CIVL 7/8117 Chapter 4  Development of Beam Equations  Part 2 1/34. Chapter 4b Development of Beam Equations. Learning Objectives


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1 CIV 7/87 Chapter 4  Development of Beam Equations  Part /4 Chapter 4b Development of Beam Equations earning Objectives To introduce the workequivalence method for replacing distributed loading by a set of discrete loads To introduce the general formulation for solving beam problems with distributed loading acting on them To analyze beams with distributed loading acting on them To compare the finite element solution to an exact solution for a beam To derive the stiffness matrix for the beam element with nodal hinge To show how the potential energy method can be used to derive the beam element equations To apply Galerkin s residual method for deriving the beam element equations General Formulation We can account for the distributed loads or concentrated loads acting on beam elements by considering the following formulation for a general structure: F=KdF where F are the equivalent nodal forces, expressed in terms of the globalcoordinate components. These forces would yield the same displacements as the original distributed load. If we assume that the global nodal forces are not present (F = ) then: F =Kd
2 CIV 7/87 Chapter 4  Development of Beam Equations  Part /4 General Formulation We now solve for the displacements, d, given the nodal forces F. Next, substitute the displacements and the equivalent nodal forces F back into the original expression and solve for the global nodal forces. F=KdF This concept can be applied on a local basis to obtain the local nodal forces in individual elements of structures as: f=kdf Example 5  oad Replacement Consider the beam shown below; determine the equivalent nodal forces for the given distributed load. The work equivalent nodal forces are shown above. Using the beam stiffness equations: w f y 6 6 v w m fy 6 6v w m w
3 CIV 7/87 Chapter 4  Development of Beam Equations  Part /4 Example 5  oad Replacement Apply the boundary conditions: v 6 6 v v We can solve for the displacements w 6v w w v 8 w 6 Example 5  oad Replacement In this case, the method of equivalent nodal forces gives the exact solution for the displacements and rotations. To obtain the global nodal forces, we will first define the product of Kd to be F e, where F e is called the effective global nodal forces. Therefore: w e F 6 6 y e 5w M e 4 F 6 6 w y 8 w e M w 6 w
4 CIV 7/87 Chapter 4  Development of Beam Equations  Part 4/4 Example 5  oad Replacement Using the above expression and the fixend moments in: w w w F y 5w w w M F=KdF F y w w M w w Fy M Fy w w w Example 6  Cantilever Beam Consider the beam, shown below, determine the vertical displacement and rotation at the freeend and the nodal forces, including reactions. Assume is constant throughout the beam. We will use one element and replace the concentrated load with the appropriate nodal forces.
5 CIV 7/87 Chapter 4  Development of Beam Equations  Part 5/4 Example 6  Cantilever Beam The beam stiffness equations are: P fy 6 6v P m fy 6 6v P m P 8 Apply the boundary conditions: 6 6 v v v Example 6  Cantilever Beam The beam stiffness equations become: P 6v P P v 48 P 8 To obtain the global nodal forces, we begin by evaluating the effective nodal forces. e F 6 6 y e M e F P y 48 e M P 8 P P 8 P P 8
6 Example 6  Cantilever Beam Using the above expression in the following equation, gives: F=KdF 8 8 P P P P e Kd = F F F P P 8 8 P P P P y y F M F M F Example 6  Cantilever Beam In general, for any structure in which an equivalent nodal force replacement is made, the actual nodal forces acting on the structure are determined by first evaluating the effective nodal forces F e for the structure and then subtracting off the equivalent nodal forces F for the structure. Similarly, for any element of a structure in which equivalent nodal force replacement is made, the actual local nodal forces acting on the element are determined by first evaluating the effective local nodal forces for the element and then subtracting off the equivalent local nodal forces associated only with the element. f=kdf CIV 7/87 Chapter 4  Development of Beam Equations  Part 6/4
7 CIV 7/87 Chapter 4  Development of Beam Equations  Part 7/4 Comparison of FE Solution to Exact Solution We will now compare the finite element solution to the exact classical beam theory solution for the cantilever beam shown below. Both one and twoelement finite element solutions will be presented and compared to the exact solution obtained by the direct doubleintegration method. et E = x 6 psi, I = in 4, = in, and uniform load w = Ib/in. Comparison of FE Solution to Exact Solution To obtain the solution from classical beam theory, we use the doubleintegration method: Mx ( ) y where the double prime superscript indicates differentiation twice with respect to x and M is expressed as a function of x by using a section of the beam as shown: F y V( x) w wx M wx w Mx ( ) wx
8 CIV 7/87 Chapter 4  Development of Beam Equations  Part 8/4 Comparison of FE Solution to Exact Solution To obtain the solution from classical beam theory, we use the doubleintegration method: Mx ( ) y w x y x dxdx w x x x y C dx Boundary Conditions 6 y() y() 4 w x x x y Cx C w x x x y Comparison of FE Solution to Exact Solution Recall the oneelement solution to the cantilever beam is: 4 w v 8 w 6 Using the numerical values for this problem we get: 4 lb in in 6 4 v 8 psi in.8in lb in in. rad psi in
9 CIV 7/87 Chapter 4  Development of Beam Equations  Part 9/4 Comparison of FE Solution to Exact Solution The slope and displacement from the oneelement FE solution identically match the beam theory values evaluated at x =. The reason why these nodal values from the FE solution are correct is that the element nodal forces were calculated on the basis of being energy or work equivalent to the distributed load based on the assumed cubic displacement field within each beam element. Comparison of FE Solution to Exact Solution Values of displacement and slope at other locations along the beam for the FE are obtained by using the assumed cubic displacement function. vx ( ) x xv xx The value of the displacement at the midlength v(x = 5 in) is: v( x 5 in).78in Using beam theory, the displacement at v(x = 5 in) is: v( x 5 in).95in
10 CIV 7/87 Chapter 4  Development of Beam Equations  Part /4 Comparison of FE Solution to Exact Solution In general, the displacements evaluated by the FE method using the cubic function for v are lower than by those of beam theory except at the nodes. This is always true for beams subjected to some form of distributed load that are modeled using the cubic displacement function. The exception to this result is at the nodes, where the beam theory and FE results are identical because of the workequivalence concept used to replace the distributed load by workequivalent discrete loads at the nodes. Comparison of FE Solution to Exact Solution The beam theory solution predicts a quartic (fourthorder) polynomial expression for a beam subjected to uniformly distributed loading, while the FE solution v(x) assumes a cubic (thirdorder) displacement behavior in each beam all load conditions. The FE solution predicts a stiffer structure than the actual one. This is expected, as the FE model forces the beam into specific modes of displacement and effectively yields a stiffer model than the actual structure. However, as more elements are used in the model, the FE solution converges to the beam theory solution.
11 CIV 7/87 Chapter 4  Development of Beam Equations  Part /4 Comparison of FE Solution to Exact Solution For the special case of a beam subjected to only nodal concentrated loads, the beam theory predicts a cubic displacement behavior. The FE solution for displacement matches the beam theory solution for all locations along the beam length, as both v(x) and y(x) are cubic functions. Comparison of FE Solution to Exact Solution Under uniformly distributed loading, the beam theory solution predicts a quadratic moment and a linear shear force in the beam. However, the FE solution using the cubic displacement function predicts a linear bending moment and a constant shear force within each beam element used in the model.
12 CIV 7/87 Chapter 4  Development of Beam Equations  Part /4 Comparison of FE Solution to Exact Solution We will now determine the bending moment and shear force in the present problem based on the FE method. Mx ( ) Mx ( ) d y dx B d Nd d dx Nd 6 x 4 6x 6 x 6x B 6 x 4 6x M v 6 x 6x v Comparison of FE Solution to Exact Solution We will now determine the bending moment and shear force in the present problem based on the FE method. Position M FE M exact X = 8, lbin , lbin X = 5 in , lbin 5, lbin X = in 6,667 lbin
13 CIV 7/87 Chapter 4  Development of Beam Equations  Part /4 Comparison of FE Solution to Exact Solution The plots below show the displacement, bending moment, and shear force over the beam using beam theory and the oneelement FE solutions. Comparison of FE Solution to Exact Solution The FE solution for displacement matches the beam theory solution at the nodes but predicts smaller displacements (less deflection) at other locations along the beam length.
14 CIV 7/87 Chapter 4  Development of Beam Equations  Part 4/4 Comparison of FE Solution to Exact Solution The bending moment is derived by taking two derivatives on the displacement function. It then takes more elements to model the second derivative of the displacement function. Comparison of FE Solution to Exact Solution The shear force is derived by taking three derivatives on the displacement function. For the uniformly loaded beam, the shear force is a constant throughout the singieelement model.
15 CIV 7/87 Chapter 4  Development of Beam Equations  Part 5/4 Comparison of FE Solution to Exact Solution To improve the FE solution we need to use more elements in the model (refine the mesh) or use a higherorder element, such as a fifthorder approximation for the displacement function. Beam Element with Nodal Hinge Consider the beam, shown below, with an internal hinge. An internal hinge causes a discontinuity in the slope of the deflection curve at the hinge and the bending moment is zero at the hinge.
16 CIV 7/87 Chapter 4  Development of Beam Equations  Part 6/4 Beam Element with Nodal Hinge For a beam with a hinge on the right end: [ K] [ K] fy 6 6v m fy 6 6v m [ ] [ K ] K The moment m is zero and we can partition the matrix to eliminate the degree of freedom associated with. f K Kd f KdKd f KK d f K d K d Beam Element with Nodal Hinge For a beam with a hinge on the right end: fy 6 6v m fy 6 6v m d v v d f f y m f y f m d K f K d K d K K f K d f Kd Kd f c kcd fc fkk f m c k K K K K
17 CIV 7/87 Chapter 4  Development of Beam Equations  Part 7/4 Beam Element with Nodal Hinge We can condense out the degree of freedom by using the partitioning method discussed earlier. k K K K K c 6 6 kc Therefore, the condensed stiffness matrix is: kc Beam Element with Nodal Hinge The element forcedisplacement equations are: f y v m f y v Expanding the element forcedisplacement equations and maintaining m = gives: fy v m fy v m
18 CIV 7/87 Chapter 4  Development of Beam Equations  Part 8/4 Beam Element with Nodal Hinge Once the displacements d are found, d may be computed: d K f K d m v K m K v In this case, d is (the rotation at right side of the element at the hinge. Beam Element with Nodal Hinge For a beam with a hinge on the left end: fy 6 6v m fy 6 6v m Rewriting the equations to move and m to the first row: m f y 6 6 v fy 6 6 v m 6 6 4
19 CIV 7/87 Chapter 4  Development of Beam Equations  Part 9/4 Beam Element with Nodal Hinge For a beam with a hinge on the left end: [ K] [ K] m f y 6 6 v fy 6 6 v m K [ ] [ K ] The moment m is zero and we can partition the matrix to eliminate the degree of freedom associated with. f K Kd f KdKd f KK d f K d K d Beam Element with Nodal Hinge For a beam with a hinge on the right end: m f 6 6 v f 6 6 v m d K f K d d f m d v v f fy f m y f K d K d K d K K f K d m c kcd fc f KK f kc K KK K f
20 CIV 7/87 Chapter 4  Development of Beam Equations  Part /4 Beam Element with Nodal Hinge We can condense out the degree of freedom by using the partitioning method discussed earlier. k K K K K c 6 6 kc Therefore, the condensed stiffness matrix is: k c Beam Element with Nodal Hinge The element forcedisplacement equations are: fy v f y v m Expanding the element forcedisplacement equations and maintaining m = gives: fy v m fy v m
21 CIV 7/87 Chapter 4  Development of Beam Equations  Part /4 Beam Element with Nodal Hinge Once the displacements d are found, d may be computed: d K f K d m v K m K v In this case, d is (the rotation at right side of the element at the hinge. Beam Element with Nodal Hinge For a beam element with a hinge at its left end, the element forcedisplacement equations are: fy v f y v m Expanding the element forcedisplacement equations and maintaining m = gives: fy v m fy v m
22 CIV 7/87 Chapter 4  Development of Beam Equations  Part /4 Example 7  Beam With Hinge In the following beam, shown below, determine the vertical displacement and rotation at node and the element forces for the uniform beam with an internal hinge at node. Assume is constant throughout the beam. Example 7  Beam With Hinge The stiffness matrix for element (with hinge on right) is: fy v m fy v m v v fy a v m a a a fy a a v m
23 CIV 7/87 Chapter 4  Development of Beam Equations  Part /4 Example 7  Beam With Hinge The stiffness matrix for element (with no hinge) is: f 6 6v m f 6 6v m v v f 6b 6bv m 6b 4b 6b b f b 6b 6b v m 6b b 6b 4b Example 7  Beam With Hinge The assembled equations are: Element f a a a y v m a a a 6 6 f y a a b a b b b v m b b b b f 6 6 y v b b b b m b b b b Element
24 CIV 7/87 Chapter 4  Development of Beam Equations  Part 4/4 Example 7  Beam With Hinge The boundary conditions are: v v f a a a y v m a a a 6 6 f y a a a b b b b v m b b b b f 6 6 y v b b b b m b b b b Example 7  Beam With Hinge After applying the boundary conditions the global beam equations reduce to: 6 a b b v P 6 4 b b abp v b a abp b a
25 CIV 7/87 Chapter 4  Development of Beam Equations  Part 5/4 Example 7  Beam With Hinge The element forcedisplacement equations for element are: f y a m a a a a f y a abp b a bp b a f y ab P m b a f y bp b a Example 7  Beam With Hinge The slope on element may be found using the condensed matrix: m v d K f Kd K m K element v a 4 a abp b a 6a a 6a abp b a
26 CIV 7/87 Chapter 4  Development of Beam Equations  Part 6/4 Example 7  Beam With Hinge The element forcedisplacement equations for element are: abp f y 6b 6b b a m 6b 4b 6b b abp f y b 6b 6b b a m 6b b 6b 4b ap b a fy m f y ap m b a ba P b a Example 7  Beam With Hinge Displacements and rotations on each element. v abp v v b a abp b a abp b a
27 CIV 7/87 Chapter 4  Development of Beam Equations  Part 7/4 Potential Energy Approach to Derive Beam Element Equations et s derive the equations for a beam element using the principle of minimum potential energy. The procedure for applying the principle of minimum potential energy is similar to that used for the bar element. The total potential energy p is defined as the sum of the internal strain energy U and the potential energy of the external forces : U p Potential Energy Approach to Derive Beam Element Equations The differential internal work (strain energy) du in a onedimensional beam element is: U x x dv For a single beam element, shown below, subjected to both distributed and concentrated nodal forces, the potential energy due to forces (or the work done these forces) is: S V TvdS Pv m y iy i i i i i
28 CIV 7/87 Chapter 4  Development of Beam Equations  Part 8/4 Potential Energy Approach to Derive Beam Element Equations If the beam element has a constant crosssectional area A, then the differential volume of the beam is given as: dv da dx The differential element where the surface loading acts is given as: ds = b dx (where b is the width of the beam element). Therefore the total potential energy is: da dx T vb dx P v m p x x y iy i i i xa x i Potential Energy Approach to Derive Beam Element Equations dv The straindisplacement relationship is: x y dx We can express the strain in terms of nodal displacements and rotations as: x 6 6x4 x 6 6x x y d [ ] x yb d x 6 6x4 x 6 6x [ B]
29 CIV 7/87 Chapter 4  Development of Beam Equations  Part 9/4 Potential Energy Approach to Derive Beam Element Equations The stressstrain relationship in onedimension is: [ E] x x where E is the modulus of elasticity. Therefore: [ ][ ] x ye B d [ ] The total potential energy can be written in matrix form as: T T T da dx bt v dx d P p x x y xa x x yb d Potential Energy Approach to Derive Beam Element Equations If we define, w bt y as a line load (load per unit length) in the y direction and the substitute the definitions of x and x the total potential energy can be written in matrix form as: E p y d B B d dadx w d N dx d P T T T T T [ ] [ ] [ ] A Use the following definition for moment of inertia: I Then the total potential energy expression becomes: p T T T T T d [ B] [ B] ddx wd [ N] dx d P A y da
30 CIV 7/87 Chapter 4  Development of Beam Equations  Part /4 Potential Energy Approach to Derive Beam Element Equations Differentiating the total potential energy with respect to the displacement and rotations (v, v, and ) and equating each term to zero gives: The nodal forces vector is: T T [ B] [ B] dx d w[ N] dx P T f w[ N] dx P The elemental stiffness matrix is: T k [ B] [ B] dx Potential Energy Approach to Derive Beam Element Equations Integrating the previous matrix expression gives: T k [ B] [ B] dx k
31 CIV 7/87 Chapter 4  Development of Beam Equations  Part /4 Galerkin s Method to Derive Beam Element Equations The governing differential equation for a onedimensional beam is: 4 dv w 4 dx We can define the residual R as: 4 dv w N,,,and4 4 i dx i dx If we apply integration by parts twice to the first term we get: xxxx i xx i, xx i xxx i, x xx v N dx v N dx N v N v where the subscript x indicates a derivative with respect to x Galerkin s Method to Derive Beam Element Equations Since v N d to x is: or v xx xx [ ], then the second derivative of v with respect x 6 6x4 x 6 6x v B d [ ] Therefore the integration by parts becomes: Nixx, B dxd Nw i dx NV i Nix, m d d [ ] i,,, and 4
32 CIV 7/87 Chapter 4  Development of Beam Equations  Part /4 Galerkin s Method to Derive Beam Element Equations The above expression is really four equations (one for each N i ) and can be written in matrix form as: x T T T T [ B] [ B] dx d [ N] w dx [ N] m [ N] V The interpolation function in the last two terms can be evaluated: [ N] x [ ] [ N] x [ ] x [ N] x [ ] [ N] x [ ] x Galerkin s Method to Derive Beam Element Equations Therefore, the last two terms of the matrix form of the Galerkin formulation become (see the figure below): i V() i m() i V( ) i 4 m( )
33 CIV 7/87 Chapter 4  Development of Beam Equations  Part /4 Galerkin s Method to Derive Beam Element Equations When beam elements are assembled, as shown below: Two shear forces and two moments form adjoining elements contribute to the concentrated force and the concentrated moment at the node common to both elements. Problems: 7. Verify the four beam element equations are contained in the following matrix expression. T T [ B] [ B] dx d w[ N] dx P 8. Do problems 4., 4., 4.8, 4., 4.4 and 4.47 in your textbook.
34 CIV 7/87 Chapter 4  Development of Beam Equations  Part 4/4 Problems: 9. Work problem 4.6 in your using the SAP. Attempt to select the lightest standard W section to support the loads for the beam. The bending stress must not exceed 6 MPa and the allowable deflection must not exceed ( = 6 m)/6 End of Chapter 4b