When residents of West Texas awakened on April 17, 2013, they had no idea of what was to come. At the close of a run-of-the-mill Wednesday, the town was rocked by the equivalent of a 2.1 magnitude earthquake, not caused by faults in the Earth but an explosion.
That evening, the town's major employer and supplier of chemicals to farmers was the source of a massive ammonium nitrate explosion, killing 15, including 12 first responders, and injuring more than 300. More than 150 residences, schools, and care facilities were damaged or destroyed. These are significant numbers for any town, but when a community covers only 1.6 square miles, they're devastating.
At its epicenter, the blast left a crater 93 feet wide by 13 feet deep. The plant itself was decimated. The closest house, 300 feet from the blast, was obliterated.
Forensic engineer Erik Nelson (B.S. 81, M.S. 83, Ph.D. 86) has his own perspective on this event. His firm, Nelson Forensics, arrived within a week of the explosion to evaluate blast distress. Regulars on the scene of natural disasters and high-profile engineering failures, the firm’s engineers provide forensics and consulting services to the legal, insurance and private sectors.
“At first, access in West was limited while the ATF and other agencies were determining the cause of the plant’s fire and if there was criminal involvement,” he says. “With a special permit, we were allowed in the first two zones and within the second week, we were allowed in the ‘ground zero’ zone.”
Once Nelson and the team were granted access to damaged structures, they evaluated residences, commercial buildings, and a hospital authority over several months.
They learned that blast distress takes on some unique nuances as it radiates out from the fertilizer plant. To better understand these, enter the concept of "blast loading."
Overview of Blast Loading
An explosion (blast) instantaneously creates a shock wave of highly compressed air, which travels radially outward from the explosion origin in an effort to reach equilibrium with the surrounding air. Unique characteristics of blast loading include:
- Quickness – Materials, like wood, can resist higher instantaneous loads than static loads; the duration of loading for most explosions is less than a second.
- Decay – Blast pressures decay very rapidly with time and distance from the origin.
- Bounceback - The initial positive phase is followed by a negative phase. The negative phase is the result of a vacuum being created near the explosion origin due to the displaced air from the shockwave.
- Equal Opportunity - All faces of a structure can be positively loaded or "squeezed" almost simultaneously.
What are the Mechanisms of Blast Loading?
- Overpressure—Similar to submerging the structure in water, overpressure can "squeeze" all surfaces positively; however, due to the rapid decay of the blast wave, the overpressure at the blast-facing side may be much larger than the overpressure on the opposite side.
- Reflected Pressures—Loosely comparable to loading from ocean waves, the highest contribution of blast loads on a structure usually occurs from reflected pressures.
- Dynamic or "Blast Wind" Pressure – Resulting from air movement as the blast wave radiates outward, blast wind usually makes a minor contribution to blast loading. Roof shingles, which are relatively weak cladding items, are rarely lost due to blast wind.
- Ground Motions – Similar to seismic "shaking," ground motions from blasts usually have a minor impact on structures relative to the other loading mechanisms but can be dominant further out as the overpressure decays rapidly.
The evaluation of distress after an explosion like that in the West takes on unique characteristics related to blast loading, which include distress patterns, unlike most other catastrophic events. Understanding these nuances helps forensic engineers distinguish between distress related to the blast and damage due to long-term deterioration, weather events, or other perils.
The damage assessments made by Nelson and his firm will be used by property owners so that they can rebuild or repair as need be.
“Although we can never replace the dedication, courage, and lives of the first responders, we take great pride in being able to do our part in the rebuilding process,” says Nelson. “And through education of our findings, we can further contribute to the design and construction of future buildings by learning important lessons from these catastrophic events.”
Contributors: Erik Nelson (BS 81, MS 83, PhD 86) and Jeffrey L. Hull (BS 2009, MS 2012)
