Combine PDF
Combine PDF
Fig shows 2d truss structure and each node has two degrees of freedom.
The only difference between bar element and truss element is that in
bars both local and global coordinate systems are same where in truss
these are different.
50
A load on a truss can only be applied at the joints (nodes)
Due to the load applied each bar of a truss is either induced
with tensile/compressive forces
The joints in a truss are assumed to be frictionless pin joints
Self weight of the bars are neglected
Consider one truss element as shown that has nodes 1 and 2 .The
coordinate system that passes along the element (x l axis) is called
local coordinate and X-Y system is called as global coordinate
system. After the loads applied let the element takes new position
say locally node 1 has displaced by an amount q 1l and node2 has
moved by an amount equal to q2l.As each node has 2 dof in
global coordinate system .let node 1 has displacements q1 and q2
along x and y axis respectively similarly q 3 and q4 at node 2.
Resolving the components q1, q2, q3 and q4 along the bar we get two
equations as
51
Or
U = ½ qTKq
For a truss element we can write
U = ½ qlT K ql
Where ql = L q and q1T = LT qT
52
Therefore
U = ½ qlT K ql
Taking the product of all these matrix we have stiffness matrix for truss
element which is given as
53
Stress component for truss element
= E
Therefore
54
When orientation of an element is know we use this angle to calculate l
and m as:
55
3
2
= 901.3 mm
Similarly calculate all the parameters for element 2 and tabulate
56
Calculate stiffness matrix for both the elements
Element 1 has displacements q1, q2, q3, q4. Hence numbering scheme
for the first stiffness matrix (K1) as 1 2 3 4 similarly for K2 3 4 5 & 6 as
shown above.
Global stiffness matrix: the structure has 3 nodes at each node 3 dof
hence size of global stiffness matrix will be 3 X 2 = 6
ie 6 X 6
57
From the equation KQ = F we have the following matrix. Since node 1
is fixed q1=q2=0 and also at node 3 q5 = q6 = 0 .At node 2 q3 & q4 are
free hence has displacements.
In the load vector applied force is at node 2 ie F4 = 50KN rest other
forces zero.
58
Solution: Node numbering and element numbering is followed for the
given structure if not specified, as shown below
59
Global stiffness matrix: the structure has 4 nodes at each node 3 dof
hence size of global stiffness matrix will be 4 X 2 = 8
ie 8 X 8
60
Solving the matrix gives the value of q3, q5 and q6.
61
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V N tNL V, + N, V t Ny ve
= -(-a)4, - (#), +(ea) y +(1-a),>0-
--)o () (*n) o
(tr) o
-(1-E.) (1) o
(&)
= o -C-)
o
(-) o
o
(1) O
(ttn)o (
o - -+)
O
Sushe s"
ARoo StAuss
fr-J-fe}
Numerca TL on
ALd neulkal plo blmo & devaton has ben
donn in Pant A ( chaptes )