Loads and Forces Acting on Structures

Loads and forces are usually classified into two broad groups: dead load and imposed loads and forces. For the purpose of structural analysis, any load can be idealised into concentrated loads (single forces acting over a small area) and line loads (closely placed concentrated loads along a line, like a set of train loads or weight of a partition wall on a floor etc.). Distributed loads are loads which act over an area.

1. Dade Load

Dead loads include the weight of all permanent components of the structure, such as beams, columns, floor slabs, etc. and any other immovable loads that are constant in magnitude and permanently attached to the structure. Dead load is perhaps the simplest of all loading types, since it can be readily computed from given dimensions and known unit weight of materials. However, exact structural dimensions are not known during the initial design phase and assumptions must first be made which may be subject to changes later as the structural proportions are developed. In some structures, such as plate girders and trusses, dead weight assumptions can be expressed by general formulae. Obviously such formulae are derived from known weights of previously built structures. The Indian Standard schedule of unit weights of building materials (first revision) (IS: 1911 1967) gives the average unit weight of materials for the purpose of dead load calculations.

2. Imposed Loads and Forces 

Imposed loads are the forces that act on a structure in the use of the building or structure due to the nature of use, activities due to people, machinery installations, external natural forces, etc. These are: 

1. live load
2. wind load
3. seismic force
4. snow load
5. loads imposed by rain
6. soil and hydrostatic forces
7. erection loads and
8. other forces.

Live Load

Live load is categorised as: (1) live load on buildings, and (2) live load on bridges. Live Load on Buildings The character of use of occupancy of a structure together with the detail of any specific installations would suggest the live load on the structure. In buildings, these loads include any external loads imposed upon the structure during its service, such as the weights of stored materials, furniture and people. The estimation of live loads based on any rational basis is still not possible. To aid the designer, codes usually describe uniformly distributed live loads or equivalent concentrated loads that represent the minimum loads for that category of use. IS: 8751964 provides conservatively superimposed loads on floors and roofs.

Live Load on Bridges Another type of live load is that of moving vehicles on highways and railway bridges. As in the case of buildings, these are the minimum specified values to be used for the design of bridges. The live loads on a highway bridge are prescribed in the Indian Roads Congress Standard Specifications and Codes of Practice for Road Bridges: Section IL The loadings have been classified as class AA, class A and class B. The code also specifies hypothetical vehicles with wheel loads and wheel bases for the classification of vehicles and road bridges. The code also specifies the impact factor, centrifugal forces, longitudinal forces due to the tractive effort of vehicles or due to braking.

Similar information is available for loading on a railway bridge. The nature and magnitude of the loads to be taken for railway bridges in India are given in the Bridge Rules of the Ministry of Railways, Government of India. In moving live loads such as those on bridges and in crane gantries, the critical positions of moving vehicles or wheel loads that produce maximum forces at various points of the structure have to be determined. This is usually done with the help of influence lines discussed elsewhere in the book. also low level light structures in coastal areas. Wind forces are based upon the maximum wind velocity, which in turn depends upon the region and location. It also depends upon the shape of the structure. In the absence of any meteorological data, the wind pressure may be taken from IS: 875-1964 The code gives two basic wind maps of India: one giving the maximum wind pressure including

Wind Loads

Wind loads are very important in the case of tall structures and winds of short duration as in squalls, and the other excluding winds of short duration. The code recommends the same wind pressure for all heights up to 30 m and thereafter gives values at intervals of 5 m up to 150 m. The code recommends the use of only the map giving the maximum pressure for squall conditions. But the allowable stresses can be increased by 33 to 50% depending upon the ratio of the wind pressures given by both maps for any particular area.

Earthquake Forces 

Earthquake forces should be considered for the design of structures in areas of seismic activity. The highly irregular or random shaking of the ground transmits acceleration to structures and the mass of the structure resists the motion due to inertia effects. The total inertia force (usually equal to the horizontal shear at the base of the structure) ranges from about 0.02 to 0.12 W or more for most buildings, where W is the total weight of the structure, The Indian Standard Recommendations Criteria for Earthquake Resistant Design of Structures (third revision) (IS: 18931975 divides the whole country into five seismic zones depending on past experience and the probability of the future occurrence of earthquakes. The inertia force based on the seismic coefficient as appropriate for seismic zones depends on the type of soils and foundation system a smaller value for hard soils and a larger value for soft soils Buildings provided for accommodating essential services which are of post-earthquake importance, such as emergency relief stores, food grain storage structures, water works and power stations should be designed taking into account the importance factor"

Snow and Rain Loads 

Snow and rain loads affect the design of roofs. The design loads corresponding to the highest accumulation of snow can be found in IS: 8751964 and other forms of design information. These values are based on past weather records maintained by the Meteorological Department. If storm water is drained properly, rain does not contribute to any load on the structure. However, structural failures have occurred when rain water got accumulated on roofs due to choked storm water drains. The accumulation of water causes additional load and hence deflection which permits more water to accumulate. This progressive deflection and accumulation of water may continue, leading to structural failure.

Soil and Hydrostatic Forces

Structures below the ground, such as foundation walls, retaining walls or tunnels are subjected to forces due to soil pressure. The pressures may be estimated according to established theories. The force exerted by a fluid is normal to the surface of the retaining structure. The magnitude of the force depends on the hydrostatic pressure which is taken as p=vh where v is the unit weight of the fluid and a is the height of the fluid retained. This linear pressure distribution occurs in tanks, vessels and other structures under fluids.

Erection Loads 

All loads required to be carried by a structure or any part of it due to the placing or storage of construction materials and erection equipment, including all loads due to the operation of such equipment, shall be considered as erection loads.

Other Forces 

Impact, vibrations, temperature effects, shrinkage, creep, settlement of foundations and other such phenomena produce effects on structures, some of which may be similar to those caused by external loads and forces. These forces may sometimes be surprisingly large and should be taken into consideration while designing.

3. Load Combinations

Engineering judgement must be exercised when determining critical load combinations. It is not necessary to superpose all maximum loads. For example, a simultaneous occurrence of an earthquake and high velocity winds will have negligible statistical probability. Critical load combinations are usually specified by codes.