Modern Steel Construction • January 2023 • Forging a New Steel Story

A recently completed multi-use complex helps lift an old steel town to new heights.

ONCE A THRIVING STEEL MILL TOWN, Conshohocken, Pa., has long been an area in transition.

Its proximity to Center City Philadelphia—a 15-minute drive—and accessibility to mass transit has perpetually made it an attractive commercial hub, but up until recently, growth had been haphazard. “Conshy,” as the locals call it, lacked a town center, and little thought had been given to walkability or parking.

Keystone Development + Investment had a vision to change that and proposed SORA West, a multi-use complex that includes a new 14-story office building, a hotel, a parking garage, and a historic firehouse adapted into a restaurant, all built around a public plaza that hosts concerts and other events.

At the same time, pharmaceutical distributor AmerisourceBergen wanted to combine two locations into a single headquarters and increase its brand identity. The company, with a top ten ranking on the Fortune 500 list, studied labor conditions, trends, and workplace dynamics and chose Conshohocken and SORA West as its new corporate home.

The company’s 1,500 Pennsylvania-based employees have recently moved into the 14-story, 524,838-sq.-ft office building, which was completed in late 2021. The space offers 11 floors of collaborative office space, including the lobby and ground-floor amenities; a two-level, 76,372-sq.ft basement parking facility with 173 parking spaces; a 16,000-sq.-ft rooftop terrace with a mechanical, electrical, and plumbing penthouse; and a high roof.

The chosen framing material for a project that reimagines a steel town? Steel, of course—3,500 tons of it.

“Structural steel allowed earlier design of the base structure so an early bid package could be issued,” said Mal Bland, PE, principal and project executive/operations manager for IMEG (formerly The Harman Group), the structural engineer of record for the core and shell. “This allowed the structural steel fabricator to begin their work earlier. The base structure is normally on the critical path, so accelerating the steel fabricator and detailer results in an earlier turnover to the developer. In turn, this allows the developer to deliver the core and shell of the building to the corporate tenant sooner.”

“The use of steel allowed an efficient column grid of 30 ft by 45 ft that works well to maximize the efficiency of office layouts for corporate office buildings,” Bland continued. “And the use of structural steel resulted in approximately $20-per-sq.-ft savings in the structural cost.”

The site presented several challenges that the design team of IMEG, architect Gensler, and general contractor Intech were able to solve using the structural steel with slab on metal deck building, including working with a difficult slope and maintaining the durability of the steel-framed parking levels in the basement.

Five braced frames were needed to laterally brace the 184-ft-tall structure—three in the long direction and two in the short direction. The braced frames, mostly made up of W14 wide-flange chevron braces, were strategically placed within the interior of the floor plates, next to the stairs/elevators, to maximize open floor space and to offer unobstructed views around the perimeter of the building. To limit the lateral drift, moment frames were placed at the far ends of the building. These frames used partially restrained beam-to-column moment connections to keep service-level wind drift values within a code limit of H/400.

The team chose to frame the underground parking structure with structural steel as well. The two parking levels comprise one slab-on-grade level, one slab-on-metal deck level, and a parking speed ramp to access it from one level higher due to the steeply sloping terrain.

Vehicles can track in water and deicing salts, which puts the supported parking levels at risk for deterioration and corrosion. During the steel detailing stage, the team paid particular attention to the durability considerations highlighted in AISC Design Guide 18: Steel-Framed Open-Deck Parking Structures (aisc.org/dg) and ACI 362.1: Guide for the Design and Construction of Durable Concrete Parking Structure in an effort to limit stagnant water, protect the slab from water seeping in, and supply a path for water to exit.

The first step was for the steel framing and slab on metal deck to slope to drains in two directions rather than building a flat slab with varying thicknesses, which helped reduce the weight of the slab. The minimum design slope of the slab is 1.5% or 3∕16 in. per foot diagonally, which allowed for a minimum of 1% due to construction tolerances and beam camber and helped avoid any potential water ponding.

The team chose G-90 vented galvanized metal decking for the slab-on-metal deck. The decking is a stay-in-place form only, with the slab having top and bottom reinforcing steel, and the perforations in the decking allow trapped water to be released from above the decking. The slabs were designed as continuous spans with negative bending reinforcing over the supports, with detailing that included additional top reinforcing over all the supports and at slab edges to minimize cracking above the supports. All reinforcing steel in both the supported slab on metal deck and slab-on-grade was epoxy coated. Additionally, slab on metal deck is susceptible to cracking, which gives water an entryway into the slab, so a urethane traffic membrane coating was applied throughout the supported parking levels to bridge the cracks and protect the slab.

The perimeter of the structure has a 3-ft-wide, 2-in.- tall concrete wash that slopes the top of the concrete away from the edge of the slab. The wash keeps water from the edge of the structure, walls, and façade elements and also prevents water from entering the elevator lobby.

Another challenge with the garage was the perimeter retaining walls. In a typical underground structure, the slab and steel framing resist the soil loads and pass the loads through the diaphragm to the opposite side. Because of the ramping on this structure, each bay of the parking level is split and does not present a direct load path from one side to the other. In addition, the building sits on a sloping site with a two-story retaining wall on one side and a three-story retaining wall on the opposite side. Lastly, a parking access ramp spans almost two-thirds the length of the building next to the three-story retaining wall that drops down two stories. This required several split diaphragms with unique details in order to transfer lateral loads from soil loads to the braced frames and surrounding walls.

To access the underground parking levels, a structural speed ramp on the north side of the building slopes down from the entrance at Level 2, through the Level 1 slab, to the first level of underground parking. The ramp bisects the Level 1 floor diaphragm below grade, leaving nothing to resist the soil pressures retained by the three-story basement retaining wall. Ultimately, the speed ramp itself was used to resist these soil forces. An in-plane galvanized steel truss was used to distribute the nearly 2,400 kips of soil load from the retaining wall at the north through the 24-ft-wide ramp slab and into the floor diaphragms at Level 1 and the lower parking level, where the load is eventually counteracted by the passive soil pressure formed at the south side of the building.

There were varying layouts at the upper office levels and lower parking levels, so several columns had to be transferred out at the ground level. A series of 58-in.-deep to 68-in.-deep built-up steel plate girders were used to transfer out four columns with a range of factored loads from 1,800 kips to 2,250 kips. The heaviest of these plate girders weighed nearly 8.5 tons and was made of 3.5-in.-thick Gr 50 steel flange plates.

The south face of the building has a setback in the curtain-wall façade, a prominent architectural feature that affected the structural steel framing. To accommodate this 2-ft setback, a sloping column was introduced between Levels 3 and 4. The gravity load in this column of more than 1,200 kips translated into nearly 200 kips of horizontal thrust at each level. To resolve these forces, a series of diagonal WT braces were installed between the primary floor beams, from the work points at the top and bottom of the sloping column to the nearest braced frames. These forces were combined with the overall lateral forces in the building and incorporated into the final design of the braced frame elements.

The façade is a single-story glass curtain wall system attached to the top of the slab at the perimeter. Block-outs were provided in the slab on metal deck at the curtain wall mullions and infilled with grout after façade installation. It is easiest to connect the façade to the top of the slab, but this can cause a detailing problem with the finishes, so IMEG included a pocket within the concrete slab edge to hide the connection, which was grouted over after installation to be flush to the surrounding slab.

At the top of the building, the rooftop mechanical units reside behind a 20-ft-tall screen wall nearly 110 ft wide, braced on either side by two penthouses with individual braced frames. Since the wall did not align with the columns below, kickers could not be used. The team solved this issue with a horizontal ring truss at the top of the wall, and the lateral load from this truss transferred directly into the braced frames or into the slab on metal deck diagram of the penthouses.

From top to bottom, the development pays homage to steel history with modern steel framing, thus continuing the town’s steel story into the future.

Brian Sherman and Sean Pousley are both project engineers – structural with IMEG.