Project risk assessment is an effective tool for planning and controlling cost, time and achieving the technical performance of a building construction project. Construction projects often face a lot of uncertainties, which places building construction projects at the risk of cost, time overruns as well as poor quality delivery. Considering the limited resources of developing countries, there is need to complete building projects on-time, on-budget, and to meet optimal quality hence, risk management is an important part of the decision making process in construction industry as it determines the success or failure of construction projects. In line with this need, this research aims to establish a system to improve the time, cost and quality performance of building construction projects in developing countries, through a comprehensive risk management model that ensures the expectations of clients are met. To achieve the aim of this research, a mixed methodological approach was adopted. Through the review of literature, a conceptual risk management framework suitable to elaborate risk assessment of building construction projects especially for developing countries was developed. A questionnaire survey using a nonprobability sampling technique was conducted to elicit information from construction professionals in Nigeria to assess their perception of 79 risk factors identified from literature review based on the likelihood of occurrence and impact on projects using a five point scale. Responses from 343 construction professionals were drawn from 305 contractors and subcontractors and 38 clients (private and public) within the Nigerian construction sector. Response data was subjected to descriptive statistics to depict the frequency distribution and central tendency of responses. Subsequently, the risk acceptability matrix (RAM) was adopted to categorise and prioritise risk factors. 27 critical risks that affect building construction projects were identified. A Bayesian Belief Network (BBN) model was developed by structural learning and used to examine the cause and effect relationship amongst the 27 critical risk factors. The developed BBN model was subjected to validation using a multiple case study of two building construction projects in Nigeria. The result showed the interrelation between the 27 risk factors and how they contributed to cost and time overruns as well as quality problems. The critical risks directly affecting the cost of building construction project were: fluctuation of material prices; health and safety issues; bribery and corruption; material wastage; poor site management and supervision; and time overruns. The critical factors identified to directly affect quality were: supply of defective materials; working under harsh conditions; improper construction methods; lack of protective equipment; ineffective time allocation; poor communication between involved stakeholders; and unsuitable leadership style. Time overruns on building construction projects was directly caused by: quality problems; low productivity; improper construction methods; poor communication between involved parties; delayed payments in contracts; and poor site management and supervision. As a consolidation of the findings of this research, a BBN model for identifying risk factors that directly affect time, cost and quality on building construction projects has been developed which has the potential for assisting construction stake holders to manage risks on their projects. In view of the findings, a best practice system for risk management in building construction projects in Nigeria has been developed with an implementation guide to help building construction practitioners to successfully implement risk management on their building construction projects. Suitable risk responses, also in the form of recommendations have been identified. The strategies include actions to be taken to respond to risks based on their perceived significance or acceptability as well as some positive risk responses, such as exploiting, sharing, enhancing and accepting, and other negative risk responses, such as avoidance, mitigation transfer and acceptance.

Les systèmes complexes ferroviaires étant de plus en plus contraints par des autorités de décision placées à un haut niveau d'abstraction, il devient problématique d'imposer des critères à une autre échelle que fonctionnelle. Ainsi, dès lors que l'on descend plus bas, nous sommes confrontés à des spécificités des systèmes nationaux qui font perdre la généralité du travail des décisionnaires Européens. Le problème est qu'à chaque niveau d'abstraction, des méthodes d'évaluation du risque existent, mais sans être compatibles entre elles. Par ailleurs, la combinaison des couches et la vision fonctionnelle du système ne prennent pas en compte l'impact des fonctions les unes sur les autres, ni le lien entre le niveau global et les composants afin d'allouer la sécurité.Nous proposons donc une démarche harmonisée d'évaluation du risque, capable de répartir les contraintes définies au niveau fonctionnel abstrait sur les entités qui implémentent les systèmes avec leurs spécificités.Notre contribution est méthodologique. Elle part d'un modèle fonctionnel du système ferroviaire constitué en couches. Le but étant de représenter ce système sans dépendance entre les fonctions, il a fallu les traduire indépendamment des autres en faisant apparaître les entrées/sorties comme des places/transitions d'un réseau de Petri. A chaque couche de la décomposition correspond une classe de réseau de Petri. Ainsi, à la couche structurelle, nous associons les réseaux de Petri Temporels; à la couche fonctionnelle les réseaux de Petri stochastiques et à la couche logique les réseaux de Petri Prédicats Transitions / The railway systems are being more and more forced by decision authorities. As they are placed at a high level of abstraction, it becomes problematic to impose another criterion or scale. In fact, since we come down lower, we are confronted with specificities of the national systems which make lose the majority of the work of the European decision-makers. The issue is that, at every level of abstraction, risk assessment methods exist, but without being compatible. Besides, the combination of layers and the functional vision of the railway system do not take into account the impact of some functions on the others, nor the link between the global level of risk and the components to assign the safety.Thus, we propose a harmonized approach for risk assessment. This approach allows us to distribute the constraints defined at the abstract functional level on the entities which implement both the systems and their specificities.Our contribution is methodological. It leaves a functional model of the layered railway system. The purpose is to represent this system without any dependencies between the functions. For instance, it was necessary to translate them independently by creating entrances/exits as places/transitions of a Petri net. A Petri net class corresponds to each layer. To the structural layer, we associate the time Petri net; to the functional layer, the stochastic Petri nets and to the logical layer, the predicate transitions nets