Civil Engineering Reference
In-Depth Information
one-off pier segments. The maximum length of segments is generally controlled by the
cost of lifting and moving them, which depends on the scale of the site.
As most box section bridges of medium span have an average thickness of the order
of 0.5 m, a 12 m wide by 3.5 m long segment would weigh some 53 tons. However,
the segments at mid-span are likely to be lighter than those near the piers, where webs
are usually thickened. Thus the weight of typical span segments will vary between 45
and 70 tons. In general, a weight of segment that does not exceed 60-70 tons allows
the use of readily available cranes and low-loaders.
When the deck is wide, segments will inevitably be heavier. For instance, the
segments for a 20 m wide single box will weigh typically between 25 and 35 tons per
metre. Thus a 3.5 m long segment will weigh of the order of 90-120 tons.
Clearly, longer span bridges have a greater average thickness of concrete (
8.4
), and
consequently the segments will be heavier.
In most box section bridge decks there is a pier diaphragm. In order to limit the
weight of the pier segment to that of standard segments, it is common practice to
reduce its length, typically to 2-2.5 m. It is also often necessary to reduce the weight
of the diaphragm by thinking clearly about its various functions (
9.6
), and removing
all redundant concrete. Generally, the pier segment is used as the origin of the counter-
cast run, and is sometimes cast in advance in a simple fi xed mould.
For long decks, the length of segment adopted may critically determine the number
of casting cells that have to be mobilised; an increase in length may reduce the number
of cells, at the cost of increasing the weight of the segments. The use of lightweight
concrete has proved to be economical when it has allowed the length of the segments
to be increased and the number of casting cells to be reduced without increasing the
segment weight beyond some critical limit.
By advancing or withdrawing the stop-end segment so it penetrates more or less
into the steel shutter of the casting cell, the length of segments may be adjusted slightly
during casting, in order to suit slightly varying span lengths. However, this may well
be impossible if the bridge deck is sharply curved, as the sealing of the shutter onto the
stop-end segment would be compromised, causing grout leaks and untidy joints.
The designer also has the option of adopting two or more lengths of segment, either
to adapt them to varying spans, or to limit the weight of the heavier segments for a
variable-depth deck. However, if the segments within one casting run have different
lengths, the casting programme will be disrupted to some degree, as the mould has to
be adjusted. It is good practice to minimise such changes, and where possible keep all
the segments of a casting run at the same length.
In general, the maximum span that may be built economically by the precast
segmental method is infl uenced by the height and cost of the mould, and by the weight
of segments to be lifted. For instance, a 160 m span would need a mould some 8 m
high, and the deepest span segments for a 12 m wide deck are likely to weigh some
50 tons per metre, or 175 tons for a 3.5 m long segment. Clearly for very long viaducts,
where the moulds and lifting gear will be well amortised, the size and weight of the
segments may not be an obstacle to choosing this method of construction.
In some special cases, very long, heavy segments may be justifi ed. In Benaim's
proposed alternative design for the Storabaelt Approach Viaducts, consisting of a 4 km
long series of 164 m spans, match-cast segments 23.7 m wide and up to 8 m long
were planned, weighing up to 600 tons. This was possible because powerful fl oating
