Architecture and engineering firm Skidmore, Owings & Merrill (SOM) says its completion of testing of a timber-concrete composite system provides "strong evidence" that such materials can satisfy code requirements and compete with traditional construction methods.

SOM's research has focused on a structural system called a concrete jointed timber frame, which employs mass timber for main structural elements and reinforced concrete for connections. In the final round of testing, a floor specimen—36 feet long by 8 feet wide—was modeled on a portion of a typical structural bay. The tested element was a cross-laminated timber (CLT) deck topped with a thin layer of reinforced concrete to enhance the structural, acoustic and fire performance of the system.

The reinforced concrete topping slab was thickened at the supporting CLT beam to form a rigid connection between CLT decks, a feature that allows floors to span between beams with a relatively thin cross-section. For the test, the specimen was loaded with a hydraulic actuator and recorded by 48 different sensors over the course of two hours.

The floor system supported 82,000 pounds—approximately eight times the required design load. Image credit: SOM. SOM reports that the floor system provided greater stiffness than required by code and supported an ultimate load of 82,000 pounds—approximately eight times the required design load. The results will serve as the basis for verification testing—a series of tests that will address issues such as fire resistance—which will be required before the system can be used in high-rise buildings.

According to SOM Associate Benton Johnson, the test highlights the "real benefits" of the composite timber approach. "We took a small amount of concrete that was necessary for acoustic and fire performance and used it to enhance the structural performance of the floor," he says. "This move allows mass timber to reach its full potential, allowing it to compete in the market while also reducing the carbon footprint of cities.”

Since 2013, the firm has been examining use of concrete jointed timber frames as a potential means to lower the carbon footprint of high-rise buildings. The testing is part of the Timber Tower Research Project, which seeks to utilize mass timber as a primary structural material with the goal of reducing the embodied carbon footprint of buildings by 60% to 75% compared to a benchmark concrete building.

While timber-concrete systems such as CLT have been in use in Europe since the 1990s, they are only now being looked at in the U.S., and their entry into the market is being opposed by some in the concrete industry.

“Before designers, builders and even legislators proactively encourage the use of wood products in construction, especially in the low- to mid-rise residential sector, greater testing must take place,” says Kevin Lawlor, of Build with Strength, a coalition of the National Ready Mixed Concrete Association.