Resilient Design Considerations

When selecting a structural framing system and material, there are many factors that must be considered.

Building Code Requirements

The resilience of a building’s structural framing system is a function of both the material and the design of that system. Structural framing systems that meet the requirements of the building code can be built using steel, concrete, or wood. The purpose of building code provisions is to provide safety for occupants in the event of an extreme event. To ensure the best possible outcomes, all buildings are subject to building codes, such as the International Building Code (IBC).

In general, building codes do not directly address the resilience of the building by referencing either a subjective evaluation of the four Rs or an objective evaluation of the time and cost of repair and recovery. Section 1604 of IBC states that building structures and parts of the building must have the following considerations built into the design: strength, load and resistance factor, allowable stress, and empirical design or construction methods as permitted by the material.

However, IBC section 1604.5 begins to address the resiliency of buildings by including enhanced design requirements for high-rise buildings in risk categories III and IV. In those cases, structural integrity is evaluated independently. This means deformations in the material are allowed as long as failure does not occur. The goal is to allow for the redistribution of loads in the event of damage. This is possible to accomplish using structural steel, concrete, or wood. However, the question isn’t whether it’s possible to accomplish this goal; the question is, what is the most efficient way to meet this goal by maximizing design properties of the material specified in the design, getting the best return on investment from the system, achieving redundancy, and creating a system that is easy to repair after an extreme event?

BUNKER APPROACH VS. SYSTEMS APPROACH

Photo by James Steinkamp, Steinkamp Photography—IDEAS2 Award submission

The STEM Wing of Mundelein High School in Mundelein, Illinois.

When designing a structural framing system, there are two different philosophies on how to increase the structure’s ability to handle extreme events. They are commonly referred to as a “bunker” philosophy and a “systems” philosophy. The bunker philosophy attempts to handle an elevated level of potential loads by increasing the mass of the structure (i.e., build the bunker with thicker walls to handle larger forces applied to it). The increase in mass requires an increase in materials and an increase in cost. The systems philosophy maximizes redundant load paths by using a material’s natural ductility and reserve strength. During the design process, the systems approach can integrate serviceability considerations. This allows for an emphasis on modularity and rapid repair.

A building designed following the bunker philosophy is not ideal because it is incredibly difficult to repair. By contrast, the systems approach provides a technical solution that addresses the challenges of resilient design. A systems solution would provide multiple options for lateral load resistance by utilizing a highly ductile environment that allows adequate member deformation while still keeping access to critical services intact and operational. The design of a system with special connections and buckling-restrained braces as structural fuses allow a structure to withstand an extreme event, such as an earthquake or an event resulting from high winds or a blast. If damage occurs to the structural system, these fuses can be efficiently removed and replaced, returning the structure to full functionality in a short period of time without major structure demolition or extensive retrofit.

In short, the bunker philosophy is a material-intensive solution and is not ideal for most applications, while the systems philosophy provides a technical solution and is much more practical.