Determining Structural Systems

  • 22 May, 2020

One of the most fundamental architectural design processes is to decide on the most appropriate structural system for a project. There are several factors involved and, let's face it, it's not always a clear-cut decision.

In fact, sometimes considerations compete with others making the weighing of priorities even more important. Is "first cost" the most important consideration for the project? How about life-cycle costs? Can those two be at odds with each other? The answer is often yes. Now about sustainability? Would a material's recycled content outweigh the other considerations like durability or aesthetics? Certainly, there is a lot to process in the early stages of architectural design, and there almost always is a bit of give and take (compromise).

In my career, the prominent factors in determining the most appropriate structural system include:

  1. The building code: Fire-resistive, non-combustible, heavy timber, ordinary/composite system construction, etc.
  2. Costs: Both first costs and life-cycle costs
  3. Fit with the architectural concept and design criteria
  4. Sustainability: Material content, resource extraction, contribution to carbon footprint, and thermal performance, etc.
This is not the complete list of considerations, but simply a very general summary of the ones that I have experienced the most often.

To better understand the decision process, here are a couple of examples of very different projects with widely different drivers for selecting the appropriate structural system:

Recently, I worked on a fire station project in a rural community of about 25,000 residents just a few miles outside of Seattle. Most of the buildings in the community were of a variety of materials, but wood is the most predominant. This community also had a general appreciation for what they called "the Northwest style," which is a term given to buildings with sloped roof forms and a fair amount of exposed wood structure. These drivers were generally compatible with several other considerations including lower first costs, sustainability (locally and sustainably resourced), and code requirements as the building footprint, overall size, and occupancy would allow for wood construction.

However, there were also other factors at odds with the considerations mentioned above such as life cycle costs and durability. These were in conflict as wood buildings can deteriorate faster over time than those of other materials and can require greater levels of maintenance. In this case, the Fire District established their priorities and opted for a wood structure with the acknowledgement that they may have a greater level of maintenance (or even a shorter lifespan for the building).

Another project with a very different focus was a high-tech, clean technology building that prioritized flexibility over all else. The client for this project often retools their manufacturing floor to adapt to changes in technology and to meet market demands for their products. In this case, the materials under consideration were mainly steel or concrete. Ultimately, concrete was preferred for its mass and resistance to vibration, but which concrete structural system (of the many possibilities) to use? It turns out that this client needed to have a floor slab that allowed for the flexibility to make penetrations over most of the floor area from underneath without diminishing the capacity of the slab. Very quickly, the choice narrowed to a two-way waffle slab to satisfy the criteria for penetrations. With the waffle slab system, the client could visibly see where they could make penetrations (from the floor below) without compromising the capacity of the slab. Penetrations could occur in the pan areas as the steel reinforcing was primarily located in the two-way beams and at the column capitals.

This decision to use a two-way waffle slab was not driven by costs or code requirements. In fact, the waffle slab was one of the most expensive first cost options available. But for this client, the flexibility meant everything, and totally outweighed any of the differences in cost.

With all the many factors to consider when determining the appropriate structural system, for the architect, it becomes a process of exploration of options balanced against the priorities of the project. The key is to navigate the design team and client to achieving the highest priorities first and establish a willingness to accept compromise.

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