Thinking Outside the Wood Box

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While mass timber is gaining traction in the United States for its sustainability benefits, it is still often treated as a boutique material limited to short spans and particular market sectors. The new University of Washington (UW) Health Sciences Education Building (HSEB) flips that narrative. A high-performance academic facility, the HSEB combines cross-laminated timber (CLT), composite steel, and concrete to achieve long spans, control vibration, and reduce embodied carbon without compromising constructability or aesthetics. The project was also used to validate the U.S. Mass Timber Floor Vibration Design Guide, opening the door to using mass timber for more vibration-sensitive applications and showcasing what is possible when structural engineers think outside the wood box.

Located on the UW’s Seattle campus, the four-story, 98,000-square-foot HSEB houses large lecture halls, skills labs, and classrooms designed to teach and inspire future healthcare professionals. From the start, the University prioritized sustainability, but would not compromise programming, functionality, or schedule. The large classroom spaces required 50-foot clear spans with good floor vibration performance, which created a unique design challenge.

With only six months to finalize the design, the University chose to use a progressive design-build (PDB) delivery model, bringing together architects (Miller Hull Partnership), structural engineers (KPFF), the contractor (Lease Crutcher Lewis), and fabricators for an integrated design and construction approach. This integrated delivery method allowed real time pricing and constructability feedback and enabled the team to explore a range of structural systems, including some that were outside of conventional construction methods. By leveraging the PDB principle of “choosing by advantage,” the team identified the top priorities of the project, which included program, performance, sustainability, cost, and local materials. Using these project priorities as a decision metric, the team selected a hybrid composite steel-CLT-concrete structural framing system over a more traditional composite steel structure or a full mass timber structure. The hybrid system maintained the health and sustainability benefits of wood, while keeping the long span and efficient structural depth of steel. This was the University’s first CLT project, and it not only achieved a significant reduction in embodied carbon but also expanded the industry’s understanding of hybrid systems and vibration performance.

The University’s embodied carbon goals were a driving force in selecting CLT for the HSEB’s floor system. The project was an early example of the UW’s effort to reduce embodied carbon, specifically setting targets to achieve a minimum of LEED Gold certification and to limit embodied carbon to ≤500 kg CO₂e/m² in the core and shell. These targets were later adopted campus wide in the UW Green Building Standards.

Using CLT significantly reduced the embodied carbon of the building. Embodied carbon refers to the upfront energy used to extract, process, and ship materials to the site. Miller Hull completed a whole building life cycle assessment (WBLCA) for the project, which quantified the carbon reduction of the HSEB’s CLT floors when compared to those of a conventional steel building. The results showed that the CLT floors achieved a 150% reduction in upfront carbon, or a 50% reduction in full lifetime embodied carbon. The building achieved LEED Gold in May 2024, underlining the University’s commitment to balancing performance and carbon reduction.

The regional benefits were also compelling: sourcing CLT from regional suppliers (Structurlam), using renewable forestry practices, and supporting local manufacturing reinforced the project’s environmental mission and commitment to the local timber economy and forest management goals. This in turn supports the region’s efforts to mitigate wildfire risks by responsibly managing and protecting natural resources. Using a regional supplier also helped limit shipping cost and the CO₂e expended during transport to the site.

While steel and CLT are often paired together, the HSEB went a step further and used a composite steel, CLT, and concrete system to achieve the long spans and vibration performance of traditional composite steel framing, without adding structural depth and still maintaining the aesthetic and biophilic benefits of wood. The composite hybrid floor system is made up of:

The composite action is achieved by leaving gaps between the CLT panels at each beam line. A 3- to 4-inch gap or trough is left between the panels, which allows enough room to weld shear studs running down the centerline of the top of the steel beam. When the concrete topping is poured, the concrete flows into the trough, engaging the shear studs and creating composite T-beam action between slab and beam. This dramatically increases stiffness and reduces vibration.

One of the clever yet seemingly simple strategies the team used was to design the column grid on a 10-foot module. This module was compatible with both the single-span capacity of 3-ply CLT and standard concrete on metal deck. This approach allowed the team to carry two schemes forward and delay making the decision to use CLT until later in the project without risking major redesign. The PDB team was able to progress the design to a point where there was sufficient pricing certainty and design contingency could be released to fund the CLT. If the team had been forced to decide whether to use CLT in the early design phases before there was time to bid and price the project, CLT would have been removed during value engineering phases. The 10-foot module also worked for a wide variety of suppliers, which gave the team flexibility during procurement and enabled cost-competitive bidding.

To aid the cost model, the design team worked closely with the contractor to reduce labor and increase erection speed. One example of a labor-saving decision was to use a spandrel panel. Standard CLT layups typically orient the strong axis of the panel in the long direction. Since panels were broken at every beam, this approach would have resulted in many small panels, driving up the piece-count. The HSEB team instead used spandrel panels, which have the strong axis oriented in the short direction. Using these panels allowed 40-foot CLT panels to be placed parallel to the steel beams, along their weak axis, reducing labor to just 25% of typical panels. One consequence of this piece-count optimization was that the panels themselves were very flexible during erection. To make this work, KPFF and Lease Crutcher Lewis developed a custom rigging-frame that allowed the long, weak-axis panels to be hoisted safely and efficiently. The rigging frame was adjustable to work with all panel sizes. The prefabricated panels flew into place rapidly, and the topping slab tied everything together.

The building’s lateral system consisted of buckling restrained braced frames around the perimeter, which integrated seamlessly with the steel gravity framing. The concrete topping was also used as the structural diaphragm at typical floors, although a bare CLT diaphragm was used at the roof. The seismic base occurs at level 1, with a single-story basement below. Traditional concrete-on-metal deck was used at the seismic base due to below-grade timber durability concerns and to brace the concrete basement walls. The building was founded on conventional spread footings.

When the project started, the design of the hybrid system was not covered in the AISC Steel Construction Manual or the National Design Specification for Wood Construction (NDS). Because the hybrid system fell outside of prescriptive-code pathways, KPFF conducted performance testing to confirm that the system complied with the International Building Code (IBC) chapter 17, section 1709, “Preconstruction Load Tests,” for alternate construction materials, and met the requirements of the local authorities having jurisdiction (AHJ). The design team partnered with the University’s own Large-Scale Structural Engineering Testing Laboratory to test full-scale 34-foot composite beam specimens. The testing consisted of two phases:
The beam was loaded to 200% of the design load and held for 24 hours. The beam was then unloaded and checked that it rebounded to within 25% of the initial unloaded deflection.
The beam was tested to failure or to 250% of the design load.

The beams performed well in the tests, meeting the IBC strength and deflection requirements. Further, during testing, a narrow crack formed in the topping slab down the centerline of the beam. When testing was complete, the topping slab was chipped away, and it was observed that the shear studs on the beam had deformed in a gentle S-curve, which is consistent with composite behavior. This testing allowed the team to use the initial optimized beam design.

Unlike concrete and steel structures, which have 100 years of built examples to draw from, the vibration performance of mass timber is still relatively unexplored. While KPFF coauthored the U.S. Mass Timber Floor Vibration Design Guide (the Guide), there are limited constructed projects to validate it. This lack of performance data has led to limited adoption of mass timber in structures where vibrations could be of concern, either for vibration-sensitive equipment or occupant comfort. In the HSEB project, the team saw a unique opportunity to advance vibration research on CLT’s performance, and they applied for and received a Wood Innovations Grant from the U.S. Forest Service to study floor vibrations.

The HSEB was therefore one of the first full-scale implementations to align lab tests, field measurements, and modeling predictions with the Guide. The team completed the vibration study in three stages:

Using the method outlined in the Guide, the team used SAP2000 to run a finite-element modal analysis to estimate modal frequencies and mode shapes. The model used modified shell elements for the CLT and assumed full composite action of the topping slab and steel beams. The results of the modal analysis were post-processed in KPFF’s proprietary vibration software called FIVE (Footfall-Induced Vibration Evaluation). Material damping was assumed to be consistent with the Guide (2-4%). The vibration performance was then predicted under walking loads and group motion for both peak accelerations and root-mean-squared velocities. These results were calculated and compared to threshold values, with a target acceleration of <0.5% of gravity, to align with industry guidelines for classrooms and offices.

Basic structural properties of CLT, are still unknown. The HSEB team saw this as a unique opportunity to study some of those basic properties and behaviors such as damping ratios. The team tested bare CLT panels, concrete-topped CLT panels, and the previously mentioned full-scale beam specimens. Lab testing focused on the following areas:

The lab tests for 3-ply panels closely matched predicted stiffness values from PRG 320. The 5- and 7-ply panels tested slightly lower (by 7–14%), which was attributed to shear deformation effects in low span-to-depth ratios (13 to 19). Test results confirmed the assumptions behind the design models and allowed the team to confidently reduce overdesign.

The team took field vibration measurements during various stages of construction. They collected accelerometer data while conducting measured walks in representative labs and classrooms, then verified that the measured natural frequencies and accelerations matched or exceeded predicted performance at three time points:

This in-situ data was the final piece of validation, closing the loop from design to lab testing to the final as-built condition.

A few highlights of the study include:

Long spans and wood floors typically spell trouble for vibration. But the HSEB proved that a hybrid approach can meet stringent comfort standards even in classrooms and labs and can be reliably predicted by following the methodology outlined in the Guide.

The HSEB project shows that the use of mass timber does not have to include compromises. When paired intelligently with steel and concrete in a hybrid system, mass timber can deliver good vibration performance at long spans, reduce embodied carbon, and be erected quickly—all within budget and on schedule. Furthermore, the project’s extensive testing and published results should help future designers expand the use of mass timber. The HSEB proves that when we think outside the wood box, we don’t lose what we know: we build on it. ■

About the Author

Jessica Westermeyer, PE, SE, is an associate and licensed structural engineer at KPFF focused on progressive design-build, mass timber, and higher education projects. Her projects include the DBIA 2021 Project of the Year (UW HRC) and a 2024 NCSEA SEE finalist (UW HSEB). (jessica.westermeyer@kpff.com)

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