Thoughts on Wood Construction
by Robert L. Johnson, PE, SE
A recent Columbus Dispatch article picked-up on the growing use of wood construction for mid-rise buildings that’s occurring in the Columbus area. The article is spot on as it is now commonplace for multifamily buildings to utilize the advantages of the newer codes that permit up to five stories on a two-story podium structure. In some cases, it’s possible to obtain more density on the same footprint by adding another level in the form of a mezzanine above, or by adding subterranean parking below.
One of the key differences in understanding these taller wood structures is that the Construction Type changes between four and five stories of wood framing. Four stories of framing falls under Type 5 Construction, whereas five stories falls under Type 3 Construction.
The primary structural difference is that Type 3 Construction is more stringent and requires Fire Retardant Treated (FRT) lumber in the exterior walls. Special attention to bearing on these exterior walls is certainly required and addressed in the code. Architects and plan reviewers now have a good understanding of best practices for these conditions.
Podiums can be constructed from a range of materials, including steel, concrete, precast, and sometimes even wood framing. There is always a “best” solution for every project and I have yet to see the same solution implemented on the countless projects we have completed over the years.
Sometimes the thinner, two-way flat plate post-tensioned concrete slab is a good solution that provides the inherent required fire rating, while maintaining the lowest possible floor-to-floor height needed to stay below a height limit. Often a precast solution is needed for speed of construction as is needed in winter conditions. Or, a cost-effective steel podium on a smaller footprint project is more appealing to Owners.
Regardless, of the solution, it is always beneficial to work through the crucial fire rating requirements, column layouts, and podium details early with your design team.
It will be interesting to see over the next decades how the code adapts to even taller wood structures. As cross-laminated timber (CLT) and other advanced engineered wood products become more readily available. As other countries are doing, it’s possible that in the Midwest markets, we’ll see wood framing used in high-rise buildings.
Will our code change to adapt these opportunities? What will be the next engineered wood product? How will we continue to increase building density with wood? All exciting questions that we will get to see unfold in our time.