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NEW CITY BRIDGE.

Design Approved by City Council. ESTIMATED COST £79,000. A design for a bridge to replace the present City Bridge, prepared by the former City Engineer (Mr. R. M. Scott), was approved at a special meeting of the City Council on   Friday. It is estimated that the cost of the bridge which will be of reinforced concrete, will be £79,000.

After meeting of the general pur- poses committee of the Adelaide City Council on Friday, its recommendation that the design for the new City Bridge

should be approved, was adopted, on the motion of Alderman Isaacs. An amendment that the council should allow the City Engineer to put in a tender for the bridge was moved by Councillor Bardolph. He said in the interests of the ratepayers their engineering department should tender for the work. It would not bind the council to accept me tender, but would provide a check on the other contrac- tors and would prevent them from mak- ing an overcharge. If the council adopted that course the City Engineer would follow the same methods as those of private tenderers and would get prices from engineering works and even should his be the highest bid, the coun- cil would have all particulars of the job before considering the tenders. A Councillor—It would cost us more to do it that way than by private enter- prise. Councillor Bardolph—At the present time under the contract system the council has to employ highly paid and skilled officers to supervise the work of the contractors. A Councillor—Only a waste of time. Councillor- Bardolph—When a road   contract was let not only was there a council superviser, but a man was put on the concrete mixer to prevent the contractor robbing the council. (Up- roar.) It was all very well for certain members of the council to support the contract system. He did not say that there was an ulterior motive, but it was apparent to him that while some mem- bers of the council objected to the ex- penditure of £2,000 in beautifying North- terrace, which would be done depart- mentally, they were prepared to put £100,000 of the ratepayers' money hi the pockets of private enterprise. (Further uproar). Alderman Rees pointed out that all tenders would have to come before the City Engineer. The amendment was put, and a divi- sion called for. Councillors Bardolph, Edwards, Jones, and Lundie voting for the amendment which was lost. The Old Bridge. The new bridge will take the place of the present structure, which was bunt by Mr. M. C. Davis in 1877, when Mr. Caleb Peacock was mayor, and Mr. John L. Hyndham city engineer. This structure is of wrought Iron plate gir- der type spanning the river between two masonry abutments approximately 98 feet apart at water level. On the introduction of electric trams in 1909, additional steel lattice girders were added to carry the heavier loading. The width of the present roadway be- tween handrails is 54 ft. 6 inches, whereas the width of King William- road is 132 feet between fences. The effect of this construction of both carriageway and footpaths has been felt for some years, and with the growth of the city, and the ever-in- creasing motor vehicle traffic over it, the bridge has become more and more congested, reaching a peak density whenever there is an important match on the Adelaide Oral. To overcome this congestion, the council adopted the progressive policy of widening the roadway over the bridge to the full width of King Wil- liam-road. It was therefore decided to construct a new bridge wholly of reinforced concrete conforming with the alignment of King William-road. Such a bridge has now been designed, and in addition to providing for a road to carry effectively all future road and tram traffic, It also gives an increase of approximately 17 feet in width to the river beneath the bridge, and provides for footpaths under the bridge on each bank to obviate the necessity of people walking beside the river crossing over King William-road. The loading adopted as a basis for the design is the theoretical loading set out by the Ministry of Transport, England, and is considered as being adequate for all normal increases in vehicular traffic for at least a hundred years to come. The New Bridge. The new bridge is to be wholly of reinforced concrete, and is to consist of a main span over the river, and two approach spans, one on each side. The main span is to be constructed of ten three-hinged arch ribs giving a clear span 3 ft above water level of 115 ft. 6 in., and the approach spans will each consist of ten arch ribs of the bow- string type having a clear span of 31 ft. 6 inches. The hinges for the main web ribs are to be of cast steel. The width of the bridge is to be 132 ft. be- tween inside faces of handrails, whilst the total length of the new structure will be 221 ft. 4 in. The width of each footpath will be 20 ft., and that of the roadway 92 ft. between kerbs. The total increase in width of the bridge is 77 ft. 6 inches, or about two and a half times wide as the existing struc-   ture. The centre line of the new bridge will coincide with the centre line of the two existing tram tracks. The balustrades will consist of concrete posts and handrails, and the baluster panels are to be of bronze. To allow free movement of the main arch ribs at the three hinges, the deck- ing of the bridge has been broken for the full width of the bridge on a line vertically above the line of the binges in the arch ribs. This means that there are three expansion joints, one situated over the centre of the main span, and one approximately over the river face of   each abutment. These joints allow free movement of the decking, due to move-   ments of the arch ribs, and at the same time provision is made in the form of plastic joints for a continuous surface the traffic over the bridge. The bridge will be lit from four concrete standards which will also serve to sup- port the overhead trolly wires for the trams, by four bronze bracket lights at- tached to each standard. On the line of each handrail, two pylons and two pilasters with refuges are to be con- structed, the pylon being at the ends of the bridge, and the pilasters between the approach and main arches. The main abutments are to be of mass con- crete and each is supported on 39 bents of ironbark piles of six piles per bent. This decision was made after much pre- liminary work at the site and many different design investigations. Determining Nature of Foundation. As a first step to ascertain the nature of the foundation at the bridge site, shafts were sunk and four bores were drilled and samples taken of the dif- ferent classes of material at different depths, one bore being drilled at each corner of the present structure. The type of material encountered in each bore varied, particularly near the sur- face, on account of the fact that there had been a considerable amount of filling at the time of constructing the existing bridge. Generally, however, the material varied to a depth about 4 ft. below water level, below which for 20 to 25 ft. was sand and gravel at which depth a band of yellow clay 2 to 5 ft. thick existed, and then, sand and gravel again. The results of this boring indi- cated that no solid rack could be counted on for foundations so that the loads of the new bridge would have to

be transmitted to the ground through pile foundations or direct bearing on to sand and gravel. This fact also de- cided the type of bridge as a three hinged arch as against a hingeless arch as the effect of a possible settlement of the foundations is relatively negli- gible whereas in the case of a hingeless arch a foundation settlement might lead to disaster. On account of its ex- treme Importance, a great deal of con- sideration has been given to the ques- tion of the design of the foundations of the new structure, and many schemes were investigated before a final basis for design was adopted. In treating the foundation, each scheme had to take account of three different types of loading under different parts of the bridge, that is loading due to tram, vehicular, and foot traffic, each with four different conditions of loading. Widest Concrete Bridge in World. Further, the new bridge, being the widest known reinforced concrete bridge in the world, and having ten ribs sup- porting its superstructure, necessitated special consideration being given to the abutment on account of its length. In order that It should act In uniformly distributing the various loads to the pile supports throughout its length. Ac- cordingly, instead of a uniform section throughout, the abutment has been ap- portioned to give a mass distribution corresponding to that required by the forces acting on the abutment at the part concerned. The design finally adopted was one with a mass concrete abutment supported by 39 bents of six piles per bent of ironbark; piles driven at an angle of 25 degrees from the verti- cal. Altogether 483 piles will be re- quired. Order of Construction. Provision has been made to ensure a minimum interference with traffic dur- ing construction. An essential condi- tion is that two lines of tram traffic must be kept continuously in operation dur- ing the whole of the work. The bridge is to be constructed in two main sec- tions. The existing bridge east of the girders supporting the present up tram track will be removed, and the founda- tions and four of the ten arch ribs of the new bridge, together with their superstructure, will be built. During the construction of this portion, both tram tracks and the western road track and footpath will be available for transit of pedestrian and road traffic across the river. In tte second section both of the tram tracks will be moved east tem- porarily, a distance of 24 ft. 2 in., to the portion of the structure completed in the first section. Road and foot traffic will also be transferred to the new structure. The remainder of the present bridge will then be demolished, and the new struc- ture completed, after which the trams may be moved to their final position In the centre of the bridge. Architectural Treatment.   In the architectural treatment of the bridge, the aim has been to provide a structure which will fit naturally into the park and garden landscape. The difference in level between King Wil- liam-road and the summer level of the water in the lake does not allow of a high rise for the main arch spanning the river, consequently vertical lines have been used where possible to avoid the effect of flatness, which is liable to per- tain when horizontal lines predominate in an arch with a low rise. To give a pleasing effect of stability, the main structural members have been enhanced by suitable lines, but at the same time the spandrel of the arch has been kept open to allow as much light as possible to penetrate beneath the bridge, and at the same time to give a slender and graceful appearance in elevation, al- though the bridge is designed for the heaviest vehicular traffic likely to even- tuate in the future. The actual rise from the south end of the deck slab to the centre of the bridge is 1 ft. 1 in., while the fall from the centre to the north end is 11 in. The bridge will be finished in plain rubbed or surface treated concrete, specifically avoiding the flatness given by a rendered finish. The consultants who collaborated on the type of design and system of founda- tions to be adopted were Professor R. W.   Chapman, Messrs. R. H. Chapman and H. G. Jenkinson. The details and cal- culations were carried out by Mr. W. H. James, assisted by Mr. I. Watts, the work being under the supervision and control of the former city engineer and surveyor (Mr. R. M. Scott).

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