UNIT-III ARCHES Introduction: Arch: What is an arch? Explain. What is a linear arch?

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Bridget Bryant
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1 UNIT-III RCES rches as structural forms Examples of arch structures Types of arches nalysis of three hinged, two hinged and fixed arches, parabolic and circular arches Settlement and temperature effects. Introduction: Mainly three types of arches are used in practice: three-hinged, two-hinged and hingeless arches. In 19 th century, three-hinged arches were commonly used for the long span structures. Then development in structural analysis, for long span structures starting from late nineteenth century engineers adopted two-hinged and hingeless arches. Two-hinged arch is the statically indeterminate structure to degree one. Usually, the horizontal reaction is treated as the redundant and is evaluated by the method of least work. rch: n arch is a curved beam or structure in vertical plane and subjected to transverse loads which act on the convex side of the curve and re-sights the external loads by virtue of thrust. It is subjected to three restraining forces i.e., Thrust Shear force ending Moment What is an arch? Explain. n arch is defined as a curved girder, having convexity upwards and supported at its ends. The supports must effectively arrest displacements in the vertical and horizontal directions. Only then there will be arch action. What is a linear arch? If an arch is to take loads, say W1, W, and W3 (fig) and a ector diagram and funicular polygon are plotted as shown, the funicular polygon is known as the linear arch or theoretical arch.

2 The polar distance ot represents the horizontal thrust. The links C, CD, DE, and E will be under compression and there will be no bending moment. If an arch of this shape CDE is provided, there will be no bending moment. For a given set of vertical loads W1, W..etc., we can have any number of linear arches depending on where we choose O or how much horizontal thrust (or) we choose to introduce. State Eddy s theorem. Eddy s theorem states that The bending moment at any section of an arch is proportional to the vertical intercept between the linear arch (or theoretical arch) and the centre line of the actual arch. M x = Ordinate O O 3 x scale factor What is the degree of static indeterminacy of a three hinged parabolic arch? For a three hinged parabolic arch, the degree of static indeterminancy is zero. It is statically determinate. Explain with the aid of a sketch, the normal thrust and radial shear in an arch rib. Let us take a section X of an arch. (fig (a) ). Let q be the inclination of the tangent at X. If is the horizontal thrust and the vertical shear at X, from the free body of the RS of the arch, it is clear that and will have normal and radial components given by, N = cosɵ + sinɵ R = cosɵ - sinɵ

3 Difference between the basic action of an arch and a suspension cable n arch is essentially a compression member which can also take bending moments and shears. ending moments and shears will be absent if the arch is parabolic and the loading uniformly distributed. cable can take only tension. suspension bridge will therefore have a cable and a stiffening girder. The girder will take the bending moment and shears in the bridge and the cable, only tension. ecause of the thrusts in the cables and arches, the bending moments are considerably reduced. If the load on the girder is uniform, the bridge will have only cable tension and no bending moment on the girder. Distinguish between two hinged and three hinged arches Sl. No Two hinged arches Three hinged arches 1 Statically indeterminate to first degree Statically determinate Might develop temperature Increase in temperature causes increase in stresses Central rise. No stresses. 3 Structurally more efficient Easy to analyse. ut in costruction, the central hinge may involve additional expenditure. 4 Will develop stresses due to Since this is determinate, no stresses due sinking of supports to support sinking. Types of rches a). ccording to the support conditions (structural behaviour arches) or hinges i. Three hinged arch inged at the supports and the crown 3-hinged arch is a statically determinate structure. inged at the crown Springing Rise ii. Two hinged arch inged only at the support inged at the Support It is an indeterminate structure of degree of indeterminacy equal to 1

4 Rib of the arch Rise Span iii. iv. Single hinged arch Fixed arch (or) hingeless arch The supports are fixed It is a statically indeterminate structure. The degree of indeterminancy is 3 b), ccording to their shapes v. Circular or curved or segmental arch vi. vii. viii. Parabolic arch Elliptical arch Polygonal arch c), ccording to their basis of materials i. Steel arches, ii. iii. Reinforced concrete arches, Masonry arches (rick or Stone) etc., d), ccording to their space between the loaded area and the rib arches i. Open arch ii. Closed arch (solid arch). Three inged rch Three hinged arch is statically determinate. Third hinge at crown and the other two hinges at each abutments Mostly used for long span bridges nalysis of three inged Parabolic rch ending moment at the crown hinge is zero rch have two reaction at support (One horizontal & one vertical) Need for four equation to solve and find the unknown reaction. inges at the support

5 We can use three static equilibrium conditions and in addition to that the.m. at the crown hinge is equal to zero. Crown Springing Rise inged at the Support For Symmetric Parabolic rch: 1. Rise: Where, y c = r = Radius (or) Rise of arch L = Length of rch or Span. Internal forces (Fx, Fy & Mz) a. Normal Thrust (N x ) N x = x SinƟ + CosƟ b. Radial Shear (R x ) R x = x SinƟ - CosƟ c. Slope of arch (Ɵ) θ = tan -1 [(4h/L ) (L x) d. Resultant (R) R = ( + ) Where, F x or R x = shear force in the arch F y or N x = thrust in the arch θ = Slope of arch axis at P. = Shear at P C = Thrust at P M = ending moment at P

6 1. three hinged parabolic arch of 0 m span and 4 m central rise as shown in figure carries a point load of 40 kn at 4 m horizontally from left support. Compute M, SF and F at load point. lso determine maximum positive and negative bending moments in the arch and plot the bending moment diagram..m.d

7 nalysis of two-hinged arch typical two-hinged arch is having four unknown reactions, but there are only three equations of equilibrium available. ence, the degree of statically indeterminacy is one for two-hinged arch. Rib-shortening in the case of arches. In a two hinged arch, the normal thrust which is a compressive force along the axis of the arch will shorten the rib of the arch. This in turn will release part of the horizontal thrust. Normally, this effect is not considered in the analysis (in the case of two hinged arches). Depending upon the importance of the work we can either take into account or omit the effect of rib shortening. This will be done by considering (or omitting) strain energy due to axial compression along with the strain energy due to bending in evaluating.

8 Strain energy due to bending (U b ) Where, M = ending moment E = Young s modulus of the arch material I = Moment of inertia of the arch cross section s = Length of the centreline of the arch Strain energy due to axial compression (U a ) Where, M = ending moment N = xial compression. = Cross sectional area of the arch E = Young s modulus of the arch material s = Length of the centreline of the arch Total strain energy of the arch Symmetrical two hinged arch Consider a symmetrical two-hinged arch as shown in figure. Let C at crown be the origin of co-ordinate axes. Now, replace hinge at with a roller support. Then we get a simply supported curved beam figure as shown in below.

9 Since the curved beam is free to move horizontally, it will do so as shown by dotted lines. Let M o and N o be the bending moment and axial force at any cross section of the simply supported curved beam. Since, in the original arch structure, there is no horizontal displacement, now apply a horizontal force as shown in figure. The horizontal force should be of such magnitude, that the displacement at must vanish. ending moment at any cross section of the arch M = M o (h-y) The axial compressive force at any cross section N = N o + cosɵ Where, θ the angle made by the tangent at D with horizontal Substituting the value of M and N in the equation Temperature effect Consider an unloaded two-hinged arch of span L. When the arch undergoes a uniform temperature change of T C, then its span would increase by α L T if it were allowed to expand freely. α is the co-efficient of thermal expansion of the arch material.

10 Since the arch is restrained from the horizontal movement, a horizontal force is induced at the support as the temperature is increased. nalysis of 3-hinged arches It is the process of determining external reactions at the support and internal quantities such as normal thrust, shear and bending moment at any section in the arch. Procedure to find reactions at the supports Sketch the arch with the loads and reactions at the support. pply equilibrium conditions namely F x 0, F y 0 and M 0 pply the condition that M about the hinge at the crown is zero (Moment of all the forces either to the left or to the right of the crown). Solve for unknown quantities. 1. Find the M,RS,NT catch 4m from left hand side,m from right hand side of the three hinged parabolic arch shown in fig. Step1: Find,,, Take = (6/+6) = 0 = 55 KN Take = (10 6 /) = 0 = 45 KN (LS) 6-( 4)- (10 6 /)=0 = 37.5 KN

11 (RS) = 0 = 37.5 KN (i) M, RS, NT (4m from LS) Y D = 4r/L (Lx-x ) = 4 4/1 (1 4-4 ) Y D = 3.56 m ending moment M@D = ( 4)-( 3.56)-(10 4 /) M D = 6.5 KNm Normal thrust NT = x Sinθ + Cosθ x = -(10 4) = = 15 KN θ = 4r/L (L-x) = 5.46 NT = 15 cos sin 5.46 = 40.3 KN Radial shear RS = x cosθ - sinθ = 15 cos sin 5.46 =.58 KN (ii) M, RS, NT (m from RS) Y E =. m X = 5 KN ending moment M@E = (- ) +. M@E = -6.75KNm Normal thrust NT = x Sinθ + Cosθ x = -0 = 45-0 = 5 KN θ = 4r/L (L-x) = 50.9

12 NT = 5 cos sin 55.9 = 43KN Radial shear RS = x cosθ - sinθ = 5 cos sin 55.9 = KN. Three hinged circular arch, a find support reaction, M, RS, NT at 4m from L..S and 5m from R..S. Solution: Find,,, Take = = 0 = 105 KN Take =0 = 0 KN (LS) 10-( 6)- (50 8) - (75 6) = 0 = KN (RS) = KN Find Y D r (R-r) = L /4 L = 0m, r = 6m 6(R-r) = 0 /4 R = m R = x +(R-r+Y D ) x =6m = 6 +(11-6+ Y D ) 9.61 = Y D Y D =4.8 m θ = sin -1 (x /R) = sin -1 (6/11.33) = 31.98

13 (i) M, RS, NT (4m from LS) ending moment = ( 4)-(50 )-( Y D ) = KNm Normal thrust NT = x Sinθ + Cosθ x = -(75+50) = 0 KN NT = 0Sin Cos31.98 = KN Radial shear RS = x cosθ sinθ = 0 cos sin31.98 = KN (ii) M,RS,NT(6m from RS) x = 5 m θ = 6.18 ending moment Y E = 4.83 M@E = ( 5)+( Y E ) = (0 5) + ( ) = KNm Normal thrust NT = x Sinθ + Cosθ x = 0 KN NT = 0 Sin Cos6.18 = KN Radial shear RS = x cosθ sinθ = 0 cos sin6.18 = - 3 KN

14 3. two hinged parabolic arch of span 15m and a point load of 0 KN at a distance of 4m from L..S. Find the M, RS, NT 4m from L..S and 3m from R..S. since r = 5m. Solution: Find, Take M@ = = 0 = KN Take M@ = = 0 = 5.3 KN = 0 l (μ y dx)/(18r L/15) μ 1 = x 1 μ = -5.33x +80 Substitute the μ,μ 1 values to above equation and we get = 11.8 KN (i) M,RS,NT(4m from LS) ending moment Y D = 3.9 m M@D = ( 4)-(0 0)-( Y D ) = 1.6 KNm Normal thrust NT = x Sinθ + Cosθ x = - 0 = KN θ = NT = -5.33Sin Cos35.65 = 6.5 KN Radial shear RS = x cosθ sinθ = cos sin35.65 = KN (ii) M, RS, NT (5m from RS) x = 5 m θ = 8.65 ending moment Y E = 3.75 M@E = 0

15 Normal thrust NT = x Sinθ + Cosθ x = 5 KN NT = 5Sin Cos8.65 = KN Radial shear RS = x cosθ - sinθ = 5 cos sin8.65 = -.04 KN 4. 3-hinged arch has a span of 30m and a rise of 10m. The arch carries UDL of 0.6 kn/m on the left half of the span. It also carries concentrated loads of 1.6 kn and 1 kn at 5 m and 10 m from the rt end. Determine the reactions at the support. (sketch not given). 0.6 kn/m C 1 kn 1.6 kn 5 m 5 m h = 10m = 4.75 = 4.75 = 7.35 L = 30m = 4.5 F x (1) To find vertical reaction. F y M x x x 51x 0 (0.6 x15) () 4.5 kn kn

16 To find horizontal reaction. M C 0 1x5 1.6x10 4.5x15 x10 0 M C kN 4.75kN OR 7.375x15 x10 (0.6x15) kN 4.75kN To find total reaction = 7.35 kn R = 4.5 kn R = 4.75 kn = 4.75 kn R kN 7.35 tan R 6.0kN tan

17 5. 3-hinged parabolic arch of span 50m and rise 15m carries a load of 10kN at quarter span as shown in figure. Calculate total reaction at the hinges. 10 kn C 15 m 1.5 m 50 m F x 0 To find vertical reaction: Fy 0 M (1) x x kN To find orizontal reaction M C 0 5 To find total reaction kN = 7.5 R R = 4.5 = 4.17 = 4.17

18 tan 4.861kN R R.9 60 tan 8.581kN R R 4.17 kn Problem: Determine the reaction components at supports and for 3-hinged arch shown in fig.

19 10 kn/m C 180 kn.5 m 10 m.4 m 8 m 6 m To find orizontal reaction F x (1) To find vertical reaction. F y x () M 0 x4 x x1810 x 10 x (3) M C 0

20 180 x8 x14 x x (4) dding and kN 103kN 10 x kN symmetrical 3-hinged parabolic arch has a span of 0m. It carries UDL of intensity 10 knm over the entire span and point loads of 40 kn each at m and 5m from left support. Compute the reactions. lso find M, radial shear and normal thrust at a section 4m from left end take central rise as 4m. 40 kn 40 kn 10 kn/m M C 4 m m 3m 0 m F x F y x (1) ()

21 M 0 40 x 40 x 5 (10 x 0)10 x kn 166 kn M c 0 (10 x10)5 x 4114 x kN 160kN ERTICL 160 kn 10 kn/m 40 kn 4 m m 166 kn NORML M = kn ORIZONTL y = kn REDIL M at M = x x 4 40 x - (10 x 4) = knm 4hx y L x L 4 x 4 x 4 0 y.56m 0 4

22 4h L tan L x 4 x 4 0 = 0 x Normal thrust = N = Cos Cos = kn S = 160 Sin x Sin S = kn

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