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FIXED BEAMS CONTINUOUS BEAMS
INTRODUCTION
A beam carried over more than two supports is known as a continuous beam. Railway bridges are common examples of continuous beams. But the beams in railway bridges are subjected to travelling loads in addition to static loads. We will only consider the effect of static, concentrated, and distributed loads for the analysis of reactions and support moments. Figure 1 shows a beam ABCD, carried over three spans of lengths L1, L2, and L3, respectively. End A of the beam is fixed, while end D is simply supported. At the end D support moment will be zero, but at end A, supports B and C there will be support moments in the beam, to be determined.
Figure 1 Continuous beam
Prof. Clapeyron has provided a theorem showing the relationship
between three support moments of any two consecutive spans
of a continuous beam and the loads applied on these two spans.
This theorem is generally known as the “Clapeyron’s theorem
of three moments”.
Engineers are generally responsible with the job of analyzing
support moments, support reactions, and bending moments in a
continuous beam for the optimum design of beam sections.
CLAPEYRON’S THEOREM OF THREE MOMENTS
This theorem provides a relationship between three moments of two
consecutive spans of a continuous beam with the loading arrangement on these
spans.
Let us consider a continuous beam A′ ABCC′ supported over five supports, and
there are four spans of the beam with lengths L1′ L1, L2, and L2′, respectively,
as shown in Figure 2. In this beam let’s consider consecutive spans AB and BC
carrying a uniformly distributed loading (udl) of intensities w1 and w2,
respectively, as shown in Figure 2.
Figure 2 Continuous beam
CLAPEYRON’S THEOREM OF THREE MOMENTS
Let’s say that support moments at A, B, and C are MA, MB, MC, respectively. If
the bending moment on spans AB and BC is positive, then support moments
will be negative.
Now let’s take two spans AB and BC independently and draw the bending
moment (BM) diagram of each considering simple supports at ends. Maximum
bending moment of AB will occur at its centre and be equal to: (Fixed beams,
distributed loading, SS case, page 24)
Figure 2 Continuous beam
8
2
11Lw
Similarly, maximum bending moment at the centre of span BC will be
as shown in Figure 3.
Figure 3 BM diagrams over two supports
8
2
22Lw
Span AB (Independently)
Origin at B, x positive towards left, bending moment at any section X-X is
Support moments MA, MB, MC are shown in the diagram. Bending moment at this section due to support moments (using the top triangle)
222
2
11111
xwx
LwxxLwM x
xM BM
AMxxL 1
AABBAx
ABAx
ABAx
MLx
MMLx
MMM
MML
xLMM
L
MM
xL
MM
11
1
1
11
BABx MMLx
MM 1
Resultant bending moment at the section (when AB is a part of a continuous beam)
or
Integrating this equation, we get
where C1 is a constant of integration.
At support B, where x = 0, slope (say)
EIiB = 0–0+0+0+C1
or constant of integration, C1 = +EIiB
so,
BABxxx MMLx
MxwxLw
MMM 1
2
111
22
BAB MMLx
MxwxLw
dx
ydEI
1
2
111
2
2
22
Now
Integrating this, we get
where C2 is another constant of integration.
At support B, x = 0, deflection y = 0,
so, 0 = 0 − 0 + 0 + 0 + 0 + C2
Constant C2 = 0
Finally
At the support A, x = L1, deflection y = 0, so,
or
Similarly considering span BC, origin at B, x positive towards right and proceeding in the same manner as before, we can write the equation
The slope i′B = –i′B because in portion AB, x is taken positive towards left
and in portion BC, x is taken positive towards right.
Adding and we get
This is a well-known “Clapeyron’s theorem of three moments” (for
support moments having three consecutive supports for a continuous
beam carrying uniformly distributed loads). If there are n supports for a
continuous beam and if the two ends are simply supported, there will be
(n – 2) intermediate supports and (n – 2) equations will be formed so as
to determine the support moments at intermediate supports.
But i′B = − i′B
∴ 2MB (L1 + L2) + MAL1 + MCL2
Example A continuous beam ABCD is carried over three equal spans of length L each. It carries a udl of intensity w through its length as shown in the figure. Determine the (i) support moments, (ii) support reactions, (iii) deflection at centre of span BC, if EI is the flexural rigidity of the beam. Draw the BM diagram of the continuous beam.
The beam is simply supported at ends A and D, so bending moments
MA = MD = 0
Moreover, the beam is symmetrically loaded about its centre; therefore, MB = MC and reactions RA = RD and RB = RC. Using Clapeyron’s theorem of
three moments for spans AB and BC
MAL + 2MB (L + L) + MCL (from page 11)
But MA = 0, MB = MC,
so,
Taking moments at support B of the beam, only on left side of B
or
Reaction,
Reaction, RA = 0.4wL = Reaction, RD
Total load on beam = 3wL
= RA + RB + RC + RD
But RB = RC, due to symmetry, therefore,
2RA + 2 RB = 3wL or 2 × 0.4wL + 2RB = 3wL
Reaction, RB = 1.1wL = Reaction, RC
Bending moment at centre of each span
The figure above shows the BM diagram for the continuous beam with four points of contraflexure P1, P2, P3, and P4, respectively. Bending moment at centre of span AB and CD
= 0.125wL2 − 0.05wL2
= +0.075wL2
Deflection at the Centre of Span BC
Considering span BC only, with known values of moments
MB = MC = −0.1 wL2
Reaction RB = RC = 1.1 wL
If we take a section XX at a distance of x from B, the bending moment
Integrating this equation, we get
where C1 is constant of integration.
At the centre of middle span, slope is zero due to symmetric loading, so
Constant,
Now,
Integrating this equation we get
where C2 is another constant of integration.
At x = 0, y = 0, therefore, EI × 0 = −0 + 0 − 0 − 0 + C2 Constant C2 = 0
Finally,
Deflection at centre,
We should note that in this problem we have taken uniform section throughout. The section of the beam can change from span to span or within the span, depending upon the requirement.
Exercise 5.1 A continuous beam ABC is carried over two spans AB and BC, in which AB = 6 m and BC = 4 m. Span AB carries a udl of 4 kN/m and span BC carries a udl of 6 kN/m, as shown in the figure. Calculate the support reactions and support moments.
THEOREM OF THREE MOMENTS—ANY TYPE OF LOADING
So far we have considered only uniformly distributed loading over the spans of a
continuous beam. But a beam can be subjected to any type of loading and the
Clapeyron’s theorem can be modified for any type of loading on a continuous
beam. Let us consider two spans AB and BC of spans lengths L1 and L2,
respectively, of a continuous beam. Spans carry the combination of loads as
shown in Figure 4.
Figure 4 Loads on a continuous beam
THEOREM OF THREE MOMENTS—ANY TYPE OF LOADING
Considering the beam of spans AB and BC, independently as simply supported,
the bending moment diagrams of two spans can be drawn. Then support moment
diagrams will be as shown by AA′B′C′C. Consider span AB, origin at A, x positive
towards right, section X-X, bending moment at the section will be
M = M′x + M″x
M= BM considering span as simply supported
+ BM due to support moments
Figure 4 Loads on a continuous beam
or
Multiplying both the sides by x dx and integrating
or
where a′1 = Area of BM diagram of span AB, considering this as simply
supported,
a″1 = Area of BM diagram of span AB due to support moments,
= Distance of centroid of a′1 from end A, and
= Distance of centroid of area a″1 (due to support moments from A).
Moreover, from A
(from the figure, lined area))
a′ BM diagram support moments From , we can write
Similarly considering the span CB, taking origin at C and x positive towards left, another equation can be written:
But slope at iB = –i′B because in portion AB we have taken x positive towards right and in portion CB we have taken x positive towards left. Adding these last two equations:
or
Area a′1 and a′2 are positive as per convention and support moments MA, MB, MC are negative.
Figure 4 Support moments diagram
Example A continuous beam ABCD, 12 m long supported over spans AB = BC = CD = 4 m each, carries a udl of 20 kN/m run over span AB, a concentrated load of 40 kN at a distance of 1 m from point B on span BC, and a load of 30 kN at the centre of the span CD. Determine support moments and draw the BM diagram for the continuous beam.
The figure shows a continuous beam ABCD, with equal spans. AB carries a udl of 20 kN/m. Span BC carries a concentrated load of 40 kN at E, 1 m from B. Span CD carries a point load of 30 kN at F, at centre of CD.
Let us construct a′ diagrams (BM diagrams considering each span independently as S.S.)
Span AB Maximum BM
For parabola AG′B, area
Span BC Maximum BM
Triangle BE′C, area
(about B)
(about C)
Span CD, Maximum BM
Using the equation of three moments for spans AB and CB
4MA + 16MB + 4MC = −320 − 210 = −530 or MA + 4MB + MC = −132.5 (i)
But MA = 0
So, 4MB + MC = −132.5 (ii)
Using the equation of three moments for spans BC and DC
But
MD = 0
4MB + 16MC = −150 − 180 = −330 4MB + 16MC = −330 (iii)
From Equations (ii) and (iii)
15MC = −197.5
MC = −13.166 kNm MB = −29.8335 kNm
Taking
BB′ = −29.8335 kNm CC′ = −13.166 kNm
Drawing lines AB′, B′C′, C′D, the diagram AB′C′DA, is the BM diagram due to support moments. There are four points of contraflexure in the BM diagram for continuous beam when a′ diagrams are superimposed over a″ diagram, the BM diagram due to support moments. Positive and negative areas of the BM diagram are also marked.
SUPPORTS NOT AT SAME LEVEL There are two spans AB and BC of lengths L1 and L2 of a continuous beam. Supports A, B, and C are not at same one level. Support B is below support A by δ1 and below support C by δ2 as shown in Figure 5 (a). These level differences are very small in comparison to span length, say of the order of 0.1% of span length. For spans AB and BC, bending moment diagrams are plotted considering the spans independently as shown by the shaded diagrams. Say
Figure 5 Continuous beam
MA
MB
L1
MC
MB
L2
Span AB: Consider a section X-X, at a distance of x from A
or
or
and .
Similarly consider span CB, origin at C, x positive towards left
It may be noted that iB = –i′B slope, because for span AB, we have taken x
positive towards right but for span CB, we have taken x positive towards left.
So, iB + –i′B = 0
Adding and
or MA L1 + 2MB (L1 + L2) + MCL2
From this equation MA, MB, and MC can be determined, after determining
Example A continuous beam ABC, 10 m long, spans AB = 6 m, span BC = 4 m, carries a udl of 2 kN/m over AB and a concentrated load of 6 kN at centre of BC. Support B is below the level of A by 10 mm and below the level of C by 5 mm. Determine support moments and support reactions, if EI = 6,000 kNm2.
Figure (a) shows the loads on the beam. Let us draw BM diagrams considering the spans as independently supported.
Span AB
area
Span CB
area
Support moments MA = MC = 0
Using the equation derived in Fixed Beams, Support Moments:
Total load on beam = 6 × 2 + 6 = 18kN
Moreover MB = RA × 6 − 6 × 2 × 3
−1.95 = 6RA − 36
Reaction, RA = + 5.675 kN
also MB = 4RC − 6 × 2
−1.95 = 4RC −12
Reaction, RC = +2.5125 kN
Reaction, RB = 18 − 5.675 − 2.5125 = 9.8125 kN.
CONTINUOUS BEAM WITH FIXED END For a continuous beam with a fixed end, the equations for support moments can be derived considering the slope and deflection at fixed end to be zero. Figure 6 (a) shows two consecutive spans AB and BC of a continuous beam. End A of the beam is fixed. Bending moment diagrams a′1 and a′2 are plotted considering spans AB and BC supported independently as shown in Figure 6. At any section if: M′x = BM due load on span, considering span independently M″x = BM due to support moments, then
Figure 6 Support moments
Considering origin as B and x positive towards left:
But at fixed end:
or
where MA is the fixing couple at fixed end A.
This relationship can also be obtained by considering an imaginary span A′A of zero length and bending moment at A′, M′A = 0 using Clapeyron’s theorem for two spans A′A and AB.
or
If the other end of the continuous beam is also fixed, a similar equation can be made by considering an imaginary span to the right of the other fixed end, and then applying the theorem of three moments.
Example A continuous beam ABCD 14 m long rests on supports A, B, C, and D all at the same level. AB = 6 m, BC = 4 m, CD = 4 m. Support A is a fixed support. It carries two concentrated loads of 60 kN each, at a distance of 2 m from end A and end D as shown in the figure. There is a udl of 15 kN/m over span BC. Find the moments and reactions at the supports. The figure shows the continuous beam ABCD, with fixed end at A. Let us first draw BM diagrams considering each span to be simply supported.
Span AB Maximum BM,
Origin at A,
Origin at B,
Span BC
Span CD
Considering imaginary span AA′ of zero length and using Clapeyron’s theorem for two spans A′A and AB,
or
Spans AB and BC
Spans BC and CD
But 4MB + 16MC = −600 (iv)
Putting the value of MA in equation (iii)
MB + 4MC = −150 From eq. (iv) (vi) Solving eqs. (v) and (vi), we get 16 MB = −330
Moment, MB = −20.625 kNm, Putting the value of MB in eq. (vi) 4 MC = −150 + 20.625 Moments,
The figure shows shaded diagrams are a′1, a′2, and a′3 BM diagrams. Bending moment diagram due to support moments is superimposed on these diagrams to get resultant BM at any section.
MC = 4RD − 60 × 2 = −32.334
Reaction, RD = 21.9 kN
MB = 8RD + 4RC − 6 × 60 − 60 × 2 = −20.625
8RD + 4RC = 459.375
8 × 21.9 + 4RC = 459.375
Reaction, RC = 71.0 kN
Support Reactions
Moments about A 14RD + 10RC + 6RB −12 × 60 − 4 × 15 × 8 − 60 × 2 = MA = −56.354 kNm
Reaction, RB = 41.1 kN
Total load on beam = 60 + 4 × 15 + 60 = 180 kN
Reaction RA = 180 − 41.1 − 71.0 − 21.9
= 46 kN
Putting the values in the solution:
Example A continuous beam ABC, fixed at end A, supported over spans AB = BC = 6 m each. There is a udl of 10 kN/m over AB and a concentrated load of 40 kN at centre of BC as shown in the figure. While the supports A and C remain at the same level, the level of support B is 1 mm below due to sinking. Moment of inertia of beam from A to B is 18,000 cm4 and from BC it is 12,000 cm4. If E = 210 kN/mm2, determine support moments and draw BM diagram. Let us first draw a diagram for both spans area,
a′2 is a triangle with
Moment, MC = 0, because end C is a simple support.
Span AA´B Imaginary span AA′ and AB equation of three moments.
Taking I1 common throughout
(because level of A is higher than level of B by 1 mm)
12MA + 6MB = − 577.8 (i) Now using the theorem of three moments for spans AB and BC, and noting that I1 is different than I2, equation can be modified as
Multiplying throughout by I1,
But I1 = 1.5I2, putting this value, we get
6MA + 30MB + 9MC = −540 −810 + 37.8 + 37.8 6MA + 30MB + 9MC = −1274.4 kNm But MC = 0
So, 6MA + 30MB = −1274.4 kNm (ii)
6MA + 3MB = −288.9 kNm From eq (i) (iii)
From these two equations Support moments,
Superimposing diagram over this, we get resultant bending
moment diagram. Shaded parts show positive BM. There are three
points of contraflexure as shown.