EXPANSION JOINTS

During severe weather or seismic events, buildings are often subjected to loads that cause them to move in various ways. This movement is planned for in normal circumstances using building expansion joints. Since those joints are often vulnerable to the impacts of severe events, it is important they are properly addressed as part of a resilient design. Otherwise, the building itself can be directly impacted since the joints are meant to be the “relief valves” for forces allowing the building to appropriately move, absorb, or otherwise react and remain safe to use.

There are three fundamental types of forces that can act on buildings where expansion joints are needed. These include thermal expansion and contraction, seismic activity, and wind loads. Each of these are discussed further in the following sections with a focus on avoiding or minimizing damage to the building and/or harm to people.

Thermal Movement

Thermal movement in buildings is caused by daily environmental temperature changes in and around the structure. Thermal movement is primarily “one-directional” in nature and is the result of the expansion and contraction of a building as it is affected by heat, cold, and humidity levels. The amount of movement thermal joints must accommodate is defined by a structural engineer throughout the building but typically has a movement of plus or minus 10-25 percent of the nominal joint size. This aspect of resilience shouldn’t be ignored since it’s been noted that if a proper expansion joint isn’t present, there’s a good chance that Mother Nature will put one in for you – just not in ways that we are happy with.

Climate directly impacts thermal movement, so different locations will likely experience different amounts of thermal movement. A building near the equator with fairly constant year-round weather will likely experience much less variability in temperatures and humidity than, say, a building in New York City or Northern Germany. For example, if we look at a place like Ponce, Puerto Rico, which is near the equator, the temperature moves only a few degrees above or below the average of 77°F (25°C). By contrast, New York City has a 10-year average temperature swing from a low of 27°F (-3°C) to a high of 86°F (30°C) – almost 60 degrees F in difference on average. Of course, there can be days where these averages are exceeded, meaning that the total temperature swing can be well over 60 degrees throughout the year.

The significance of bigger temperature swings is that expansion joints need more area to move – the bigger the temperature swings, the bigger the need for the building to expand or contract accordingly. With average temperatures changing in many locations, that means that more expansion and contraction capacity may be needed, particularly in areas that are receiving new episodes of extended periods of extreme heat. A classic example of this type of problem is evidenced in pavement buckling in summer – the combination of ambient high heat and solar load causes the pavement to expand, and at weak points it buckles or literally “explodes.” Buildings may not experience as violent an event, but if expansion joints aren’t properly sized, designed, and installed, thermal expansion and contraction can cause buckling of surfaces such as roofs and interior floors. That can open the building to the elements and cause more weather-related damage.

The larger the structure (such as stadiums and airports) the more area that is exposed and vulnerable to thermal effects. This was a concern for architects and engineers designing a terminal expansion at the Manchester Airport in the United Kingdom. The $1.36 billion expansion of Terminal 2 made it 150 percent larger than the original to boost passenger capacity and provide gate and tarmac space for the largest jets now in operation. The new terminal also features 32 new shops and food and beverage outlets.

Architects were concerned about thermal movement creating a ⁷⁄₈-inch (20mm) difference in floor heights across the slabs or sections of the terminal. This much difference would present a significant trip hazard for pedestrians transiting the terminal. Even worse, it could potentially be present for weeks or even months during the hottest or coldest times of the year. The solution selected by the design team was to create a project-specific “glide plate” floor joint. This custom solution had to move in three directions while simultaneously addressing the rigorous heavy-duty loads encountered in this major transportation hub. By properly accounting for all this movement, the expansion joint system was able to prevent both damage to the building and harm to people.

Photo courtesy of Inpro; Source: Prairie State Wire

Hot pavement surfaces that are not protected with proper expansion joints can buckle and “explode,” as shown on the left. Buildings with properly installed control joints, as shown on the right, help prevent such damage.

Seismic Movement Section

Seismic activity is caused by shifting of the earth’s tectonic plates causing earthquakes, tremors, etc., along fault lines. Among the complicating factors for resilient building design, seismic movement may be horizontal, vertical, in shear, or a combination of all three. That means seismic expansion joints need to be able to accommodate all these different movements. For taller buildings, the higher floors may actually move more than the lower levels as the building is caused to sway from its base. All of this means that seismic expansion joints must have the capacity for movement associated with them of plus or minus 50-100 percent.

In the United States, California and the San Andreas Fault are often thought of as the prime location for seismic activity. In reality, there are numerous seismic zones all across the country that require seismic design attention. The reasons for that activity can vary based on location, but it remains a genuine concern and resiliency threat.

It’s important to recognize that minor earthquakes are far more prevalent than severe or very large quakes such as the magnitude 6.7 Northridge quake in California in 1994 or the 9.1 magnitude earthquake (and tsunami) off Sumatra in 2004. Instead, most earthquakes are well below these levels but strong enough to cause temporary or permanent damage. Therefore, seismic expansion joint systems are best selected as products that can “reset” themselves after a minor seismic event and allow cover panels to be repositioned easily.

Note, too, that some geographic locations have seen an increase in seismic activity that has been linked to oil and gas exploration and production. Hydraulic fracturing (or fracking) may be causing pocket collapses and slippage between layers of rock that trigger small seismic events. In other cases, it appears to be something different. For example, the U.S. Geological Survey reports that beginning in 2009, Oklahoma experienced a surge in seismic activity. This surge was so large that its rate of magnitude 3 and greater earthquakes exceeded the rate in California from 2014 through 2017. While these earthquakes have been induced by oil and gas related processes, few of these earthquakes were induced by fracking. Rather, most earthquakes in Oklahoma were determined to be caused by the industrial practice known as "wastewater disposal". This is a process in which fluid waste from oil and gas production is injected deep underground far below ground water or drinking water aquifers. In Oklahoma over 90 percent of the wastewater that is injected is a byproduct of oil extraction process and not waste frack fluid.

Images courtesy of Inpro; Source: U.S. Geological Survey

The areas of the United States affected by seismic activity, and the risk of damage to buildings, are documented and available through the U.S. Geological Survey.