Manual Knex 78640 - Education Intro to Structures Bridges Teachers Guide (page 6 of 83) (English)

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bridges

website: www.knexeducation.co.uk

A Quick Guide to Structures: Bridges

river and tidal currents and earthquakes on the

bridge. The Tacoma Narrows Bridge collapsed,

for example, because engineers did not fully

take into account the effect of wind, (Page 58),

while cable-stayed and suspension bridges,

(Pages 17-20), are not designed to carry trains

because of their susceptibility to shock load.

When the total load capacity of the bridge

is taken into consideration, the longest single

span for a beam bridge is approximately 80 –

100 metres.

In order to solve the problem of spanning

wider gaps safely, all the forces that act on

a bridge must be taken into account. A suc-

cessful bridge is one in which the forces are

balanced.

In other words, the action of the load pushing

down is balanced by the reaction of the bridge

structure, and the materials from which it is

made, pushing in the opposite direction. If the

action is greater than the reaction, movement

will take place in the direction of the larger

force – the bridge will fail.

Bridge basics

FORCES ACTING ON STRUCTURES

Fig. 2 Strengthening a beam by making it thicker

Fig. 3 The bridge itself is too heavy

But will this work over even wider spans?

The problem with the bridge shown in Fig. 3

is that it has been thickened so much that

its own weight has become greater than the

strength of the materials from which it is made.

Although it looks strong, it is unable to support

its own weight and it will fail (collapse) or, at

best, it will be very weak.

The weight of all the materials used to make

a bridge is called the dead load and this

weight must be taken into consideration

when designing a bridge. The weight of all

the objects carried on a bridge is called the

live load.

Additional considerations that engineers must

factor into their bridge design calculations

include the shock load, which is the result of

a sudden high impact, such as that caused by

a train or heavy truck crossing a bridge, and

the environmental load, resulting from the

effects of strong winds, ice and snow build-up,

When, however, people attempted to cross

wider barriers, carrying heavier loads and using

wheeled transport, bridges had to be stronger,

more stable and made to stay rigid over wider

spans. One way to strengthen a beam is to

make it thicker.

All structures have forces – pushes, pulls, twists

– acting upon them. The most important ones

affecting bridges are those of compression and

tension, although torsion and shear forces can

play a role.

Education

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Others manual(s) of Knex 78640 - Education Intro to Structures Bridges Teachers Guide

Knex 78640 - Education Intro to Structures Bridges Teachers Guide Instructions - German - 94 pages

Knex 78640 - Education Intro to Structures Bridges Teachers Guide Instructions - Dutch - 81 pages

Knex 78640 - Education Intro to Structures Bridges Teachers Guide Instructions - French - 94 pages

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Knex 78640 - Education Intro to Structures Bridges Teachers Guide Manual

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