Variable Load Distribution

In the reliability calculation of the current structural codes, the variable load distribution is Gumbel set for one-year loads. This calculation includes two deficiencies: (1) the variable load distribution must be set for the working life, normally for 50 years; and (2) the standard Gumbel distribution is questionable as it has a robust upper tail which unrealistically affects the calculation. The first error is unsafe while the second is safe. The overall effect is unsafe. A feasible option is that the upper tail of Gumbel distribution is cut at about 1.5-times the mean 50-year load, corresponding to about 1000-year return loads. This modified distribution has virtually the same lower tail as standard Gumbel distribution. The normal distribution as another option, however, is probably somewhat unsafe. The variable load safety factors of the Eurocodes are given here based on Gumbel distribution, modified Gumbel distribution and normal distribution set for 1-year and 50-year locations.