Arash Behnia

A Comprehensive Study of the Polypropylene Fiber Reinforced Fly Ash Based Geopolymer

As a cementitious material, geopolymers show a high quasi-brittle behavior and a relatively low fracture energy. To overcome such a weakness, incorporation of fibers to a brittle matrix is a well-known technique to enhance the flexural properties. This study comprehensively evaluates the short and long term impacts of different volume percentages of polypropylene fiber (PPF) reinforcement on fly ash based geopolymer composites. Different characteristics of the composite were compared at fresh state by flow measurement and hardened state by variation of shrinkage over time to assess the response of composites under flexural and compressive load conditions. The fiber-matrix interface, fiber surface and toughening mechanisms were assessed using field emission scan electron microscopy (FESEM) and atomic force microscopy (AFM). The results show that incorporation of PPF up to 3 wt % into the geopolymer paste reduces the shrinkage and enhances the energy absorption of the composites. While, it might reduce the ultimate flexural and compressive strength of the material depending on fiber content.

Finite Element Analysis of the Dynamic Response of Composite Floors Subjected to Walking Induced Vibrations

The applications of composite materials have been widely practiced in modern construction. Structural engineers are often urged to consider aesthetic values as well as the financial aspects in their work, which results in structures that have long span, lightweight and low natural frequencies. These structures exhibit excessive vibrations that cause major discomforts to the occupants. The purpose of this study is to establish a methodology using finite element analysis for assessing the dynamic responses of composite floors and determining the corresponding level of comfort. Linear elastic finite element analysis was conducted using more realistic load models with respect to the application of different geometries of concrete slab and fiber reinforced polymer materials. The composite floor investigated included FRP deck, FRP beams, and concrete slabs of various thicknesses. The resulting maximum peak accelerations indicated the need for more realistic load models to generate a time function including space, time and heel impact descriptions. The FRP deck or beam was satisfactory in terms of serviceability and comfort level. There were no significant differences between the results when fiber reinforced polymer materials or common concrete-steel composite floors were used.

Effects of coordinated crowd motion on dynamic responses of composite floors in buildings

This paper explores the vibration behaviour of composite floors in an existing building using a comprehensive study of the modal dynamic responses. Different panels are subjected to loads induced by human motion. The computed fundamental natural frequency and vibration modes are first verified against experimental and numerical results from previous studies. Departing from close correlation established by comparisons, this study investigates the effects of coordinated passive live loads as additional stationary mass due to crowds jumping. In another case, the effects of different intensities and the loads created by different jumping crowd sizes are investigated. Thirty modes of vibrations are selected to obtain all the possible excitations and to make a third harmonic load frequency available to excite the critical modes. In addition, the presence of different coordinated passive live loads on the composite floor results in different behaviours for each particular panel that is associated with location of load and passive live load intensities. This study shows that an increase in crowd size and the intensity of the active load are not directly proportional to the dynamic responses especially for displacement. It is also found that the synchronisation of active load plays a part in reducing structural dynamic responses, an observation that has not been highlighted before in previous studies.

An improved version of Inverse Distance Weighting metamodel assisted Harmony Search algorithm for truss design optimization

This paper focuses on a metamodel-based design optimization algorithm. The intention is to improve its computational cost and convergence rate. Metamodel-based optimization method introduced here, provides the necessary means to reduce the computational cost and convergence rate of the optimization through a surrogate. This algorithm is a combination of a high quality approximation technique called Inverse Distance Weighting and a meta-heuristic algorithm called Harmony Search. The outcome is then polished by a semi-tabu search algorithm. This algorithm adopts a filtering system and determines solution vectors where exact simulation should be applied. The performance of the algorithm is evaluated by standard truss design problems and there has been a significant decrease in the computational effort and improvement of convergence rate.

Finite element analysis of high modal dynamic responses of a composite floor subjected to human motion under passive live load

Light weight and long span composite floors are common place in modern construction. A critical consequence of this application is undesired vibration which may cause excessive discomfort to occupants. This work investigates the composite floor vibration behavior of an existing building based on a comprehensive study of high modal dynamic responses, the range of which has been absent in previous studies and major analytical templates, of different panels under the influence of loads induced by human motion. The resulting fundamental natural frequency and vibration modes are first validated with respect to experimental and numerical evidences from literature. Departing from close correlation established in comparison, this study explores in detail the effects of intensity of passive live load as additional stationary mass due to crowd jumping as well as considering human structure interaction. From observation, a new approach in the simulation of passive live load through the consideration of human structure interaction and human body characteristics is proposed. It is concluded that higher vibration modes are essential to determine the minimum required modes and mass participation ratio in the case of vertical vibration. The results indicate the need to consider 30 modes of vibration to obtain all possible important excitations and thereby making third harmonic of load frequency available to excite the critical modes. In addition, presence of different intensities of passive live load on the composite floor showed completely different behavior in each particular panel associated with load location of panel and passive live load intensity. Furthermore, implementing human body characteristics in simulation causes an obvious increase in modal damping and hence better practicality and economical presentation can be achieved in structural dynamic behavior.

Development of a Tomography Technique for Assessment of the Material Condition of Concrete Using Optimized Elastic Wave Parameters

Concrete is the most ubiquitous construction material. Apart from the fresh and early age properties of concrete material, its condition during the structure life span affects the overall structural performance. Therefore, development of techniques such as non-destructive testing which enable the investigation of the material condition, are in great demand. Tomography technique has become an increasingly popular non-destructive evaluation technique for civil engineers to assess the condition of concrete structures. In the present study, this technique is investigated by developing reconstruction procedures utilizing different parameters of elastic waves, namely the travel time, wave amplitude, wave frequency, and Q-value. In the development of algorithms, a ray tracing feature was adopted to take into account the actual non-linear propagation of elastic waves in concrete containing defects. Numerical simulation accompanied by experimental verifications of wave motion were conducted to obtain wave propagation profiles in concrete containing honeycomb as a defect and in assessing the tendon duct filling of pre-stressed concrete (PC) elements. The detection of defects by the developed tomography reconstruction procedures was evaluated and discussed.