Deflections

This post explains the basic concepts behind deflection.

Note that the terms Deflection and Displacement have a very similar meaning, and define the movement of a body from it's original position. Deformation however, has a slightly different definition.

Types of Deflection

There are four main types of deflection considered in the context of structural engineering:

• Horizontal
• Vertical
• Torsional
• Interstorey Drift

Horizontal and vertical deflections can be considered as translational displacements.

The final one, interstorey drifts, is related to horizontal deflections but is discussed in a separate post:

※ What is Interstorey Drift? Explained in 1-Minute 

Horizontal Deflection

When there is translational movement of a structure, then this is referred to as horizontal deflection. Since the deflections of structures are usually small, they are commonly expressed in units of mm.

Horizontal deflection may be caused by Wind Loading, Seismic Loading, or other types of lateral loads/instabilities. 

※ What are the Horizontal Deflection Criteria? Explained in 1-Minute

Vertical Deflection

Vertical deflections describe motions of the structure in (or opposite to) the direction of gravity. Like horizontal deflections, they are commonly expressed in units of mm.

Vertical deflections may be caused by the self-weight of the structure or imposed loads (e.g. finishes, Live Loading). Other important load considerations for vertical deflections include:

• Wind Uplift / Downforce
• Snow loads
• Vertical Seismic Ground Motion 

※ What are the Vertical Deflection Criteria? Explained in 1-Minute

Torsional Deflection

Torsion describes the twisting about the longitudinal axis of a member. This is usually expressed in units of radians.

Torsional deflection can be cause when the loading is eccentric/unbalanced, and when there are no other structure members which frame into the section.

※ What are the Torsional Deflection Criteria? Explained in 1-Minute

※ What is Compatability Torsion? Explained in 1-Minute

Concluding Remarks

Remember, deflection is an important serviceability requirement for buildings. Excessive deflection may lead to building service issues and level issues (windows not opening properly), as well as discomfort for building occupants. In the worst-case, they can present instability problems.

Finally, deflection is not the only important serviceability requirement in terms of the building movement. Other important considerations include velocities and accelerations, such as:

※ Wind-Induced Acceleration of the Building

※ Peak/RMS Accelerations due to Pedestrian Footfall Vibration

※ Peak Accelerations due to Seismic Loading

※ Storey Torsional Velocities due to Wind Loading