Structural Damage

Structural Damage

THE NORTHRIDGE EARTHQUAKE

At 4:31 a.m. Pacific Standard Time on January 17, 1994, an Mw 6.7′ earthquake struck southern California’s San Fernando Valley. The epicenter, about one mile southwest of the Northridge area of the valley, was at a depth of 14 km and on a plane, or earthquake fault, which dipped about 40° southward. The portion of the earth’s crust above this plane was forced northward over the portion beneath.

Because the fault was not clearly observable prior to the earthquake, and indeed did not clearly emerge to the surface in this event, it is termed a “blind thrust fault.” Only in recent years has it become clear that blind thrust faults exist over a large portion of the Los Angeles Basin, providing the potential for events similar to the Northridge Earthquake.

While the duration of strong shaking, around 20 seconds, was not unusual for an event of this size, ground motions associated with the earthquake were unusually severe. Several earthquake instruments in the epicentral area recorded accelerations of about 0.9g. That is, horizontal forces due to the earthquake were about equal to the weight of the structure undergoing the forces- analogous to a man’s being hit in the stomach with a sandbag of a weight equal to his own, once per second, for 20 seconds. For comparison, the building code is predicated on design ground motions of about 0.4g, although it is anticipated that buildings designed to this level should fare reasonably well even if subjected to 0.6 to 0.7g.

The San Fernando Valley lies about 20 miles northwest of downtown Los Angeles and has a residential population of more than a million people. It is relatively builtout (fully built) with single-family dwellings; low-rise apartments; and commercial, light-industrial, and mid-rise buildings of up to 20 stories, including the 28,000-student California State University campus at Northridge and the multibuilding headquarters of Great Western Bank. Nonbuilding structures include several major freeways with attendant complex interchange overpasses, the terminus of the Los Angeles Aqueduct, major electric transmission intertie substations, oil and gas pipelines, rail lines, and Burbank Airport.

Damage to these buildings and infrastructure was moderate to severe over much of the valley and extending into the Los Angeles Basin, particularly Santa Monica. A number of modem buildings collapsed, and literally thousands of buildings sustained significant damage. At least 57 people died, and 9.158 were seriously injured.’ Overall, property damage is estimated at approximately $15 billion, equivalent to about $1,000 per capita for the entire southern California population.

Particularly significant nonbuilding damage occurred to freeway overpasses and water and gas pipelines. Freeways collapsed at the north end of the valley; at the 15/1405 interchange; and on the east-west running I-10 (Santa Monica) freeway, the busiest freeway in the world, over which 300.000 vehicles travel daily. Several fatalities resulted at the 15/1-405 collapse, including a police motorcyclist on an earthquake-related response, who drove off the overpass at the collapsed section. However, the search and rescue effort for these collapses was minimal compared with the major effort required in the case of the 1989 Loma Prieta Earthquake at the Cypress structure collapse in Oakland.

Underground water and gas pipelines broke in numerous places in the valley, resulting in loss of hydrant pressure for most of the valley’s north and west portions. Responding fire units resorted to drafting residual water from mains and backyard swimming pools. However, the majority of fire service responses were to building collapses. since buildings sustained the majority of property damage. Because an understanding of damage typical to buildings is critical to fire department emergency planning and response, this article first briefly reviews how earthquakes affect buildings and then discusses the specific damage observed to wood, masonry, concrete, and steel buildings in the Northridge Earthquake.

HOW EARTHQUAKES AFFECT BUILDINGS

When earthquake shaking occurs, a building gets thrown from side to side and/or up and down-that is, while the ground is violently moving from side to side, the building tends to stay at rest, similar to the way a passenger stands on a bus that accelerates quickly. Once the building starts moving, it tends to continue in the same direction, but by this time the ground is moving back in the opposite direction (as if the bus driver first accelerated quickly, then suddenly braked). Thus, the building gets thrown back and forth by the motion of the ground, with some parts of the building lagging behind and then moving in the opposite direction. The force (F) that the building sustains is related to its mass (m) and the acceleration (a), according to Newton’s law. F = ma. The heavier or more massive the building, the more force is exerted. Therefore, a tall, heavy, reinforced-concrete building would be subjected to a much greater force than would a lightweight, onestory, wood-frame house given the same acceleration.

(Photo by Keith D. Cullom, IFPA.)

(Photo courtesy of EQE International.)

Damage can result from structural members (beams and columns) being overloaded and/or differential movements between different parts of the structure. If the structure is sufficiently strong to resist these forces or differential movements, little damage will result. If the structure cannot resist these forces or differential movements. structural members will be damaged, and collapse may occur.

Building damage is related to the duration and severity of the ground motion. Larger earthquakes tend to shake longer and harder and therefore cause more damage to structures. Earthquakes with magnitudes less than five rarely cause significant damage to buildings, since acceleration levels and the duration of shaking for these earthquakes are relatively small. In addition to damage caused by ground shaking, damage may result from buildings pounding against one another, ground failure that causes the degradation of the building foundation, landslides, fires, and tidal waves (tsunamis).

The level of damage that results from a major earthquake depends on how well a building has been designed and constructed. Most buildings are designed with a lateralforce resisting system (LFRS) to resist the effects of earthquake forces. In many cases, a building’s LFRS makes it stiller and thus minimizes the amount of lateral movement and, consequently, damage. The l.FRS generally consists of axial(tension and/or compression) bracing, shear-resistant elements, and/or bending-resistant elements. Large overhangs, many wings (H. E, and other building plan shapes, typical of hospitals and high-rise apartments), or “softstory’’ types of complex building configurations tend to introduce discontinuities in the LFRS, leading to stress concentrations that cannot always be foreseen. A “soft story” is a building floor or story whose structural lateral stiffness is substantially less than that of the floor above (and/or below). An example is apartment buildings with ground-floor parking, which typically have many fewer walls on the ground floor than the floors above. Unless specially detailed, this “vagrant architecture” (no visible means of support) will sustain extreme displacements in the soft story, leading to structural failure and possibly collapse.

WOOD FRAME

In pre-World War II wood-frame houses, the LFRS typically is taken to be oneby six-inch wood bracing, although the lath and plaster walls actually provide considerable strength (as do stucco or other exterior claddings). In newer homes, plywood siding typically replaced the bracing used to prevent excessive lateral deflection. Without the extra strength provided by the plywood, walls would distort excessively, or “rack,” resulting in broken windows and stuck doors. The building code permits gypsum wallboard to be used in lieu of plywood, if there are sufficient walls adequately nailed with this material. However, use of this material as the LFRS is being reexamined as a result of the Northridge event.

(Photo by Keith D. Cullom, IFPA.)

In the Northridge event, damage was widespread to unbolted homes, due to sliding, and to homes with etipple walls (see Figure I). Newer homes in some cases still sustained major damage due to simple overloading, given the very high ground motions. Many homes in the affected area sustained considerable non structural cracking of walls, broken glass, and damaged contents, but the majority of homes still were not seriously damaged structurally, and relatively few collapsed (although many chimneys came down).

Apartments. One type of building that sustained considerable damage in the Northridge event was the twoand three-story apartment building, mostly built in the 1960s to 1980s. Literally hundreds of apartment buildings of two distinct types, described below, sustained major damage.

• Older buildings built completely of wood, with parking beneath. These lack a well-designed LFRS and in many cases simply collapsed as a “house of cards” onto the cars beneath due to their “soft story.” The greatest single fatal incident was in a building of this type, in which 16 people in ground-floor apartments were killed when the lowest story “pancaked.”
• More modern, larger buildings, typically consisting of a ground-floor parking structure of reinforced concrete (a shearwall structure with a post-tensioned slab) on top of which is built a twoor three-story wood stud wall apartment building. In many cases, the slab is designed so that the wood buildings above can be built virtually at will on the slab: however, this results in the wood walls’ not aligning with concrete walls below, overloading the slab. The wood buildings’ LFRSs in Northridge generally are composed of gypsum wallboard. Many of these fared very poorly, sustaining the following two main areas of damage.

-The wood stud wall structure atop the reinforced-concrete parking structure was violently shaken, due both to the ground motions as well as the amplification of the ground motions by the reinforced-concrete parking structure. The result was that these structures sustained accelerations well in excess of l.Og-perhaps as great as 2.0g. Without plywood or other structural bracing. the gypsum wallboard cracked badly and pulled away from its nailing, allowing the buildings to rack, or lean significantly. The result was that many of these buildings lost all structural integrity and had to be vacated for extended repairs, if not demolition.

(Photo at left courtesy of EQE International; photo at right by Glenn P. Corbett.)

The second area of damage was to the reinforced-concrete structure itself , primarily to the columns supporting the slab, due to a combination of excessive vertical motions as well as lateral deflections.

UNREINFORCED MASONRY BEARING WALL

Due to the major damage that occurred to unreinforced masonry(URM-) bearing wall structures in the 1933 Long Beach Earthquake. these types of structures have not been permitted in California since 1933. It should be noted that this date is nominal; URM construction continued into the 1940s in portions of California lacking strict building code enforcement (at last count, 24,000 URM buildings still existed in California). In some areas of the United States, this type of construction continues to this day.

These buildings usually range from one to six stories in height and typically function as commercial, residential, and industrial buildings. The construction varies according to the type of use. although wood floor and roof diaphragms are common. Smaller commercial and residential buildings usually have light wood floor/roof joists supported on perimeter URM wall and interior wood load-bearing partitions. Larger buildings, such as industrial warehouses, have heavier floors and interior columns, usually of wood. The bearing walls of these industrial buildings tend to be thick, often as much as 24 inches or more at the base. Wall thicknesses of residential buildings range from nine inches at upper floors to 18 inches at lower floors.

The LFRS typical to the URM is the “box” type-the roof and walls form a “box.” Lateral loads due to the earthquake are distributed by the roof diaphragm to walls, which resist these forces via in-plane shear action. Where these buildings existed in the area affected by the Northridge Earthquake, such as in Santa Monica, they sustained major damage, replicating patterns observed in all previous earthquakes (see Figure 2). Typical problems included the following.

(Photo courtesy of EQE International.)

(Photos courtesy of EQE International.)-

(Photo by Glenn P. Corbett.)

• Insufficient anchorage. Because the walls, parapets, and cornices are not positively anchored, they tend to fall out. The
• collapse of bearing walls can lead to major building collapses. Some of these buildings have wall anchors as a part of the original construction or as a retrofit. These anchors provide some benefit, but even many retrofitted buildings have subsequently been demolished.
• Excessive diaphragm deflection.
• Because most of the floor diaphragms arc constructed of wood sheathing, they are very flexible and permit large out-of-plane deflection at the wall transverse to the direction of the earthquake. The large drift, occurring at the roof line, can cause masonry walls to collapse under their own weight.
• Low shear resistance. The mortar used in these older buildings often is made of lime and sand, with little or no cement, and has very little shear strength. Bearing walls, are therefore weak in-plane and will be heavily damaged.

TILT-UP

In traditional tilt-up buildings (see Figure 3), concrete wall panels are cast on the ground and then lilted upward into their final positions. More recently, wall panels have been fabricated off-site and trucked in. The wall panels are welded together or held in place by cast-in-place columns or steel columns, depending on the region. Tilt-up LFRSs are conceptually identical to URMs- that is. a “box” system in which the tilt-up wall panels are resisting loads from the roof through in-plane shear action. The roof beams are often glue-laminated wood or steel open-webbed joists that are attached to the wall panels; these panels may be load-bearing or nonload-bearing, depending on the region. These buildings tend to be low-rise industrial or office buildings and are very common in the area affected by the Northridge Earthquake.

Before 1973 in the western United States, many tilt-up buildings did not have sufficiently strong connections or anchors placed between the walls and the roof and floor diaphragms (see Figure 4). During an earthquake, tension in the roof diaphragm results in crossgrain bending in the ledger. Because wood is very weak in cross-grain bending, the ledgers tended to split horizontally, failing the wall-roof connection. The connections between concrete panels are also vulnerable to failure. Without them, the building loses much of its lateral-force resisting capacity. For these reasons, many tilt-up buildings were damaged in the 1971 San Fernando Earthquake. Since 1973, tiltup construction practices have changed in California and other high seismicity regions, requiring positive (direct) walldiaphragm connection.

Prior to the January earthquake, a large number of these older, pre-1970s vintage tilt-up buildings still existed in the Northridge area and had not been retrofitted to correct this wallanchor defect. Damage to several hundred tilt-up buildings was observed, both to older as well as newer tilt-up buildings, in this earthquake. Damage to newer buildings was due either to anchors failing or to portions of the plywood roof diaphragm failing in tension or shear, allowing roof panels to collapse.

REINFORCED CONCRETE

Reinforced concrete is a very common building material. From a structural or LFRS viewpoint, reinforced-concrete buildings can be divided into two major categories.

• Frame. In this type, the LFRS is composed of beams and columns, and the lateral forces are resisted by bending in these elements (see Figure 5).
• Shear walls. In this type, the LFRS is composed of reinforced-concrete walls, and the lateral forces are resisted primarily by the in-plane stiffness of these walls.

Concrete moment-resisting frame. Concrete is a material with excellent compressive strength but weak in tension. It therefore must be reinforced with steel. An earthquake will cause a concrete frame’s beams and columns to flex back and forth. This flexing tends to shear the concrete beams and columns-that is. place them in tension diagonally to their length, so that they also require reinforcing steel to wrap or “confine” the beam or column core, resisting this diagonal tension.

Recognition of the need for extensive confinement grew during the 1960s and was underscored by the failure of buildings in the 1971 San Fernando Earthquake in the same vicinity as that affected by the 1994 Northridge event. As a result, requirements for proper confinement of reinforced-concrete frame elements, termed “ductility requirements,” were introduced in the 1970s. However, many reinforced-concrete frame buildings built before these requirements were introduced still existed in 1994. and several collapsed.

Another problem seen in recent earthquakes and resulting in a dramatic collapse is that of “punching shear,” in which steel reinforcing is insufficient or lacking in the concrete column-to-slab connection, and the alternating flexure of the building, combined with vertical accelerations, causes the concrete directly around the column-slab interface to fail in diagonal tension, allowing the slab to be “punched through” and collapse (see Figure 6).

Concrete shear-wall buildings. Shearwall buildings have a concrete LFRS consisting of cast-in-place concrete walls that are relatively long compared with their height, making them very stiff. Before the 1940s, these systems were used in schools, churches, and industrial buildings. Frame buildings with shear walls tend to be commercial and industrial. A common example of the latter type is a warehouse with perimeter concrete walls.

Concrete shear-wall buildings constructed since the early 1950s tend to be institutional, commercial, and residential buildings, ranging from one to more than 30 stories in height. The shear walls in these newer buildings can be located along the perimeter, as interior partitions, or around the service core.

These buildings generally perform better than concrete-frame buildings. They are quite heavy relative to steel-frame buildings, but they also are rigid due to the presence of shear walls. Some types of damage observed in the Northridge earthquake were

• shear cracking and distress that occurred around openings in concrete shear
• walls (see Figure 7);
• shear failure that occurred at wall construction joints, usually at a load level below the expected capacity: and
• bending failures that resulted from insufficient chord steel lap lengths.

None of these problems resulted in structural collapse during the Northridge Earthquake.

Forking garages. A number of reinforced concrete parking structures experienced partial collapse in the Northridge Earthquake, including a dramatic collapse at Cal State’s Northridge campus. This structure’s LFRS consisted of a perimeter ductile reinforcedconcrete frame, which performed very well, exhibiting almost “taffy”-Iike resistance to deformation. However, while the building’s LFRS enabled it to sustain very large displacements. it also was its literal downfall: Only the perimeter beams and columns were detailed for ductility-the interior beams and columns were not designed to undergo these displacements. As a result, when they failed due to excessive displacements, they dragged the perimeter frames down with them.

In contrast to this dramatic failure, most reinforced-concrete parking garages have shear-wall LFRSs. In many cases, however. the shear walls are the hare minimum, and loads not considered in the design are thrown onto shorter columns, which fail in shear and drop their beams and girders.

STEEL

Steel-frame buildings generally may be classified as either moment-resisting frames or braced frames, based on their LFRS. In braced frames, the lateral forces or loads are resisted by the tensile and compressive strength of the bracing, while momentresisting frames resist lateral loads and deformations by the bending stiffness of the beams and columns (i.e., there is no diagonal bracing).

Steel-frame buildings tend to be satisfactory in their earthquake resistance compared with other structure types because of their strength, flexibility, and lightness. Collapse in earthquakes has been very rare, although steel-frame buildings did collapse, for example, in the 1985 Mexico City Earthquake. In the United States, these buildings have performed quite well and generally have not been considered susceptible to collapse unless subjected to extremely severe ground shaking.

However, a surprising number of steel moment frames built in the 1980s exhibited a type of failure in the Northridge event not previously observed. This failure was shearing at beam-column moment connections, primarily in a type of connection in which the moment (or bending) is transferred via welding of the upper and lower beam flanges to the columns, while the gravity or vertical load is transferred via a bolted connection (see Figure 8).

This type of failure was observed in more than 50 buildings in the affected area and has led to major concerns about this previously well-regarded type of construction. It should be noted that there are literally thousands of buildings with this type of connection in California and other seismically active parts of the United States. On the other hand, although this damage is very serious, no steel buildings collapsed; and. indeed, it took several weeks for this damage even to be noticed. Research on this problem is actively underway.

In conclusion, the Northridge Earthquake is the most damaging earthquake, at least from a property point of view, to have occurred in the United States since the 1906 San Francisco Earthquake. In recent times, the only disaster to rival it in terms of property loss has been Hurricane Andrew in 1992. While Andrew caused greater monetary loss and much more damage to housing. it actually caused comparatively little damage to larger buildings. In contrast, the Northridge event is typical of earthquakes in that larger, multistory buildings were not immune from major structural damage and even collapse. Such damage poses severe challenges to firefighters responding to post earthquake fires and/or search and rescue emergencies.

(Photos by Glenn P. Corbett.)

Various structural systems employed in different kinds of buildings perform differently from a seismic perspective. Knowing these differences will help emergency responders better understand when a building poses a major collapse peril and prepare for it. This discussion also illustrates how much we need to continue to study earthquakes and their effect on our constructed environment.

Endnotes

1. Mw termed moment magnitude, is a measure of total earthquake energy or size, similar to the bettcrknown Richter magnitude. Mw and Richter magnitude are numerically similar up to about magnitude 8. where Richter magnitude saturates, or “maxes out.” whereas Mw continues to accurately reflect the overall energy released in the earthquake.
2. R.S. Stein and R.S. Yeats. “Hidden Earthquakes.” Scientific American, June 1989: p48-57.
3. OES. 1994. Northridge Earthquake. Interim Report. California Governor’s Office of Emergency Services. Sacramento.