Passage 8A steel frameHidden behind

Passage 8
A steel frame
Hidden behind the outside walls of many large, modern structures is a framework of columns and beams. In the early days of framework construction, iron was sometimes used for the framework. But steel proved to be more durable and adaptable. Most of the famous skyscrapers in New York City were built on steel frames. Today, prestressed concrete can also be used for long bridges and medium-sized buildings.
Structural engineers design the framework of a building. They chose the type of steel to be used and decide how the framework will fit together. Most of the steel parts, such as beams and girders, are joined together before being fitted into the whole framework.
Passage 14
The most common types of foundation systems are classified as shallow and deep. Shallow foundation systems are several feet below the bottom of the building; examples are spread footings and mats. Deep foundations extend several dozen feet below the building; examples are piles and caissons. The foundation chosen for any particular building depends on the strength of the rock or soil, magnitude of structural loads, and depth of groundwater level.
The most economical foundation is the reinforced-concrete spread footing, which is used for buildings in areas where the subsurface conditions present no unusual difficulties. The foundation consists of concrete slabs located under each structural column and a continuous slab under load-bearing walls.
Mat foundations are typically used when the building loads are so extensive and the soil so weak that individual footings would cover more than half the building area. A mat is a flat concrete slab, heavily reinforced with steel, which carries the downward loads of the individual columns or walls. The mat load per unit area that is transmitted to the underlying soil is small in magnitude and is distributed over the entire area. For large mats supporting heavy buildings, the loads are distributed more evenly by using supplementary foundations and cross walls, which stiffen the mat.
Piles are used primarily in areas where near-surface soil conditions are poor. They are made of timber, concrete, or steel and are located in clusters. The piles are driven down to strong soil or rock at a predetermined depth, and each cluster is then covered by a cap of reinforced concrete. A pile may support its load either at the lower end or by skin friction along its entire length. The number of piles in each cluster is determined by the structural load and the average load-carrying capacity of each pile in the cluster. A timber pile is simply the trunk of a tree stripped of its branches and is thus limited in height. A concrete pile, on the other hand, may be of any reasonable length and may extend below groundwater level as well. For extremely heavy or tall buildings, steel piles, known as H-piles because of their shape, are used. H-piles are driven through to bedrock, often as far as 30 m (100 ft) below the surface. H-piles can be driven to great depths more easily than piles made of wood or concrete; although they are more expensive, the cost is usually justified for large buildings, which represent a substantial financial investment.
Caisson foundations are used when soil of adequate bearing strength is found below surface layers of weak materials such as fill or peat. A caisson foundation consists of concrete columns constructed in cylindrical shafts excavated under the proposed structural column locations. The caisson foundations carry the building loads at their lower ends, which are often bell-shaped
Passage 15
With low-buildings the variety of possible shapes is much greater than with taller buildings. In addition to the familiar box shape, which is also used in very tall buildings, low buildings may use cathedral-like forms, vaults, or domes. A simple single-story structure might consist of a reinforced-concrete slab laid directly on the ground, exterior masonry walls supported by the slab (or by a spread footing cast continuously around the perimeter of the building), and a roof. For low buildings, the use of interior columns between masonry load-bearing walls is still the most common construction method. Spaced columns supported by the slab or by individual spread footings may be used, however; in that case the exterior walls can be supported by or hung between the columns. If the roof span is short, abutting planking made of wood, steel, concrete, or other material can be used to form the roof structure.
The structure of a simple one-story building may also consist of the wall and roof framing combined by being either fastened together or shaped in one piece. The possible structural shapes are almost infinite and include the three sides of a rectangle fastened together into a unit called a bent, the familiar church form of vertical sides and sloping roof, the parabola, and the semicircle or dome.
The supporting structure and exterior walls, floor, and roof may also be made as a unified whole, much like a rectangular pipe with closed or open ends. These forms may be cast in reinforced plastic.
Passage 16
Exterior Walls (Facades) and Roofs
The curtain wall, the most common type of nonload-bearing wall, may be assembled either on or off the site. It consists of an exterior skin backed with insulation; a vapor barrier; sound-deadening materials; and an interior skin that may be part of the curtain wall or may be attached separately. The exterior skin may be made of metal (stainless steel, aluminum, bronze), masonry (concrete, brick, tile), or glass. Limestone, marble, granite, and precast concrete panels are also used for facades.
The traditional method of constructing a roof is to lay down, over a steel or concrete deck spanning the framing members, rolls of roofing felt laminated with tar and topped with gravel. Synthetic materials are also being used increasingly in place of felt and tar. New grasslike and ruglike materials made of plastic enable recreation areas to be built on top of roofs at little expense.
Curtain wall in medieval architecture, the outer wall of a castle, surrounding it and is usually punctured by towers or bastions. A non-load-bearing wall can be applied in front of a framed structure to keep out the weather. In 1917 Willis Polk (1876-1924) gave the Hallidie Building in San Francisco a continuous curtain wall of steel and glass which hung in front of the main frame, thus separating “structure” from “cladding”. There are now many types manufactured from a variety of materials; sections may include windows and spaces between.


Passage 20
Geodesic domes
After World War II, an American architect, Richard Buckminster Fuller, came up with a new idea for a domed structure. He designed a dome-shaped frame made of triangles of steel rods. The circular base of the dome could rest on the ground or on columns. The triangular spaces were filled in with light panels.
This kind of structure is called a geodesic dome. Its curved shape makes it pleasing to look at. But it is most useful because it holds itself up. No extra supports are needed inside.
Most geodesic domes are built to last, but they are so easy to set up and take down that they are sometimes used only for a short time. They make good halls for exhibitions, concerts, and some sports events. A geodesic dome was erected in the Philippines in just 22 hours.