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Multiple Continuous Beams

The deflection of Continuous Beams with more than one span.
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Continuous Beams

When a Beam is carried on three or more supports it is said to be Continuous. It is possible to use an extension of the Moment-Area method ( See "Bending of Beams Part 3") to obtain a relationship between the Bending Moments at three points (Usually Supports.)

A beam is a horizontal structural element that is capable of withstanding load primarily by resisting bending. The bending force induced into the material of the beam as a result of the external loads, own weight, span and external reactions to these loads is called a bending moment.

23287/Multiple-Continous-Beams-108.png
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On the drawing the areas A_1 and A_2 are the Free Bending Moment areas obtained by treating the Beam as over two separate spans l_1 and l_2 . If the actual Bending Moments at these points are M_1, M_2 and M_3. Then a Fixing Moment diagram consisting of two trapezia can be introduced and the actual Bending Moment will be the Algebraic sum of the two diagrams.

In the lower figure the Elastic Line of the deflected Beam is shown.

The deflections \delta _1 and \delta _2 are measure relative to the left hand support and are positive upwards. \displaystyle \theta is the slope of the beam over the central support and Z_1 and Z_2 are the intercepts for l_1 and l_2

\therefore\;\;\;\;\;\;\theta  = \frac{Z_1 + \delta _1}{l_1} = \frac{Z_2\;+\[\delta _2 - \delta _1]}{l_2}

Note. This assumes that the slopes everywhere are small.

\frac{A_1\bar{x}_1 - (\displaystyle\frac{M_1\;l_1}{2})(\displaystyle\frac{2\;l_1}{3}) - (\displaystyle\frac{M_2l_1}{2})(\displaystyle\frac{2\;l_1}{3})}{E\;I} + \frac{\delta _1}{l_1}

\;\;\;\;\;\;\;= - \frac{A_2\bar{x}_2 + (\displaystyle\frac{M_3 l_2}{2})(\displaystyle\frac{2\;l_2}{3}) + (\displaystyle\frac{M_2l_2}{2})(\displaystyle\frac{2\;l_2}{3})}{E\;I} + \frac{\delta _2 - \delta _1}{l_1}

Note that \displaystyle Z_2 is a negative intercept.

The above equation can be written as:

If \displaystyle I_1 = I_2

If the supports are at the same level:

If the Ends are Simply Supported then \displaystyle M_1 = M_3 = 0

Clapeyron's Equation Or The Equation Of Three Moments

Span is the distance between two intermediate supports for a structure, e.g. a beam or a bridge.

Equation (2) is the most general form of The Equation of Three Moments. Equations (3) (4) and (5) are simplifications to meet particular needs. Of these Equation (4) is the form most frequently required.

Example:

[imperial]
Example - Example 1
Problem
A Beam Ad 60 ft. long rests on supports at A, B, and C which are at the same level. AB = 24 ft. and BC = 30 ft. The loading is 1 ton/ft. throughout and in addition a concentrated load of 5 tons acts at the mid-point of AB and a load of 2 tons acts at D

Draw the Shear Force and Bending Moment diagrams.
Workings
M_a = 0

M_c = 2\times6 + 6\times3 = 30\;tons-ft.

Applying Equation (3) to the span A\;B\;C

2M_b\times 54 + 30\times30 =
= 6\left[\left(\frac{1}{2}\times\frac{5\times24}{4}\times24 \right)\times\frac{12}{24} + \left(\frac{2}{3}\times\frac{24^2}{8}\times24 \right)\times\frac{12}{24}\; +\;\left(\frac{2}{3}\times\frac{30^2}{8}\times30 \right)\times\frac{15}{30} \right]

= 6\times1881

\therefore\;\;\;\;\;\;\;M_b = 96.2\;tons-ft.

The Bending Moment at mid-point of AB

= 5\times \frac{24}{4} + \frac{24^2}{8} - \frac{M_b}{2} = 53.9\;tons-ft.

The Bending Moment at the mid-point of BC

= \frac{30^2}{8} - \frac{1}{2}\left(M_b + 30 \right) = 49.4\;tons-ft.

To find the reactions at the supports:

M_b\;= - R_a\times 24 + 24\times 12 + 5\times 12 for A\;B

M_b\;= - R_c\times 30 + 36\times 18 + 2\times 36 for B\;C\;D

\therefore\;\;\;\;\;\;R_a = \left(\frac{288 + 60 - 96.2}{24} \right) = 10.49\;tons\;\;\;\;say \;10.5\;tons

And R_c = \left(\displaystyle\frac{540 + 72 - 96.2}{30} \right) = 21.5\;tons

23287/Multiple-Continous-Beams-109.png
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By difference,

R_b = 60 + 5\;+2 - 10.5 - 21.5 = 35\;tons

From the Shear Force diagram it can be seen that the maximum Bending Moment occurs either at a distance of 13.5 ft. from C where:

M = 21.5\times 13.5 - \frac{19.5^2}{2} - 2\times19.5 = 62\;tons-ft.

Or at a distance of 10.5 ft. from A where:

M = 10,5\times 10.5 - \frac{10.5^2}{2} = 55.2\;tons-ft.

The combined Bending Moment diagram is shown at the bottom of the sketch.

Beams With More Than Two Spans.

Where a Beam extends over more than three Supports the Equation of Three Moments is applied to each group of three in turns. In general if there are n Supports there will be n - 2 unknown Bending Moments ( excluding the Ends) and n - 2 equations to solve simultaneously.