Widespan Membrane Roof Structures 175 Cause % Inadequate appreciation of loading conditions 43 or structural behaviour Mistakes in drawings or calculations 7 Inadequate information in contract documents 4 or instructions Contravention of requirements in contract 9 documents or instructions Inadequate execution of erection procedure 13 Unforeseeable misuse,abuse and/or sabotage,catastrophe, 7 deteriora tion(partly“unimaginable'”?) Random variations in loading,structure, 10 materials,workmanship,etc. Others 7 Table 1.Prime causes of failure.Adapted from Walker(1981) All these factors fit very well in the field of long span structures involving oftenly something "unusual"and clearly have an influence affecting human interaction. In Table 1,the prime cause of failure gives 43%probability (Walker,1981)to inadequate appreciation of loading conditions or structural behaviour.Apart from ignorance and negligence,it is possible to observe that the underestimation of in- fluence and insufficient knowledge are the most probable factors in observed failure cases (Matousek Schneider,1976). Performance and serviceability limit states violation are also directly related to structural reliability.Expertise in structural detail design,which is normally consid- ered as a micro task in conventional design,have an important role in special long span structures:reducing the model and physical uncertainties and avoiding chain failures of the structual system. According to the author,knowledge and experience are the main human inter- vention factors to filter gross and statistical errors in the normal processes of design, documentation.construction and use of structures. The reliability of the design process in the field of special structures must be checked in the following three principal phases:the conceptual design,analysis,and working design phases. Fig.1.Montreal Olympic Stadium-A cable stayed roof solutionWidespan Membrane Roof Structures 175 Cause % Inadequate appreciation of loading conditions 43 or structural behaviour Mistakes in drawings or calculations 7 Inadequate information in contract documents 4 or instructions Contravention of requirements in contract 9 documents or instructions Inadequate execution of erection procedure 13 Unforeseeable misuse, abuse and/or sabotage, catastrophe, 7 deteriora tion (partly “unimaginable”?) Random variations in loading, structure, 10 materials, workmanship, etc. Others 7 Table 1. Prime causes of failure. Adapted from Walker (1981) All these factors fit very well in the field of long span structures involving oftenly something “unusual” and clearly have an influence affecting human interaction. In Table 1, the prime cause of failure gives 43% probability (Walker, 1981) to inadequate appreciation of loading conditions or structural behaviour. Apart from ignorance and negligence, it is possible to observe that the underestimation of in- fluence and insufficient knowledge are the most probable factors in observed failure cases (Matousek & Schneider, 1976). Performance and serviceability limit states violation are also directly related to structural reliability. Expertise in structural detail design, which is normally consid￾ered as a micro task in conventional design, have an important role in special long span structures: reducing the model and physical uncertainties and avoiding chain failures of the structual system. According to the author, knowledge and experience are the main human inter￾vention factors to filter gross and statistical errors in the normal processes of design, documentation, construction and use of structures. The reliability of the design process in the field of special structures must be checked in the following three principal phases: the conceptual design, analysis, and working design phases. Fig. 1. Montreal Olympic Stadium - A cable stayed roof solution