There are generally three aspects to the performance of a structure. The first is structural. The structure needs to resist the gravitational, wind, and other loads to which it will be subject without failing. This is perhaps the easiest to understand. As Amy Klobuchar, the senior senator from Minnesota stated, bridges should not just fall down in the United States. But there’s more to performance than not collapsing. Serviceability and durability matter as well.
Being able to resist loads is necessary but not sufficient. The second aspect is serviceability. This refers to the performance of the structure while it is resisting the loads. A serviceable structure is one that does not excessively deflect, or damage things that are attached to it. For example, the floor of a building may bounce up and down while the occupants are walking. The structure may not collapse under the walking human beings. However, the vibration renders it unserviceable.
The third aspect of structural performance is durability. The structure must be able to resist the loads from the environment, including freezing and thawing, sulfate attack, chemical attack, internal degradation, and corrosion of the reinforcing steel. A structure that cannot withstand its environment will deteriorate to the point where it is either unsafe, unserviceable, or both. It’s incumbent on us as designers to ensure that we have characterized the service environment with the same diligence we would use for the loads.
Building codes for serviceability and durability
Herein lies the problem. Building codes clearly describe the load requirements. The actions of wind, rain, snow, gravity and seismic events are visible. Serviceability failures are apparent in broken windows, swaying structures or sticking doors. But durability problems can be harder to see—and may even act unseen until it is too late.
Our codes are not as advanced with regard to durability as they are for structural and serviceability requirements. ACI 318 has just started incorporating concrete exposure classes, for example. As we convert to more sustainable materials we will no doubt find new deterioration mechanisms by experience, much as we discovered alkali aggregate reaction and corrosion of reinforcing steel.
Examples of unseen deterioration include the collapse of the parking structure in Elliot Lake, Ontario, and the condominium collapse in Surfside, Florida. Good models for predicting the extent of damage are still in development, and durability problems do not allow some of the simplifying assumptions such as treating concrete as a continuum we use in structural and serviceability design.
Future developments of concretes that behave in a less brittle fashion, in structures that self repair and/or self report, and the continuous monitoring of structures with the internet of things will change design practice. Corrosion monitoring in particular is a field that needs much attention. Until then we have to depend on periodic inspections.
