final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 110
Summary of reports of the failure of falsework for the viaduct
over the river Loddon, Berkshire 1972
General
On 24 October 1972 the temporary structure supporting a road bridge under
construction over the river Loddon at Woodley, near Reading, Berkshire,
collapsed killing 3 men and injuring 10 others.
The investigation was particularly difficult because a good deal of the falsework
was buried in the river bed and under reinforced concrete which had fallen on
top of it and set hard.
A concrete viaduct of the post-tensioned type and continuous over 13 spans
formed part of the A329 Relief Trunk Road which is a link road to the M4
motorway.
The viaduct had two separate carriageways each 53 ft 8 in wide with a gap of
2 ft 10 in between each carriageway. The north carriageway span which collapsed was being constructed over the river Loddon. At that place and time
the river was approximately 90 ft wide 4nd 2-3 ft deep. The distance between
and normal to the piers was 93 ft and each pier was 35 ft 6 in wide at approximately 33 ft above the water level. The deck of the viaduct was to be formed
by in-situ concrete 4 ft deep over the 31 ft 6 in wide main central 'spine' section
and of a mean depth of 1 ft at either side which cantilevered from the spine.
The deck was skewed longitudinally in relation to the piers at an angle of
approximately 37 degrees and had a 1 in 29·25 crossfall; sketches on pages 112
and 113 show the arrangement.
Falsework
The falsework over the river consisted of fabricated steel lattice trusses 98 ft
4 in in length, spaced at 1 ft 8 in centres under the spine section and at 4 ft
centres under the cantilevered sections. Each truss consisted of a top and
bottom boom of 6 in x 6 in broad flange beam section with tubular diagonal
web members mainly of 3½ in, 3 in and 1 ¼ in diameters. The top booms were
braced together by bolted 2½ in x 2 in x ¾ in mild steel angles in a horizontal
plane. Other bracing in the form of steel scaffold tubes and fittings, was provided in a vertical plane, laterally across the trusses at approximately 20 ft
intervals longitudinally. The bottom booms were also connected laterally by
other scaffold tubes. Each truss was made up of four intermediate sections
14 ft 9 in long x 7 ft deep and two 19 ft 8 in long end sections connected by
means of two 1 i in diameter high tensile bolts in each of the top and bottom
boom joints. Each end section had a short vertical member approximately
13 in long connected to the top flange, attached to which were rounded end
plates to form rocker-type bearings. The connections between the sections
in the bottom boom could be adjusted to provide the requisite camber for the
trusses.
The trusses were supported on a complex arrangement of beams and prefabricated trestles, the main component of which was a system of tubular steel
trestles. These trestles were approximately 24 ft high and were arranged in two
groups of four, one on each side of the river adjacent to its concrete pier. The
total width of each group of trestles was approximately 62 ft overall.
The base of the trestles sat on parallel 33 in x 11 ½ in steel beams approximately
80 ft in length. These in turn rested on short 10 in x 10 in x 12 in x 12 in uni111
