Achieving Higher Quality in High-Density Residential: the Strengths of Structural Steel
Maximize space, versatility, and quality in mid-rise and high-rise apartments and condominiums
Continuing Education
Use the following learning objectives to focus your study while reading this month’s Continuing Education article.
Learning Objectives - After reading this article, you will be able to:
- Compare the advantages of structural steel framing to other building materials
- Explore system concepts that allow the architect greater flexibility in programming and design of apartments and condominiums
- Determine economical methods of design
- Analyze faster methods of construction in using structural steel framing
Forward-thinking architects are developing designs that maximize usable area, provide versatility, and create high-quality long-lasting buildings using structural steel systems. Structural steel framing has recognized benefits in apartment and condominium design, effectively addressing the challenges of limited program space, speed of construction and future adaptability to changing needs. It is an ideal solution for mid-rise and high-rise structures where economy can be gained through repetition of the structural members. The most successful projects are a result of a collaborative team effort. As the building structure is always on the critical path, the benefit of designing with structural steel is a reduced overall construction schedule with effective planning.
The Strengths of Structural Steel
Structural steel has historically found great efficiency in the design of office buildings, warehouses, and most other non-residential building types. Apartment and condominium projects constructed using conventional or innovative steel systems are now being recognized for high performance and quality. These buildings take full advantage of the following benefits of using structural steel:
Highly sustainable. Steel is the most recycled material in the world.1 The recycled content of structural steel beams and columns produced in the United States averages 90 percent. A striking 98 percent of structural steel is recovered at end-of-life for reuse or recycling. The structural steel industry continues to reduce its low greenhouse gas emissions and energy intensity. Results of these efforts are evident in recent findings on greenhouse gases, which show the iron and steel industry reduced carbon emissions by 37 percent and energy intensity by 32 percent between 1990 and 2013 on a per ton basis.2
Image courtesy of Neil Denari Architects
More usable space. Longer spans and smaller columns allow for more usable floor area and fewer structural intrusions in open spaces. Two key selling factors in apartments and condominiums are open views and maximum free space. For example, in a 10-story residential building with 30-foot structural bays, 13 inch by 13 inch steel columns would translate to approximately 18 inch by 18 inch concrete columns for the equivalent loads. Steel columns have a reduced structural footprint compared to concrete, allowing for the greatest flexibility in furniture layout and more unobstructed views.
Fast erection and assembly. Structural steel is fabricated and erected in nearly all weather conditions with no formwork requirements, which significantly reduces the time for construction. An excellent choice for fast-track construction, structural steel framing meets owner needs to bring apartments and condominiums to market on time. A shorter schedule allows owners to reduce financing costs and generate revenue earlier.
Durable and long-lasting. The combination of a high-strength, low-weight material and resistance to rot and insects are prime elements contributing to the durability of structural steel. Apartment and condominium buildings utilizing structural steel framing inherently have a long life span due to the long-term performance of the material. Structural steel can be recaptured from demolition sites and reused multiple times prior to being recycled. The framing components may be disassembled and salvaged, a cleaner process than the demolition of concrete buildings which leads to more dust and pollution in the air.
Versatile. Structural steel framing handles changes to load and occupancy requirements with ease, extending the functional life of buildings. Additional reinforcement is simple and straightforward with steel and can be designed to support changes of use, increased loads, additional floors, and longer spans. An ideal system for unit conversion or adaptive reuse, the versatility of structural steel highlights the benefits for apartment and condominium designs.
Aesthetic options. Structural steel offers a wide range of shapes and sizes that allow a multitude of options for building cladding and expression. Recognized for its slenderness and grace, steel can be shaped into tight curves and broad arches, and utilized for long cantilevers. Steel may be exposed architecturally and should be protected from fire and corrosion where required. Methods of fire protection range from intumescent coating to cementitious spray-proofing, while design for corrosion includes options such as galvanizing or painting depending on the exposed condition.
Strong seismic performance. A ductile material such as steel is excellent for high-density residential buildings in seismic zones, where safety is critical for large numbers of occupants. Structural steel systems offer the highest level of safety in seismic design. Seismic design is required in approximately half of the geographic area of the United States. The lightweight nature of steel reduces the seismic base shear by about one-third compared to an all-concrete structure, which results in a significant reduction in foundation loads.
Establishing Design Goals
In the apartment and condominium market, real estate trends are a key factor in influencing the program needs and design of a building. As part of a market that caters to a variety of end users, high-density residential buildings increasingly require a fast track project design and construction schedule. Structural steel is similar to a kit of parts in which all the components come to the site ready for assembly and erection in nearly all weather conditions. Time is saved and labor costs are reduced with prefabricated structural components. The earlier an owner can occupy a building, the shorter the time required for construction loans with the added benefit of earlier revenue generation.
In addition, architects should not feel limited by the span and depth of the structural system. The programs for apartments and condominiums involve a wide range of unit mixes, room areas, and ceiling heights. Understanding the marketplace underscores the value gained when designing with structural steel systems to optimize space and flexibility, and allow faster building completion.
Plan for flexibility. The market trends of today may be vastly different from those 50 or just 15 years from now. Program design often requires a future plan for converting micro units to conventional units, or for modifying apartments into condominiums. Structural steel framing can handle subsequent changes in load requirements and can be reconfigured if needed, including new penetrations in floor deck systems without any need for the extensive time required for scanning to locate rebar or post-tensioning cables. Buildings often outlast current program needs and structural steel systems make them more adaptable over time.
Reduce the structural footprint. Steel columns take up less floor area than concrete columns, are higher in strength, and significantly lighter in weight. Thin floor systems, long-span deck, open-web joists, staggered trusses, and conventional beams with composite deck all offer solutions that can be used to reduce building weight overall. Cost savings are also achieved with decreased foundation loads. Providing larger clear space with maximum design efficiency leads to greater design flexibility and higher demands for occupancy to the owner’s benefit.
Maintain precision and quality. Since fabrication occurs off site, steel framing is created with tolerances typically measured in sixteenths and eighths of an inch, where competing systems have field construction with tolerances measured in whole inches. The fears of dealing with poor craftsmanship and creep with cast-in-place concrete, or creaky floors and shrinkage in wood-frame construction, can be eliminated by utilizing structural steel systems.
Photo by Bruce T. Martin Photography, courtesy of Elkus Manfredi Architects
BUILDING SUSTAINABLE COMMUNITIES
The American Brewery Lofts in Boston, Massachusetts, was originally a timber structure that housed a former beer brewing company. Elkus Manfredi Architects designed a renovation of the existing structure to house 34 residential units and expanded the building to include 45 units over a two-story parking structure. To accommodate the changes, the existing timber frame structure was strengthened with architecturally exposed structural steel. Lofty ceilings were created up to 23 feet high, supported by an open and airy frame of structural steel beams and columns.
Urban infill is growing in popularity as people move towards cities. Adaptive reuse projects complement city growth with respect to this movement. An ideal solution for building reuse, structural steel requires little disruption to healthy communities to revitalize existing structures. It is a high-quality material which performs well over the test of time. Steel strengthens existing structures with ease, allowing for changes in loads, vertical expansion, and conversions from different occupancies.
Solutions in Steel
Understanding the overall concepts of each structural steel system can strengthen decisions made during the design process and allow for productive discussion and collaboration with the project team. The choice of structural system is not just based on material costs, but also accounts for the methods of construction, amount of on-site labor, and complexity of design. Systems that are efficient in design often prove to be economical in the construction phase.
Thin Floor Systems
Architects are frequently faced with solving the owner’s desire to maximize the number of residential stories in a limited building height allowance. Specialized steel beam and precast hollow-core plank systems can help the building industry to address this need. Reducing the floor plate thickness in a building provides many benefits: lower floor-to-floor heights, savings on overall building energy, and a lighter structural weight, to name a few. Steel systems are available that allow the top and bottom planes to include both the structural floor and the structural beams in an 8-inch or 10-inch-deep system. Lightweight floor systems as slim as 8 inches span 30 feet without the need for beams projecting below the slab. The equivalent flat plate system in an all-concrete solution requires rebar and formwork, adding to the on-site labor requirements. Wood framing typically requires deep joists to meet the average spans for mid-rise and high-rise apartments and condominiums. Buildings implementing a structural steel thin floor system have saved enough overall height in some applications to add additional floors and maximize the building total square footage.
In an efficient and economical thin floor system scenario, steel beams support precast hollow-core planks. Grout fill is used to lock the beam and plank together and create composite action on-site. The underside of the plank is commonly used as a finished ceiling that is smooth and regular. Savings in both time and material are achieved by exposing the underside of the structure. Compared to an all-concrete framing solution, a structural steel system utilizing a thin-floor system is 50 percent lighter in weight. Reducing the loads supported by foundations leads to reduced project costs. The thin floor system allows for a rapid speed of erection due to the prefabrication of plank and steel. Reduction in on-site labor and the elimination of formwork are major benefits in saving time and costs. Other trades are able to access floors faster to complete their work, condensing the overall construction schedule. In a market where speed governs, architects must factor in the time for building completion in their overall design services to meet the owner’s schedule. A number of buildings have implemented specialized thin floor systems in North America.
Image courtesy of Stantec Architecture
THIN FLOOR SYSTEM: GLASSHOUSE
In 2014, the GlassHouse project broke ground in Winnipeg, Canada, using the Deltabeam thin floor system developed by the Peikko® Group. Designed locally by the international practice Stantec Architecture, this high-end 21-story residential building promises floor-to-ceiling glass in a cool modern aesthetic for 196 condominium units. To meet the need for speed, the project team implemented a thin floor structural steel system to advance the erection process in the most economical way. The modified beams, known as Deltabeams, have been UL tested to achieve 2-hour, 3-hour, and 4-hour ratings without the need for additional fire protection on the beam itself. The underside of each exposed beam is painted black in contrast with the exposed hollow core plank ceilings, stark white walls, and warm walnut flooring. Steel columns are architecturally exposed and coated with a black intumescent paint to meet the required fire protection. The typical floor-to-floor height is 9 feet 10 inches separated by thin 10-inch horizontal bands on the facade. Architect and Senior Associate Michael Banman along with Fletcher Noonan, design leads of GlassHouse at Stantec, are well-versed in the Deltabeam system. Banman elaborates, “In our practice, it is vital to consider concept, construction, and structure simultaneously, holistically. The organization, clarity, repetition, and rhythm of the exposed structural steel and of the construction assembly facilitate the construction schedule. This in turn lowers construction costs and reduces the cost of owning while providing a more sophisticated space in which to dwell.” The steel is tracking on schedule and the building is set for completion in 2016.
Images courtesy of Girder-Slab® Technologies, LLC
THIN FLOOR SYSTEM: TROY BOSTON
Troy Boston is a 378-unit apartment complex located in Boston’s South End at 275 Albany Street designed by ADD Inc., now with Stantec. The project team is pursuing LEED Gold certification for the dense urban development. Two residential towers at 12 and 20 stories anchor the corner of a tight site, wrapped with a podium comprised of outdoor rooftop amenities, three levels of parking, and ground-level retail space. The project team performed an analysis to compare costs and schedule for multiple structural options: a conventional steel-framed system, a cast-in-place concrete system, and a thin floor system known as the Girder-Slab® system. All three systems were competitively bid solutions. Senior Associate Principal, Jon Lannan, AIA, of ADD Inc., noted “getting this building open before the competing neighbors” was a high priority for the developers, especially in the growing Boston market. Weather related issues during construction had to be nearly eliminated to maintain the critical path. The Girder-Slab® system was ultimately chosen as it held the advantage of fast erection which led to a shorter schedule, resulting in a more economical solution. Designed to accommodate bay sizes of 25 feet by 30 feet, the thin floor system can achieve floor-to-floor heights as low as 8 feet 10 inches when using 8-inch hollow core planks. The design team of Troy Boston achieved the desired clear ceiling heights of 9 feet 2 inches by setting the floor-to-floor heights at 9 feet 10 inches.
The 330,000-square-foot project is on schedule to complete the structural framework within 10 months. The underside of the hollow core planks is exposed and the modified beams, D-beams®, are treated with spray-applied fire protection. By achieving low floor heights, material costs are reduced in addition to saving building energy usage. Flexibility of space planning was also desired to lay out the floor plans with a dense unit mix of studios, one-bedroom, and two-bedroom apartments. To create larger bays with fewer columns, the structural team at Odeh Engineers designed the beams to span longer than 25 feet with the implementation of tree columns that were integrated within partitions. The thin floor system ultimately represents a simple design that allows for low floor-to-floor heights within a fast schedule. According to Lannan, “[We] would happily use Girder-Slab® again.”
Long-Span Composite Deck
Composite metal deck economically spans 8 to 10 feet between beams. An effective design solution is to implement long-span deck with structural steel framing to avoid intermediate framing on spans much shorter than a typical apartment or condominium unit. High-strength long-span steel deck is used with concrete cover to create spans of 28 feet and greater. The depth of the floor system is typically 10 to 12 inches for spans up to 35 feet, but can be designed to be slimmer. Shoring is required to support the long span deck at approximately 10-foot intervals while the metal deck replaces formwork for the poured concrete. Long-span composite deck systems may be prefabricated in large bay sizes, allowing for modular methods of construction, which often are of benefit in fast-track construction projects. Unprotected long-span deck achieves 1-hour and 2-hour UL-approved ratings when provided with concrete cover of a specified design thickness. Deck may be left exposed or finished with gypsum board depending on the desired aesthetic. This system, when finished with a gypsum board ceiling, is lighter in weight than a typical all-concrete solution while still meeting necessary performance criteria for sound control, impact isolation, and floor vibration.
Image courtesy of ODA New York
LONG SPAN DECK: 100 NORFOLK ST
100 Norfolk Street, in New York City, is a 12-story luxury condominium building designed by ODA New York that began construction in 2014. Located on a narrow urban site, the form extends over an adjacent grocery story to create larger floors. A series of cantilevers become more apparent as the building rises in height, expressing the braced framing used for lateral support. Long-span deck was ultimately selected to maintain a lightweight building to reduce critical loads on the foundation. The Deep-Dek® Composite Versa-Floor system maintains a low-profile floor plate just barely visible behind the glassy exterior. Interior spaces take advantage of generous ceiling heights, clear long spans, and panoramic views of the skyline. Many of the 38 units, ranging from studios to three-bedroom units, have already sold well in advance of the building completion.
In-Wall Beam
It is natural for architecture and structure to require extensive collaboration prior to completion of the design. In apartment and condominium buildings, the room sizes are set on modules that guide, or are governed by, the column spacing of the structure. The long-span capability of structural steel framing allows for the architectural layout to effectively inform the spacing of the structural grid.
In-wall beam framing is a classic example of a system in which the framing is developed to align with the partition layout of the interior. Wide flange beams are designed to span the distances required to reduce intermediate framing in room interiors. The modules of beam-free space allow for higher floor-to-ceiling heights while room partitions enclose the beams that support the floor system. Composite deck as thin as 5¼ inch total thickness is able to achieve a 2-hour rating with unprotected deck, benefitting a low floor-to-floor solution. With an in-wall beam framing system, the program layout and design comes first followed by support from the steel structure.
Photo by DeSimone Consulting Engineers
SAVE SPACE WITH STEEL PLATE SHEAR WALLS
When space is tight and concrete shear walls are demanding large amounts of floor area, steel plate shear walls can play a major role in the design of a building. Steel plate shear walls (SPSW) are thin plates located between columns that save valuable floor space in lieu of using thick concrete shear walls. These steel shear walls are commonly used in slender buildings that require great stiffness with limited floor area usage. Prefabricated steel panels are installed on site with minimal field work, increasing the speed of construction rapidly compared to cast-in-place concrete shear walls. Reducing the time for installation is a key time saving factor, which can significantly reduce associated labor costs. A study performed by structural engineers Sellie and Hooper concluded that an approximate 2 percent gross area savings was achieved by implementing steel plate shear walls instead of a typical concrete shear wall.3
Photo by DeSimone Consulting Engineers
IN-WALL BEAM: HIGHLINE 23
Highline 23, in New York, is a perfect example of a condominium building which had multiple space constraints. Designed by Neil M. Denari Architects with DeSimone Consulting Engineers, the 14-story luxury residential tower is built upon a narrow 40 by 49-foot site area. The floor area increases as the building grows taller. Large trusses are used to support the longer spans and also define the building aesthetic. The project utilized the in-wall beam framing method and steel plate shear walls to create more efficient space and to construct the building economically.
Conventional Solutions
Conventional methods of steel beam and column framing are commonly known for use in nonresidential building types such as offices and schools. Structural steel framing is once again proving to be beneficial in high-density residential projects as the speed of erection allows for building completion faster than all-concrete framing solutions. Similar to the in-wall beam framing system, conventional beam layout can span long distances. High floor-to-ceiling heights, common in luxury apartment and condominium design, do not have to sacrifice floor space by using heavy, massive concrete columns. Ceilings may be exposed to lofty structure or finished to house recessed lighting. New buildings take advantage of the speed and versatility of steel framed construction while maintaining the high quality and strength only found in structural steel.
Photo by Kessler Photography
CONVENTIONAL SOLUTIONS: THE CANOPY LOFTS
The Canopy Lofts rise seven stories in the historic haymarket district of Lincoln, Nebraska. Designed to entice the young professional market, 70 apartments provide 9-foot clear ceilings in the living spaces with 11-foot-4 inch floor-to-floor heights. The ground floor is made up of retail spaces with six floors of one-, two-, three-, and four-bedroom apartments above. Symbolically representative of the railway system which used to run across the site, architecturally exposed structural steel is a common theme throughout the development. The units contain a mixture of exposed beams, coated with a dark gray intumescent paint, and finished ceilings with recessed lighting. According to the co-developer and co-owner of the property, Chief Industries, Inc., fast erection with an all-weather system of construction was highly desirable to bring the building to market on time. “We wanted the best out there,” states Roger Bullington of Chief Industries, Inc. From an owner’s perspective, a long-lasting building of high quality makes the project a valuable investment and serves the community well. Due to the visible speed of construction of the Canopy Lofts, other local buildings in development have reconsidered using traditional concrete framing and revised their designs to incorporate structural steel.
Staggered Truss
The structural concept for the staggered truss system was developed with flexibility in mind, as column-free and beam-free spaces 30 feet by 60 feet or larger are possible. Story height trusses are typically spaced 60 feet apart, running along the short dimension of the building and alternating on column grid lines on every other floor. With the alternating, or staggering, structure, the building contains fewer structural elements which enhances the speed of construction and reduces loads on the foundation compared to an all-concrete frame. Trusses allow for horizontal penetrations as needed for mechanical, plumbing, electrical and fire protection. Precast hollow core plank flooring is commonly used and allows for floor-to-floor heights as low as 8 feet 9 inches. Repetitive truss designs are economical and efficient for the structural system, easily accomplished with the same floor-to-floor heights and stacked unit layouts. Hanging the second floor from the level above allows for a ground floor free of interior columns. Ideal for buildings greater than five stories, the speed of construction is known for saving time in the project schedule and as a benefit to owners desiring early occupancy.
A common design challenge is addressing an efficient parking grid that is coordinated with residential floor plates above. Transferring structure to accommodate residential unit modules above a parking grid often comes at a cost. The staggered truss system eliminates the need for structural transfer levels, which often take up critical space, and works well in mixed-use building types that include parking, high-density residential and hotel occupancies.
There is a long history of buildings which take advantage of the vast amount of clear floor space offered by the staggered truss framing system. Towers on the Park consists of a nine-story and 20-story building and was one of the first large apartment buildings in New York City to use the staggered truss system in 1986. A luxury vacation complex of five- to seven-story buildings, the Seascape Point Condominiums in Osage Beach, Missouri, was completed in 2006 along Lake of the Ozarks. In 2007, the Macallen Building Condominiums offered 140 luxury units in the Boston market with distinct floor plans that were integrated with the staggered truss system. The Summer House apartments will be a 22-story building comprised of 226 units, five levels of parking, and ground level retail and restaurant space in downtown Stamford, Connecticut.
Image courtesy of Stantec Architecture in association with Office DA
Macallen Building Condominiums, Boston, Massachusetts. The exterior fins represent the locations of story height staggered trusses that span across each floor.
Prefabrication and Modular Construction
Prefabricated and modular systems using efficient construction techniques have been developed to speed up building construction. Reducing the amount of labor spent on site begins with choosing the right systems during the design phase.
Photo by Kessler Photography
PREFABRICATED LOAD-BEARING WALL PANELS
2040 Market Street in Philadelphia, completed in 2012, utilized modular load-bearing Integrity Wall™ Panels and lightweight Ecospan composite joist floors to convert a vacant office building into apartments over a ground level retail space. The load-bearing wall system combines 4-inch hollow structural steel shapes (HSS) in the form of prefabricated panels that are designed to fit within interior partitions. The lightweight nature of open web steel joists and their cost effectiveness make them a viable solution for projects with a limited construction budget. Floor joists designed for high-density residential construction are typically spaced at a minimum 24 inches on center, supported by beams or bearing walls which transfer loads to columns. A 12-inch joist with a 2.5-inch concrete slab on metal deck can span approximately 30 feet in average conditions. Columns are smaller when supporting lighter weight, allowing for greater usable floor space. Reduced loads on the building foundation translate to greater cost savings. The lightness of the structural system for 2040 Market Street allowed eight additional floors to be built on top of the existing building. The floor plate was also expanded to maximize the usability of the site area.
Modular framing systems. Architects gain economy in design through repetitive floor layouts on stacked floor plates. It is often commonplace in apartment and condominium design to take advantage of repetition by creating typical details, fewer drawings, and a building which owners can easily market with standard rates to future occupants. Repetitive, modular design is most efficient with modular production. Compared to concrete- and wood-framed structures, modular steel framing has achieved cost savings due to its lightweight advantage, reduced material costs, and speed of erection.
Modular connections. Another innovative approach is a modular system based on prefabricated moment connections. With structural steel, the speed of erection can be further increased by simplifying the assembly process in the field. While mainly used in high seismic areas, these modular moment connections may also be applied in zones not governed by seismic activity. A bolted configuration is less labor intensive and more economical than the welding process. The process in the field is a simple assembly from a kit of parts. This method of framing is 35 to 45 percent faster than wood frame residential construction and is also recognized for speed superior to concrete construction.
Photo courtesy of ConXTech®
ConXTech® is a solution for fast erection with modular connections that are lowered, locked into place, and then bolted. The construction sequence takes less than half the time required for alternative conventional systems.
Strong Design Lasts Long
Structural steel systems exhibit a lasting quality which allows for infinite possibilities in the design of apartment and condominium buildings for the present and future. Each site, program, and design intent of a building is unique. Determining the best structural solution for a project takes into account multiple aspects of a project. The material strengths and weaknesses, availability of resources, construction schedule, and sequence of operations are all vital to determining the best structural system. The construction schedule and labor typically are the longest and most costly portions of the building process. Architects must plan for efficient structural systems which reduce time and labor in the field as well as bring the owner value in the short term and long term. Advances in structural steel systems are depicted in the endless span options, speed of erection, innovative technologies, and ability to endure the tests of time and adapt easily to future uses. For the apartment and condominium market, structural steel has great advantages over other material solutions.
Endnotes
- World Steel Recycling in Figures 2009-2013, Bureau of International Recycling (BR), 2014
- 2013 Annual Statistical Report, American Iron and Steel Institute
- Seillie, Ignasius F. and John D. Hooper. Steel plate Shear Walls: practical design and construction. Modern Steel Construction. April 2005. Online.
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The American Institute of Steel Construction is a not-for-profit technical institute and trade association representing the structural steel industry. AISC provides technical assistance and complimentary conceptual solutions to architects, engineers, code officials, and educators to promote better, safer, and more economical buildings, bridges, and other structures framed with structural steel. www.aisc.org |