The Golden Gate Bridge

Different Structures and Their Challenges

Just because a building survived the 1906 San Francisco quake or the 1989 Loma Prieta quake does not necessarily mean that it will be safe in the next major Bay Area earthquake. The EQ damage that a building experiences is a function of the level of shaking it experiences, the type of soil it sits on, and the nature and quality of construction.

The intensity of shaking that a structure will experience during an earthquake is a function of three main factors:

  • The magnitude of the earthquake--the larger the quake, the stronger the shaking.
  • The distance from the fault that ruptured--the nearer you are to the fault, the greater the shaking.
  • The type of ground materials beneath the structure--soft soils amplify the shaking; hard bedrock does not.

Structural and geotechnical engineers, who have studied earthquakes and how buildings react to them for the last 100 years, know that an earthquake can even cause a wide variation of impact at building sites close to each other. A lot depends on the quality of the construction, the material used and whether the building meets the seismic building codes and standards.

The 1989 Loma Prieta earthquake caused more than 16,000 units to be uninhabitable throughout the Monterey and San Francisco Bay Area. Scientists and other groups who have analyzed potential earthquakes have determined that most future earthquakes here will have a greater impact than Loma Prieta. In fact, they predict future temblors will have an even greater impact than the 1994 Northridge earthquake in the Losa Angeles area where over 46,000 housing units were made uninhabitable. (See abag.ca.gov)

Single-Family Homes

Older houses (usually pre-World War II) that are not bolted to their foundations or lack bracing of walls enclosing a crawl space (the cripple wall) accounted for over 2,800 of the 16,000 housing units made uninhabitable by the Loma Prieta earthquake in 1989. Most Structural Engineers believe that we will probably experience more damange when an earthquake strikes the Bay Area more directly with a closer epicenter. (The Loma Prieta quake had its epicenter in the Santa Cruz Mountains, approximately 60 miles south of San Francisco.)

Home in Santa Cruz mountains

This home in the Santa Cruz Mountains, California, collapsed in the 1989 magnitude 6.9 Loma Prieta earthquake (USGS photo).

Pre-World War II homes are expected to account for over 11,000 of the expected 156,000 uninhabitable housing units that will result from a 7.1 or bigger earthquake on the Hayward fault.

An additional 2,200 post-World War II homes are also expected to be uninhabitable. These are homes with living areas over a garage, on hillsides, or lacking adequate cripple wall bracing.

Structural retrofit of older houses typically includes a combination of adding anchor bolts to prevent sliding off their foundations and adding plywood sheathing along portions of the cripple walls to prevent a collapse of those walls. In most cases, a Structural Engineer or qualified design professional should be consulted to address these elements listed above and other potential deficiencies.

Mobile Homes

Mobile homes that are not well tied to their foundations are prone to shifting off their supports in damaging earthquakes.

Mobile homes accounted for only a couple of hundred of the uninhabitable units resulting from the Loma Prieta earthquake. However, they accounted for almost 4,600 of the uninhabitable units resulting from the Northridge earthquake.

mobile home

Mobile home damage in the 1980 magnitude 5.8 Livermore earthquake (photo courtesy NISEE).

Mobile homes are expected to account for over 11,000 of the almost 156,000 uninhabitable units that will result from an earthquake on the entire Hayward fault. These units house a disproportionate share of the elderly and poor, particularly in the North and South Bay.

Three common methods can ensure that mobile homes are structurally safe, that is, that they are properly tied to their foundations:

  • Owners can consult a design professional and install a conventional foundation similar to that for a wood-frame home;
  • For new mobile homes or existing homes being relocated, they can install an Engineered Tiedown System using Standard Plan Approvals; or
  • For existing homes, they can install a Certified Earthquake Resisting Bracing System.

Wood-Frame, Multifamily Apartments

Multifamily wood-frame residential buildings, particularly with all or part of the first floor used for parking ("soft-story" buildings) are a huge problem in earthquakes. They were responsible for 7,700 of the 16,000 housing units rendered uninhabitable by the Loma Prieta earthquake and over 34,000 of the housing units rendered uninhabitable by the Northridge earthquake.

crooked basement

Source -- Courtesy of the California Office of Emergency Services, Coastal Region

The potential for deaths in these buildings is significant. For example, 16 people were killed in the Northridge Meadows apartment complex as a direct result of its soft story. Wood-frame multifamily apartment buildings are expected to account for 103,000 of the projected 156,000 uninhabitable housing units from an earthquake on the entire Hayward fault.

A large portion of San Francisco's housing stock, for example, is made up of soft-story buildings. There may be as many as 5,000 such buildings in the City, each one housing many families. The risk is due to the garage door openings and the extent of window openings at street level. Particularly at risk are corner buildings with retail establishments on both sides or streets. These retail/commercial stores usually have few interior partitions or shear walls but many large windows. These conditions weaken the structure's ability to withstand earthquake motion.

Structural Engineers expect these same units to create a disproportionate share of the potential public shelter population because they tend to be occupied by people with limited resources - the very poor, the very old, and the very young.

The structural retrofitting for apartments should involve a specific solution designed by a Structural Engineer or other design professional with past experience in this type of work.

Retrofitting multifamily buildings with large openings for parking involves adding bracing elements, like steel frames or shear walls at the lowest story level, and tying this bracing into the floor above that level. In taller buildings, some upper story walls may also need strengthening if the goal is more than reduction of life-safety risk. Because a building's height, shape and existing construction materials may vary, these factors can require additional retrofit measures to prevent collapse.

Unreinforced Masonry Buildings

Unreinforced masonry buildings were responsible for almost 2,000 of the uninhabitable units resulting from the Loma Prieta earthquake, even though they were only 1 percent of the Bay Area's housing stock. They accounted for 3,100 of the uninhabitable units resulting from the Northridge earthquake. This number would have been higher except for the aggressive program of the City of Los Angeles to retrofit these structures for life safety.

Many of these unreinforced masonry buildings in other cities still pose a serious life-safety concern. Unreinforced masonry buildings are expected to account for close to 13,000 of the almost 156,000 uninhabitable units resulting from a magnitude 7.1 Hayward fault earthquake. Again, however, these units house a disproportionate share of the poor, particularly in San Francisco.

santa cruz

Unfortunately for Santa Cruz, the lessons of the past had been forgotten by 1989. These photographs show damage in Santa Cruz from the 1906 San Francisco quake. The root cause of the damage in 1906 was the same as in 1989: unreinforced masonry buildings constructed on loose sediments subject to intense shaking and liquefaction.

As with apartment buildings, the retrofit for unreinforced masonry buildings should be designed by a Structural Engineer or other qualified design professional. Some buildings may need added sheathing on the roof or floors. In taller buildings, some of the masonry walls may need to be strengthened with spray-applied concrete. Adding steel frames at storefront openings is also a common retrofit method.

Some buildings may need added sheathing on the roof or floors. In taller buildings, some of the masonry walls may need to be strengthened with spray-applied concrete. Adding steel frames at storefront openings is also a common retrofit method.

Non-Ductile Concrete Moment Frame Buildings

Non-Ductile Concrete Moment Frame buildings can best be described as interconnected concrete beams and columns lacking sufficient confining re-bar. As a result the buildings tend to behave in a brittle manner when subjected to earthquakes.

There is a significant number of buildings in the Bay Area that are non-ductile concrete moment frame buildings and have not been seismically upgraded to withstand a major earthquake. The number exceeds 500 in San Francisco alone. The building code provisions governing concrete frame buildings were completely revised in 1967 after the collapse of the hospital in the San Fernando quake.

These are primarily multistory structures of both residential and commercial occupancy. Most of these have not been mitigated and pose a high risk of major damage and potential collapse in the event of a strong earthquake,

Structural Engineers who have analyzed these non-ductile concrete moment frame buildingg believe that the beam and column proportions and reinforcing details of these structures are not adequate to prevent brittle (non-ductile) failure in the event of a major earthquake.

Similar buildings have been toppled as a result of serious seismic motion in every major earthquake around the world, since the quake in 1967 in Caracas Venezuela and up to more recent events in Iran, Algeria, Mexico City and Morocco.

As these are primarily multistory buildings in the Bay Area, the impact of a major earthquake on this type of building is partial or total collapse through the "pancaking" effect of the floor slabs. The result will be the entrapment of occupants and the loss of many lives.

Most of these at-risk buildings were constructed between 1950 and 1970 and sit primarily on firm soil and high ground. They have not yet been exposed to a strong, intense earthquake. The Loma Prieta Earthquake in 1989 produced little shaking in the areas where these buildings are situated, leaving owners and residents with a false sense of security.

TILT-UP

Tilt-up buildings consists of concrete wall panels tilted into place and either wood or light metal roofs. When an earthquake occurs, forces perpendicular to the walls must be resisted at the base and the top of the walls. If the forces generated are too large, the connections (called wall anchors) between the walls and roof framing can fail.

Along with unreinforced masonry buildings, older wood frame buildings with parking below, and nonductile concrete frame buidlings, older tilt-ups have proven to be among the worst performing building types. Only through good fortune (i.e. earthquake occuring in the morning prior to buildings being occupied) have deaths in tilt-ups in past earthquakes been avoided.

Prior to the 1971 San Fernando Earthquake, the connection between the walls and the roof were not engineered. Building collapses in the San Fernando Earthquake led to building code changes including requiring steel hardware designed for wall anchor forces. Over the years the building code provisions related to wall anchors were made more and more strict due to research and observations of damage in earthquakes. Relatively few tilt-ups were damaged in the Loma Prieta earthquake because ground motions were not severe in areas where tilt-ups were loctated. However, it was not until the 1994 Northridge Earthquake when approximately 400 tilt-ups were badly damaged that is was noted that even more modern tilt-ups are high risks. Wall anchor design requirements were dramatically increased in the 1997 Uniform Building Code.

Depending on the age of the tilt-up and type of wall anchors provided, a pre-Northridge earthquake tilt-up has anywhere from a high to moderately-low chance of partial collapse during the strong ground shaking associated with even a nearby, moderate earthquake. Factors that increase the likelihood that seismic upgrade is needed include:

  • Age – older tilt-ups are more likely to require retrofit.
  • Fault proximity.
  • Building configuration – older tilt-ups with irregularities typically have serious deficiencies.
  • Poor construction quality – older flexible strap wall anchors were often poorly installed.
  • Poor wall anchor detailing – flexible strap wall anchors or wall anchor located on one side of roof framing member only (eccentric) are vulnerable. Glulam beams supported on pilasters (columns monolithic with the wall) without independent wall anchors can fail catastrophically.

Fortunately most tilt-ups are relatively easy and inexpensive to retrofit.