We’re all used to ball bearings that fit in the palm of your hand. But, ball bearing sizes range from the tiniest, used within miniature computer hard drives, to the most massive, used within buildings and other huge structures. So, out of all the possible ball bearing sizes, where on the face of the entire planet can we find the largest ball bearings now in active operation?

A case can be made for the ball bearings used in Miller Park, the athletic stadium built in 2001 in Milwaukee, Wisconsin, USA, in the mechanism used to open and close the stadium roof. However, though over seventeen feet high, the pivot mechanism used to move the roof only incorporates ball bearings, and, though big, none of these bearings are the biggest in the world.

The largest ball bearings in the world are in the San Francisco Bay area of the USA, within the Benicia-Martinez Bridge, three parallel spans that carry 100,000 vehicles a day across Carquinez Strait west of Suisun Bay. As part of California Interstate Highway 680, these bridges link the town of Benicia at the northern end to the town of Martinez at the southern end. This segmented concrete structure was originally built in 1962 as a replacement for the car ferry services across the strait; the newest span was constructed alongside the original by the T. Y. Lin International construction firm, and was opened to the public on August 25, 2007.

The 2007 Benicia-Martinez Bridge, a segmented concrete structure with sixteen spans (twelve over water), cost US$ 1.3 billion to build — the 1962 bridge, with seven concrete spans, by contrast, only cost US$ 25 million. The 1962 bridge is still used to carry four lanes of southbound vehicle traffic, and has a pedestrian/bicycle lane; the new bridge has five lanes for northbound vehicle traffic only. There is also an older railroad bridge in between the two, built 1929-30, which carries Union Pacific freight trains and Amtrak passenger trains.

The seismic features added to both of the vehicle bridges alone cost US$ 122 million. In addition to the special huge ball bearings added to the bridges for seismic safety, special concrete was used that weighs 20% less than normal concrete, but still has a 6500 psi design strength and a stronger modulus of elasticity, both of which limit the deflection of the structure.

Both the superstructure and substructure of the ten steel spans on both bridges were seismically retrofitted: steel members were strengthened or replaced, all expansion joints were replaced with larger steel joints, expansion hinges were reinforced, the lateral bracing system was strengthened, anchors were used to fasten the concrete decks more securely to the steel beams, and a seismic monitoring system was installed.

Then there were the seismic isolation bearings, the largest friction pendulum bearings ever made. Each bearing, with a smaller articulated slider mechanism around its circumference, is at least twelve feet in diameter, and weighs between 40,000 and 50,000 pounds. These bearings, encased at the ends of the concrete segments, and at the ends of each of the bridges’ spans where it meets land, will, when an earthquake occurs, allow up to six FEET of horizontal movement with minimal superstructure displacement. That’s right — SIX FEET!

Daniel Mackelburg
Assistant Engineer
www.nmbtc.com