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More than 120 years ago, Frederick Lothrop Ames, grandson of the founder of the successful Ames Shovel Works, financed the construction of the Ames Building in downtown Boston, which was billed in a marketing piece as “a notable example of recent commercial architecture, and the most prominent and eligibly situated of Boston’s great buildings.” Ames hired Shepley, Rutan, and Coolidge, a firm headed by protégés of legendary and then-recently deceased architect H.H. Richardson, to complete the design in what now is labeled “the Richardsonian Romanesque style.” The building, constructed in 1889, is Boston’s first “skyscraper” — rising 14 stories above Court Street — and it remained the tallest non-ecumenical building in the city until the construction of the Custom House Tower in 1915. It became listed on the National Historic Register in 1974.
 Simpson Gumpertz and Heger engineers worked with three different developers to create an economically viable project from the renowned but outdated Ames Building. www.vitopalmisano.com |
Ames Hotel
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The Ames Building remains one of the tallest commercial load-bearing masonry buildings in the United States, surpassed perhaps only by the Monadnock Building in Chicago. Built soon after the Great Boston Fire of 1872 that destroyed 65 acres of the city’s downtown, its “fire proof” construction featured interior steel columns and floor beams that were encased entirely in terra cotta tile.
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For more than 100 years, the Ames Building served as an office building; unfortunately, by the 1990s, the building was showing its age. In 1998, a developer purchased the building and determined that the small, irregular floor plates were better suited for hotel than office use. The developer engaged Simpson Gumpertz & Heger Inc. (SGH) to address the building’s structural issues, and we began a decade-long relationship with the structure, culminating with the opening of the Ames Hotel in late 2009. We worked for three developers/owners during this period, as the potential market for a boutique hotel rose and fell over time. Our knowledge of the building structure and our broad capabilities in building restoration convinced each subsequent developer to include us as part of their design team.
Getting started
The conversion to a hotel required a complete gutting of the building, including the removal of six original bank vaults from the lower floors. An existing 10-story lightwell at the rear of the building was filled with new circulation space. Of the four original elevator shafts, one was converted to a mechanical shaft, and another was expanded and extended to the basement. The original sidewalks covering basement vaults on two sides of the building were extremely corroded by decades of winter salt applications and leakage and had to be reconstructed. Perhaps the greatest engineering challenge of the project was finding a way to squeeze nearly 70 tons of mechanical equipment onto 2,400 square feet of available roof space.
We designed the building’s additions and alterations using the 6th edition of the Massachusetts State Building Code, which exempted historic buildings from the seismic provisions required for renovations to non-historic buildings. Two rigid requirements applied: The building must be safe for urban, Exposure A wind-loading; and the lateral-load capacity of the building could not be reduced. Our lateral-load analysis confirmed that the building’s masonry shear walls could safely resist the required wind load — no surprise there, since it had survived multiple historic hurricanes before several tall buildings providing beneficial shielding were erected in the vicinity. Since the hotel required multiple new openings in the massive brick walls, we filled abandoned openings to replace some lost strength. Although the new openings weakened some walls, they were not the limiting elements and were not in critical locations, so the overall lateral-load capacity was not compromised.
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A critical consideration in renovating a building from this era is the weldability of the existing metal structural components. Mills were still rolling wrought iron shapes in 1889, so we were not certain that the beams and columns were steel. Even if steel was used, its chemical composition varied widely in this era; some early steels contained high levels of carbon, phosphorous, and other substances that could preclude welding. We knew it would be difficult to design and construct this project using only mechanical fasteners for existing framing, so we collected steel filings from several beams and columns distributed throughout the building and sent them to a lab for analysis. We provided the chemistry results to a metallurgist, who confirmed that the structure was steel and wrote a welding procedure for the project. Fortunately, he determined that the existing steel could be welded using E7018 electrodes with preheat from 100-150°F, depending on the thickness of the section. For quality assurance, we specified testing three sample welds of new to existing steel using non-destructive procedures.
Taking the challenges
Among the many challenges we faced over the decade, the sidewalk vaults, the unique floor construction, and the improvements to the buildings services including HVAC and transportation were the most interesting to solve.
Sidewalk vaults — One of the first challenges on the project was addressing the severely deteriorated sidewalk slabs on the two public facades of the building. The first developer replaced the sidewalks as a condition of sale to the second developer. The sidewalks span between granite foundation walls at the perimeter of the basement vaults and the 5-foot-thick granite piers supporting the building facades. We addressed durability by specifying galvanized steel framing, epoxy-coated reinforcement, a sacrificial steel deck, additional concrete cover, and a waterproofing system. We designed the sidewalk to meet the loading requirements of the Massachusetts Building Code (250-pound-per-square-foot distributed loading or 8,000-pound concentrated loading). After construction, the city surprised us by revealing an unpublished design requirement that sidewalks must have the capacity to resist AASHTO (American Association of State and Highway Transportation Officials) HS-20 load requirements (32,000-pound axle load). Fortunately, the structure only required minor modifications to meet these requirements.
Flat-arch floors — The terra cotta fire-proofing effectively concealed the structural frame from observation and complicated our work. Figure 1 shows a diagram of a typical flat-arch floor from the 1906 edition of “The Architect’s and Builder’s Pocket-Book” by Frank E. Kidder. The blue arrows show the lines of compressive thrust developed within the floor system. According to Kidder, these systems are extremely strong; the handbook publishes safe allowable live loads of far more than 200 pounds per square foot for arches spanning less than 5 feet.
The architectural program for a hotel requires the construction of numerous bathrooms and associated plumbing lines. Each bathroom required cored holes in the floor. The terra cotta flat-arch floor system completely concealed the steel framing. The terra cotta was topped with cementitious fill and wood flooring and covered by plaster on the underside. To help the architect lay out the bathrooms, we surveyed the steel framing on each floor using a pachometer. The architect used the survey results to produce an as-built framing plan, and used the plan to locate the bathrooms, avoiding cores at beam locations.
Care must be taken when making openings in flat-arch tile floors. The floor system’s reliance on internal arching action has two implications: making a small opening that does not extend to the beams leaves in place terra cotta tile that no longer has an arching mechanism available; and making a large opening that extends to the beams could destabilize the terra cotta in adjacent spans because the steel beams may not have sufficient weak-axis strength and stiffness to resist the thrust from the adjacent spans. We developed a detail for large floor openings that required the contractor to add steel tie bars to the underside of the first two beams adjacent to each side of the opening to resist the arching thrust from the terra cotta; we provided criteria for the maximum size and minimum spacing of small openings (less than 6 inches in diameter) not requiring reinforcement; and for openings that were too large to be made without reinforcement, but less than the beam spacing, we specified beam-to-beam removal of the terra cotta and reconstruction using steel sub-framing and concrete on steel deck.
Rooftop equipment — Perhaps the biggest challenge on the project was finding a way to support the new rooftop equipment. To reduce visibility from the street, the screenwall and mechanical equipment had to be set back from the public east and south facades, leaving only about 2,400 square feet to place nearly 70 tons of mechanical equipment. We developed a two-tier steel-framing system to support the equipment. The upper tier of level platforms posted down to a lower tier of sloping members that carried the loads directly to the perimeter-masonry-bearing walls and interior columns; the roof framing had no capacity to carry additional loads. We located the bottom of the lower tier framing about 12 inches above the roof surface to provide for future re-roofing. The steel fabricator did an excellent job detailing the complicated geometry. To accommodate the new mechanical system loads, we strengthened the interior columns at the upper stories.
Elevators — Extending the service elevator to the basement level also presented challenges. The building is supported on massive tiered granite block foundations. The elevator shafts are adjacent to a bearing wall, so extending an elevator to the basement required removing a portion of the granite foundation to accommodate the pit. The first developer wished to bring three elevators to the basement, which would have required constructing heavily reinforced-grade beams between each elevator pit to maintain the foundation’s integrity. At our recommendation, the second developer eliminated the center elevator, greatly simplifying the foundation requirements. Since the grade beams were no longer required, vertical loads could be transferred to the soil-bearing surfaces by means of arching action within the remaining foundation left intact below the two pits. The final developer elected to extend only the service elevator to the basement level. Even this simplified approach required creating a large notch in the granite foundation. We developed a monitoring program designed to detect any vertical or rotational movement of the walls in the vicinity of the pit during removal of the granite blocks and construction of the new pit. We did not detect any significant movement during these operations.
Conclusion
Although the Ames Building is one-of-a-kind, the types of challenges the project presented are common to many renovation projects, where antiquated materials, hidden conditions, code upgrades, and strengthening strategies test the structural engineer’s ingenuity. We are pleased we had the opportunity to apply our knowledge and creativity to help reinvigorate this architectural icon. Although it took some time for all the pieces to fall into place, seeing the building occupied and alive again convinces us that the project was 10 years well-spent.
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Spotlight: Simpson Gumpertz & Heger Inc.
JG: How did you solve the most unique problem on this project? SG: The design team was challenged with creating a modern interior that could acknowledge the great historic and heroic bones of the building and also inspire hotel guests to return again and again. As well, integrating state-of-the-art mechanical systems into an old building with no outdoor space was a challenge. The design team was able to integrate new systems in a seamless and delicate way into the structure. JG: What new design innovations were employed by the design team? SG: A collaborative effort between Morgans Hotel Group, Rockwell Group, and Cambridge Seven Associates allowed for functional and playful details throughout public spaces and guestrooms. Working with a vaulted flat arch structure, the idea of traditional shaft space was not an option. We took advantage of great ceiling heights to run mechanical systems horizontally through the structure as opposed to the traditional vertical stacked systems typical of the hospitality industry.
JG: What was the first task you needed to do to get started on the design? MDW: Our first task was to begin to understand how the building was constructed. Although we had original framing plans, they lacked detail, and we needed to determine how well the plans met the as-built conditions. We did extensive field work measuring and documenting the building structure. JG: What software did the design team use for the project design? MDW: We considered using finite element model to perform the lateral analysis, but instead settled upon an elaborate multi-page spreadsheet to distribute lateral loads to the wall lines and piers in proportion to their stiffnesses using a rigid diaphragm approach. We linked the lateral load worksheets to gravity load worksheets to calculate the combined vertical and shear stresses in each masonry pier. We used RISA-3D for many of the structural elements, including the lightwell infill framing and the rooftop mechanical support framing. JG: What was the most unique problem you solved on the project? MDW: One unusual problem arose when the architect determined that we needed a new egress door through a heavily loaded 30-inch-thick brick shear/bearing wall at the first story. We reinforced the wall by notching out the corners of the remaining piers and headers and adding W8x40 columns and W12x50 lintels. The steel framing had to be installed before the new opening was made. JG: What sustainable aspects were pursued by the architectural team? MDW: Since we were reusing an existing building, the project had inherent materials with sustainability benefits. Wherever possible, we worked to preserve and retain these historic materials rather than replace them with new materials that would consume additional resources.
Firm Facts
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By the numbers: Ames Hotel
Number of square feet: 90,000 Number of stories: 14 Structural system: Exterior bearing wall masonry including stone and brick, interior steel columns and beams, flat-arch terra cotta floor system Foundation type: Spread footings constructed of granite blocks stacked “wedding cake” style Unique project aspects
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Q&A with the architect
Q&A with the structural engineer
Mark D. Webster, P.E., LEED AP BD+C, SECB, senior staff II – structures at Simpson Gumpertz & Heger Inc. was the project manager on the Ames Building. He can be reached at mdwebster@sgh.com. Joseph J. Zona, P.E., SECB, is senior principal at the company and can be reached at jjzona@sgh.com.
