Steel Roof

The great superiority of mild steel over timber in stress-resisting properties, the ease with which they can be packed and transported, and their resistance to fire and insects have caused them to gradually displace timber as the material for roofs of large or even moderate dimensions.

Every member of a truss should be in either direct tension or compression; but in smaller roofs the purlins are often distributed along the length of a principal rafter, causing the member to be in transverse stress; in that case the distance between the struts should not be great, otherwise the principal rafter will have to be wastefully large.

Struts should be arranged as short as possible, and as many braces as possible be in tension.

There are four general types of steel trusses for roof:

• these for supporting ceilings;

• open trusses;

• arched truss;

• mansard truss.

Text 8 Walls

Walls are constructed to enclose areas and to support the weight of floors, roofs, earth or water. They are classified as follows: « walls to resist vertical pressures;

• walls to resist oblique thrusts.

The first section of heading includes all house walls, solid or hollow, supporting single floors, and couple close raftered roofs. The second section includes all walls carrying the girders of framed floors and the trusses of framed roofs. Inside walls serve as parti­tions or divisions for several rooms inside the dwelling. Inside walls may or may not support other parts of the structure.

An outside wail rests directly on the foundation wall forming a beanng unit for the upper floors and the roof and an enclosure for entire inner construction. Outside walls are made of wood, steel (for retaining walls), brick, stone, concrete blocks or concrete, or combi­nation of two or three of the foregoing materials, cane and adobe.

Classification of stone walling. This is classified as follows:

• rubble;

• block-in-course;

• ashlar.

Rubble walls are those built of thinly-bedded stone, generally under 9 inches in depth, of irregular shapes as in common or ran­dom rubble, or squared as in coursed rubble.

Block-in-course is composed of squared stones usually lar­ger than coursed rubble, and under 12 inches in depth.

Ashlar is the stone from 12 to 18 inches deep, dressed with a scabbling hammer, or sawn to blocks of given dimensions and carefully worked to obtain fine joints. The length of a soft stone for resisting pressure should not exceed three times its depth; the length in harden stones, four to five times its depth; the breadth, from one-and-a-half to twice its depth and the breadth in harder stones, three times its depth.

Reinforced brickwork. It is brickwork which has been strengthened by the introduction of steel or wrought iron in the form of either flat or rod bars, woven wire or expanded metal. Such brickwork is capable of resisting tensile and shear stresses, in addi­tion to compressive stress. Reinforcement of brickwork also im­proves the longitudinal bond of thick walls. The brick walls are rein­forced at every third course with steel meshed strips.

Cavity walls. This type of construction is now very common and is generally preferred to solid wall construction for many types of buildings, especially houses. A cavity wall is usually an external wall. It consists of two separate walls of brickwork, having a cavity between, and connected together by metal ties. This double wall is generally 11-in. thick, consisting of 4.5 in. inner and outer leaves and 2-in. cavity. Such a wall is adequate for a two-storied building of the domestic type. The chief merits of cavity walls are:

• they prevent rain from penetrating to the internal face;

• they have a high insulating value;

• They are economical.

Text 9 Partitions

Partitions are walls which are used to divide buildings into rooms, corridors and cubicles. They also often act as deep trusses to support the joists of floors, purlins and ceiling joists of roofs, etc. Partitions may be classified under following: timber, clay and terra­cotta, concrete, sawdust concrete, plaster, wood-wool cement, as­bestos-cement, and metal.

Timber partitions. They consist of vertical members called studs, which are secured to two horizontal length of timber, the up­per being the head and the lower, the sill. One or both sides may be either lather and plastered or covered with boarding, plywood sheets, wall boards, etc.

This class of partition is in common use. Because of its light­ness, it is usefully employed when there is no supporting wall below. A disadvantage is its deficient fire-resisting quality.

Clay and terra-cotta brick and block partitions. The common­est type of clay partition is the ordinary solid brick wall of 4 14 in. or more in thickness. Such walls are relatively strong and fire-resisting, but their weight precludes their use for partitions on upper floors unless provision in the form of girders or lower wall is made for their support. Hollow clay units, either bricks or blocks, are comparatively light and yet are sufficiently strong for the construction of non-load bearing partitions. In general, the blocks are built in either cement mortar or compo and are bonded in the usual way with staggered vertical joints. The following are the merits of these blocks: satisfac­tory mechanical strength, lightness in weight, good heat and sound insulation, fireproof, non-shrinkage and vermin proof.

Precast concrete block partitions. - Both solid and hollow blocks are made. There are three types of precast concrete blocks, namely:

• Concrete blocks used for loadbearing walls and made of dense aggregate such as broken stone, heavy blastfur­nace slag and crushed bricks.

• Concrete blocks used for loadbearing wall and made of lighter aggregates including foamed slag, clinker, ex­panded slate and pumice.

• Concrete blocks for non-loadbearing partitions and made of aggregates similar to those for type 2.

Text 10 Subterranean Heat Serves Man

Coal, oil and gas have been man's main sources of heat from ancient times. At the same time the earth contains inside it a practically inexhaustible supply of thermal energy, which without doubt can be used with great profit for economic needs.

Long, long ago magma, surrounding the core our planet, rose towards the surface but could not burst through the crust and started cooling down gradually. This cooling process has been going on for hundreds of thousands or millions of years. By transferring heat to the rock around it, the magma heats the water in artesian basins and this is how subterranean deposits of hot water origi­nated.

Our country has tremendous quantities of hot subterranean water. Such water can be found both in the subtropical areas and in the Arctic.