Published in the ECHO Journal, November 2007
A Safety Issue Uncovered
Throughout the last fifty years, there has been an increasing awareness of the need to provide a higher level of safety to building occupants in seismically-active regions. Knowledge gained from observations of earthquake damage, testing, and computer modeling has resulted in stricter building codes and, consequently, a more stringent design and construction approach. Public and commercial buildings now have more shear panels, tie-down bolts, welds, deeper sections and robust ductile framing systems. In the multi-family and single family residential realm, units that would normally be composed of merely wood, nails, and plywood sheathing are now being built with metal straps, metal clips, larger foundations, and even steel walls.
During this time that safety awareness has increased, there has also been a large, unprecedented, building boom. Urban growth has accelerated to a point where demand for quick, affordable construction is higher than ever. In the housing market, for example, increasingly large numbers of custom and tract houses are being built in record time and at lower costs. It is theorized that at one point, building officials could not perform inspections as often as desired, because they did not have the resources to face the building boom.
Unfortunately, the end result is a significant amount of construction that has not been built per the original engineer’s intended design. In other words, it is fair to say that many structures may be missing the critical elements needed for seismic restraint. As a result, if enough components are missing during a large enough earthquake, excessive damage and even collapse may occur.
This article explores two case studies of residential buildings where seismic restraining elements were omitted during construction. Both buildings were built within the last 25 years and both are in the highest seismic zone (zone 4). We will discuss the field analysis methods, the missing critical components, and the required repairs to solve these issues.
Case Study 1: Condominiums, San Francisco, California
The first case study is a group of condominiums in a large development just south of San Francisco, California (see Figure 1). The buildings were undergoing a series of improvements that included new windows, siding, and roofing. During the course of removal of the existing exterior finishes, it was noted that many seismic hold-downs were not installed in critical areas as highlighted in red in Figure 2.
Hold-downs are steel ties that prevent the building from overturning during an earthquake (see Figures 3a and 3b), which is achieved by anchoring the structure to the ground.
To address the problem, a repair was engineered consisting of a strap nailed to the walls of the building and anchored to the foundation via epoxy anchored bolts (see Figures 4a and 4b). Special care had to be taken regarding the bolt locations because the foundation had tension-cable reinforcement (also known as a “post-tension” foundation).
These buildings had additional structural compromises including shear transfer clips not being installed in many locations; specifically the red-lined area illustrated in Figure 5. Shear transfer clips (see Figure 6) are steel plates that allow earthquake forces to be transferred from each floor level into the walls at each adjoining level.
Simply put, shear transfer clips prevent sliding (see Figure 7). The repair method involved installing the clips in the appropriate locations as indicated in the red outline in Figure 8.
Case Study 2: Single-family Residence, Southern California
This case study consists of a single-family residence located in Southern California (see Figure 9).
Various metal seismic restraining components were observed to be missing in the basement of the house. This prompted an extensive review of the building’s seismic continuity. However, the owner did not want the interior or exterior finishes to be removed or damaged for the investigation. Because the components under scrutiny were made of steel, X-ray technology was used to determine the existence of the hardware. X-ray locations were randomly chosen by a professional statistician to avoid bias in the assessment.
X rays are a form of high energy, short-wave radiation. They have the ability to pass through relatively dense materials with light-like behavior. In the case of a building assembly, the radiation passes through wood and lightweight finishes but is absorbed by denser materials like steel. As a result, X-ray photography can give a clear representation of the metal components in a wall assembly.
In this investigation, the X rays were taken on site using a small, low power, battery powered X-ray tube. An X-ray sensitive reusable film was placed behind the object of interest. The region around the machine was cleared of occupants and the X-ray tube was energized, thus exposing the film to the radiation. The image was then developed on site with a digital scanner and stored on a laptop computer. Figure 10 shows a resulting X ray depicting a hold-down.
In another location, X rays showed that no hold-down existed. Multiple X-ray images were taken and eventually it was concluded that the hold-down wasn’t installed (see Figure 11).
Similarly, X-ray photography was used to look for horizontal seismic steel straps. In an earthquake, a building will tend to “tear” apart between regions of different mass and geometry as illustrated in Figure 12a. Steel straps of proper length and thickness can prevent this from occurring (see Figure 12b).
The X rays revealed the existence and non-existence of seismic straps. Figure 13a shows a seismic strap installed, whereas Figure 13b shows no strap was installed in the intended location.
With regard to the straps and hold-downs mentioned, it was decided in court that each missing component be installed per the original engineer’s plans.
Conclusion
The seismic restraining components discussed in these two case studies, such as the hold-downs, clips, and straps are essential for any building in a seismic zone. Generally, the absence of these components is not visible without removing exterior finishes to make that determination. However, non-evasive methods for determining the existence/non-existence of these critical items using X-ray technology. Upon determining the location of the missing components, the removal and remediation of the exterior finishes was very minimal and a cost effective way to improve structural integrity and preserve aesthetic appearance.
The proper installation of these components is affordable and the money spent is small relative to the potentially large amount of damage that can result if they are not installed. Additionally, components may be inadvertently omitted and a building without the proper seismic hardware can be unsafe in an earthquake, jeopardizing the lives of the inhabitants. Hiring an engineering consultant for design during construction, renovation, or remodeling of your building(s) to ensure that it has the proper hardware detailed and installed is highly recommended to avoid structural problems in the future.
Justin Bettner, P.E. is a structural engineer with the engineering firm of Allana, Buick and Bers. He holds degrees in civil and structural engineering from the University of California, Berkeley, and has worked extensively on improving the structural safety of condominium developments throughout California.