Storey Drifts

Introduction to Drift and Deflection

Lateral deflection is the predicted movement of a structure under lateral loads; and story drift is defined as the difference in lateral deflection between two adjacent stories. 

During an earthquake, large lateral forces can be imposed on structures; both the 1997 UBC (the basis of the 2007 Building Code Pakistan) and ASCE 7-02 requires the designer to assess the effects of this deformation on both structural and nonstructural elements. Lateral deflection and drift have three primary effects on a structure; the movement can affect the structural elements (such as beams and columns); the movements can affect non-structural elements (such as the windows and cladding); and the movements can affect adjacent structures. Without proper consideration during the design process, large deflections and drifts can have adverse effects on structural elements, nonstructural elements, and adjacent structures.

Effect of Drift on Nonstructural Elements

Since lateral deflection and drift affect the entire building or structure, design of nonstructural elements is also governed by these parameters. The nonstructural elements should be designed to allow the expected movement of the structural system. If the nonstructural elements are not adequately isolated from the movements of the lateral force resisting system, adverse effects are likely. For example, in a large earthquake, the cladding may become damaged or fall off the structure, posing a life-safety hazard to passers-by. Even in smaller earthquakes, if the cladding does not permit lateral movement of the structure, the cladding may experience premature damage, resulting in water intrusion and/or economic loss. Similarly, if windows do not permit movement of the structure, the windows may break, posing a potentially significant falling hazard.

The effects of deflections and drift on stair assemblies are sometimes neglected. Without proper detailing that permits adequate inter-storey movement to occur, stair assemblies have the potential to act as a diagonal brace between floors; the stair assemblies resist the movement of the structural frame until damage to the stair assemblies or their connections occurs. If the vertical support for the stair assembly breaks or is damaged, the stairs can collapse during the earthquake or even after the earthquake as the occupants attempt to exit.

Finally, if the nonstructural elements are not adequately isolated from the structural elements, the nonstructural elements may interfere with the structural elements and cause adverse effects to the structural elements themselves, creating short columns, torsion, or stiffness irregularities.

Effect of Drift on Adjacent Structures 

Pounding of Adjacent Structure

Under lateral loads from a large earthquake, the expected movements of a structure can be significant. Consequently, both the 1997 UBC and ASCE 7-02 require that adjacent structures be isolated from each other by a prescribed distance so that contact between adjacent structures is minimized. If adjacent buildings or structurally separate portions of the same structure do not have adequate separation, they may “pound” against each other during an earthquake. Pounding can have significant adverse effects, especially when the floors are not co-planar. Pounding of structures with non-co-planar floors can result in the floors of one building impacting the columns of another building at mid-height. This impact induces large shears and bending moments into the impacted columns, potentially causing the columns to fail and the structure to collapse. When adjacent structures have coplanar floors, pounding may be advantageous in some respects. If floors are coplanar, the two adjacent structures will have a more difficult time resonating with the earthquake. Since pounding is a highly non-linear response, pounding will tend to damp out vibrations and reduce the responses of the two structures. However, the pounding is likely to increase floor accelerations and is likely to result in significant localized damage between the structures.

Effect of Drift on the Structure


In terms of seismic design, lateral deflection and drift can affect both the structural elements that are part of the lateral force resisting system and structural elements that are not part of the lateral force resisting system. In terms of the lateral force resisting system, when the lateral forces are placed on the structure, the structure responds and moves due to those forces. Consequently, there is a relationship between the lateral force resisting system and its movement under lateral loads; this relationship can be analyzed by hand or by computer. Using the results of this analysis, estimates of other design criteria, such as rotations of joints in eccentric braced frames and rotations of joints in special moment resisting frames can be obtained. Similarly, the lateral analysis can also be used and should be used to estimate the effect of lateral movements on structural elements that are not part of the lateral force resisting system such as beams and columns that are not explicitly considered as being part of the lateral force resisting system.

Design provisions for moment frame and eccentric braced frame structures have requirements to ensure the ability of the structure to sustain inelastic rotations resulting from deformation and drift. Without proper consideration of the expected movement of the structure, the lateral force resisting system might experience premature failure and a corresponding loss of strength. In addition, if the lateral deflections of any structure become too large, P-Δ effects can cause instability of the structure and potentially result in collapse.

Structural elements and connections not part of the lateral force resisting system need to be detailed to withstand the expected maximum deflections and drifts. Though these elements are generally ignored during the design lateral analysis, they must effectively “go along for the ride” during an earthquake, meaning that they experience deflections and rotations similar to those of the lateral force resisting system.

Consequently, both the 1997 UBC and ASCE 7-02 require that the structural elements not part of the lateral force resisting system be designed to maintain support of design dead and live loads under the expected deformations, including any PΔ effects.

Effect of Drift on the Structure