f1.—35

18

the water sufficiently by pumping to enable divers to do the necessary excavation to permit of the caissons being entered into the rock and thereafter pumped out. In adopting for the bridge a vertical clearance of 135 ft. we have to some extent disregarded the weight of evidence, believing that the modern tendency in the design of cargo-vessels is not towards an increase in. the height of masts. Further, we are advised by the naval authorities that such warships as have higher masts could without difficulty lower their top hamper should it be desired to proceed under the bridge at the top of high water. It must be remembered that the depth of water in the main channel is such that no cargo-ship woidd have any difficulty in proceeding up the harbour at dead low water, which would give another 5 ft. of clearance at neap tides, increasing to 12 ft. at spring tides. The cost of the bridge would be materially increased if the clearance was made, say, not less than 150 ft., and the proposed grades maintained. It will be necessary to protect the channel piers with heavy fendering, so that no damage will be done to the bridge and so that the damage to any vessel touching them would be minimized. Owing to the great depth of water this will be an expensive matter, and it will, we think, be advisable to place rubble filling around these piers up to a level of about 30 ft. below ordinary low-water spring-tide level, into which a sufficient number of piles to support the fendering can be driven for this purpose. An outline plan has been prepared which shows the type of bridge we recommend as being the most suitable for the site and conditions obtaining (Appendix No. 2). On the bridge we have provided for a width of 40 ft. of clear roadway and two 6 ft. footpaths, the latter to be carried outside the trusses on the channel spans. The details of the design have not been worked out sufficiently to enable us to fix the entire over-all width of the bridge, but it should approximate 62 ft. on the channel spans and 52 ft. on the balance. In placing the 800 ft. span we have not shown this span in what is at present the best position for navigation, because the sailing course now is diverted to the northern side of the harbour by reason of a shoal which exists between Birkenhead and Point Erin. However, borings made show that this shoal consists of easily dredged material, and therefore we have been able to place the 800 ft. span of the bridge fairly centrally in the harbour, thereby assisting the grading down on the northern side, which presented some difficulty upon first consideration of the problem. The dredging of this shoal will enable a first-class sailing-course to be obtained under the bridge as we have indicated it. The extent of this dredging is not large, being approximately 300,000 cubic yards to 35 ft. at low water, springtide (Appendix No. 5). The total length of the bridge is 4,740 ft. in the main structure, together with 1,700 ft. of bridging across Shoal Bay and Ngataringa Bay in connection with the causeways. The total length of the works from the present end of Fanshawe Street to the proposed junction with the Devonport roading opposite Stanley Bay wharf is 19,840 ft. With regard to the loading to which the bridge should be designed : In calculating the stresses in the stringers, transomes, and hangers we have adopted the British Ministry of Transport standard loading for each 10 ft. width of roadway. This gives a greater loading than would be given by two lines of tram-cars and two traction-engine trains side by side, so that although no special attention was given to the tram aspect the bridge would be amply strong enough to carry tram traffic. In addition to the above a load of 100 lb. per square foot has been adopted for the footways. For calculating the stresses in the trusses and plate girders a roadway load varying from 100 lb. per square foot of roadway on the 60 ft. spans to 50 lb. per square foot in the 800 ft. span has been adopted, and a footway load of 25 lb. per square foot on both sides throughout. The above loading will allow of the bridge being used by 20-ton tractionengines and trailers in any probable combination. The New Zealand standard loading is less than the British Ministry of Transport standard, but in a structure expected to last for a great many years it is desirable to make allowance for possible future developments. Some bridges in industrial centres in England have been