ROOF AND ROOFING
The covering of the top of a building, or roof, consists of three major components: the
structural framing members; a stiff membrane, or roof deck, spanning structural members;
and a waterproof outer layer of roofing. The earliest roofing materials were probably mud
and sod supported by logs and woven reeds. The early Greeks and Romans
manufactured kiln-dried clay tile to cover their public buildings and dwellings; clay tile is
still a popular roofing material, particularly in warmer climates. Bundles of straw or reeds
tied to horizontal framing timbers formed the roofing for the thatched cottages in medieval
Europe, and slate was used throughout the Middle Ages as a covering for the great
cathedrals of Europe. By the 1500s copper sheets were pounded out by hand and used in
limited quantities as roofing, and in 1750 the first copper sheets were rolled. After that time
much of the slate and tile roofing was replaced by copper, which is still widely used to
produce a long-lasting roofing surface. Sheet lead has been used as a roofing material for
centuries.
Roofing shakes are split from straight-grained woods such as red cedar; wooden roofing
shingles are sawed into a uniform size. Both are popular roofing materials, although in
some areas fire restrictions bar the use of untreated wooden roof coverings. Metals such
as steel, aluminum, and copper are used as flat or corrugated sheets, or are formed to
resemble clay or slate tiles. Metal roofing is made with joints that allow for expansion and
contraction during temperature changes.
The bitumens asphalt and coal tar pitch came into general use as roofing materials after
1892, when a chemist developed an asphalt-impregnated roofing paper. Both substances
are durable, malleable, and provide a high degree of waterproofing. Roll roofing and
asphalt shingles are made from heavy felted materials composed of shredded wood, rag,
or glass fibers that have been saturated with bitumens. To form a finished roof, roll roofing
is nailed or fastened to the roof deck--the solid, usually plywood surface laid over the
wooden slats of the frame--with hot bitumens or an adhesive. Asphalt shingles are nailed
or stapled to the roof deck. A "built-up" roof is produced by applying alternate layers of
roofing felt and hot asphalt or pitch. The top layer is given a hot flood coat of the bitumen;
granules of rock, gravel, slag, or ceramic particles may be embedded while the flood coat
is still hot. Roofs with a comparatively low slope are sometimes sprayed with a
polyurethane foam that is coated with gravel when it hardens.
ROADS AND HIGHWAYS
A road or highway may be defined as an overland route between two points. To clarify
some terminology, a road is generally a narrow route in a rural area; a street is an urban
route; and a highway is a wide road that can carry more traffic at higher speeds. In the
United States, a major highway--a multilane divided roadway with limited access (relatively
infrequent entrances and exits)--is called an expressway.
France is generally credited with the first successful laying of pavements of asphalt mastic
in the early 19th century. In the middle of the century both France and England used
compressed rock asphalt for city streets, and in the United States, bitumen (a natural
asphalt) was used. The rapid growth of motoring in the early 20th century encouraged the
use of asphalt because it produced a dustproof surface, but all the forms then in existence
were apt to become slippery and cause skidding when wet. From about 1929, while a
surface was being constructed and the material was still hot, a dressing of bituminouscoated chippings was added, which gave the asphalt road a rough surface.
A great asset to the modern highway engineer has been the development of aerial
photography which greatly assists the preliminary ground surveys for new roads and the
planning of routes on the map. Before construction of a road begins, estimates are made
of the likely cost of possible alternatives, taking into account the geological features,
economic needs of the area, and possible damage to the environment. The expensive
stage of earth moving must be kept to the minimum, and in areas where a road
necessarily has an undulating course, the aim is to balance the cut (the excavations)
against the fill (the embankments). Detouring around natural obstacles, such as difficult
strata or unstable subsoil conditions, is sometimes preferable to constructing a straight
road.
Large numbers of laborers built the railroads; in road construction, however, much of the
work is done by sophisticated machinery. The process starts with the loosening of the
topsoil by a scraper; next, a bulldozer pushes the loosened earth to one side, so that the
line of the road is clear. At this point the road is discernible from the air, even though it is
nowhere near its final form. Excavations may have to be made, embankments
constructed, and sharp curves straightened out. Once the road has its predetermined line
and gradients, excavation can begin for the foundations, which must be capable of bearing
the expected weight of traffic on the road. Since the mid-century it has become possible in
some situations to stabilize the subsoil itself rather than create a foundation of crushed
rocks or concrete, provided that the subsoil is fairly uniformly graded. Cement, lime, or
bitumen is used as an additive. After the subsoil has been loosened, a binding agent is
added, and the mixture is watered, tamped down, rolled, and, when hard, covered with a
waterproof surface.
Above the foundation, or stabilized soil, comes a base course, usually of concrete, and
then the top surface, or pavement, of asphalt or concrete. Shuttering--the use of wooden
forms for support--is usually needed to hold concrete in position until it sets, but machines
are now available that make this step unnecessary by producing firmly compacted
concrete. Concrete can be laid by a spreader that runs on tracks; this step can be followed
with the laying of a steel-mesh reinforcement by a crane. Another spreader then puts on
the top surface, and a vibrator compacts the surface. Another machine roughens the
surface and checks that the camber--which is the slight arch upward in the middle of the
road--is proper for good drainage.
FIRE SAFETY
In the United States more than 2.4 million fires occur each year, causing thousands of
deaths (some 6,000 annually), hundreds of thousands of injuries, and billions of dollars in
property damage. Despite a dramatic drop in recent years, the U.S. rate of fire-caused
deaths is the highest of the industrialized nations. Comparative figures are telling:
Chicago, half the size of Hong Kong, suffers triple the number of fire-caused deaths;
Baltimore has 13 times as many deaths by fire as Amsterdam, a city of the same size.
Until the mid-18th century, city dwellers had only the fire watchman and the volunteer
bucket brigade to protect them from fires. Benjamin Franklin organized the first permanent
fire company in 1736 in Philadelphia; New York City followed suit in 1737. By the late
1800s, however, a series of devastating fires had claimed thousands of lives and
destroyed whole sections of many U.S. cities. The principal causes for these
conflagrations were poor building construction and shoddy materials, insufficient water
supplies, and a fragmented system of fire fighting that created fierce rivalries between
volunteer fire departments, which often fought one another rather than the fires they were
called to put out.
Modern fire communications systems range from the pull-alarm in street boxes that alert a
local department to a fire's location, to voice boxes from which a caller can talk directly
with the department, to radio systems between departments.
Throughout the industrialized world, the basic fire-fighting vehicle is a self-propelled truck,
the fire engine, adapted for a variety of functions and carrying an extensive assortment of
tools and equipment: pumps, hoses, water tanks, ladders, and portable tools and
appliances. The type of engine equipment will vary with the intended service. A pumper
has a hose, water tanks, and pumps and is used as an auxiliary source of water at a fire. A
ladder truck houses the long aerial ladders used for rescue and fire fighting and also
carries such special equipment for rescue and salvage operations as breathing
apparatuses and tools for forcible entry.
Other fire-fighting equipment that may be brought to the scene of the fire, or fireground,
includes the portable fire extinguisher, radio equipment, electric lights and generators,
portable pumps, and water-additive equipment to make foam for smothering fires burning
in flammable liquids. Minimum protective equipment for the fire fighter includes a suitable
helmet, protective coat, boots, and gloves.
Well over three-quarters of all U.S. homes now have at least one smoke detector, and
local fire ordinances usually require the installation of detectors in all new homes. Heat
and fire sensors are additional devices for detecting fire and alerting people to the danger.
The most effective means of controlling fire in large buildings are automatic fire sprinkler
systems. Water pipes are positioned behind the ceilings; at intervals, sprinkler heads
protrude into the room. Sufficient heat destroys the seals in the sprinklers, releasing a
steady stream of water. The newest sprinklers have automatic shutoff valves and may be
equipped to transmit fire alarms to local fire departments.
Water is still the most commonly used extinguishing agent. It cools the burning material
and smothers the fire. For special situations, however, other extinguishing agents are
more effective. Fire-fighting foam is used primarily to fight fires involving flammable liquids.
Carbon dioxide suppresses fires involving gas, flammable liquids, electrical equipment,
and ordinary combustible material, such as paper and wood.
Three principal elements determine degree of fire hazard of a building: its structure, its
contents and the interior finishes. Every element of the structural framework must be
planned with fire safety in mind. The interior finishes, such as wood paneling, wallboard,
acoustical tile, furniture, and carpeting fabrics must all be evaluated for their flammability
characteristics. In many cases, local codes give minimum fire-resistance standards for
interior-finish materials; many materials carry flammability information.
A building's contents may also determine the type and severity of a fire. Local fire
departments should be made aware of the storage of volatile chemicals or other highly
flammable materials, or of products that produce toxic gases when they burn.
CONTEMPORARY WOODEN HOUSE CONSTRUCTION
In terms of its basic construction, a contemporary house is composed of a foundation; the
framing, or superstructure; an exterior skin; interior finishes; electrical wiring; heating
systems; and plumbing.
The base of the foundation, the footing, must be sunk below the frost line--the depth to
which the ground freezes--to insure that it will not be moved by prolonged frost. The
foundation walls are usually made of poured concrete or concrete block, and waterproofed below ground level. If the surrounding soil is poorly drained, drainage tile is used
to divert underground water away from the foundation. Anchor bolts are set into the top of
the foundation wall and are used to anchor the wooden house frame to the foundation.
In a conventional frame house, the frame--the skeleton that supports all the major
elements of the house--is almost always of wood, usually of relatively small dimensions: 2
by 4 in., 2 by 8 in., or 2 by 12 in. in various lengths. The frame is fastened to the
foundation walls by the anchor bolts; door and window frames, siding, roof covering, and
flooring are fastened to the frame.
Although framing nomenclature varies widely in different areas, it includes several
universal terms for those framing members which are present in almost every house.
The sill plate is the wood plank that is anchored directly to the foundation wall and
supports the exterior house wall. The roof plate anchors the roof rafters to the house
frame.
Posts or corner studs are the main vertical supports of the frame.
Studs are smaller vertical members and provide support for exterior siding and interior
paneling or wallboard.
Braces are diagonal members used to brace the studs.
Girders, or beams--often of steel--are horizontal members that carry the weight of the
house.
Joists support the weight of the floor and ceiling.
Girts and plates are horizontal ties holding the frame together at the second floor level and
on top of the studs at roof level.
Headers are members placed over a door or window opening. They are used to support
the ends of studs that have been cut off to make the opening.
Rafters provide support for the roofing material.
Framing practices differ according to the type of house being built. Conventional eastern,
or braced, framing is the oldest framing type and is characterized by the use of solid
corner posts and studs that run the full height of the house from foundation to roof. In
western, or platform, framing, each floor level acts as a platform for the posts and studs
above it.
Floor framing consists of joists strengthened by short stiffening members, or bridging.
Rough flooring, or subflooring, may be plywood or rough boards laid diagonally over the
joists; the actual, or finish, floor--wood, vinyl, or tile--is then laid over this substructure.
Roof framing differs according to the shape of the roof. The most common shape is the
gable, or pitch, roof, which is a simple triangular section: the two sloping sides meet at the
center, or ridge. Most roof shapes are variations of the gable. Roof frames consist of
rafters that form the support for the roof covering. They are attached to the roof plate and
slant upward to meet the ridge board. They may be reinforced by interior braces.
Interior walls, or partitions, are made up of studs covered with panels. The hollow space
left within the wall will contain some of the plumbing, electrical wiring, and ductwork for
certain types of heating and air-conditioning systems. Other parts of these systems will be
run through the exterior walls, the floors, and the ceilings.
Finish flooring and ceilings are now put in place. Interior trim such as doors, stairs,
baseboards, and moldings is installed, along with finish plumbing and electric units:
fixtures, switches, radiators, sinks, tubs, and so on.
Prior to exterior finishing, insulating materials are placed over or between the studs.
Exterior plywood sheathing is then nailed over the studs, followed by building paper and
the exterior finish material.
WALLS
The two types of wall are (1) load bearing, which supports the weight of floors and roofs;
and (2) non-bearing, which at most supports its own weight.
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
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 multistorey 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 as well. 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 joined 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.
The non-bearing wall (excluding the independent garden variety) 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, wood products, etc., can be equally efficient. Thus freedom of
choice extends also to the form of walls and offers greatly expanded opportunities for
creative expression.
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.
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 sort, while
lintels in materials that are strong in bending, such as steel, may span 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 slabs) 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 post than as a lintel; under heavy load 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.
STRUCTURES
A structure is a part of a building that carries its weight. We should also remember that
anything built is a structure. A structure may be a dwelling house or a pyramid in Egypt, or
a dam built across a Canadian river. A building is a structure with a roof, and much of civil
engineering structural design is the design of building structures.
Every structural design includes the foundation design. The structural design itself includes
two different tasks, the design of the structure in which the sizes and locations of the main
members are settled, and the analyses of this structure by mathematical or graphical
methods or both, to work out how the loads pass through the structure with the particular
members chosen. For a common structure such as a building frame, many methods have
been developed for analysis.
For the typical multi-storey structure in a city, whether it is to be used for offices or
dwelling, the most important member to be designed is the floor for two reasons: it repeats
all the way up the building and it has the greatest effect on the dead load of the building.
The dead load, in fact, can be fairly exactly calculated by assuming that the floors are the
only dead load. These floors are generally of reinforced or prestressed concrete because
they resist fire better than steel or wood, which is an important consideration in a tall
building. There are two main types, the solid floor and the hollow-tiled floor (or ribbed). In
the latter type of floor part of the lower half of the slab is hollow: it is a great advantage,
because this concrete would not strengthen the floor but would be heavy. Ribbed floors
are therefore lighter than solid floors but it is more difficult to cast them with holes through
them unless these holes are carefully planned beforehand.
Suspended structures are among the most interesting structures. The first large ones were
built in London and other great cities in the sixties. In all these structures, the columns or
stanchions are made fewer and larger so as to reduce the buckling effect on them and to
increase their effective length. In two that were erected in London there is only one column
in the centre of the building, and this is a hollow concrete tower some 12 m square which
carries the lifts, stairs; ducts, pipes and cables are within it or attached to its wall. The
tower may be called the core of the building and on its top is a bridge overhanging in all
directions, from which high-tensile steel bars drop to carry the floors below. These bars
are very thin and can be hidden in a door frame or window frame so that for such a
building there need be no noticeable obstruction to sight or horizontal movement in any
direction outward the core.
But this is only the beginning of suspended construction. If it is successful and if the
world’s large cities continue to become more crowded the idea will grow. It seems possible
that the whole city may be a few of these vast buildings, carried on pairs of towers, joined
by light weight bridge structures, possibly suspension bridges. To reduce sway and
buckling, the columns will be massive, and the floors will hang from the bridges by thin
high-tension steel suspenders in the same way as a suspension bridge deck hangs from
its cables.
FOUNDATIONS
Foundation Ioads and pressures. Foundations should be designed to support the
weight of the structure, live load and wind load and also the following loads when they are
present: earth pressure, weight of fill on elements of the structure, traction forces,
centrifugal forces, snow and ice, seismic forces, hydrostatic and hydrodynamic loads, and
surcharge or load transmitted to the supporting soil other than through structural
components. Also the design should provide for the moments produced by the loads.
Lessons from failures. When foundations give trouble, the cause usually is one or more
of the following: inadequate site investigation, improper interpretation of exploration
findings, faulty foundation design, and poor workmanship during construction, and
insufficient provision for the effects of exceptional thermal and biological conditions,
rainfall, and floods. Analyses of reported foundation failures indicate that, in addition to the
precautionary steps necessary for good, safe superstructure design and construction,
certain steps are required to ensure good, safe and economical design and construction.
These include the following:
Make a thorough site investigation. This applies to small as well as large structures.
Experience shows that there is more trouble with one-storey buildings, such as houses,
shopping centers, and small factories, than with major structures, because many
engineers do not consider a small building to be heavy enough to settle.
See that soil samples are properly extracted and handled. Take special care with those for
cohesionless or stratified soils.
Determine whether groundwater is present. If it is, determine the elevation of the
groundwater table and estimate future variations in level.
Investigate the effects of uplift, seepage and water flow.
Design foundations to avoid differential settlement, since such settlements may seriously
affect the structure, or take them into account in design.
Select a foundation appropriate both for the superstructure to be supported and the soil it
rests on.
Investigate the possibility of stress superposition on the soil under a foundation causing
overstressing. Such a condition may arise when footings are placed so close that high
pressures from one overlap those from others.
When a structure is to be supported on inclined soil strata, consider the possibility of the
soil movement under the load of the structure.
Either avoid foundation construction that may seriously affect adjacent structures or
provide adequate support for them, both during and after construction.
Keep construction under close supervision to prevent shoddy work, unsuitable methods
and use of inadequate equipment. See that excavations are adequately braced. Bracing
should resist lateral pressure of retained earth, nonuniform pressure distribution from
different soil layers, vibrations from vehicles and machines, and other loads that may
induce oblique stresses.
Elements of a Building
The major elements of a building include the following: (1) the foundation, which supports
the building and provides stability; (2) the structure, which supports all the imposed loads
and transmits them to the foundation; (3) the exterior walls, which may or may not be part
of the primary supporting structure; (4) the interior partitions, which also may or may not be
part of the primary structure; (5) the environmental-control systems, including the heating,
ventilating, air-conditioning, lighting, and acoustical systems; (6) the vertical transportation
systems, including elevators, escalators, and stairways; (7) communications, which may
include such subsystems as intercommunications, public address, and closed-circuit
television, as well as the more usual telephone-wiring systems; and (8) the power, water
supply, and waste disposal systems.
Environmental Control
Perhaps the greatest improvements in building construction have been in heating, cooling,
ventilation, lighting, and sound control. In most large buildings complete, year-round air
conditioning is now standard. Some areas in a building may need to be cooled even in
winter, depending on the distance from exterior walls and the heat generated by lighting,
electrical equipment, or human occupancy. The level and quality of lighting have improved
greatly. Largely as a consequence of these changes, the cost of the mechanical and
electrical systems in buildings has increased at a greater rate than other individual building
costs; such costs currently account for a quarter to a third of total construction expenditure.
Increasingly since the late 1970s all these systems are automatically operated and
controlled by computers that are programmed to maximize efficiency and minimize waste
and energy consumption.
Communications and Power Systems
The growing use of power, telephone, and facsimile transmission equipment and of closed
circuit television, intercommunication, and security and alarm systems has increased the
amount of wiring that must be installed in buildings. Main cables run vertically in open
shafts, with branches at each floor running through conduits located either in the hung
ceiling space or embedded in the floor slab itself.
The electrical power required in buildings has increased with the number and complexity of
environmental systems in operation. Because a power outage cannot be tolerated,
emergency power generators are installed in an increasing number of buildings. Some
buildings, particularly in remote locations, are equipped with their own primary electrical
generating systems. Diesel and gas turbine generators are used. The heat generated by
these engines, instead of being wasted, is sometimes utilized for other purposes within the
building.
Vertical Transportation
Elevators, especially high-speed, automatically controlled, cable-operated elevators, are
the major form of vertical transportation in high-rise structures. Low-rise buildings and the
lower floors of commercial buildings may also have escalators. For fire protection, it is
necessary to provide at least two means of egress from every major space in a building.
Therefore, in addition to elevators and escalators, all buildings, even the tallest, have two
enclosed and protected stairways for their entire height.
Water Supply and Waste Disposal
Buildings must have a piped-in water supply for a variety of purposes: drinking, washing,
cooking, waste disposal, internal fire fighting (either through standpipes and hoses or
through automatic sprinklers), and service to air-conditioning systems or boilers.
Disposal of wet and dry wastes in buildings is accomplished by a variety of devices, such
as incinerators, shredders, and garbage compactors. There are also devices that assist
waste-pickup and disposal systems. The usual method of carrying away waterborne waste
is through piping connected to the sewer system outside the building. New technology is
aimed at recycling water to reduce waste and pollution.
Building construction
Building construction means procedures involved in the erection of various types of
structures. The major trend in present-day construction continues away from handcrafting
at the building site and toward on-site assembly of ever larger, more integrated
subassemblies manufactured away from the site. Another characteristic of contemporary
building, related to the latter trend, is the greater amount of dimensional coordination; that
is, buildings are designed and components manufactured in multiples of a standard
module, which drastically reduces the amount of cutting and fitting required on the building
site. A third trend is the production or redevelopment of such large structural complexes as
shopping centers, entire campuses, and whole towns or sections of cities.
A proliferation of building types reflects the complexity of modern life. The attention is more
and more focused on the overall qualities manifested by aggregates of buildings and parts
of cities as being more significant than individual structures. As the total building stock
grows, conserving buildings and adapting them for changes in use becomes more
important. It is also necessary to take into account the internal functional equipment of
modern buildings. In recent decades, elaborate systems for vertical transportation, the
control of temperature and humidity, forced ventilation, artificial lighting, sanitation, control
of fire, and the distribution of electricity and other services have been developed. This has
added to the cost of construction and has increased expectations of comfort and
convenience.
When masonry materials are stacked vertically, they are very stable; every part is
undergoing compression. The real problem of construction, however, is spanning. Ways
must be found to connect walls so as to provide a roof. The two basic approaches to
spanning are post-and-lintel construction and arch, vault, and dome construction. In postand-lintel construction, lintels, or beams, are laid horizontally across the tops of posts, or
columns; additional horizontals span from beam to beam, forming decks that can become
roofs or be occupied as floors. In arch, vault, and dome construction, the spanning
element is curved rather than straight. Arches may be supported by piers or columns to
form an arcade, half-cylinder (or barrel) vault, or hemispherical dome.
Post-and-lintel solutions can be executed in various materials, but gravity subjects the
horizontal members to bending stress, in which parts of the member are in compression
while others are in tension. Wood, steel, and reinforced concrete are efficient as beams,
whereas masonry, because it lacks tensile components, requires much greater bulk and
weight. Vaulting permits spanning without subjecting material to tension; thus, it can cover
large areas with masonry or concrete. Its outward thrust, however, must be counteracted
by abutment, or buttressing. Trussing is an important structural device used to achieve
spans with less weighty construction. Obviously, a frame composed of three endconnected members cannot change its shape, even if its joints could act as hinges.
Fortunately, however, the principle of triangulation—attaching a horizontal tie beam to the
bottom ends of two peaked rafters—can be extended indefinitely.
Spanning systems of almost any shape can be subdivided into triangles, the sides of
which can be made of any appropriate material—wood, rolled steel, or tubing—and
assembled using suitable end connections. Each separate part is then subject only to
either compressive or tensile stress. In the 18th century, mathematicians learned to apply
their science to the behavior of structures, thus making it possible to determine the
amounts of these stresses. This led to the development of space frames, which are simply
trusses or other elements arrayed three-dimensionally. Advances in the art of analyzing
structural behavior resulted from the demand in the 19th century for great civil engineering
structures: dams, bridges, and tunnels. It is now possible to enclose space with
suspension structures or pneumatic structures, the skins of which are held in place by air
pressure. Sophisticated analysis is particularly necessary in very tall structures, because
wind loads and stresses that could be induced by earthquakes then become more
important than gravity.