important role in the nonlinear behaviour and ultimate strength of the vault. In this study, a detailed mesoscale finite element mesoscale approach is used to model slender unreinforced barrel vaults subjected to cyclic quasi-static and dynamic load-ing. According to this approach, 3D solid elements connected by 2D damage-plasticity inter-faces are used to represent the arrangement of bricks and mortar present in the masonry. Theproposed numerical description is first validated against the results from physical tests on a barrel vault under quasi-static cyclic loading. Subsequently, the shear response of a prototype vault is analysed by performing nonlinear simulations under prescribed horizontal displace-ments at the supports, considering also the influence of previous damage induced by earth-quakes with different magnitudes

Simplified modal analysis of multi-storey RC buildings for application in seismic retrofitting

This paper presents a novel simplified approach for determining the fundamental vibration characteristics of reinforced concrete (RC) buildings for application in the seismic retrofitting of substandard multi-storey RC building structures. Even when retrofitted, such structures would likely respond inelastically under a major earthquake, where the seismic response is most realistically represented by computationally expensive nonlinear dynamic analysis. Towards reducing such computational demands, the capacity spectrum method (CSM) offers a practical design and assessment approach which utilises nonlinear static pushover analysis in conjunction with the dynamic characteristics of a structure to establish whether the seismic capacity of the structure meets the demands of seismic ground motion. In this respect, the dynamic characteristics of a structure can be ordinarily obtained by performing conventional direct modal analysis which requires the assembly of mass and stiffness matrices to solve the eigenvalue problem. This paper presents the development of a simplified numerical approach that determines the translational modes of vibration and the associated periods for a retrofitted building structure in both horizontal directions in a simplistic manner without assembling the mass and stiffness matrices. The accuracy of the proposed simplified approach is verified for realistic structures via comparisons against direct eigenvalue analysis. This, along with planned research on simplified pushover analysis, paves the way for the practical seismic retrofitting of substandard reinforced concrete buildings within the CSM framework

High Temperature Oxidation Behavior of Fe-Cr Steel in Air at 1000-1200 K

The high temperature oxidation behavior of Fe-Cr steel was studied in air at elevated temperatures of 1000, 1100 and 1200 K for up to 72 ks. The mass change of all samples was recorded in order to evaluate their oxidation kinetic. The structure of oxide scales was investigated by mean of X-ray diffraction and SEM-EDX. According to oxidation kinetic curve, the mass gain of oxidized sample increases with increasing oxidation time and temperature. At 1000 and 1100 K, the Fe-Cr steel exhibits an excellent oxidation resistance. As oxidation temperature increase to 1200 K, however, the accelerated oxidation occurred. This is considered due to breakaway oxidation. The Fe-Cr steel forms a duplex oxide layer consisting of Fe-rich oxides in the outer layer and Fe-Cr oxides in the inner layer. The obtained results suggest that the oxidation temperature strongly affects the oxidation resistance of Fe-Cr steel and the structure of formed oxide layer on the steel surface. The influence of oxidation temperature on the oxidation resistance and scale structure is discussed in this paper