final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 99
Hypotheses advanced
(a) The cross head timbers were substandard and out-of-square. Although
tests showed that these were weak it was eventually accepted that they
were strong enough for their purpose.
(b) It was suggested that due to the camber in the bridge deck, the fork heads
were extended too far.
(c) Inadequate cross bracing of the centre tower scaffolding.
(d) A settlement in the centre tower ground bearers.
(e) A substantial section of the report is devoted to a suggestion that due to
the large overlap of the steel joists, the centre tower became eccentrically
loaded at its full design load. The sketch on page 99 explains this and
suggests that due to the uplift of the over-lapping lengths of joists, these
joists - and consequently the centre tower - supported more plastic
concrete than was allowed for in the design.
Conclusion
The engineers involved in the investigation suggested that the design and
erection of the falsework was adequate and that the engineers carried out all
reasonable supervision.
The foregoing points in the hypotheses were the only factors discovered that
could have initiated the collapse. The investigators could not be more precise
than to suggest that either one of these points or a combination of several
caused the failure.
Summary of the report of the Supervising Coroner of Ontario
on the failure of falsework at the Heron Road Bridge 1966
General description of the project
The bridge was to span the Rideau River, Colonel By Drive and the Rideau
Canal as part of the extension of Heron Road.
During summer flows the river is contained within a shallow stream bed 50 t
100 feet wide. A flood plain, 200 to 300 feet wide, forms the western bank
the river. The plain is exposed except during periods of flood. Immediateh
south of the bridge, the surface till of the flood plain is one or two feet thick.
but the bedrock surface drops rapidly in a northerly direction, resulting in ti
thicknesses of up to 20 feet in the vicinity of the north side of the bridge. ·
The proposed design required the construction of two separate 3-lane concret
bridges of seven spans each. Each structure was 877 feet 6 inches in length fro
centre to centre of abutment bearings and comprised five intermediate span
of 147 feet 2 inches plus two end spans, one of 52 feet 6 inches at the we
abutment and one of 112 feet 9 inches at the east abutment. The general arrangement of the bridge is shown opposite.
· The abutments and piers Nl and Sl, N2 and S2 and N3 and S3 were support
on steel H-piles driven to rock. Piers N4 and S4 bearing directly on bedroc
whereas piers N5 and S5 and N6 and S6 were supported on spread footing
founded in dense granular material in the bed of the Rideau River.
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