final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 92
Appendix I
Case Studies
Summary of the report of the British Columbia Royal
Commission, Second Narrows Bridge Inquiry 1958
Brief history and description
The bridge was being constructed in the Greater Vancouver area between the
north and south shores of Burrard Inlet.
The bridge consisted of four northern approach spans leading from a viaduct, a
main cantilever section with a central span of 1100 ft and two anchor spans of
465 ft. The spans were numbered from north to south, 1 to 7 inclusive, span
5 being the north span, span 6 the central span, and span 7 the south anchor
span. The bridge deck, about 80 ft in width, was designed to carry six lanes
of highway traffic and two sidewalks.
The piers of the sub-structure, which had been completed at the time of the
collapse were numbered consecutively from north to south, 1 to 17. Pier 14
supported the south end of span 4 and the north end of span 5. Pier 15 supported
the south end of span 5.
The erection scheme called for two temporary piers known as false bents,
numbered N4 and N5, between piers 14 and 15, to provide temporary support
for the cantilevering of span 5 from pier 14 towards pier 15.
Prior to the time of collapse, the four approach spans had been erected. Span 5
was in process of erection. This work had proceeded to a stage where the two
top chords, the cast bottom chord, the two diagonal members and the two
vertical members had been connected in place. At that time there were located
on the deck of span 5 No 1 Traveller or Crane (155 tons), a diesel locomotive
and two railway trucks (38 tons), the bottom west chord (52 tons), and miscellaneous erection equipment (85 tons), making a total superimposed load of
330 tons. A sketch is shown overleaf.
Preparations were being made for the crane to move the west bottom chord
upward and outward from the railway trucks, to clear the bridge deck, and then
downward to its position on the west side of the structure where it was to have
been connected up. At the actual moment of the collapse of span 5 none of
the equipment on the span was in active motion and the actual lifting of the
chord from the railway trucks had not commenced.
It was at this stage on the afternoon of 17th June, 1958 that without any
warning, the south end of span 5 collapsed into the water. This was followed
almost instantaneously by the collapse into the water of the south end of span 4.
Eighteen of the men working on the structure at the time lost their lives.
Some conclusions from the evidence
Collapse sequence
(1) The collapse of span 5 took place in three stages:
(a) a short initial dip or drop, variously estimated by eye-witnesses as from
from 1 to 6 ft
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