Special text: Unusual railways Special text: Underground railways

THEME :BRIDGE BUILDING

TEXT A. BRIDGE BUILDING
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(1) Streams and rivers have always been obstacles in the way of man's travel and people had to overcome these obstacles by erecting bridges, these remarkable examples of civil engineering.
(2) Nobody knows when the first bridge was built and when the idea of throwing a log over a stream came to a pre-historic man. Most probably a tree blown by the wind happened to fall across a stream and thus suggested the bridge idea accidentally. Sometimes the swift rivers washed away the soil and stone thus forming natural bridges. In tropical countries, lianas growing from one tree to another formed natural suspension bridges used by a traveller for getting across a river or a stream. One thing is quite evident today—primitive people used natural bridges thousands of years ago.
(3) Very often, however, these bridges did not provide needed strength. Then people began laying several logs together, making a wider and less dangerous path. Encour­aged by this success, they tried to bridge wider streams of the banks of the stream were high, they sometimes built suspension bridges of woven lianas, the latter being fast­ened to the tree at each side of the stream.
(4) Modern bridges are of several types. The simplest and the oldest type of bridge is the beam bridge, its pro­totype being a fallen tree or a log laid across a stream or a valley. Some of the beam bridges have a single span or beam while others have a lot of spans. The end of each span rests on concrete piers rising from the bed of the river, the end spans being laid on the river banks. Most of the larger bridges are complicated steel structures, many smaller ones being made of reinforced concrete.
(5) The most interesting type seems to be a suspension bridge. This bridge consists of two towers built on the opposite banks of a river or a valley and used as supports for cables. A roadway, often double-deck, is suspended from the cables by means of vertical connections.
(6) The first modern suspension bridge to carry railway tracks was designed and built by John Roebling, who later gained world-wide fame as the builder of New York's Brooklyn Bridge. In 1855, a most complicated task faced the builders of bridges. They were to span the 1,000-foot (300 m) wide Niagara river. The rapid waters prevented from building piers. Most men, including experienced builders, refused the job. Then a brilliant idea came to J. Roebling—why not to build a suspension bridge? Although engineers had known the principle of suspending a bridge long before,they did not believe such a bridge to be stable enough to carry trains. It was J. Roebling who was the first to prove the possibility of using suspension bridges for railway traffic Mftjie idea to apply a suspension bridge had not come to the famous builder, the rapid waters of the Niagara river and many other swift rivers would not have been bridged.
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(7) The early bridges were all designed empirically, from experience gained by previous failures and successes. The pioneers who used this empirical method made some guesses as to strength required and built accordingly. Should the structure collapse⁴ or shake dangerously, they ma3e the next one a little heavier .Cldt could withstand the force of the wind and carried its loads for a reasonable length of time, the builders felt safe in constructing a larger one with the same proportions. It was not until recent years that the strength of even small structures was possible to calculate.
(8) The history of bridge construction knows many examples when bridges were built without basic knowledge of mathematics or the force the wind can exert on the bridge. This often resulted in great tragedies, one of these being the collapse of the Tay Bridge in Scotland, the worst bridge accident since the advent of modern engineering.
(9) The bridge was designed by Thomas Bouch, one of the leading bridge builders, and opened in June, 1878. Being two miles long and having eighty-four spans, this bridge was spoken of as one of the wonders of the world. After carrying its loads for about two years the bridge suddenly collapsed on December 29, 1879. It happened on a stormy night when the wind reached the greatest force. The people who awaited the arrival of the evening train began to be nervous of what might happen to it in such a weather. Some of them even went to the end of the bridge. Soon they saw a lighted train crossing the bridge. A few moments later, a shower of fire was seen to fall into the river below. All aboard the train were lost, the exact number being difficult to estimate as some may have been carried to sea.
(10) When the matter was being investigated the experts found the designer not to have taken the force of the wind int6 consideration. Had the principles of aerodynamics been known to Thomas Bouch and other designers, the tragedies like these might not have happened at all.
(11) Nowadays bridge engineering is closely connected with other engineering sciences and applied mathematics. The advances in other branches of science have necessitated the advances in the science of bridge building.
(12) Bridges have played a great part in the development of railroading. In some countries, for example, many miles of railroad track do _rnot touch the ground for they are laid on bridge structures.i Were all the bridges in the USA placed end to end, they wotrhf stretch from New York to London and beyond. On the other hand, the development of the railways has influenced the art of bridge building necessitating the erection of stronger bridges and the replacement of the existing ones by the structures of increased strength and carrying capacity.
(1) Daryolar va daryolar har doim insonning sayohati yo'lida to'siq bo'lib kelgan va odamlar qurilish muhandisligining ajoyib namunalari bo'lgan ko'priklar qurish orqali bu to'siqlarni engib o'tishlari kerak edi.
(2) Birinchi ko'prik qachon qurilganini va daryoga yog'och tashlash fikri tarixdan oldingi odamga qachon kelganini hech kim bilmaydi. Ehtimol, shamol tomonidan urilgan daraxt daryo bo'ylab tushib ketgan va shuning uchun tasodifan ko'prik g'oyasini taklif qilgan. Ba'zan tez daryolar tuproq va toshlarni yuvib, tabiiy ko'priklarni hosil qilgan. Tropik mamlakatlarda bir daraxtdan ikkinchisiga o'sadigan lianalar sayohatchilar tomonidan daryo yoki oqimdan o'tish uchun ishlatiladigan tabiiy osma ko'priklarni hosil qilgan. Bugungi kunda bir narsa aniq - ibtidoiy odamlar ming yillar oldin tabiiy ko'priklardan foydalanganlar.
(3) Biroq, ko'pincha, bu ko'priklar kerakli kuchni ta'minlamadi. Keyin odamlar kengroq va kamroq xavfli yo'l qilib, bir nechta ignabargli yog'ochlarni birlashtira boshladilar. Ushbu muvaffaqiyatdan ruhlanib, ular daryoning baland qirg'oqlaridan kengroq oqimlarni ko'prik qilishga harakat qilishdi, ular ba'zan to'qilgan lianalardan osma ko'priklarni qurishdi, ikkinchisi esa daryoning har bir tomonidagi daraxtga mahkamlangan.
(4) Zamonaviy ko'priklar bir necha turdagi. Ko'prikning eng oddiy va eng qadimgi turi to'sinli ko'prik bo'lib, uning pro¬totipi qulagan daraxt yoki daryo yoki vodiy bo'ylab yotqizilgan logdir. Ba'zi to'sinli ko'priklar bitta oraliq yoki nurga ega, boshqalari esa juda ko'p. Har bir oraliqning oxiri daryo tubidan ko'tarilgan beton tayanchlarga tayanadi, oxiri daryo qirg'oqlariga yotqiziladi. Kattaroq ko'priklarning aksariyati murakkab po'lat konstruktsiyalar, ko'plari esa temir-betondan qilingan.
(5) Eng qiziqarli turi osma ko'prik kabi ko'rinadi. Bu ko'prik daryo yoki vodiyning qarama-qarshi qirg'og'ida qurilgan va kabellar uchun tayanch sifatida ishlatiladigan ikkita minoradan iborat. Yo'l yo'li, ko'pincha ikki qavatli, vertikal ulanishlar orqali kabellardan osilgan.
(6) Temir yo'llarni olib o'tish uchun birinchi zamonaviy osma ko'prik Jon Roebling tomonidan loyihalashtirilgan va qurilgan bo'lib, keyinchalik u Nyu-Yorkdagi Bruklin ko'prigi quruvchisi sifatida dunyo miqyosida shuhrat qozongan. 1855 yilda ko'prik quruvchilar oldida eng murakkab vazifa turardi. Ular kengligi 1000 fut (300 m) bo'lgan Niagara daryosini bosib o'tishlari kerak edi. Tez suvlar iskala qurishga xalaqit berdi. Aksariyat erkaklar, jumladan, tajribali quruvchilar ham ishdan bosh tortdilar. Shunda J. Roeblingga ajoyib g‘oya keldi — nega osma ko‘prik qurmaslik kerak? Muhandislar ko'prikni to'xtatib turish printsipini ancha oldin bilishgan bo'lsa-da, ular bunday ko'prik poezdlarni tashish uchun etarlicha barqaror bo'lishiga ishonishmagan. Aynan J. Roebling temir yo'l harakati uchun osma ko'priklardan foydalanish imkoniyatini birinchi bo'lib isbotlagan Mftjie osma ko'prikni qo'llash g'oyasi mashhur quruvchiga kelmagan, Niagara daryosining tez suvlari va boshqa ko'plab tez daryolar bo'lmagan. ko'prik qilingan.
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(7) Ilk ko'priklar oldingi muvaffaqiyatsizliklar va muvaffaqiyatlar natijasida olingan tajriba asosida empirik tarzda ishlab chiqilgan. Ushbu empirik usuldan foydalangan kashshoflar talab qilinadigan kuch haqida ba'zi taxminlar qildilar va shunga mos ravishda qurdilar. Agar struktura qulab tushsa yoki xavfli silkinsa, ular keyingisini biroz og'irlashtirsa. Cldt shamol kuchiga bardosh bera oladi va uning yuklarini ma'lum vaqt davomida ko'taradi, quruvchilar bir xil nisbatda kattaroq qurilishni xavfsiz his qilishgan. So'nggi yillargacha hatto kichik tuzilmalarning mustahkamligini hisoblash mumkin edi.
(8) Ko'prik qurilishi tarixi ko'p misollarni biladi, ko'priklar matematikadan boshlang'ich bilimsiz yoki shamol ko'prikka ta'sir qiladigan kuchsiz qurilgan. Bu ko'pincha katta fojialarga olib keldi, ulardan biri Shotlandiyadagi Tey ko'prigining qulashi, zamonaviy muhandislik paydo bo'lganidan beri eng yomon ko'prik avariyasi.
(9) Ko'prik yetakchi ko'prik quruvchilardan biri bo'lgan Tomas Bush tomonidan loyihalashtirilgan va 1878 yil iyun oyida ochilgan. Uzunligi ikki milya va sakson to'rt pog'onali bu ko'prik dunyo mo''jizalaridan biri sifatida tilga olingan. Taxminan ikki yil davomida o'z yukini ko'targandan so'ng, ko'prik 1879 yil 29 dekabrda to'satdan qulab tushdi. Bu shamol eng katta kuchga ega bo'lgan bo'ronli kechada sodir bo'ldi. Kechki poyezdning kelishini kutgan odamlar bunday ob-havoda uning taqdiri nima bo'lishidan asabiylasha boshladilar. Ulardan ba'zilari hatto ko'prikning oxirigacha borishdi. Ko'p o'tmay ular ko'prikdan o'tib ketayotgan nurli poezdni ko'rdilar. Bir necha daqiqadan so'ng pastdagi daryoga olovli yomg'ir yog'ayotgani ko'rindi. Poyezddagilarning hammasi yo‘qolgan, ularning aniq sonini aytish qiyin, chunki ba’zilari dengizga olib ketilgan bo‘lishi mumkin.
(10) Vaziyat o'rganilayotganda, mutaxassislar dizayner shamolning int6 kuchini hisobga olmaganligini aniqladilar. Agar aerodinamika tamoyillari Tomas Buch va boshqa dizaynerlarga ma'lum bo'lganida, bunday fojialar umuman sodir bo'lmasligi mumkin edi.
(11) Hozirgi vaqtda ko'prik muhandisligi boshqa muhandislik fanlari va amaliy matematika bilan chambarchas bog'liq. Ilm-fanning boshqa sohalaridagi yutuqlar ko‘prik qurish ilmining rivojlanishini taqozo etdi.
(12) Ko'priklar temir yo'lning rivojlanishida katta rol o'ynadi. Ba'zi mamlakatlarda, masalan, ko'p milya temir yo'l erga tegmaydi, chunki ular ko'prik tuzilmalariga yotqizilgan. AQShdagi barcha ko'priklar uchi uchi joylashgan bo'lsa, ular Nyu-Yorkdan Londonga va undan tashqariga cho'zilgan. Boshqa tomondan, temir yo'llarning rivojlanishi ko'prik qurish san'atiga ta'sir ko'rsatdi, bu esa kuchliroq ko'priklar qurish va mavjudlarini mustahkamligi va yuk ko'tarish qobiliyati yuqori bo'lgan inshootlar bilan almashtirishni talab qildi.