An Improved Life-Cycle Design Theory Of Engineering Structures And Its Life-Cycle Cost Model

Life-cycle design(LCD)of engineering structures aims to develop structural design schemes,maintenance plans and disaster response programs by considering the loads,environmental effects and hazards that may occur during the lifespan of structures as well as the potential economic,environmental and social impacts induced by engineering activities,so as to ensure that all structural performance requirements are satisfied and all adverse impacts are minimized.However,some defects have been witnessed in current LCD theories of engineering structures.The multiple design objectives of LCD approach prevent its application in real engineering projects.Sustainable requirements have not been effectively addressed or satisfied in current LCD methods.The life-cycle cost(LCC)model is also incomplete.In light of the abovementioned problems,this paper tends to explore and modify the LCD theory and methods of engineering structures;to set up the life-cycle green design indicator system of structures;to develop the hierarchical LCD method that can cover multiple design objectives;to convert the environmental impacts and social impacts to environmental costs and social costs,respectively;to build the life-cycle total cost model that contains direct costs,environmental costs and social costs based on classical life-cycle cost model;and to put the life-cycle total cost model into practice via software development and case study.The following research work has been carried out in this paper.'(1)The pros and cons of different structural design methods were analyzed by retrospecting the evolution of structural design theories.Based on the LCD objective system in previous studies and the current design concepts,a modified LCD objective system was proposed,where the traditional design objectives include structural performance,service life and economic efficiency objectives,and the green design objectives consist of local environmental,social and global environmental objectives.The advantages of LCD method with respect to design objectives,time horizon,dynamic design and durability-based design were analyzed,and the associated indicator system was proposed.(2)The life-cycle green design indicator system of engineering structures was proposed based on the modified LCD theory and the green building evaluation systems from home and abroad.It contains the local environment-oriented environmental evaluation indicators,the human-oriented users and social satisfaction indicators,and the global environment-oriented sustainable development indictors.The detailed life-cycle green design indicators were set up by hierarchy analysis,indicator classification and weight analysis.The life-cycle green design indicators of coastal highway bridges were developed considering specific structural styles,functions and environmental characteristics.(3)The hierarchical LCD method was proposed considering the traditional design objectives and green design objectives.The design process was divided into six levels that covers the aspects of structural safety and reliability,durability,economic efficiency,local environment,social impacts and global environment.The hierarchical LCD method was applied to a reinforced concrete highway bridge in marine environment,and a comprehensive comparison between traditional design and the hierarchical LCD approach is made within six design levels to emphasize the advantages of the proposed method.(4)Following the mainline of structural durability,the theoretic framework of an improved LCD method was restructed to include the reliability and sustainability indicators.The upgraded LCD method was carried out through dynamic performance-based design concept.It solved the problems of vague concepts and repeated indicators in current LCD methods by setting up the sustainability indicators,and also replenished the contents of LCC at the same time.The upgraded LCD indicator system possesses a more complete and reasonable framework,and applies to a wide variety of structural types and service conditions.(5)The environmental cost model based on the pollution prevention measures was presented.The environmental costs of common construction materials,energy,transportation,and construction equipment were calculated.A structural type selection between reinforced concrete girders and steel girders of a bridge structure was carried out based on the environmental cost-incorporated initial cost.The environmental cost-incorporated life-cycle cost model was introduced,and a life-cycle cost analysis(LCCA)of steel bridge girders considering the initial cost and future maintenance cost was performed.Uncertainties arising from the direct cost and environmental cost were analyzed.The effects of discount rate of environmental cost on total LCC were examined using sensitivity analysis.(6)The social impacts of engineering structures were classified into two categories:personal-level impacts,including physical conditions,psychological conditions and personal economic conditions;and social-level impacts,comprising human settlements,social-economic development and social resources.The social impacts were transferred into social costs,and detailed computational equations were presented with an emphasis on civil infrastructures.A deteriorating bridge was used to illustrate the calculation of various components of social costs,and a social-cost incorporated LCCA was performed based on the proposed deterioration model,maintenance plans and accident information.(7)The life-cycle total cost of structures is composed of direct costs,environmental costs and social costs.Based on the environmental cost model and social cost model,a Life-cycle Cost Calculator for Engineering Structures was developed on MATLAB platform to perform life-cycle total cost analysis and evaluation.A bridge structure in Ningbo,Zhejiang Province was used as an example to illustrate the life-cycle total cost analysis,where two sets of maintenance plans of the bridge were compared.The uncertainties associated with three types of costs were investigated.Utility theory was used to normalize the life-cycle direct costs,environmental costs and social costs of the bridge,so as to select the more sustainable maintenance plan.