Structural LCC design of concrete bridges

This doctoral thesis comprises chapter 1–6, appendices and four Papers. The thesis is based on the axiom that the structural engineer ought to design bridges to as great benefit and as low cost as possible for the society. The thesis focuses on the possibilities, and responsibilities, the structural designer has to design cost-effective concrete bridges for the society. It is questioned, if the long service lives, prescribed in the design codes, comply with the axiom. In the thesis, it is examined, if, LCC analysis, life-cycle cost analysis, may be a possible tool for cost-optimisation of the durability aspect in structural design. Further, the possibility to choose design parameters based on LCC is investigated. In purpose to evaluate the usefulness of the method, examples are given. The calculations in the thesis are only intended to show examples of economical optimisation with respect to durability of concrete structures, they do not intend to present results which can be directly used in structural design. The results from the calculations are strongly dependent on input. Therefore, the choices of economical models, discount rates and durability models for bridges and bridge structural members, are critical for the results. The examples show that LCC can be used for cost optimisation of design parameters and service lives. Instead of prescribing a fixed design life, an interdisciplinary approach is suggested, where design life and durability is chosen from lowest annuity cost, based on LCC. In the calculation examples considering chloride induced reinforcement corrosion, a high concrete quality, small crack widths, and moderate concrete covers were optimal in general. The most important parameter that influences the optimal service life was found to be the discount rate. Calculated service life was found to be about 100 years at discount rate 4% and about 60 years at 7%. However, for other examples, where other deterioration models or LCC models are used, the results may be totally different. It is suggested, that the structural engineer should have the possibility to choose optimal combination of design parameters, in each specific case. The structural engineer shall be able to, for the bridge manager, present different durability designs, including cost calculations, so that the most economical design for the society may be chosen. For public investments, also societal costs like traffic delay cost may be considered in the LCC calculations. LCC analysis can be seen as a valuable tool for cost-effectiveness in the design stage in the future. By the use of LCC, the structural designer will be able to estimate the consequences of his/her decisions, and it will promote forward thinking and provide a basis for evaluation. However, the way of calculating design parameters, presented in the papers included in this thesis, is not useful for the structural engineer in every design object. However, for larger objects, for future authors to design codes, or for researchers, it may be useful.