Skidmore, Owings & Merrill continues to explore the viability of a timber-and-concrete composite floor system and opens its study to a composite floor system supported by a steel structure.
One reason mass timber construction remains absent from high-rise construction in the United States is because the system remains largely untested in performance and strength. An effort led by Skidmore, Owings & Merrill (SOM) and Benton Johnson, an associate director and structural engineer based in SOM's Chicago office, has been steadily chipping away at the many unknowns of wood as structure through with its Timber Tower Research Project, which began in 2012 and proposed a hybrid timber-concrete composite system for an existing 42-story building. (The ongoing effort received an honourable mention in ARCHITECT’s 2014 R+D Awards).
Today, the firm released two new studies in its tall timber design and construction research. “Physical Testing Report #1: Composite Timber Floor Testing at Oregon State University” reviews and documents the objectives and setup, protocol, and results of a testing program completed in 2016 on full-scale timber-and-concrete specimens. The second report, “American Institute of Steel Construction (AISC) Steel and Timber System for High-Rise Residential Buildings,” examines whether a system utilizing structural steel columns and beams with a composite cross-laminated timber (CLT) and concrete floor system could be competitive in the high-rise residential market.
“Steel, timber, and concrete each have natural advantages and disadvantages,” Johnson says. “Sustainable structures aim to use a minimal amount of materials and minimize embodied carbon footprint. Hybrid structures use each material where they are most effective, reducing overall consumption. The hybrid/composite approach is often the most economical solution in terms of both cost and carbon footprint. These reports recognize that fact and explore how timber structures can benefit from a composite approach.”
Highlights: A Composite Timber-and-Concrete Floor System
As ARCHITECT previously reported, SOM and Oregon State University developed a testing program to explore the structural behaviour and capabilities of a composite timber-and-concrete floor system with a typical bay size of 20 feet by 24 feet that could be used in a high-rise residential building. The project was funded in part by the Softwood Lumber Board.
The team explored different methods to model the behaviour of a composite system based on common analysis techniques already in use for conventional systems, including finite-element analysis and SAP2000 software. The report also documents the team’s setup and equipment for physical testing.
In evaluating different shear connection mechanisms between a 2.25-inch-thick concrete topping slab and 6.75-inch-deep CLT floor—to ensure the composite system lives up to its name—the team found HBV perforated metal, epoxied plates to provide “the greatest amount of composite action with little shear slip.” Inclined VG cylinder screws were also acceptable. The team also conducted tests to assess two-way stiffness, long-term deflection, and loading capacity.
The last of these tests was conducted on a full-scale floor panel measuring 8 feet wide and 36 feet long. The specimen failed at approximately 82,000 pounds, or eight times the design service load. As a result, the team anticipates strength will not be the limited factor in using a composite floor panel except for cases in which “charring of the wood” is required to contribute to a system’s fire rating.
Courtesy Benton Johnson / SOM Load test of full-scale mass-timber composite floor system, with applied load at eight times that of the design service load.
As with most scientific studies, SOM’s investigation generates more research questions, which include refining analytical methods for studying composite CLT-concrete floor systems, developing design guidelines and code provisions for a hybrid floor system, and more structural and non-structural testing, that need to be answered before the approach “can be adopted by design codes and used in the market without restriction.”
Highlights: The Steel and Timber Study
SOM’s original Timber Tower Research Project posited the “viability” of a structural steel system with a composite floor system to leverage the “superior spanning capabilities” of steel and “lightweight properties” of the composite mass-timber floor system. In a study funded by the AISC, SOM examined the impact of using this system for a benchmark nine-story-tall residential project that the firm designed and recently completed in California.
The firm modelled the system using asymmetrical wide-flange steel beams onto which the CLT panels would nest and sit on the beams’ bottom flanges to create a flat-soffit ceiling below (the CLT panels would be notched to ensure the steel flanges and panel faces sat flush). The concrete-topping-slab-to-steel intersection was assumed to create the system’s composite action, but “will require testing to confirm this behaviour.” While the benchmark building, which uses an 8-inch post-tensioned concrete flat plate, has a typical column bay of 27 feet 6 inches by 32 feet, the modelled composite system assumed a typical column bay of 27 feet 6 inches by 24 feet 3 inches, the maximum span of the composite timber system with a 10.5-inch floor system depth. According to the report, the modelled composite system bay size is still larger than what many markets require.
After running through different connection scenarios and assumptions regarding scope of work, loading conditions, and scheduling considerations, the report found that “the viability of the [proposed composite] system will depend on its ability to support marketable and serviceable residential units, efficiency of construction, and overall project cost.” Furthermore, the team found that the steel and mass-timber building could be constructed faster than a concrete framed building if the hybrid elements were prefabricated, and that the building would weigh 65 percent less than a comparable concrete-framed building, which “reduces foundation costs, seismic loading, and construction time.” The study also concludes that the two building types could be comparable in cost—within 10 percent of each other—depending on project specifics and current market conditions.
Image Courtesy Benton Johnson / SOM
Original link - Architect Magazine
