What causes structures to move? – In a word, gravity. Gravity is constantly working to get things closer to the ground. Strong structures resist gravity.
There are two common ways a structure may give in to gravity.
a) If it is sitting on something that is not strong enough, the ground below it will fail. Better to build on bedrock than quicksand.
b) If the structure itself is weak, it will not support the loads imposed on it. The total load is made up of the following – Dead load – the weight of the structure itself, Live load – furniture, people, wind, snow and earthquakes.
Wind acts intermittently on structures. Wind forces can push, pull or lift buildings. Buildings must be strong enough to resist the lateral and uplift forces of wind as well as the downward force of gravity. Hurricanes and tornadoes are extreme wind conditions. These often result in mechanical damage caused by projectiles. The structure of a home is the skeleton, which includes the foundations and footings as well as the floors, walls, and roof. Structures are judged by how well they are able to stand still. Successful structures do not move; unsuccessful ones do, sometimes dramatically.
EARTHQUAKES AND EROSION
Earthquakes also create forces, which can cause structural failures. Like wind, these forces are intermittent and variable and can push, pull or lift buildings. Erosion is a slower form of earth movement, but it can have a devastating effect on structures as well.
House components may fail because they were poorly built with improper materials, or the materials were poorly assembled. Rot, insects, fire and mechanical damage can cause well-built structures to fail. Rust can attack metal components.
COMPRESSION AND TENSION
What forces affect individual structural components? The two basic forces are compression and tension. A material is under compression when it is being pushed from both ends. A material is under tension if it is pulled on. Components in compression tend to get shorter or are squashed. Components under tension tend to get longer or are pulled apart. Many building components feel a combination of compression and tension. Some building materials are good in compression, others work well in tension and some perform well in both. A pile of bricks is very good in compression; you can stand on it. However, it is very poor in tension. A child can pull the pile apart. A chain, on the other hand, is very good in tension. You can pull quite hard on both ends and nothing will give, but the moment you try to push on it, the chain collapses. It is not very good in compression.
SHEARING AND BENDING
Different materials fail in different ways. Shearing and bending are common modes of failure. Shear occurs when adjacent faces of a material move in opposite directions. When a beam splits, or a brick cracks, it is because of shear. Bending is movement without shearing. A plank spanned between two chairs will bend if someone stands on it, particularly if they stand near the middle. The upper half of the plank is pushed together under compression; the bottom half gets slightly longer because it is in tension. Building components that fail by bending are said to sag or buckle. Some materials can bend a significant amount without losing their strength. Brittle materials, however, do not bend much before they break. Ductile materials do. Ceramic tile is brittle, rope is ductile. Some ductile materials are elastic. This means they will go back to their original shape after being bent. A rubber ball is elastic; a nail is not.
Deflection is a mild form of bending. If structures deflect just a little, people do not mind. Building codes stipulate how much deflection is acceptable. A typical floor joist, for example, is allowed to deflect 1/360th of its span.
What makes a good building material? It should be good at resisting the forces of tension and compression. It should be cheap, easy to work with, light, long lasting, water, rot and fire resistant, and stable under different temperature and humidity levels. No one material does it all. That is why houses are made of many materials. Wood is one of the better materials for small buildings. It is relatively good in both tension and compression. Steel is also good in both tension and compression.
Building materials are chosen based on cost-effectiveness. The goal is to assemble a structure that will perform well for as small a cost as possible. This can lead to some very small margins of safety and, of course, some failures. As new materials are developed, they are tried; in some cases, with great success; in other cases, with very poor results. The structure is by far the most important part of the house. The safety and usability of the entire home depends on its structural integrity. Since many structural components are buried below grade or behind finishes, much of the structural inspection is done by looking for evidence of movement. Where no movement has occurred, imperfections may go undetected.
New interior or exterior finishes and patching work may conceal imperfections over the short term. In these cases, problems will not be identified.
Structural repairs can be very costly, and in some cases the problem is so severe that the building is torn down. In many cases, a structural engineer should be consulted before making repairs. An incomplete understanding of a problem may lead to incorrect solutions and a life-threatening situation.