Foundations

Date: 2015-10-07; view: 521.

The loads that a structure imposes, on the ground normally reach the ground through walls,

piers, or columns. Ideally, if the ground surface is a firm stratum of natural rock, able to take the loads directly without noticeable settlement, the walls, piers, or columns can simply be ended when they reach it. They can also be built up directly from it after some preliminary leveling. Unfortunately, such strata have rarely been found in the places where men have wanted to build. Therefore, some means have to be provided to spread the loads more widely to carry them down to rock or firmer ground at a lower level.

A foundation is Structure that transmits loads from a building or road to the underlying ground. A footing is a slab element that acts as the foundation, transferring loads from the superstructure to the ground.

Most foundations extend underground, and the foundations of large buildings often penetrate to the bedrock. One common type of foundations consists of walls that extend below the frost line and transfer the weight to wider footings. Other kinds of foundations include Slab-on grade foundations, pier and beam foundations and piles.

The primary dangers to a foundation are movement and uneven support. Change in ground water table is a common cause of foundation failure. Also flowing water can remove supporting soil from underneath a bridge foundation and freezing water can heave the supporting soil in one direction and then in the other direction when it thaws.

Changes in soil moisture can cause 'reactive' clay soil to swell and shrink. This swelling can vary across the footing due to seasonal changes or the effects of vegetation removing moisture. The variation in swell can cause a footing sitting on the reactive foundation soil to distort, cracking the structure over it. This is a particular problem for house footings semi-arid climates such as South Australia, Southwestern USA, Israel, and South Africa where wet winters are followed by hot dry summers. Raft slabs with inherent stiffness have been developed in Australia with capabilities to resist this movement.

When heavy structures are built in arctic areas where the ground is permanently frozen, special refrigeration units must be used to keep the pressure of the structure from melting the supporting soil.