Xiaoxiao Wu, PE, And David Himelman 2023-01-07 06:22:34
A new steel-framed museum in Charleston, S.C., floats above its waterfront location and tells the stories of the people it celebrates.
THE INTERNATIONAL AFRICAN AMERICAN MUSEUM (IAAM) is devoted to telling the stories of African peoples captured and brought across the Atlantic, the cultures they developed, and their continuing impact on our world—and the building’s steel frame helps manage the impact of the elements in its high-wind, high-seismic location.
The new museum’s steel-framed home is situated on the former site of Gadsden’s Wharf in Charleston, S.C., where some historians estimate 40% of enslaved Africans entered the United States. As articulated by Henry N. Cobb, Pei Cobb Freed and Partners’ lead designer for the project, “The special design challenge of the museum was to build on this site without occupying it.”
The building is conceived as a one-story volume measuring 84 ft wide (north-south), 426 ft long (east-west), and 24 ft high, and is raised 13 ft above the ground on a double row of 18 monumental columns, which are set in approximately 18 ft from the volume’s perimeter and arranged in 48-ft by 48-ft square bays.
Elevating the building preserves the hallowed ground it sits on, shelters the African Ancestors Memorial Garden below, and lifts the occupiable interior spaces out of the floodplain. The principal museum building volume is clad in warm, beige Petersen Tegl Brick on its long sides, while the short sides feature approximately 20-ft-deep balconies shaded by angled, vertical louvers. Two off-center cast-in-place concrete cores and a monumental stair situated between them provide access from the Memorial Garden and the gallery level above. The monumental stair is open to the air and configured as a skylight-covered atrium, and above the museum volume is a 48-ft by 192-ft penthouse—set back from the museum parapet to limit its visibility from grade—that houses administrative spaces.
Design Criteria
The IAAM’s proximity to the coast and location in a seismically active region places significant demands on the structure. In addition to high seismic loads, the structural design accounts for soils prone to liquefaction, hurricane winds of up to 157 miles per hour, the waterfront flood zone, and breaking wave loads. To address concerns regarding wind loading, the client engaged Rowan William Davies and Irwin (RWDI) to conduct a cladding wind load study that included wind tunnel testing.
To cope with the location’s poor soil, the building foundation system consists of precast, prestressed concrete piles supporting cast-in-place (CIP) concrete pile caps connected by CIP concrete grade beams. The piles range in length from approximately 75 ft to 90 ft, depending on the top of finish grade, to extend down to a marl layer bearing stratum. The pile caps are typically 11 ft by 11 ft and situated beneath each of the 18 5-ft diameter CIP concrete columns. The columns, which are clad in 6-ft-diameter glass fiber-reinforced concrete (GFRC) column covers, are sized to withstand hurricane/ flood loads and support the elevated steel structure above. Larger pile caps support the shear walls forming the building’s two cores, which are asymmetrically placed along the building’s east-west axis.
The cores are a primary component of the building’s lateral force-resisting system and the only path for services, egress stairs, and elevators to enter the building from below. The shear wall design required particular attention to maintaining the cores’ structural integrity while accommodating the large number of openings required for services. Wall beams and boundary elements are located at the wall’s corners and intersections with the steel floor framing. The shear wall design also follows ACI 318 code requirements for special reinforced concrete shear walls to lower the seismic loads on the structural system.
Structural Steel Framing
Each CIP column rises approximately 14 ft to 16 ft to a consistent datum about 12 ft above the finish grade, above which the structural steel framing, approximately 900 tons in total, begins (the project includes an additional 200 tons of steel comprised of miscellaneous and deck elements). These concrete columns support fabricated steel box columns, which, along with the two CIP cores, provide the building’s gravity support. The fabricated steel columns are 16 in. square and were fabricated by Lyndon Steel using plate thicknesses varying from 1.25 in. to 2 in. in order to maintain a slender profile that coordinated with the architectural finishes. Columns that extend above the museum roof (Level 3) to support the more lightly loaded penthouse roof transition to standard wide-flange sections.
The typical floor framing at each level consists of a two-way grillage of wide-flange steel beams spanning between columns to form a series of 48-ft square two-way moment frames that pair with the shear walls are the structure’s lateral force-resisting system. Due to the region’s relatively high seismic loads and the asymmetrical core locations, which induce torsion on the structure, the moment frames are designed as steel intermediate moment frames, per the AISC Seismic Provisions for Structural Steel Buildings (ANSI/AISC 341, aisc.org/specifications) with reduced beam-to-column (or “dog bone”) connections. This detail also includes provisions for diagonal “beam lateral braces” made of double-angle members that connect the moment frame beams’ bottom flanges to the secondary framing, which help resist torsion within the deep beams. A secondary wideflange section at the 48-ft bays’ center spans north-south to support infill wide-flange sections that, in turn, provide intermediate support for the slabs, which are composite concrete on metal deck.
At Level 02, the floor framing cantilevers from the column lines 16 ft on the long sides and 20 ft on the short sides to provide the deep eves and balconies, respectively. Connections of the cantilevered beams are standard moment frame details; the typical column bay’s deep beams provide a back span. On the north and south sides, where perimeter beams support the brick façade, custom sections are used for torsional resistance. The fabricated composite section consists of a WT welded to the top of the HSS tube and achieved the greater depth required to support the bottom of the brick façade, which extends about 3.5 ft below the top of the steel while also resisting the torsion caused by the heavy façade loads. These perimeter fabricated sections support slender hollow structural section (HSS) posts spaced 24 ft apart that span to the simply supported perimeter framing at the roof above and control deflections at the façade.
The penthouse roof framing consists of moment frames arranged in the east-west direction only. The frames consist of wide-flange columns and beams and support an unfilled metal deck. Due to the asymmetrical position of the penthouse, some of the wide-flange columns do not align with the 48-ft square grid and instead transfer their load to the deep beams below. In the absence of the slab diaphragm and a two-way moment frame system, the penthouse roof framing uses in-plane, horizontal HSS diagonal bracing. The penthouse roof also includes a large central skylight slightly elevated above the roof that provides cover for the monumental stair atrium. This framing consists of a moment-connected 7-ft by 5-ft grid of HSS beams supported by the CIP concrete shear walls and HSS posts bearing on the primary penthouse roof steel framing below. The skylight structure is connected to the concrete shear walls at four distinct support points for gravity and lateral stability; each of the four connections is detailed to release movements in different directions (allowing maximum flexibility of the steel framing under temperature loading in order to minimize temperature-induced stresses in the steel). At the exterior four corners of the skylight, four posts extending down to the steel beams at the penthouse roof level provide additional vertical support. Teff on coatings are applied in the base connections of these posts to avoid lateral interaction between the skylight framing and the base building structure.
Façade Work
The building is entirely clad by a brick façade, and the sensitivity of the bricks to movement required close collaboration between the design of the primary structure and the façade structure. Structural engineer Guy Nordenson and Associates (GNA) worked closely with façade design consultant Thornton Tomasetti to confirm that the primary structural deflections were acceptable. Multiple load combinations were analyzed that considered the construction sequence of the brick façade panels and various live load patterns, and GNA provided detailed vertical and lateral deflections to Thornton Tomasetti at every façade panel connection point to the primary structural beams for these load combinations. Because of the design team’s close attention to the detailing and installation planning of the brick system throughout the design phase of the project, the façade installation went smoothly during construction.
Coordination Successes
Throughout the construction documents phase, coordination between the architectural ceiling build-up, MEP systems designed by Arup, and steel floor framing received significant design team attention, which was facilitated by using AutoDesk Revit and producing more traditional plan overlay drawings to track clashes. A typical architectural floor section of 50 in. and the building’s elevation above the ground, which required all services to issue from the two central building cores, made coordination particularly important and challenging since penetrations through the steel beams would be required. The reduced beam flange moment frame connection imposed further constraints, as no architectural wall connection or other welded connections could be made to affected beams within 8 ft of connections to the columns. To ensure that design criteria were adhered to and that beam penetrations were well documented going into bidding and construction, GNA elevated each primary line of structural framing to document the “protected zone” per the AISC Seismic Provisions and the penetration locations.
The combination of demanding design constraints imposed by the site—from seismic and hurricane activity to proximity to the Atlantic Ocean—and the elegant directness of the building’s architecture required the development of an equally simple, pragmatic, and robust structure. Delivering such a solution required close design and steel team coordination early on, and these collaborative efforts, which guided the development of the project’s structural approaches, ensured the successful realization of the new home for IAAM’s important work.
Addressing Thermal Bridging
Egress from the museum’s north side is provided by two steel staircases whose second and fourth landings are tied into the building’s interior wide-flange steel beams, posing a potential thermal bridging problem. Without thermal breaks, heat energy would flow unabated through the otherwise insulated building envelope, wasting heat or air-conditioning efforts and allowing condensation and mold to form on adjacent interior structures during cold winter months.
To mitigate thermal bridging, the team specified Schöck Isokorb steel-to-steel structural thermal breaks to be installed in line with the building envelope at the point where the landing frames connect to interior beams. Each thermal break consists of stainless steel threaded rods and bolts penetrating an R-15 insulation block, providing requisite structural integrity while reducing heat energy transfer by up to 75%. Not only do the breaks satisfy gravity loads, but they also withstand lateral forces from earthquakes, floods, and hurricanes.
Owner
International African American Museum
General Contractor
Turner Construction/Brownstone Construction Group
Architect of Record
Moody Nolan
Design Architect
Pei Cobb Freed & Partners
Structural Engineer
Guy Nordenson and Associates
Façade Consultant
Thornton Tomasetti
Steel Team Fabricator
Lyndon Steel Company, Winston-Salem, N.C.
Erector
CAS Steel Erectors, LLC, Hendersonville, N.C.
Detailer
Prodraft, Inc. , Chesapeake, Va.
Xiaoxiao Wu is an associate partner and David Himelman is a senior associate, both with Guy Nordenson and Associates.
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