The objective of a structural engineer is to design a structure that will be able to withstand all the loads to which it is subjected while serving its intended purpose throughout its intended life span. In designing a structure, an engineer must, therefore, consider all the loads that can realistically be expected to act on the structure during its planned life span. The loads that act on common civil engineering structures can be grouped according to their nature and source into three classes:
(1) dead loads : due to the weight of the structural system itself and any other material permanently attached to it.
(2) live loads : which are movable or moving loads due to the use of the structure.
(3) environmental loads : which are caused by environmental effects, such as wind, snow, and earthquakes. In addition to estimating the magnitudes of the design loads, an engineer must also consider the possibility that some of these loads might act simultaneously on the structure. The structure is finally designed so that it will be able to withstand the most unfavorable combination of loads that is likely to occur in its lifetime. The minimum design loads and the load combinations for which the structures must be designed are usually specified in building codes. Building codes vary from country to country and also, owing to geographical variations, from region to region within a country. The US national codes providing guidance on loads for buildings, bridges, and other structures include: ASCE Standard Minimum Design Loads for Buildings and Other Structures (ASCE/SEI 7-05) ,* Manual for Railway Engineering , Standard Specifications for Highway Bridges , and International Building Code .
Although the load requirements of most local building codes are generally based on those of the national codes listed herein, local codes may contain additional provisions warranted by such regional conditions as earthquakes, tornadoes, hurricanes, heavy snow, and the like. Local building codes are usually legal documents enacted to safeguard public welfare and safety, and the engineer must become thoroughly familiar with the building code for the area in which the structure is to be built. The loads described in the codes are usually based on past experience and study and are the minimum for which the various types of structures must be designed. However, the engineer must decide if the structure is to be subjected to any loads in addition to those considered by the code, and, if so, must design the structure to resist the additional loads. Remember that the engineer is ultimately responsible for the safe design of the structure. The objective of this chapter is to describe the types of loads commonly encountered in the design of structures and to introduce the basic concepts of load estimation. We first describe dead loads and then discuss live loads for buildings and bridges. We next consider the dynamic effect, or the impact, of live loads. We describe environmental loads, including wind loads, snow loads, and earthquake loads. We give a brief discussion of hydrostatic and soil pressures and thermal effects and conclude with a discussion about the combinations of loads used for design purposes. The material presented herein is mainly based on the ASCE Standard Minimum Design Loads for Buildings and Other Structures (ASCE/ SEI 7-05).
Dead loads are gravity loads of constant magnitudes and fixed positions that act permanently on the structure. Such loads consist of the weights of the structural system itself and of all other material and equipment permanently attached to the structural system. For example,the dead loads for a building structure include the weights of frames, framing and bracing systems,floors,roofs,ceilings,walls,stairways,heating and air-conditioning systems , plumbing, electrical systems,and so forth. The weight of the structure is not known in advance of design and is usually assumed based on past experience . After the structure has been analyzed and the member sizes determined,the actual weight is computed by using the member sizes and the unit weights of materials .The actual weight is then compared to the assumed weight,and the design is revised if necessary . The unit weights of some common construction materials are given in Table2.1. The weights of permanent service equipment, such as heating and air-conditioning systems,are usually obtained from the manufacturer.
Live loads are loads of varying magnitudes and/or positions caused by the use of the structure. Sometimes, the term live loads is used to refer to all loads on the structure that are not dead loads, including environmental loads, such as snow loads or wind loads. However, since the probabilities of occurrence for environmental loads are different from those due to the use of structures, the current codes use the term live loads to refer only to those variable loads caused by the use of the structure. It is in the latter context that this text uses this term. The magnitudes of design live loads are usually specified in building codes. The position of a live load may change, so each member of the structure must be designed for the position of the load that causes the maximum stress in that member. Different members of a structure may reach their maximum stress levels at different positions of the given load. For example, as a truck moves across a truss bridge, the stresses in the truss members vary as the position of the truck changes. If member A is subjected to its maximum stress when the truck is at a certain position x, then another member B may reach its maximum stress level when the truck is in a different position y on the bridge. The procedures for determining the position of a live load at which a particular response characteristic, such as a stress resultant or a deflection, of a structure is maximum (or minimum) are discussed in subsequent articles.
Live loads for building
Live loads for buildings are usually specified as uniformly distributed surface loads in kilo newton per meter. Minimum floor live loads for some common types of buildings are given in Table 2.2. For a comprehensive list of live loads for various types of buildings and for provisions regarding roof live loads, concentrated loads, and reduction in live loads, the reader is referred to the ASCE 7 Standard.
Live Loads for Bridges
Live loads due to vehicular traffic on highway bridges are specified by the American Association of State Highway and Transportation Officials in the Standard Specifications for Highway Bridges , which is commonly referred to as the AASHTO Specification. As the heaviest loading on highway bridges is usually caused by trucks, the AASHTO Specification defines two systems of standard trucks, H trucks and HS trucks, to represent the vehicular loads for design purposes. The H-truck loadings (or H loadings), representing a two-axle truck, are designated by the letter H, followed by the total weight of the truck and load in tons (1 ton 0.9 tonne) and the year in which the loading was initially specified. For example, the loading H20-44 represents a code for a two-axle truck weighing 20 tons (18 tonnes) initially instituted in the 1944 edition of the AASHTO Specification. The axle spacing, axle loads, and wheel spacing for the H trucks are shown in Fig. 2.2(a). The HS-truck loadings (or HS loadings) represent a two-axle tractor truck with a single-axle semitrailer. These loadings are designated by the letters HS followed by the weight of the corresponding H truck in tons and the year in which the loading was initially specified. The axle spacing, axle loads, and wheel spacing for the HS trucks are shown in Fig. 2.2(a). Note that the spacing between the rear axle of the tractor truck and the axle of the semitrailer should be varied between 14 ft (4.6 m) and 30 ft (10 m), and the spacing causing the maximum stress should be used for design.
The particular type of truck loading to be used in design depends on the anticipated traffic on the bridge. The H20-44 and HS20-44 are the most commonly used loadings; the axle loads for these loadings are shown in Fig. 2.2(a). In addition to the aforementioned single-truck loading, which must be placed to produce the most unfavorable effect on the member being designed, AASHTO specifies that a lane loading, consisting of a uniformly distributed load combined with a single concentrated load, be considered. The lane loading represents the effect of a lane of medium weight vehicles containing a heavy truck. The lane loading must also be placed on the structure so that it causes maximum stress in the member under consideration. As an example, the lane loading corresponding to the H20-44 and HS20-44 truck loadings is shown in Fig. 2.2(b). The type of loading, either truck loading or lane loading, that causes the maximum stress in a member should be used for the design of that member. Additional information regarding multiple lanes, loadings for continuous spans, reduction in load intensity, and so on, can be found in the AASHTO Specification. Live loads for railroad bridges are specified by the American Railway Engineering and Maintenance of Way Association (AREMA) in the Manual for Railway Engineering . These loadings, which are commonly known as Cooper E loadings, consist of two sets of nine concentrated loads, each separated by specified distances, representing the two locomotives followed by a uniform loading representing the weight of the freight cars. An example of such a loading, called the E80 loading, is depicted in Fig. 2.3. The design loads for heavier or lighter trains can be obtained from this loading proportionately increasing or decreasing the magnitudes of the loads while keeping the same distances between the concentrated loads. For example, the E40 loading can be obtained from the E80 loading by simply dividing the magnitudes of the loads by 2. As in the case of highway bridges considered previously, live loads on railroad bridges must be placed so that they will cause the most unfavorable effect on the member under consideration.
When live loads are applied rapidly to a structure, they cause larger stresses than those that would be produced if the same loads would have been applied gradually. The dynamic effect of the load that causes this increase in stress in the structure is referred to as impact. To account for the increase in stress due to impact, the live loads expected to cause such a dynamic effect on structures are increased by certain impact percentages, or impact factors. The impact percentages and factors, which are usually based on past experience and/or experimental results, are specified in the building codes. For example, the ASCE 7 Standard specifies that all elevator loads for buildings be increased by 100% to account for impact. For highway bridges, the AASHTO Specification gives the expression for the impact factor as:
I= 15/L+ 38.1< or equal to 0.3
in which L is the length in meters of the portion of the span loaded to cause the maximum stress in the member under consideration. Similar empirical expressions for impact factors to be used in designing railroad bridges are specified in .
Wind loads are produced by the flow of wind around the structure. The magnitudes of wind loads that may act on a structure depend on the geographical location of the structure, obstructions in its surrounding terrain, such as nearby buildings, and the geometry and the vibrational characteristics of the structure itself. Although the procedures described in the various codes for the estimation of wind loads usually vary in detail, most of them are based on the same basic relationship between the wind speed V and the dynamic pressure q induced on a flat surface normal to the wind flow, which can be obtained by the application of Bernoulli’s principle.