FIXED BEAMS CONTINUOUS BEAMS

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Job Harrell
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1 FIXED BEAMS CONTINUOUS BEAMS

2 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 L 1, L 2, and L 3, 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

3 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.

4 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 L 1 L 1, L 2, and L 2, 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 w 1 and w 2, respectively, as shown in Figure 2. Figure 2 Continuous beam

5 CLAPEYRON S THEOREM OF THREE MOMENTS Let s say that support moments at A, B, and C are M A, M B, M C, 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) 2 w 1 L 1 8 Figure 2 Continuous beam

6 Similarly, maximum bending moment at the centre of span BC will be as shown in Figure 3. 2 w 2 L 2 8 Figure 3 BM diagrams over two supports

7 Span AB (Independently) Origin at B, x positive towards left, bending moment at any section X-X is Support moments M A, M B, M C are shown in the diagram. Bending moment at this section due to support moments (using the top triangle) w x x w L x x L w M x x M B M M A x L x 1 A A B B A x A B A x A B A x M L x M M L x M M M M M L x L M M L M M x L M M B A B x M M L x M M 1

8 Resultant bending moment at the section (when AB is a part of a continuous beam) or M x M x 2 d y EI 2 dx w1 L1 x M x 2 w1 L1 x 2 w1 x 2 2 w1 x 2 2 M B M x L 1 B x L M A 1 M M A B M B Integrating this equation, we get where C 1 is a constant of integration. At support B, where x = 0, slope (say) so, EIi B = C 1 or constant of integration, C 1 = +EIi B

9 Now Integrating this, we get where C 2 is another constant of integration. At support B, x = 0, deflection y = 0, so, 0 = C 2 Constant C 2 = 0

10 Finally At the support A, x = L 1, deflection y = 0, so, or

11 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

12 But i B = i B 2M B (L 1 + L 2 ) + M A L 1 + M C L 2 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.

13 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 M A = M D = 0

14 Moreover, the beam is symmetrically loaded about its centre; therefore, M B = M C and reactions R A = R D and R B = R C. Using Clapeyron s theorem of three moments for spans AB and BC M A L + 2M B (L + L) + M C L (from page 11) But M A = 0, M B = M C, so, Taking moments at support B of the beam, only on left side of B or

15 Reaction, Reaction, R A = 0.4wL = Reaction, R D Total load on beam = 3wL = R A + R B + R C + R D But R B = R C, due to symmetry, therefore, 2R A + 2 R B = 3wL or 2 0.4wL + 2R B = 3wL Reaction, R B = 1.1wL = Reaction, R C Bending moment at centre of each span

16 The figure above shows the BM diagram for the continuous beam with four points of contraflexure P 1, P 2, P 3, and P 4, respectively. Bending moment at centre of span AB and CD = 0.125wL wL 2 = wL 2

17 Deflection at the Centre of Span BC Considering span BC only, with known values of moments M B = M C = 0.1 wl 2 Reaction R B = R C = 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 C 1 is constant of integration.

18 At the centre of middle span, slope is zero due to symmetric loading, so Constant, Now, Integrating this equation we get where C 2 is another constant of integration. At x = 0, y = 0, therefore, EI 0 = C 2 Constant C 2 = 0

19 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.

20 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.

21 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 L 1 and L 2, respectively, of a continuous beam. Spans carry the combination of loads as shown in Figure 4. Figure 4 Loads on a continuous beam

22 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 or Figure 4 Loads on a continuous beam

23 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).

24 Moreover, from A (from the figure, lined area)) From, we can write a BM diagram support moments Similarly considering the span CB, taking origin at C and x positive towards left, another equation can be written: But slope at i B = 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:

25 or Area a 1 and a 2 are positive as per convention and support moments M A, M B, M C are negative. Figure 4 Support moments diagram

26 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

27 For parabola AG B, area Span BC Maximum BM Triangle BE C, area (about B) (about C)

28 Span CD, Maximum BM Using the equation of three moments for spans AB and CB 4M A + 16M B + 4M C = = 530 or M A + 4M B + MC = (i) But M A = 0 So, 4M B + M C = (ii)

29 Using the equation of three moments for spans BC and DC But M D = 0 4M B + 16M C = = 330 4M B + 16M C = 330 (iii) From Equations (ii) and (iii) 15M C = M C = knm M B = knm Taking BB = knm CC = knm

30 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.

31 SUPPORTS NOT AT SAME LEVEL There are two spans AB and BC of lengths L 1 and L 2 of a continuous beam. Supports A, B, and C are not at same one level. Support B is below support A by δ 1 and below support C by δ 2 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

32 M B M A M B L 1 M C L 2

33 Span AB: Consider a section X-X, at a distance of x from A or or

34 Similarly consider span CB, origin at C, x positive towards left It may be noted that i B = 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, i B + i B = 0 Adding and or M A L 1 + 2M B (L 1 + L 2 ) + M C L 2 From this equation M A, M B, and M C can be determined, after determining and.

35 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 knm 2. Figure (a) shows the loads on the beam. Let us draw BM diagrams considering the spans as independently supported.

36 Span AB area Span CB area

37 Support moments M A = M C = 0 Using the equation derived in Fixed Beams, Support Moments: Total load on beam = = 18kN Moreover M B = R A = 6R A 36 Reaction, R A = kn also M B = 4R C = 4R C 12 Reaction, R C = kn Reaction, R B = = kn.

38 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

39 Considering origin as B and x positive towards left: But at fixed end: or where M A 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.

40 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.

41 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.

42 Span AB Maximum BM, Origin at A, Origin at B,

43 Span BC Span CD

44 Considering imaginary span AA of zero length and using Clapeyron s theorem for two spans A A and AB, or

45 Spans AB and BC Spans BC and CD But 4M B + 16M C = 600 (iv)

46 Putting the value of M A in equation (iii) M B + 4M C = 150 From eq. (iv) (vi) Solving eqs. (v) and (vi), we get 16 M B = 330 Moment, M B = knm, Putting the value of M B in eq. (vi) 4 M C = Moments,

47 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.

48 Support Reactions M C = 4R D 60 2 = Reaction, R D = 21.9 kn M B = 8R D + 4R C = R D + 4R C = R C = Reaction, R C = 71.0 kn Moments about A 14R D + 10R C + 6R B = M A = knm

49 Putting the values in the solution: Reaction, R B = 41.1 kn Total load on beam = = 180 kn Reaction R A = = 46 kn

50 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 cm 4 and from BC it is 12,000 cm 4. If E = 210 kn/mm 2, determine support moments and draw BM diagram. Let us first draw a diagram for both spans area,

51 a 2 is a triangle with Moment, M C = 0, because end C is a simple support.

52 Span AA B Imaginary span AA and AB equation of three moments. Taking I 1 common throughout (because level of A is higher than level of B by 1 mm) 12MA + 6MB = (i) Now using the theorem of three moments for spans AB and BC, and noting that I 1 is different than I 2, equation can be modified as

53 Multiplying throughout by I 1, But I 1 = 1.5I2, putting this value, we get 6M A + 30M B + 9M C = M A + 30M B + 9M C = knm But M C = 0

54 So, 6M A + 30M B = knm (ii) 6M A + 3M B = 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.

FIXED BEAMS IN BENDING

FIXED BEAMS IN BENDING INTRODUCTION Fixed or built-in beams are commonly used in building construction because they possess high rigidity in comparison to simply supported beams. When a simply supported