Interstorey Drift

For buildings, horizontal deflections are a large concern. However, the relative horizontal deflections of one floor to the next is also a very significant concern. In this post, we will look at the relevance of interstorey drift and why it is important to control it within certain limits.

What is Interstorey Drift?

Consider the building below which is subjected to some seismic forces.

All the floors will deflect, some more than others. However, we need to control the relative deflection between floors. This relative deflection (for example, δRF - δ4F) is known as the interstorey drift.

If one floor moves too much compared to the floor below, then there could be significant damage to the facade and secondary elements which cannot follow the movement. 

Interstorey Drift Angle

By itself, the interstorey drift is not a very useful indicator. For example, if the interstorey drift is 10 mm, it's difficult to judge whether it is excessive or not. Therefore we look at the interstorey drift in relation to the height of the storey being considered by calculating the interstorey drift angle.

The interstorey drift angle can be calculated as:

However, for very small values of deformation, this can be simplified to:

This is also known at the storey drift ratio. Let's consider the simple example below.

Example

We can calculate the interstorey drift angle as:

You can also describe it as a 0.5% interstorey drift. By the way, in Japan, the interstorey drift for steel structures is usually limited to 1/200 for the 1st Stage Design (elastic design). Reinforced Concrete (RC) structures are usually much stiffer, so interstorey drift is unlikely to govern design decisions.

※ What is the Interstorey Drift Criteria?