Determination of the loads for which a structure has to be proportioned is an important task in a design. The various loads that are likely to act on a structure and the possible combinations of such loads that can act are all points to be considered.
Dead load refers to the weight of a structure. It is necessary to estimate reasonably the dead load before a structural analysis is made. After a preliminary design, the actual weight should be determined and compared with the originally estimated values and the necessary corrections should be made.
Loads other than dead loads may be considered as live loads. The live loads may be either steady or unsteady i.e., they may be fixed or movable or moving. They may be applied slowly or suddenly.
The usually considered live loads are the following:
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(i) The weights of people, goods, furniture and any machinery in a building.
(ii) The weight due to traffic on a bridge.
(iii) The weight due to accumulation of snow.
(iv) Dynamic forces caused by moving loads.
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(v) Forces due to wind action.
(vi) Pressure forces exerted by liquids in storage tanks.
(vii) Temperature variations creating forces when expansions and contractions are prevented.
Besides these, earthquakes can induce dynamic forces.
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Gravity loads determined from weights of structures are static loads. Loads in motion produce greater forces than when they are static. A dynamic force produced by sudden change in velocity is an impact load which can be lateral or longitudinal. Motion on a curved path can also cause a lateral force. Longitudinal forces are caused due to acceleration or deceleration of vehicles.
The IS 875 Code on loading standards gives details of loads to be considered on structures.
Brief details of the same are given below:
A structure is subjected to various types of loads like dead loads, live loads, wind loads etc.
Dead Load:
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This consists of the weight of walls, partitions, floors, roofs including the weights of all other permanent constructions in the building. A schedule of unit weights of building materials is given in I.S.: 1911 code.
The I.S. recommendations for dead load are given below:
Provision for Loads from Partition Walls:
Loads due to partitions shall be assessed on the basis of the actual constructional details of the proposed partitions and their positioning in accordance with plans, and the loads thus assessed shall be included in the dead load for the design of floors and the supporting structures.
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Where, however, the actual loads of the partitions cannot be assessed beforehand owing to lack of knowledge of the final positioning of the partitions, the floors and the supporting structures shall be designed to carry, in addition to other loads, a uniformly distributed load per square metre of not less than 33⅓ per cent of the weight per metre run of finished partitions over the entire floor area subject to a minimum uniformly distributed load of 1000 N/m2 in the case of floors used for office purposes.
No partition shall be allowed to be erected which may, in effect, result in stresses greater than those allowed for in the design.
Live Loads on Floors:
Live Loads on Floors of Various Types of Buildings:
Live loads on floors shall comprise all loads other than dead loads. The minimum live loads on different floors for different uses shall be as given in the Table. The loads specified in this table are uniformly distributed static loads N/m2 on the plan area and provide for normal effects of impact and acceleration, but do not take into consideration special concentrated loads, snow loads and other loads.
Reduction in Floor Live Loads:
The following reductions in assumed total live loads on floors may be made in designing columns, walls, peirs, their supports and foundations:
No reduction shall be made in the case of warehouses, garages and other buildings used for storage purposes and for factories and workshops designed for 5000 N/m2. However, for buildings, such as factories and workshops, designed for a live load of more than 5000 N/m2 the reductions given in the table may be made provided that the loading assumed for any column, etc. is not less than it would have been if all the floors had been designed for 5000 N/m2 with no reduction.
Where, a single span of a beam or girder supports not less than 50 metre2 of floor at one general level, the live load may be reduced in the design of the beam or girder by 5 per cent, for each 50 metre2 supported, subject to a maximum reduction of 25 per cent.
This reduction or that given in the table, whichever is greater, may be taken into account in the design of columns, supporting such a beam, but, shall not be made where the floors are used for storage purposes nor in the weight of any plant or machinery which is specifically allowed for.
Live Loads on Roofs:
Live Loads on Various Types of Roofs:
On flat roofs, sloping roofs and curved roofs allowance for live load shall be are given below:
Roofs of buildings used for promenade or incidental assembly purposes shall be designed for a minimum load of 4000 N/m2 or heavier, if required.
Snow Load:
If a roof is subjected to snow load, it should be designed for the actual load due to snow or for the live loads specified in table on Table 3.2 whichever is more severe. Actual load due to snow will depend upon the shape of the roof and its capacity to retain the snow and each case shall be treated on its own merits.
In the absence of any specific information, the loading due to the collection of snow may be assumed to be 25 N/m2 per 10 mm depth of snow. The possibility of total or partial snow load should be considered, that is one-half of the roof fully loaded with the design snow load and the other half loaded with half the design snow load. In the case of roofs with slopes greater than 50°, snow load may be disregarded; where, however, there are possibilities of formation of snow pockets, these should be taken into account.
Load Due to Rain:
On surfaces whose positioning, shape and drainage system are such as to make accumulation of rain water possible, load due to such accumulation of water and the live loads for the roofs as given in the table shall be considered separately and the more critical of the two shall be adopted in the design.
Horizontal Loads on Parapets and Balustrades:
Where it is desired to design parapets and balustrades against horizontal pressures, loads shown in table below expressed as horizontal loads acting at handrail or coping level shall be provided for. The values given are for guidance only and where values for actual loadings are available, they shall be used instead.
Impact and Vibrations:
For structures carrying live load which induce impact or vibration, as far as possible, calculations shall be made for the increase in the live load due to impact or vibration.
In the absence of sufficient data for such calculation, the increase in the live load shall be as follows:
Concentrated Live Loads with Impact and Vibrations:
Concentrated live loads with impact and vibration which may be due to installed machinery shall be considered and provided for in the design. The impact factor shall not be less than 20 per cent which is the amount allowable for light machinery.
Concentrated Imposed Loads with Impact and Vibration:
Concentrated imposed loads with impact and vibration which may be due to installed machinery shall be considered and provided for in the design. The impact factor shall not be less than 20 per cent which is the amount allowable for light machinery.
Provision shall also be made for carrying any concentrated equipment loads while the equipment is being installed or moved for servicing and repairing.
Impact Allowance for Crane Girders:
For crane gantry girders and supporting columns, the following allowances shall be deemed to cover all forces set up by vibration, shock from slipping or slings, kinetic action of acceleration, and retardation and impact of wheel loads:
Forces specified in (c) and (d) shall be considered as acting at the rail level and being appropriately transmitted to the supporting system. Gantry girders and their vertical supports shall be designed on the assumption that either of the horizontal forces in (c) and (d) may act at the same time as the vertical load.
Overloading Factors in Crane Supporting Structures:
For all cranes and charging cranes, where there is possibility of overloading from production considerations an overloading factor of 10 per cent of the maximum wheel loading shall be taken.
Crane Load Combinations:
In the absence of any specific indications, the load combinations shall be indicated below:
Vertical Load:
In an aisle, where more than one crane is in operation or has provision for more than one crane in future, the following load combinations shall be taken for vertical loading:
(a) Two adjacent cranes working in tandem with full load and with overloading to the previous clause.
(b) For long span gantries, where more than one crane can come in the span, the girder shall be designed for the crane fully loaded with overloading plus as many loaded cranes as can be accommodated on the span but without taking into account overloading according to the previous clause to give the maximum effect.
Lateral Surge:
For design of columns and foundations, supporting crane girders, the following crane combinations shall be considered:
(a) For Single-Bay Frames:
Effect of one crane in the bay giving the worst effect shall be considered for calculation of surge, and
(b) For Multi-Bay Frames:
Effect of two cranes working one each in any of two bays in the cross-section to give the worst effect shall be considered for calculation of surge force.
Tractive Force:
When one crane is in operation with no provision for future crane, tractive force from only one crane shall be taken.
Where more than one crane is in operation or there is provision for future crane, tractive force from two cranes giving maximum effect shall be considered.
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Lateral surge force and longitudinal tractive force acting across and along the crane rail respectively, shall not be assumed to act simultaneously. However, if there is only one crane in the bay, the lateral and longitudinal forces may act together simultaneously with vertical loads.
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In the case of guard parapets on a floor of multi-storeyed car park or crash barriers provided in certain buildings for fire escape, the value of imposed horizontal load (together with impact load) may be determined.
Impact Allowance for Lifts, Hoists and Machinery:
The imposed loads specified above shall be assumed to include adequate allowance for ordinary impact conditions. However for structures, carrying loads which induce impact or vibration, as far as possible, calculations shall be made for increase in the imposed load, due to impact or vibration.
In the absence of sufficient data for such calculation, the increase in the imposed loads shall be as follows:
Live Loads on Roofs:
The I.S. code has stipulated the allowance for live load on flat roofs, sloping roofs and curved roofs (Table 3.4).
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For special types of roofs with highly permeable and absorbent material, the contingency of roof material increasing in weight due to absorption of moisture shall be provided for.
The loads given above do not include loads due to snow, rain, dust collection etc. The roof shall be designed for imposed loads given above or snow/rain loads whichever is greater.
Provision shall also be made for carrying any concentrated equipment loads while the equipment is being installed or moved for servicing and repairing.
For crane gantry girders and supporting columns, the allowances in the above table shall be deemed to cover all forces set up by vibration, shock from slipping of slings, kinetic action of acceleration and retardation, and impact of wheel loads.
The horizontal forces specified in (c) and (d) shall be considered as acting at the rail level and being appropriately transmitted to the supporting system for both electric and hand-operated cranes.
Gantry girders and their vertical supports shall be designed on the assumption that either of the horizontal loads (c) and (d) may act at the same time as the vertical load.
Wind Load on Buildings:
The I.S. 875 specification has stated that for structures of various plan shapes other than rectangular plan shape, the external pressure acting on the projected area in the plane perpendicular to the wind shall be the product of basic pressure stipulated by the code and the shape factors given in the table 3.5.
Note 1:
In the case of projections above the general roof level, the base width shall be taken as the width of the construction where it rises from the surface of the roof; and the height governing the ratio of height to base width shall be that from the roof surface to the top of the construction.
Note 2:
The division of wind load into that acting on windward and leeward sides shall be the same as that for rectangular buildings as affected by the openings.