Dr. Mahalingam College of Engineering & Technology Pollachi- 642003 CONTINUOUS ASSESSMENT TEST β III - KEY Class & Branch Sub. Code & Name Semester : III B.E. - CIVIL : STRUCTURAL ANALYSIS - I :V Max. Marks Time Date : 50 : : 29/10/2011 PART β A (10 x 2 = 20 Marks) Answer all questions : 1. What is moment distribution method? The moment distribution method is a structural analysis method for statically indeterminate beams and frames developed by Hardy Cross in 1930. In the moment distribution method, every joint of the structure to be analysed is considered as fixed so as to develop the fixed-end moments due to loads. Then after release the known moments at joints, each fixed joints having more than one member are balanced for unbalanced moment. The balance moments are distributed according to the distribution factors of members and moments are carried over to other end. The iteration is repeated until equilibrium is achieved. 2. What is the difference between absolute and relative stiffness? Absolute stiffness is represented in terms of E (youngβs modulus), I (Moment of Inertia) and L (Length of member) to indicate the moment/force required 4πΈπΌ such as πΏ to produce unit slope/deflection/deformation. Whereas relative stiffness is represented in terms of I and L, omitting constant E (when structural elements connected to joints are made of same material). 3. Briefly explain stiffness factor and distribution factor. Stiffness factor Stiffness factor is the moment required to produce an unit rotation at that end without producing translation at the far end. Distribution factor When several members meet at a joint and a moment is applied at the joint to produce rotation without translation of the members, the moment is distributed among all the members meeting at that joint proportionate to their stiffness. stiffness of the member Distribution factor = Sum of stiffness of members at the joint 4. What is carry over moment and carry over factor? Carry over moment It is defined as the moment induced at the fixed end of the beam by the action of a moment applied at the other end, which is hinged. Carry over moment is the same nature of the applied moment. Carry over factor ( C.O.F) A moment applied at the hinged end B β carries overβ to the fixed end A, a moment equal to half the amount of applied moment and of the same rotational sense. C.O =0.5 5. Explain term balancing in the moment distribution method. Algebraic sum of clockwise and counter-clockwise moments at any internal joint should be zero for equilibrium. When this sum is not zero, equivalent and opposite moment is applied at that joint to achieve the equilibrium. This step is called balancing. 6. Three members meeting at a rigid joint carry moments 5 kNm(clockwise), 10kNm (clockwise) and 9kNm (counter-clockwise). What moment should be applied to balance? 6kNm (Counter-clockwise) 7. Distribute an applied balancing moment of 12kNm at a joint among three members whose relative stiffnesses are 1/6, 1/3 and 1/2. 2kNm, 4kNm and 6kNm respectively for the members having relative stiffnesses of 1/6, 1/3 and 1/2. 8. An udl of 50kNm acting on one the span of two span continuous beam which is symmetrical and each span length is l. Sketch the equivalent load interms of symmetric and anti-symmetric loads. 25 kN/m A 25 kN/m 25 kN/m B l C A l l C B l 25 kN/m Symmetrical loads Anti-symmetrical loads 9. Sketch the reduced structure with support condition for the two span symmetric portal whose columns are fixed at the ground. B B C C For symmetrical loading For Anti-symmetrical loading A A 10. What is Naylorβs method? In the naylors method (also called cantilever moment distribution method), for symmetrical frames, columns are assumed as one end is free to rotate and translate and other end as fixed (similar to cantilever). The end moments are calculated based on this assumption and then the balancing and carry over is iteratively done by suitably modifying the member stiffness. In this method, sway analysis is completely avoided. PART β B ( 3 x 10 = 30 Marks) Answer any three of the following 11. Analyse the portal frame shown in fig. 1 using slope deflection method and sketch the bending moment and shear force diagram. (10) Solution Outline 48 kN 1. Find fixed end moments due to loads 2. Write slope-deflection equations B ( Io ) 3. Obtain joint equilibrium equations 2m 2m 6 kN/m 4. Obtain shear equation. 4m 5. Solve joint equilibrium equations and shear equation to get the slopes and deflections (Io) 6m 6. Find final end moments by substituting slopes and deflections in slope-deflection equations. A 7. Draw the bending moment diagram Fig. 1 8. Draw the shear force diagram C 10 kN/m (2Io ) D DETAIL SOLUTION Step 1: Find fixed end moments due to loads ππΉπ΄π΅ = β ππΉπ΅πΆ = β π€π2 12 ππ 8 =β =β 6×42 12 48×4 8 = β8 πππ = β24 πππ ; ; ππΉπ΅π΄ = + ππΉπΆπ΅ = + π€π2 12 ππ 8 =+ 6×42 12 = +8 πππ = +24πππ ππΉπΆπ· = β π€π2 12 =β 10×62 12 = β30πππ; ππΉπ·πΆ = π€π2 12 = 10×62 12 = 30πππ Step 2: Slope-deflection equations In this problem 4πΈπΌπ ππ΄ 2πΈπΌπ ππ΅ 6πΈπΌπ πΏ EπΌπ ππ΅ 3EπΌπ Ξ΄ + β 2 = β8 + β ππ΄ = 0 πππ π π π 2 8 ππ· = 0 2πΈπΌπ ππ΄ 4πΈπΌπ ππ΅ 6πΈπΌπ πΏ 3EπΌπ Ξ΄ ππ΅π΄ = ππΉπ΅π΄ + + β 2 = 8 + EπΌπ ππ΅ β π π π 8 4πΈπΌπ ππ΅ 2πΈπΌπ ππΆ πΈπΌπ ππΆ ππ΅πΆ = ππΉπ΅πΆ + + = β24 + πΈπΌπ ππ΅ + π π 2 2πΈπΌπ ππ΅ 4πΈπΌπ ππΆ πΈπΌπ ππ΅ ππΆπ΅ = ππΉπΆπ΅ + + = 24 + + πΈπΌπ ππΆ π π 2 4πΈ(2πΌπ )ππΆ 2πΈ(2πΌπ )ππ· 6πΈ(2πΌπ )πΏ 4πΈπΌπ ππΆ πΈπΌπ πΏ ππΆπ· = ππΉπΆπ· + + β = β30 + β 2 π π π 3 3 2πΈ(2πΌπ )ππΆ 4πΈ(2πΌπ )ππ· 6πΈ(2πΌπ )πΏ 2πΈπΌπ ππΆ πΈπΌπ πΏ ππ·πΆ = ππΉπ·πΆ + + β = 30 + β π π π2 3 3 Step 3: joint equilibrium equations ππ΄π΅ = ππΉπ΄π΅ + Joint B ππ΅π΄ + ππ΅πΆ = 0 ο β16 + 2πΈπΌπ ππ΅ + πΈπΌπ ππΆ 2 β 3πΈπΌπ πΏ 8 =0 Joint C ππΆπ΅ + ππΆπ· = 0 ο β6 + πΈπΌπ ππ΅ 2 + 7πΈπΌπ ππΆ 3 β πΈπΌπ πΏ 3 =0 Step 4: Shear equation π»π΄ + π»π· + 24 β 60 = 0 HA = MAB +MBA 4 β 24×2 4 and HD = MCD +MDC 6 + 60×3 6 Substituting π»π΄ πππ π»π· and Substituting moments interms of slope and deflection, the shear equation is obtained as follows: 3πΈπΌπ ππ΅ πΈπΌπ ππΆ 43πΈπΌπ πΏ + β β 18 = 0 8 3 144 Step 5: Solve joint equilibrium equations along with shear equation to get the slopes and deflections 1523 5421 14103 πΈπΌπ ππ΅ = β 386 , πΈπΌπ ππΆ = β 772 and πΈπΌπ πΏ = β 193 are obtained by solving joint equilibrium equations and shear equation. Step 6: Final Fixd end moments Substituting πΈπΌπ ππ΅ = β3.946, πΈπΌπ ππΆ = β7.022 and πΈπΌπ πΏ = β73.073 in slopedeflection equations, we get, ππ΄π΅ = 17.43πππ ; ππ΅π΄ = 31.46πππ ππ΅πΆ = β31.46πππ; ππΆπ΅ = +15.0πππ ππΆπ· = β15.0πππ; ππ·πΆ = +49.68πππ Step 7: Draw the bending moment diagram Step 8: Draw the shear force diagram 17.43+31.46 24×2 HA = β 4 = 0.22kN 4 HD = β15.0 + 49.68 60 × 3 + = 35.78kN 6 6 Find RA and RD, then draw shear force diagram 12. Analyse the structure loaded as in fig. 2. by the moment distribution method and sketch the bending moment and shear force diagram (10) Solution Outline 20kN/m 1. Find fixed end moments due to loads 80 kN 2. Determine distribution factors for members B A at each internal joint (2I) (I) 3m 2m 2m 3. Tabulate initial fixed end moments, release or apply known moments 4. Balance the unbalanced moment and carry over (2I) 4m Fig. 2 the moments. 5. If the carry over moments unbalances the D joint equilibrium, do step 4 and 5 repeatedly till unbalanced moment is negligible. 6. Find out the final end moment by summing all the initial, balanced moments and carry over moments for each end of the span. 7. Draw the bending moment diagram 8. Draw the shear force diagram DETAIL SOLUTION Step 1: Find fixed end moments due to loads ππΉπ΄π΅ = β ππΉπ΅πΆ = β π€ππ 2 π2 π€π2 12 =β =β ππΉπ΅π· = 0πππ; 80×3×22 52 20×22 12 = β38.4πππ; = β6.67πππ ; π€π2 π ππΉπ΅π΄ = π2 π€π2 ππΉπΆπ΅ = 12 = = 80×32 ×2 52 20×22 12 = 57.6πππ = 6.67πππ ππΉπ·π΅ = 0πππ Step 2: Determine distribution factors for members at each internal joint Joint Member BA B BC BD Stiffness Total Stiffness 3πΈπΌ 3πΈ 2πΌ = = 1.2πΈπΌ π 5 4πΈπΌ 4πΈ πΌ = = 2πΈπΌ π 2 4πΈπΌ 4πΈ 2πΌ = = 2πΈπΌ π 4 Distribution Factor 0.2308 5.2πΈπΌ 0.3846 0.3846 Steps 3 to 6 Joint A Member C.O.F AB BA BD 0 0.5 0.5 D.F 1 0.2308 0.3846 FEM Release C/O -38.4 Initial Moment B 57.6 C D BC CB DB 0.5 0.5 0.5 0.3846 1 1 0 -6.7 6.7 0 0 -6.7 6.7 0 -27 -27 +38.4 19.2 0 76.8 Balance 1 C/O 1 Final End 0 moment -16.2 -13.5 60.6 -27 -33.7 -6.8 -13.5 -13.5 C Step 7: Draw the bending moment diagram Step 8: Draw the shear force diagram Find π π΄ , π π΅ , π πΆ πππ π»π· Taking moment about B, considering left side. π π΄ × 5 β 80 × 2 = β60.6 ο π π΄ = 19.88ππ Taking moment about B, considering right side. 20×22 π πΆ × 2 β 2 + 6.8 = β33.7 ο π πΆ = β0.25ππ From β V = 0 π π΅ = 120 β 19.88 + 0.25 = 100.37ππ Taking moment about B, considering forces and moments below. π»π· × 4 + 13.5 = β27πππ ο π»π· = β10.25ππ (ie) 10.25kN(ο) 13. Analyse the structure loaded as in fig. 3. by the moment distribution method and sketch the bending moment and shear force diagram (10) 50 kN/m C B (I) 50 kN/m (I) 3m E 3m Fig. 3 3m ( I ) (2I) (2I) 4m 4m D F A Solution Outline 1. Considering symmetric structure, analyzing half of the structure is enough to find all the necessary design parameters (BM and SF). So, Consider half of the structure with fixed end at C, since there is support at C (Modification of stiffness is not necessary). 2. Find fixed end moments due to loads 3. Determine distribution factors for members at each internal joint 4. Tabulate initial fixed end moments 5. Balance the unbalanced moment and carry over the moments. 6. If the carry over moments unbalances the joint equilibrium, do step 4 and 5 repeatedly till unbalanced moment is negligible. 7. Find out the final end moment by summing all the initial, balanced moments and carry over moments for each end of the span. 8. Draw the bending moment diagram 50 kN/m 9. Draw the shear force diagram B STEP 1 C DETAIL SOLUTION (I) 3m Step 1: Consider half of the structure with fixed end at C Step 2: Find fixed end moments due to loads (2I) ππΉπ΄π΅ = 0πππ; 4m ππΉπ΅π΄ = 0πππ ππΉπ΅πΆ = β ππΉπΆπ΅ = π€π2 12 π€π2 12 = =β 50×32 50×32 12 12 = β37.5πππ ; A = 37.5πππ Step 3: Determine distribution factors for members at each internal joint Joint B Member BA BC Stiffness 4πΈπΌ 4πΈ 2πΌ = = 2πΈπΌ π 4 4πΈπΌ 4πΈ πΌ = = 1.333πΈπΌ π 3 Total Stiffness Distribution Factor 0.6 3.333πΈπΌ 0.4 Steps 4 to 7 Joint A Member AB BA C.O.F D.F 0.5 1 B Initial FEM 0 Balance 1 C/O 1 11.25 Final End 11.25 moment C C E F BC CB CE EC 0.5 0.5 0.5 0.6 0.4 1 0 -37.5 37.5 22.5 15 -45 22.5 -22.5 EF CB 7.5 22.5 -22.5 45 Step 8: Draw the bending moment diagram Step 9: Draw the Shear Force diagram Find π π΄ , π πΆ πππ π»π΄ Taking moment about C, considering left side forces and moments. 50×32 π π΅ × 3 β 22.5 β 2 = β45 ο π π΅ = 67.5ππ. Because of symmetry, π πΈ = 67.5ππ From β V = 0 π πΆ = 300 β 67.5 β 67.5 = 165ππ Taking moment about B, considering forces and moments below. π»π΄ × 4 + 11.25 = β22.5πππ ο π»π΄ = β8.44ππ (ie) 8.84kN(ο) -11.25 14. Analyse the portal frame shown in fig. 4 using Naylorβs method and sketch the bending moment and shear force diagram. (10) By modifying the stiffness of member in the symmetric line, 100 kN number of iterations can be reduced. Hence, advantage of the 50 kN/m symmetrical structure is also used in the solution. (I) B C Solution Outline 2m 1. Equivalent loads in terms of symmetric and 3m anti-symmetric loading Fig. 4 2. Analysis for symmetric loading (Moment Distribution method) 4πΈπΌ 3. 4. 5. 6. 3πΈπΌ a. Find stiffnesses (for fixed end , for hinged ) and 4m (2I) (2I) π π 4m modify the stiffness of member which is at symmetric line if no support (multiply with 0.5 for symmetrical loads) b. Determine distribution factors for members and c. carry over factors (0.5 for fixed end, 0 for hinged) D d. Find fixed end moments due to symmetrical loads A e. Tabulate initial fixed end moments f. Balance the moment and carry over the moments. g. If the carry over moments unbalances significantly the joint equilibrium, repeat balancing process. h. Find out the total end moments for symmetrical loads by summing all the initial, balanced moments and carry over moments for each end of the span. i. For other half of the structure, take the opposite moment at the corresponding ends. Naylor method for anti-symmetric loading a. Find stiffnesses (treat vertical members as cantilever β Stiffness for the cantilever is πΈπΌ 4πΈπΌ 3πΈπΌ , for fixed end , for hinged and modify the stiffness of member which is at π π π symmetric line if not support (multiply with 1.5 for anti-symmetrical loads) b. Determine distribution factors for members c. carry over factors (0.5 for fixed end, 0 for hinged and -1 for cantilever) d. Find fixed end moments due to anti-symmetrical loads e. Tabulate initial fixed end moments f. Balance the moment and carry over the moments. g. If the carry over moments unbalances significantly the joint equilibrium, repeat balancing process. h. Find out the total end moments for anti-symmetrical loads i. For other half of the structure, take the same moment at the corresponding ends. Obtain final end moment by adding moments from step 2 and step 3 Draw the bending moment diagram Draw the shear force diagram Step 1 : Equivalent symmetric and anti-symmetric loading for a given load 50 kN 50 kN 50 kN 50 kN/m C B B 50 kN 3m (I) 2m 3m (2I) A 4m C (I) (2I) D Symmetrical Loading 4m (2I) A 4m (2I) 4m D Anti-Symmetrical Loading Step 2 : Analysis for symmetric loading (Moment Distribution method) 4πΈπΌ 3πΈπΌ 3a. Find stiffnesses (for fixed end π , for hinged π ) and modify the stiffness of member which is at symmetric line if no support (multiply with 0.5 for symmetrical loads) 3b. Determine distribution factors for members Joint B Member BA Stiffness Total Stiffness 4πΈπΌ 4πΈ 2πΌ = = 2πΈπΌ π 4 4πΈπΌ 4πΈ πΌ BC = (Unmodified) π 3 4πΈ πΌ BC* β 0.5 = 0.667πΈπΌ (Modified) 3 Distribution Factor 0.75 2.667πΈπΌ 0.25 3c. carry over factors (0.5 for fixed end, 0 for hinged) 3d. Find fixed end moments due to symmetrical loads ππΉπ΄π΅ = 0πππ; ππΉπ΅π΄ = 0πππ ππΉπ΅πΆ = β π€π2 12 β π1 π1 π12 π2 β π2 π2 π22 π2 =β 50×32 12 β 50×1×22 32 β 50×2×12 32 = β70.83πππ (Moment for one-half the structure is enough when advantage of symmetric structure is considered) Steps 2e to 2i Joint A Member AB BA BC C.O.F D.F 0.5 0.5 0.5 1 0.75 0.25 0 -70.83 53.12 17.71 B Initial FEM 0 Balance 1 C/O 1 26.56 Total End 26.56 moment 53.12 C CB -53.12 53.12 D CD CB D -53.12 22.5 -26.56 Step 3 : Naylor method for anti-symmetric loading 3a. Find stiffnesses (treat vertical members as cantilever β Stiffness for the cantilever is 4πΈπΌ 3πΈπΌ πΈπΌ , π for fixed end π , for hinged π and modify the stiffness of member which is at symmetric line if not support (multiply with 1.5 for anti-symmetrical loads) 3b. Determine distribution factors for members Joint Member Stiffness Total Stiffness Distribution Factor πΈπΌ πΈ 2πΌ BA 0.2 = = 0.5πΈπΌ π 4 4πΈπΌ 4πΈ πΌ BC B = (Unmodified) π 3 4πΈ πΌ BC* β 1.5 = 2.0πΈπΌ (Modified) 3 3c. carry over factors (0.5 for fixed end, 0 for hinged) 3d. Find fixed end moments due to symmetrical loads 2.5πΈπΌ 0.8 ππΉπ΄π΅ = 0πππ; ππΉπ΅π΄ = 0πππ ππΉπ΅πΆ = β π1 π1 π12 π2 + π2 π2 π22 π2 =β 50×1×22 32 + 50×2×12 32 = β11.11πππ (Moment for one-half the structure is enough when advantage of symmetric structure is considered) Steps 3e to 3i Joint A Member AB BA BC C.O.F -1 -1 0.5 D.F 1 0.2 0.8 0 -11.11 2.22 8.89 B Initial FEM 0 Balance 1 C/O 1 -2.22 Total End -2.22 moment 2.22 C CB -2.22 -2.22 D CD CB D 2.22 22.5 -2.22 Step 4: Obtain final end moment by adding moments from step 2 and step 3 A Joint Member B AB BA C BC CB Moment for symmetrical load 26.56 53.12 -53.12 53.12 Moment for Antisymmetrical load -2.22 2.22 -2.22 -2.22 55.34 -55.34 50.90 Final End moment for a given load 24.34 D CD CB D -53.12 22.5 -26.56 2.22 22.5 -2.22 -50.90 22.5 -28.78 Step 5: Draw the bending moment diagram No loads at Vertical members. 2 loads are acting at beam, hence, sagging moments due to load at salient points in the beam are as shown below: Considering simply supported beam, reaction at left support is 75+66.67=141.67kN π΅ππ₯ππππ ππππ‘ π΅ππ₯ππππ ππππ‘ 50 × π₯ 2 = 141.67π₯ β πππ π₯ < 1 2 2 50 × π₯ = 141.67π₯ β β 100 × (π₯ β 1)πππ 1 β€ π₯ β€ 3 2 Mathematica β Code snippet to plot piecewise function Plot[Piecewise[{{141.67π₯ β 50π₯ 2 50π₯ 2 , π₯ <= 1}, {141.67π₯ β β 100(π₯ β 1), π₯ > 1}}], {π₯, 0,3}] 2 2 100 80 60 40 20 0.5 1.0 1.5 2.0 2.5 3.0 Step 6: Draw the Shear Force diagram Find π π΄ = 141.67 π π· = 108.33 HA = HD = 23.34+55.34 4 β50β28.78 4 = 19.67kN (ie) 19.67kN(ο ) = β19.67kN (ie) 19.67kN(ο) ---- End ----
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