The Forces of Nature
Earthquakes are violent shaking of the ground caused by sudden movement of a small part of the Earth's crust. Earthquakes cause shaking of the ground not only horizontally, but vertically as well. As a design action, houses not only have to resist shaking caused by an earthquake, but also, in some areas (like New Brighton) they must also be able to withstand liquefaction, which can cause houses to unevenly "sink" into the ground.
For a house to resist earthquake shaking it needs to be:
- Flexible but strong so that the house can sway but not collapse under horizontal shaking.
- Very stiff and strong using walls and bracing so that it effectively moves with the ground.
- Disconnected from the ground so that when the ground shakes the house doesn't move with the ground. This solution is called base isolation.
A house presents an obstacle to the clear flow of wind. The amount by which a house blocks the flow of wind affects how much force from the wind the house needs to resist this is called drag. The flow of wind around a house pushes the house on one side (windward side) due to its flow path being blocked while pulling the house on the other side (leeward side) due to the formation of a low pressure region.
For a house to resist wind it needs to be:
- Stiff, strong and well attached to the ground - this is the opposite to designing a house to be flexible in an earthquake or separated from the ground to isolate it from ground shaking.
When snow falls on a roof it doesn’t move off the roof like rainwater but instead stays in place accumulating more snow as it continues to fall during a snowstorm. Hence, the effect of snow creates extra weight for the roof and house to support.
To design a house to resist snow three methods need to be used:
- Have an angled roof to encourage any fallen snow to slide off the roof, reducing the weight of snow which needs to be supported
- Have strong roofs to cope with any extra weight they need to carry
- Have strong columns or walls supporting the roof .
Depending on the steepness of the seabed near the shore a tsunami can appear as either a very tall wave, crashing near the shore and not moving far inland, or as a massive temporary rise in sea level causing mass inundation many kilometers inland.
It is very difficult to design a house for tsunami - in fact there are no tsunami proof houses. A tsunami is destructive not only because of the depth and speed of flowing water it generates but also because of the debris it pick up (ships, houses, trees, cars, etc.) and "throws" into anything in its path.
The best way to design for tsunami is to:
- Allow it to freely flow through the building, but design the house to be tall and strong so that people can utilise the roof for safety. - otherwise another solution is to protect the community by building a flood defence wall.
Low lying areas of land, or land near waterways, are prone to flooding during heavy rainfall. Flooding itself is not a critical design load for a house, but it is a nuisance for the occupants as the floodwaters can ruin house contents.
The best way to design against flooding is to:
- Build on stilts to prevent water from getting inside the house in the first place.