the use of a single line of columns to support the superstructure. These spans, therefore, consist of precast, pretensioned concrete hollow box beams, generally 31m long and 15 per span, supported on and made compo- site with welded steel portals.
The superstructure is divided into two independent continuous struc tures, one of 16 spans and the other 17, each with a central fixed anchor span,
Detail of span over railway lines
דוז וזז
Completed crossing of Grand Union Canal
38
Sliding bearings of up to 1170t safe working load at the base of the portal legs accommodate thermal movement and comb block joints are located at the terminal abutments and the common joint between the two struc- tures.
Since the portals are composite, temporary works were required to sup- port the 2030t dead weight of a span until the concrete crossheads were
DBF
completed. The portals were carried by beams spanning between pairs of trans- verse Autofab girders. These in turn were supported on steel trestles built around the portal legs on the permanent foundations, since supports for the trestling could only be located at the legs.
The beams were mainly placed by a 58m long erection transporter, designed and built by Laing from Bailey bridge sections. Having placed a span, the transporter was launched forward 31m on to the supporting girders of the next portal.
An interesting exception to this procedure was adopted for a three span structure over the Grand Union Canal. Here 120 box beam segments were lifted by crane on to temporary staging, joined in-situ and post-tension- ed in stages. The transporter was only used to place the 71t, 32m long drop in segments for the suspended span. The clear span between the portals over the canal is 51m,
Heating
Road heating is provided on gradi- ents of 1 in 25 or more and for the carriageways of the elevated round- about. Provision is made along the structure for a water main and hy- drants, carriageway lighting and traffic gantries with direction and surveyance signs.
Considerable thought was given to the design of the safety barrier to be incorporated in the central reservation and at the outer edges of the structure. The design adopted by the consulting engineers consists of a concrete upstand 609.6mm high in the central reservation and 762mm high on the edges, faced with a resin-based material on the traffic side and surmounted by a steel rail.
Lighting of the dual carriageways, both two and three lanes, of the ele- vated road consists of 12.2m high steel lighting columns mounted on top of the concrete wall of the median safety barrier. Slip roads elsewhere have light- ing columns mounted on the walls of outer safety barriers.
Consultants
The consulting engineers are G. Maunsell and Partners in association with Mr. P. F. Stott, MA, FICE, FInstHE, Joint Director of Planning and Transportation to the Greater Lon- don Council. Consulting architects were William Holford and Partners in asso- ciation with Hubert Bennett, FRIBA, Architect to the GLC.
Far East BUILDER, November 1970
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