Earthquake Performance of Nonstructural Components

Structural versus Nonstructural Elements

Structural elements are typically components associated with the primary building structure used to provide the support and environmental enclosure for the facility functions. Nonstructural items support the function of the facility and typically include the following.

• Architectural components
  • Cladding
  • Interior partition walls
  • Ceilings and lights
  • Raised computer floor systems
  • racks and shelving
• Equipment and systems
  • Electrical power and distribution systems
  • Heating, ventilation, and cooling systems
  • Fire protection systems
  • Emergency power generation
• Building contents and inventory
  • Record storage
  • Production equipment and systems
  • Supplies/inventory
  • Computer equipment

For many facilities, particularly manufacturing or production facilities, the value of nonstructural components, equipment, and systems will typically exceed the value of the building structure itself. In a moderate earthquake, damage to critical equipment and contents may be more important than damage to buildings. In addition, damage to such equipment can lead to extended business interruption due to lost production and even a loss in market share. In many cases, business interruption may pose a corporation's greatest earthquake financial risk.

Historical Performance of Nonstructural Components

Past earthquakes can teach us valuable lessons regarding the vulnerabilities of nonstructural systems to even moderate levels of ground motion. For example, the 1994 Northridge Earthquake caused significant nonstructural damage to a number of area hospitals. In these instances, the hospitals remained structurally sound, but required closure due to significant damage to nonstructural components—primarily water damage and loss of emergency utility function. The problem lies in the treatment of these commodities in the building design codes.

The primary types of failures experienced by nonstructural components can be classified as either inertial failures or displacement/deformation failures.

Inertial failures are failures caused by the following.

• Excessive shaking of the component
• Component rocking due to unanchored or marginally anchored conditions
• Component sliding due to unanchored conditions

Good examples of inertial failures are shown below with the sliding rooftop AC units on the left and the overturned computer equipment shown on the right.

Displacement/deformation failures are failures caused by the following.

• Excessive building inter-story displacements or drift
• Incompatible stiffness between the building structure and component
• Interaction between adjacent structural systems and nonstructural systems
• Multiple structure connection points

Good examples of displacement/deformation type failures are shown below with the displaced ceiling grid on the left and a deformed architectural glazed wall on the right.

Losses Due to Nonstructural Damage

The impact from earthquake damage to a building or facility owner can frequently go well beyond the typical damage that is often depicted in post-earthquake photographs. The damage can be classified as either direct property damage or indirect property damage. Indirect property damage may include the following.

• Loss of operations
• Loss of service
• Loss of market share
• Business continuity or interruption

The photographs below illustrate the type of events that can lead to additional indirect losses above and beyond that of the direct damage loss that initiated the event. The photograph on the left depicts the flooding that occurred in a facility after a sprinkler pipe failure. Note, there was no fire following the earthquake for this facility.

The photograph on the right shows a common type of electrical panel failure that can often lead to further business interruption. Extended business interruption can result if specialty equipment is damaged due to the potential long lead times for equipment procurement, construction, shipping, and installation.

The losses due to business interruption, which are greatly influenced by nonstructural damage, can often equal or exceed losses due to the actual damage to the structure and equipment. The chart below compares the projected earthquake losses for a Midwest manufacturing facility. As shown by the chart, the loss due to business interruption, cleanup, and recovery is roughly equal to the total expected direct damage loss for the facility. Furthermore, the total expected loss exceeds the total value—buildings, equipment, and inventory, of the facility.

Building Codes and Nonstructural Components

Traditional model building codes, such as the Standard Building Code, BOCA, and UBC have not concentrated on the seismic design of nonstructural components, equipment, and systems. In fact, the model codes are defined as a minimum design requirement for the purpose of protecting life-safety. The model codes are not designed for the purpose of providing property damage protection to a building and its contents. In fact, acceptable building performance under the model codes can be a damage state that allows for the safe egress of building occupants from the building with no life-safe injuries, but the building sustains significant damage such that it is uneconomical to repair and return to service.

Probably the most important link in having the ability to quickly repair and resume facility function is the proper performance of equipment and systems. And, this is where the model codes contain less in their proper specification of design provisions.

The current interpretation and application of the model codes for equipment and systems is to treat them as independent components.

Most vendor equipment and system design agencies believe that if the equipment has been installed with seismically designed anchorage, it will perform adequately during and following a seismic event. This is true for approximately 75-80 percent of equipment installations. However, it is the internal component support and the interdependencies of the equipment systems that have often shown the greatest vulnerability for damage when subjected to earthquake ground motion. The identification of these key vulnerabilities and the specification of the proper seismic design criteria is imperative in order to ensure that the performance objective of continued function following a major earthquake is satisfied.

Since the 1964 Alaska and 1971 San Fernando earthquakes, the codes have attempted to increase both the scope and strictness of nonstructural seismic design provisions in an attempt to achieve better performance. It is within the last several code editions that the seismic design provisions for these commodities have begun to address the real issues in assuring the proper performance of nonstructural components, equipment, and systems when subjected to major earthquake events.

The new International Building Code (IBC) issued to replace the three model building codes in use throughout the United States incorporates more stringent design requirements for nonstructural components, which should aid in reducing the damage to nonstructural components. Specifically, the IBC does the following.

• Uses increased design forces relative to most of the model building codes
• Incorporates significantly greater prescriptive requirements for nonstructural components than most model building codes
• Specifies additional drift design provisions
• Incorporates additional and more specific requirements for anchorage design
• Requires the component itself to be seismically designed when the equipment is designated to have a higher level of importance (Ip>1.0); i.e., for hospitals, fire stations, police stations, etc.

Summary

The importance of properly designing, constructing, and installing nonstructural components in order to reduce the losses due to earthquakes cannot be overstated. As history has demonstrated, damage to nonstructural components in past earthquakes has resulted in the majority of the direct property losses. Additionally, the damage to nonstructural components can contribute to increased indirect losses due to business interruption and loss of market share. It therefore becomes incumbent on the building owner to understand the intended purpose of the model building design codes related to the expected, and acceptable, earthquake performance of the facility in establishing earthquake risk tolerance.