Chapter 6 Special footings and beams on the elastic foundations
Chapter 6 Special footings and beams on the elastic foundations
6.1 Introduction This chapter will take up the design of several of the more complicated foundation members such as those required to support several columns in a line or from industrial loadings. When a footing supports a line of two or more columns, it is called a combined footing. A combined footing may have either rectangular or trapezoidal shape or be a series of pads connected by narrow rigid beams called a strap footing. The types of combined footing-Fig.6.1 Both the conventional "rigid"and the beam-on-the- foundation method of combined footing analysis will be presented
6.1 Introduction ▪ This chapter will take up the design of several of the more complicated foundation members such as those required to support several columns in a line or from industrial loadings. ▪ When a footing supports a line of two or more columns, it is called a combined footing. ▪ A combined footing may have either rectangular or trapezoidal shape or be a series of pads connected by narrow rigid beams called a strap footing. ▪ The types of combined footing-Fig. 6.1 ▪ Both the conventional “rigid” and the beam-on-thefoundation method of combined footing analysis will be presented
6.2 Rectangular combined footings Columns located off-center will usually result in a non-uniform soil pressure. To avoid the non-uniform soil pressure,an alternative is to enlarge the footing and place one or more of the adjacent columns in the same line on it. The footing geometry is made such that the resultant of the several columns is in the center of the footing area. This footing and load geometry allows the designer to assume a uniform soil pressure distribution
6.2 Rectangular combined footings ▪ Columns located off-center will usually result in a non-uniform soil pressure. ▪ To avoid the non-uniform soil pressure, an alternative is to enlarge the footing and place one or more of the adjacent columns in the same line on it. ▪ The footing geometry is made such that the resultant of the several columns is in the center of the footing area. ▪ This footing and load geometry allows the designer to assume a uniform soil pressure distribution
6.2 Rectangular combined footings The footing can be rectangular if the column that is eccentric with respect to a spread footing carries a smaller load than the interior columns. The basic assumption for the design of a rectangular combined footing is that it is a rigid member,so that the soil pressure is linear. The pressure will be uniform if the location of the load resultant coincides with the center of area. ◆ This assumption is approximately true if the soil is homogeneous and the footing is rigid
6.2 Rectangular combined footings ▪ The footing can be rectangular if the column that is eccentric with respect to a spread footing carries a smaller load than the interior columns. ▪ The basic assumption for the design of a rectangular combined footing is that it is a rigid member, so that the soil pressure is linear. ▪ The pressure will be uniform if the location of the load resultant coincides with the center of area. ▪ This assumption is approximately true if the soil is homogeneous and the footing is rigid
6.2 Rectangular combined footings In actual practice it is very difficult to make a rigid footing,for the thickness would have to be great. In recognition of the over-design using the conventional (or "rigid")method,current practice tends to modify the design by a beam-on- elastic-foundation analysis. ■ The conventional (or rigid)design of a rectangular combined footing consists in determining the location of the center of footing area.Next the length and width can be found
6.2 Rectangular combined footings ▪ In actual practice it is very difficult to make a rigid footing, for the thickness would have to be great. ▪ In recognition of the over-design using the conventional (or “rigid”) method, current practice tends to modify the design by a beam-onelastic-foundation analysis. ▪ The conventional (or rigid) design of a rectangular combined footing consists in determining the location of the center of footing area. Next the length and width can be found