final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 119
Summary of the principal features from the study of a
selection of falsework collapses
Case 1
An in-situ concrete bridge of 65 ft span of flat slab construction was being
built for a by-pass road over a canal.
The falsework consisted of Hunnebeck trusses 8 ft deep and spaced at 4 ft
centres across the width of the bridge. The tresses were supported on timber
trestles approximately 7 ft deep and sat on the abutments of the bridge which
had already been cast. The timber formwork for the deck slab was suspended
f.rom the bottom tie members of the trusses. The trestles were made from baulks
of timber and were similar in form to vierendeel girders with some scaffold
tubes to maintain the rectilinear shape of the panels. The timber members
were nailed together. No bracing or shoring was provided to prevent sideways
movement of the trestles, their stability being dependent upon the width of the
timbers. There was no horizontal bracing between the top booms of the Hunnebeck trusses but some vertical bracing in the form of scaffold tubes was provided between the trusses at intervals along their lengths.
Collapse occurred when concreting was being carried out at one edge of the
bridge. Concrete had been placed at the centre of the span and work was
proceeding towards the abutments when one of the trestles collapsed sideways.
The cause of the collapse was considered to be the instability of the trestles
against horizontal forces arising from the concreting operations.
Case 2
A concrete building 100 ft O in long x 80 ft O in wide and two storeys high was
being constructed. The first floor, which was 15 ft above ground level, was
under construction.
The falsework consisted of adjustable steel props, on timber sole plates, with
flat heads to support the timber formwork bearers. The bay size of the floor
was 33 ft O in x 20 ft O in. In one bay the props had been arranged in pairs in
three lines parallel to the 33 ft O in length of the bay giving formwork spans
of 10 ft O in. The adjacent bay had two lines of props parallel to the 33 ft O in
length with a centre line at right angles to these, thereby giving formwork spans
of 17 ft 6 in.
The bay with 10 ft span bearers was poured successfully, but during the concreting of the adjacent bay the floor collapsed, the centre line of props failing
under load.
The cause of the collapse was considered to be overloading of the centre props.
These carried more load than those in the adjoining bay because of the different
spacing. No drawings had been provided for the scheme and a change in lay-out
had been made by the site staff without any consideration of the consequences.
In addition there were the following serious defects in construction:
(a) no bracing or ties were provided to the propping arrangements
(b) props were out of alignment and eccentrically loaded
(c) a mixed assortment of lengths of props had been used, and it was probable
that one prop had been placed on top of another in some instances
( d) pieces of reinforcing bars were used instead of proper pins in the props.
120
