The flying buttress is strongly associated with Gothic church architecture. The buttresses resist the force pushing a wall outward by redirecting it to the ground, resisting the outward push of the interior arches and vaulted ceiling.
Flying buttresses “fly” because the buttress is not in contact with the wall all the way to the ground; the lateral forces are transmitted across an intervening space. Flying buttresses have two key parts: a massive vertical masonry block on the outside of the building and an arch bridging the gap between that buttress and the wall.
Some of the building elements that suffered the most damage as a result of the earthquake were the six flying buttresses on the Cathedral’s east end. When construction began in 1907, there was no reinforcement used between stones. The seismic motion caused stones in the buttresses to shift and separate and loosened and rotated the pinnacle stones.
The engineering to repair the east end flying buttresses was not that different from the principle of putting reinforcing steel rods within a concrete wall — except that at the Cathedral, the work was done retroactively. A hole, three inches in diameter and 22 inches long, was drilled into the upper and lower flyers of each buttress with the last 18 inches penetrating the exterior wall of the apse. A threaded stainless steel rod was then inserted into the full length of the hole.
Once the rod was in place, grout was injected in the void around the rod; when cured, the grout enables the rod and stone to form a cohesive structure that will prevent major collapse should another earthquake occur. A similar strengthening technique also was used to reinforce the top three to four stones on each buttress pinnacle.
During the reinforcement procedure, other weaknesses in the stone construction were discovered. When one of the 22-inch holes on a north buttress flyer was injected with grout, it took more than 70 gallons of the liquid material—30 gallons more than was used to fill the other flyers — to fill. The grout traveled through unseen voids within the exterior wall and emerged on the interior, near the high altar.