Chapter 7Special Loads

NBC 2016 (SP 7:2016) Volume 1

2016 · PDF page 512

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7.1 This clause gives guidance on loads and load effects due to temperature changes, soil and hydrostatic pressures, internally generating stresses (due to creep, shrinkage, differential settlement, etc), accidental loads, etc, to be considered in the design of buildings as appropriate. This clause also includes guidance on load combinations. The nature of loads to be considered for a particular situation is to be based on engineering judgment (see also 3.6)

7.2 Temperature Effects

7.2.1 Expansion and contraction due to changes in temperature of the materials of a structure shall be considered in design. Provision shall be made either to relieve the stress by the provision of expansion/contraction joints in accordance with good practice [6-1(21)] or design the structure to carry additional stresses due to temperature effects as appropriate to the problem.

7.2.1.1 The temperature range varies for different regions and under different diurnal and seasonal conditions. The absolute maximum and minimum temperature which may be expected in different localities in the country are indicated in Annex B of good practice [6-1(22)]. These figures may be used for guidance in assessing the maximum variations of temperature.

7.2.1.2 The temperatures indicated are the air temperatures in the shade. The range of variation in temperature of the building materials may be appreciably greater or less than the variation of air temperature and is influenced by the condition of exposure and the rate at which the materials composing the structure absorb or radiate heat. This difference in temperature variations of the material and air should be given due consideration.

7.2.1.3 The structural analysis shall take account of changes of the mean (through the section) temperature in relation to the initial (st) and the temperature gradient through the section.

PART 6 STRUCTURAL DESIGN — SECTION 1 LOADS, FORCES AND EFFECTS

a) It should be borne in mind that the changes of mean temperature in relation to the initial are liable to differ as between one structural element and another in buildings or structures, as for example, between the external walls and the internal elements of a building. The distribution of temperature through section of single-leaf structural elements may be assumed linear for the purpose of analysis.

b) The effect of mean temperature changes $t_{1}$ and $t_{2}$ , and the temperature gradients $v_{1}$ and $v_{2}$ in the hot and cold seasons for single-leaf structural elements shall be evaluated on the basis of analytical principles.

1 For portions of the structure below ground level, the variation of temperature is generally insignificant. However, during the period of construction, when the portions of the structure are exposed to weather elements, adequate provision should be made to encounter adverse effects, if any.

2 If it can be shown by engineering principles, or if it is known from experience, that neglect of some or all the effects of temperature do not affect the structural safety and serviceability, they need not be considered in design.

7.3Hydrostatic and Soil Pressure

7.3.1 In the design of structures or parts of structures below ground level such as retaining walls and other walls in basement floors, the pressure exerted by the soil or water or both shall be duly accounted for on the basis of established theories. Due allowance shall be made for possible surcharge from stationary or moving loads. When a portion or whole of the soil is below the free water surface, the lateral earth pressure shall be evaluated for weight of soil diminished by buoyancy and the full hydrostatic pressure.

7.3.1.1 All foundation slabs and other footings subjected to water pressure shall be designed to resist a uniformly distributed uplift equal to the full hydrostatic pressure. Checking of overturning of foundation under submerged condition shall be done considering buoyant weight of foundation.

7.3.2 While determining the lateral soil pressure on column like structural members such as pillars which rest in sloping soils, the width of the member shall be taken as follows (see Fig. 18):

Actual Width of Member	Ratio of Effective Width to Actual Width
Less than 0.5 m	3.0
Beyond 0.5 m and up to 1 m	3.0 to 2.0
Beyond 1 m	2.0

The relieving pressure of soil in front of the structural member concerned may generally not be taken into account.

FIG. 18 SKETCH SHOWING EFFECTIVE WIDTH OF PILLAR FOR CALCULATING SOIL PRESSURE

7.3.3 Safe-guarding of structures and structural members against overturning and horizontal sliding shall be verified. Imposed loads having favourable effect shall be disregarded for the purpose. Due consideration shall be given to the possibility of soil being permanently or temporarily removed.

7.4Fatigue

7.4.1 General

Fatigue cracks are usually initiated at points of high stress concentration. These stress concentrations may be caused by or associated with holes (such as bolt or rivet holes in steel structures), welds including stray or fusions in steel structures, defects in materials, and local and general changes in geometry of members. The cracks usually propagate if the loading is cyclic and repetitive.

Where there is such cyclic and repetitive loading, sudden changes of shape of a member or part of a member, especially in regions of tensile stress and/or local secondary bending, shall be avoided. Suitable steps shall be taken to avoid critical vibrations due to wind and other causes.

7.4.2 Where necessary, permissible stresses shall be reduced to allow for the effects of fatigue. Allowance for fatigue shall be made for combinations of stresses due to dead load and imposed load. Stresses due to wind and earthquakes may be ignored, when fatigue is being considered, unless otherwise specified in relevant Codes of practice.

Each element of the structure shall be designed for the number of stress cycles of each magnitude to which it is estimated that the element is liable to be subjected during the expected life of the structure. The number of cycles of each magnitude shall be estimated in the light of available date regarding the probable frequency of occurrence of each type of loading.

NOTE — Apart from the general observations made herein, the Section is unable to provide any precise guidance in estimating the probabilistic behaviour and response of structures of various types arising out of repetitive loading approaching fatigue conditions in structural members, joints, materials, etc.

7.5Structural Safety During Construction

7.5.1 All loads required to be carried by the structures or any part of it due to storage or positioning of construction materials and erection equipment including all loads due to operation of such equipment, shall be considered as erection loads. Proper provision shall be made, including temporary bracings, to take care of all stresses due to erection loads. The conjunction with the temporary bracings shall be capable of sustaining these erection loads, without exceeding the permissible stresses specified in respective Codes of practice. Dead load, wind load and such parts of imposed load, as would be imposed on the structure during the period of erection, shall be taken as acting together with erection loads.

7.6Accidental Loads

The occurrence of which, with a significant value, is unlikely on a given structure over the period of time under consideration, and also in most cases, is of short duration. The occurrence of an accidental load could, in many cases, be expected to cause severe consequences, unless special measures are taken.

The accidental loads arising out of human action include the following:

a) Impacts and collisions,

b) Explosions, and

c) Fire.

Characteristic of the above stated loads are that they are not a consequence of normal use and that they are undesired, and that extensive effects are made to avoid them. As a result, the probability of occurrence of an accidental load is small whereas the consequences may be severe.

The causes of accidental loads may be,

inadequate safety of equipment (due to poor design or poor maintenance); and
wrong operation (due to insufficient teaching or training, indisposition, negligence or unfavourable external circumstances).

In most cases, accidental loads only develop under a combination of several unfavourable occurrences. In practical applications, it may be necessary to neglect the most unlikely loads. The probability of occurrence of accidental loads, which are neglected, may differ for different consequences of a possible failure. A data base for a detailed calculation of the probability will seldom be available.

7.6.1 Impact and Collisions

7.6.1.1 General

During an impact, the kinetic impact energy has to be absorbed by the vehicle hitting the structure and by the structure itself. In an accurate analysis, the probability of occurrence of an impact with a certain energy object hitting the structure and the structure itself at the actual place shall be considered. Impact energies for dropped object should be based on the actual loading capacity and lifting height.

Common sources of impact are:

a) Vehicles;

b) Dropped objects from cranes, fork lifts, etc;

c) Cranes out of control, crane failures; and

d) Flying fragments.

The codal requirements regarding impact from vehicles and cranes are given in 7.6.1.2 and 7.6.1.3.

7.6.1.2 The requirements for loads as a result of collision between vehicle and structural or non-structural elements shall be as per 7.6.1.2.1 and 7.6.1.2.2.

7.6.1.2.1 Collisions between vehicles and structural elements

In road traffic, the requirement that a structure shall be able to resist collision may be assumed to be fulfilled, if it is demonstrated that the structural element is able to stop a fictitious vehicle, as described below. It is assumed that the vehicle strikes the structural element at a height of 1.2 m in any possible direction and at a speed of 10 m/s (36 km/h).

The fictitious vehicle shall be considered to consist of two masses $m_1$ and $m_2$ which during compression of the vehicle, produce an impact force increasing uniformly from zero, corresponding to the rigidities $C_1$ and $C_2$ . It is assumed that the mass $m_1$ is broken completely before the breaking of mass $m_2$ begins.

The following numerical values should be used:

$m_{1} = 400 \, kg, C_{1} = 10\,000 \, kN/m,$ the vehicle is compressed.

$m_{2} = 12\ 000 \text{ kg}, C_{2} = 300 \text{ kN/m}, \text{ the vehicle is compressed.}$

PART 6 STRUCTURAL DESIGN — SECTION 1 LOADS, FORCES AND EFFECTS

NOTE — The described fictitious collision corresponds in the case of a non-elastic structural element to a maximum static force of 630 kN for the mass $m_{1}$ and 600 kN for the mass $m_{2}$ irrespective of the elasticity, it will therefore be on the safe side to assume the static force to be 630 kN.

In addition, breaking of the mass $m_{1}$ will result in an impact wave, the effect of which will depend, to a great extent, on the kind of structural element concerned. Consequently, it will not always be sufficient to design for the static force.

7.6.1.2.1.1 For structural elements in stilts and car parking levels

For structural elements in stilts and car parking levels, where driveways and car parking are provided in the buildings which have limited vehicle speeds, the design shall take care of the following considerations:

a) Speed of the vehicle to be 20 km/h maximum (since within a building and inside the car parks, speed of the vehicle may not generally be more than 20 km/h).

b) Impact to be considered at a height of 450 mm to 600 mm.

Structural elements such as columns, RC walls or vertical load resisting elements shall be designed for loads due to the above unless protected by separate resisting system.

7.6.1.2.2 Collision between vehicles and non-structural elements

Non-structural elements, such as vehicle barriers for car parks shall be designed to resist the impact and the momentum as outlined below:

a) The horizontal force F (in kN), normal to and uniformly distributed over any length of 1.5 m of a barrier for a car park, required to withstand the impact of a vehicle shall be as given below:

F = \frac {0 . 5 m v ^ {2}}{\delta_ {\mathrm{c}} + \delta_ {\mathrm{b}}}

where

$m =$ gross mass of the vehicle, in kg;

v = velocity of the vehicle normal to the barrier, in m/s;

$\delta_{\mathrm{c}} =$ deformation of the vehicle, in mm;

$\delta_{\mathrm{b}} =$ deflection of the barrier, in mm.

b) Where the car park has been designed on the basis that the gross mass of the vehicles using it will not exceed 2 500 kg, the following values shall be used to determine the force F:

\begin{array}{l} m = 1 5 0 0 \mathrm{kg}; \\ v = 4. 5 \mathrm{m/s}; \end{array}

\delta_ {\mathrm{c}} = 1 0 0 \mathrm{mm};

For a rigid barrier, for which $\delta_{b}$ may be taken as zero, the force, F, appropriate to vehicles up to 2 500 kg gross mass, shall be taken as 150 kN.

c) Where the car park has been designed for vehicles whose gross mass exceeds 2 500 kg, the following values shall be used to determine the force F:

m = actual mass of the vehicle for which the car park is designed, in kg;

$v = 4.5\mathrm{m / s};$ and

\delta_ {\mathrm{c}} = 1 0 0 \mathrm{mm}.

d) The force determined as in (b) or (c) may be considered to act at bumper height. In the case of car parks intended for motor cars whose gross mass does not exceed 2 500 kg, this height may be taken as 375 mm above the floor level.

e) Barriers to access ramps of car parks have to withstand one-half of the force determined in (b) or (c) acting at a height of 610 mm above the ramp.

Opposite to the ends of straight ramps intended for downward travel, which exceed 20 m in length, the barrier shall be designed to withstand twice the force determined in (b) or (c) acting at a height of 610 mm above the ramp.

NOTE — The mass of 1 500 kg is taken as being more representative of the vehicle population than the extreme value of 2 500 kg.

7.6.1.3 Safety railings

With regard to safety, railings put up to protect structures against collision due to road traffic, it should be shown that the railings are able to resist the impact as described in 7.6.1.2.

NOTE — When a vehicle collides with safety railings, the kinetic energy of the vehicle will be absorbed partly by the deformation of the railings and partly by the deformation of the vehicle. The part of the kinetic energy which the railings should be able to absorb without breaking down may be determined on the basis of the assumed rigidity of the vehicle during compression.

7.6.1.4 Crane impact load on buffer stop

The basic horizontal load $P_{\mathrm{y}}$ (tonne), acting along the crane track produced by impact of the crane on the buffer stop, is calculated by the following formula:

P _ {\mathrm{y}} = M V ^ {2} / F

where

$V =$ speed at which the crane is traveling at the moment of impact (assumed equal to half the nominal value) $(\mathrm{m / s})$

$F =$ maximum shortening of the buffer, assumed

equal to 0.1 m for light duty, medium-duty and heavy-duty cranes with flexible load suspension and loading capacity not exceeding 50 t, and 0.2 m in every other cranes.

$M =$ reduced crane mass, $(\mathrm{t.s}^2 /\mathrm{m})$ ; and is obtained by the formula:

M = \frac {1}{\mathrm{g}} \left[ \frac {P _ {\mathrm{h}}}{2} + \left(P _ {\mathrm{t}} + k Q\right) \frac {L _ {\mathrm{k}} - l}{L _ {\mathrm{k}}} \right]

where

\begin{array}{r l} \mathrm{g} & = \text {acceleration due to gravity} \\ & (9. 8 1 \mathrm{m} / \mathrm{s} ^ {2}); \end{array}

$P_{\mathrm{h}} =$ crane bridge weight (t);

P _ {\mathrm{t}} = \text { crab bridge weight (t); }

$Q =$ crane loading capacity (t);

k = coefficient, assumed equal to zero for cranes with flexible load suspension and to one for cranes with rigid suspension;

$L_{\mathrm{k}} =$ crane span (m); and

$l =$ nearness of crab (m).

7.6.2 Explosions

7.6.2.1 General

Explosions may cause impulsive loading on a structure. The following types of explosions are particularly relevant:

a) Internal gas explosions which may be caused by leakage of gas piping (including piping outside the room), evaporation from volatile liquids or unintentional evaporation from surface material (for example, fire);

b) Internal dust explosions;

c) Boiler failure;

d) External gas cloud explosions; and

e) External explosions of high explosives (TNT, dynamite).

The codal requirement regarding internal gas explosions is given in 7.6.2.2.

7.6.2.2 Explosion effect in closed rooms

Gas explosion may be caused, for example by leaks in gas pipes (inclusive of pipes outside for room), evaporation from volatile liquids or unintentional evaporation of gas from wall sheathings (for example, caused by fire).

1 The effect of explosions depends on the exploding medium, the concentration of the explosion, the shape of the room, possibilities of ventilation of the explosion, and the ductility and dynamic properties of the structure. In rooms with little possibility for relief of the pressure from the explosion, very large pressures may occur.

Internal over pressure from an internal gas explosion in rooms of sizes comparable to residential rooms and with ventilation

areas consisting of window glass breaking at a pressure of $4\mathrm{kN / m^2}$ (3-4 mm machine made glass) may be calculated from the following method:

a) The over pressure is assumed to depend on a factor $A / V$ , where $A$ is the total windows area, in $\mathrm{m}^2$ , and $V$ is the volume, in $\mathrm{m}^3$ , of the room considered;

b) The internal pressure is assumed to act simultaneously upon all walls and floors in one closed room; and

c) The action $q_{\mathrm{o}}$ may be taken as static action.

If account is taken of the time curve of the action, the schematic correspondence between pressure and time is assumed (see Fig. 19), where $t_1$ is the time from the start of combustion until maximum pressure is reached and $t_2$ is the time from maximum pressure to the end of combustion. For $t_1$ and $t_2$ , the most unfavourable values should be chosen in relation to the dynamic properties of the structures. However, the values should be chosen within the intervals as given in Fig. 20.

2 Figure 19 is based on tests with gas explosions in room corresponding to ordinary residential flats and should, therefore, not be applied to considerably different conditions. The figure corresponds to an explosion caused by town gas and it might, therefore, be somewhat on the safe side in rooms where there is only the possibility of gases with a lower rate of combustion.

The pressure may be applied solely in one room or in more rooms at the same time. In the latter case, all rooms are incorporated in the volume $V$ . Only windows or other similarly weak and light weight structural elements may be taken to be ventilation areas even though certain limited structural parts break at pressures less than $q_{o}$ .

Figure 19 is given purely as guide and probability of occurrence of an explosion should be checked in each case using appropriate values.

7.6.3 Vertical Load on Air Raid Shelters

7.6.3.1 Characteristic values

As regards buildings in which the individual floors are acted upon by a total characteristic imposed action of up to $5.0 \, kN/m^{2}$ , vertical actions on air raid shelters generally located below ground level, for example, basement, etc, should be considered to have the following characteristic values:

a)	Buildings with up to 2 storeys	: 28 kN/m $^{2}$
b)	Buildings with 3-4 storeys	: 34 kN/m $^{2}$
c)	Buildings with more than 4 storeys	: 41 kN/m $^{2}$
d)	Buildings of particularly stable construction irrespective of the number of storeys	: 28 kN/m $^{2}$

In the case of buildings with floors that are acted upon by a characteristic imposed action larger than $5.0 \, kN/m^{2}$ , the above values should be increased by the difference between the average imposed action on all storeys above the one concerned and $5.0 \, kN/m^{2}$ .

1 By storeys it is understood, every utilizable storey above the shelter.

2 By buildings of a particular stable construction, it is understood, buildings in which the load-bearing structures are made from reinforced in-situ concrete.

PART 6 STRUCTURAL DESIGN — SECTION 1 LOADS, FORCES AND EFFECTS

Chart

FIG. 19 SKETCH SHOWING RELATION BETWEEN PRESSURE AND TIME

Chart

FIG. 20 SKETCH SHOWING TIME INTERVAL AND PRESSURE

7.6.4 Fire

7.6.4.1 General

Possible extraordinary loads during a fire may be considered as accidental actions. Examples are loads from people along escape routes and loads on another structure from structure failing because of a fire.

7.6.4.2 Thermal effects during fire

The thermal effect during fire may be determined from one of the following methods:

a) Time-temperature curve and the required fire resistance (in min), and

b) An energy balance method.

If the thermal effect during fire is determined from an energy balance method, the fire load is taken to be:

q = 1 2 t _ {\mathrm{b}}

where

q = \text { fire action (kJ / m} ^ {2} \text { floor), and }

\begin{array}{r l} t _ {\mathrm{b}} & = \text {required fire resistance (in min)} \\ & \{\text {see good practice [6 - 1(23)]} \}. \end{array}

NOTE — The fire action is defined as the total quantity of heat produced by complete combustion of all combustible material

in the fire compartment, inclusive of stored goods and equipment together with building structures and building materials.

7.7 Vibrations

For general details on loads due to vibrations, reference may be made to Annex M.

7.8 Blast Loads

For provisions related to blast loads, see Annex N.

7.9 Other Loads

Other loads not included in this Section, such as special loads due to technical process, moisture and shrinkage effects, etc, should be taken into account where stipulated by building design codes or established in accordance with the performance requirement of the structure.

7.10 For additional information regarding loads, forces and effects about cyclone resistant buildings and landslide control aspects, reference may be made to good practices $[6-1(6)]$ and $[6-1(24)]$ to $[6-1(28)]$ , respectively.