Wassim Raphael

Reliability of prestressed concrete structures considering creep models

The reliability of prestressed concrete structures subject to viscoelastic behaviour is investigated regarding the creep model defined by the Eurocodes. A probabilistic phenomenological model is proposed for long-term creep strains on the basis of large database of creep tests. The uncertainties in the geometrical and mechanical parameters are modelled by random variables. The proposed model considers also the statistical fitting error in creep strain predictions. The reliability analysis is performed on a prestressed concrete deck, in order to show the large impact of time-dependent phenomena on the reliability of prestressed structures, and consequently the importance of considering appropriate viscoelastic models in the design of this kind of structures. Moreover, the errors related to creep models are shown to play a very important role in the structural safety assessment.

Incorporating Bayesian networks in Markov Decision Processes

This paper presents an extension to a partially observable Markov decision process so that its solution can take into account, at the beginning of the planning, the possible availability of free information in future time periods. It is assumed that such information has a Bayesian network structure. The proposed approach requires a smaller computational effort than the classical approaches used to solve dynamic Bayesian networks. Furthermore, it allows the user to (1)take advantage of prior probability distributions of relevant random variables that do not necessarily have a direct causal relationship with the state of the system; and (2)rationally take into account the effects of accidental or rare events (such as seismic activities) that may occur during future time periods of the planning horizon. The methodology is illustrated through an example problem that concerns the optimization of inspection, maintenance, and rehabilitation strategies of road pavement over a 14-year planning horizon.

Seismic Soil Structure Interaction of a Midrise Frame Structure

Earthquakes have occurred for millions of years and will continue in the future affecting millions of lives, thousands infrastructure and costing billions of dollars. Seismic soil structure interaction is the process in which the structure rested on the ground and subjected to an earthquake is affected by the response of the soil medium beneath it having its own characteristics. Thus, soil structure interaction response is dictated by the interaction between the structure, foundation and underlying soil or rock. To clarify the contribution of the seismic soil structure interaction, 3D time history finite element models of 15 story concrete frame building were performed using Abaqus under Kocaeli’s Mw = 7.5 strong ground motion. The effects of soil boundary limits, mat thickness and soil linearity were investigated. To demonstrate the importance of modeling correct soil structure interaction problems, the numerical analysis was carried out for three cases: (1) a fixed-base structure, (2) a structure rested on silty sandy soil, and (3) a structure supported by raft foundation and rested on silty sandy soil. The results, presented in terms of the structural lateral deflection and base shear versus time, show that soil structure interaction effects should be evaluated when the maximum lateral deflection is obtained at top of the structure regardless when it occurred as well as at maximum absolute lateral deflection at each level. Moreover, excluding soil structure interaction effects may result in inadequate structural safety for the frame building rested on silty sandy soil and considering linear soil properties may result in non-economical structural design.

Numerical analysis and investigation of short- and long-term behavior of unidirectional composites

This study gives a detailed analysis on estimating the ultimate tensile strength of unidirectional fiber reinforced composites and its creep behavior under sustained tension load. We develop two different micromechanical models that allow us to estimate the longitudinal tensile strength and the evolution with time of fiber and matrix stresses around arbitrary array of fiber breaks. The first model is based on the shear-lag theory while the second one is developed using the software Abaqus. The comparison of the above models allowed to validate the fundamental assumptions of the shear-lag theory (first model) as well as several numerical issues related to time integration and spatial discretization. The Monte–Carlo method was used in order to account for the stochastic fiber strength and its impact on the ultimate tensile strength (short-term) and creep (long-term behavior) of unidirectional composites. Finally, a parametric investigation on the fiber type and the load level on the long-term behavior of unidirectional composites was performed showing an accelerating creep effect for fibers of inferior quality such as glass fibers compared to carbon fibers.