If I were to make a broad generalization for the way that we typically design the structural systems for residential buildings and office buildings here in Toronto it would be as follows: office buildings tend to have a big structural core with perimeter columns and residential buildings tend to have a smaller core accompanied by both columns and shear walls (long structural walls essentially). There are a myriad of other differences, but for the purposes of this post, I’m going to run with this broad classification.
When something is typically done a certain way it often means that it is generally what the market wants and it is a cost effective solution. In the case of office buildings, this sort of structural system is essential for maintaining open plans and future flexibility. You can’t have shear walls interrupting your floor plates. And because big office buildings also tend to have a lot of elevators, the structural core is usually what provides lateral stability to the building (or at least this is what the structural engineers tell me).
But this same imperative for open plans isn’t usually there for residential buildings. In this case, the unit demising is often fairly fixed and the individual resident/tenant spaces tend to be smaller than in office buildings, which makes frequent structural elements a lot more palatable. And since the elevator cores also tend to be smaller (fewer elevators), there is usually a need to introduce other structural elements that can provide the building with lateral stability. (Again, this is what the engineers tell me.) So enter all the shear walls.
But every now and then, somebody in Toronto will ask: Is this the right way to be building? Other cities don’t build their residential buildings with all of these shear walls and so should we really be limiting the future flexibility of our multi-family housing supply by constructing in this way? These are good questions. The short answer is that it tends to be easier/cheaper to build this way. Our market is used to it. And generally end-users are just fine with it.
However, this method of building isn’t necessarily a universal truth. The structural system for One Delisle, for example, is far closer to that of an office building than it is to that of a typical residential tower. Much of this was driven by the building’s architecture and its continually changing floor plates. I have also heard of instances where purpose-built rental developers are choosing to go column over shear wall so that there’s greater flexibility in the future. There’s certainly a case to be made for this.
As developers, it is impossible to know all there is to know about any one discipline. You need the right team in place for that. But we do have to look at the bigger picture, weigh all of the constraints, and then hopefully make a reasonably good decision. This is one example of that.
Image: Bay-Adelaide Centre North, Toronto
The other differences are the floor to floor heights, distance from perimeter to core, fire exiting/separations and HVAC distribution. Pre-designing a potential conversion of an office building to residential is do-able due to having columns and not shear walls, and it provides fabulous high rooms but this resilience comes at a greater cost. Changing use of an an apartment building to office has a whole host of challenges. The floor to floor heights don’t work as there is no height for HVAC, and the distance between and location of the fire exits can be problematic amongst other issues.
I can recall years ago being asked to create the most flexible residential solution which we did at some expense. Decades later it turns out nothing ever happened to warrant providing this flexibility in the first place, everything remained the same. The built-in flexibility was just to change from one residential layout to another not to switch uses altogether. Just how much flexibility are the rental developers you mention looking to provide?