Civil Engineering Reference
In-Depth Information
14.2 Long line casting
The long line precasting method may be used if the deck is straight or of constant
curvature over its entire length. A soffi t form is erected in the casting yard either one
span or a half span long, depending on the method of erection of the deck, Figure 14.1.
This soffi t form must represent the fi nished alignment of the bridge deck, taking into
account any pre-cambers that may be necessary to compensate for defl ections due to
dead loads and prestress.
Deck segments are then cast successively within a travelling shutter. The length of
segments is controlled by the lifting capacity available, but is typically 3-5 m. Each
segment is cast against the previous one, to create a joint that will be a perfect fi t when
the segments are erected. A suitable bond breaker is applied to the hardened face of a
segment before casting the new segment.
As each segment is cast, the foundation of the soffi t form will defl ect under its weight,
and the pressure of the fresh concrete may rock the adjacent segment backwards. These
defl ections will distort the vertical alignment of the span. If this distortion is to be kept
within tolerable limits, it is important that the soffi t form is founded very rigidly.
The segment removal must be done carefully, to avoid damaging the matching faces.
Shear keys or other features on the matching faces must be designed to facilitate this
separation. As a segment is removed the soffi t form will rebound, and consequently
a buffer of two or three segments should be left adjacent to the construction head to
avoid causing further distortions.
If the deck is to be erected span-by-span with a cast-in-situ stitch in each span, the
soffi t form would represent a length between construction joints.
If the deck is to be erected in balanced cantilever with a cast-in-situ stitch at mid-
span, the simplest arrangement is for the soffi t form to represent a double cantilever.
However, space in the casting yard and cost may be saved by a soffi t form that is
only half a span long. With this arrangement the pier segment is cast fi rst. When a
complete half span has been cast and the segments have been removed to storage, the
pier segment is turned through 180° and used as the origin of the other half span of
the balanced pair.
Figure 14.1
Typical long line casting bed
