Walls are constructions of stone, brick, or other materials, and serve to retain earth or water, or in buildings to support the roof and floors and to keep out the weather. The following points should be attended to in the construction of walls: The whole of the walling of a building should be carried up simultaneously; no part should be allowed to rise more than about 3 feet above the rest; otherwise the portion first built will settle down to its bearings before the other is attached to it, and then the settle ment which takes place in the newer portion will cause a rupture, and cracks will appear in the structure. If it should be necessary to carry up one part of a wall before the other, the end of that portion first built should be racked back,-that is, left in steps, each course pro jecting farther than the one above it.
Work should not be hurried along unless done in cement mortar, but given time to settle to its bearings.
Thickness of Walls. The thickness necessary to be -given walls depends upon the height, length, and pressure of the load, wind, etc., and may be determined from that section of applied me chanics termed 'Stability of Structures." In practice, however, these calculations are rarely made except for the most important structures, for the reason that if a vertical wall be properly con structed upon a sufficient foundation, the combined mass will retain its position, and bear pressure acting in the direction of gravity, to any extent that the ground on which it stands and the component materials will sustain. But pressure acting laterally has a tendency
to overturn the wall, and therefore it must be the aim of the con structor to. compel as far as possible, all forces that can act upon an upright wall to act in the direction of gravity.
In determining thickness of walls the following general prin ciples must be recognized: 1. That the center of pressure (a vertical line through the center of gravity of the weight), shall pass through the center of the area of the foundation. If the axis of pressure does not coincide exactly with the axis of the base, the ground will yield most on the side which is pressed most; and as the ground yields, the base assumes an inclined position, and carries the lower part of the structure with it, producing cracks, if nothing more.
2. That the length of a wall is a source of weakness and that the thickness should be increased at least 4 inches for every 25 feet over 100 feet in length.
3. That high stories and clear spans exceeding 25 feet require thick walls.
4. That walls of warehouses and factories require a greater thickness than those used for dwellings or offices.
5. That walls containing openings to the extent of 33 per cent of the area should be increased in thickness..
6. That a wall should never be bonded into another wall either much heavier or lighter than itself.
In nearly all of the larger cities the minimum thickness of walls is prescribed by ordinance. .
The accompanying table gives the more usual dimensions: