A WAY OUT

Dominion, Volume 11, Issue 137, 27 February 1918, Page 5

A WAY OUT

• MASSED CONCRETE BRIDGES TO EVADE EXPENSIVE STEEL WORK RAILWAY WBRKS IN CENTRAL OTAGO How are our engineers faring willi steel, : if neb entirely linprnciiraoln, ai. a proliibitivo priced is ii question which may cross the mind of the thoughtful citizen in these times. A. very satisfying answer was given to the f|iiery at the annual conference of the iVcw'Zealand Society of Civil Wngiiieers per medium of a paper by Mr. C. J. M'Kenzie, M. lust. 0.K., on the concreto bridges wliicli have been constructed on the Otago Central railway lino since the beginning of 1916. AVlint •the discussion evolved was the expression of opinion that, after all _ reinforced concrete had not many, if any,

points over massed concrete, whilst at. the present time it was two and a hall times as expensive to use. "Originally," said the paper, "it was intended that these bridges should be of the built-up steel girder typo on concrete or timber -piers, but owing to the over-increasing price of steel and the. difficulty of obtaining supplies it was decided to construct them in massed concrete, and as this particular part of the country is subject to sudden heavy rainfalls, and the grades of the creeks are exceptionally steep, it was deemed inadvisable to erect piers in tho beds of the stream; the spans were therefore made long enough to cross ; the stream bed. "At the Leaning Rock Creek Bridge, the first erected, tho foundations on the Cromwell side were all on rock, but on the Clydo side, rock was not found within reasonable distance; tho foundation of- the main piers was therefore carried down through the gravel to wbut was considered a. safe distance below the bed of the creek. At the other bridges erected to date rock was encountered near the surface. Gravel for concrete work was obtained m a. pit alongside the railway, and in each instance the rails were laid to tho bridge site before commencing concreting.: The cost of- this item was fairly" low, An oil-driven concrete batch mixer of New Zealand manufacture was used, the output of which averaged 40 cubic yards per day, or five cubic yards per hour. At Leaning Rock Creek Bridge the cost of mixing and placing the concrete seemed fairly high, probably owing partly to the inexperience of the workmen with the plant. On the Sonora and Nine Mile Creek Uridses this item was reduced by nearly 50 per cent. - Two months after the concreting was completed a test load of 42 tons was applied on an area eight feet wide, extending from the crown of the second jSmull arch 12 feet towards the crown of tho main arch. Levels were taken before and after the application of the load, and a very slight deflection was noticeable at the crown of tho main arch only. ' The Design. As tho departure of building such structures in massed concrete is a new one in this country, the design was all-important. Details were supplied m a paper read by Mr. J. E. Cull. Of the' Leaning Rock Bridge he read:— "This bridge differs from al] the others in having secondary arches to transfer tho load on to the mam arch. Theso secondary arches havo a clear span of 13 feet and a rise of 4 feet, the main arch having a clear span of 50 feet with a rise «f 23 feet, this construction was more economical in concrete than spandrel walls, on account of the big riso iu ,tho arch, and the consequently great 'thickness at'the Base required for such' .-walls.- The .parapet.wall, with a length, of 94 feet, is divided into three parts, the joints being joggled and filled with two thicknesses of felt, to prevent cracking due to chango of temperature. The maiii arch is 2 feet 6 inches thick at the creVwn, and 4 feot thick at the haunches. The sequenco used in placing the concrete in the main aroli is shown on the figure by the numbers I, 2, 3, and 4. Tho centring was designed with heavy walings longitudinally to the span to resist tho tendency of the lower sections of tho concrete to close together before the key was placod. Nevertheless a small crack developed duo to this cause, at one of the haunches, and in future spans of this character hinges are being provided to admit of a slight closing in of the lower sections."

Mr. F. W. Furkort showed the manner in which the concrete used in the bridge was worked. All material at the bridgehead was moved by gravity from the road to the mixer, and from thence to the point of deposit. He also referred to a crack that developed in the main arch, and illustrated the ingenious method of repairs. Mr. R. W. Holmes (Engineer-in-Chief) referred to the claims of massed concrete work, and said that he was afraid that its just claims had been obscured, The-reinforced concrete idea, heing new, was rushed. As things Were at present, with the price of steel so extremely high, it might be of i-alue to members to readjust _ tboir ideas as to the- respective merits of massed and reinforced concrete. At the present -time-massed concrete could bo laid for 40s. per cubic yard, aiiereaa reinforpod concrete cost £5 per cubic yard; in other words, for every yard of reinforced concrete, 2V yards of massed concrete could be laid.' Mr. Holmes said there wiis a great advantage in having a continuous unobstructed track over bridges, and they had.been able to do this. The ballasted road was carried across the bridge, which was ontirely without any overdecking. Holes were- left here and there in the deck for any water to run free.

' Mr. J. M'Lcan, Chief Locomotive Engineer, said that in bridge construction work for railway purpoeos thero was a'continuous fight between the locomotive engineers and the civil engineers, as to provision in tho futuro lor additions to the loading. It was on record whore tho loading of a nar-row-gauge railway bad been increased as much as 25 per cent. From that it would he cloar that as tho load was increased their duty was to ascertain whether the bridges could stand tho extra strain, and if the margin'was notsufficient to strengthen them. Then in railway bridge-building they had to carefully consider for how far ahead in loading they were to provide for, and to consider to what extent the provision for requirements ahead was economically justifiable. It might be quite unjustifiable over a long term of years. The experience of the Railway Department showed that roughly the cost of their works were twothirds labour and one-third -material, «o that any measures that eliminated labour enormously reduced the cost of maintenance. For that reason the Department was trying to adopt permanent material, such as massed and reinforced concrete wherever possible. He could endorse what Mr. Holmes had said about the serviceability of massed concrete, and conceded that its use in many instances was a sound proposition. ' At present they wore trying experiments as between the old type of reinforced concrete against n type that was being used by the New York Rapid Transit Corpora lion, particularly in places where the roof or overhead snace was restricted. This was the embedding of the. ordinary steel joists in rich concrete. They had found that the strength of a ioisl. sn embedded hacl increase from 50 In 75 per cent.