Safety issues in bridge demolition projects
a case study
by Osama Y. Abudayyeh
DURING THE 1989 Loma Prieta earthquake, major transportation structures in the San Francisco Bay area also sustained damage. In some instances, the extent of the damage was substantial, making the structures unstable or unsafe. These structures, including the Cypress Viaduct in Oakland and the Central Freeway Viaduct in San Francisco, had to be demolished. This article is a case study of the Central Freeway Viaduct demolition.
The Central Freeway Viaduct is a bilevel multispan bridge located in heavily populated downtown San Francisco. In a number of locations the structure was so close to adjacent buildings (less than six feet) that it imposed a major challenge in planning the demolition process. Additionally, the City of San Francisco has a number of utilities (for example, the electrical wiring to run its electric muni bus system) that required protection, presenting a second challenge to the planning process. And the viaduct crossed both commercial and residential areas, each having restrictions on vibration and sound levels, imposing a third challenge.
Planning the demolition of the Central Freeway Viaduct was a major project. Creative solutions were proposed and implemented to manage the demolition and to provide a safe environment that would protect adjacent structures, workers, the public, and municipal utilities.
Managing the demolition while ensuring safety was a major task for both the California Department of Transportation (Caltrans) and for its contractors.
Protecting Adjacent Structures. During demolition, protecting structures adjacent to the Central Freeway Viaduct in the project vicinity posed a particular challenge. Their proximity to the bridge required diligent planning and special care to avoid damaging or even collapsing such buildings. Several measures were used to protect the buildings adjacent to the viaduct. Steel cables or rods restrained the hinges on the spans of the double-deck viaduct to prevent a premature collapse from a bridge span slipping off its hinge seat. In addition, a California-registered civil engineer analyzed all possible loads on the viaduct structure to establish a safe loading range before demolition commenced. Caltrans representatives exercised a strict inspection and enforcement policy to ensure that spans were not overloaded by debris and/or heavy pieces of equipment. A deflection monitoring system continuously recorded the deflection of the bridge spans throughout the demolition process.
Note: This article was peer-reviewed.
Exhibit 1. When viaducts in the San Francisco Bay Area were damaged by a 1989 earthquake, Caltrans had to find some creative solutions to keep demolition projects from causing further damage. Steel structures like these were lowered into place as a safety measure to prevent freeway columns from collapsing toward nearby buildings.
Another safety measure consisted of temporary column-restraining steel structures and/or cables that restrained all columns. Such restrainers were designed to prevent the premature collapse of a column in the direction of adjacent buildings. The direction of collapse for these columns was carefully controlled and monitored.
To further protect adjacent buildings, a vibration monitoring instrument monitored each building or structure within 200 feet of the demolition activity. Each monitoring instrument displayed a flashing yellow light when safe vibration levels were exceeded. When vibration exceeded imposed limits, the contractor was instructed to stop the operation and change procedure. Daily logs of vibration results were filed in Caltrans’ field office.
Exhibit 2. Caltrans’ plan specified a sequence of steps for the demolition process. Girders from the upper deck were clipped and eased down to the lower deck. There, they were pulverized, with the rubble being frequently pushed down to the ground to maintain safe load limits.
Exhibit 3. Almost finished: the bent cap of the lower deck being pushed down by the pulverizer. The demolition project was considered a success once the bent cap was on the ground and no accidents or injuries had occurred.
As an added protection, Caltrans established a photo survey program. using still pictures and videos, they surveyed all buildings adjacent to the viaduct before and after demolition. This created a detailed record of existing damage to the buildings as well as damage due to the demolition activities.
Protecting Workers and the Public. Demolishing a major concrete structure such as the Central Freeway Viaduct is an extremely dangerous operation. To ensure adequate protection of the workers and the public the contractor on this project was required to prepare a demolition plan showing the demolition sequence, staging, equipment location, restraints and falsework for structure stability, and traffic control. The demolition plan, prepared and signed by a California-registered civil engineer, included detailed engineering calculations showing load determinations and structural analyses. The submittals were carefully reviewed by Caltrans in accordance with its specifications and with the State of California safety regulations. Demolition plans were reviewed and modified by Cal-trans engineers and the contractor before every major demolition activity to ensure that all field representatives knew the plans and procedures. (See the brief description of the sample demolition plan presented later in this article.)
The contractor also had to submit a comprehensive “Code of Safe Practice” for review by Caltrans before demolition could proceed. Caltrans required that this code include a plan for the use of personal protective equipment (hard hats, safety glasses, construction boots, etc.) and a maintenance plan for keeping all pieces of equipment on the job in good working condition.
In addition, the contractor prepared a dust control plan to protect the public from dust and debris. Watering was the method of choice to reduce dust during demolition. The contractor used nonpotable water for dust control, and protected workers from the mist by providing special protective clothing. Signs were posted to alert the public and workers that nonpotable water is unsafe to drink and that contact with such water should be avoided. To prevent debris from flying onto road or pedestrian traffic, debris nets were always installed around the portion of the structure being demolished.
To protect the public from noise, work-hour schedules were specified and were strictly enforced. The maximum sound levels were also specified in the special provisions and periodically monitored.
Protecting Utilities. The Central Freeway Viaduct demolition project had both underground and overhead utilities in its vicinity. A number of measures were taken to protect underground utilities (gas mains, water pipes, and sewer lines), some of which were directly beneath the structure.
Debris piles were built on top of such lines to provide a cushion against impact from falling objects, while steel plates were used as covers to further protect against impact. As extra protection against damaging impact, the contractor was not allowed to drop large pieces of debris on the ground. In addition, high-pressure water lines were shut down within the demolition zone.
Overhead utilities included the electric lines used by the San Francisco muni bus system. To protect these lines, the contractor and Caltrans were in continuous contact with the responsible agency, which was cooperative in temporarily removing their lines in the immediate vicinity of the portion of the structure being demolished until demolition was completed. To minimize service disruption and inconvenience to the public, accurate schedules were always sent to this agency so that the lines would be out of service for the least amount of time necessary.
Demolition Plans. Throughout the project duration the overall sequence of steps for the demolition process remained the same. However, some variations were necessary to deal with special conditions that arose on some spans; for example, a span with a hinge or a span too close to an existing structure.
During the demolition of an upper-deck span two pieces of equipment were used: a breaker positioned on the upper deck, and a pulverizer positioned on the lower deck. First, the breaker slotted the roadway deck and soffit between the girders. Next, girders were clipped and dropped to the lower deck. Then, the girders were pulverized on the lower deck (see Exhibit 2). To maintain the safe load limits established during the planning phase of the demolition process, and to reduce the loads on the lower deck, demolition debris was always pushed down to the ground. After the roadway slab (deck, soffit, and girders) was completely pulverized, the upper deck's bent cap was removed. This was accomplished by positioning the breaker on the lower deck and gradually decreasing the depth of the cap by breaking it in layers. Then, the columns were demolished, followed by the removal of the column restrainers.
Once the upper deck was removed, the pulverizer was positioned on the ground and the entire lower deck was pulverized one box girder (two girders and the soffit and deck between) at a time until the lower deck was completely demolished. The bent cap was then pushed down to the ground and pulverized (see Exhibit 3).
THE INVOLVED PROCESS of bridge demolition requires careful planning, execution, and inspection to establish and maintain a safe working environment. Caltrans and its contractors developed a safe working environment that addressed the needs of all parties involved. This team effort resulted in the successful demolition of the San Francisco Central Freeway Viaduct—a project that had no major accidents or injuries.
Osama Abudayyeh, Ph.D., is an assistant professor of construction engineering and management at Western Michigan University. Before joining Western Michigan he was with North Dakota State University. Prior to that he was a bridge engineer for the California Department of Transportation.
PM Network • January 1997