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June 22, 2017
As with all technology, modular frame building systems (MFBS) continue to advance.
In 2009, Swenson Say Faget completed the 10-story Ballard at the Park mixed-use project with Security Properties and Bumgardner Architects (two levels of underground parking, ground floor retail, one floor for resident amenities, and six floors of apartments). We used the Inter-Steel Structures Inc. (ISSI) modular frame building system. At that time, it was the tallest ISSI project.
Today, we are constructing the tallest ISSI building again, this time a 17-story building in downtown Portland called Sky3 Place.
Sky3 Place is considered a high rise as defined by the building code and therefore requires the use of primarily noncombustible materials in construction. Anyone who has seen the 1974 classic “The Towering Inferno” may realize this is just common sense, but the restriction is actually based on the effective height limitations of a standard fire truck.
The most common types of non-combustible framing for high-rise buildings are concrete floor slabs and columns, or steel beams and columns supporting concrete floors. Both of these systems meet the noncombustible material requirement; however, they require the placement of large columns throughout the building. This can be a considerable disadvantage when every square foot is ultimately important to the bottom-line of the project, and where columns interfere with architectural design.
Additionally, these traditional framing systems require a large amount of time and labor to construct on site. Each piece of rebar and each steel beam must be placed before completing the floor and moving on to the next level.
An ideal candidate
During design development, the team determined that Sky3 was an ideal candidate for the use of a modular frame building system. The ISSI system was selected for the structural walls supporting the upper 13 stories of the building. Factors that influenced our approach include:
• Per building code, the non-combustible material requirement immediately rules out traditional wood stud wall framing, and limits the use of steel-framed walls to those that meet minimum fire resistance ratings. ISSI modular steel walls meet code and are fire-rated by Underwriters Laboratories for up to four-hour assemblies.
• The use of ISSI framing allows the Sky3 project to overcome the 75-foot building height restrictions placed on wood-framed buildings, without the bulky columns found in traditional steel or concrete framing.
• The upper 13 apartment floors are virtually identical on every level, such that the walls align vertically and carry the structural load by stacking on each other the full height of the building. Any offset of the walls would require large and costly beam additions to support the weight of the concrete and steel floors.
• A 2-foot-thick post-tensioned concrete slab provides a base for the 13 floors of stacked bearing walls. This size accommodates the open architectural layout of the parking, retail, and amenities spaces at the ground level and below grade floors.
• The ISSI system consists of roughly 8-foot-long pre-assembled wall panels that are fabricated in an off-site, controlled environment and arrive at the site ready to install. Each panel contains load-bearing structural steel framing with sound-resistant steel furring and is ready for site-applied wall finishes. Floors are simple metal deck and concrete placed directly on the wall panels.
• Craning the modular wall panel systems into place means faster construction and that translates to less financial downtime for the owner.
• Panels typically utilize powder-actuated fasteners to connect the wall panel to the panel directly below. This installation process allows for small adjustments as needed to maintain proper alignment.
• The earthquake-resisting system consists of heavily reinforced concrete walls around the full height stair and elevator cores of the building.
Engineers at Swenson Say Faget worked closely with Ankrom Moisan Architects, Absher Construction and the off-site fabricators during development of each ISSI floor plan to ensure that the wall locations were well coordinated. Autodesk Revit and Rhino were used to create detailed three-dimensional models of nearly every component of the wall system. This model was shared with the design and fabrication team to ensure that everyone was working from the same datum.
The model also enabled sequenced installation drawings for use by the construction team on site, kind of like instructions for a 13-story tall bookshelf from Ikea.
Even with their many advantages, modular frame building systems are not without limitations. There is a practical limit to the number of stories that can be supported on the walls, as the thickness of the wall supporting many stories can become excessive.
The 13 ISSI stories of Sky3 are supported on walls of a total thickness of 8½ inches, including the architectural finishes. This thickness is not unusual for an apartment wall system and was easily incorporated into the floor plan. Above about 13 stories, however, requires a thicker system and is potentially less cost-competitive to conventional concrete and steel systems.
Since the structural components of MFBS projects must be identical on repeating levels, architectural design is then limited to these parameters. For some projects, that is not ideal.
Undoubtedly improvements to the technology of building systems will continue to evolve over the next decade. We look forward to seeing how it unfolds. In the meantime, we continue to design MFBS projects throughout the Northwest to accommodate taller residential projects with quicker construction times for our clients.
Another ISSI example is the 13-story St. Helens Apartments in Tacoma. This project is currently in design and has two levels of underground parking, ground floor retail, one floor for resident amenities and nine floors of apartments.
Evin Gibson, PE, SE, is project manager with Seattle-based structural engineering firm Swenson Say Faget. He has 12 years of experience with SSF designing steel, concrete, masonry and wood structures. Heidi Maki, marketing manager at the firm, has 20 years of AEC marketing experience.