Advanced Structural
Designs ACN 097 789 87
92 Vasey Cres CAMPBELL ACT 2612 Telephone
61612171
Facsimile 61612170 Email mal@structuraldesigns.com.au
What was so innovative
about Impulse’s Big Space hanger at the
The Structure
The structure’s clear span is about 82 metres and you can park a 22m
high Boeing 717 in it. It is
predominantly constructed of square hollow sections and consists of trussed
arches supported on trussed cantilevered support frames with a knee brace added
to transfer some moments out of the arch.
The major innovation in the structure was the use of bundled
prestressing wires in the bottom chord of the trussed arches and in the outer
chords of the support trusses. These
prestressing wires were stressed to impart compression into the truss chords
effectively cancelling out the dead load deflections. While the columns where stressed before
erection, the arch bottom chords where stressed in two stages. Both columns and the bottom chords where
grout filled (columns from the base up) which both adds to their compressive
strength and increases their fire performance.
The major benefit of this approach was to save weight in the structure
since steel SHS’s used have an ultimate strength of 450MPa compared with
1280MPa for the stressing wires. This
benefit along with the ability to control the initial compressive stress in the
truss top chord and the support outer chord led to a claimed overall weight
saving on the project of around 40%.
The approach was not without certain penalties as the bottom chord was
very slender and required a great deal of additional lateral bracing before
pairs of trusses could be lifted into place.
A special lifting jig was also required as the large spans could only be
lifted at the extremities and the tendency for the paired trusses to flip over
was quite pronounced.
The final product looks slender and attractive but on the day of
inspection the trusses could be seen to move up and down 20 mm under a moderate
breeze. In normal circumstances this
would be expected but given the gantry cranes hanging overhead we could not
help but wonder whether this might become a serviceability issue.
The Hardstand
The hardstand was a design and construct job by Austress
Freyssinet. Since the tensile capacity
of the concrete is generally the limiting factor in the strength of the
pavement it is not that surprising that stressing the slab was found to be a
viable solution. We were surprised to
hear that a major saving was reduction in base coarse thickness (of 400mm?) as
base coarse under a rigid pavement is normally only deep enough to control
pumping etc (say 200). In any case the
potential creep and shrinkage of such a pavement is considerable so the
structure was isolated from the pavement and all services penetrating the
pavement were fitted with compressible collars.
The construction was overseen by Construction Controls
As an interesting footnote the new larger hanger being constructed on
the RAAF side of theCanberra