Design Wind Speeds for South Australia

BASIC REGIONAL WIND SPEED

Typical Building/Structure

V[R] = 45 m/s [162 km/h]
mean return period 500 years
5% risk of exceedance for 25 year design life

Post Disaster Building/Structure

V[R] = 48 m/s [173 km/h]
mean return period 2000 years
1.25% risk of exceedance for 25 year design life

These wind speeds are typically as measured at an airfield 10m above the ground in terrain category 2. (M[z,cat]=1 by definition). These speeds are adjusted at a local site to allow for differences in terrain, and height, topography, shielding and direction. These differences are allowed for in AS1170.2 by multipliers: M[z,cat], Mt, Ms, Md.

Vzu = V[R] x M[z,cat] x Mt x Ms x Md

This is then converted into a site reference design pressure

qzu = 0.5 x density[air] x Vzu²

The site reference pressure is then multiplied by a pressure coefficient (Cp) to get the pressure experienced by a given surface of a building. These are the ultimate strength design loads. At ultimate strength, materials are permitted to deform but not fracture. When the load is removed the material remains deformed. Whilst do not expect a structure to collapse at the ultimate strength load, we do expect the building will no longer be suitable for purpose. The building will require major repair or replacing. At serviceability loads, the structure remains suitable for purpose. Expect a post-disaster facility to remain suitable for purpose when typical building is no longer functional. (eg. Hospital, CFS building) That is the serviceability load for a post disaster facility should at least be equal to the ultimate strength loads of normal buildings.

{On the other hand the limit state codes are a confusing mess. Ultimate strength is the load at fracture. Our structural codes give no real consideration for over load and mode of failure. All structures have the potential to experience greater than their design loads, when they do it is important that they fail in a controlled manner, or that over load is otherwise guarded against and prevented. Buildings are for the most part not designed to be failsafe. Picking a big load with a low probability of being exceeded is not fail-safe design. Over load due to natural phenomena is not practical to prevent: it is therefore important to be able to monitor loading and control modes of collapse. If we are using probabilistic design then the ultimate strength load should be the load at fracture and/or collapse. Ultimate strength loads are not operational loads, they are the loads at which operation stops, and we cease to have a useful system. Ultimate strength loads are the loads at which we need to replace the system. We cannot get an extra 10% out off the system because we no longer have a system: it is dead, deceased, no more, pushing up the proverbial daisies.}


Revisions:

  1. [08/10/2016] ; Original