final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 106
An absence of wedges in the forkheads was noted despite the obvious crossfall.
Concrete consolidation was by the use of six shutter vibrators in addition to
poker vibrators. Three men did the job and at the time of the collapse were in
the centre of the span and under the middle of the deck moving the vibrators.
The vibrators were clamped to the 6 in x 3 in timbers under the soffit.
There is evidence of movement of the shutters ( or the falsework) on two occasions
during the pour. Witnesses reported a slight movement of the deck at about
5.0 a.m. This was not reported to any of the engineers present.
About an hour before the collapse it was noticed that small pavement shutters
on the east side of the bridge were beginning to lean over, the bottom spreading
towards the centre of the bridge. This was investigated and apparently rectified
by the carpenters.
Summary of report of the collapse of falsework to the
Arroyo Seco Bridge, California 1972
Description of bridge and falseworks
The bridge is located in the City of Pasadena, California and carries the highway
known as Route 210.
The Arroyo Seco bridge was designed as two parallel bridge structures separated
by an expansion joint. The bridges are three spans in· length and prestressed by
a reinforcement system of tensioned steel strands embedded in the concrete.
(A sketch is given on pages 108 and 109).
The bridge superstructure was a box girder constructed in two stages. The
bottom concrete slab and the vertical stems were constructed in the first phase,
then the side forms were removed from the sterns, and forms were constructed
across the tops of the stems forming the hollow cells. In the second stage the
concrete deck slab was placed across the cells and made integral with the stems.
This sealed off the cells and the buried top slab forms remain in place for the
life of the bridge.
The bottom surface of the Arroyo Seco bridge was arched. Because of this, the
contractor constructed the falsework of two structural forms. The lower portion
consisted of heavy duty steel towers supporting steel cap beams and steel
stringer girders. On this a planked working platform was laid. The falsework
above the platform was constructed of timber posts and beams. (See page 109).
The collapse
The collapse occurred at about 13.30 on 17 October 1972. The first phase of
the concreting was nearly completed on the south structure and just starting
on the north structure. Falsework for both structures was in place. On the day
of the failure concrete was being placed in the bottom slab and stems of the
centre portion of the bridge and the day's operation was about 80 % complete.
A conveyor belt system was in use to transport concrete from transit-mixers
to the required position (three witnesses stated "that the pour was being made
faster than usual"). Concrete was placed in the stem forms in 3 lifts, each lift
progressing from south to north and then vibrated with hand held vibrators.
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