Limit of Deflection & Why It Is Provided ?

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Limit of Deflection & Why It Is Provided ?
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Deflection have simple meaning ; an extent to which a member get displaced when subjected to the load. It can be expressed by means of angle (Fig. 1 : Beam is deflected by the angle \theta_B ) or a distance (Fig. 1 : Beam is deflected by the distance \Delta_B ). 

Deflection in Beam
Fig. 1 : Deflection in Beam

Same as the every term is structural engineering, deflection also have some limiting criteria known as Limit Of Deflection.

Limit of Deflection

According to IS code the standard limit for deflection is as follows;

DLL < L/360

DTL < L/240

Where, DLL= Deflection due to Live Load

DTL = Deflection due to Total Load

L = Length of member

The above limits were originally formulated for the member with brittle finishesh such as cement plaster (1:6). But considering the way things are changing in construction field, new products aRe introduced everyday, this limit of deflection can be optimized according to site conditions. Yet the specific criteria of deflection should comply with the regulations provided by the IS code.

Why There Is A Limit For Deflection

The main reason behind setting the limit for deflection is to minimise The possible damage and provide a reasonable comfort for the building occupants. Thus the limit of deflection comes under the limit state of serviceability (IS 456-2000).

As we know in limit state method we go one step ahead of the working stress method, that’s why the structure designed by the limit state method tends to be more slender and hence they become more prone to the highee deflection and thus they are needed to be checked for deflection & cracking. This can be done by comparing the original value of deflection with the limiting value of deflection. This limiting valuE of deflection assures that the various aspects of building are not affected due to the given deflection. The main four aspects of buikding are as follows;

  • Appearance of building
  • Finishing of surface (plasters, tiling etc.)
  • Efficiency of building (by means of utility purpose)
  • Partition of building (partition can be defined as the temporary or permanent light weight walls builf for dividing the big space into the small cabins or desks)

The above aspects are affected when There is a deflection to the great extent.

For example, a floor in a building is finished with tiling and when the load is applied to this slab/floor the slab get deflected. Thus now there will be reduction in the available length of top surface (compression at top & tension at bottom) resulting in formation of compressive stresses. The tiles have some sort of compressive strength hence they will resist The stresses to some extent but when that limit is crossed, the tile may crack & upset (Fig. 2) or there will be uplifting of some tiles(Fig.3)

Cracking of Beam Due to Deflection of Slab
Fig. 2 : Cracking of Beam Due to Deflection of Slab
Lifting of Tiles Due To Deflection of Beam
Fig. 3 : Lifting of Tiles Due To Deflection of Beam

Whereas in case of long span beam, if the beam is deflected to great extent, the general public will be critical about its safety also the astetic view of building will be disturbed and In case of godown building it may cause obstacle to some tall machinery or equipments or moving goods.

Thus to avoid aforementioned all sort of problems, we have to check the beam for deflection.

Udayram Patil

Udayram Patil

Udayram Patil is an enthusiastic graduate civil engineer with an eye for innovative & technical writing.His wish is to combine his knowledge and experience about his field, to deliver the best practical cum accurate information to his audience.
Udayram Patil

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