It may be stated here for reinforced walls the area of rein­forcing steel should be not less than 0,1 per cent of the area, both horizontally and vertically.

Although it is preferable from theoretical consideration to use small rods at close spacing, constructional difficulties will often ne­cessitate the use of larger bars at greater spacing. When this hap­pens, it is desirable to increase somewhat the percentage of rein­forcement.

Text 6 Civil Engineering

The term "civil engineering" was first used to distinguish the work of the engineer with a non-military purpose from military engi­neering. But increasing specialization has led to subdivision of engi­neering into civil, mechanical , electrical, and other forms, and the term "civil engineering" is usually applied to such as excavation and embankment, the construction of railways, bridges, docks, and har- bours, the control of water by dams and reservoirs, canals, aque­ducts, and pipelines, and the reclamation of land.

Civil engineering did not develop until the rise of Rome. The Cloaca Maxima, the great drain of Rome, was built in the sixth cen­tury В. C. It is in existence today, but the oldest part still standing probably dates from the third century В. C. Like most ancient drains and sewers, it was at first open, but was later enclosed. The original purpose of Rome's sewers was to drain off waste waters. For effi­ciency, the sewers were built along the lines of the natural streams. Of the approximately 200 aqueducts the Romans built throughout their Empire, at least nine were in Rome. Though they were able to deliver 85 million gallons of water per day, consumption averaged only 40 million. Like other ancient systems, the Romans' set-up had no main pipe leading from the reservoir to the town. Instead, the wa­ter flowed from the source partly along conduits which were often of considerable length, and partly over aqueducts to the water tower. The water tower interior was divided into four compartments - the tank itself and three subsidiary tanks. Two of the smaller tanks sup­plied the baths and the private houses of the rich. The third tank supplied the public fountains, where the majority of the people drew their water.

Lead or clay pipes were used to conduct water to the houses. The lead pipes (4 ^ЛА inch in diameter), were made from sheet-lead, bent around a core. Walls were V* inch thick.

The Romans, although they did not invent paved roads, ad­vanced road building to a new height. The total length of the roads built by the Romans in Britain is estimated at over 47,000 miles. They were constructed to last forever and many are in use today; some have simply been resurfaced.

In the construction of their road network, the Romans aimed at the shortest route, regardless of obstacles. Rocks were cleared away, tunnels were dug through hills, and swamps were drained. At first the Romans built timber roads, then somewhat later, a timber road mounted on stakes, many having a covering of pavement.

Roman bridges, at first made of wood, were later built of stone. Typical Roman style was a semicircular arch and short span. The bridge builder's chief problem was to provide solid foundations.

Town building was based on camp tradition, and some towns arose out of army camps. A typical town built this way was Man­chester, England.

Fusion of Roman and North European traditions was re­flected in many ways. Buildings combined the Roman arch and the steep peaked roof of Northern Europe. Roman traditions were con­tinued in an architectural form knows as Romanesque.

The first significant advance over Roman methods, however, was the invention of the ribbed vault. The ribs were built independ­ently of the wall and supported the stone-vault web.

London bridge, finished in 1209, took thirty-three years to be built. It consisted of nineteen irregular pointed arches, its piers rest­ing on broad foundations designed to withstand the Thames' cur­rent.

The use of pointed arches was another advance upon Ro­man methods, yet the medieval bridge was not as great an engi­neering achievement as was the cathedral. Providing for only one­way traffic, the typical bridge was narrow. It was not adapted to heavy vehicles.

Text 7 Composite Roof

Roof trusses built up of timber and steel members are known as composite trusses. The tensional members of trusses in compos­ite roofs are often constructed of steel, because the latter has much greater tensile strength than timber.

In open roofs steel ties may be used, but ceiling joists are more easily fixed to timber tie-beams. The principal rafters in com­posite roofs are always of timber, owing to the facility with which purlins may be fixed and to the high resistance of rectangular sec­tion of timber to compression.

Trusses designed to support the ceilings usually follow the lines of wood trusses, having the compression members in wood. Where no ceilings are required then the whole of the tentional members are formed of steel.