Types Of Loads On Superstructures | Different Types Of Loads On A Structure | LCETED

As already defined superstructure is the part of the structure above ground level which is visible easily. In a building column, walls, beams, slabs, doors, window etc. form the superstructure while in bridges piers and decks form superstructure. The visible part of dams and water tanks are superstructures of dam and water tank respectively. In this chapter, various loads acting on the superstructure are discussed and types of construction are discussed.

 

TYPES OF LOADS ON SUPERSTRUCTURES

To get safe structures at the same time without ignoring the economy of the structure, it is necessary to estimate the various loads acting suitably. Indian standard code IS: 875–1987 specifies various design loads for buildings and structures. They have grouped various loads as under:

1. Dead loads

2. Imposed loads

3. Wind loads

4. Snow loads

5. Earthquake loads

6. Special loads

Details of earthquake load are covered in IS 1893 – 1984 which should be considered along with other types of loads given in IS-875. The code also gives various load combinations to be considered in the design.

 

DEAD LOADS

The dead load in a building comprises the weight of roofs, floors, beams, columns, walls, partition walls etc. which form a permanent part of the building. It is to be found by working out the volume of each part and then multiplying with unit weight. Unit weight of various materials is listed in part-I of IS: 875. Unit weights of some of the common materials are presented in the table below.

 

Material	Weight
Brick Masonry	18.8 kN/m3
Stone Masonry	20.4 – 26.5 kN/m3
Plain cement concrete	24.0 kN/m3
Reinforced cement concrete	25.0 kN/m3
Timber	5 to 8 kN/m3
Wooden  floors hard wood        16 mm thick	160 kN/m3
Wooden  floors hard wood        20 mm thick	200 N/m3
Terrazo paving	240 N/m3
Country Tiles (single) including battens	700 N/m3
Mangalore Tiles with  battens	650 N/m3
Mangalore tiles with flat tiles	785 N/m3
A.C. Sheets       6 mm thick	170 N/m3
A.C. Sheet 5 mm plain	110 N/m3

 

IMPOSED LOADS (IL)

The loads which keep on changing from time to time are called imposed loads. Common examples of such loads in a building are the weight of the persons, weights of movable partition, dust loads and weight of the furniture. These loads were formerly known as live loads. These loads are to be suitably assumed by the designer. It is one of the major loads in the design. The minimum values to be assumed are given in IS 875 (part 2)–1987. It depends upon the intended use of the building. These values are presented for square metres of floor area. The code gives the values of loads for the following occupancy classification:

(i) Residential buildings–dwelling houses, hotels, hostels, boiler rooms and plant rooms, garages.

(ii) Educational buildings

(iii) Institutional buildings

(iv) Assembly buildings

(v) Business and office buildings

(vi) Mercantine buildings

(vii) Industrial buildings, and

(viii) Storage rooms.

The code gives uniformly distributed loads as well as concentrated loads. The floors are to be investigated for both uniformly distributed and worst position of concentrated loads. The one which gives the worst effect is to be considered for the design but both should not be considered to act simultaneously.

In a particular building, imposed load may change from room to room. For example in a hotel or a hostel building, the loads specified are,

	udl	Concentrated load
(a) Living rooms and bedrooms	2 kN/m2	1.8 kN
(b) Kitchen	3 kN/m2	4.5 kN
(c) Dining rooms	4 kN/m2	2.7 kN
(d) Office rooms	2.5 kN/m2	2.7 kN
(e) Storerooms	5 kN/m2	4.5 kN
(f)Rooms for indoor games	3 kN/m2	1.8 kN
(g) Bathrooms and toilets	2 kN/m2	–
(h) Corridors, passages, staircases etc. and  (i) Balconies	4 kN/m2	1.5 kN concentrated at the outer edge.

 

Some of the important values are presented in the table below which are the minimum values and wherever necessary more than these values are to be assumed.

 

Minimum Imposed Load to be Considered

s.no	Occupancy	UDL Load	Concentrated
1.	Bathrooms and toilets in all types of building	2 kN/m2	1.8 kN
2.	Living and bed rooms	2 kN/m2	1.8 kN
3.	Office rooms in
	(i) Hostels, hotels, hospitals and business buildings with separate store	2.5 kN/m2	2.7 kN
	(ii) In assembly buildings	3 kN/m2	4.5 kN
4.	Kitchens in (i) Dwelling houses	2 kN/m2	1.8 kN
	(ii) Hostels, hotels and hospitals	3 kN/m2	4.5 kN
5.	Banking halls, classrooms, x-ray rooms, operation rooms	3 kN/m2	4.5 kN
6.	Dining rooms in (i) educational buildings, institutional and mercantine buildings	3 kN/m2	2.7 kN
	(ii) hostels and hotels	4 kN/m2	2.7 kN
7.	Corridors, passages, stair cases in
	(i) Dwelling houses, hostels and hotels	3 kN/m2	4.5 kN
	(ii) Educational institutional and assembly buildings	4 kN/m2	4.5 kN
	(iii) Marcantine buildings	5 kN/m2	4.5 kN
8.	Reading rooms in libraries
	(i) With separate storage	3 kN/m2	4.5 kN
	(ii) Without separate storage	4 kN/m2	4.5 kN
6.	Dining rooms in (i) educational buildings, institutional and mercantine buildings	3 kN/m2	2.7 kN
	(ii) hostels and hotels	4 kN/m2	2.7 kN
9.	Assembly areas in assembly buildings
	(i) With fixed seats	5 kN/m2	..
	(ii) Without fixed seats	5 kN/m2	3.6 kN
10.	Storerooms in educational buildings	5 kN/m2	4.5 kN
11.	Storeroom in libraries	6 kN/m2 for a height of 2.24 + 2 kN/m2 for every 1 m additional height	4.5 kN
12	Boiler rooms and plant rooms in
	(i) hostels, hotels, hospitals, mercantile and industrial buildings	5 kN/m2	4.5 kN
	(ii) Assembly & storage buildings	5 kN/m2	4.5 kN

imposed loads to be considered on various roofs are presented in the table below

Must read: How Do We Calculate The Dead Load In The Slab?

 

TABLE. Imposed  Loads  on Various Qcs of Roofâ (Table 2 of National Building Code - i98Sl

s.no	Types of roof	Imposed load measured	Minimum Imposed load
(i)	Flat, sloping or curved roof with slopes up to and including 10 degrees
	[a) Access provided	1.5 kN/m2	3.75 kN uniformly distributed over any span of one-metre width of the roof slab and  9 kN uniformly distributed over the span of any beam or truss or wall.
	(b) Access not provided except for maintenance.	0.75 kN / na2	1.9 kN., uniformly distributed over any span of one-metre width of the roof slab and 4.5 kN uniformly distributed over the span of any beam of truss or wall.
(ii)	Sloping roof with a  slope greater than 10 degrees.	For roof membrane sheet or purlins - 0. 75 kN/ m2 less	Subject to a minimum of 0.4 kN/m2
(iii)	Curved roof with a slope of line obtained by joining springing point to the crown with the horizontal, greater than 10 degrees.	0.75 — 0.52 o’) kN/m2 where h —— the height of the highest point of the structure measured from its springing: and I = chord width of the roof if singly curved and shorter of the two sides if doubly curved. Alternatively, where structural analysis can be carried out for curved roofs of a11 slopes in a simple manner applying the laws of statistics, the curved roofs shall be divided into minimum of 6 equal segments and for each segment imposed load shall be calculated appropriately of each segment as given in (i) and (ii)	Subject to a minimum of 0.4 kN/m2

 

NOTE 1. 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 load, whichever is greater.

NOTE 2.  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.

However, in multi-storeyed buildings chances of full imposed loads acting simultaneously on all floors is very rare. Hence the code makes provision for the reduction of loads in designing columns, load-bearing walls, their supports and foundations as shown in the table below

Reductions in Imposed Loads on Floors in Design of Supporting Structural Elements

Number of Floors (including the roof) to be carried by Member Under Consideration	Reduction in Total Distributed Imposed Load in Per cent
1	0
2	10
3	20
4	30
5 to 10	40
Over 10	50

 

WIND LOADS

The force exerted by the horizontal component of wind is to be considered in the design of buildings. It depends upon the velocity of wind and the shape and size of the building. Complete details of calculating wind load on structures are given in IS-875 (Part 3) -1987. A brief idea of these provisions is given below:

(i) Using colour code, basic wind pressure ‘Vb’ is shown in a map of India. The designer can pick up the value of Vb depending upon the locality of the building.

(ii) To get the design wind velocity Vz the following expression shall be used:

Vz = k₁ k₂ k₃ V_b

Where,    

k1 = Risk coefficient

k2 = Coefficient based on terrain, height and structure size.

k3 = Topography factor

 

(iii) The design wind pressure is given by

p = 0.6 V²

 

where pz is in N/m² at height Z and V 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.

 

SNOW LOADS

IS 875 (part 4) – 1987 deals with snow loads on roofs of the building. For the building to be located in the regions wherever snow is likely to fall, this load is to be considered. The snow load acts vertically and may be expressed in kN/m² or N/m². The minimum snow load on a roof area or any other area above ground which is subjected to snow accumulation is obtained by the expression

S =  mS₀

Where, S = Design snow load on plan area of roof.

m = Shape coefficient, and

S₀ = Ground snow load.

Ground snow load at any place depends on the critical combination of the maximum depth of undisturbed aggregate cumulative snowfall and its average density. These values for different regions may be obtained from Snow and Avalanches Study Establishment Manali (HP) or Indian Meteorological Department Pune. The shape coefficient depends on the shape of roofs and for some of the common shapes, the code gives these coefficients. When the slope of a roof is more than 60° this load is not considered.

It may be noted that roofs should be designed for the actual load due to snow or for the imposed load, whichever is more severe.

 

EARTHQUAKE FORCES

Earthquake shocks cause movement of the foundation of structures. Due to inertia, additional forces develop on the superstructure. The total vibration caused by an earthquake may be resolved into three mutually perpendicular directions, usually taken as vertical and two horizontal directions. The movement in the vertical direction does not cause forces in the superstructure to any significant extent. But the movement in a vertical direction does not cause forces in the superstructure to any significant extent. But the movement in horizontal directions causes considerable forces.

The intensity of vibration of ground expected at any location depends upon the magnitude of the earthquake, the depth of focus, the distance from the epicentre and the strata on which the structure stands.

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. IS: 1893– 1984 gives the details of such calculations for structures standing on soils that will not considerably settle or slide appreciably due to earthquakes. The seismic accelerations for the design may be arrived at from seismic coefficients, which is defined as the ratio of acceleration due to earthquake and acceleration due to gravity. For the purpose of determining the seismic forces, India is divided into five zones. Depending on the problem, one of the following two methods may be used for computing the seismic forces:

(a) Seismic coefficient method

(b) Response spectrum method

The details of these methods are presented in IS 1983 code and also in the National Building Code of India. After the Gujarat earthquake, (2000) Government of India has realized the importance of structural designs based on considering seismic forces and has initiated training of the teachers of a technical institutions on a large scale (NPEEE).

There are a large number of cases of less importance and relatively small structures for which no analysis be made for earthquake forces provided certain simple precautions are taken in the construction. For example

(a) Providing bracings in the vertical panels of steel and R.C.C. frames.

(b) Avoiding mud and rubble masonry and going for light materials and well braced timber-framed structures.

 

OTHER FORCES AND EFFECTS

As per clause 19.6 of IS 456 – 2000, in addition to the above load discussed, account shall be taken of the following forces and effects if they are liable to affect materially the safety and serviceability of the structure:

(a) Foundation movement (See IS 1904)

(b) Elastic axial shortening

(c) Soil and fluid pressure (See IS 875, Part 5)

(d) Vibration

(e) Fatigue

(f) Impact (See IS 875, Part 5)

(g) Erection loads (See IS 875, Part 2) and

(h) Stress concentration effect due to point load and the like.

 

LOAD COMBINATIONS

A judicious combination of the loads is necessary to ensure the required safety and economy in the design keeping in view the probability of

(a) Their acting together

(b) Their disposition in relation to other loads and severity of stresses or deformations caused by the combination of various loads.

The Recommended Load Combinations by National Building Codes

 

1.	DL	7.	DL + IL + EL
2.	DL + IL	8.	DL + IL + TL
3.	DL + WL	9.	DL + WL + TL
4.	DL + EL	10.	DL + EL + TL
5.	DL + TL	11.	DL + IL + WL + TL
6.	DL + IL + WL	12.	DL + IL + EL +TI

 

Where,   

DL = dead load

IL = imposed load

WL = wind load

EL = earthquake load

TL = temperature load.

NOTE: When snow load is present on roofs, replace imposed load by snow load for the purpose of above load combinations.

 

TYPES OF CONSTRUCTION

Walls are an important part of the superstructure. They are commonly constructed with stones, bricks or hollow concrete blocks. Walls enclose and divide the space in the building. In addition to it if they are made to carry load from roof/floor apart from self-weight it is called load-bearing construction.

If reinforced cement concrete or steel frame consisting of columns, beams, slabs are built first and walls are built only to enclose the area, the load transfer is mainly by beams and columns walls carry only self-weight. These walls serve as filler material. Such structures are called framed structures.

 

Load bearing construction

Load-bearing walls are built with stone, brick or concrete blocks joined together by cement mortar of 1 cement to 6 sand (1: 6). The walls are built course by course. The height of a course in stone masonry, brick masonry and hollow concrete block masonry are 150 mm, 100 mm and 200 mm (or 100 mm) respectively. In load-bearing walls, the verticality of the wall should be strictly ensured and vertical joints should be broken. The thickness of the wall should be sufficient to transfer the load safely, without exceeding permissible stress. The critical portions in masonry from consideration of stresses are near the openings for doors and windows and the portion where concrete beams rest.

Minimum thicknesses used are 375 mm, 200 mm and 200 mm in case of stone, brick and hollow block constructions respectively. It is also recommended that the slenderness ratio of the wall defined as the ratio of effective length or effective height to thickness should not be more than 27. National building code of India (NBC – 1983) defines the effective height and effective length as given in table 5.5 and 5.6 [for full details refer to NBC – 1983.]

 

Effective Height of Walls in Terms of Actual Height H

Sl. No.	End Condition	Effective Height
1.	Lateral as well as rotational restraint	0.75 H
2.	Lateral as well as rotational restraint at one end and only lateral restraint at other	0.85 H
3.	Lateral restraint but no rotational restraint at both ends	1.0 H
4.	Lateral and rotational restraint at one end and no restraint at other ends (Compound walls, parapets etc.)	1.5 H

 

Effective Length of Walls of Length L

Sl. No.	Condition of support	Effective Length
1.	Continuous and supported by cross walls	0.8 L
2.	Continuous at one end and supported by cross wall at other end	0.9 L
3.	Wall supported by cross walls of each end	1.0 L
4.	Free at one end and continuous at other ends	1.5 L
5.	Free at one end and supported by cross wall at other end	2.0 L

Comparison between stone masonry and brick masonry

The merits and demerits of stone masonry and brick masonry are compared in table

Merits and Demerits of Stone Masonry and Brick Masonry

Description	Stone Masonry	Brick Masonry
1. Strength	High	Much Less
2. Durability	Excellent	Less
3. Appearance	Beautiful. No treatment is necessary with age.	Not so good. Needs plastering and colour washing.
4. Danger from dampness	No danger	May disintegrate.
5. Skill	Skilled Labour is required for dressing and placing stones.	Ordinary skill is enough.
6. Handling	Heavy. Hence handling cost is more.	Easy to handle. Hence handling cost is less.
7. Fire resistance	Less	More.
8. Moulding to the desired shape	Needs skilled labour	Convenient
9. Uses	For foundations, walls in buildings, dams, piers and abutments.	For building load-bearing and position walls.

 

 

Framed constructions

Framed construction starts with foundations for columns. Columns are then raised. Beams and floors are built simultaneously in the case of R.C.C. Construction goes floor by floor. After the skeleton of the second floor is ready construction of walls is taken up. Construction of multistorey buildings is possible in this type of construction.

The advantage of framed construction is an interior alteration of rooms is possible by removing or by constructing additional walls.

In factories, steel frame structures are also used. In these cases, flooring is by R.C.C. and roofing is usually with trusses supporting A.C. sheets.

 

Composite Construction

If facing and backing of walls are made using different materials it is called composite wall construction. The facing material used is always good in appearance.

The following types of composite constructions are used:

1. Stone slabs facing with brick masonry backing.

2. Dressed stone facing and brick masonry backing.

3. Brick facing with rubble stone masonry.

4. Tile facing and brick backing.

5. Brick facing and concrete backing.

6. Stone facing and concrete backing.

In all these constructions proper bond between facing and backing should be achieved. For this purpose, GI or aluminium clamps may be used. In the case of brick facing alternate courses of bricks are projected inside the backing. Rich plaster is used between facing and backing materials.

Shows stone slab facing with brick masonry backing.

Composite masonry—stone facing with brick masonry backing

 

Must read: What is Lintel? | Types of Lintel | Uses | lintel length Calculation | lintel bearing

 

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