Does Mass Timber offer a new opportunity to create taller, longer-lasting housing, all while saving costs? To find out, SFHAC invited Dean Lewis of SFHAC-member DCI Engineers to speak to our Regulatory Committee on the topic.
First, what is is Mass Timber?
Mass Timber is a revitalized type of wood construction that is leading the way towards taller wood frame buildings. Different than the traditional stick frame, Mass Timber uses wooden panels that range from 4” to 20” in thickness and are 10’ wide and up to 60’ long and are made out of the traditional 2” x 4” & 2” x 6”. There are many types of Mass Timber, however only the main three which Dean sees being applied to the bay area were covered. They three include:
- Glue Laminated Timber (GLT);
- Nail Laminated Timber (NLT); and
- The new hot thing, Cross Laminated Timber (CLT).
CLT is exceptionally popular because the pieces of wood are laid across one another and laminated to create a strong block of wood. CLT panels have comparable strength to steel and concrete and weigh significantly less than both.
Some of the advantages of the new CLT are pushing the limits of what some cities and countries can do. The Computer Numerical Controlled, or CNC fabrication, is used to manufacture Mass Timber products within a 1/16” tolerance when being shipped on site. The level of prefabrication allows a smaller crew to add floors in a matter of days. The level of enhanced precision with Mass Timber also creates tighter assemblies and creates an airtight environment that is far more energy efficient. There have even been studies on Biophilia, which indicates that humans have a more positive emotion towards the exposed aesthetic of wood. There’s a reason we have tree huggers, not steal beam huggers.
Why hasn’t this caught on?
So, with all the positive aspects, what’s not to like? The largest concern, according to the team at DCI, is the building departments limitations due to fire safety. There is a difference between something being flammable and being fire resistant. Everyone remembers the Mission Bay fire only a couple of years ago. However, Mass Timber burns differently than stick frame construction. The outside of a the wood does burn, forming a layer of charring. This layer of charred wood lacks oxygen which is used as fuel for the fire and insulates the interior portion of wood. This internal portion of wood is structurally adequate and is protected by the charring layer.
Another concern at this point in time is price, CLT is still relatively expensive. This is partially driven by the fact that there are no manufactures in Bay Area, so it doesn’t seem that we’re quite ready for the revolution. On top of that, the concrete construction industry certainly has concerns about the loss of business – is Mass Timber an industry disrupter that some expect? The City of Seattle’s famed Bullitt Center was the first project to gain notoriety in the United States using Mass Timber. There is a project in Austria that breaks ground this year that will go up to 24 stories.
Go taller
So how far are we away from this in San Francisco? Presently, the building code allows five stories of wood frame construction. The building department has been open to allowing more stories of Mass Timber construction, however they still do not want anything over the 85 foot height limit. Unfortunately, the construction costs are higher for that eight story building compared to five floors of wood over three stories of concrete. So what’s the answer? DCI is proposing we go higher.
Ten stories of Mass Timber over one story of concrete can be economically more feasible than five stories of light frame construction over three stories of concrete.
This past year DCI Engineers teamed up with SFHAC-member Kwan Henmi Architecture & Planning to complete a proof of concept Mass Timber structure. This proof of concept was worked out in some interested sketches. One of the intriguing aspects of this type of construction is the aggregation method.
If they plan was to build to 28 stories over the next 100 years, Mass Timber allows the construction to be implemented over time. Six stories to start, another five in 10 years, and so on. As long as the foundation is built with the endgame in mind, they can keep going up with the height limits. Imagine the possibilities…
***Photos courtesy of Kwan Henmi Architecture, Woodworks, Rethink Wood and DCI Engineers.
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