tHlllHWll l I L UlHlHIHNlHNIHI 144 518 ”THS SHORT SPAN SUSPENSION BRIDGES Highway Loads H-IS Thesis {cr'theDegrec of C. E. f ' Eric Edmund Bottoms. 1935‘ ____/"-— ‘ ,r qur/1717‘33d6 Y0 - S~nflv I " -..".‘ .t‘l‘-..‘lu’ . 1.10:. ‘ I. (til. I...\ .l... N. «nil! I c . . , . . .. .. .| Iran} .4...» vii. flu... afi.d3J£..J....Mm41u....l.zl ”dill“. 44$1§9wgfi1gqsflll¥ibpilflifi§filflfi‘uw 5.331%“;1‘ .. Mu... It 1.!"n i' Ilvl'IiIr \p‘lrl .9. SHOUT" SPAN SURPEIISTON BIT/(‘37?) (Hiifh'iYRV Load 3 H-15 ) TIESIS FOP. DZGZ’ZE OF C.E. “uric 3dr. 1d Bottoms V'" THES‘S sans-n am N assert-33107.: BHDCTEF: (airway Loads 11-15) INTRODUCTION OR PEEFACE The main reason for choosin: this subject mav be traced back to certain advice which Tr. J. A. L. uaddell so generously bestowed to the young and aseiriny en ineers in his _Hagnus Ouus. "Bridge Engineering" and its comeanion volume. "Vconomics of Bridge hork". His writinps have been the inepir- ation for this attempt to obtain a means of estimating accurately and quickly the quantities of material in superstructures, sub— structures and approaches of short span susnension bridpes de- signed for light highway loadinas. It is believed that a lifiht himhwey suspension bridge is a most satisfactory way of connecting farms, estates. factories and communities that are just across the river from the main highway. A suspension bridge is of scenic beauty in any locality. It is quickly erected: the comnarativelv lirht weight of material used, making heavy expensive equipment unnecessary. Eric E. Bottoms Chicago. Illinois June. 1955 _2- 98887 SHORT SPAN SUSPENQION BRIDG7S m “’m‘- — The original design of the suspension bridge is one of the simplest. our ancestors stretched ropes or chains across a river or ravine. and laid a floor directly upon them. From that time to the present. there has been a gradual increase in the development and design of the suspension bridge. In recent years there has been a noticeable increase in number of short span suspension bridges built. Although few have been built. they have shown. in many cases. new and uncommon expedients. This trend is important as designers are more apt to in- corporate new and novel ideas in small or less costly bridges. than in more eXpensive ones, thus. the art of suspension bridge design will be accelerated. zany engineers would propose a suSpension bridge in place of a simple span if they could, without complete detail design,compare the rel- ative costs. This paper intends to show a method and means to secure the quantities of materials for all parts of short span suspension bridges by an easy and accurate method. Unit prices are purposely o- mitted due to the variation in cost of material and erection in the various parts of the country. Let us look back in history and trace the development of the sus- pension bridge. Ihe first bridge engineers were our arboreal an- cestors tho formed living chains of their own bodies from tree to tree and bank to bank over which weaker members of their tribes passed. Vatives of trOpical jungle countries often times fashioned crude suspension bridges by using twisted vines and matted fibers. Xerxes is credited with building a small suspension bridge in cross- ing the Kellespont during his invasion of Greece in 480 B. C. It is said to have consisted of ropes on which beams were laid transversely and suspended between ships. All early suspension bridges were made of planks laid directly on chains. The first of this type was built in China. 65 A. D. Although the suspension bridge is the oldest and most picturesque type of all bridges. and after completion. is the safest structure because of its simplicity5known to bridge engineers, the modern type is an American invention. and its greatest development has been in this country. James Finlay built the first regular modern suspension bridge in 1796, and obtained patents on this type of bridge from the United States Covernment. Hand forged chains were used on all Finlay's bridFEs, the largest of which was the bridge across the Schuyskill River. built in Ihiladelphia in 1809. The total span of 306 feet was made up of two individual spans of 153 feet each with an intermediate pier. This bridge collapsed in 1811 from the weight of an excessive load of cattle. It was replaced with another bridge, which also col- lapsed,under a load of snow and ice. A third, a foot bridge, was Opened in June. 1816. with a Span of 408 feet and a width of 18 inches; the cables were composed of six three-eighths inch wires. and a wooden floor without stiffening. -his also failed under a load of snow and -3... ice. It was, however, the first wire suspension bridge in any country. The most famous of the old chain briljes desi~ncd under the Tinlty patent was the one built in 171C across the Yerrinac fiver. three miles from fie burysnort, 7assachusetts. It had a snan of 244 feet, and was 40 feet above the water. it had two roadways fifteen feet wide and “was strong enouxh to allow for the passage of horses and carriages. Whatever their speed. "he railing W's stout and STTOHT, which contributed much to the stiffness of the floor". It is of in- terest to note that in l 09. a hundred years later. the original chains were replaced with parallel wire cables conforming with the present day practice. The balance of the bridge was also replaced without chanying the appearance of the structure, so it would re- tain its original outline. Lhe Kenai Strait Bridge at Banror. Horth Hales. between the islands of_Anglesey and fiarnarvonshire was built in 1826 by 3r. Thomas Telford and is still in use today. It consists of a central span 580 feet lonr. a side span of 280 feet. four fifty-foot stone arches at one end and three at the other. a total strength of 1.710 feet. The floor. 50 feet in width, contains two roadways and a four foot walk. is supported by sixteen main cables arranged in four sets ver- tically above one another. one set at each side of each roadway. The masonry towers are 152 feet high and 29 feet thick at the level of the roadway. Erch chain consists of five iron bars, 5 1/4 inches by 1 inch. 10 feet long. united by 8x16 inch links and 3 inch bins. When Telford planned this bridge. he investigated the major forces by means of models. It was unquestionably a remarkable accomplish- ment in bridge enrineering, not only because of its unprecedented Span. but because of many other unusual features, its splendid conception and its ingenious erection. Von Kites built a chain bridge over the snubs Canal with a span of 354 feet in 1828. .sbles were flat bars of open hearth steel. Shis was the first use of steel for bridge building in any country. I. Vicat in 1%31 Wove the first cables in place during the erection of a suspension bridge across the Rhone. Irevious to this, wire cables were woven on the {round and then lifted. In 1860. the Von Kites bridge was taken down and replaced with another designed by Schnirch having a span of 225 feet. It was noted for being the first. and at that time. the only railroad suspension bridge in Eurone. John A. Poebling started on his lons career of bridge building with the P ttsburch Anueduct in 1844. This was composed of seven indiv- idual spans, each 182 feet long and supported by two seven-inch cables. Each cable was composed of seven stra.ds made up of £36 Wires 0.148 inch diameter. totaling 1,652 wires in each cable. -4- A combined railwav and “I"hwu" brid {vs built by ocrl~v~ rare": t‘ Piapara River in 1874 , I.’hich had been the“ght en impossiblwé rat, was the first long Span, o 1 feet, to he cozierueted with stiffez~an trusses insuring a rL:id Iloor. Ctiffenin" trusses had h:en :sed a few years before on a small suspension bridge over the KenTU‘TV River at Frankfort, Kentucky, but its Sps.n no.3 only :00 feet. .he stiff- ening truss has develOped because of Lhe e) wee ve vibration on the floor and the possible dazirer 0: being overL urned by heavy win” . Roeblins introduced the aerial epixining process on the Zingera River Bridge. It consisted of pulling: loop after loop of single tire across the river, from anchorage to anchorage over the towers by means of a travelling wheel. :he wires were laid up in strands, which unon com- pletion. were compacted tor ether into a cylindrical cable and ti5htlv wrapped with wire. ;his method has been used on a l lar; sas;e; sion bridges erected since. Another bridge was built across the Iia{m are Fiver n short distezce below the Falls in 18C? by Samuel Keefer of Ottawa. It hed a main scan of l, 260 feet. The cables were supported on wooden t overs. lbs famous BTOOKIVH Brid~e bu 8‘3 was made possible by drM l : .n steel wire. L11 previous srsp . bridge cables were fabricsfe 1 From 0 \ er (0 E- .0 wire drawn Irom charcoal irozt, and f r the first time on say hrirge, the cables \.ere protect:d with a 1e30f galvanized wire. In 192:3, the Car. den-:hiladelphie Prinflze :93 completed. It had a center span of 1,750 feet. th e lon; est at that time. “he sifie st.ans are each 716 feet long. Its 13.E foot xiidth is taken ap by a W feet roadnay. 6 lanes, and 4 tracks I'oz rapid trarsit trai as. There are also two ter- foot tide elevated foot 31:3. Shi v.hcle rcea.‘y is su_pnorted by two wire cables eech thirtyi I hes in diazneter on etc 61 toz rs Zoe feet shove the water. The Georse Washin ton Bridge across the Hudson Iiver at Fort Lee, amain span of u,500 feet, was com leted in 1931, forty-eight years 19% r :fter the °°mP19t10n 0f the Brook l"n Bridge. It represents more th‘ hundred-per—cent increase in main gp‘» n Jennth or: both the Brecklyn ans Camden-Philadelphia Bridges. The accompanying graph plate Io. l mry be of interest as it increase of main span lengths from 1813 when the Schuxe ill arii built by Finlay up to and including th Golden-Cr te Brid e scheduled for completion early in 1958. “his graph clearly show the Cincinnati Bridge over the Ohio iiver was the forerunner of t 4 he Brooklvn Brid;e, and that the Brooklyn Brid,~e hell the record re ‘1 its span was excedded by five feet When the illiamsburr Bri 're ans built in 1903, taenty yea rs la ter. he line COPlectlnf the peaks is interveniny. In the lower right hand corner of flats No. 1 are several modern short Span highway bridges which indicate the revived interest in SIIpensioa bridges for short Spams. “hey will be discussed in turn. The ?;ondout Creek Bridge at Kins ton, New York, was onencd to the traffic in 19?2. It was the first of the so-cellrd “odern Vhort Span Susnension Bridges" with a main span of 705 feet. The roed my is supported by two nine inch csbles each rpde up of seven strcncs, each composed of 3 8? galvanized wires C. 133 inches in diameter. hue to the steepness of the side snnn cables, it was necessrry to rdd extra strands from the tower to the anchornre to take un the lsrfer load. The strength of the main span.cablcs is 6,330 tons each and for the side Spans, 6,803 tons. It is clear that the construction of tr is brid -3e incited the in- terest of many engineers and highway departmznts because five years later the General Ulvsses S. Grant Bridge was bui t st Iortsnouth. Ohio. to be followed by seven or more spans under 1, C? feet in the next seven years. The General U. S. (Irant Bri‘ e onened in 19?? has a nt“in span of 700 feet. The designers celc l:t:d the stresses 9nd sections bv tie common elastic theory, and then Pssnmed 9 reduction of ten percent for deflection correction, because hev lr ked en ocean te th or? for exact anelvsis of continore srzns. ‘hev believed a {Teeter re- duction w rld be justified but thcv hsd no ray of determinins the preper amount. Had Tr. te1n.en'° theories been Pvailtble at thst time, a reduction of eighteen p r-cent vould hrve been availnblt. The Grand Vere Bridge. -nebec. was onened in 1929. 't that time it was the longest structure, 949 foot span,of its type. 7he cs ropes were out and socketed in the shop to erect dimensions. 'h roadway is eighteen feet V do and Cesitned for 3-13 lO'dinf. or 60 lbs. per sounre foot on the road and 12 lbs. t.r sgusre foot on the sidewalks, which corresponds to the f-EO lendinr of the Cans.dien Enrineering Standards Association. ”ind loads of 35 and 25 lbs. aere used on the unloaded ani loaded portions of the Gran. The cables were designed for a dead 1026 of 7,130 lbs. and a live load of 1,152 lbs. per linear foot of bridge. ho Lain span is nrde up of 39 panels, L4 feet.4 inches each. The backstavs, unloaded, are straight. ble ‘3 ) The "aumee “iver Bridge, “oledo. Ohio, built in 1?:9-71 hrs five traffic lanes end two side Plks. The main Span is 785 feet. ghe roadway is supported by two 13 llé inch (before wrnnpinr) cetles composed of 19,3 inch strands, The individurl fires are .155 inch in diameter. The strands are made up of sinrle wires laid in 9 nor- allel formation and the strands are laid parallel in the csblcs. The towers are 220 feet hi:h. The suspenders are 1 e/a inch in diameter. The San Rafael Bridge. completed in 1933 over the ‘io o'ue dcl ort near Nae. in San Dominro. has a mein even of ‘50 fret. . he- too lanes nine feet wide 11-,sned for V-lS loadir s, This bridre is noticeable for three specirl features:--- (1) The floor is o: intcr~ lockinr steel channels, (2) @he main cables are prestressed parallel strands of open type construction, (3) The main saddles are built into the tower thereby placing the centerline of the cables at the intersection of the main tower members and eliminating the usual eccentric wind loadings at the top of the towers. Chis method brinrs r11 resultant forces to a common point, penzittinr a clean cut tover tOp devoid of artificialities. The towers are of the fixed flexible type, 57 feet, 11 inches high. “he seiffenine truss, nodified warren, with a 1 to 75 ratio. has 30 panels, each 14 feet, 11 3/4 inches lonr. The San Domingo fienublic completed another short span suspension bridre in 1934. fhis bridge, which spans the Hiruano Ziver near San Pedro de incoris. is called the ianfis Bridre. The central Span is 554 feet lon‘-and the side snens ere 196 feet lone. The design of this bridge is also for H-lE loadine. ”he same three features in- cernorated in the Len Refeel Bridre are used. Test of the suspenders are of prestressed cableo ‘he main cables are made up of nine strands in open construction, This open type of construction imparts to the uninitiated, added strength. The Civil Conser°ation Corps in the state of California in 1934 con- structed hit: their own personn 1, two short Span suSpension bridges, one across the Kalemath at Happy Cnmn of 300 feet, the other across the Sacramento Fiver at Sins of 130 feet. Both have stiffening trusses, modified Warren type, and are designed accerdin: to the Govern :nt 15 ton leading, which provides for a string of 15 ton trucks following each other at reasonable intervals. “he stares of deveIOpment of the suspension bridge is clearly divided into four parts: (1) construction, (2) analysis, (3) economics, and (4) esthetics. In early cons: iction the bridge mas composed of a li ht platforn, unstiffened, suspended from a cable made up of chains, eye-bars, sinrle wires, Hire renes, and Spun strands, the loads passinp dir- ectly from the floor into the cable. ”hen the bridges constructed in this fashion became unsatisfactory for concentrated loads. the builders constructed heavy railinss, the forerunner of the stiffeniny truss. Later the roadway vas made up of a series of simple spans, panel length, Which were supported by means of suspenders to the main cable. Is the science of bridge build'ng developed and the designers exnanded their knowledge of analysis, the simple trusses were made continous throu3h the supporting points and the stiffening trusses extended from one tower to the other. Tlastic and deflection theories Mere exnounded with the trend towards the deflection theory and designing of the bridbes by exact methods eliminating all inieterminate stresses. conomical bridge construction is made up of three parts: (1) adequate strength and capacity (correct designing). (2) durability and minimum cost of maintenance, (3) flexibility and movability. these three points stablish the usefulness and the value of the bridge, and not its cost of construction - the cost of a bridge and the value have no relation to each other. Edequate strensth 9 nd capacitv of a bridge are merely the result of correct desisninj and construction. ‘xnerience has demonstrated the need for designing the modern brid; to include provisions for the future increase of its strength and ecapacity. These provisions can be made with a small increase in the first cost. if we knew the future demands of t’ansportation, such preca tions would not be necessary. Increased traffic in nvelves ‘.ei ht and velum hurabilitv and minimum cost of mrintenanee of a bridge depends en- tirelJ qun that type of construction and kinds of mat -rirls in- corporat ed into the structure and method of protection. ides ptability do: 3 not neces srrily apply to a suspension bridée for few he ve been taken apart and reconstructed elsthere, but if we desirn ‘ith true sooner" this must be considered. his is one ad« vantage that the Open T"pc of cable construction, as used in the San Rafael Bridge, has over those Hltl cables spun and compacted in place, as were the cables in the _aumee Bridre. On the fourth or esthetic stare, bridge engineers have bareLy entered. 'he rrofcssion- eblijntion of making bridqes structures of beauty is beinr realized more and more. Lhe follOVing'factors are l. he back “round is taken into consideration. Tor an exalee, the foners of St. Johr's Bridre in Oreron are desirncd to harmonic Lith t.e background of hills covered with evsrcreen trees. ?. “hes esi.n ad o1ted is a unit: tie parts of \hich must be\in proportion. 3. Thrhasis should be placed upon the function of the structure, and it should be ornanented only with that in view. Put in snother view, this step on the counoeition is intended to dramatize the import nt features of the des ién. he best appearinr brid ;e is al'eys the one in which the curl ineerinr solution is correct, vith the architectural treatment 3 rvin; rerely to emphasize the important features of desian. Any desirn of true beauty must neceSearily originate with the enrineer, leaving the arch itect only in the role of collaborator. contributing to the attractiveness of the enmineer's creation. Bridges have tto functions: To satisfy the demands of traffic, and to have an acceptable appearance. The engineer who fails to design for these two requisites does not perform a complete service. Before discussing the design or method used in computing the various - a - curves that appear later, it is well to notice the theories ad- vocated by the engineers at the top of the profession. Dr. J. A. L.‘ e.ddell states in his "Tcenonies of Bridte cr‘:”. "Ehe theorv of stre s detenzination sdopted has the approxinrte method given in 0hr son, Brvs n and urnesure's ' odrrn 7rsecd Structures'. Part 2, instead of the elder methoi of 'r. ”illivm H. Burr, which, for convenience and simplicity W;s taken ‘9 standard by the author in ”ritin; ‘hsnter L? of 'BIidge -ngincer- ing'". The results of the t o theories do not differ C?€3 l? sepecially for bridges with end trusses anchored, but the 1 theory requires a little less metal. foebling carefullv Worked out stresses for the ~3roo‘:l;rz» Erii.r from facts thst he himself had observed. Yis first s1 pension structure, the Fittsburgh heusduct mes Hithcut precedent, but because he we as confident that his figures were rifiht, he nillin 13 took the risk._Since thnt time, many theories hrve been expounded. Professor J. helan in his ”Eiserne Boqenbrflcken und 3: we e~r Chen” devised a deflection theory, which is applicable to desi‘ns having either continous or hir ged trusses. It yields lover 3021c ts and shears and consequentlv savings up to 65 per—cent. in the weirht of the metal of the s iffenin; truss. -his theory is applintile to continous and multiple Spsn cesisn, as tell as to the Slxrle area, two—hinsed tyne. Cteinmr.n's “encrs.l cc) ltsion is that t‘ type of suspension briige of-ers advent fies ovcr the t U_ for Spans under 1.000 feet when designed for hihhwry lo'iinxr, and longer Spans when designed for railroad loadinyr. Johnson. Bryan and Turneaure' s for ulae are based on ‘elwn's theories. Yheir exact method tnres into cons1idcration ieflection, which, as stated above, cuts down the moment and hence the co :. The theory later used in conputins various mirves is that of {onus Bryan and Turnesure's: the type of tridre is the two—sin ed stif“eniir truss with unloaded buckstsys. he live 10? ds used are in accord with the A. P. F. A. Steeificftion for highway bridges (Plate Lo. 3 . All the Esta o~t°1n11 * ends to show that it is not necessary to consider the actual dietri but ion of live loads to the cable “hen uniform loads are used. - - Jhe unit weights t9.ken for verious deed 10: 6.3 are as follows: - - - . Creosoted lumber 4.5 to 5 lbs. per borrd foot. 1 P. Hardwood ( 09h) 4.25 ” 3. Yellow Pine 5.75 " " " " 4. 50ft "food 2.73 H n n n (Shite Pine) 5. Concrete 15c. " " cubic " (Regular) 6. Concrete 110. lbs. ncr cubia feet (Hryiite) 7. Ashnhalt 1°C. " " suuere " pavement - 2 inches thick 8. Steel 490. lbs. n0? cubic " 9. Earth 100. " " " " 10. Snow (Compacted) 50. " " " " Various trees of cables have been used, described as fellows:-- 1. Cables made up of a number of individual wires all laying parallel in the finished cable. 2. Those made up of a number of twisted wire strands, the strands laying parallel in the finished csblc. 3. Those made up of a number of vire rcpes, the roues aging Parallel in the finished cable. They differ in these unites-- 1. Eire (Individual) 2. Strands (sires twisted into a rone) 3. Bones (Ftrends twisted into 3 reps) Then 3 y be more simply named:-— 1. Parallel wire cables (usually spun in flace) 2. Parallel strand cables. 3. Parallel rcpe cables. Twisted strands or rcpe strands should be distinguished from wire roses° A rope is made up by twisting a number of rope strands around a central strand, a substantial loss in the masnitude of "3" (modulus of elssticity) results from this additional onerotion. The cables used on the Tan Eafsel Bridge are nrestresscd, Darcllel strand, using oyen type of cozstruction. fhe onen construction Hives an impression of a larger cable (about 80*) lfirger. end in- creases the anpesrence. The individurl strands ere fer enough apcrt to permit ready inspection at all times, end to “110w psinhiné. trapping costs are saved. 7he strands are in lovers in the saddles. With zinc fillers separating the layers. ith tnis ar'enrenent strand bearing pressures are neclireble, for no strend bears on the dile I ss with the pressure other than its own, whereas. in a closed crble. the lower strands are subject to a comyreseive load from all upper Strands. Nearly all suspension bridges have followed the precedent set by the Brooklyn Bridge in using parallel wire cables. with wires of about 0.2 square inch cross section. Although the diameter of Wire used his remained almost constant, the quality of wire has steadily infroved. - 10 _ as shown bv he folloxins tmbilation giving the ultimate 3 ran th of tires zsed in repress l Msti e bridges. Bridae Tons per square inch. I'Tiar'flra o o o e o o o o o o o o o o o 58 Brvoklin’l. o o o a o o o o o o o o o o 80 1 11184713131175. 0 o o o o o o o o o o o 90 "(Lullattfln . o a o o o o o o o o o o o 96 Delaware. 0 o o o o o o o o o o o o o 100 George lashin ton . . . . . . . . . . 104 GOIden Gate 0 o o o o o o o o I o o O 104 San Rafe-€10 o o o o o o o o o o o o O 11-2 r:his comparision shows the econoz.‘" of prestressing bricige cables. (The San -afael Bridge cables were prestressed, as neted above). On the same bridges, the stiffening trusses have been gradual reduced in depth and weight. A tabulation of this 1:111 be :iv;n later. ”be time needed for Spinning the parallel wire cables has decreased in the past years. Brooklyn Brid re, 1835, 3,600 tons in 21 rionths Lanhattan Bridge. 1909. 6,400 " " 4 " George Iashington 1931, 28,100 " " lO " fridge , Th "ei"ht freely suSpended between towers has increased according to the Span length. the time of erection has also varied. tear 11 St aits Bridg e, 650 tons Breezlyn Bridre, 8,120 " Georre asuinfi Ion Bridge, 68, 30 " The George “ashinrton Bridge was constructed in one-third the time that it took to construct the Brooklyn Bridge. "he Brooklyn Bridge took twice as long as the flenai Straits Bridge to construct. For the St. Johns Bridge in Portland. Oregon. the specifications called for twisted strand cables, as an alternative to the parallel wire cable design. is received. the bids showed a savings of $42,000 by adoptin: the twisted strand design. “ereover. this pr0posal announced an expected savin“ of two months in the time of co. pletion of the bridge, this time beine n.de possible by diSpensina with the construction of foot br idjes, and by shortening the time required for cable strincin_ -‘he diameter of these cables was 16 1/2 inch. Port Oxford Cedar Les used for fillers. The wrapping wire yes No. 9 soft annec led double reliviizcd xire. Er. .addell states in his 'Bconomics of Bridge .ork". p? :6 M6 "The selection of the versed sine for the cables is a matter of economic importance. Increasing it reduces the sectional area of the cables and _ 11 - backstays, but aucments slishtly their lenrths: it adds to the heiL‘ ht anl ‘eight of toxer columns and their bracinr. On tie other hand. it affec: s a lejht sc Vin-s in mas s a 5 cost of anchorages due to the reduction of over 1rnin; moricnt that is caused by the dial nurtion of stress in the oactstsys. ‘Iperience has shown that the depth of catenai y equzl to one-ninth of the span will usually give the rost satisfa.ctor5 res :lts: but there is no hard and fa st rule about this, and it is p: rsissable to use any depth between the limits of on—eighth and one-tenth of the span". However, for all practiable purposes, when the loads are to be con- sidered uniform, the main cable may be taken as a parabola. The length of the cable between tower saddles may be computed b.:-- L' - 1 ¥ 8d a.“ 31 The back stays hsving no say. L3 3 K see B. L - Cable lenrth between towers. 1 - Horizontal distance between towers. d - Sag in feet. f : T”Wt-10 0f 38.? - dill. L.l : Cable length for b ckstays. K : Horizo- trl distance anchoraee to tower. B - antle of backstsy rith horiz onta There is a great Opportunity for study in the desirn of economic anchor- ages. There are three mein causes to be considered: - (1) Is the f un: ation to be on bed rock, (2) on piles, (S) clay or siwiliar butrri 1 without pilinr. If bed rock is close to tie surfs.ce. it will be wise to use it, but otherwise it will be rore ec onomic to rut in a shsllow anchorage either on pilinr or by spreadine the base. jhe main economic expedient in so fer as practiable, is to concentrate the weight at the rear of the azwchcr ‘e and the syread of the surface at the front. fihis tends to increase the resistin~ resent a tinst and to reduce the intensity of hearing at the toe. -Lis means that it is economic, there— fore. to make the anchorace low and narrow in front, aid high in the rear. It may be one of severrl such buttresses, insiwe d of one solid block of concrete. _hc toes Lny be joined to increase bearing value and the backs Fained by a mall to increase its feights. In reneral. th e economic choice is shaped more or less like a wedge. The towers of the Brcoklrn Brides are of massive masonry and because this was the first lsrge bridge, zany engineers imitated it closely to have their desi ns accepted. She towers of the George 'e.shin ton Bridge are designed to be covered vithu .tsonry at a lzter date, hon- ever. their present ru: ed beauty is due to the true desirn ithout a thought to beauty. It would be a she me if their honest des igns should -12 be hidden by a false front of cut stone. fhe towers of the fan Pafael Brides have fixed column type of towers of steel. 57 feet 11 inches hiph With a batter of 51 inches. hese towers are securely anchored to the main pier concrete. he stiff: ess f"ctcr may be described as follows:- A 734 1b. pull horizonu'lly ‘ill CIlect the column one inch, no vertical load considered. ”is det: Aination of the stiffness factor under varying loads is es sentis l in :rrivin: at unstressed lengths and preper adjustments of cable strcnd 3. Fr. Steinman's design of the :t. John's Bridge towers is different ex- pedient. ‘he towers have ver rticsl legs to carry the direct loads of the cable in con June ion U'ith better 1e 3 for bracing and stability. with either straight or better le 5 clone , it is difficult to secure a pleasing affect. In one ,2 se .hc tower appears heavy. the other, - it assumes an awkward angularity. The combination is most sstisfyin . Three towers are of the fixed flexibLe type. The most economical type of towers are those of the fixed flexible type with vertical columns and better legs With the saddles recessed into the column to pass all stresses through a common point. ncrete ~:ith1 r carried down to rock or with a Spread base supported 0. ile s. In sone crees, tith extremely short snans, it may be practical :o rely on spreadin' the base alone. The pier may either be of one solid mass, or. upon rolid substrate in two parts, one under each tower leg. .he most economical for small Spans is solid concrete with a Spread base upon piles. Local conditions are the deciding factor. The main pier substructure is conwonly of mass co The depth of the stiffening trusses hes gradually been reduced. in is- dication of the respective depth ratio is given by the following table:- Bridge Date Tatio Lilliamsburg . . . . . . . . 1903 . . . . . . . . Ehrmnttnn o o o o o o o o o 1909 o o o o o o o 0 Bear TTOUAI‘L-‘iln. . . . g . g g 1924 o o o o o o o o TGIP‘H‘T-I‘e £11751. 0 o o o o o o lgne o o o o o o o o finbessador . . . . . . . . . 19?8 . . . . . . . . Crand' Vere. o o o o o o o o Ian-9 o o o o o o o o ‘ f ' f “u 'P'?‘ 00000000 ‘2. 1 r S . 501111 0 o o o o o o o O o 19:21 a o o o o o o O 1.112,,8‘71 116 o g o o o o o o 0 Q 1 92.1 0 O O o 0 O O O ‘ «1' cf. Hi-‘HHHHr’b-JHH f George is shin,.on. . . . . . 19?l . . . a . . . . to l? * G'Oldencateooooooooolg'zloooooooo 1:01”) * This ratio obtains when the lover deck is built, vt im~c cnt tine there is no stiffening truss. The general tendency both in the United StrAes and abroad has been towards a rigid stiffening system, and the text books 5nd fi°R3 noderx treatises on snapension bridges had confined thenselvcs entirely Lo that system and to the electic thecrv, Lithout respect to Spin length, Tend weight of the bridge, or character of the traffic. The desi’ner of the ”cor“e Trshingtcn Enid e "The ner;mi sebility of en 91 est flc: cible s; pleted br ii (.2 0 fl — "J- . g \Q' Dr. --. .13. . 561.111.1911, Snips ‘e:1'1'£re sets of 'PU? con— b it 1 Y: - thr t i3. - uith rapid transit t1 sins_runnin~ over the - 13 - briife on the 10 er deck or an entirel" flexible system in case the briiye carries only vehicular trsizic on t :e utter finch, wts not obvious to the writer at the inception of his stuafl1cs. "fixtensive studies 0 nvinccd the HT ter thh for long span sus- pension bridges, a rigid system was not iccesssry. Te Was 2110 familiar with the feet that by the agrlicntion ofo :he corrrct or so-cslled ”deflection theory" as distinruish:df elastic theory" to a more or less flexible s stem, material economies can be affected. ' "“his is inherently due to the stiffenin: affect of the deed load. hich sff set is i~nored1 n the so-called ”elastic theory". 7he latter fact had been pointed out by various ”riters, notihl" Irof "elsn of "iennn. Maistris. it had also been rroved hr 'he an lie- I. Lu ation of a no iIied ceflection theory oy Leon :. .zosssiff to the design 01 the Lanhsttsn and - elswnre Fiver Bridges. "”he dead losd resists the live 10“d def or~w tion of tiie cs hie poly- gon and, becsuse of its great mat: zitude of :ers smile stii:‘e:1in, .3 1' EfLGCt . The foregoing discussion is agplicnble to lon Span nri es, how- ever. it shows that in general the tens ew1c c for short 9 pension bridgns to have a rigid stif: e-i ; 'ys ratio of dcyth of stizfeninrgtr1ss to the :u in 1,000 feet is l to 55. This ratio ill {wie an of metrl. The floor svstem on sue tension bridges is ro di:;cr‘1t from the son- ventionrl floor system consisting of wearing-surface, stringers, floor beams and lateral stiffeninr. 3n interestinr e2? erirunt i.rs tried on the Sen fisfzel :rii;e flooring consists of in. erlockin" steel c1m1nels. l cad alt r- netely ups end down s.nd covered n th s iinorsled arises asrhslt plank. it he a total ‘ei ht of 45 lbs. per square foot. way ares. 7he channels are lrid st riyht enrles To tle stri to which they are {lug relied. :his tyre of floor o:r.it mission of the usual dis onnl members of a wind s:steu. In a it is a plate girder vith the floor as the web. There are various tynes of arrroaohes: - fiie six is and economical tyne being the trestle and viedtct. If esthetics or locsl conditions demand. short snans: throujh or deck, supported by piers or rocker bents, with the truss the same depth of the stiffeninf trus s, c one next in economic cons iderstion. “he least econoric is .o susfiend the approaches from the beckstays because:- 1. Far greater weight of the tstrriel required for the stiffening trusses and hszgers as compared with that I r a trestle approach. , 2. Far greater cost of anchorages due to the large lever arm for the overturning moment, the cable pull being horizontal and applied near the elevation of the floor. ”he only case in.thich it is economic to susyend the anproach from the backsteys is Vhen there is deep water beneath that is required for navigation. If the water is deep and is not required for nav- igation. the economic choice is a series of deck spans. of as nearly as may be. economic lenrth. iconomic length being determined by the cost per linear foot of superstructure and substructure taken to- gather. ‘fter consideriné the aforementioned subjects and the judiciously application of formula and adhering to the best engineering practice. many computations have bzen made and the results plotted on plates 2 to 18 inclusive. Various designs which were selected as typical. and used as a basis for this work are shown on five plates, A to F inclusive. An attempt was made to select a typical situation from which to est- imate the quantities of materials necessary for a modern short span suspension bridge withan H-lS highway loading. Vsing this settins, a problem has been set up and worked out. ?otations are shown on the computation forms which tlate was consulted for that particular Dart or item. It is the writer's belief that short span suspension bridges will be utilized more and more in coming years and that an accurate system of preliminary estimating as presented here will be of great use to the average engineer who is not a specialist in suspension bridge design. THE PH“ - 15 - No. 5 Sheet No. W¢4Z Bridge over jaw". 8% fl SPECIFlCATION or Bmdgg \QZPB 0F brcdg e W cH’H of mag MLOACIING ”/5 C C trusses—133i n- C C towers 500% Vertical clearance 90M 1?; of foundation WM ”WW Approach spans git/91w: V7401] Approaches i W Remarks Mwi‘é/fiMW WWW MW ”jag/ea é WW 4% 30% PROBLEM ll‘em MW Sheet No. ’1' M log gm cablesless cables f3: lineal FOAL Qfmgm spend. ou de. lee Loodji’ldbi) 56X 10 / / 20 POUI’Icls lmPOCl‘ (PM‘Q v.9 X10 Go ., Concrel'e roadway slab§"0l¢") MM I 000 Melol in lloor sysle m.(P'°l“j/3'S% ’1 Z9 7 Melal In sliflenine frugséwcvlwxzxu 43 Average lenglb OF bangzr5.6"a4e?)mw Welglsl of cable bangers, “mar/Ia” 4 ~ Mlscelloneous' melal, MW .230 n “TOTAL LOAD PER F00T64) 25.5.4 " lofal ngd per Eggl on one ca bl_e_ One half Il'em (A) ’2‘ 77 pounds Assumed welgbl' per Fool cl l cable 72 u Halal weigbl on l cable /3 4? u Use a cable welq hflflS 72 pouncls PerFool. IO PROBLEM. ITEM 777m aw shes No.32; RTAL WEIGHT 9F MATERIAL Floor system 247 X °°° ‘ /921°°'pounds. SttlFenmg trusses 43”” 25:3”. Miscellaneous meldl 3°“ 6°” ”5"” .. 4 x (poo ~ 2,400 Hanger cable Main ca ble, including backslays From plal'e 9 "bxnxz. = /26,7za ConCrele 5’05 /000 X400 690,000 - TOTAL WEIGHT (B) 423,310 pounds Welql'l‘l' per lineal fOO‘l' 460362900 (CL /éao+ '. SP0” are /;Lo’ Type’dfdfl“, [/Hsj Structural metal . pounds. plole no. /f- /32,000 XL. Zélgua u Concrete slob Plate/1 /soox/2o’ ”9°" “ TOTAL WEIGHT (0) 446°” pounds. ITEMW ' Load on lows r: W W pounds. //;ao xeoo 471,000 .. lee loacl, maln span L; V8 load’ approach SP0 n 76 x10 X’Zo’ / 72,400 u lmPaC+’ main span 60 {$00 36,000 u 15-5- X10 XIzo' 5‘4“, .. l mpact, approach span Welgbl' mom span (3) ““3” " Weigbl approacl') span/W 444",” __,__,__ OTAL. WEIGHT ‘3’71’Upoumls. PROBLEM ITEM WWW Shaman Blol dlvlclecl by 2 For WEIgll‘l’ On one lower 1,310,120 +1 = glasswPOUNds. 5+eel per lineal Fool plale I5 4’0 u Talal mel'alln one l'ower 4”” mo 44’” u TOTAL FOR TWO TOWERS 5721“" ITEM 2W Weight on one l’ower: “5‘5“" ouncls. Welqlwl otone l‘ower: 44"” u TOTAL WEIGHT ON ONE PIER 4’77’5“ From lol'e lb read:- PJZS 4'5 e we res Cofferclom 3"- .. Conc rel‘e "5° cu.ycls. Welqbl per lineal Fogton cables lncluclfng cables (C) “’00 uncls. 4359900 Tension on onabomge (plale n) .. Concrete m one anchorage(plale I6) 32" cu.yc.ls. TOTAL concRsTE 32°” " 54o " “ PROBLEM. Main cable 0 “u"! 720‘ lbe Main span. ' ‘ Floor system. “37120" lbs Stillenlng l’russ. 253500 “36? Miscellaneous metal. ’5’000 lbs Hanger cable 2,400 lbs Concrete slab(l'layclil’e) £00,000 lbe Ap roach span. Elruclural rnel‘al 264°”le Concrete slab /8°’°°°lb.@ Tower sl’eel 523°00le Piers I Piles 405° Flo ‘ Cofferclam 8‘5— He Comer-5+3 I've/MW; [14' m/ /.5'0 CXQ m, TOTAL Percentage Forenglnee rung _—————____— TOTA L. COST PROBLEM. L| b n I v sv- " I ' ‘ v V ‘7 \' \vr/ \ “l 50’ ' ~- ’ead=300'——-l 60' l—— NATURAL SCALE l"=l00' HOR. SCALE ' w=roo'~o" O -h———- ': ' 1 6 I0@30 30° '1 VER. SCALE ‘00: 25:0» TYPlCAL LAYOUT Plale ’A‘ fl ‘ ....__. HANGER SPAC | NG _____ 5TRIN6fifi__.+.___STRI NGE LENGTH LENGT TYPE OF STlFFENlNG TRUSS ~ HQ’YGER Center to ocular +russn o l '16 u . L Wld‘l‘h OF roadway a." 1"::.b'." 1.v.v.' I .:u ‘xchu-I-vgv‘" - ‘- I':"l.":'w" . . It: U.. 0- U-i .‘Ir‘.-‘ 5‘" ' Olen"; .7": 95:4: 2‘ °':‘-'. 1>53 :‘.'-.*.'s='~."~':"5 "a ‘«I '3 $.95”??? ~ 3.. BIS-.5":"':O.I 6.91231? - .. - ....9“o '7 LSTRINGERS - FOR SPACING SEE PLATES3 854.- ADJUSTABLE HANGER couwscnon TYPlCAL DETAlLS Plate "5“ ADJUSTABLE HANGER CONNECTION BLE ANGER CABLE CLAMP ‘acc :V/vr v v \v ' ..> V II; ‘ 5', ‘ I “least (2.. §§§h$ ‘7‘. AV \.V :’ at least 22" TYPICAL CONCRETEL I CABLE ANCHGRAGE atleasflz” DETAILS Pla le "C“ TYPICAL TOWER 7 Fla +9.. D. Cihhr +0 [card-cg. ower legs *3" -—— l8‘ rcfi'kg i°L%‘§'3‘.°' —- PIER SUPPORTED ON 20 TON PILES. ONE FILE TO ,9 SQUARE FEET. SEE PLATEIé TYPICAL PIE R PIaI‘e "“E / II // HORIZONTAL COMPONENT OF ACTUAL CABLE TENSION RESISTED BY EARTH BEARING PRESSURE ON FRONT SURFACE OF ANCHORAGE - EARTH PRESSURE VALUE TAKEN AT 2000 POUNDS PER SQUARE FOOT. TOTAL WEIGHT OF ANCHORAGE EQUAL TWICE THE VERTICAL COMPONENT OF' CABL E TENSION. ANCHORAGE MAY 5g mvmsp AS ILLUSTRATED OR ONE MASS AS ASOWN BY DASHED LINE. Ceni‘cr +0 gel-fingr Gab IGS "~~~~ fl ‘3 :J ' 4» N\/ala/I/H\’AS / w///T;NCH0RAGE PIo‘I’e “F“ LENGTH OF MAIN SPAN 4500 4000 3500 2000 I500 I000 600 o I. \ ”TEA/o z b is j \ 9-«74 - . / $ / / s” a» d? /'////// 09.1: / k ,m 0, ,/ o 0 ° f ‘ O l/ 90 g I . g 1800 cf I830 I850 ‘I390 1920 ‘3 9 WAR ‘CONSTRUCTED o SUSPENSION SPIDERS PIoIe I. I950 90' 8 6’ 8 Ca 0 POUNDS PER SQUARE FOOT ROADWAY ”3 O \ \ M LE I00 300 LIVE sbo 700 900 SPAN H‘IS LOADING LOADS AND IMPACT PIqu 2. OI A // / (fl N SPACING OF STRINGERS OR FLOOR BEAMS IN FEET 0 \ O 20 4O 60 60 I00 PERCENT OFCONCENTRATED LOAD ' TO EACH STRINGER LOADS TO STRINGERS OR FLOOR BEAMS PIOI'Z 3. THICKNESS OF STRUCTURAL SLAB IN INCHES Go (1| 2 4. s 5 IO SPAN IN FEET H-IS LOADING CONCRETE SLAB ON STEEL JOISTS PIoI'e 4. POUNDS PER SQ. FT. BETWEEN TRusses. I6 I5 I3 fig 40"" g) 00 / / 20 ’ DISTANCE BETWEEN HANGERS 30 4O L—C'C HANGFRS .4 I {miffnins H‘IS Loadingjzn frossfofwss. STEEL IN FLOOR SYSTEM SIrimcr :Irtruer- PIQ‘I‘B 5. 51‘3““)?- bca J7 m WEIGHT IN POUND PER SQUARE FOOT 25 r [I 24 23 ‘15, / 22 089/ I5 20 25 30 35 40 DISTANCE BETWEEN CENTER LINE OF TRUSSES LATERAL AND SWAY BRACING INCLUDED. STEEL IN FLOOR SYSTEM. PIaI’e. 5A. INCHES IN DEPTH or BEAM IO cue Fd CK SECTI DR SHEAR SEISTION SAFE FROM SHEAR 5 IO MINIMUM SPAN I5 IN FEET 20 25 I AND GIRDER BEAMS LIMITING SPANS Made 6. POUN DS PERLINEAL FOOT PER TRUSS 80 7O 5O 4O 3O 20 I0 300 mm __, :90 700, so OWER TO TOWER 0 \ H ' ‘I5 Ioadmg'ZZ H has: Io Imss METAL IN SIIFFENING TPUSS PIOI’e Z 40 30 / / 'é‘ / / / L- LENGTH IN FEET m. / ao’o b.0337 1 *N when Io-JA Na Min. Ienqflv PO" 500 SPAN 709 900 AVERAGE LENGTH OF HANGERS PIaIe 5. 3600 1400 I I / I200 / 1.. I: u. Iooo / Z I I~ q; 900 z / In .I II .J 600 300 . soo rroo 900 L* I—n*ZL,. SPA N LM=1+8 2 31 LENGTH OF MAIN CABLES L,=(d"’+232)z Mafia 9. .IPQOH 4> 300 0. o. o. a a o z 5 I o o 4 3 fl 2 M 7 w I 0 OI 4 2 ’I20 100 .2. .0m 2. > 5 Z 6. 2 L75 w 3 0 Q00 v? ,w \5 v I5 3ch Au 3 -72 5. 5. A .2. .wm I wwwwz <1 “.0 3 > 2000 O O a W CONCRETE BRIDGE FLOORS pIaIe I2. ,7 yo 6. q 7, «c/ 00 In /. ,/ / / 2"” Q 5.0 V / //// O O 3 .4 00 350 O 5 2 0 O 2 O O WOZDOA. 7: FOOu Emu mmbmh MWQ 4314:). mOhImEg I00 I75 I50 SPAN 75 0 5 H'I5 LOADING METAL IN TRUSS PIaI'e I3. 'Iaqooo . 2 I6 000 q / E 8 \ \ Izqooo ’ / aqooo , / TOTAL WEIGHT OF STRUCTURAL METAL IN POUNDS .8 o 8 60,000 / / 40,000/ 50 75 I00 I25 I30 SPAN I-HSLMDING ‘ ZOFOOT ROADWAY) 6INCH SLAB. STRUCTURAL METAL IN BRIDGES DESIGNED IN ACCORD WITH PI + BUREAU or PUBLIC RdADs q 3 [4, I5- TON LOADING. 2609 I / I500 / I000 500 WEIGHT OF STEEL PER LINEAL FOOT OF TOWER IOOO ‘ 2900 3000 4000 5000 LOAD ON TOWER‘ THOUSANDS OF POUNDS. TOWER STEEL PIoI-e I5. .1 / , .4 z \ \ \/ <4; /\ 24o \ ”I “If” I . / CONCRETE PER PIER CUBIC YARDS Q) 0 \ 9 \ /// . \L \ 400 300 ' I2 00 I6 00 20 00 2400 SUPERIMPOSED LOAD ON PIER - THOUSANDS OF POUNDS. SEE PLATE ”E“ CONCRETE PIERS PM he I6. OI: pouwos, ON A NCHORAG a“. PULL 0 I R TENSIONIIN THOUSANDS 7000 6000 5000 4000 3000 2000 I000 9/ “00/ / I4 VD . 0%.?“ ’ / 140/ /R/ / / 300 500 ' 700 SPAN IN FEET. 900 PULL 0N ANCHORAGE PIaI'e I7. CUBIC? YARDS PER ANCHORAGE. I400 I 200 I 000 800 600 400 / - 200 0 I000 2000 3000 4000 5 000 6 000 TENSION IN THOUSANDS OF POUNDS CONCRETE IN ANCHORAGE PIaIe I5. III“ ‘1 4- Iii... HI «I! i1. III! I l I IIIIIIIII I I