final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 105
The drawing previously referred to showed all bracings on the line of the
standards. The drawing also specified the use of chairs with right angle couplers.
No chairs were found on site.
Investigations showed that the bracing system was not uniform throughout the
span and that connections were made at levels which meant that some bracings
were unrestrained for relatively long distances.
From examination of the wreckage of the main span it was clear that the
majority of standards had all failed in one direction and that the mode of
failure was in a single curve from top to bottom. This suggested that the actual
effective length of the standards in the collapse condition was not related to
the distance between intermediate horizontal members. There was very little
evidence of contraflexure between these members, i.e. between nodes.
Calculations had not been obtained for the falsework prior to the collapse.
The preliminary drawing gave an indication of design loads but a copy of the
original calculations was not available.
Calculations to BS 449, assuming standards to be loaded concentrically and
with an effective height between restraints of six feet, showed that there was a
slight overstress but not sufficient to cause failure.
With the same restraint conditions but with a simple ecce ntricity applied at
the forkhead and the resulting moment transferred into the vertical standard
and the horizontal members according to their relative stiffnesses, a factor for
combined bending and axial stress of 1 ยท24 was obtained. (The appropriate
factor recommended in BS 449 is unity).
The condition of an effective length based on the total height of the standard
acting in single curvature and concentrically loaded, showed that the standard
would fail. Similarly the assumption of full fixity at top and bottom again
showed that the standard would fail.
Enquiries established that the falsework had been inspected in the week prior
to the pour. As a result some additional bracings and standards were added.
A pile of unused bracing units were discovered in the wreckage. These were
surplus to requirements. It was observed that in places the erection of the
units was not in strict accordance with recommendations in respect of bracing
and that in order to make up differences in height across the width of the
bridge (due to crossfall), ordinary mild steel scaffold tubes had been coupled
(with single couplers) to the tops of the towers.
An attempt had been made to connect together adjacent towers but it was
difficult to establish the extent of this tying.
Some additional falsework supports below the cantilever at the side of the
bridge had been fixed with the prime purpose of varying working platforms. It
was connected to the rest of the falsework. Additional standards in the side span
at the north end were added again connecting to the falsework. Scaffolds were
also provided at the extreme ends of the bridge.
Calculations on the timber members comprising the formwork showed them to
be stressed between 500 and 700 lb per sq in.
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