Types of Loads

As described above, structures must be proportioned so that they will not fail or deform excessively under the loads they may be subjected to over their expected life. Therefore, it is critical that the nature and magnitude of the loads they may experience be accurately defined. Usually there are a number of different loads, and the question as to which loads may occur simultaneously needs to be addressed when specifying the design loading. In general, the structural engineer works with codes, which specify design loadings for various types of structures. General building codes such as the “International Building Code”  specify the requirements of governmental agencies for minimum design loads for structures and minimum standards for construction. Professional technical societies such as the American Society of Civil Engineers (ASCE), the American Concrete Institute (ACI), the American Institute of Steel construction (AISC), and the British Standards Institute (BSI) publish detailed technical standards that are also used to establish design loads and structural performance requirements. In what follows, we present an overview of the nature of the different loads and provide a sense of their relative importance for the most common civil structures

Source of Loads

Loads are caused by various actions: the interaction of the structure with the natural environment; carrying out the function they are expected to perform; construction of the structure; and terrorist activities.

Interaction with the Environment

Interaction with the natural environment generates the following types of loads:

· Gravity—gravitational force associated with mass

· Snow—gravity type loading

· Wind—steady flow, gusts

· Earthquake—ground shaking resulting from a seismic event

· Water—scour, hydrostatic pressure, wave impact

· Ice—scour, impact

· Earth pressure—soil–structure interaction for foundations and underground structures

· Thermal—seasonal temperature variations

The relative importance of these sources depends on the nature of the structure and the geographical location of the site. For example, building design is generally governed by gravity, snow, wind, and possibly earthquake loads. Low-rise buildings in arctic regions tend to be governed by snow loading. Underground basement structures and tunnels are designed for earth pressure, hydrostatic pressure, and possibly earthquake loads. Gravity is the dominant source of load for bridge structures. Wave and ice action control the design of offshore platforms in coastal arctic waters such as the coasts of Alaska and Newfoundland. Structures located in California need to be designed for high seismic load. Structures located in Florida need to be designed for high wind load due to hurricanes. Thermal loads occur when structural elements are exposed to temperature change and are not allowed to expand or contract.

Function

Function-related loads are structure specific. For bridges, vehicular traffic consisting of cars, trucks, and trains generates gravity-type load, in addition to the self-weight load. Office buildings are intended to provide shelter for people and office equipment. A uniformly distributed gravity floor load is specified according to the nature of the occupancy of the building. Legal offices and libraries tend to have a larger design floor loading since they normally have more storage files than a normal office. Containment structures usually store materials such as liquids and granular solids. The associated loading is a distributed internal pressure which may vary over the height of the structure.

Construction

Construction loading depends on the process followed to assemble the structure. Detailed force analyses at various stages of the construction are required for complex structures such as segmented long-span bridges for which the erection

Fig. 1.8 Millau viaduct

loading dominates the design. The structural engineer is responsible for approving the construction loads when separate firms carry out engineering and construction. A present trend is for a single organization to carry out both the engineering design and construction (the design-build paradigm where engineering companies and construction companies form a joint venture for the specific project). In this case, a team consisting of structural engineers and construction engineers jointly carries out the design. An example of this type of partnering is the construction of the Millau Viaduct in southwestern France, shown in Fig. 1.8. The spans were constructed by cantilevering segments out from existing piers, a technically challenging operation that required constant monitoring. The bridge piers are the highest in the world: the central pier is 280 m high.

Terrorist Loads

Terrorist loads are a new problem for structural engineers, driven primarily by the need to protect essential facilities from terrorist groups. Design criteria are continuously evolving, and tend to be directed more at providing multilevel defense barriers to prevent incidents, rather than to design for a specific incident. Clearly, there are certain incidents that a structure cannot be designed to safely handle, such as the plane impacts that destroyed the World Trade Center Towers. Examining progressive collapse mechanisms is now required for significant buildings, and is the responsibility of the structural engineer

Table 1.3 Loading attributes

Fig. 1.9 Temporal variation of loading. (a) Impulsive. (b) Cyclic. (c) Quasi-static