THE TRAFFIC BRIDGE.
Mataura Ensign, Rōrahi 15, Putanga 1303, 7 Hōngongoi 1893, Page 3
THE TRAFFIC BRIDGE.
Following is the . full text of Mr Hovrorth's report upon the cost of bridging the Mataura at the two proposed sites at Gore, and in view ef to-night's meeting in the Town Hall it should be oarefuljy perused by all ratepayers :—
Invercargill, July 5, 1893. To the Clerk Gore Borough Council.
Sir,— ln compliance with your request ! I have the honor to furnish the following report on the cost of bridging the Mataura rirer at the two proposed sites :— • IST— THE MEDWAT STREET OB UPPEE SITE. Bridge. — The bridge proposed for this site is 450 feet long, in six spans of 75 feet each, on part concrete and timber piers. Suf er 'structure. ~Th o superstructure would be built on that known as the Howe truss principle, whijh is recognised as being the most suitable for bridges such as the one in question, strength being combined with lightness, economy, and ease of construction. The width between the trusses proposed is 18 feet, of which 17 feet will be available for traffic, which will allow two vehicles to pass with comfort on the bridge. The following timbers are recommended to be used in the work : — The lower chords and floor joists (in tens'on) of ironbark, upper chords and diagonal bracing (in compression) of totara, horizontal bracing and 'flooring of blue gum. Owing to the greater strength (in tension) of Australian over New Zealand timber, a smaller quantity can be used, and therefore' the extra price of Australian over $ew Zealand timber is somewhat equalised; As regards the durability of blue gum a proof of this may be seen in the Oreti bridge at the ferry on the old Biverton road, which has been laid for over 25 years and is in very good order yet.
Foundation and Piers. — The level of the rock has been ascertained by boring, and is found to be conveniently near the surface for building on, therefore the most suitable pier for this site (always bearing in mind that funds are .not unlimited) is one having for its base a concrete foundation, built up from' the rock to a height sufficient to clear ordinary low water level, and on this would be secured a well braced trestle, carried up to the necessary height to take the superstructure and clear flooJ level. Many railway bridges are built thus to save the first cost, which is considerably less than with the masonry carried up the whole way, and with the foundation or base in, at some future time when it becomes necessary to renew the bridge, and if funds permit, the masonry can then be completed. I have no hesitation in recommending this plan. - Approaches. — The approaches would require to be formed 24 feet and metalled 20 feet wide, and run out at an easy gradient from the floor of the bridge. Estimates. — The estimated cost of this bridge, together with approaches, is L 3200. THE MERSEY BTBEET OB LOWEE SITE. Bridge.-— The length of bridge required for this site is 560 feet, consisting of 14 spans of 40 feet each on pile piers driven into the shingle. Superstructure. — The superstructure is similar to that proposed for the upper site, with of course the necessary modification in the timbers to suit the smaller spans. The length of the span is also guided by the cost of the foundation, it being found cheaper to put up 40ft spans than 20ft spans, the greater number of piers costing more than the extra timber required for bracing, etc., in 40ft trusses ; just in the same way 75ft Bpans are cheaper for the upper site than spans of half; that length owing to the extra cost of foundations over superstructure. These facts are given, as gome diversity j
of opinion might be caused by the difference in the designs. - . Width ofßoctdway and Approaches, — These are the same as that proposed for the upper site. '■'' Piers, etc. — It has been ascertained by boring that piles can be diiven to a sufficient depth to reach beyond scour, therefore the most suitable foundation is that composed of timber piles. The timber recommended is ironbark, as it is very durable and drives easily to a depth of 20ft in heavy shingle (this being the depth attained at' Mataura Island and Pyramid bridges), whereas native timber requires careful handling to get it down 12ft without splitting— an insufficient depth in a shingle-bearing river, where scour is likely to lake place. Protective Works.— The west bank is a sound one and not likely to be encroached upon by the river, but the east bank is threatened by the inroads of the river, and if the Council decide upon this site some further protective works vrill be necessary. The existing groins will require strengthening and a small groin erected below the bridge. Estimate. — The estimated cost of a bridge at this site, which includes approaches and protective works, is L 2500. CONCLUSION. It is quite possible, however, to reduce the cost b^low the estimates furnished if the Council would be satisfied with timber of less durability for the floor. In such case' miro could be substituted for blue gum, which would represent a saving of L2OO on the first cost for the upper site and about L 250 for the lower site, thus reducing the estimates to JL3OOO and L 2250 respectively. Miro lasts well in such places, and has to be renewed for wear only at the end of about seven years. As the planking would be laid lengthways of the bridge a good portion of the timber would last much longer, as tin wear at the sides would not be great. The estimates are based on the cost of similar work io Southland, such as the Oreti river bridges at Winton arid Hillend, and the Mataura bridges at the Island, Otama and Pyramid.
A sum sufficient to cover engineering and inspection during construction is also added. It is probable, however, that owing to the anxious inquiries that are made after this work, it will be keenly competed for and the work, let under the estimate.
Should there be any further information required I shall be most happy to supply it. — I have the honor to be, etc., C. H. Howobth, Assoc. M, Inst., C.E.