Read the text about different building materials. Compare them and say which one is more suitable for permanent or temporary structures.

Early bridges were made of timber and stone because those building materials could be easily found anywhere (fig. 6.1, 6.2.). Materials were free and abundant, and there were low labour costs, since slaves, soldiers, or natives did the work. The Romans built many wooden structures, but their reputation rests on masonry bridges. Roman bridges are famous for using the circular arch form, which allowed for much longer spans and provided more permanence than wood. Stone is strong in compression but weak in tension. Its primary application has been in arches, piers, and abutments. Besides, stone is durable, weatherproof and resistant to freezing and thawing. The most suitable rock for this purpose is granite, basalt, dolomite and the widespread and rather cheap sandstone or limestone. The bridge builders also used artificial stone, i.e. concrete made from a mixture of water, sand, gravel, and a binder, such as cement. Concrete is strong in compression and weak in tension.

The Romans used cement for piers in rivers. They developed a cofferdam, a temporary enclosure made from wooden piles driven into the riverbed to make a sheath, often sealed with clay. Concrete was poured into the water within the ring of piles. Although most surviving Roman bridges were built on rock, the Pont Sant’Angelo Bridge, the finest surviving bridge in Rome, stands on cofferdam foundations, built in the Tiber River 1, 800 years ago. It consists of seven stone arches and five main spans about 18 m each, supported on piers seven meters high. Mounted on the parapets are statues of Saint Peter and Paul and ten statues of angels.

Concrete with steel bars embedded in it is called reinforced concrete. The steel carries all the tension; the concrete protects the steel from corrosion and fire. Reinforced concrete has become an economical substitute for stone. In addition to its price and load-carrying advantages, reinforced concrete can be molded into a variety of shapes, allowing for much creative expression.

Masonry bridges offer the following advantages: long-lasting performance (only wars and earthquakes can destroy them); the aesthetic merits of masonry bridges adorn many urban landscapes; greater ability to resist overloads; considerable reduction of maintenance costs. Possible disadvantages of masonry bridges are: greater dead weight as stone density is between 2 and 2.7 t/m³; only the arch structure is allowed (it produces the horizontal force – the thrust, which requires powerful foundations and solid ground to rest on (fig. 6.1); construction of masonry bridges requires a lot of manual labour.

Wood is relatively weak in both compression and tension, but it has usually been widely available and is inexpensive. It has been used effectively for small footbridges. Nevertheless, in the 18^th century, designs with timber reached new span length, and Swiss builders used trusses with spans of 58 m over the Rhine River. Engineers now incorporate laminated wooden beams and arches into some modern bridges. Timber bridges usually serve as short-term structures lasting 10-15 years (fig. 6.2). Timber piles and cribs are often used for bridge foundations. The best types of wood used for spans and piers are pine, spruce, fir, and other softwoods such as larch or cedar. This sound wood is easily treated and does not decay. Expensive wood species such as oak, hornbeam and beech are applied only to the most important elements – the caps and dowels. To increase its waterproof capability timber is impregnated with antiseptics. It extends the service life up to 25-30 years. Plywood spans are widely used because they are more durable, rigid and lighter than those made of logs and squared beams.

Exercises:

Try to guess the meaning of the following word combinations:

Frost-resistant building material; corrosion-resistant metal; rot-resistant timber; fire-resistant covering; water-resistant concrete; wind-resistant structure; torsion resistant bridge element.