BRIDGING THE WAIHOU

Hauraki Plains Gazette, Volume XXXIII, Issue 4391, 17 March 1922, Page 3

BRIDGING THE WAIHOU

AT KIRIKIRI. ENGINEER’S REPORT. The following report on the proposed bridge over the Waihpu River at Kirikiri was submitted to theHauraki Plains Conuty Council at the last meeting by Mr F. E. Powell, C.E.; “It is with pleasure I am able to report- as a result pf actual tests and surveys, that there appears to be no serious difficulty in the way of constructing a first-class traffic bridge across any part, of the Waihou River between Kopu and the old Kirikiri wharf. I have, therefore, •selected as the site the -narrowest part of the river, and understand that this is satisfactory to the other parties concerned, including the P.ublic Works Department. This site is about midway between t'lie ferry landings on the two sides of the river, and is shown on the accompanying tracing, numbered B/294. I am huthoritatively informed that at this place no appreciable alteration ofl the banks has taken place since observations have been made, and this is probably due to the fact that the whole of the area explored by borings shows a comparatively hard surface, varying in thickness from about 4 to 8 feet. Contrary to many expectations, the borings reveal a most satisfactory condition, for the driving of piles to carry great weight, with the result that I do not.hesitate to recommend that the bridge be of reinforced concrete, of the type shown in general form in the accompanying plan. I estimate the total cost o" such a bridge, together with approach roads compensation, fenc ing, and all engineering charges, but not including loan expenses, at 1 the sum of £58,000 (fifty eight thousand poupds). ALTERNATIVE SCHEMES. “I have considered several alternative schemes of construction, but the only one that appeal’s to offer a substantial saving is of the type discussed by Messrs Adams- and Muir in their report on Waihou bridge proposals. Although it is true th,at timber bridges of this type should last many years if properly looked after, it is generally admitted that l.ong before the expected life is reached decay will have set in>, rendering the structure a matter of anxiety to the responsible authorities. Usually,, although repairs are possible, they are costly, and the controlling authorities naturally look askance at the expenditure of much money on a decaying structure. I have no doubt that the estimates given for this class of bridge were correct, but I believe the extra cost involved in:my design will be justified on the score of permanence, less cost of upkeep, and the absence of any feeling of anxiety.

“The type of bridge recommended in my <report on t'be Nethertonl-Hikn-taia bridge, with steel joists and timber 1 decking was definitely decided upon as giving a more permanent class of' structure than the timber truss bridge, and for its comparative lightness where weight was a serious matter.. It does not work out appreciably lower in cost than the de_sign before you. at current rates, and as weight has been proved not to be a strong objection, there is. no need to discuss any but the m'ost permanent type suited to the conditions. PERMANENCE OF THE WORK. Reinforced concrete has been objected to in the past on the score of doubt, as to its lasting qualities, and because some failures or partial failures have been recorded, It is safe to say, however, that with our present knowledge of the subject a proeprly designed structure in this material should be one" of the most permanent, short of massive stone or mass concrete, By the method I propose to adopt in this case good work can be assured. All parts with the exception of the decking will be made ashore and . transported . to their place, "thus enabling . greater care to be taken in the construction, and, furthermore, should result in a better finish. GOVERNMENT REQUIREMENTS. "The requirements of the Public Works and Marine Departments as t'o clearances, widths of roadways width of opening span, loadings and factors of safety, and the dimensions of- bridge generally, have all been provided for. DIMENSIONS OF BRIDGE. “The total length of the bridge proper, that is, the*, actual opening of the waterway, is 1740 ft, or approximately one-third of a mile. The opening span will have a clear way of 50ft, and headroom under the opening span girders will be at least Bft at maximum high water. Both ends of the bridge will, be graded down from the opening span to the road approaches!, at very easy grades. “The roadway width between wheelguards will be 12ft except at three places, where it will.be widened out to 18ft. The spans generally will be 66ft centre to centre, but the passing places will have spans of 40ft only. "Roadway approaches ,are designed to he formed 24ft on top of fill, and will have 15ft 1 width of metalling, each side of which the formation will be coated with shell or metal waste. BRIDGE FOUNDATIONS'. “The foundations will be reinforced concrete piles from 16in to 18in square, according to length. They will be driven hard, and will, be capable of carrying with perfect safety a load of 30 tons each. The longest piles may be nearly 70ft, but 1 the majority will be less. The character of the river, bottom, as disclosed by' all the borings, is the same, anfi consists erf a layer of very firm material overlying fairly soft mud of varying depths. This gradually changes to a close-grained material of great bearing power, which may be expected to dose tightly round'the files and thus, add to their stability. "Above low water a system of walIngs, braces, and panels will be used to bind the piles together in the

usual manner to give the necessary lateral stiffness.

FIXED SUPERSTRUCTURE SPANS“The system: of superstructure adopted after careful consideration of various types is as follows: Each span will have two main girders reinforced concrete, which 1 , as already mentioned, will be built ashore. They will weigh close on 30 tons each, and will be transported and Landed on the previously driven pile sets, to which they will be bolted down. The t.ransomes will rest on the lower chords of the girders, and will be rolled steel H joists encased in concrete. They also will be built ashore and carried to their places. Transoms will be about) sft centres.

“With the girders and transoms all in place and secured, falsework will be set up and the steel bars assembled for the decking, which will tlmn be concreted in. A joint will be made in the decking at the end of each bay, thus not- only providing for expansion and contraction, but in the remote event of any slight settlement anywhere will obviate cracking of the concrete. Wheel guards will be concrete-, apd suitable drainage holes will be provided. The' deck will be given sufficient camber: it is not intended to coati it in .any way, as I believe it will be very many years before such- will be necessary.

“There are several advantages in the proposed method of constructing the girder system, For one thing the heigiht of the girders yili give a considerable protection against weather which pedestrians especially will appreciate. At the ends of the spans it will be easy to construct refuges for pedestrians at practically no extra cost, so that protection will be afforded against cattle or other possible dangers at very frequent intervals. One advantage of the weight attending the use of concrete is that relatively the moving loads are small, so that vibration is reduced to a minimum. In addition the appearance of. the bridge will bn neat and substantial. OPENING SPAN. “It. will be seen that I have again adopted the type of drawbridge opening span designed in the first place for the -Netherton-Hikutaia bridge This has been very fulliy considered, and at the ruling rates for materials will. I think, still be the least expensive of the better tlypes of moving spans. The double-deaf type, although probably less costly to construct, has many objections, and I would not adopt! it unless absolutely compelled by pressing financial reasons. There is not in this case however, any objection to the use of a swing span—of the style of the Puke bridge—and in the event of engineering workmanship and labour generally becoming less costly, it may still be possible to. adopt that Hype .at no greater expense than that estimated for the drawbridge. “The operation pf the drawspan will be by means of a 121vp. benzine engine, but can be converted to electric power when this becomes available, the engine then being a standby. One man will be able to do all that is necessary, and the time of opening or 'closing should hot be more than four minutes., “A system of stop bars will be provided for the protection of road traffic, and will be interlocked with the span. For river traffic semaphore signals, visible day or night, will also be provided, and similarly interlocked. “The opening span will be constructed in steel, with hardwood decking. “An operator’s cabin will be provided above the centre of the girders to enable bhe-operator to have an unobstructed view of both road .and river. The machinery will be housed on one side ofi the structure. FENDER PILING.

"Provision will be made by means of fende£, piling and dolphins for the •safe guidance of vessels through the .bridge. The piles will be Australian ‘turpentine,’ and will have efficient bracings and walings. ROAD APPROACHES. "Road approaches have been designed in the positions-shown on the tracing. On the east side the road is shown parallel to tfhe river and joined up to an existing road at Kopu. 'This avoids another railway crossing, and takes the traffic direct to the factory, while not in any way lengthening the road to Thames. A couple of- small timber bridges will be necessary, and have been allowed for. On the Plains side the road is also sh6wn parallel to the river, Connecting to the existing road to the ferry, landing. In each case it is proposed to cut off all angles, so that turns will not be sharp. Owing to the character of the country and the length of the bridge there will, be good visibility for all traffic using the bridge. "The roads will be built up to a sufficient height, and side drains provided. The top formation width is proposed to be 24ft, of which 15ft width will' he metalled, the margins being covered with" shell or metal waste. The metalling, to be 9in o p 2%1n metal, well blinded and rolled, and a camber of, say, 6in, to be given. It is not proposed to treat the metalling in any way, hs the formation will be new, and should have time to settle before 'such is odne. CONCLUSION, “Tn conclusion, I desire to say that the estimates have been prepared with great care, so that while not placing them too high, I believe they will- be fully sufficient to cover all the work named- No allowance has been made for z a possibility of a fall in prices of material and labour, and if such takes place it is reasonable to suppose the cost of the bridge will be less in prop'drMon. “I have only to add that I very much appreciate assistance very kindly given in the compilation of data and in various ways to enable judgments to be formed. In this conne tion my thanks arc especially due to the engineering staff of the' Public Works Department, to Mr E. F. Adams, engineer, and not. least of all to yourself, your Council, find 't’ county clerk."