Torsion in building

 

What is torsion?

Today we will talk about the effects of torsion in the civil engineering field. Torsional force affects not only the civil engineering structures but also other mechanical structures too.

What causes the torsion? Basically, torsion is caused by the force called torque on the free end of the structure when the other end is fixed.

In buildings, torsion is created when the center of mass and center of stiffness (rigidity) doesn’t coincide. The distance between these two points is referred as torsion eccentricity. The lateral load acts on the centre of mass and the opposing force acts through centre of stiffness. Due to this a couple is developed which tries to twist the building. Hence whenever we design a building either manually or using software we must check the torsional eccentricity.

Center of mass is generally the geometric center of the floor or building.

Location of the center of stiffness depends upon characteristics of the components of the lateral load resisting systems. E.g.: - Shear walls, moment frames, braced frames.

Torsion in the buildings: -

                Let us suppose a building whose right part deflects more compared to its left part when the lateral load is applied. In this case, the building undergoes rotation, this rotation of building is called torsion. Simply torsion in a building is the rotation of the building due to the unequal displacement of the structural parts of the building.

Cases of torsion occur in the building:

1)      Irregular shape of the building.

                                     


In this building if applied by lateral load then the L portion of the building undergoes torsion differently as compared to the R portion of the building. We can clearly see in the figure the stressed region.

 

2)      Asymmetry in mass of building.         

Assume we got a regular building. Let us assume the R portion of the building has high mass/load as compared to the L part. In this case due to the asymmetry in the mass of the building whenever there is lateral load acting on the building the R portion has more displacement then the L portion. Due to this it undergoes torsion.

 

3)      Due to uneven wall infills: -

                                                                        


  Assume a regular building. Here we have wall infills and open space as according to the figure shown above. Here when acted by the lateral loading column (1) undergoes more deflection as compared to the column (2).

 

4)      False Symmetry building: -

                                                                               
Assume a regular building with a void for lift or staircase at one side and shear wall at two adjacent sides as shown in above plan. The above building looks symmetrical geometrically by actually it is called false symmetry. These types of buildings undergo torsion. So the best practice is that better build these staircases and voids at the Center of the building near the center of mass of the building and shear wall symmetrically on opposite sides.

 

Q)   How do we know that the building might fail in torsion?

Suppose a building of width ‘W’ with the center of stiffness ‘K’ and Centre of mass ‘M’ at the positions as shown in the above plan. Here the horizontal distance between the Centre of stiffness and Centre of mass is denoted by ‘d’. This ‘d’ is also called as the torsional eccentricity. If the value of ‘d’ is greater than 20% of the total width of building ‘W’ then we need to redesign the building doing some architectural or some structural changes.

                                i.e.  If d> 20% of W    then the building will fail.

 

Hence if torsion is not considered while designing a building then there might be severe damage in our structure during earthquake.

 

Photo of torsion failure in the building: -

                                                                 


                                                                                                          Prepared by: - Er. Biplov Sapkota

 

 


 


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