Erection of a precast two-lane unit.
ETH
A
Fourteen such units form a typical 105ft. span.
of 14 precast concrete slices 7 ft. 3 in. long with in situ concrete joints 3 in. wide, making a 7 ft. 6 in. module. The dual carriageways are formed by two such structures placed side by side, initially independent but later united by an in situ concrete median strip. cast at the same time as similar edge beams on the outer cantilevers.
The superstructure was analysed as a continuous beam for live and super- imposed dead loads while the neces- sary modifications were made in cal- culating the effects of the basic dead load and the secondary prestressing moments because of the span-by-span method of erection.
As the basic structure is too shal- low to consider attaching the pres tressing cables externally to the con- crete within the hollow box, the webs must be wide enough to accommod- ate an economic layout of cables and allow concreting. The webs are con- sequently thicker than necessary from a stress viewpoint, but this was con- sidered justifiable in that it allowed the cable layout and fabrication of the units to be considerably simplified.
con-
The in situ spans of 60 ft. are de- signed partly in reinforced concrete while retaining full structural tinuity with the main prestressed con- crete superstructure. The structural activity in these small spans is condi- tioned by the 105 ft. spans on each side producing a small sagging mo- ment at midspan. Here the prestress- ing cables enter the span from both sides and are anchored in the soffit near the columns.
There are advantages in this ar- rangement. With the span by span method of construction and the par- ticular cable layout adopted it is de- sirable that the cables should flow in the direction of construction. From this in situ section at the top of the anchorage ramps, construction pro- ceeds in opposite directions, eastwards
Far East Architect & Builder February, 1967
along the three lane section and west- wards along the two lane structure.
The arrangement in the 60 ft. spans provides a simple means of changing the leading direction of the prestressing cables using the most economic number of cables, and with- out cable paths crossing each other or using capping cables.
In this way
the column moments are sustained by prestress, while the midspan moments are resisted by high tensile reinforce-
ments.
Precast Segments
Since adequate access is available along most of the route a purpose- made handler for the units is not re- quired. The main factor in the sizing of the segments was to produce a unit that could convenietly be handled by a mobile crane. The weight of the standard three lane beam segment is about 25 tons and the two lane and
HANDRAIL AND SAFETY BARRIER WITH SPLASH GUARD
13'-0"
+
ASPHALT
ramp segments both weigh about 19 tons each.
To facilitate precasting and achieve the most economical use of form- work, there are only three external shapes of precast unit ramp, two lane and three lane. All units in each section are of uniform cross-section, with the exception of the one above each column which differs internally to provide a manhole access to the drainage system within the spine.
All precasting can be achieved us- ing the same external formwork be- cause the structural depth and the profile of the webs and soffit of the cantilever slabs are the same on every unit. The only external dimensions which vary are the width of the spine bas and the length of cantilever slab. The units are cast right way up and are provided with mild steel lifting hooks in the webs which are burned off after erection.
On the structure of segmental con- struction, a joint of reasonable thick- ness is of considerable value in ac- commodating any small irregularities in the units. On the curved sections the units are not tapered to the exact mathematical shape but to the nearest 14 in. The joints are therefore of 3 in. nominal thickness to allow the use of poker vibrators, and are con- sidered invaluable in allowing some latitude on erection.
While making the in situ joints, the continuity of the ducts between the units is preserved by inflatable rubber tubing which can be pulled along to complete each joint in turn.
Stressing
on
After positioning the precast units the staging and jointing, the Freyssinet 12/0.6 in. multi-strand prestressing cables are threaded through ducts which are totally en- closed within the two webs. Each cable is anchored on the soffit at the leading cantilever end and on the deck at the corresponding quarter point position two span lengths be- hind.
One set of cables is stressed as each
IN SITU
MEDIAN STAIR
WATERPROOF MEMBRANE
ALUMINIUM SCREEN AROUND BEARINGS
SORED
4-4 DIA
CYLINDERS
IN SITU
EDGE BEAM
SECTION THROUGH
3 LANE
STRUCTURE
47