Kevin Reynolds, PE, And Alex Font, PE 2023-09-01 14:24:59
Bike right up to your workspace in this steel-framed office building with six stories of exterior cycling ramps.
A TOWN WHERE COFFEE SHOP CONVERSATION often centers around the newest mountain bike trail was a fitting location to build the world’s first bikeable office building.
Ledger is a six-story, 230,000-sq.-ft mixed-use office structure that provides fixed and flexible co-working space, located in Bentonville, Ark., just a few blocks south of the city’s historic square. The building features a community-centric design focused on public gathering areas and outdoor access. All told, Ledger is Bentonville’s latest project geared toward solidifying the title as the mountain biking capital of the world, and steel is its backbone.
Six stories of bikeable ramps take cyclists from the ground to the roof. Anyone can bike or walk the ramp, which provides access to all floors of the building. The trip from the first floor to the roof is approximately ¾ mile. The building features bike-friendly amenities, including indoor lockers on each floor for secure bike storage, eBike charging stations, onsite showers on each floor, and a bikers’ lounge. Its outdoor communal terraces allow occupants to be part of the downtown Bentonville experience.
General contractor Nabholz and the project developer engaged steel fabricator W&W|AFCO Steel in a design-assist role during the initial design phase, which allowed the design and construction teams to work together to develop positive solutions to the many challenges the project presented.
Chief among those hurdles was Bentonville’s 100-ft maximum height restriction on building structures, which required the teams to develop a steel design alternate that would be more economical than the originally proposed concrete structure.
The teams collaborated to develop and perform cost analysis on numerous conventional and unconventional steel framing options that minimized the floor-to-floor spacing, and they determined that a conventional steel moment frame system using composite floor slab over shallow floor members was the best option. On top of that, Nabholz’s comprehensive cost analysis concluded the proposed steel framing system would be more economical than the originally planned concrete building frame.
Maximizing Floor-to-Floor Heights
In response to the height restriction, the teams sized the typical girders and floor beams to minimize depth, with the steel members sized about 6 in. shallower than typical framing on an office building with the same structural grid. The building columns are laid out on a 30-ft by 30-ft grid with W18×97 primary floor girders and W10×54 infill beams spaced at 10 ft on centers. Cambers measuring 1¼ in. were specified for the W10 infill beams to help offset the estimated dead load deflection. The typical W18 girders were also raised 2 in. to the top of floor deck elevation, further reducing the overall structure depth. For the typical framing, the overall floor depth (with a 6-in. floor slab) was 225∕8 in. at the girders and 161∕8 in. at the infill beams. This allowed space for MEP and sprinklers with headroom to spare over the open office floor plan.
A Lateral System with Erection Efficiency
As with any building project, the schedule was a concern from the beginning. Due to the limited laydown space available on site, an efficient structural system and thoroughly planned erection sequence were focal points early during the design stage. A consistent system of moment frames throughout the structure in lieu of braced frames or shear walls appeared to offer the most advantages for the erection of the building, and welded moment connections were not considered due to the schedule. Instead, all moment connections were designed as extended end plate bolted connections, as per AISC Design Guides 4 (Extended End-Plate Moment Connections Seismic and Wind Applications) and 16 (Flush and Extended Multiple-Row Moment End-Plate Connections), both available at aisc.org/dg. Not only did the moment frames provide instant stability and plumbness of the structure upon erection, therein eliminating the need for temporary guy cables, but also the erection sequence was not influenced by having to plan around the erection of braced cores. Each column line of the building provides stability, ultimately allowing for more flexibility with the erection sequence planning.
Minimizing the “Junk”
Bolted moment connections are framed into the column web and flanges from all sides at many of the column locations. It was important to design the moment connections in a way that did not require column web stiffeners or doubler plates. Eliminating the column web stiffeners and doubler plates enabled the extended end plate bolted moment connections to fit consistently and cleanly into the webs of the columns. The typical column size of W14×120 was selected to carry the typical floor girder moment connections without additional reinforcement required to the column. When considering the cost of extra material and fabrication time for added web stiffeners and doubler plates to the columns at the floor framing, increasing the column sizes to eliminate the additional reinforcement is often the more economical solution.
Architectural requirements necessitated the perimeter slab edge extending 1 ft, 4 in. from the centerline of the spandrel beam. The metal floor deck was extended over the bent plate slab edge to eliminate the need for kicker bracing at the slab edges, and Nelson studs were welded through the deck to the bent plate below to create a composite steel section to carry the weight of the slab and floor. Temporary angle kickers were installed to steady the bent plate during the floor slab pours and removed later prior to installation of the building envelope. The absence of kickers outside the perimeter beams offered an ideal space for running mechanical ducts along the perimeter glass façade.
Standing Out
Structural steel also served as the perfect solution to support the undulating ramps and terraces that traverse the perimeter of the building. A combination of shallow steel framing, framing steps, cantilevers, and built-up trusses support the ramps and terraces without inhibiting the interior spaces below.
In addition to the exterior climbing ramp to the roof, the building expresses itself with some eye-catching cantilevers. On its west side, the floors from Level 3 to the roof cantilever almost 12 ft. Bolted moment connections through the perimeter column webs support the west side cantilever, effectively cantilevering the typical W18×97 floor girders at each column line. At the end of the cantilever beam, a continuous vertical W16 column is attached with bolted moment connections at each floor level. The moment-connected vertical columns stiffened the individual cantilever beams and tied each level together, redistributing the floor live loads along the height of the building and holding a uniform deflection at the end of the cantilever.
On each end of the building the floors cantilever from 14 ft to 19 ft to provide column-free cover to the terraces below. The 14-ft cantilevers on the terraces work similarly to the west side cantilevers, with bolted moment connections through the perimeter columns. However, at the terrace levels, the cantilevered beams make a vertical step down at the column to accommodate the roof build-up needed at the exterior terraces. For the nearly 19-ft cantilever on the north end, custom plate girder shapes were designed to carry the Level 3 floor plate and roof terrace above. The plate girders supporting Level 3 were also tapered to reduce the depth toward the end of the cantilever and provide greater clear height to the exterior terrace below. These girders were constructed with 2-in. thick steel plates, 16-in. wide flanges, and a maximum depth of 26 in., with the tapered plate girders reducing to a minimum 19 in. depth.
All cantilever members were sized to produce an estimated ½ in. deflection under dead load. A tip camber of 3/8 in. to ½ in. was specified for all cantilevers, with the goal of leveling the floors as much as possible once the floor slabs were placed and cured. Through collaboration between structural engineer Engineering Consultants, W&W|AFCO, and the steel erector, the team determined that shop installation of a 1∕16-in. steel shim near the low side of the bolted moment connections where the cantilever beams attach to the perimeter columns would produce the specified tip camber at the end of each cantilever. A 1∕16-in. steel shim was also added near the top side of the bolted moment connection at the end of the cantilever. The shim solution proved to be more efficient for the fabricator than alternate methods of producing camber, such as heat-treating or rolling of the steel beams.
The typical building column spacing was spread out to 60 ft to create the interior open atrium and lobby space. Steel Vierendeel trusses were designed to support the 60-ft spans of the exterior ramps above. The Vierendeel design was chosen for its simplicity and lack of diagonal members, which allow for more uninterrupted light to enter the building. The truss members consist of W16×100 verticals and W18×130 or W18×175 chords. The connections, both between truss verticals to truss chords and truss chords to support columns, are made with bolted moment connections. The team worked to incorporate bolted moment connections in lieu of field welding as much as possible to ease and speed up the steel erection.
The design team also found a solution in steel to the project’s acoustical challenges. W&W|AFCO and metal decking supplier New Millennium proposed a cellular acoustical metal deck, which eliminated the originally planned spray-on acoustical treatment. The metal decking option proved to be more economical and provided a clean, flat surface that the developer and architect desired for the interior ceilings.
The final design incorporated approximately 2,200 tons of structural steel and 230,000 sq. ft of metal decking. Fabrication began in September 2020 and was completed in April 2021. Erection began in November 2020 and was largely complete by July 2021. Now open, Ledger serves as a new symbol for Bentonville that walks the walk (and rides the ride) as an employee-centric building that encourages wellness and activity.
Owner
Center City, LLC
Architect
Michel Rojkind and Callaghan Horiuchi + Marlon Blackwell Architects
General Contractor
Nabholz
Structural Engineer
Engineering Consultants, Inc.
Steel Team Fabricator
W&W|AFCO Steel
Detailer
S. P. International, Inc.
Kevin Reynolds (kreynolds@wwafcosteel.com) is the Executive Vice President of W&W|AFCO Steel, and Alex Font (alex.font@ecilr.com) is a principal engineer with Engineering Consultants, Inc.
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