final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 114
totalling 30 men were attending to the placing of the concrete in the formwork.
Concrete was first deposited near the centre of the span, then pouring proceeded outwards towards the piers. The rate of pour during the first hour was
84 cubic yards dropping to 53 cubic yards in the second hour, after which it
rose to 88 cubic yards an hour prior to the collapse.
Consolidation of the concrete was by means of 14 poker vibrators.
The collapse
At about 13.35 a downward movement occurred (one estimate being of
6 in) towards the east end of the span. A few seconds later the span collapsed
into the River Loddon. From an inspection of the damage it appeared that
the falsework deck fell in the line of the skew, the trusses overturning sideways.
The trestles at the east side of the river overturned and fell on top of the trusses,
whilst at the west side, only one of the trestles overturned, namely that at the
south end. The others at the west side remained vertical but in places had moved
laterally 12 in towards the adjacent concrete pier. All the grillages of steel
beams at the east side of the span were badly twisted or buckled, particularly
the 12 in x 6½ in x 31 lb universal beams which sat in the trestle forkheads.
Some of the grillage beams at the west end remained in position on the trestles
but were damaged.
Possible causes of the accident
Having eliminated the possibility of foundation movement or defective formwork as potential causes of the accident, attention was concentrated on the steel
falsework. The overall stability of the falsework was suspect in that the pinjointed type of framework was not firmly anchored. In other words, although
the uprights were standing freely on the foundation beams at their bases and
the trusses were merely resting on the bearer plates at the top, the uprights
were not tied back to or braced against the concrete piers alongside them.
In addition, the lateral stability of the connected trusses was doubtful. However,
there was insufficient evidence to suggest that these two factors substantially
reduced the stability of the structure.
It was noted that the system had been used without incident on the southern
carriageway span under a heavier load than that on the span which collapsed.
Thus weaknesses might have been introduced in moving the falsework from its
first position and re-arranging it under the northern carriageway.
During the striking of the north-east trestle under the southern carriageway
span two of the lower legs had been damaged. However, the actual loads in the
legs of this trestle were relatively small.
Although the moving of the falsework was reported as a relatively smooth
operation it had been necessary to weld side plates at the rocker bearings to
maintain them in position. There had also been difficulties in aligning the
trestles when the falsework was re-assembled. As a result of these alterations
the actual support conditions for the trusses on the grillage beam assemblies
bore little resemblance to those assumed in calculations, namely vertical loads
applied centrally on the bearings without horizontal forces. Other .effects which
had not been allowed for included frictional forces in the rocker bearings.
the inadequate fixing and the eccentricity of the bearing pads on the 10 in x
10 in x 49 lb header beams, and the poor seating of the grillage , beams in the
forkheads.
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