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Seismic Upgrades Completed on the Rock’s Guardhouse

By Catherine A. Cardno, Ph.D.

The seismic rehabilitation of the guardhouse complex that serves as the point of entry to the main cell block on Alcatraz Island has been completed.

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Work to seismically rehabilitate the guardhouse complex on Alcatraz—the primary route through which visitors pass on their way to the island’s main cell house—was completed last month. Dusterhoff/NPS

August 4, 2015—Alcatraz Island, located within San Francisco Bay in California, is known for many things, not least among them being the federal prison that operated on the island for 30 years. A one-time military installation during the U.S. Civil War, the rocky island housed what is now referred to as a "supermax" prison from 1934 to 1963. For the last 40 years, however, the island has been a national park, and maintaining the aging buildings intact—and safe—for the more than 1.8 million visitors who wander the island annually is a priority. That task now falls to the National Park Service (NPS), which in July completed a seismic rehabilitation of the guardhouse complex—the primary route through which visitors pass on their way to the island's main cell house. 

"Most visitors know about the Bureau of Prisons era from films and television," said David Dusterhoff, the NPS project manager for Alcatraz, who wrote in response to questions posed by Civil Engineering online. "They arrive with the intent of visiting the cell house to view Al Capone's and Robert "Birdman of Alcatraz" Stroud's cells and to learn about the various escape attempts." But the island has more to offer than its prison, he explained, and the NPS's primary goal is to "expand the public's understanding of the multiple layers of Alcatraz history, including the Civil War/post-Civil War army era and the Native American occupation era," Dusterhoff explained.

The recently completed $3-million project focused on the guardhouse complex, a set of four clustered buildings constructed over the course of many years. To accommodate the island's rocky sandstone terrain, the buildings are perched together on the side of an outcropping, extending over-and at one time guarding—the primary route that visitors to the island take from the ferry landing terminal to the main island. (A separate pedestrian route around the island passes by the upper stories of the complex). 

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Prior to the current project, the guardhouse complex comprised four buildings constructed over time: the guardhouse (1857), the library (circa 1869), the schoolhouse (1917), and the boathouse (1918). NPS/Lerner and Associates AHSR

"This project not only seismically stabilized the guardhouse complex for future generations but also restored and revealed elements such as a cannon port, a dry defensive moat structure, and a granite Alcatraces 1857 entablature," Dusterhoff noted. "These additions to the project allow for a greater understanding by the visitor of the guardhouse as part of a Civil War-era third system fortification," he noted. "The combined elements of structural stabilization, new interpretive opportunities, and access to areas not previously available have added value and an increased the level of safety to the visitor experience."

Before the rehabilitation work began, the complex included four buildings: the guardhouse, the library, the schoolhouse, and the boathouse, which had been built over the generations, seemingly clinging to each other and the cliffside like barnacles. As with any structure that has stood for centuries, the buildings have had many names and many functions over the years. The schoolhouse was once a chapel, for example. And the library was at one time used as a shooting range for prison guards, gun storage maintained in the attic space, according to Brian Kehoe, P.E., S.E., R.L.S., F.ASCE, an associate principal in the Emeryville, California, office of the structural engineering firm Wiss, Janney, Elstner Associates, Inc.

The guardhouse—also referred to as the "Sallyport"—is a one-story pre-Civil War-era brick building standing on rock. It measures 50 by 25 ft and is constructed of 4 ft thick unreinforced brick masonry walls, two of which extend over the main route from the island's landing area to the rest of the island. The dockside wall gives the structure its name because of an opening—a sally port—that controlled ingress to the main portion of the island from its single landing point. 

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Alcatraz Island housed a military installation during the U.S. Civil War and what is now referred to as a “supermax” prison from 1934 to 1963. It has been designated as a national park for the last 40 years, and its buildings—including the guardhouse complex (center)—are carefully being rehabilitated. Wikimedia Commons/Sidvics

The building is the oldest building on the island, dating back to 1857. Remnants of its original moat and drawbridge pulleys are still visible if you know where to look, according to Kehoe. "Because it's kind of built into the hillside a bit, one side of the building is actually retaining, and then there is actually an extension of that retaining wall that continues on," Kehoe says. 

The second building, currently referred to as the library, is a two-story building that was built circa 1869 against the side of the sallyport building. It has unreinforced masonry perimeter walls with wooden floors on its second and attic levels. Its first story is an open-air space through which the road to the rest of the island passes. 

The most significant challenge to be resolved with the seismic repair work was the susceptibility of the unreinforced brick masonry to failure during an earthquake, Kehoe says. "The most important was the wall people walk right through," and that is the library gable wall, he says. "It was a very tall masonry wall and they had really no anchorage for out-of-plane forces. The most important thing for the safety of the public was to tie that brick masonry back to the [library] floor structure and the roof structure so that it had some adequate out-of-plane anchorage." 

The library wall was strengthened with steel framing, anchor bolts, and a concrete bond-beam located atop the wall, Kehoe explains. 

The second story of the library has a thick wood diaphragm floor, which acts as the ceiling for the open-air ground level. "Normally, what you would do for a seismic retrofit would be to assume that that wood diaphragm is acting as a structural element and would transfer shear forces into the end wall," Kehoe notes. "That would be the normal way of evaluating and designing the strengthening. But in this case, we couldn't do it that way." 

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The current work on the guardhouse complex uncovered a granite “Alcatraces Island 1857” sign, which had been blocked from view when the library was built abutting the guardhouse nearly 150 years ago. Dusterhoff/NPS

Water damage had decayed the wooden framing, requiring repair work. But a larger issue was caused by how the library had been built atop the sally port wall. The open-air room through which visitors pass is enclosed by the wooden diaphragm of the library's second-story floor. "The floor …abutted the portion of that wall where the original sign for the entrance to the island was located. [It] was a granite sign that says "Alcatraces Island," the old spelling of the name of the island," Kehoe explains. "So the floor diaphragm couldn't be connected to that wall there."

A portion of the floor diaphragm was removed as part of the project, so that the public can now view the sign—which once hung above the long-gone drawbridge—as they walk through the entrance to the island. "What we had to do was come up with a way of getting the seismic forces out," Kehoe explains. Instead of anchoring the floor diaphragm into the library's end walls, the design team added long steel rods that extend across the width of the floor to anchor back into the original 4 ft thick defensive wall of the sally port. This will transfer any potential seismic-induced shear forces back to the retaining wall.

The seismic improvements of the library also included steel framing that was added to the interiors of the second floor and attic to protect the walls from crumbling. These areas are unused and unavailable to the public, so the framing runs along the interior walls, preserving the integrity of the look of the exterior of the building.

The third building within the complex—currently referred to as the schoolhouse—is a two-story reinforced concrete building built in 1917 with wooden roof joists. It features concrete beams and a floor slab at the second floor. This building was built atop the guardhouse, through which the road passes.

For this project, braced steel frames were added to the interiors to strength the building, and the roof membrane was removed because of water damage and decay. A new plywood diaphragm was installed with roof-to-wall anchors. "It needed a new roof and this was a good opportunity to do some seismic strengthening," Kehoe notes.

The fourth building in the complex was a two-story, badly deteriorating wooden boathouse that was built alongside the library. Constructed in 1918, the building was constructed atop concrete beams and columns that extended to the island's water line. Rather than repair the building, it was removed entirely. As a separate, lightweight building, removing it did not impact the seismic performance of the complex, according to Kehoe. The removal of the boathouse also restores ocean views that it had blocked from the open-air room through which the visitors pass.

The island's main cell house has already been seismically upgraded but additional seismic work on the island's buildings is ongoing. (For more information on preserving structures on Alcatraz Island, read "Preserving Concrete on the Rock," on Civil Engineering online.)



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