I am trying to analyze a bridge truss. The geometry includes truss members,
cables and a pulley. I have attached the geometry (Truss.jpg) for your reference.
The bridge design has a cable passing over a pulley. The objective is to avoid
tension in the bottom chord members of the truss by pretensioning the cables.
The pretension cables should produce a compressive stress in the bottom chord
members and thus balance the existing tension developed in them under a live
load. How do I model the pulley and apply the load?
Please see the ANSYS 9.0 input below. BEAM189 elements were used for the
truss structure, LINK10 elements were used for the cable, CONTA175 elements
were used for the nodes to surface contact between the cable and the pulley,
and a TARGE170 rigid target cylinder was used to model the pulley. By including
an initial strain (1%) in the LINK10 cable elements, the bottom members of the
bridge truss were put in compression to begin the analysis. In a second load
step, the live load was added. All of the dimensions, section properties, loads,
etc., were made up, so please only use this example for procedural purposes...
= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
/title, Truss Design
/RGB, INDEX, 100, 100, 100, 0
/RGB, INDEX, 80, 80, 80, 13
/RGB, INDEX, 60, 60, 60, 14
/RGB, INDEX, 0, 0, 0, 15
et,1,BEAM189 ! truss members
keyopt,1,4,2 ! combined shear stress output
secdata,0.1,0.1,0.2,0.01,0.01,0.01 ! w1, w2, w3, t1, t2, t3, meters
mp,ex,1,2.07e11 ! Young's modulus, Pa (N/m^2)
mp,prxy,1,0.30 ! Poisson's ratio, unitless
mp,dens,1,7.83e3 ! mass density, kg/m^3
keyopt,2,2,0 ! no stiffness when cable slack
keyopt,2,3,0 ! tension-only cable
rmodif,2,1,0.0004909 ! area (m^2) for 0.025m diameter cable
rmodif,2,2,0.01 ! initial strain (1%)
mp,ex,2,2.07e11 ! Young's modulus, Pa (N/m^2)
mp,dens,2,7.83e3 ! mass density, kg/m^3
et,3,CONTA175 ! node-to-surface contact
keyopt,3,5,3 ! "Adjusted to Touch" ...
keyopt,3,7,1 ! automatic bisection of increment
keyopt,3,10,2 ! update contact stiffness each iteration
rmodif,3,1,0.50 ! rigid cylinder radius, meters
mp,mu,3,0.01 ! small amount of friction
et,4,TARGE170 ! rigid target cylinder (pulley)
keyopt,4,2,1 ! user-specified constraints
et,5,BEAM188 ! pulley support
keyopt,5,3,2 ! internal mid-node used
keyopt,5,4,2 ! combined shear stress output
secdata,0.05,0.05 ! width and height, meters
mp,ex,5,2.07e11 ! Young's modulus, Pa (N/m^2)
mp,prxy,5,0.30 ! Poisson's ratio, unitless
mp,dens,5,7.83e3 ! mass density, kg/m^3
k, 1, 0.0, 0.0, 0.0
k, 2, 3.0, 0.0, 0.0
k, 3, 6.0, 0.0, 0.0
k, 4, 9.0, 0.0, 0.0
k, 5, 12.0, 0.0, 0.0
k, 6, 3.0, 3.0, 0.0
k, 7, 6.0, 3.0, 0.0
k, 8, 9.0, 3.0, 0.0
k, 9, 6.0, -1.0, 0.0
larc,1,9,7,18.5 ! arcs for cable ...
lmesh,1,13,1 ! mesh truss with I-beam sections
lmesh,14,15 ! mesh cables
e,ncurr ! create node-to-surface contact
n,, 6.0, -0.5, -0.1
n,, 6.0, -0.5, 0.1
npilot=node(6.0,-0.5,-0.1) ! pilot node for rigid cylinder (pulley)
e,npilot,n2 ! rigid target cylinder (pulley)
e,npilot ! pilot node element
e,ntruss,npilot ! pulley support ...
nlgeom,on ! large displacement solution
d,nfix,all,0.0 ! fix left end of truss
d,nroll,uy,0.0 ! allow right end of truss to roll
d,all,uz,0.0 ! make problem 2D ...
solve ! establish initial strain in cable
f,node(3,3,0),fy,-50000.0 ! downward concentrated loads ...
acel,0.0,9.8,0.0 ! add gravitational load, m/sec^2
/title, Bottom Beams in Compression from Cable Preload (Initial Strain)
/title, Bottom Beam Compression Reduced form Additional Loading ...