Can you define allowable drift and how it affects a contractor?
Drift is the lateral movement of buildings caused by wind or earthquakes. According to Don Allen, P.E., S.E., director of engineering at Super Stud Building Products, Inc., “If not addressed in design, drift can lead to problems such as cracking, serviceability of systems and even structural issues.” This discussion is limited to wind forces.
Buildings move with the wind. The higher the velocity of the wind and the taller the building, the more the building will move. Older high-rise buildings were designed to be stiffer. Newer high-tech building materials, analysis methods and wind research has resulted in more flexible buildings, as well as damping systems that reduce both deflection and acceleration in high-rise buildings. The early high-rise buildings measured drift in inches whereas today it is measured in feet. There are two ways the concept of drift is applied. The first is that the entire structure is evaluated for drift. The second is localized to one floor and how it behaves in relation to the floor above or below. This is called inter-story drift.
Up to the 2018 International Building Code, the only reference to drift was in section 1604.3: “Structural systems and members thereof shall be designed to have adequate stiffness to limit deflections and lateral drift.” The current version of the IBC references ASCE 7-16 Minimum Design Loads and Associated Criteria for Buildings and Other Structures. The standard does not establish drift limits for wind but does give guidance in the section on serviceability considerations in the appendix. For the amount of permissible drift, the appendix suggests an overall building limit between h/400 and h/600 (where h = building height in feet), or a single-story limit for inter-story drift of 0.4 inches to limit damage to nonstructural components.
Allowable drift is an engineering concept similar to allowable deflection that is used as a structural consideration for wall and floor design. In wall design, the concept is to limit how far the wall can deflect when subject to a load without destroying any attached cladding such as gypsum panels or plaster. The allowable drift relates to the height of a structure and limits the amount of horizontal sway of the entire building, or adjacent floors, when it is subjected to a transverse load such as wind. This becomes a design consideration for mid- and high-rise construction.
Similar to wall and floor deflection, a ratio of height or span to a given variable is utilized. For a wall, a common limit on deflection is L/240, where L is the height of the wall in inches. For allowable drift, a common ratio is h/500. This translates to the top of a 500-foot-tall building being allowed to move one foot.
From a gypsum panel installation standpoint, an important concept is to accommodate this drift. For an interior nonstructural wall, the gypsum panels should not be rigidly fastened to the top track, or other elements rigidly attached to the floor above. This same principle applies to nonstructural walls that terminate into a structural column. Gypsum panel ceilings should also be isolated from the structure. For structural load-bearing walls, this is not an issue. If wind drift exceeds the recommended 0.4 inches, and if nonstructural partition finishes are rigidly attached to structure, these finishes will likely crack and fail. For structures where the load-bearing system is not cold-formed steel, connectors are available that allow for drift. These connectors are generally specified for the larger magnitude drifts due to earthquakes and not for the smaller drifts caused by wind loading.
For load-bearing cold-formed steel framing, drift is controlled by shear walls or X-braced strapping, or a combination of the two. Inspectors are often concerned with loose diagonal straps, but Don Allen states that this has been shown to contribute relatively little to inter-story drift. The contractor should work with the specialty structural engineer on all the details that will be required, and understand their importance and how best to install.
The contractor should be confident in the knowledge that cold-formed steel is ideally suited to allow for both wind and seismic drift. Material properties of steel can provide resilience against drift through elasticity. However, what is needed to capitalize on these properties is a good design coupled with proper installation.
The construction documents should contain information relative to loading and structural requirements. A contractor should be aware of the impact of drift and accommodate it in the bid package and subsequent installation. For load-bearing applications, the SSE will have drift embedded in the design. For the nonstructural condition, contractors should seek information from construction documents and work out any missing drift isolation details with the design team.
Robert Grupe is AWCI’s director of technical services. Send your questions to grupe@awci.org, or call him directly at (703) 538.1611.
