This post is the first of two exploring lessons learned from designing a tall timber building proto-model called PMX. We recommend reading the introduction first.
Timber is nimble and light compared to other structural building materials, and that gives it many advantages, such as making it easy to work with in a factory. But these same properties also create new challenges for timber’s use in designing taller buildings. When engineers design a building, they must create a structure that can resist different types of forces, including vertical forces like gravity and lateral forces like wind. The lighter the building, the more susceptible it usually is to lateral forces.
To gain insight into the performance of our 35-story mass timber proto-model, PMX, the team modeled how it would compare to a traditional concrete building of the same size. PMX was approximately 2.5 times lighter than its concrete counterpart, and when we began doing wind analysis, we discovered that PMX was reacting more like a building of 40 or 50 stories. That’s one of the reasons why many of the tall timber buildings completed to date are actually hybrids, reinforced at their cores by concrete walls or steel bracing.
We preferred to work exclusively with timber, if possible, to capture its sustainability benefits and to prove the potential for manufacturing. So, to respond to lateral pressure on our 35-story timber building, we drew from engineering tactics more typical of super-tall building design: the cross brace frame and a tuned mass damper.
The cross brace frame
Constructing a structural core out of timber was a non-starter on PMX: to create sufficient stiffness, the core wall sizes would have had to be a whopping 5 feet thick each. Not only would timber that thick be almost impossible to fabricate and very difficult to assemble on site, it also would cause loss of valuable floor space: our estimate was a loss of 615 square feet, or approximately one two-bedroom unit on every 8,450-square-foot floor.
After the team evaluated a series of options, it became clear that the only pure-timber structural system that would work for PMX was an exoskeleton form.
The PMX exoskeleton system consists of big timber beams crossing the facade of the building. This distinctive zig-zag is a derivative of a design tool pioneered by iconic structural engineer Fazlur Khan, whose steel exoskeleton on Chicago’s 100-story John Hancock Center in 1965 laid the foundation for modern skyscraper design. By transferring bracing to the exterior of the building — instead of having chunky walls and partitions throughout — the exoskeleton system opened up interior floor plans, allowing for far more usable space at taller heights.
The same logic applies on PMX. Because the exterior lateral bracing is now doing the lion’s share of the work to keep the building standing, the core walls become lean — dropping from 5 feet thick to just 10 inches — and the interior is left open and flexible. The entirety of the 8,450-square-foot floor is preserved for tenant use, and future tenants would have the ability to knock down the non-structural wall between rooms.
The tuned mass damper
The exoskeleton allowed us to design a safe, efficient, and flexible building, but not necessarily a comfortable one. Some people are surprised to learn that tall buildings are actually designed to sway to a certain extent — usually several feet — much as a tree gently sways in the wind. But when buildings sway too much, occupants report feelings of airsickness. The building won’t topple over, but people won’t be comfortable staying inside, especially on the higher floors.
Even with the exoskeleton, PMX was particularly prone to swaying due to its lightness. To reduce movement, we explored three different solutions: The first was to add significant amounts of concrete throughout the structure, which seemed like a poor option after all our efforts to keep concrete out of the core. The second was to make the timber pieces thicker, which would add cost and take up significant interior space throughout the building. The third was to deploy an engineering innovation called a “tuned mass damper.”
A tuned mass damper is typically a heavy piece of steel, connected by springs to the structure of the building at the penthouse level. It helps absorb shock, moving back and forth and acting as a countervailing force to wind or earthquakes. Where Khan’s exoskeleton was a favorite of tall building design in the 20th century, tuned mass dampers have become a favorite of structural engineers working on the super-tall buildings of the 21st century. Usually they are hidden with a building’s mechanical equipment, though occasionally they are integrated into the building’s architecture, like the golden pendulum at the top of Taipei 101.
For PMX, using a tuned mass damper became an obvious choice: without one, we would have had to more than double the amount of timber material used, in a bid to make PMX nearly as heavy as a traditional concrete building. But by using a 70-ton steel tuned mass damper — the equivalent of nine African elephants — we could reduce lateral sway to a level where people would feel comfortable inside the building.
This targeted chunk of steel plays an essential role in making PMX comfortable for tenants, while remaining less carbon-intensive than using concrete throughout the building.
Curious for more? Continue to part two of this series: How to design a timber building that’s easy to make in a factory.
If you’re a developer or timber professional who wants to work together on advancing tall timber building design, contact Sidewalk’s buildings team at firstname.lastname@example.org.