Tie bars aren’t just a pretty interface. Here’s how to help them withstand earthquakes in Charleston

Some brick buildings survived the devastating 1886 Charleston earthquake. Their secret was an ingenious structural element borrowed from Greek and Roman construction.

By Meghomita Das, colleague of Palomar (@meghomita)

Quote: Das, CE, 2022 Tie bars aren’t just a pretty front. Here’s How They Help Resist Earthquakes in Charleston, Temblor, http://doi.org/10.32858/temblor.275

The tie rods are secured by a tie plate. Credit: lcm1863, via Flickr, CC BY-SA2.0

On the night of August 31, 1886, the residents of Charleston, South Carolina, were surprised by a powerful earthquake that would go down in history as one of the most powerful that shook the East Coast of the United States. Sixty people were killed, and the earthquake left a trail of devastation across the city, totaling $5 million in economic losses ($165 million in 2021).

Several factors made the event significant. Not only was it one of the largest earthquakes to hit the East Coast in history, but the proximity of the epicenter, the high population density of the city at the time, the presence of loose soil and the use of bricks over timber in buildings all contributed to the widespread damage.

The effect could have been much worse, had it not been for an ingenious centuries-old engineering solution used to cement buildings in the hurricane-prone south, preventing them from collapsing and saving many lives.

There is no history for much history

Historical records indicate that the Charleston earthquake of 1886 was of 6.9-7.3 on the Richter scale, which at the time was unusual for a region that did not have records of such a large earthquake. Scientists have since discovered evidence of prehistoric earthquakes that occurred near Charleston in the last 5,000 years. The epicenter of the 1886 earthquake is believed to have been in Charleston itself. The shock was so severe that it was felt along the east coast of New York City, as far east as Bermuda and as far west as Louisiana. Since then, several earthquakes of varying intensity have occurred in the area, with reports of damage to chimneys and collapsed buildings.

In the fine floodplain soils of downtown Charleston, strong ground shaking has led to soil liquefaction and sand boiling, phenomena in which waterlogged soil or sand behaves like clay. Many historic buildings and townhouses collapsed when the ground below became too soft to bear its weight.

About 400 aftershocks followed the quake over the next 30 years, according to the US Geological Survey (USGS). More earthquakes have occurred since then. On average, 10-20 are logged each year, with up to five of them large enough to be felt. The largest earthquake of 1913 of magnitude 4.8 was in Union County.

During the first half of 2022, a swarm of earthquakes rocked Elgin, South Carolina — about 135 miles from Charleston — with tremors ranging in magnitude from 1.3 to 3.5. The USGS reports that these new earthquakes have nothing to do with the Charleston-area seismic zone.

These swarms may be aftershocks of an undocumented prehistoric earthquake that occurred in the area near Elgin, says John Ebel, a seismologist at Boston College. Likewise, clusters of earthquakes we observe in various parts of the central and eastern United States could be aftershocks from prehistoric earthquakes that would have occurred within the past 1,000-2,000 years, he says.

Explain the mysterious earthquakes

Scientists have classified the 1886 earthquake and all subsequent events in this region as internal earthquakes – tremors that strike far from the edge of the tectonic plate. Similar earthquakes of varying degrees are felt every year along the east coast and into Quebec, Canada. Despite the number of earthquakes, the forces driving the slip in this region are unclear. “There is no direct answer to the underlying causes of these earthquakes,” says Thomas Pratt, a USGS geophysical researcher. “It’s kind of a mystery, but we have some competing hypotheses about what might be the cause of some earthquakes,” adds Anjana Shah, a research geophysicist at the USGS.

Collapsed buildings after the 1886 earthquake in Charleston, Photo: Henry de Saussure Copeland via Flickr, CC BY-NC 2.0

One hypothesis behind seismic activity involves the release of energy along weak levels. Hundreds of millions of years ago — when dinosaurs and other reptiles dominated the land masses during the Mesozoic period — divisions began to dissect the eastern North American plate. These splits pushed the plate apart when the giant supercontinent, Pangea, separated to form smaller continents. Over millions of years, these faults formed basins filled with sediment. The faults that define the faults are now buried deep under mounds of sediment, but they still form potentially weak areas along which earthquakes can occur. As the North American plate moves away from Europe and Africa, these former fault zones serve as an ideal location for stress stresses that are released in the form of earthquakes, Ebel says.

The earthquakes strike within linear regions roughly parallel to the east coast. Other hypotheses proposed to explain seismic activity include crustal thinning that creates weak fault slip zones, water intrusion into basement rock that may cause some earthquakes, and the presence of other local stress-focused geologic features. However, both Shah and Pratt cautioned against identifying a unifying cause behind these earthquakes. We still don’t understand why it is likely to occur in one area versus another or what causes this localization, Shah says.

Tie it all together

When the rupture erupted in 1886 in downtown Charleston, it destroyed buildings and chimneys and caused roofs to collapse. Almost all of the city’s eight thousand buildings at the time had internal damage and windows were shattered. Brick buildings sustained the most damage due to their weight and unsupported masonry construction. It is baffling that some of the historic brick buildings have been left with only minor damage.

These buildings were safer because they were built with graceful panels and tie bars, according to an assessment by the Executive Relief Committee of architects and engineers, which formed after the earthquake. These low-tech structural elements dot the facade of many historic Charleston buildings and are used to reinforce the brick buildings against the devastating impact of hurricanes and storms.

Patrice panels are round or decorative cast iron panels that are attached to the exterior walls of masonry buildings. It serves as a washboard for wrought iron (currently steel) connecting rods that connect the building’s external facade to the building’s internal structure. The use of frill plates and iron tie rods comes from earlier Greek and Roman engineers, who used iron to support load-bearing masonry structures and hold arched piers in place.

Decorative tie plate fixes tie rod to brick building. Credit: Leo Reynolds, via Flicker, CC BY-NC-SA 2.0

Over time, this practice has been adopted and improved by various civilizations. Dutch cast iron is shaped into a variety of shapes to develop decorative motifs for albatrosses. When people migrated to the Americas, they brought their practice of using decorative trim panels and tie rods with them to enhance the exterior masonry facade of their new homes in its interior timber frame.

These panels and tie rods provide superior positions for attaching floors and ceilings to masonry walls in historic homes, says Craig Bennett, an experienced preservation engineer in South Carolina Historic Building. His company uses this mechanism to reinforce and strengthen many historic buildings in Charleston and throughout the Southeast. When a building shakes during an earthquake, unreinforced masonry walls tend to fall outward, destabilizing the roof and floors.

Other methods of reinforcement, such as epoxy fasteners, are not effective at securing construction, Bennett says. “The important thing about tie plate and tie rods is that they pull the building walls back over a larger area, creating less pressure unlike epoxy anchors that work on a limited area and create a higher concentration of pressure.”

These forms of reinforcement not only work with earthquakes but also for any type of natural disaster that can cause a building to shake or sway violently, such as hurricanes and tornadoes. However, installing them the wrong way will reduce their effectiveness, says Bennett. Ideally, the baseboard and rails are installed within the floor system with spacing between the rails depending on the location of the windows. Installing bars above or below the window is ineffective because the brick-free window areas do not exert the same amount of weight on the surrounding bricks and create areas of low pressure stress in the masonry walls.

Spencer means, via Flickr CC BY-SA 2.0 width = “1500 height = “1683″ class = “size-full wp-image-14513″ /> Patrice panels are spread on the building’s exterior wall. The tie rod screws into the frill plate and secures the masonry wall to the floor frame. Credit: Spencer Mean, via Flickr CC BY-SA 2.0

For the plank and tie rod mechanism to be effective, the rod must be in an area where the normal weight of the masonry can provide good pressure to the rods and hold them in place. Having this means that a greater amount of shear force—the force that pushes an object in opposite directions, such as during vibration—will be required to move this rod out of place.

In Charleston, some buildings already had such installations of panels and tie rods to modify and reinforce construction affected by disasters such as the hurricane of 1811 and the hurricane of 1885. Buildings with structures withstood the vibrations caused by the earthquake, while those without structures collapsed. After the earthquake, tie bars and frill plates were widely used to reinforce damaged masonry buildings – so much so that they became popular among the stately buildings of downtown Charleston. Thus, they got their nickname from “earthquake screws”.

A tie-rod installation in the Nathaniel Russell home after the 1886 earthquake. This floor plan shows orange lines indicating the tie-rod installation that secures masonry walls to the building’s inner frame. Credit: Historic Charleston Foundation, drawn by Glenn Keyes Architects, with modification by Jamie Lynn Wiedman. From Clemson University theses by Jamie Lynn Weidman, 2012

In 1886, Charleston was a booming city recovering and rebuilding itself after the Civil War. Its population was about nineteen thousand. Today, Charleston hosts one hundred and fifty thousand residents. If a similar event occurred in Charleston today, it would cause an estimated $20 billion in damage and nine hundred deaths. Whether or not the city will experience another major earthquake in the future, earthquake nails will ensure that Charleston’s historic buildings remain bound and tied together to stand the test of time.

Megumita Das is Palomar’s classmate at Temblor. She is a Ph.D. Candidate at McGill University in Montreal, Canada, where she studies ancient earthquake signals and slow slip events (www.meghomita.com). Palomar Holdings sponsors a science writing fellow to cover important earthquake news across the United States

references

Weidman, Jamie Lynn, “Links That Give: The Emergence of Iron-Tie Reinforcement in Load-Bearing Masonry Buildings in Charleston, South Carolina” (2012). All messages. 1340.

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