National Information Service for Earthquake Engineering
University of California, Berkeley
By Charles James and Lydia Hernandez
National Information Service for Earthquake Engineering
Starting with a small tremor on October 3, the city of Helena, Montana suffered through a devastating series of several hundred earthquake shocks in the month of October, 1935 including three damaging earthquakes on October 12th, 18th, and the 31st. Previous to this cluster of tremors there had been little recorded seismic activity in the area of Helena. (1) The earthquakes disproved a then popular misconception that all seismic activity within the United States occurred solely in California and Alaska. Before October 1935, the spurious sense of immunity from natural disaster contributed to an atmosphere of uncontrolled construction in Helena. Earthquake hazard and earthquake- resistant design were disregarded. Older, antiquated construction in Helena behaved predictably during the tremors and offered few new lessons to structural engineers. Damage was largely a repetition of that from past shocks, both in this country and elsewhere; collapsed chimneys, fallen parapets, gables and end walls, shattered walls parallel to interior framing, with partial or total collapse of structures as the ultimate end. Most buildings with unreinforced masonry bearing walls were severely damaged within the month-long barrage of seismic activity. Likewise, industrial smoke stacks built almost entirely of brick fell down. However, public records indicate that prior to the earthquake swarm of 1935, impartial appraisal of many of these older buildings would have led to their condemnation. (2) Some newer buildings in Helena complied with local structural design requirements and adhered to common practice. Structural failure could not be placed on poor construction. The inadequacies of existing structural design requirements became painfully obvious after a large earthquake. The October 18th earthquake brought serious damage to City Hall as well as the area to the east of the mercantile district along Main Street. There, many chimneys were down, brick dwellings were seriously damaged or partly collapsed, brick veneer was thrown off, and many commercial, school, and public buildings were greatly affected, some destroyed. The worst wreckage occurred in structures on the softer alluvial soil toward the valley notably the new High School and the Bryant School. ( See accompanying images )
The last large shock of October 31st caused the collapse of parts of buildings which previously had been seriously affected but which remained standing, including the new High School and the Kessler Brewery. It also caused new damage in many structures not previously seriously affected. The failure of the high school is directly attributable to deficiencies in design. The skeleton frame was designed for vertical (not horizontal) loads and reinforced for such loads only. Walls could offer no stability to the frame. As a result the walls broke up and shattered, and the frame was cracked or ruptured in many places. Of scientific significance an early version of a strong ground motion seismograph was deployed and high quality records were obtained in part because of the duration of the earthquake swarm. In 1940, the Montana earthquake records were used by M.A. Biot to develop the first earthquake response spectrum curves. (3) In summing up his report to the Board of Fire Underwriters of the Pacific on the Helena series of earthquakes of October, 1935, the structural engineer H.M. Engle (4) wrote, "the earthquakes considered individually were not notably great in duration, destructive intensity, or area affected. ... the amount of damage will probably exceed $4,000,000, which is serious in a city the size of Helena. The people of Helena face a serious situation in the rehabilitation of their city. It is no disgrace to admit the inadequacy of past practice in design and construction, but it will be a disgrace if engineers, architects, and residents of the district generally fail to profit from their lesson. Rehabilitation to be intelligent, must be more than repair; it must include strengthening and additional safeguard against future shocks." (5)
1. Subsequently F.P.Ulrich published "Helena Earthquakes" in the Bulletin of the Seismological Society of America, vol.26, no.4, October, 1936, p.323-339 outlining a history of "known" information about earlier earthquakes in the region.
2. See the report "Unusual After-Shock at Helena Causes More Damage, Wrecking New School" (based largely on the work of H.M. Engle) provided to Engineering News Record, November 7, 1935, p.653.
3. M.A. Biot. "A Mechanical Analyzer for the Prediction of Earthquake Stresses." Bulletin of the Seismological Society of America, vol.31, no.2, April, 1941. p.151-171. NOTE: Response Spectrum can be defined as a plot of maximum acceleration, velocity or displacement amplitudes of a single-degree-of-freedom oscillator as the natural period of the oscillator varies across a range of periods (time) or frequencies.
4. Some of Harold Engle's field notes, records of damage, and copies of photographs exist in the Steinbrugge Collection at the EERC Library, University of California at Berkeley.
5. H.M. Engle. "The Montana Earthquakes of October, 1935: Structural Lessons. Bulletin of the Seismological Society of America, vol.26, no.2, April, 1936, p. 109.
Updated January 23, 1998.
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