final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 116
(f) The bearings of the 12 in x 6½ in x 31 lb universal beams in the forkheads on the trestles were not fiat as had been assumed in the design.
In many cases, deformation of the bases of the forkheads over the jacking
screws had resulted in near knife-edge supports, thus reducing the buckling and twisting strengths of the universal beams.
(g) The bearing pads on some of the 10 in x 10 in x 49 lb universal column
sections in the grillage assemblies were only clipped or tack-welded in
position. This was not good practice bearing in mind the horizontal
forces they had to sustain.
(h) Some of the bearing pads on the 10 in x 10 in x 49 lb universal column
sections were found to be eccentrically located by up to 1 ½ in. Such
eccentricity was not allowed for in the design and resulted in unintentional eccentric loadings.
(i) The main design assumption of pin joints at the bearings between the
trusses and the trestles was not realised due to the crudity of the rocker
arrangement. This relied on two rough curved surfaces, exposed to the
elements and not lubricated which did not provide the low resistance to
rotation assumed in the design.
(j) The diagonal bracing provided between the trusses was not sufficient to
ensure the high standard of stability necessary to resist the horizontal and
dynamic forces which may have occurred.
Additional comments
It was considered that the flexural shortening of the compression boom of the
trusses would have applied a horizontal force to the top of the trestles.
There was some concern over the variation in dimensions caused by inaccurate
rolling of some of the steel sections.
The verticality of the structure after the moving operation was questioned.
The effects of vibration from a nearby railway were considered but were not
thought to contribute to the failure.
Although German and French operatives were involved in the erection of the
falsework no language problem was evident.
Summary of the investigation of the collapse of the
Skyline Plaza, Fairfax County, Virginia 1973
Description of structure
The Skyline Centre Complex which was located near Baileys Crossroads,
Fairfax County, Virginia was a development planned to contain eight apartment buildings, six office buildings, a hotel and a shopping centre. The building
which collapsed was designated A-4 and was of reinforced concrete flat plate
construction supported on a 4 ft O in thick foundation mat. The completed
structure was to have had 26 storeys of apartments plus a penthouse and a four
storey basement. The typical storey height above ground was 9 ft O in from
top of slab and the floor slabs were 8 in thick. The building was to be
approximately 386 ft long by 76 ft wide. Normal weight aggregate concrete of
varying strengths was used in the columns and lightweight coarse aggregate
concrete in the floors. Inspection on 2 floors showed that the lightweight
aggregate concrete floor slab passed through the columns. The floors were
cast in 4 sections which are shown on a plan view on page 118.
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