building, foundation, types
The portions of a building which rest upon the soil or rock are called the foundations or footings. The foundation of a structure generally serves to transfer the loads from the structure to underlying materials (such as soil or rock) which are known as the foundation materials. Foundations must keep the walls and floors from contact with the soil, to protect against the action of the frost, to prevent from sinking down and settlements. To make the foundations strong and durable it is necessary to study their area, strength and also the characteristic and resistance of the bed upon which they rest. Before foundation design and construction, the foundation material is usually investigated by means of samples from test borings or pits. This sample may be tested for their physical characteristics in laboratory or at the construction site. The purpose of this test program is to establish the nature and arrangement of the foundation material, its strength and compressibility. The position of the water table is also determined and a load plan for the proposed structure should be prepared. This plan should include sketches showing the nature, weight and sizes of all materials in the walls and in the floors. This helps to prevent the overloading of the foundation and makes the necessary corrections easier. The load plan should indicate the concentrated) loads of Individual columns and linearly distributed loads of bearing walls. a) Foundation Types Foundations for structures are classified as footings, mats, piles or piers, depending on their form and the method of the construction. Footings are enlargements of the lower portion columns or walls. They distribute the load to the foundation matt a rather small pressure to make the settling tolerable and prevent the footing from breaking into the ground. Since old times masonry piers have been used as footings, but heavy loads on weak soils they require much space and increase load. Grillages of steel rails and I beams were used i.1890overcome these disadvantages in early skyscraper construction.
1900 these types were replaced by reinforced-concrete footings. Proper area of the footing is determined by dividing the load by allowable soil pressure — a value often specified in local building regulations. This value is determined by the character of foundation material and the principles of the soil mechanics. Centre of gravity of the footing must usually be in line with the point of application of the load. b) Types of Footings The most common types of foundation structures are:.
1) Isolated spread footings (one to each column);.
2) Combined footings (of two columns); and.
3) Mat, or raft foundations (all columns rest upon heavy slab). These forms may sometimes rest directly on the bearing soil they may rest on the heads of piling. Sometimes the columns anon large cylinders of plain concrete, from 4 to 8 ft. in diameter extend to rock or other hard stratum at some depth. This form called «piers or "caisson" foundation. Spread footings may bounder walls or columns and may be of timber, steel, plain masonry, or reinforced concrete. The problem is to enlarge the base in order to secure bearing or carrying loads. Wall Footings In this case the footings act as a cantilever beam with the load acting upward and the bearing area at the middle. The bearing must be uniformly distributed over the bearing area, but the bearing may be a little greater at the middle than at the edges. Continuous footings under columns When footings are built continuously under columns, the footing has the nature of a beam carrying concentrated loads at column points acting downwards and distributed loads acting upward. The distribution of the pressure cannot be calculated directly, but the problem is to determine the width of the footing slab so that the slab acting as a continuous beam under a distributed load upward will have upward reactions at column points not less than the column loadings. c) The Selection of a Suitable Type of Foundation In the selection of the foundation type it is necessary to consider the following main points:.
1. The loads of the structure should be transferred to soil layers which can safely support them..
2. The deformations of the soil layers under the foundation should correspond to the deformations which the foundation itself, its super- structure and adjoining structures can safely withstand..
3. The construction operations should not be dangerous for the structures which are located near. It is very important to study the building codes and regulations of the city where the structure must be built. All plans should include the means for the necessary control of construction operations. Specifications should also include provisions for the control of the foundation during its construction and after completion of the whole structure. The settling of structures located above deep beds of soft clay is undesirable to foundation engineers; as such settlings often continue to increase during many years after the completion of the structures. If the weight of any building is greater than the bearing resistance of the material on which it rests, the material will give way and the building will settle. In this case the walls will not be vertical and the floors will not be level and some fractures may be formed under the different stresses. The settlement of a new building can damage an old building resting on compressible soil. As a result of this settlement, the cracking of the walls in the old building20 may take place. In such cases it is recommended to build the part of the structure a little later, when most of the settlements of the heavy part have already taken place. It is also necessary to post the plaster work or the stone lining of the facade as many dam take place in brittle surface materials. Settlement analyses and settlement control help to work out the details of the structural design and the construction operations.
