A Rectangular combined footing is most commonly used in the following circumstances: when two parallel pillars are close enough together to cause their foundation to overlap; when the ground-floor load-bearing capability is relatively low. A Rectangular combined footing is implemented when the surface on which the combined-staircases are standing is relatively soft. Low ground-water and low soil humidity conditions are favorable for such installations. When installing the foundation on a rectangle (or any other shape), make sure that there are no overhangs on either side of the standing surfaces. In addition, check that the corners of the foundation are at right angles to the vertical.
The actual installation process does not require any special skills, but you should be aware that installation of a rectangular combined footing involves the application of a leading-edge ground-fill which should extend to the very farthest point on either side of the standing shaft. The bearing clearance should then be computed according to the geometric aspect ratio of the standing shaft. For the purpose of this discussion, let us assume that the bearing clearance is thirty inches. The resultant angle of attack for each pair of pillars, A and B, is then obtained by multiplying the actual angle by the slope of the floor and the appropriate equation:
The first set of trapezoidal lines represents the leading edge of each pair of standing floor beams. The second set of lines represents the second leading edge of each set of standing beams. The actual property line on which the Trapezoidal combined footing is based is actually the earth’s normal plane. This allows easy computation of the angles of attack for the foundation and the standing floors. The equations to be computed are:
It is easy to see from these equations that the vertical loads required to support the upright load at the top of the standing floor is equal to the sum of the vertical and horizontal weights at the bottom of the upright beam. Now suppose that the trapezoidal top beams are placed one quarter of a circle above the ground. This would give us the second set of columns carrying equal weights to support both the upright and the horizontal loads. This is equivalent to the rectangular combined footing.
Now suppose that the leading and the second sets of columns are both installed at right angles to the axes of both lateral and longitudinal planes. If these columns were not of equal length, then the trapezoidal footings would have to be of unequal lengths. This would produce the three-sided structure known as a pancake. The problem would then become how to construct a perfectly round and symmetrical pancake. To solve this problem, designers of buildings usually build rectangular footings.
The Diagram of a rectangular combined footing has slopes at the top and bottom, which represents the downward pressure that is exerted on the floor system when the upright weight is placed on top of the horizontal lateral load. This downward pressure is equal to -BO, where BO is the sum of all the vertical forces. Therefore, the horizontal loads must be balanced by two concentric ropes or webbed ropes of the same length and each containing one and a half times the lateral and the vertical tension balances the Upright and the Horizontal forces. This way, a perfectly even stress distribution is maintained. For this reason, it is very important that the angles of these ropes are such that they form a ninety degree angle when laying parallel to the walls.
In building structures, these support two separate but balanced tension loads. These particular supports are usually called granular soils. The main advantage of such a type of support system is that it does not provide the vertical pressure needed for resisting the pressures of soil movement. Such a support also does not impede the movement of water, air and moisture between adjacent surfaces.
It is generally believed that rectangular footings carry equal loads. However, this is far from being true because some types of footings carry much more weight than others. The best solution would be to find a type of supported foundation which carries equal loads. In most cases, this can be achieved by joining slabs of different heights and orientations, which are vertically aligned. The resultant structure, which is technically referred to as tandem foundations, is an excellent solution for building with rectangular footings. When carried out in depth, these foundations provide very good structural support, with no significant lateral or vertical bias.