Seismic Design of Nonstructural Building Components: The New Frontier of Earthquake Engineering

Earthquake Engineering Applied to Nonstructural Elements

With the development and implementation of performance-based earthquake engineering, harmonization of performance levels between structural and nonstructural building components becomes vital. Even if the structural components of a building achieve a continuous or immediate occupancy performance level after a seismic event, failure of architectural, mechanical or electrical components can lower the performance level of the entire building system. This reduction in performance caused by the vulnerability of nonstructural building components has been observed during recent earthquakes worldwide. Moreover, nonstructural damage has limited the functionality of critical facilities, such as hospitals following major seismic events. The investment in nonstructural building components and building contents is far greater than that of structural components and framing. Therefore, it is not surprising that in many past earthquakes, losses from damage to nonstructural building components have exceeded losses from structural damage. Furthermore, the failure of nonstructural building components can become a safety hazard or can hamper the safe movement of occupants evacuating or of rescue workers entering buildings. In comparison to structural components and systems, there is relatively limited information on the seismic design of nonstructural building components. Basic research work in this area has been sparse, and the available codes and guidelines are usually, for the most parts, based on past experiences, engineering judgment and intuition, rather than on objective experimental and analytical results. Often, design engineers are forced to start almost from square one after each earthquake event: to observe what went wrong and to try to prevent repetitions. This is a consequence of the empirical nature of current seismic regulations and guidelines for nonstructural building components.

Importance of Considering Nonstructural Components in Seismic Design

• Nonstructural components represent the major portion of the total investment in typical buildings.

• Nonstructural damage can limit severely the functionality of critical facilities, such as hospitals.

• Harmonization of seismic performance between structural and nonstructural components becomes vital during an earthquake. Even if the good performance of structural components of a building allow its continuous and immediate occupancy after a seismic event, failure of architectural, mechanical, or electrical components and contents can lower the performance level and functionality of the entire building system.

• Failure of nonstructural components can become a safety hazard or can hamper the safe movement of occupants evacuating or of rescuers entering buildings.

• Damage to nonstructural components occurs at seismic intensities much lower than those required to produce structural damage. Steel moment‐resisting frames yield at story drifts beyond 1% while gypsum partition walls show significant crack at drifts as low as 0.25%. In many past earthquakes, losses from damage to nonstructural building components have exceeded losses from structural damage.

Challenges Associated with the Seismic Design of Nonstructural Components

• In comparison to structural components and systems, there is much less information available giving specific guidance on the seismic design of nonstructural building components for multiple‐performance levels.

• Limited basic research has been done and often design engineers are forced to start almost from square one: observe what goes wrong and try to prevent repetitions. This is the reason of the empirical nature of current seismic regulations and guidelines for nonstructural components. The design information currently available for the most part is based on judgment and intuition rather than on experimental and analytical results.

• Multiple factors affecting the seismic behavior of nonstructural components such as those that affect the structural response (i.e. the local soil conditions, the dynamic characteristics of the building structure, the regional seismicity, etc.) and then supplemental factors that influence the nonstructural response (i.e. the dynamic characteristics of the nonstructural element and its bracing to the structure, the location of the nonstructural component within the building, the function of the facility, the importance of the particular component to the operation of the facility, etc.)

Causes of Seismic Damage to Nonstructural Components

• Inertia forces caused by accelerations at various level in the structures cause overturning or sliding in mainly “acceleration‐sensitive” nonstructural components. e.g. unrestrained file cabinets, emergency power generating equipment, freestanding bookshelves, office equipment, and items stored on shelves or racks.

• Distortions imposed on nonstructural components when the building structure sways back and forth. Seismic response of a building gives rise to interstory displacements or drifts causing distortions in “drift‐sensitive” nonstructural components such as windows, partitions, and other items that are tightly locked into the structure.

• Separation or pounding at the interface between adjacent structures.

• Interaction between nonstructural components may share the same space in a ceiling plenum or pipe chase. These items may have different shapes, sizes, and dynamic characteristics, as well as different bracing requirements vibrate differently from one another causing dynamic interaction. Examples: Sprinkler distribution lines interacting with the ceiling causing the sprinkler heads to break and leak water into the room below. Adjacent pipes of differing shapes or sizes are unbraced and collide with one another or adjacent objects. Suspended mechanical equipment swings and impacts a window, louver, or partition. Ceiling components or equipment can fall, slide, or overturn blocking emergency exits.

References:

Taghavi S., and Miranda E. [2002] “Seismic Performance and Loss Assessment of Nonstructural Building Components,” Proceedings of 7th National Conference on Earthquake Engineering, Boston, USA.