Probability based calibration of pressure coefficients

Normally, a consistent basis for calculating partial factors focuses on a homogeneous reliability index neither depending on which material the structure is constructed of nor the ratio between the permanent and variable actions acting on the structure. Furthermore, the reliability index should not depend on the type of variable action.

A probability based calibration of pressure coefficients have been carried out using pressure measurements on the standard CAARC building modelled on scale of 1:383. The extreme pressures measured on the CAARC building model in the wind tunnel have been fitted to Gumbel distributions, and these fits are found to represent the data measured with good accuracy. The pressure distributions found have been used in a calibration of partial factors, which should achieve a certain theoretical target reliability index. For a target annual reliability index of 4.3, the Eurocode partial factor of 1.5 for variable actions agrees well with the inherent uncertainties of wind actions when the pressure coefficients are determined using wind tunnel test results. The increased bias and uncertainty when pressure coefficients mainly are based on structural codes lead to a larger partial factor of the order of 1.8.

In order to put the partial factors determined into perspective, calculations have also been carried out for structures exposed to snow load. Assuming a target annual reliability index of 4.3, the partial factors for snow load may become as high as approx. 3.0 when the characteristic shape coefficients are based on mean values as specified in background documents to the Eurocodes. Importance of hidden safeties judging the reliability is discussed for wind actions on low-rise structures.