2.2 Causes of Nonstructural Damage
- 2.2.1 Inertial Forces
- 2.2.2 Building Deformations
- 2.2.3 Building Separations
- 2.2.4 Nonstructural Interaction
Earthquake ground shaking causes damage to nonstructural components in four principal ways:
- Inertial or shaking effects cause sliding, rocking or overturning (Section 2.2.1).
- Building deformations damage interconnected nonstructural components (Section 2.2.2).
- Separation or pounding between separate structures damage nonstructural components crossing between them (Section 2.2.3).
- Interaction between adjacent nonstructural components (Section 2.2.4) cause damage.
When a building shakes during an earthquake, the base of the building typically moves in unison with the ground. The entire building and its contents above the base experience inertial forces that push them back and forth in a direction opposite to the base excitation. In general, the earthquake inertial forces are greater if the mass of the building is greater, if the acceleration or severity of the shaking is greater, or if the location is higher than the base, where excitations are amplified. Thus, the earthquake forces experienced above the base of a building can be many times larger than those experienced at the base.
When unrestrained or marginally restrained items are shaken during an earthquake, inertial forces may cause them to slide, swing, rock, strike other objects, or overturn (see Figure 2.2.1-1). File cabinets, emergency generators, suspended items, free-standing bookshelves, office equipment, and items stored on shelves or racks can all be damaged as they move and contact other items, fall, overturn or become disconnected from attached components. The shaking can also cause damage to internal components of equipment without any visible damage or movement from its original location.
General Interest Sidebar
As a passenger in a moving vehicle, you experience inertial forces whenever the vehicle is rapidly accelerating or decelerating. If the vehicle is accelerating, you may feel yourself pushed backward against the seat, since the inertial force on your body acts in the direction opposite to that of the acceleration. If the vehicle is decelerating or braking, the inertia forces may cause you to be thrown forward in your seat.
During an earthquake, structural members of buildings can deform, bend or stretch and compress in response to earthquake forces. For example, the top of a tall office tower may lean over a few feet in each direction during an earthquake. The horizontal deformation over the height of each story, known as the story drift, might range from a quarter of an inch to several inches between adjacent floors, depending on the size of the earthquake and the characteristics of the particular building structure and type of structural system. The concept of story drift is shown in Figure 2.2.2-1.
When the building deforms, the columns or walls deform and become slightly out of square and thus, any windows or partitions rigidly attached to the structure must also deform or displace the same amount. Brittle materials like glass, plaster partitions, and masonry infill or veneer cannot tolerate any significant deformation and will crack when the space between stops or molding closes and the building structure pushes directly on the brittle elements. Once cracked, the inertial forces in the out-of-plane direction can cause portions of these architectural components to become dislodged and to fall far from their original location, possibly injuring passers-by underneath them.
There have been many notable examples in past earthquakes where rigid nonstructural components have been the cause of structural damage or collapse. These cases have generally involved rigid, strong architectural components, such as masonry infill or concrete spandrels that inhibit the movement or deformation of the structural framing and cause premature failure of column or beam elements. When a structural column is restrained by nonstructural components, it is often referred to as a "short column" or "captive column." This is a serious concern for the design of structural systems. Designers of nonstructural components must be mindful to isolate their systems from the deformations of the adjacent structural components or to make sure that the structural components have been designed to accommodate the interaction.
Another source of nonstructural damage involves pounding or movement across separation or expansion joints between adjacent structures or structurally independent portions of a building. A seismic joint is the separation or gap between two different building structures, often two wings of the same facility, which allows the structures to move independently of one another as shown in Figure 2.2.3-1.
In order to provide functional continuity between adjacent structures or between structurally independent portions of a building, utilities must often extend across these building joints, and architectural finishes must be detailed to terminate on either side. The separation joint may be only an inch or two wide in older construction or a foot or more in some newer buildings, depending on the expected horizontal movement, or seismic drift between buildings. Flashing, piping, conduit, fire sprinkler lines, heating, ventilation, and air-conditioning (HVAC) ducts, partitions, and flooring all have to be detailed to accommodate the seismic movement expected at these locations when the two structures move closer together or further apart. Damage to items crossing seismic separation or expansion joints is a common type of earthquake damage. If the size of the gap is insufficient, pounding between adjacent structures may result, which can damage structural components but more often causes damage to nonstructural components, such as parapets, veneer, or cornices on the façades of older buildings.
A special type of seismic joint occurs at the ground level of base-isolated buildings, which are separated from the ground by seismic shock absorbers or isolators, in order to reduce the transfer of earthquake accelerations to the building. The seismic joint typically occurs between the foundation below the isolator and the building above. These joints may be as much as several feet wide; special detailing is required for all the architectural finishes and building utilities that cross the joint.
An additional source of nonstructural damage is the interaction between adjacent nonstructural systems which move differently from one another. Many 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.
Some examples of damaging nonstructural interactions include:
- Sprinkler distribution lines interact 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.