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Durability Design Of Structures Essay

1298 words - 6 pages

Pick up 5 items to be considered in the service life design and describe how to include and/or assess them in the design process.
Service life design is the process of designing a structure or element to meet a client’s requirement for a specific minimum period of time during which no excessive unplanned expenditure is required on operation, maintenance or repairing. But this only defines service life design in a qualitative form so fib defines design service life in a quantitative form. According to fib service life design can be defined by 3 parameters which are: a clear definition of the relevant limit state, a number of years and a level of reliability for not passing the limit state during this period.
Thus in order to meet the service life design of structures, various factors should be considered that has direct or indirect effect on service life deign. Some of them are:
Compliance with laws and regulations
Economic efficiency
Social, environmental compatibility
Easiness of maintenance
Effect of huge seismic action
Among these factors some of these factors are explained in detail below.
Safety is one of the key issues for the design service life. Generally design processes are based on the comparison of the resistance of the structure (the R variable) with the action or load (often called the S variable). Failure occurs when the resistance is lower than the load. Since the load on a construction as well as the resistance sometimes highly variable, S and R cannot be compared in a deterministic way. The decision has to be based on maximum acceptable failure probabilities. The probability of failure, denoted pf , describes the case when a variable resistance R is lower than a variable load S. This probability is required to be lower than the target probability of failure pTarget
p{Failure} = pf = p {R − S < 0} < pTarget
pf : failure probability
pTarget : target failure probability
In this kind of design problems, it is necessary to calculate the relevant probability of failure. This procedure starts with the limit state function Z = R – S, and introduces the variables R and S to the equations including their average and standard deviation. Z is the reliability of the construction. Assuming that the variables S and R are normally distributed, the reliability Z, the difference between the variables R and S, is a variable which itself is normally distributed. Averages and standard deviations of the parameters can be calculated according to the relationship below. The reliability index is calculated using the average and the standard deviation of Z: µZ and σZ respectively. The reliability index is the difference between the mean values of R and S divided by the standard deviation of the variable Z or, alternatively, the mean value of Z divided by the standard deviation of Z.
p_f=Φ(-μ_z/σ_z )=Φ(-β)
μ_z=μ_R-μ_S,σ_z=√(σ_R^2+σ_S^2 )

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