THE ROYAL ENGINEERS JOURNAL. Vol. XXIII. No. 2. FEBRUARY, 1916. CONTENTS. I'AlhK. 1. Replacement of Destroyed Masonry Arch wA.h Three-Hinged Timber-Braced Arch. By Capt. G. C. GOWLLAND, R.'1. (W'ith Photos and Plates) 49 2. Timber Girders. By Major R. H. CTN:UINNGTON, R.E. ... .. ... 6I 3. Sieges and the Defence of Fortiteed Places by the British and Indian Armies in the XIXth Century (contclt/dd). By Col. Sir EDWARD T. TIIACK.RAY, V.C., K.C.B., late R.E.: The Siege of Ladysmith ...... 63 4. Notice of Magazine:-R-tz,ue Ailitaire Suiss¢: Supplement, "The European War": The Contest for Hill 60- The Second Battle of Ypres-The British Retreat-Tlle Franco-Belgian Counter-Attack-The Bombardment of Dunkirk. By Major W. A. J. O'MEARA, C.M.G., P.s.r., late R.E. (Barrister-at-Law of the Inner Temple) ... --- -- * - ^ ^ INSTITUTION OF RE OFFICE COPY DO NOT REMOVE _ __ _____,___.___.___v- BULLIVANT & CO., Ltd., MAKERS OF STEEL WIRE ROPES FOR CRANES, LIFTS, HOISTS, WINDING and HAULING, Etc. 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A D VERT'ISEMENTS. SOLE AGENTS for the R.E. Institute Publications (Institute Members excepted): MACKAY & CO., LTD., CHATHAM. AD VER TISE 4IENTS. BURBERRY WEATHERPROOF WAR KIT ~.' " Invaluable for winter campaigning, as . it ensures warmth, comfort and protec- ^ ' tion, and mitigates the risk to health of fC;j'/iS^ , * exposure to bad weather." BURBERRY TRENCH-WARM- Supplies *it 5' ' ,i the services of three coats in one, each of which ¢;/' can be worn separately. A Weatherproof that will stand hours of rain; a light Camel Fleece Short-Warm; and a thick Overcoat for the ._ ; R .severestweather. 1 UNIFORMS OF TENACE WHIPCORD I jil ' A Burberry cloth of prodigious strength, made of WI .*. pure botany wool. Will outwear three uniforms I'':, made of ordinary Whipcord. il THE BURBERRY-W'it' or without Detach- abl e Fleece lining. Keeps its wearer dry on wet lil illil days; luxuriously warm on chilly; and is healthful IIlll and comfortable to wear in mild weather. 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TSIBER GIRDERS. By Major R. II. Cunnington, R.E ... ... ... .. 61 3. SIl(;ES AND TIIE DIFENCK OF FORTIFIE I) PLACES BY TIlE 11KITISII AND INDIAN ARMIIS IN T'HE XIXTII CENI1URY (cozcltldiei). ]By Col. Sil Edward T. Thackerny, v.c., I.C.B.. late R.E.: The Siege of Ladysmith ... ... ... ... ... ... ... 63 4. NO(rICE OF MAGAZINE :- Rnt ie Militiret Suisse : Supplement, " The European War ": The Contest for IIill 6o-The Second Battle of Ypres-The British Retreat---The Fianco-Belgian Counter- Attack-The Bomnardment of Dunkiil;. By Major W. A. J. O'Meara, C.M.G., p.s.c., late R.E. (Banristel-at-Law oi the Inner Temple) ... 7S .4Auhots alone are responsible /or /lestatempet/s mtade and the opinio,s erxpressed in theit papets. Replacement of Destroyed Masonry Arch 1916. 49 REPLACEIMENVT OF DESTROYED MIASOVRY ARCH WITH/ THREE-IIATGED TIIMBER-BRACED ARCII. By CAPT. G. C. GOWLVLAND, R.E. PRIOR to the advent of heavy motor transport, heavier mobile guns, etc., it was as a rule unnecessary, when making a road report, to go into much detail as to the actual strength of the road bridges. It was generally assumed that all existing bridges, including bridges on 2nd and 3 rd class roads, would safely bear any train of military vehicles. Bridges on Ist class roads may usually be assumed to be strong enough to carry all modern military transport, but this is not always the case with bridges on 2nd and 3rd class roads. In England nearly all the bridges on by-roads have notices fixed to them stating what are the greatest axle loads they are capable of carrying. These permissible weights often fall short of the requirements of the heavy train of a modern army. In reporting on roads, it may therefore often be necessary for R.E. officers to quickly decide what weights can be safely carried by existing road bridges. Steel girder bridges are of modern design and will usually be safe, but if their carrying capacity is doubtful they can be quickly measured up and their strength calculated. Reinforced concrete bridges being still more modern than the steel are still more likely to be safe. The difficulty arises when old masonry bridges are under consideration. Many of these bridges have been in existence for a long time aiid were designed at a period when a farm waggon or a stage coach was the heaviest vehicle on the road. An officer of the Corps may be called upon to decide what load any one of these old bridges can carry with safety. The quickest guide in making a decision of this kind is to use some empirical formula, such as is found in Trautwine's or other engineering pocket books. If a masonry arch and its abutments are safe according to the formula tle bridge can be passed as safe for all military traffic. These formulae are of course only a guide for use in the field. To get more accurate information the line of resistance in the arch ring under consideration must be found and must of course fall within certain limits. This sort of work however cannot be done by the roadside and hence the value of some formula which can quickly be applied to any segmental or elliptical arch. Besides deciding whether existing masonry bridges are safe for military traffic it may be necessary for the R.E. to quickly replace 50 THE ROYAL ENGINEERS JOURNAL. [FEBRUARY masonry arches that have been destroyed by the enemy. In cases of this sort empirical rules are not much use as it is necessary to find out what is going on in abutments, etc., when an arch ring has been demolished. Take as an example an ordinary brick arch road bridge. Consider the abutment G A F E (Fig. I) with the arch intact and no G A B W R' R2 FIG. I. rolling load on the bridge. The abutment is acted on by :-R the minimum arch thrust, i.e., the arch thrust due to the dead load, T the earth thrust, and W the weight of the abutment itself. These three forces have a reaction R, which should pass inside the centre third of the bed joist F E if the bridge is properly designed. Suppose that the arch A B C D is blown down. This means that R the minimum arch thrust is removed and that R2 the resultant of all the forces acting on the abutment wvill act nearer to E as indicated in Fig. 2, R, being quite near to E and passing outside the centre third of the bed joint F E. This means there is tension in the mortar in the left face of the abutment. The abutment will be shaken by the explosion and probably the mortar will not be very reliable when under tension. The result will be that the abutment will be on the point of collapse under the earth thrust only. Suppose that a girder bridge is erected in place of the demolished arch as indicated in Fig. 2. The abutment G A E F is still on the point of collapse, though it will be slightly assisted in holding up by the weight of the girder bridge, but it is quite possible that any rolling load advancing to cross the girder bridge may cause the abutment to collapse before the load reaches the bridge itself. With the rolling load in the position shown in Fig. 2 T the earth thrust is maximum while W the weight of the 19'6.] REPLACEMENT OF DESTROYED MASONRY ARCH. 51 Rol,Iinaq load A C XIIX[XIv ov o v v \ '"ss N N F F FIG. 2. CILVATION. ?.l.O IA ^* 80f 100 10IM 0 180it l0 »Th -'1, .