With mass timber projects on the rise around the United State, Skidmore, Owings & Merrill (SOM) and Oregon State University (OSU) have partnered to produce two new reports on how timber buildings can overcome their technical limitations by integrating steel and concrete. The new composite systems being proposed would allow timber construction to rise higher than before, with longer floor spans.

The OSU Testing Report, released earlier this month, looked into the possibility of combining cross-laminated timber (CLT) floor systems with a concrete topper, to improve the strength of the flooring as well as lengthen its span. To accurately represent real-world conditions, the SOM team first drew up plans for a “typical” 11-story residential building and indicated where the wood columns would normally be. With the floor span determined, the CLT flooring was stress tested for load, bending, cracking and shearing, before and after the application of a concrete slab. A 2.25-inch thick concrete layer was applied over a 6.75-inch thick CLT floor for the experiment.

After testing smaller, individual sections, an eight-foot-by-36-foot full-sized mockup was created and subjected to load testing, only failing after engineers applied eight times the normal service load, or around 82,000 pounds of pressure. One complicating factor is that CLT can be charred for a higher fire rating at the expense of its strength, and any real-world application of CLT would need to be thicker than in testing conditions. Still, the results are a promising first step to increasing floor spans in timber buildings as well as improving their acoustic properties.

A full-scale timber and concrete composite floor system undergoing a load test. (Courtesy SOM)

The second report was produced in conjunction with the American Institute of Steel Construction (AISC) and examined how steel framing can best be integrated with timber floor systems. Because steel framing can span much greater distances than timber with smaller columns, and because CLT is lighter than concrete, a building that uses both should get the best of both worlds. In SOM’s modeling, this combination model was equally as strong as a steel and concrete building while offering window bays of the same size as a typical residential building.

Ideally, high-rise timber construction of the future would combine both of these techniques, as the concrete slab topper adds extra seismic protection. With timber construction offering the potential for more sustainable, durable and quickly assembled towers, hybrid research could be a stepping stone towards bringing mass timber construction into the mainstream.

All of SOM’s timber research reports can be found here.

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