Design for Serviceability - A probabilistic approach

Sammanfattning: In many design situations the acceptable performance of structures is defined by serviceability requirements. To estimate the probability of serviceability failure three main design aspects should be considered: (1) the relevant exposures, (2) the structural response and (3) the performance criteria. This thesis presents background information and new findings on structural serviceability related to all three aforementioned aspects. To study current practice concerning serviceability design two surveys with experts are presented: (1) serviceability issues in present Swedish design practice and (2) research interviews with international experts working with glass structures. The main conclusion of the two surveys is the general need for better understanding and guidance about serviceability, both in education and design codes. Since load history and long-term deformations may have a large influence on serviceability, an advanced finite element model has been developed to estimate the deflections of structural sized timber beams in natural environment. The model is capable to take into account the combined effect of the variations of relative humidity in the surrounding environment and the time-variant mechanical loading. Based on the results of the model, possibilities of simplified modelling are investigated to predict long-term deflections of timber structures. The reliability of serviceability limit states in current design codes is investigated mainly focusing on static deflections. Simple models are used to compare various construction materials – steel, concrete and timer – in terms of serviceability reliability, based on design according to Eurocode prescriptions using second order reliability methods. It is shown that the reliability of Eurocodes is not consistent. The inconsistency exists among different materials, variable to total load ratios and loaded areas. The proposed method may provide a basis for code calibration with regard to serviceability. As a final step the time-dependent reliability for serviceability design for beams is estimated using Monte Carlo simulation. The study presented in the time-variant analysis is restricted to timber floor beams in offices and residential buildings. However, the presented method provides a framework to calibrate proper deflection limits, load factors and creep coefficients based on a probabilistic investigation involving uncertainties in all important aspects of structural serviceability.