Loma Prieta Is Still a Factor for Demolition
March 26, 2024
This article originally appeared in the May/June 1998 issue of DEMOLITION.
It has been over eight years since the 1987 Loma Prieta earthquake wrecked havoc with the freeway system in the San Francisco Bay Area. The destroyed Cypress Freeway in Oakland has long since been removed, but the politics of rebuilding took several years and the new structures were eventually built right through the old Southern Pacific Rail Yards in West Oakland. To accomplish this construction, a temporary connector structure consisting of two double-lane bridges nearly one-half of a mile long were constructed. Also, a section of the new freeway was to pass right under a section of BART’s (Bay Area Rapid Transit) elevated rail line.
During 1996-1997, NADC member ICONCO Inc. completed two separate contracts with a total value of over $1.5 million related to this massive reconstruction project often referred to as the most costly freeway section ever built.
The Cypress “F” contract for Kiewit-Kasler, a joint venture, required the phased removal of the overhead structure consisting of steer girders and a concrete deck. The structure itself ranged from grade to an elevation of about 40’ and varied from 32’ to 88’ wide. Several of the spans crossed over active Amtrak main lines, which could be shut down only for short periods of time in the late evening and early morning hours. Track protection had to be installed and removed each time this work was performed.
The entire structure was recycled by ICONCO. All concrete from the 10” thick decks to the 1 6’ x 1 6’4” pile caps were crushed for road base, while the rebar was salvaged as scrap and the 36” wide flange beams were salvaged for resale as used structural steel. Overall, approximately 4,000 tons of steel and 20,000 CY of concrete were removed and recycled.
ICONCO’s second demolition contract in this complex rebuilding scheme was the Cypress “B” project for the Kasler Corporation that included moving a connector ramp to a location under the existing BART line.
The plan called for the elimination of seven supporting columns and replacing them with three new ones, thereby increasing the spans to accommodate the new freeway.
The removal of the columns posed a number of challenges to ICONCO. First, the work could only be performed during non-revenue hours, which for BART is a three-hour window each night during the week and a five-hour window on Saturday and Sunday evenings.
The new columns and support beams were poured and stressed prior to the removal of the old columns. During demolition, movement of the BART rails up or down would be limited to 1/4 of an inch. Furthermore, a contractual restriction on vibration limited demolition tools to hand-held breakers, or non-impact devices such as saws and coring bits until the column was physically disconnected from the structure.
The columns themselves were 5 feet in diameter with two rows of No. 18 reinforcing bars on 6” centers. Because the BART engineers were very concerned that excess track movement would shut down the line for the morning commute, ICONCO was faced with the task of removing a column in approximately two hours or less so that enough time would be available for track adjustments prior to the morning commute.
ICONCO’s plan called for chipping sufficient concrete to expose and remove a section of the reinforcing bar approximately 1 foot below the bent cap on one shift. This left the column structurally sound and enabled a wire saw to cut completely through the remainder of the column in sufficient time for the track to be adjusted.
Subcontractor and NADC member Cal-West Concrete Cutting Inc. performed this task without a hitch. Luckily, the design of the new support was right on the money and no track movement occurred. Once the column was saw cut, excavator-mounted breakers made short work of the remaining columns and footage.
The last obstacle was completed and shortly thereafter the cypress connector reopened some eight years after the earthquake induced collapse in 1989.