Pier 15, the future home of the Exploratorium, was built almost 100 years ago, in 1915. Although Pier 15 was rebuilt in 1931 and survived the city’s 1989 quake, when the Exploratorium broke ground on its new campus in 2010, the pier was in need of a full-scale seismic overhaul.
Thankfully, building codes and construction techniques have come a long way in the last 100 years, and the new Exploratorium campus is built to withstand an earthquake of up to magnitude 8.2.
A team of builders and engineers, led by Project Manager Jay Yin of Rutherford & Chekene, undertook the enormous task of making Piers 15 and 17 earthquake-ready. Workers sank (or rather vibrated into place) four sets of massive steel pilings 135 feet long and 6 feet in diameter. More than a thousand dilapidated pilings were repaired or replaced. Crews demolished non-historic structures, ripped out old pavement, and went to work preserving the impressive truss structure within Pier 15’s interior, which spans the 820-foot length of the pier, the equivalent of a New York City block.
One of the most impressive but least visible underpinnings of the seismic retrofit is an enormous expansion joint within the building. At two feet wide and about 300 feet long, it sits between the piers and what is called the marginal wharf, which runs parallel to the old seawall and from which the piers jut out into the bay.
Imagine a line drawn along the shore, from the western edge of where Pier 17 begins to the eastern edge of the land from which Pier 15 extends into the water. The line skirts the entrance to the building on Pier 17, crosses open ground between the piers, and then hits a literal wall when it gets to Pier 15. It’s the north wall of the historic bulkhead that will house the new museum store, forum, classrooms, and Learning Commons. Workers had to effectively slice through that part of the building—floor, walls, and ceiling—to accommodate the joint.
Just what is a seismic expansion joint? It’s essentially a gap between different parts of a structure that allows separate elements to move independently of one another in case of an earthquake.
Pier 15 extends out almost entirely over water, but it’s also connected to land at the street. Because the rolling waves of an earthquake travel at different rates through land than through water, a building anchored at both is at risk of being pulled apart.
When a structure has one foot on the land and the other in the seabed, such a joint is especially important. “The pier is more flexible and will move more than the land,” says Jay. The joint allows land-based structures to move as they need to while the pier goes its own way, accommodating up to 24 inches of difference in movement.
If a structure can shift and move when stressed by the underground forces of an earthquake, it’s less likely to break. Bridges, stadiums, parking garages, freeway overpasses, buildings—in earthquake country, they’re all supposed to have seismic joints.
At Pier 15, this split between land and sea won’t be obvious to visitors, but it will do its job when it’s needed. The joint itself is covered so there aren’t big holes in the wall and ceiling or along outdoor walkways. But the observant visitor will be able to see where the two parts of the joint come together, a reminder of how we are now better prepared for the ground (and seabed) shifting underfoot.