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  • James Brokenshire

Histories Engineering Lessons

An engineering failure may be defined as an unforeseen, substandard, performance of an engineered element, ranging in catastrophic failure to minor functional shortcomings.

Failures are both our greatest enemy but also ironically, our foremost educator.

One of the most famous examples of engineering failures is Tower of Pisa and less known the Transcoma Grain Elevator.

The Tower of Pisa in Italy is about 60 tall from, 20 m in diameter and weighs approximately14,500 tons. It was constructed in three phases. Four floors were built over a 5 year period from 1173 to 1178. Following an almost 100 year hiatus, in the second phase of construction, three additional floors were constructed between 1272 and 1278. The third construction phase occurred more than 80 years later between 1360 and 1370 when the bell tower was added.

The tower’s foundation is inclined at almost 5.5 degrees to the south; the tower overhangs the ground about 6 m out of plumb. The value corresponding to the eccentricity on the loads on the foundation is 2.3 m.

Evidence indicates that the phased construction was mandated by the performance of the structure as it was being built.

Further, records indicate that during construction the tower appeared to move sufficiently so that the builders used obliquely cut stones in an effort to maintain the floor of each successive story approximately horizontal.

Construction of a million-bushel grain elevator began in 1911 at North Transcona), Manitoba, Canada.

The Transcoma elevator structure consisted of a reinforced concrete work-house and an adjoining bin-house. In plan, the work-house measured 21 by 29 The structure was 55 m high and was founded on a raft foundation 3.6 m below grade. The bin-house consisted of five rows of thirteen bins each 4.3 m diameter and 28 m high that rested on a concrete framework supported by a reinforced concrete raft.

The bin-house raft measured 23 m by 59 m and was also founded at a depth of 3.7 m below grade. Upon completion in September 1913 filling of the elevator commenced and the grain was distributed as evenly as practicable to the bins. On October 18, 1913, after 31,500 cu m (875,000 bushels) of wheat had been placed in the elevator settling was noted and increased uniformly to about 0.3 m per hour.

Tilting of the elevator then began to occur; it ceased 24 hours later when the inclination reached 26 degrees 53 minutes from vertical. The subsoil below the elevator’s foundation consisted of a uniform deposit of clay that resulted from the sedimentation in waters of glacial Lake Aggassiz.

The clay was a varved, slickensided, highly plastic material varying from about 9 to 15 m (30 to 50 ft) in depth. It overlaid glacial till over limestone bedrock. From the ground surface to a depth of about 9 m , the clay had a stiff consistency that gradually decreased with a decrease in depth. A water table was encountered at a depth of about 9 m.

In 1951, a comprehensive geotechnical investigation led to the conclusion that the elevator foundation failed due to a bearing failure in the underlying clay. Unfortunately, for the design engineers, the state-of-the-art in geotechnical engineering in 1911 had not reached the point that the ultimate bearing capacity could be computed. No borings were known to have been made for the design of the elevator. Lessons Learned The development of soil mechanics after the Transcona failure eventually provided a basis for computing the ultimate bearing capacity of soils. It was subsequently realized, therefore, that the Transcona failure served as a "full-scale" check of the validity of such computations. In hindsight, had the Transcona engineers had access to soil mechanics theory, the failure could have been averted.

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