http://dspace.cityu.edu.hk:80/handle/2031/704 Tue, 30 Apr 2013 15:24:42 GMT 2013-04-30T15:24:42Z http://dspace.cityu.edu.hk:80/handle/2031/6561 Title: Durability of FRP-concrete interface Authors: Yun, Yanchun ( 惲燕春) Abstract: Research on ways to retrofit, strengthen and repair reinforced concrete (RC) structures using fiber reinforced polymer (FRP) materials has made significant progress in recent years. However, attention has been focused on short-term mechanical properties of retrofitted structures. One important aspect that needs to be carefully investigated to ensure the safety of this technology is evaluation of longterm durability of adhesively bonded joints. This study investigated bond characteristics of FRP-concrete joints subjected to fatigue loading and freeze-thaw cycling. To achieve the objectives of the study, an improved double-face shear test (direct pullout test) was developed for different bonding systems under fatigue loading. The influence of the load amplitude and number of cycles of fatigue on bond performance was examined and discussed through analysis of failure mechanism, load-slip curves and strain measurements taken during fatigue, as well as monotonic tests. Durability of the FRP-concrete bond interface under freeze-thaw cycling was investigated by single-face shear tests, with exposure condition, concrete grade and number of freeze-thaw cycles as the parameters considered. The results indicate that bond strength, bond stiffness, interfacial fracture energy and maximum slip of the joints decreased with increase in number of freeze-thaw cycles, and they were also affected by the exposure environment. Based on experimental tests and other collected test results, bond-slip and bond strength model under normal environment are developed as these models can consider different concrete strength and FRP stiffness values. The model for predicting bond strength of long FRP-concrete joints was assessed by comparing it with experimental test data collected from extant literature. Finally, by using the proposed constitutive model of FRP-concrete interface under normal environment to analyse the experimental test data, bond-slip and bond strength models for FRPconcrete joints under freeze-thaw cycling were developed. Bond-slip and bond strength models provide an insight into the long-term performance of FRP-concrete interface subjected to freeze-thaw cycles, which is important and necessary for design and construction of FRP retrofitting systems. Notes: CityU Call Number: TA444 .Y86 2011; xx, 320 leaves : ill. 30 cm.; Thesis (Ph.D.)--City University of Hong Kong, 2011.; Includes bibliographical references (leaves 262-277) Sat, 01 Jan 2011 00:00:00 GMT http://dspace.cityu.edu.hk:80/handle/2031/6561 2011-01-01T00:00:00Z http://dspace.cityu.edu.hk:80/handle/2031/6560 Title: Improved social force model for building evacuation simulation Authors: Yuen, Kit Yan ( 袁潔欣) Abstract: Over the last few decades, there were numerous model developments in the field of pedestrian flow simulation. Most of these models try to simulate the pedestrian's evacuation in buildings in case of fire or emergency situations. They also look at capacity issues at bottlenecks such as rail interchanges and bi-directional flow. A number of models for pedestrian movement have been developed in a variety of disciplines. There are two distinct groups of models in macroscopic and microscopic perspectives. The macroscopic models focus on the system as a whole, while microscopic models study the behaviour and decisions of individual pedestrians, their effect on other pedestrians around them, and the system as a whole. There is a need to model the pedestrian behaviour for a range of applications including event planning, resource usage, and urban planning. For instance, the organizers of a large event in an exhibition hall require information on what areas are likely to be congested so that management strategies can be developed and tested before starting the event. Similarly, the designers of a shopping mall might be interested in how people move around their intended design so that they can place shop entrances and seating in useful locations. Recently, computer based analysis for pedestrians movement in buildings is widely adopted by the fire researchers or engineers in performance-based fire engineering study. However, the pedestrians exhibit different behaviours depending on their knowledge of the environment and other personal characteristics. Unlike the rules that govern vehicular traffic, there are few formal procedures or rules that govern the pedestrian movement, resulting in often complex and chaotic movements. Pedestrians are not restricted to lanes or specific routes. In general, they are restricted by the physical boundaries around them such as the width of doorways or presence of walkways and also the movements of their neighbours. As a consequence, the modelling of pedestrian movements presents some specific problems not encountered in other forms of transport modelling. A full understanding of crowd behaviours normally requires exposing real people to the specific environment for obtaining empirical data, which is difficult because of such environments are often dangerous in nature especially in emergency situations. Moreover, the major deficiency of the existing pedestrians modelling is the adaptation of crowd behaviour that is extremely difficult to be described by mathematics. In addition to studying the crowd behaviour based on the observations and the historical records, computer simulation may be a useful alternative that can provide valuable information to evaluate a building design, to help planning process, and for dealing with emergencies. In this thesis, a modified computing pedestrian model, namely Improved Social Force Model, has been developed to simulate the perception and the cognition of a pedestrian in case of emergency evacuation. The algorithm of the model is implemented based on the Social Force Model introduced by Helbing and Molnar (1995). This model examines pedestrian movements as either positive or negative social fields, in which a pedestrian behaves as if acted upon by external forces. However, the decisions and interaction between pedestrians is an extremely flexible and intelligent process. To provide more accurate results in pedestrian behaviours, the physical features of pedestrian movements such as walking speeds, acceleration, queuing, and herding behaviour must be accurately reproduced. The motivation of this research comes from the need to understanding pedestrian psychology and modelling pedestrian behaviour accurately. Some parameters involving human behaviours will be introduced into the original Social Force Model in order to improve the accuracy for the computing modelling. In summary, two human behaviours will be added into an individual's walking performance: herding behaviour and visual angle. By adding a herding parameter into a pedestrian in an evacuation simulation is proved to be obtained a more accurate result. However, herding behaviour could not be the same in all age of pedestrians. In an Improved Social Force Model, the younger pedestrian, the larger herding behaviour; as younger pedestrians have lower judgement in the wayfinding (searching for a suitable escape route) so that they will follow the actions of others as a guide to determine how they might act. In addition, under evacuation situation, pedestrians will more concentrate on finding their destination (e.g. an exit) in order to leave the building as soon as possible. Therefore, applying a visual angle of ±85° to the pedestrians can achieve a more accurate simulation result. Applying the concept into the algorithm of the proposed model, the predicted values for each pedestrian and time step are calculated. The model consists mainly of three terms. These terms are the desired velocity of motion of a pedestrian, the interactions between pedestrians, and the interactions between pedestrian and boundaries. Human behaviours, herding parameter and visual angle, are applied to the second term when processing the algorithm. Then the last step in the algorithm is to update of the position, velocity and acceleration of an individual for the next time step. In proofing the performance of an Improved Social Force Model, two other computational pedestrian movement models, Social Force Model and Simulation of Transient Evacuation and Pedestrian MovementS (STEPS), were applied. Social Force Model is a well-known pedestrian movement modelling and it was published in the Journal of Nature in 2000 by Helbing et al.; Improved Social Force Model is proposed by the author in order to improve the prediction of pedestrian movement modelling which is based on the Social Force Model; STEPS is a commercial application of simulation in pedestrian movement and it is well-validated in academic and well-adopted in industry. Therefore, STEPS is being a benchmarking model in the research so as to evaluate the performance of the Improved Social Force Model. From the computational results of the pedestrian simulation for various scenarios, it was found that the simulated escaping time of pedestrians in the proposed model from the fire room is longer than the original Social Force Model by comparing to the results from STEPS. The agreement of the proposed model is not expected to be perfect since there are important variations among collected by different authors under different layouts, situations, and cultures. However, an improvement is clearly visible. Notes: CityU Call Number: NA2543.P4 Y83 2010; xviii, 187 leaves : ill. (some col.) 30 cm.; Thesis (Ph.D.)--City University of Hong Kong, 2010.; Includes bibliographical references (leaves 147-155) Fri, 01 Jan 2010 00:00:00 GMT http://dspace.cityu.edu.hk:80/handle/2031/6560 2010-01-01T00:00:00Z http://dspace.cityu.edu.hk:80/handle/2031/6559 Title: A decision support system for the determination of concession period in transportation project under Build-Operate-Transfer contract Authors: Yu, Chenyun ( 喻晨韻) Abstract: The determination of the concession period length is one of the most important decisions undertaken during the life span of a Build-Operate-Transfer project. The concession period length directly affects both the involved government and private investors' financial returns and risks, and offers significant economic and social benefits. In existing methods, the concession period is usually determined by the concessionaire, depending on their expected investment return using the payback period method or it is predicted without comprehensive analysis of the influential factors. In view of this, a Decision Support System for concession period length determination (CPLD) was developed and, as demonstrated herein provides a possible way of solving the concession period problem, especially under the impact of various influential factors. In the proposed Decision Support System, an overall model for CPLD, and two sub-models for predicting the key variables of CPLD were developed. Since the length of the concession period is under impact from various factors, an in-depth investigation of the influential factors was firstly conducted, in which the selection, classification, and ranking of the influential factors were concerned. Thus the input variables for the models involved in this system were generated. Then two sub-models were developed to predict the key variables (Traffic flow and Toll fee) of concession period length, so as to make the overall model for CPLD reliable and perform smoothly. Additionally, a Variable Evaluation System was constructed. The input variables involved in the model were tested in this section. Thus the degree of importance of each variable was obtained. Finally, the overall model for CPLD was developed. The established model does not mean to achieve the highest profit for any side (either the government or the private sector); rather it means a balance of the two extreme sides within the same decision-making process. Two datasets of tunnel projects in Hong Kong were constructed to verify the prediction models. A dataset of a simulated highway project was employed for training the overall model of CPLD. The experimental results show that the proposed method and system can provide the decision makers with a set of alternatives, among which an optimal one can be selected after balancing the interests of both the government and private sectors. Thus the Decision Support System was developed. It is expected that the proposed DSS can assist the decision-making process of concession period length determination, and finally contribute both theoretical and practical knowledge for academics and practitioners in the field. Notes: CityU Call Number: HE196.5 .Y8 2011; xxii, 293, [58] leaves : ill. 30 cm.; Thesis (Ph.D.)--City University of Hong Kong, 2011.; Includes bibliographical references (leaves 271-293) Sat, 01 Jan 2011 00:00:00 GMT http://dspace.cityu.edu.hk:80/handle/2031/6559 2011-01-01T00:00:00Z http://dspace.cityu.edu.hk:80/handle/2031/6557 Title: Study on the atomizing mechanisms and the fire extinguishing efficiency of pneumatic-atomizing water mist generated by the steam atomizing medium Authors: Xiao, Xiukun ( 肖修昆) Abstract: The generating method and the efficiency of fire extinction are the most important issues that should be treated firstly in the research and application of water mist fire suppression system. And it is with great theoretical and realistic significances to develop new generating methods and application areas to water mist fire suppression technology. However, some problems have been faced nowadays in the processes of the development and application of water mist fire suppression technology, such as the short flow-length, the poor mobility, the low droplet momentum, the low mist flux density and so on. Therefore, much more works should be carried out to solve these problems. The previous studies reveal that the pneumatic-atomizing (twin-fluid) water mist system can discharge mist flow in a long distance. Therefore, the present study is conducting works on pneumatic-atomizing water mist technology. The contributions and works of the present studies are summarized as follows: 1) The generating method and atomizing mechanism of a pneumatic-atomizing water mist with longer flow-length and larger mist flux density were developed in the thesis. Firstly, a new atomizer with adjustable annular orifice was designed to generate pneumatic-atomizing water mist with long flow-length and large mist flux density. The gas and liquid was mixing in the way of 'Y' type. Secondly, the water mist fire suppression system was set based on the systems designs of water supply and gas supply. The steam was taken as the gaseous atomizing medium to study the promotion of the inert gas in fire extinguishing. And the water mist with good spray performance was then generated. The water mist spray generated was steady and symmetrical. The flow-length was about 10m. Thirdly, the calculation of the GLR value and the condensation of the steam are measured to study the flow characteristics of the atomizer. Fourthly, the atomizing mechanism of the pneumatic-atomizing atomizer was studied. The interactions between liquid sheet rings or liquid sheet ring and liquid sheet were studied with a high speed camera. And the reasons to the formation of the 'tree and root' structure were explained firstly. 2) The spray performance of the pneumatic-atomizing water mist generated with a new designed atomizer was measured. Firstly, the temperature distribution of the water mist with different GLR value was measured due to the higher-temperature property of the gaseous atomizing medium. Secondly, the mean droplet diameter of water mist with different GLR value have been calculated based on the binary analysis of the images which were captured by a high-speed camera with a frequency of 2000 fps. Thirdly, the droplet velocity was measured based on the DPIV methodology. And the two dimensional velocity vector field of water mist with different GLR was rebuilt. Fourthly, the cone angle of the spray was measured based on the binary analysis of the images. Fifthly, the mist flux was measured with a device of 'collecting tubes' in case of water mist with different GLR value. 3) Experiments have been conducted to pool fire extinguishing with water mist in test case of local application and total-flood application. Firstly, the experimental setup was set for fire suppression in local application. And the heat release rate of the ethanol, gasoline and diesel in different oil pan was calculated. Secondly, the experimental results were studied. Different kind types of the fuel were compared to study the effects of the fuel. And the typical processes of pool fire extinguished with water mist in local application were also studied based on the pictures captured with a digital video. And then the dominative mechanisms for extinguishing pool fire with pneumatic-atomizing water mist in local application were studied. The lifetime of the droplet with various diameters in case of different flame temperature was calculated. Thirdly, the experimental setup for fire suppression in total-flooding application was set. And different kind types of the fuel were compared to study the effects of the fuel. And the effects of the water mist with different GLR values were also studied. And then the dominative mechanisms for fire extinction with water mist in enclosed space were studied. 4) The flame expansion phenomenon of pool fire extinguishing with steam jet and water mist was studied. Firstly, the existence of a fuel vapor rich core is a prerequisite for observing a flame expansion phenomenon caused by an impinging steam jet with sufficient momentum. This is also the reason responsible for the phenomenon that no significant flame expansion phenomenon was observed in Class A fire. The axial length of the fuel vapor rich core of a medium size pool fire is Lc = 1/5Lf (Lf is the visible flame height). Secondly, the root cause responsible for the flame expansion is the fuel vapor convection after impacting with the steam jet. An expanded fire ball is formed due to the burning of the convected fuel vapor, which gives rise to a flame expansion phenomenon. Thirdly, the flame expansion phenomenon could be divided to three typical stages: the flame height-decreasing stage, the fire-expanding stage and the burn out stage. The flame height-decreasing stage is the stage in which the steam jet impacts the flame and reaches the fuel vapor rich core. The fire-expanding stage is the stage in which the fuel vapor is convected and burnt as a fire ball after the steam impacting the fuel vapor rich core. The fire is then immediately suppressed for lacking of fuel in the burn out stage. The lacking of fuel is the key for the transient nature of the flame expansion phenomenon. Fourthly, the experimental results reveal that, the expanded flame shape was very close to the theoretical analysis proposed and the typical three stages are also be observed in the experiments. Fifthly, the quantitative features of the flame expansion phenomenon, the expansion ratio of the flame width φl, the expansion ratio of the flame volume φv and the expansion ratio of the flame radiation φr, are defined to characterize the level of the expansion. Sixthly, experiments have been conducted to confirm the quantitative features. Notes: CityU Call Number: TH9336 .X53 2011; 195 leaves : ill. 30 cm.; Thesis (Ph.D.)--City University of Hong Kong, 2011.; Includes bibliographical references (leaves 173-190) Sat, 01 Jan 2011 00:00:00 GMT http://dspace.cityu.edu.hk:80/handle/2031/6557 2011-01-01T00:00:00Z