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 consider all the loads that can realistically be expected to act on the structure during its planned life span. The loads that act on 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; and (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.
1) Dead Loads
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 etc. The calculation of dead loads of each structure are calculated by the volume of each section and multiplied with the unit weight.
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 Table 1. The weights of permanent service equipment, such as heating and air-conditioning systems, are usually obtained from the manufacturer.
Table 1: Unit Weights of Construction Materials
|
Sl. No.
|
Material
|
Unit Weight (kN/m^3)
|
|
1
|
Aluminum
|
25.9
|
|
2
|
Brick
|
18.8
|
|
3
|
Plain Cement Concrete
|
24
|
|
4
|
Reinforced Cement Concrete
|
25
|
|
5
|
Structural Steel
|
77.0
|
|
6
|
Wood
|
6.3
|
2) Live Loads/ Imposed loads
Live loads are loads of varying magnitudes and positions caused by the use of the structure. Live loads are either movable or moving loads without any acceleration or impact. These loads are assumed to be produced by the intended use or occupancy of the building including weights of movable partitions or furniture etc. Live loads keep on changing from time to time. Sometimes, the term live loads are 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.
The magnitudes of design live loads are usually specified in building codes. In India, the minimum values of live loads to be assumed are given in IS 875 (part 2)–1987. It depends upon the intended use of the building. 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.
Table 2: Minimum Floor Live Loads for Buildings
|
Sl. No.
|
Occupancy or Use
|
Live Load (kPa)
|
|
1
|
Hospital patient rooms, residential dwellings,
apartments, hotel guest rooms, school classrooms
|
1.92
|
|
2
|
Library reading rooms, hospital operating roomsand laboratories
|
2.87
|
|
3
|
Dance halls and ballrooms, restaurants, gymnasiums
|
4.79
|
|
4
|
Light manufacturing, light storage warehouses,wholesale stores
|
6.00
|
|
5
|
Heavy manufacturing, heavy storage warehouses
|
11.97
|
Buildings Subjected to Environmental Loads
Because of the inherent uncertainty involved in predicting environmental loads that may act on a structure during its lifetime, the consequences of the failure of the structure are usually considered in estimating design environmental loads, such as due to wind, snow and earthquakes. In general, the more serious the potential consequences of the structural failure, the larger the magnitude of the load for which the structure should be designed.
a) Wind Loads
Wind loads are produced by the flow of wind around the structure. Wind load is primarily horizontal load caused by the movement of air relative to earth. 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 vibration 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 applying Bernoulli’s principle and is expressed as
q = 1/ 2 rV^2
where ‘r’ is the mass density of the air
Wind load is required to be considered in structural design especially when the height of the building exceeds two times the dimensions transverse to the exposed wind surface. For low rise building up to four to five stories, the wind load is not critical because the moment of resistance provided by the continuity of floor system to column connection and walls provided between columns are sufficient to accommodate the effect of these forces. The horizontal forces exerted by the components of winds are to be kept in mind while designing the building. The calculation of wind loads depends on the two factors, namely velocity of wind and size of the building. In India, calculation of wind load on structures is given below by the IS-875 (Part 3) -1987. Using colour code, basic wind pressure ‘Vb’ is shown in a map of India. Designer can pick up the value of Vb depending upon the locality of the building. To get the design wind velocity Vz the following expression shall be used:
Vz = k1.k2.k3.Vb
Where
k1 = Risk coefficient
k2 = Coefficient based on terrain, height and structure size
k3 = Topography factor
The design wind pressure is given by
pz = 0.6 V^2 * z
where pz is in N/m^2 at height Z and Vz is in m/sec.
Up to a height of 30 m, the wind pressure is considered to act uniformly. Above 30 m height, the wind pressure increases.
Table 3: Risk Categories of Buildings for Environmental Loads
|
Risk category
|
Occupancy or Use
|
Importance Factor
|
|
Snow Loads (Is)
|
Earthquake Loads (Ie)
|
|
I
|
Buildings representing low risk to human life in the
case of failure, such as agricultural and minor storage facilities.
|
0.8
|
1.0
|
|
II
|
All buildings other than those listed in Risk
Categories I, III, and IV. This risk category applies to most of the
residential, commercial and industrial buildings (except those which have
been specifically assigned to another category).
|
1.0
|
1.0
|
|
III
|
Buildings whose failure would pose a substantial risk
to human life, and/or could cause a substantial economic impact or mass
disruption of everyday public life. This category contains buildings such as:
theaters, lecture and assembly halls where a large number of people
congregate in one area; elementary schools; small hospitals; prisons; power
generating stations; water and sewage treatment plants; telecommunication
centers; and buildings containing hazardous and explosive materials.
|
1.1
|
1.25
|
|
IV
|
Essential facilities, including hospitals, fire and
police stations, national defense facilities and emergency shelters,
communication centers, power stations and utilities required in an emergency,
and buildings containing extremely hazardous materials.
|
1.2
|
1.5
|
b) Snow Loads
In many parts of the world, snow loads must be considered in designing structures. The design snow load for a structure is based on the ground snow load for its geographical location, which can be obtained from building codes or meteorological data for that region. Once the ground snow load has been established, the design snow load for the roof of the structure is determined by considering such factors as the structure’s exposure to wind and its thermal, geometric, and functional characteristics. In most cases, there is less snow on roofs than on the ground. In India, the code IS 875 (Part-4):1987 deals with snow loads on roofs of the building.
c) Earthquake Loads
An earthquake is a sudden undulation of a portion of the earth’s surface. Although the ground surface moves in both horizontal and vertical directions during an earthquake, the magnitude of the vertical component of ground motion is usually small and does not have a significant effect on most structures. It is the horizontal component of ground motion that causes structural damage and that must be considered in designs of structures located in earthquake-prone areas.
Fig.1 Deformation during an earthquake During an earthquake, as the foundation of the structure moves with the ground, the above-ground portion of the structure, because of the inertia of its mass, resists the motion, thereby causing the structure to vibrate in the horizontal direction. These vibrations produce horizontal shear forces in the structure. For an accurate prediction of the stresses that may develop in a structure, in the case of an earthquake, a dynamic analysis, considering the mass and stiffness characteristics of the structure, must be performed. The response of the structure to the ground vibration is a function of the nature of foundation soil, size and mode of construction and the duration and intensity of ground motion. In India, IS 1893– 2014 gives the details of calculations for structures standing on soils which will not considerably settle or slide appreciably due to earthquake.
