Mt Henry Bridge - 2012 Booklet
History of the Mt. Henry Bridge
The north south Freeway system, which provides for the bypassing of the Perth central business district, was planned and developed by Professor G. Stephenson and Mr. J. A. Hepburn in 1955. Apart from the completion in 1959 of The Narrows Bridge, which connects the north and south banks of the Swan River, the whole Freeway plan was adopted by the WA State Parliament in 1963. There have been amendments made to the plan on a number of occasions, brought about by changing demands and following further studies.
The first stage of the southern extension of the road, designated The Kwinana Freeway, saw the completion of the Canning Interchange in 1979. The second stage saw the extension reach South Street, when it was officially opened on 9 May 1982. This included the construction of the Mt. Henry Bridge across the Canning River and its completion, following 34 months of work, in April 1982.
Bridge Statistics
When completed, the bridge was (and still is) the longest road bridge in Western Australia, being 688 metres in overall length (including abutments) and 28.8 metres in width. It accommodated 6 lanes of traffic with a central concrete median barrier. It consists of nine continuous spans of precast, double cell, single box, post tensioned, concrete segments, supported by sculptured reinforced concrete piers. There are 7 spans of 76.25 metres and 2 end spans of 63 metres. The depth of superstructure is 3.6 metres. For aesthetic reasons, the soffit is parabolic. The deck has a 3 degree cross fall, each side of the centreline. An unusual feature of the bridge was the provision of a 3.15 metre pedestrian walkway and cycleway cantilevered from the bottom flange of the superstructure, on each side.
The bridge piers are made continuous with partly submerged pile caps, supported by composite piles which comprise a precast pre tensioned concrete upper section and a steel universal column lower section. The average length of pile was 42 metres, being driven through river mud and sand into hard siltstone.
The abutments are reinforced concrete cellular structures. The fixed abutment on the Mt. Pleasant side is founded on universal column sections, while the Mt. Henry side, at the bridge expansion joint, is founded on a raft footing.
The bridge services include two 914mm diameter sewerage pressure mains, a 325mm diameter high pressure gas main, Telecom mains, SEC mains, drainage, water and electrical facilities for the bridge itself.
Quantities
Total quantities in the bridge consist of the following;
| Piling, Mt. Pleasant abutment, 43 no. | 1,870 m |
| Piling, composite to piers, 184 no. | 7,530 m |
| Precast deck units | 258 no |
| Concrete, precast units | 10,470 m3 |
| Concrete, in situ | 6,530 m3 |
| Reinforcing steel | 3,068 te |
| Stressing wire, 7mm diameter | 754 te |
| Stressing strand, 15.2 mm and Macalloy bar, 38mm diameter | 140 te |
Design and Construction
The bridge was designed by the Main Roads Department of Western Australia and constructed by the Clough Engineering Group. Influenced by the high cost of constructing the Stirling Bridge (1972-1974) temporary mid span falsework support piers, and backed by detailed calculations from Swiss consulting engineers Cepas Plan Ltd. of Zurich, Clough developed a cable stayed tower which partially supported the bridge falsework from above and acted as a crane to lower the bridge precast deck units onto the falsework truss. (The whole Falsework System received the Engineering Excellence Award for 1981 from the Western Australia Division of the Institution of Engineers Australia.)
River piling, pile caps and piers were all constructed using crane mounted barges, a self propelled “Schottel” barge and a “Combi” barge, made up of 20 standard interconnected modules, and together sufficient to carry all superimposed loading. A temporary jetty (which later became permanent) was built on the Mt. Pleasant side of the river to provide for the loadout of all materials and equipment.
Piling
The Mt. Pleasant abutment piles, 43 No. 310 UC 240 steel, were driven with a Kobe K35 diesel hammer mounted on a 30RB crawler crane. The piles were up to 43m in length and raking 3:1 and 4:1. The top 9m of each pile was painted with two coats of tar epoxy enamel.
Each of the 8 river piers is supported by 23 composite piles, comprising a 310 UC 240 steel lower section and a 550mm square precast, prestressed concrete upper section, driven to an ultimate capacity of 6,000 kilo Newtons. The piles in each pier were arranged with a central vertical pile and the other piles “fanning out” in all directions at rakes varying from 1 in 20 to 1 in 5. Composite piles ranged in overall length from 38 to 47 metres, with the concrete portion from 13.5 to 21 .5m. Penetration of the lower section into the Siltstone varied from 12 to 15m. The heaviest pile handled weighed 24 te. Each of the concrete upper pile sections contained 16 No. x 12.5mm dia. pre tensioned stressing strands and pulled to 128kN, prior to casting. Each section was cast about an embedded 3m x 310 UC 240 pile stub.
The river pier composite piles were driven by a Kobe KB60 diesel pile hammer, mounted on 30m leaders attached to a 61RB crawler crane. This combination was assembled on the Combi barge and manoeuvred by means of anchors and barge mounted electric winches. Pile sections were delivered by the Schottel barge.
Pile Caps
The elliptical pile caps are 10.5 x 4.0 x 2.0m depth, containing 68 m3 of 30/20/200 mPa concrete, which was super plasticised with Melment L10 (1,045mls per 100kg cement) to achieve a high slump. This was necessary to ensure placement and compaction around the congested reinforcing steel. Each pile cap contained 15 tonnes of reinforcing steel, comprising up to 13 mats of interwoven layers, together with extra pier and temporary pedestal starter bars. All up, the reinforcement amounted to approximately 315kg/m3 of concrete.
Piers
The 8 sculptured and bifurcated piers, which varied in height to suit the bridge profile, contained an average of 60m3 of 45mPa super plasticised concrete in a total of 475 m3. A retarder, Plastet No.2 (up to 500ml per 100kg cement), was added to the mix to increase the effective time in which to be able to place the concrete. These additives were phased out as the pour height increased in each pier. Cold worked and close tolerance bent reinforcement averaged 406kg/m3 of concrete for each pier, in a total of 193te. The surface of all piers was sandblasted, following the deck construction.
Precast Deck Units
The tender documents required the contractor to provide an on site batching plant that was capable of providing tight quality control of the 45mPa design strength concrete used in the manufacture of the 242 x 110 te hollow deck units and the 16 x 115 te solid diaphragm units. The batching plant chosen was a fully automatic 1m3 Marte 1040, c/w 120 te capacity air blown cement silos. Two transit mixers provided sufficient transport of concrete to the casting yard and its placement using a mobile conveyor.
Two sets of railed telescopic steel shutters, 24.4 x 3.6 x 2.55m long, were sufficient to allow for a continuous cycle of cleaning, placing prefabricated steel reinforcement cages c/w all cable ducting, casting, steam curing, lifting and storing. The whole casting area and storing of the units was straddled by a 150 te SWL portal gantry crane. Steam boilers, 3 no. x 30 HP automatic, with chart recorders, and a steam tent, allowed the units to reach their target strength, 30 mPa for lifting, within 24 hours. Units were stacked two high and at 90 deg. to their final alignment.
Falsework Truss and Tower
The falsework truss (81.6 x 15 x 14.6m) comprised three main trusses spaced 7.5m and spliced together in 20te sections using M30 friction grip bolts; all up weight of the truss was 540te. A removable 20m section was assembled around each pier in turn. The falsework tower (40 x 7.5 x 2.5m) comprised two strutted columns made up of a lattice of braced 31OUC sections on a square grid of 1.5m; all up weight of the tower was 165te. Tower raising and lowering was achieved by using 2 x 500te jacks (fixed to the completed deck) and segmented tie cables. Backstay cable tensioning was carried out at the top of the tower using 2 x 250te jacks pulling 4 cables (42 x 7mm). The cable stayed concept allowed the rear end of the falsework support truss to be suspended from the 17m cantilever of the previous stage of deck construction. The mid span of the truss was supported by cables fixed to the head of the tower and the forward end of the truss was supported by 1000te capacity guided sliding bearings seated on temporary pedestals on the leading pile cap. This configuration finally allowed 50% of the total dead weight of 4000te of each stage to be carried through the pile cap, 17% to be hung off the free cantilever and the remaining 33% to be suspended from the truss/tower front stay cables. As the dead weight deflection of the truss tended to increase as more units were placed, the head of the tower was pulled back by the four backstay cables.
Deck Unit Placing
Precast units were lifted from storage by the gantry and placed on a rail mounted hydraulic powered unit transporter. The unit was supported on rubber bearings at each web. Upon reaching the tower, a second lifting beam was lowered and attached to the top of the unit using BBR cables. The unit was raised off the unit transporter, by means of a twin 10te line pull winch and allowed to swing out over the edge of the deck. It was then rotated 90deg. by hand and lowered down onto the falsework trolley. It was again seated on rubber bridge bearings which acted as shock absorbers during transport. The unit was transported along the top of the truss to its final position before being jacked down onto pre set packers. Packing heights allowed for the gap between bridge soffit and the truss, the truss deflection under dead loads and the pre set for bridge post tensioning. Great care was need when positioning the pair of solid diaphragm units, relative to the top plate of the permanent bearings (2500te capacity), to allow for temperature movements of previous spans and the truss itself. The main post tensioning cables were reeved through the diaphragms and hollow deck units before pouring the 450mm in situ joint between the two 115te diaphragm units and the 1OOmm joints (target strength 52mPa) between the deck units.
Post Tensioning
The permanent prestress cables having been pulled through the unit ducting and made continuous with the previous stage, a strict post tensioning sequence took place, interspaced with de stressing of the backstay cables. This is a very simplified summary of a much more ordered sequence of work to complete each stage of construction. An 800te prestress jack was used to tension the larger cables. Each of Stages 2 to 8 comprised 27 main longitudinal cables (9 per web of 125 x 7mm x 5110kN ), 12 transverse diaphragm cables, 12 bottom slab cables, 18 vertical and 40 longitudinal Macalloy bars in the diaphragm. Following MRD approval of the final prestress, all cables were grouted with a water/cement ratio not more than 0.50.
Truss and Tower Handling
The use of preloaded collapsible sand jacks around each pier, as part of the packer height, enabled a positive and quick separation and release of the falsework truss away from the bridge soffit, after prestressing for a particular deck stage was completed. Handling of the falsework truss commenced, by positioning the Schottel barge underneath the 11te cantilever section, unbolting the splice joints around the pier and transporting the cantilever truss section to a parking position adjacent to the temporary jetty. The Combi and Schottel barges were then positioned underneath each end of the 430te main truss section, which was then lowered onto the barges using 8 x 60te hydraulic extended ram jacks. Following the movement and connection to the next span, the Schottel barge returned to pick up the parked cantilever truss section and manoeuvre it into place, to be reconnected to the main truss section around the next pier.
The Tower was initially lowered onto a beam supported by an ‘A’ frame mounted on two pairs of Unit Transporter bogies travelling on the completed deck. With the tower hinge disconnected from the deck, a powered transporter moved the tower along the deck until the head engaged with a second powered ‘A’ frame travelling on top of the truss. In its new location, the base was fixed to the deck, the tower was then raised, and the backstay cables reconnected.
Services
Services to the bridge comprised electrical, water, drainage, public sewerage and an external gas main. The electrical work was particularly significant, in the amount of power and lighting fixtures that were required (some 660 switchboards, lights and switches). Electrical cable lengths up to 475 metres were pulled through the structure using a combination of rollers and snatch blocks and co ordinated with the use of two way radio. All up there were some 28.5 kilometres of electrical cable installed.
The single finger plate expansion joint at the Mt. Henry abutment allows for == / 199 mm of expansion. It was fixed to the deck with high tension bolts. The rotation joint at Mt. Pleasant abutment was a single 30.7 metre Wabo heavy duty compression seal.
Guard and Hand railing to a combined length of 2,870 metres was installed on the bridge deck and on the cantilevered footways.
Tender and Final Payment
The Tender price at award, on 14 March 1979, was $10,133,000. The Final Payment at Completion, after allowing for some variations and significant escalation, was $13,971,000.
Construction on site commenced on 31 May 1979 and Contractual Completion was achieved on 24 April 1982. Not a single hour of work was lost through industrial unrest caused by site working conditions or disagreements. The Safety record was very good, with no major incidents recorded, and the percentage of hours lost, was 1.07% from some 380,000 hours worked.
Author: Tony Quinlan, FIEAust
Additional information is available in Edmonds, Leigh, The vital link – a history of Main Roads Western Australia 1926-1996, refer pages 287-289. Design of the bridge by the Main Roads Department was managed by Gilbert (Gill) Marsh with support from Dr Ken Michael for design of the foundations and piling. (Personal Communications - Ralph Moore 8 May 2020)
