The two types of wall are load bearing, which supports the weight of floors and roofs, and nonbearing, which at most supports its own weight. Load-bearing wall
The load-bearing wall of masonry is thickened in proportion to the forces it has to resist: its own load, the load of floors, roofs, persons, etc., and the lateral forces of arches, vaults, wind, etc., that may cause it to crack or buckle. Its thickness often can be reduced at the top, because loads accumulate toward the base; in high buildings this is done by interior or exterior setbacks at the floor level of upper stories. Walls that must resist lateral forces are thickened either along the whole length or at particular points where the force is concentrated. The latter method is called buttressing. Doors and windows weaken the resistance of the wall and divert the forces above them to the parts on either side, which must be thickened in proportion to the width of the opening. In multistory buildings, windows—unless they are very small—must be placed one above the other so as to leave uninterrupted vertical masses of wall between them to transfer loads directly to the ground. The number of openings that can be used depends on the strength of the masonry and the stresses in the wall. Walls in light, wood-framed structures and in reinforced-concrete construction may have a bearing function also. But the nature of the material admits other means of resisting forces than the increase of mass.
The placement of walls is determined by the type of support for floors and roofs. The commonest support is the beam, which must be jointed to walls at both ends; consequently, its maximum permissible length establishes the distance between bearing walls. All floors and coverings are most easily supported on straight, parallel walls except the dome (see below Dome). Nonbearing wall
Excluding the independent garden variety, the nonbearing wall appears only where loads are carried by other members, as in heavy timber and other skeletal structures. Modern steel and reinforced-concrete frames require exterior walls only for shelter and sometimes dispense with them on the ground floor to permit easier access. Since the wall rests or hangs upon members of the frame, it becomes a curtain or screen and admits treatment in any durable, weather-resisting material. Traditional materials are often used, but light walls of glass, plastic, metal alloys, wood products, etc., can be equally efficient. This freedom of choice extends also to the form of walls and offers greatly expanded opportunities for creative expression. Post-and-lintel
The simplest illustration of load and support in construction is the post-and-lintel system, in which two upright members (posts, columns, piers) hold up a third member (lintel, beam, girder, rafter) laid horizontally across their top surfaces. This is the basis for the evolution of all openings. But, in its pure form, the post-and-lintel is seen only in colonnades and in framed structures, since the posts of doors, windows, ceilings, and roofs are part of the wall.
post-and-lintel systemStonehenge, an example of early post-and-lintel construction.Kristian H. Resset
The job of the lintel is to bear the loads that rest on it (and its own load) without deforming or breaking. Failure occurs only when the material is too weak or the lintel is too long. Lintels composed of materials that are weak in bending, such as stone, must be short, while lintels in materials that are strong in bending, such as steel, may span far greater openings. Masonry lintels are inefficient because they must depend on the cohesiveness of mortar, which is weaker than the blocks it bonds; so, in masonry construction, lintels of monolithic (single-slab) stone, wood, and stronger materials are employed.
The job of the post is to support the lintel and its loads without crushing or buckling. Failure occurs, as in lintels, from excessive weakness or length, but the difference is that the material must be especially strong in compression. Stone, which has this property, is more versatile as a post than as a lintel; under heavy loads it is superior to wood but not to iron, steel, or reinforced concrete. Masonry posts, including those of brick, may be highly efficient, since the loads compress the joints and add to their cohesiveness. Although monolithic stone columns are used, they are extravagant to produce for large structures, and columns are usually built up of a series of cylindrical blocks called drums.
From prehistoric times to the Roman Empire, the post-and-lintel system was the root of architectural design. The interiors of Egyptian temples and the exteriors of Greek temples are delineated by columns covered by stone lintels. The Greeks opened their interior spaces by substituting wooden beams for stone, since the wood required fewer supports. The development of the arch and vault challenged the system but could not diminish its importance either in masonry construction or in wood framing, by its nature dependent on posts and beams.
Ancient uses of the post-and-lintel were refined but not fundamentally altered until the production of cast-iron columns, which, offering greater strength and smaller circumference, greatly reduced the mass and weight of buildings. Much construction in modern materials is based on the post-and-lintel system of the past. Steel and concrete skeletons restore to modern architecture the formal simplicity of the oldest structures known. But, because they are rigid frames, they abandon the fundamental concept of the duality of post-and-lintel by fusing them into a unit throughout which stresses are distributed. The “mushroom” column is a further departure, since the unit can be extended into a covering slab and becomes a ceiling as well as a support.
“Mushroom” column with fountain, supporting a cantilever, Museo Nacional de Antropología, Mexico City, by Pedro Ramírez Vázquez and Rafael Mijares, 1964.Victor Englebert
Arch
The arch can be called a curved lintel. Early masonry builders could span only narrow openings because of the necessary shortness and weight of monolithic stone lintels. With the invention of the arch, two problems were solved: (1) wide openings could be spanned with small, light blocks, in brick as well as stone, which were easy to transport and to handle; and (2) the arch was bent upward to resist and to conduct into its supports the loads that tended to bend the lintel downward. Because the arch is curved, the upper edge has a greater circumference than the lower, so that each of its blocks must be cut in wedge shapes that press firmly against the whole surface of neighbouring blocks and conduct loads uniformly. This form creates problems of equilibrium that do not exist in lintels. The stresses in the arch tend to squeeze the blocks outward radially, and loads divert these outward forces downward to exert a resultant diagonal force, called thrust, which will cause the arch to collapse if it is not properly buttressed. So an arch cannot replace a lintel on two free-standing posts unless the posts are massive enough to buttress the thrust and to conduct it into the foundations (as in ancient Roman triumphal arches). Arches may rest on light supports, however, where they occur in a row, because the thrust of one arch counteracts the thrust of its neighbours, and the system will remain stable as long as the arches at either end of the row are buttressed by walls, piers, or earth.
The size of arches is limited only by economy; large arches exert large thrusts, and they are hard to buttress and to build. The form may be varied to meet specific problems; the most efficient forms in masonry are semicircular, segmental (segment of a circle), and pointed (two intersecting arcs of a circle), but noncircular curves can be used successfully.
Arches were known in Egypt and Greece but were considered unsuitable for monumental architecture. In Roman times the arch was fully exploited in bridges, aqueducts, and large-scale architecture. New forms and uses were found in medieval and particularly Gothic architecture (flying buttress, pointed arch), and Baroque architects developed a vocabulary of noncircular forms for expressive reasons. Steel, concrete, and laminated-wood arches of the 20th century changed the concept and the mechanics of arches. Their components are completely different from wedge-shaped blocks (voussoirs); they may be made entirely rigid so as to require only vertical support; they may be of hinged intersections that work independently, or they may be thin slabs or members (in reinforced concrete) in which stresses are so distributed that they add the advantages of lintels to those of arches, requiring only light supports. These innovations provide a great freedom of design and a means of covering great spans without a massive substructure.