final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 96
Summary of a report by the Main Roads Department of
Western Australia on the collapse of falsework to the
Welshpool Road Overpass 1966
Description of structure
The bridge was a three span prestressed concrete bridge and was designed by
the Main Roads Department of Western Australia. Each end span was 36 ft
in length with the central 54 ft span clearing two sets of railway tracks. The
bridge was 38 ft in width overall and provided a final clearance of 20 ft between
the rail tracks and the underside of the bridge deck. The four piers of the
bridge each comprised three precast columns resting on a concrete footing
supported by piles (fig. overleaf).
Falsework
The falsework for the erection of the deck slab consisted of scaffolding with
timber cross beams immediately under the deck formwork on the two end
spans. Somewhat similar scaffolding was used under the central span with
gaps being provided to allow track laying trains to pass whilst the structure was
being erected. A central scaffolding tower was provided between the two sets
of tracks. Steel RSJ beams were provided to span the tracks and to support the
formwork during construction.
Test loading
Following the erection of the original scaffolding, officers of the Main Roads
Department raised doubts as to the adequacy of the scaffolding in relation to
the ground bearers, which were 12 in x 2 in karri planks. Taking into account
the then current ground bearing conditions, it was considered that these ground
bearers were inadequate in strength. A 30 ton test load was used, being equivalent to the load that a section of the scaffolding would have to carry whilst
the concrete deck was being constructed. This was applied to one end of the
central supporting scaffolding located between the sets of tracks. The scaffolding
for this load just met the limit of three-eights inch deflection in the deck formwork which had been established for the purposes of the test.
First fallure
Following the test loading, concreting of the deck slab commenced on the
eastern end. After completing the end span and a portion of the central span,
the scaffolding supporting the eastern end of the central span subsided about
2 ft. This subsidence was subsequently shown to be caused by the splitting of
the 12 in x 2 in karri planks referred to above. The scaffolding tubes used in
the falsework were placed directly on to the planks without bearing plates.
Following the first failure the contractor strengthened the scaffolding. The
Departmental site engineers, whilst being unhappy with the original falsework,
were completely satisfied with the strengthened falsework.
Second failure
The second failure occurred on 8 August 1966 during concreting of the deck
slab, the actual failure occurring when concrete had been placed over nearly
the whole length of the central span. At the time workmen were engaged in
ยท placing the concrete and a number were injured as a result of the collapse.
(It should be noted that it was the scaffolding which was test loaded that
collapsed in the second failure).
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