Foundations

The loads that a structure imposes on the ground should normally reach the ground (or the level of the lowest floor if that is below the outside ground level) through walls, piers, or columns. Ideally, if the ground surface is a firm stratum of natural rock, can 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, foundations have to be provided to spread the loads more widely or carry them down to rock or firmer ground at a lower level.

Foundations are strong bases of buildings, usually lying below ground level, on which they are built up. Foundations carry the loading of a building itself and that of the forces acting upon it, and transmit them to the ground (a clay, soil or rock base). According to the current classification, foundations can be divided into shallow and deep.

1) Shallow foundations are subdivided into:

a) strip (or continuous) footings

b) mat (or raft) footings

c) box footings

Continuous footings form the foundations under a wall. Mat footings provide foundations equal to the whole area of the substructure. Box footings are very stiff structures having one, two or three floors and strong enough to transmit the heaviest loads. All of them spread the load fairly near the surface, simply by providing each wall, pier, or column with a substantially wider base.

2) Deep foundations include piling, diaphragm walls and caissons.

Piling carries the load further down without necessitating deep excavation. The piles were almost always of timber. Once hammered into the ground, they acted as columns usually transmitting part of the load to firmer ground at the foot and spreading part of it through the intermediate strata by surface friction.

The new requirements for tall buildings were mainly met by the substitution of grillages of steel beams for the less efficient, earlier spread footings. These have since given way to footings and piles of reinforced concrete, while there have been parallel developments in piling with the substitution of steel and reinforced-concrete piles for the previously universal timber pile. The heaviest reinforced-concrete piles are nowadays cast in situ in prebored holes.

Equally significant has been the increasing exploitation of the buoyancy principle — that of creating open basements below ground level of sufficient volume to displace a weight of earth comparable with the total weight of the building, so that there is only a small net change in pressure at foundation level when construction is completed.

12. There are some notes the student made after reading the text "Foundations". Did he remember everything right? Read his notes and correct them if necessary.

1. The loads that a structure imposes on the ground normally reach the ground through walls, piers, or columns.

2. Foundations are strong bases of buildings, usually lying below ground level, on which they are built up.

3. According to the old classification, foundations can be divided into spread (shallow) and deep.

4. Box footings provide foundations equal to the whole area of the substructure.

5. Deep excavation is necessary for piling.

6. For tall buildings grillages of timber beams were used.

7. Buoyancy principle means creating open basements below ground level of sufficient volume to displace a weight of earth.

13. Match the beginnings of the sentences (1 – 4) to their ends (a – d) using the information from the text.

1. An ideal stratum have rarely been found....

2. Shallow foundations spread the load fairly near the surface….

3. These piles, hammered into the ground, acted as....

4. The heaviest reinforced-concrete piles.....

5. The new requirements for tall buildings were mainly met....

a) columns, transmitting part of the load to firmer ground at the foot;

b) simply by providing each wall, pier, or column with a substantially wider base;

c) are nowadays cast in situ in prebored holes;

d) in the places where men have wanted to build;

e) by the substitution of grillages of steel beams for the less efficient earlier spread footing.

Дата добавления: 2015-07-20