Introduction
Conventionally, the process of bridge design considers each phase of construction separately and concentrates on the construction phase itself with the primary objective of optimizing efficiency and minimizing costs during development and construction. In the past decades, a large number of bridges have deteriorated and suffered from some damages or defects which influence the durability of bridges. Traditional bridge design methodology and construction management system are the key factors that influence durability problems that current bridges are unable to achieve its design life [
1].
For sustainable development, traditional short-term method is expanded to the life cycle of the bridge. As a new design methodology, bridge life cycle design takes all aspects of construction practice of a bridge into account, from planning, design, construction and management of service phase to demolition and recycling of materials and components, which optimize all alternative solutions based on the performance targets during the entire design life of the bridge.
In recent years, considerable effort has been devoted to the research on bridge life cycle which is now accepted by the bridge engineers all over the world. Most of them focus on life cycle cost analysis [
2,
3], life-cycle maintenance and replacement modeling [
4,
5], maintenance and replacement strategies and methods [
6-
10], reliability-based life-cycle management [
11,
12], optimization of maintenance and management for deteriorating structures [
13-
15].
There are a few efforts on the life cycle design for new structures. Sarja explained how to optimize structures for their entire design life through an optimum integrated life cycle design process, and presented a new model of integrated life cycle design for the building [
16,
17]. But few engineers and researchers have concentrated on the life cycle design for new bridges.
Nowadays, the bridge life cycle design in China is still at its initial phase, and this is the start of the final formulation of bridge life cycle design. Many engineers only know the concept of bridge life cycle design, but do not know how to achieve it. To better understand and apply the methodology for bridge life cycle design, this paper presented a general framework for bridge life cycle design based on the analysis of the main phases, main processes and their tasks as well as design methods.
Concept and main phases of bridge life cycle design
Concept of bridge life cycle design
Bridge design is an important part of the construction of a bridge which translates the requirements of owners, users, and society into the performance requirements of structural systems of the bridge, creates and optimizes structural solutions to fulfill those requirements and proves that these requirements are met through analysis and calculation.
The bridge life cycle design focuses on the design and optimization of all alternative solutions based on the performance targets during the entire design life of the bridge. Therefore, the bridge life cycle design can be defined as a design concept and methodology for seeking appropriate methods and measures from all the phases including planning, design, construction, operation, maintenance and management, even demolition of a bridge to optimize the overall performance such as service performance, economy, culture, ecology, etc., as shown in Fig. 1.
Main phases of bridge life cycle design
In China, traditional capital construction projects for highway engineering commonly adopts a two-stage design, that is, conceptual design and detail design. Furthermore, construction projects such as special highway sections, large-span bridges, interchanges, and tunnels, etc. which are technically complicated and where there is lack of experience adopt a three-stage design including technical design.
According to the concept of bridge life cycle design, the design phase of a bridge starts from investment planning, so the stages of bridge life cycle design generally include project feasibility study, conceptual design and detail design. Project feasibility study is making an integrated research which focuses on the necessity, technical feasibility, economic rationality and implementation feasibility of construction projects; thus, defining the functional and service life of the project with the intention of project construction and analyzing the potential influence of environmental factors on the bridge structure in the future. Consequently, the required performance design, especially durability design, which is needed to assure the service ages of a bridge can be primary defined, and the appraisal of maintenance, management and replacement can be defined for the investment appraisal and economic evaluation. It is noted that the technical design is not included in the study. If necessary, it needs to implement some special study for technical design.
General procedure of bridge life cycle design
In the life cycle design for a bridge, the designer needs to start from investment planning and analyze owners’ and users’ needs and requirements; then translate the clients’ and users’ needs into functional life cycle requirements of the bridge, create and optimize technical solutions, which fulfills those requirements, and prove through analysis and calculation, that the performance requirements will be fulfilled over the entire service life. General procedure of bridge life cycle design can include: 1) investment planning; 2) analysis of social, clients’ and users’ needs; 3) functional specifications of bridges; 4) technical performance specifications and items; 5) creation and planning of alternative structural solutions; 6) life cycle planning and service life optimization of each alternative; 7) multiple criteria ranking and selection of alternative solutions; 8) detailed design of the selected solution.
Main processes of bridge life cycle design
The role of bridge life cycle design is to ensure that the targets and specifications defined in the phases of project feasibility study and conceptual design can be realized in construction and throughout the bridge’s life cycle. This means that a bridge must be buildable, serviceable, durable, that it can be maintained and repaired, finally that it can be demolished and the waste recycled or disposed of. Therefore, to achieve a good life cycle quality of a bridge, the designer needs to make rational service life design and planning to assure the requirements of service life will be satisfied, to implement necessary aesthetics design to meet people’s aesthetics and landscape requirements for a bridge, and to carry out performance design to meet the functional requirements of a bridge. The designer should also allow for the influence of the bridge on the environment and zoology. Furthermore, the designer should consider the demands of future inspection, maintenance and repair and implement the design for inspection, maintenance and repair schemes. In the above processes, in order to achieve optimal design solution, the designer should calculate total costs during the life cycle (including financial costs and environmental costs) through life cycle cost analysis for each process and item for each solution. Therefore, bridge life cycle design can be composed of six processes: service life design, aesthetics design, performance design, design for environment and ecology, design for inspection, maintenance and repair, life cycle cost analysis.
Service life design
A service life requirement for the design of a new structure must be formulated in a format which can be operational as part of the design basis, thus leading to a design service life. It is clear that in this process of developing a rational design for service life the owner must be directly involved and should make a number of decisions. The bridge designer should cooperate with owners to carry out life cycle planning of a bridge and thus optimize the alternative solutions during the life cycle of the bridge.
The designer should also define the service life targets of a bridge based on owners’ and users’ service life requirements for the bridge, classify different components into target service life classes, and provide expected routine maintenance and the number of times each component must be renewed or replaced during the design service life of the bridge. In the bridge, only major structures such as tower, girder and substructures e.g. pier, base and abutment, etc. are designed to resist degradation over the service life. Other components, having a shorter service life, especially deck surfacing, cable, bearing and expansion joint, will need to be frequently replaced and routinely maintained.
Finally, the design optimization of alternative service life planning should be done based on the calculation of total life cycle costs. The process of cooperation involves the short- and long-term cost implications of the decisions taken as well. This means that the LCC aspects also play a major role in the design process.
Aesthetics design
During the process of effectively utilizing the potential benefits of bridge life cycle design, aesthetics design is crucial because the decisions made in this process have a strong influence on the economic, environmental, aesthetic, functional and technical quality of a bridge. In practice, it should take into consideration the functional form and visual form together starting from the planning of a bridge. Aesthetics design should exist throughout the entire process of structural detail design, i.e. bridge design is the integration of detail design and aesthetics design, through which the overall performance for functional, technical, economic and aesthetic, etc. of bridge life cycle design can be achieved.
Bridge aesthetics design expects that the designer selects the best structural style depending on functional requirements. Structural form should be in harmony with and have good scale, be in line with and have rhythmical image. Material selection, exterior property and color application to various structural components should be in harmony with the environment [
18]. Therefore, bridge aesthetics design includes bridge structural form design and bridge landscape design. The aesthetics consideration is not limited to perceptual level in bridge structural design, so a perfect bridge design is only the integration of rational and systemic form and landscape design with structural design.
Bridge form design is designed for mechanical capability and form of a bridge. Bridge form design strives to achieve a harmonious blend of shape and function at different levels and express the unification of force and aesthetics from global layout to details. The designer defines the uppermost form based on the design concept, environmental characteristic and bridge design condition. Thus, other forms shall be centered on the uppermost form which acts as the keynote to achieve emphasis and excellence. After forming the form elements through necessary conformity, it can further change and extend by integrating it with structural performances throughout the entire bridge [
19].
In addition to satisfying traffic function, whether the bridge design is rational and pleasing, depends on the charm of the bridge and the art of relating it to the environment. The harmony of bridge form can be accomplished through two kinds of relations: the harmony of the bridge and the environment, the harmony of the bridge itself. Bridge landscape design is decided by structural types, circumstance at the site and landscape characteristics. The tasks of bridge landscape design are: define the guidelines of landscape design, design the items of landscape design, implement color design and lighting design, optimize bridge landscape solutions through landscape evaluation. The key is to define the guidelines for landscape design during the entire landscape design [
20].
Performance design
The task of performance design is to ensure the performance and durability of technical system during the entire life cycle, so performance design is very important for the achievement of good quality during the entire life cycle of a bridge, through which the designer can translate the clients’ and users’ needs into functional life cycle requirements of the bridge and the components, thus specify the performance properties based on the functional requirements, to finally create and optimize performance design solutions especially for ordinary mechanical design and durability design to meet those requirements.
Therefore, major tasks in the performance design process can include the following:
1) Functional design of structural system and components. The designer shall translate the clients’ and users’ needs into functional life cycle requirements of the bridge and the components and define the specification of functional properties of the bridge and the components.
2) Flexibility design for the changes in service or in the future. The designer shall predict the potential changes such as the needs for broadening, load limitation, etc. in service or in the future and carry out flexibility design through various measures to accommodate future changes and avoid obsolescence before the end of service life.
3) Ordinary mechanical design. Mechanical design includes the static, fatigue and dynamic design aspects. The designer shall perform ordinary mechanical design using conventional methods to determine the preliminary dimensions for the bridge.
4) Durability design. The task of design is to ensure the performance and durability of technical system during the entire life cycle. Thus, the role of durability design is important in life cycle design. Through the principle of sustainability, resistance design will be expanded into durability design, thus introducing time as a new dimension in design calculations. The detailed durability design procedure is as follows:
a) Specification of the target service life and design service life;
b) Determination of durability limit state;
c) Analysis of environmental effects;
d) Identification of durability calculation model and degradation mechanisms;
e) Selection of a durability calculation model for each degradation mechanism;
f) Calculation of durability parameters using available calculation models;
g) Possible updating calculations of ordinary mechanical design (e.g. self-weight of the bridge);
h) Transfer of durability parameters into the final detailed design;
i) Optimization of durability design based on life cycle costing.
5) Design for construction scheme. To ensure the feasibility of a design scenario of a bridge, corresponding feasible construction techniques must be considered in the design stage. Therefore, the designer needs to cooperate with experienced bridge engineers to determine feasible primary construction schemes.
6) Design for safety or health aspects. Static and dynamic safety as well as safety aspects for fire, traffic and ship collision shall be taken into account in performance design. Furthermore, the designer shall consider and optimize long-term safety for the bridge, taking into account degradation effects. The main issue for health design is to avoid adapting the materials which can cause emissions or radiation which are dangerous for health and comfort of users.
7) Bridge design for particular cases such as in the area subjected to gales, heavy fogs or frequent rainstorms.
Design for environment and ecology
The objective for the development of environmentally and ecologically-oriented design is to realize design methods and methodologies for bridge design in order to meet the requirements of sustainable development during the life cycle of bridge, which include environmental life cycle analysis, design for energy efficiency, design for recycling and reuse, etc.
Based on the consideration of reuse and recycling, bridge designers should select appropriate structural systems, component types and their connections, as well as appropriate materials which can be dismantled with different kinds of reuseability and recyclability to actively reduce the production of waste during the stage of construction, renovation and demolition. Therefore, environmental efficiency depends on the selection of environmentally-friendly local raw materials, high durability and easy maintainability of structures during use, recycling of construction wastes and final recycling of components and materials after demolition. Some components of bridges, such as parapet, bearing and expansion joint, have a shorter or moderate service life and, consequently, easy re-assembly and recycling are most important.
Design for inspection, maintenance and repair
The life cycle performance of a bridge is highly dependent on maintenance. Therefore, life cycle planning for inspection, maintenance and repair scheme shall be provided during the design phase. The basic tasks of the bridge designer are: analyze and compare inspection, maintenance and repair methods for bridges, define inspection, maintenance and repair strategy, procedure and preliminary solutions, optimize preliminary inspection, maintenance and repair solutions based on life cycle costing.
Life cycle cost analysis
In bridge life cycle design, analysis and design are expanded in two areas: financial costs and environmental costs, whose target is minimizing the total financial costs and environmental impacts during the life cycle of the bridge. Life cycle costs are calculated as the present value or as yearly costs by discounting the costs from construction, maintenance, repair, changes, modernization, reuse, recycling and disposal. Environmental costs are the use of non-renewable natural resources and the production of air, water, or soil pollution. The consequence of air pollution includes health problems, inconvenience to people, ozone depletion and global warming. These impacts dictate the environmental profiles of the bridge’s structural and operational systems. The design goal is to keep environmental costs below permitted values and to minimize them. In practice, life cycle cost analysis is inevitably needed in the above processes, and the total costs during the life cycle of bridges used to be one of important bases for scenario comparison.
General framework of bridge life cycle design
Design processes, tasks and methods of bridge life cycle design
Bridge life cycle design is an important link in construction. The objects and tasks of various bridge design phases are different, so corresponding tasks and methods differ from various processes in bridge life cycle design, as shown in Table 1.
General framework of bridge life cycle design
Bridge life cycle design provides methods and methodologies for bridge design to meet the requirements of sustainable development during the entire service life of bridges, thus achieving a good life cycle quality. On the basis of the above analysis, the paper presented a general framework for bridge life cycle design. Starting from the owners’, social and users’ needs and requirements, the bridge engineer must apply relevant methods and measures for life cycle design on three phases including project feasibility study, conceptual design and detail design and six processes including service life design, aesthetics design, performance design, design for environment and ecology, design for inspection, maintenance and repair, life cycle cost analysis, as shown in Fig. 2.
Conclusions
This paper aimed to analyze the major phases, major processes and their tasks as well as design methods for bridge life cycle design. Starting from the owners’, social and users’ needs and requirements, the bridge engineer must apply relevant methods and measures for life cycle design for three phases including project feasibility study, conceptual design and detail design and six processes including service life design, aesthetics design, performance design, design for environment and ecology, design for inspection, maintenance and repair, life cycle cost analysis to achieve a good life cycle quality. A general framework for bridge life cycle design was presented to better understand and apply the methodology for bridge life cycle design and provide a guide for the design of new bridges and the assessment and management of existing bridges.
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