No two buildings are exactly alike. In fact, some that may seem at first appearance to be identical can display significant differences based on a number of factors, including structural composition, materials, the wear and tear on the building, even the composition of the soil beneath it.
It’s impossible, then, to give specifications to a retrofit project without first doing a comprehensive engineering study that examines not only as-builts, but inspects the property itself.
That said, here are some basic approaches to retrofitting applied to various types of structures found to be vulnerable to damage or collapse in a major earthquake.
Soft-story structures are common among apartment buildings, characterized by open parking on the ground floor and dwelling units built above. In some instances, the ground floor may be used as retail space and enclosed by windows that do not provide any structural support. These wood-framed structures, when constructed prior to 1978, are considered extremely vulnerable to collapse in a major earthquake.
The composition of these buildings lacks the ability to withstand lateral forces that push the building from side to side. The swaying can cause the first floor to collapse, and the upper stories to pancake on top of it.
Retrofit construction for soft-story buildings usually entails the installation of a steel moment frame or frames set in a sturdy foundation to absorb seismic ground motion and prevent swaying.
Non-ductile concrete buildings built before 1978 are characterized as having concrete floors and/or roofs supported by concrete walls, columns and/or frames. Due to their rigid construction and limited capacity to absorb the energy of strong ground-shaking, these structures are at risk of collapse in an earthquake.
In fact, non-ductile concrete buildings make up the majority of earthquake losses around the world. Because they are frequently used for office and retail uses that draw large numbers of people, the potential for death and injury with these structures is of particular concern.
To guard against collapse from an earthquake, retrofit construction usually entails the use of shear walls and column fortification to provide a stronger framework to prevent a collapse of the heavy floors above.
Tilt-up construction began in the early 1900s, but didn’t really catch on until the post-World War II construction boom. This cost-effective technique of pouring a building’s walls directly at the jobsite and then raising or “tilting” the panels into position was and continues to be a popular way to meet California’s demand for new commercial buildings.
The walls of a concrete tilt-up building can weigh between 100,000 and 300,000 pounds. Steel plates with headed studs are positioned into the forms prior to pouring the concrete to establish viable connection points that secure the walls to the foundation and the roof trusses to hold them in place.
Many tilt-up structures built prior to the late 1970s were constructed with limited or weak connections that have been proven to fail in an earthquake, causing severe damage and/or collapse. These building defects can be easily corrected with seismic retrofitting.
Steel Moment Frame
Steel moment frame construction dates back to the 1880s with the very first skyscraper, the Home Insurance Building in Chicago, but this building technique was most commonly used in the 1960s to 1990s.
Steel moment frame construction is characterized by the use of a rigid steel frame of beams connected to columns to support the many floors of the structure.
These structures, when built before the 1994 Northridge earthquake, can sustain brittle fracturing of the steel frames at the welded joints between the beams and the columns. In fact, many moment frame buildings in Southern California reveal cracks and fissures in these frames and may be susceptible to collapse in a major earthquake.
There are several retrofit approaches to consider in these instances, depending on the building. Some included boosted beam-column connections and chevron bracing throughout or at various points in the structure.
Unreinforced masonry buildings make up many of the older structures typical in downtown communities. They are characterized by walls (both load-bearing and not) and other structures such as chimneys that are made of brick, cinderblock, or other masonry materials not braced with rebar or another reinforcing material.
URM structures are vulnerable to collapse in an earthquake, due to a general failure of the mortar or when portions of the masonry such as parapets peel from the building façade and fall onto the sidewalk below.
Most of these structures were identified as part of a California mandate for all cities within seismic Zone 4. During the late 1980s and 1990s, many cities enacted mandatory ordinances to require retrofits of these buildings.
There are still thousands of this type of building that are yet to be retrofitted.
Retrofit construction of URM buildings consists of several approaches, including securing the structure to its foundation, joining building elements to avoid independent movement of various parts of the structure, and steel bracing systems.
As one of the leading retrofit companies in California, Optimum Seismic has partnered with AIA in a series of articles designed to help inform members about trends, policy and proven techniques associated with earthquake retrofitting. This is one in an ongoing series of articles.