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ترجمه مقاله بررسی پیرامون انواع پل ها و ساختارشان Bridges' Construction

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اصول

سه نوع اصلی از پلها موجودند:

پل تیری

پل قوسی

پل معلق

تفاوت عمده ی این سه پل در فاصله دهانه ی پل است. دهانه فاصله ای است بین پایه های ابتدایی و انتهایی پل اعم از اینكه آن ستون دیوارهای دره یا پل باشد. طول پل تیری مدرن امروزه از 200 پا (60متر) تجاوز نمی كند. در حالی كه یك پل قوسی مدرن به 800 تا 1000 پا (240 تا 300 متر) همو می رسد. پل معلق نیز تا 7000 پا طول دارد.

چه عاملی سبب می شود كه یك پل قوسی بتواند درازای بیشتری نسبت به پل تیری داشته باشد؟ و یا یك معلق بتواند تقریباً تا 7 برابر طول پل قوسی را داشته باشد. جواب این سوال زمانی بدست می آید كه بدانیم چگونه انواع پلها از دو نیروی مهم فشاری و كششی تاثیر می پذیرند.

نیروی فشارینیرویی است كه موجب فشرده شدن و یا كوتاه شدن چیزی كه بر روی آن عمل می كند می شود.

نیروی كششینیرویی است كه سبب افزایش طول و گسترش چیزی كه بر روی آن عمل می كند می گردد.

در این زمینه می توان از فنر به عنوان یك مثال ساده نام برد. زمانی كه آن را روی زمین فشار می دهیم و یا دو انتهای آن را به هم نزدیك می كنیم در واقع ما آن را را متراكم می سازیم. این نیروی تراكم یا فشاری موجب كوتاه شدن طول فنر می شود. و نیز اگر دو سر فنر را از یكدیگر دور سازیم نیروی كششی در فنر ایجادشده طولفنر را افزایش می دهد.

نیروی فشاری و كششی در همه پل ها وجود دارند و وظیفه طراح پل این است كه اجازه ندهد این نیروها موجب خمش و یا گسیختگی گردد. خمش زمانی اتفاق می افتد كه نیروی فشاری بر توانایی شئ در مقابله با فشردگی غلبه كند. بهترین روش در موقع رویارویی با این نیروها خنثی سازی پخش و یا انتقال آنهاست. پخش كردن نیرو یعنی گسترش دادن نیرو به منطقه وسیع تری است چنانكه هیچ تك نقطه مجبور به متحمل شدن بخش عمده ی نیروی متمركز نباشد. انتقال نیرو به معنی حركت نیرو از یك منطقه غیر مستحكم به منطقه مستحكم است ناحیه ای كه برای مقابله با نیرو طراحی شده و منظور گردیده است. یك پل قوسی مثال خوبی برای پراكندگی است حال آنكه پل معلق نمونه ای بارز از انتقال نیروست.

پلهای تیری

How Bridges Work

by Michael Morrissey

There's no doubt you've seen a bridge and it's almost as likely that you've traveled over

one. If you've ever laid a plank or log down over a stream to keep from getting wet you've even constructed a bridge. Bridges are truly ubiquitous -- a natural part of everyday life. A bridge provides passage over some sort of obstacle: a river a valley a road a set of railroad tracks...

In this article we will look at the three major types of bridges so that you can understand how each one works. The type of bridge used depends on various features of the obstacle. The main feature that controls the bridge type is the size of the obstacle. How far is it from one side to the other? This is a major factor in determining what type of bridge to use and by the time you are done reading this article you will understand why.

The Beam Bridge

A beam bridge is basically a rigid horizontal structure that is resting on two piers one at each end. The weight of the bridge and any traffic on it is directly supported by the piers. The weight is traveling directly downward.

Compression

The force of compression manifests itself on the top side of the beam bridge's deck (or roadway). This causes the upper portion of the deck to shorten.

Tension

The result of the compression on the upper portion of the deck causes tension in the lower portion of the deck. This tension causes the lower portion of the beam to lengthen.

Example

Take a two-by-four and place it on top of two empty milk crates -- you've just created a crude beam bridge. Now place a 50-pound weight in the middle of it. Notice how the two-by-four bends. The top side is under compression and the bottom side is under tension. If you keep adding weight eventually the two-by-four will break. Actually the top side will buckle and the bottom side will snap.

How Bridges Workby Michael MorrisseyThere's no doubt you've seen a bridge and it's almost as likely that you've traveled over

one. If you've ever laid a plank or log down over a stream to keep from getting wet you've even constructed a bridge. Bridges are truly ubiquitous -- a natural part of everyday life. A bridge provides passage over some sort of obstacle: a river a valley a road a set of railroad tracks...In this article we will look at the three major types of bridges so that you can understand how each one works. The type of bridge used depends on various features of the obstacle. The main feature that controls the bridge type is the size of the obstacle. How far is it from one side to the other? This is a major factor in determining what type of bridge to use and by the time you are done reading this article you will understand why.The Beam BridgeA beam bridge is basically a rigid horizontal structure that is resting on two piers one at each end. The weight of the bridge and any traffic on it is directly supported by the piers. The weight is traveling directly downward.CompressionThe force of compression manifests itself on the top side of the beam bridge's deck (or roadway). This causes the upper portion of the deck to shorten.TensionThe result of the compression on the upper portion of the deck causes tension in the lower portion of the deck. This tension causes the lower portion of the beam to lengthen.ExampleTake a two-by-four and place it on top of two empty milk crates -- you've just created a crude beam bridge. Now place a 50-pound weight in the middle of it. Notice how the two-by-four bends. The top side is under compression and the bottom side is under tension. If you keep adding weight eventually the two-by-four will break. Actually the top side will buckle and the bottom side will snap.

How Bridges Workby Michael MorrisseyThere's no doubt you've seen a bridge and it's almost as likely that you've traveled overone. If you've ever laid a plank or log down over a stream to keep from getting wet you've even constructed a bridge. Bridges are truly ubiquitous -- a natural part of everyday life. A bridge provides passage over some sort of obstacle: a river a valley a road a set of railroad tracks...In this article we will look at the three major types of bridges so that you can understand how each one works. The type of bridge used depends on various features of the obstacle. The main feature that controls the bridge type is the size of the obstacle. How far is it from one side to the other? This is a major factor in determining what type of bridge to use and by the time you are done reading this article you will understand why.The Beam BridgeA beam bridge is basically a rigid horizontal structure that is resting on two piers one at each end. The weight of the bridge and any traffic on it is directly supported by the piers. The weight is traveling directly downward.CompressionThe force of compression manifests itself on the top side of the beam bridge's deck (or roadway). This causes the upper portion of the deck to shorten.TensionThe result of the compression on the upper portion of the deck causes tension in the lower portion of the deck. This tension causes the lower portion of the beam to lengthen.ExampleTake a two-by-four and place it on top of two empty milk crates -- you've just created a crude beam bridge. Now place a 50-pound weight in the middle of it. Notice how the two-by-four bends. The top side is under compression and the bottom side is under tension. If you keep adding weight eventually the two-by-four will break. Actually the top side will buckle and the bottom side will snap.

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