Can you explain allowable drift in seismic design?
The design of buildings to resist seismic events is regulated by the International Building Code. The standard that is referenced within the code is ASCE 7-16 Minimum Design Loads for Buildings and Other Structures. The 2021 IBC establishes four risk categories based on the type of building under consideration. A design theory for seismic response is limiting the amount of movement the building is allowed during an earthquake. The allowable drift is a prime consideration. The lower the building code defined risk, the more the building is allowed to drift. Risk Category I includes temporary structures, agriculture facilities and minor storage structures. Category IV buildings must remain functional in the event of an earthquake. Aircraft control towers, fire stations and emergency shelters are examples.
The IBC established six Seismic Design Categories, designated as Categories A through F, which will drive design considerations. This category is determined by many factors, including local soil conditions.
The same concept of allowable drift is used to control building movement from wind forces. When comparing the limits of the two, Structure Magazine, ASCE 7-16 Provisions for Lateral Drift Determination, states, “The allowable drift of other structures with Risk Category I and II is approximately 10 times the allowable drift under wind forces.” In Categories III and IV the magnitude of the drift is reduced to maintain serviceability of the building during an earthquake.
The engineering for seismic conditions is covered in ASCE 7. The methodology employed for seismic analysis focuses on energy dissapation and limiting lateral drift. ASCE 7 sets limits on drift, which is a function of the IBC defined risk category in combination with the materials and systems that will resist the seismic force. The table also considers the seismic design category. Table 12.12.1 in ASCE tabulates the allowable story drift. This drift is shown in the table as a function of the story height. For example, for Risk Category III, the allowable drift is shown as 0.015 times the story height. Engineering concepts that should be analyzed include torsional irregularity and P-Delta effects.
Movement induced by earthquakes will affect both structural and nonstructural cold-formed steel partitions. Excessive movement in cold-formed steel structural framing for either wind or earthquake could lead to catastrophic failure of the structure. Movement from earthquakes on nonstructural partitions can result in serviceability issues, property loss and potential life safety concerns. Similar to designing for movement from wind, nonstructural partitions should not be rigidly attached to the structure at the head. One significant difference between resisting seismic movement over wind is that a crack in a gypsum panel that is a result of an earthquake can be acceptable. However, the same crack would be considered a failure when resisting wind forces.
An important discussion to have between the architect and the owner is how much damage to nonstructural components in a low seismic event is acceptable. This discussion should happen prior to design of the building, according to Don Allen, P.E., S.E., director of engineering, Super Stud Building Products, Inc.
The Federal Emergency Management Agency has published several documents on earthquakes, one of which is called “Reducing the Risks of Earthquake Damage – A Practical Guide.” One topic within the guide is a discussion on nonstructural architectural components. Architectural components include both ceilings and nonstructural partitions. They cite that nonstructural damage is caused by vertical or horizontal movement.
FEMA research reveals that causes of failure to nonstructural walls include the case where the movement of adjacent architectural components impact the partition or ceiling. An example provided in the document is where a ceiling light has damaged a gypsum panel ceiling due to the movement of the fixture. Wall cabinets mounted to the partition create eccentric loading that could cause a failure of the partition during an earthquake.
The FEMA guide states that an important consideration when designing and installing nonstructural cold-formed steel framed partitions is to pay attention to the head of the wall. Two design concepts need to be addressed. The first is where the wall runs the full story height, and the other is where the wall terminates just above the ceiling plane. In both cases, special detailing is required. The practical guide provides details for both conditions. A bracing system is recommended for the wall that terminates below the structure. Several methods are suggested where the wall terminates to the structure. The various options are based on the idea of attaching the wall to the structure but allowing for limited movement of the structure.
For the contractor, it is important to understand how the local building code, and the authority having jurisdiction will interpret seismic design for both structural and nonstructural applications. Enlisting the expertise of a qualified Specialty Structural Engineer is recommended. Further, understanding the design principles for both applications is essential. The supposition of allowable drift plays a fundamental role in both wind and seismic resistant design.
Robert Grupe is AWCI’s director of technical services. Send your questions to grupe@awci.org, or call him directly at (703) 538.1611.
