Thursday, February 09, 2017

What does IS:875-1987 covers?

This code covers basic design loads to be assumed in the design of buildings, in 5 parts. Each part is discussed here under:-

IS:875-1987(Part-1)- Dead Loads:-

                                                  This part specifies the unit weights of different types of building materials and the stored materials. The dead load in building shall comprise of the weight walls, partitions, floors etc. It is recommended that the load of partition walls, be assessed, by the designer on the basis of the actual constructional details of the proposed partitions.

IS:875-1987(Part-2)-Imposed Loads:-Imposed loads not only include live loads but also loads like that of machinery etc.

Live loads based on floor usage in various types of buildings.

Reduction in floor live load for design of columns.

Live load on Roofs:-
-Live loads on various types of roofs. 
-Snow load.
-Loads due to Rain.
-Loads on members directly supporting the roof coverings.
-Loads on roof coverings.
-Horizontal loads on parapets


IS:875-1987(Part-3)- Wind Loads:-

                                            This standard gives wind forces and their effects (static and dynamic) that should be taken into account when designing buildings, structures and components. The effect of wind on the structure as a whole is determined by the combined action of external and internal pressures acting upon it. Wind speed in the atmospheric boundary layer increases with height from zero at ground level to a maximum at a height. This code gives:

Wind map: which gives Basic Maximum Wind speed in m/s (peak gust velocity averaged over a short time interval of about 3 seconds duration). These wind speeds have been worked out for 50 years return period.  
Design Wind Speed(Vz) at any height z depends on the
-Probability factor,
-Terrain, height and structure size factor and
-Topography factor.

Design Wind Pressure: It depends on the:
-Coefficient, which depends on the atmospheric pressure and air temperature.
-Design Wind Velocity.

Off Shore Wind Velocity: Cyclonic storms form far away from the sea coast and gradually reduce in speed as they approach the sea coast.

Modification factors to modify the basic wind velocity to take into account the effect of terrain, local topography, size of structure, are included.

Terrain is classified into four categories based on characteristics of the ground surface irregularities. 

Force coefficients (drag coefficients) are given for frames, lattice towers, walls and hoardings.

The calculation of force on circular sections is included incorporating the effects of Reynolds number and surface roughness.

The external and internal pressure coefficients for gable roofs, lean-to roofs, curved roofs and multi- span roofs have been rationalized.

Pressure coefficients are given for combined roofs, roofs with sky light, circular silos, etc.

Dynamic effect.


 IS: 875-1987(Part-4)- Snow Loads:

                                             This standard deals with the snow loads on roofs of buildings. Roofs should be designed for the actual snow load due to snow or for the imposed loads specified in Part 2 Imposed Loads. Ground snow load at any place depends on the critical combination of the maximum depth of undisturbed aggregate cumulative snowfall and its average density. Ice loads should be taken into account in the design of over- head contact lines for electric traction, aerial masts.


IS:875-1987 (Part-5)-Special Loads and Loads Combinations:

The different load combinations to be considered for the design of any structure are:

                               Where DL indicates dead load , LL indicates live load, WL indicates wind load, IL indicates imposed load and EL indicates earthquake load. While considering earthquake effects, EL is substituted at place of WL. It may be noted here that there will be four cases (one from each direction) due to the effect of WL or EL. For symmetrical structures, the number of cases may be reduced accordingly.