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On May 30, 1998, at 8:38pm (CDT), a violent tornado struck the town of Spencer, South Dakota, a small farm community approximately 72 km (45 miles) west of Sioux Falls, leaving 6 dead, more than 150 injured, and nearly 90 % of a total 195 structures in the six-by-seven blocks community destroyed. Following the passage of this tornado, NIST researchers visited Spencer and conducted aerial and ground surveys to document structural damage.
The Spencer tornado left a ground track about 34 km (21 miles) long and close to a mile wide at its broadest. Based on surveys at the sites of the most severe structural damage, the National Weather Service (NWS) assigned it an F4 rating. NIST researchers conducted detailed surveys of the damage sites with the intent of providing baseline damage data to corroborate the descriptive damage associated with an F4 tornado on the Fujita scale.
Within the town of Spencer, the observed damage ranged from total disintegration of structures in the direct path of the tornado, to light damage to structural envelopes of those in the outlying areas. The completely destroyed structures included one and two-story wood-frame and light gage steel-frame constructions. Among these were wood-frame constructions that were either poorly connected to or had no connection (relying on gravity alone) to the concrete or brick foundations (this is the case with most single family homes). However, structures that were totally destroyed also included those that were properly anchored to the concrete slab-on-grade foundations by ½-in (1.3 cm) anchor bolts with proper spacing as required by current building codes (post office, fire department, gas station, antique and other stores). The town's 36.6 m (120 ft) water tower collapsed, after being hit by a wind-borne automobile. Many heavy trucks, used in transporting grain to and from the town's grain processing and storage warehouses (some reportedly fully loaded with grain), were lifted and carried more than 30 m away by the wind. Trucks and houses exhibited widespread evidence of missile impacts from broken wooden power poles, tree branches, and other debris.
Post-disaster investigations provide valuable information on the responses of structures to extreme loads. Complete documentation of instances of successful or poor performance can yield valuable lessons which can be used to improve construction practices. The NIST research team was coordinated by Rainer Dombrowski of the Office of Meteorological Service, National Oceanic and Atmospheric Administration (NOAA) and Robert Dumont of the Office of the Federal Coordinator for Meteorology (NOAA), and assisted by Todd Heitkamp, D. Gregory Harmon and Brian E. Smith of NWS.
Toppled water tower, which apparently went down after its support was undercut by the sedan at center, and/or other substantial airborne missiles. (Photo credit: Roger Edwards, Storm Prediction Center.)