Foundation Design for Mass Timber Buildings: Can They be Smaller?

Take for example a six-inch thick concrete slab. We’re talking about a mass just of that slab of about 75 pounds per square foot. A similar thickness of a mass timber floor slab is going to come in at 15 to 20 pounds per square foot, so a significant reduction in mass. But it is important to point out that in a mass timber building, we are often adding a poured layer of concrete or gypsum-based poured topping layer on top of the mass timber floor panel. That is generally going to add anywhere from one to three inches of thickness to the floor assembly and also adding anywhere from 15 to 40 pounds per square foot to the floor assembly.

The second thing we’re going to examine is the lateral systems and how lateral systems have a direct impact on the foundation requirements, specifically in a mass timber structure. Two of the key factors that play into the determination of seismic forces and lateral system forces are the overall mass of the building, the weight of the building, as well as what’s called the seismic response coefficient. This is a value obtained from a document called ASCE 7.

In general, the higher the mass of the building, the higher the resulting seismic forces would be. On the opposite side of things, the higher the seismic response coefficient, the lower the seismic forces will be. So, if you’re able to have a building that has a very low mass and a very high seismic response coefficient, that’s really about the best you can do in terms of reducing your overall seismic forces.

Now, how does mass timber play out into all of this? Well, we do know, as we said earlier, that mass timber as a building system does generally have a lower mass or a lower weight than other building materials or building systems. Therefore, it does have the potential to reduce seismic forces.

On the other hand, if we’re using something like a mass timber shear wall system, as you can see here in this image, usually what we’re looking at is an applied force at the top of the mass timber shear wall, which is coming in from the diaphragm. That is creating this overturning resulting force that you’re seeing, which is causing an uplift or a tension at one end of the mass timber shear wall and a compression or a downward force at the other end of the mass timber shear wall.

One thing we can look at here is that on the uplift side of things, the mass or the weight of that wall or loads tributary to that wall can actually counteract this uplift force. So what that’s doing is actually reducing the foundation requirements that are there to help counteract this overturning force.

Now, oftentimes, we don’t have enough mass in that dead weight of the structure of the wall to resist the overturning force. So, we do still need to size the foundation in such a way that it can resist these overturning forces. It’s not just the mass of the foundation; it’s also any soil that can be engaged by that foundation system to counteract that overturning force.

The third thing that is going to impact the foundation requirements and foundation sizes in a mass timber structure is the existing soil conditions. Now, this is something that is specific to each individual project and site, so it’s hard to make across the board, broad stroke discussions about these impacts on foundation requirements for mass timber structures.

Specifically, what we’re talking about here are the existing soil conditions and their capabilities of resisting compression loads. This is often referred to as the allowable soil bearing pressure. It’s usually stated in pounds per square foot, you know, something like, say, a thousand pounds per square foot or five thousand pounds per square foot.

Now, generally, we’re going to see a bigger benefit of a lightweight structural system when we’re on the low end of the spectrum in terms of existing conditions with low allowable soil bearing pressures. If the weight of the structure is lower, therefore we don’t need as large of an area of foundation to spread out that load over the existing soils.

If their capacity to resist loads is low, we can see some benefit by using a low weight structure such as mass timber on a site that has poor soil conditions. If you’re at the other end of the spectrum where the site has good or high allowable bearing capacity for the soil, you aren’t going to see as much benefit.

The reason I say that is because oftentimes, once you start getting up into high soil bearing pressure capabilities, oftentimes what you’ll see start to control the foundation sizes isn’t so much what area is required to resist the loads, but it might actually just be minimum foundation element sizes that could be dictated by code or dictated by in-house office best practices.

One other potential benefit of using mass timber in terms of the existing soil conditions are sites in which we’re not just going to settle for what those conditions are. We’re actually going to do some soil remediation work. Now, this could be installing rammed aggregate piers. It could be installing piles, something else to either fully take the foundation loads or reinforce the existing soil that is there.

Of course, using a lower weight structure such as a mass timber system can have the benefit of either reducing the requirements, you know, the number or the depth of the installation of these piers or piles. And of course, that is going to result in some cost and schedule savings.

So, even on sites that were doing soil remediation, using mass timber could result in less requirement for soil remediation or potentially no soil remediation requirements.

We’ve seen this play out on projects such as the Forte Tower in Australia. They had a site that had very poor existing soil conditions, and they did a study. They found that they would be able to do a four-story tall concrete building, but this structure actually went forward as a ten stories tall CLT (Cross-Laminated Timber) structure because of the lighter weight of the mass timber system.