final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 104
I
Matters arising from the investigations
There was no evidence to suggest that movement of the piers and abutments
to the permanent works occurred.
Some areas of fill were more compact than others due to the movement of
heavy traffic along the line of the future carriageway.
Timber sleepers had been bedded using sledge hammers.
The day after the collapse it was noted that there was evidence of a very high
water table in the area of the bridge. Ponds could be seen either side of the span.
The immediate top surface of the sand close to the bridge was moist.
Rainfall records obtained for the period 1 November 1970 to 22 March 1971
showed a total of 52·6 mm of rain in December, 63·6 mm in January, 17·3 mm
in February and 54·2 mm up to 22 March. During the period 14-22 March a
total of 41 mm was recorded and the heaviest rainfall of the three month period
occurred on 17 March (14·6 mm). There was no rain on the day of the collapse.
On 22 March in the period of 0600 hours to 1500 hours the maximum wind
speed recorded as 21 knots with an average of 17 · 7 knots for the period.
It was suggested that there had been a spring in the area of the north'abutment
for some time.
There is evidence to show that after the rain on 17 March the ground was
waterlogged and erosion of the ground beneath some sleepers had occurred.
Concrete was placed under some sleepers on 19 March.
The nature of the filled ground and its load bearing capacity was a subject
for investigation. It was calculated that a symmetrically loaded sleeper would
impose a load of about 1 ·8 tons per sq ft on the ground, but sleepers which were
not symmetrically loaded were observed and calculations showed that in at
least one instance pressures of 3 tons per sq ft could be expected.
Sleepers in the region of the north pier were examined and it was obvious that
some were more deeply embedded in the sand than others. This suggested that
settlement had occurred but not on a large scale.
There was difficulty in establishing whether final working drawings of the
falsework provided at Birling Road ever existed. It was obvious that the structure
which remained standing, that is the side spans, was not in accordance with a
preliminary drawing produced. Enquiries suggested that the scaffolders used
this drawing to obtain broad guidance.
No evidence could be found of swivel couplers having been used. The fact that
the bridge was curved and skewed on plan meant that the angle between the
transverse and longitudinal horizontal members was not 90°. The significance
of this is that transverse bracing connected to the longitudinal members would
not necessarily be in the same place as the rows of standards across the width
of the bridge and could not be connected to them. An examination of the side
spans confirmed that bracing in those areas was somewhat haphazard and that
few connections were possible close to standards or at each lift because of the
geometry.
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