Wind Loading Risk Assessment

Spreadsheet for varying risk and life expectancy and calculation of mean return periods for wind speed. Based on the appendices to AS1170.2:1989 and the introduction of our limit state codes: the typical basis of design is 5% risk of exceedance for a given life span: in 1989 the lifespan was adopted as 50 years to give a mean return period of 1000 years (rounded). Assuming that there was no change to the acceptable risk in the 2002 revision, then the acceptable life span, taking rounding into consideration was reduced to 25 years, for a mean return period of 500 years. If however the expected lifespan remained at 50 years then the acceptable risk was increased to 10%. Given that statistical hypothesis tests using 5% or 1% are the norm, and that expect failure rates to be significantly less than 5%, don’t expect that the risk was increased, rather the lifespan decreased. Note that structural failure is dependent on a combination of inadequate strength and excessive load.

It should be noted that the building code of Australia (BCA) does not differentiate between loss of life,  loss of amenity, or economic loss. Since design loads always have the possibility of being exceeded, quality robust design would require design for failure: that is the mode of failure should be taken into consideration and the system designed to be fail-safe. Such approach requires a more qualitative design effort. The current code mandated approach is not safe, and is effectively based on statistically guessing a big number and hoping it won’t be exceeded. Each time the big number is exceeded a bigger number is chosen. It is not economical and it denies people access to housing: there is little point talking about loss of amenity, when people do not have access to the amenity in the first place.

The natural environment becomes hazardous once twigs and branches start being ripped from trees, and that occurs at wind speeds of 62 km/h [17 m/s], such speeds are exceeded every year in South Australia, where annual storm damage is typically associated with speeds from 90 km/h to 110 km/h. Therefore at wind speeds of 62 km/h, need to take shelter behind some barrier which can provide protection from airborne debris travelling at the speed of the wind. A protective barrier however can be expendable and replaceable.

It is therefore not necessary to design an entire building to a high load, if qualitatively the building can be configured, so that it has purpose made shelter, has appropriate protective shielding, can otherwise collapse in a controlled manner and in doing so can avoid becoming airborne debris. Modern civilisation is largely dependent upon mains water supply and main sewage disposal. The most important rooms in a house therefore are laundry, bathroom and kitchen, if these rooms can remain functional after wide spread damage then recovery would be vastly more rapid. Though a large percentage of the world population needs to get the facilities in the first place.

The issue isn’t so much the magnitude of the design load, but the expected behaviour of the system at that load. As wind speeds get higher it becomes more preferable to either leave, or  bend and fold with the wind and present a smaller profile.

Download: windRiskAssessment2014.xls