Conceptually, structural systems comprise the following functional elements:
• Bearing—transfers loads to stable supporting grade, generally made up of foundations, footings, grade beams, piers
• Lifting—columns, load bearing walls, arches and other members providing vertical support
• Spanning—horizontal support of beams, girders, trusses, purlins, slabs, arches, vaults, coffers, domes, space frames, and the like; also includes roof structures of cable and membrane or pneumatically supported envelopes
• Bracing—diagonal bracing and diaphragm resistance to racking and lateral loads
Structure is fundamentally intended to provide static equilibrium, just as mechanical systems are meant to supply thermal equilibrium. Where heating, ventilating, and air-conditioning (HVAC) work to maintain a constant indoor temperature by equalizing heat losses and heat gains, structure works to transfer physical forces to supporting foundations and into supporting grade. There are some important conceptual differences of course—thermal loads are dynamic and heat flow changes direction with changes in the climate. HVAC systems use a constant source of energy to balance thermal loads and are controlled to operate in response to climatic change. Structural loads are generally static rather than dynamic; they result primarily from the uniform and unchanging force of gravity. Gravity loads consist of the dead load of the structure itself and uniformly distributed live loads imposed by the interior furnishing and occupants on the floors as well as snow loads on the roof.
There are some dynamic forces that influence structural design: wind and seismic lateral loads, for example, and imbalanced loads incurred during construction. From a systems perspective, it is interesting to note that structure must be designed for the worst set of loads that code can dictate and then left to operate at full load conditions forever, whereas HVAC capacity is designed for the 95 percent worst condition and operates at partial load in all but the worst of weather. Further, because building structure is permanent and difficult to modify, and because the structural loads are only estimates, structural systems must have large safety factors built into them. They must respond statically and continuously, even to dynamic events like hurricanes and earthquakes that may never occur. This precaution is based on the fact that failure in a structural system is potentially sudden, catastrophic, and disastrous. Failure in an HVAC system may be equally dangerous, such as involving an outbreak of Legionnaires’ disease or other indoor air quality hazards, but air-conditioning disaster is usually prevented by proper maintenance. Ongoing HVAC failures are easily remedied by modifying or replacing the system. Not so with structure, which is never bimodal and seldom dynamic. Earthquake design has opened a few possibilities for dynamic dampening of seismic loads, but these are rare exceptions (study the John Hancock Building for an example of seismic dampening).
Another issue of physical integrity that structural systems must address is fire safety. Here, the selection of wood, masonry, concrete, or steel construction must be weighed against the presence of fire suppression sprinklers or the need to protect the structural members with another layer of noncombustible material. These decisions have significant first-cost impacts and thus can reduce the amount of resources available for other design features. They also, however, involve ongoing costs associated with insurance premiums. Sometimes a more expensive structural system pays for itself in the reduction of insurance costs.
Structural systems selection, then, generally involves the choice of the lightest-weight members of the most eco – nomical-grade material, allowing the most efficient configuration that is appropriate to the anticipated loads. Taken in isolation, there is nothing architectural about this procedure; it is pure engineering. In concert with other systems, however, structure acts in visually expressive ways. It creates a grid and rhythm with the envelope system, for example, through modular repetition of horizontal and vertical supports. It creates an open frame or a closed shell for the envelope to infill. Exposed to the interior, it modulates space and orders the plan. Left clear, structure contains and organizes service elements such as ductwork and lighting.
In all, the visual expression of building structure and its intentional integration with other major systems is part of what Kenneth Frampton (1995) calls the “tectonic order.” This ordering is primarily an ennoblement of how architects think about and through the construction of a building while fully considering the methods and materials to be employed. It arises from a long tradition of the challenge of structural problems before the advent of numerical method. Structure, as a means of architectural clarity, is also associated with the human sense of stability that comes from the everyday experiences of gravity: remaining upright on a top-heavy two-legged base and walking around by continually falling forward. This lifelong experience of gravity gives the structural expression of buildings an easy communication with our intuitive senses — if it looks as though it will stand up, it probably will. Comparing this to HVAC systems again, it is clear that we have no such intuitive sense about systems of thermal equilibrium. It is hard to intuitively judge the adequacy of a mechanical system by the physical size of its components.
Another visually accessible aspect of structural systems is the graphic depiction of loads, vectors, and the resultant forces in members. These diagrams take the familiar form of free-body moment and reaction, shear and funicular diagrams. At the scale of a building they become tributary load diagrams that can be used to generate the basic geometry of the structure. These graphic illustrations connect the visible geometry of structure to the invisible geometry of imposed forces. Because the illustrations are literal, they give the structure its necessary geometry. Antonio Gaudi (1865-1934) used graphic methods like these to work out the form of the huge dome over the Segrada Familia in Barcelona, Spain (begun in 1884 and still in progress). An upside-down, three-dimensional model of the dome was constructed to scale in his workshop. Small bags of lead weights were then attached to the load points to reveal the regular parabolic curves of the structure. Later, however, when Jorn Utzon proposed the irregularly curved form of roofs in his competition scheme for the Sydney Opera House (1957-1973), there was no similar empirical method for resolving the indeterminate loads. Ove Arup and his team of engineers in London, including Peter Rice, had to write massive computer programs to resolve the transfer of loads through irregular shapes. In the end, Utzon found it preferable to base all the shells at Sydney on a radius of 246 ft (75 m).
Because gravity is static, uniform, and constant, and because the dynamic forces acting on buildings are assumed to act uniformly on any surface with equal probability, certain consistencies are manifested in structural design. The first of these, hierarchical consistency, results from the cumulative transfer of loads from top to bottom of a building and from intermediate members to primary members. In terms of a tributary load diagram, horizontal structure grows larger from purlin to joist to beam. Vertical structure grows in size as loads from above accumulate in columns and load-bearing walls transferring loads to grade. The second result of the uniformity of loads is consistency of pattern. Uniform loads dictate uniform member size and uniform spacing. Uniform member size and spacing in turn creates patterns within the hierarchies: purlin spacing, joist spacing, beam grids, column modules, and so forth.
From the perspective of physical integration, structure is often made to contain the service systems. At one scale, the interstitial space between floor and ceiling layers in a building normally carry the horizontal distribution of services: HVAC ducts, electrical wiring, and recessed lighting systems. Similar but usually smaller voids in walls can carry services as well. At another scale, hollow structural members are stronger in bending than solid members of the same diameter. This allows the voids in individual members to be used for the distribution of services. Shaped steel beams, box beams, tubular frames, and hollow-core slabs are all appropriate conduits for the distribution of service elements.
Functionally, structure can often lend its massive materials or its void interstitial spaces to the mandates of other systems. Return or supply air plenums are easy examples. This is an elaboration of physically integrated ducts to the extent of structure’s actually becoming a duct rather than just housing it. Such plenums are now frequently found in ceiling or floor voids and occasionally in wall cavities.
A more dynamic example of performance integration in structural systems is the use of structural mass as thermal heat capacity. Passive heating systems normally use floor slabs as thermal batteries to store absorbed solar energy and limit interior temperature swings. In hot, arid climates, external envelope building mass is used for its flywheel effect to effectively balance the impacts of hot days followed by cool nights. Night ventilation cooling is also used in both surface-load-dominated and internal- load-dominated buildings to provide a passive cooling heat sink for daytime heat gains. In some instances, internal thermal mass in a supply air plenum is cooled continuously by the HVAC system in order to lower the peak cooling load.