Once the dimensional requirements for a structure have been defined, it becomes necessary to determine the loads the structure must support. Often, it is the anticipation of the various loads that will be imposed on the structure that provides the basic type of structure that will be chosen for design. For example, high-rise structures must endure large lateral loadings caused by wind, and so shear walls and tubular frame systems are selected, whereas buildings located in areas prone to earthquakes must be designed to have ductile frames and connections.
Once the structural form has been determined, the actual design begins with those elements that are subjected to the primary loads the structure is intended to carry, and proceeds in sequence to the various supporting members until the foundation is reached. Thus, a building floor slab would be designed first, followed by the supporting beams, columns, and last, the foundation footings. In order to design a structure, it is therefore necessary to first specify the loads that act on it.
The design loading for a structure is often specified in codes. In general, the structural engineer works with two types of codes: general building codes and design codes. General building codes specify the requirements of governmental bodies for minimum design loads on structures and minimum standards for construction. Design codes provide detailed technical standards and are used to establish the requirements for the actual structural design. Table 1–1 lists some of the important codes used in practice. It should be realized, however, that codes provide only a general guide for design. The ultimate responsibility for the design lies with the structural engineer.
Since a structure is generally subjected to several types of loads, a brief discussion of these loadings will now be presented to illustrate how one must consider their effects in practice.
Dead loadsconsist of the weights of the various structural members and the weights of any objects that are permanently attached to the structure. Hence, for a building, the dead loads include the weights of the columns, beams, and girders, the floor slab, roofing, walls, windows, plumbing, electrical fixtures, and other miscellaneous attachments.
In some cases, a structural dead load can be estimated satisfactorily from simple formulas based on the weights and sizes of similar structures. Through experience, one can also derive a “feeling” for the magnitude of these loadings. For example, the average weight for timber buildings is (1.9-2.4 kN/m2), for steel-framed buildings it is (2.9-3.6 kN/m2), and for reinforced concrete buildings it is (5.3-6.2 kN/m2). Ordinarily, though, once the materials and sizes of the various components of the structure are determined, their weights can be found from tables that list their densities.
The densities of typical materials used in construction are listed in Table 1–2, and a portion of a table listing the weights of typical building is components is given in Table 1–3. Although calculation of dead loads based on the use of tabulated data is rather straightforward, it should be realized that in many respects these loads will have to be estimated in the initial phase of design.
\These estimates include non-structural materials such as prefabricated facade panels, electrical and plumbing systems, etc. Furthermore, even if the material is specified, the unit weights of elements reported in codes may vary from those given by manufacturers, and later use of the building may include some changes in dead loading. As a result, estimates of dead loadings can be in error by 15% to 20% or more.
Normally, the dead load is not large compared to the design load for simple structures such as a beam or a single-story frame; however, for multi-storey buildings it is important to have an accurate accounting of all the dead loads in order to properly design the columns, especially for the lower floors.
Live Loads can vary both in their magnitude and location. They may be caused by the weights of objects temporarily placed on a structure, moving vehicles, or natural forces. The minimum live loads specified in codes are determined from studying the history of their effects on existing structures. Usually, these loads include additional protection against excessive deflection or sudden overload.
The floors of buildings are assumed to be subjected to uniform live loads, which depend on the purpose for which the building is designed. These loadings are generally tabulated in local, state, or national codes. A representative sample of such minimum live loadings, taken from the ASCE 7-10 Standard, is shown in Table 1–4. The values are determined from a history of loading various buildings.
They include some protection against the possibility of overload due to emergency situations, construction loads, and serviceability requirements due to vibration. In addition to uniform loads, some codes specify minimum concentrated live loads, caused by hand carts, automobiles, etc., which must also be applied anywhere to the floor system. For example, both uniform and concentrated live loads must be considered in the design of an automobile parking deck.
For some types of buildings having very large floor areas, many codes will allow a reduction in the uniform live load for a floor, since it is unlikely that the prescribed live load will occur simultaneously throughout the entire structure at any one time. For example, ASCE 7-10 allows a reduction of live load on a member having an influence area of 400ft2 (37.2m2) or more. This reduced live load is calculated using the following equation:
L= reduced design live load per square foot or square meter of area supported by the member
L0= unreduced design live load per square foot or square meter of area supported by the member
KLL= 4 = live load element factor. For interior columns
AT = tributary area in square feet or square meters
Highway Bridge Loads
The primary live loads on bridge spans are those due to traffic, and the heaviest vehicle loading encountered is that caused by a series of trucks. Specifications for truck loadings on highway bridges are reported in the LRFD Bridge Design Specifications of the American Association of State and Highway Transportation Officials (AASHTO). For two-axle trucks, these loads are designated with an H, followed by the weight of the truck in tons and another number which gives the year of the specifications in which the load was reported. H-series truck weights vary from 10 to 20 tons.
However, bridges located on major highways, which carry a great deal of traffic, are often designed for two-axle trucks plus a one-axle semitrailer. These are designated as HS loadings. In general, a truck loading selected for design depends upon the type of bridge, its location, and the type of traffic anticipated.
Railroad Bridge Loads
The loadings on railroad bridges, are specified in the Specifications for Steel Railway Bridges published by the American Railroad Engineers Association (AREA). Normally, E loads, as originally devised by Theodore Cooper in 1894, were used for design. B. Steinmann has since updated Cooper’s load distribution and has devised a series of M loadings, which are currently acceptable for design. Since train loadings involve a complicated series of concentrated forces, to simplify hand calculations, tables and graphs are sometimes used in conjunction with influence lines to obtain the critical load. Also, computer programs are used for this purpose.
Moving vehicles may bounce or sideways as they move over a bridge, and therefore they impart an impact to the deck. The percentage increase of the live loads due to impact is called the impact factor, I. This factor is generally obtained from formulas developed from experimental evidence. For example, for highway bridges, the AASHTO specifications require that
where L is the length of the span in feet that is subjected to the live load.
In some cases, provisions for impact loading on the structure of a building must also be taken into account. For example, the ASCE 7-10 Standard requires the weight of elevator machinery to be increased by 100%, and the loads on any hangers used to support floors and balconies to be increased by 33%.
When structures block the flow of wind, the wind’s kinetic energy is converted into potential energy of pressure, which causes a wind loading. The effect of wind on a structure depends upon the density and velocity of the air, the angle of incidence of the wind, the shape and stiffness of the structure, and the roughness of its surface. For design purposes, wind loadings can be treated using either a static or a dynamic approach.
For the static approach, the fluctuating pressure caused by a constantly blowing wind is approximated by a mean velocity pressure that acts on the structure. This pressure q is defined by its kinetic energy q = ½(ρV2), where ρ is the density of the air and V is its velocity. According to the ASCE 7-10 Standard, this equation is modified to account for the importance of the structure, its height, and the terrain in which it is located. It is represented as
V = the velocity in mi/h (m/s) of a 3-second gust of wind measured 33 ft (10 m) above the ground.
KZ= the velocity pressure exposure coefficient, which is a function of height and depends upon the ground terrain.
KZt= 1.0 = a factor that accounts for wind speed increases due to hills and escarpments. For flat ground
Kd= 1.0 = a factor that accounts for the direction of the wind. It is used only when the structure is subjected to combinations of loads
Specific values depend upon the “category” of the structure obtained from a wind map. For example, the interior portion of the continental United States reports a wind speed of 105 mi/h (47 m/s) if the structure is an agricultural or storage building, since it is of low risk to human life in the event of a failure. The wind speed is 120 mi/h (54 m/s) for cases where the structure is a hospital, since its failure would cause substantial loss of human life.
Earthquakes produce loadings on a structure through its interaction with the ground and its response characteristics. These loadings result from the structure’s distortion caused by the ground’s motion and the lateral resistance of the structure. Their magnitude depends on the amount and type of ground accelerations and the mass and stiffness of the structure. During an earthquake the ground vibrates both horizontally and vertically.
The horizontal accelerations create shear forces in the column that put the block in sequential motion with the ground. If the column is stiff and the block has a small mass, the period of vibration of the block will be short and the block will accelerate with the same motion as the ground and undergo only slight relative displacements.
In practice the effects of a structure’s acceleration, velocity, and displacement can be determined and represented as an earthquake response spectrum. Once this graph is established, the earthquake loadings can be calculated using a dynamic analysis based on the theory of structural dynamics. This type of analysis is gaining popularity, although it is often quite elaborate and requires the use of a computer. Even so, such an analysis becomes mandatory if the structure is large.
Hydrostatic and Soil Pressure
When structures are used to retain water, soil, or granular materials, the pressure developed by these loadings becomes an important criterion for their design. Examples of such types of structures include tanks, dams, ships, bulkheads, and retaining walls. Here the laws of hydrostatics and soil mechanics are applied to define the intensity of the loadings on the structure.
Other Natural Loads
Several other types of live loads may also have to be considered in the design of a structure, depending on its location or use. These include the effect of blast, temperature changes, and differential settlement of the foundation.
The term ‘soil’ in soil engineering is defined as an unconsolidated material, composed of solid particles, produced by the disintegration of rocks. The void space between the particles may contain air, water or both. The soil particles may contain organic matter.
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