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Portal Frame Bridges

Portal frames are generally designed for use below ground to provide waterways or pedestrian subways. The units are supported on in situ concrete foundations. (Alternatively the base may be integral with the portal frame, see “Box culverts”.) Seals are provided between successive units to ensure a water-tight structure. The dimensions of the units and their weights will be limited by the requirements for transportation and installation.

Similar units may be designed for use above ground to form integral bridge type structures or frames for building structures.

Ranges of sizes are available, with spans from 1.2m to 6m and internal heights varying between 600mm and 3m. Usually spigot and socket joints are formed at the ends of the units. Although the joints between abutting units can be rebated and left open within culvert construction, it is common practice to fill the joints with a hydrophilic sealant and/or a compression seal. The face of joints can be finished using polysulphide sealant.

Using a configuration of two inverted ‘T’ wall units and a roof unit comprising an inverted ‘U’ section, spans from 4m to 12m with clearance heights of up to 6m can be manufactured. The horizontal and longitudinal joints formed by abutting units can be treated similarly to those between box sections. An inverted ‘U’ may be mounted on another ‘U’ to form a box, with a horizontal ball and socket joint in the walls. Spans up to 12m can be factory produced and internal clearance heights of 6m can be achieved.

Acknowledgement: The Concrete Society

Other references:Current Practice Sheet 142, Precast reinforced concrete arch and portal units, CONCRETE, March 2005, p.6.
(Note that Concrete Society Current Practice Sheets can be downloaded from the Members Area of the Concrete Society web site.)

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