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... = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = fini /clear /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 /plopts,info,1 /view,,1,2,3 /pnum,line,1 /dscale,,1 /eshape,1 /prep7 et,1,BEAM189 ! truss members keyopt,1,4,2 ! combined shear stress output sectype,1,beam,i 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 et,2,LINK10! cable keyopt,2,2,0 ! no stiffness when cable slack keyopt,2,3,0 ! tensiononly cable r,2 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 ! nodetosurface 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 r,3 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 ! userspecified constraints et,5,BEAM188 ! pulley support keyopt,5,3,2 ! internal midnode used keyopt,5,4,2 ! combined shear stress output sectype,5,beam,rect 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 l,1,2 l,2,3 l,3,4 l,4,5 l,6,7 l,7,8 l,1,6 l,2,6 l,2,7 l,3,7 l,4,7 l,4,8 l,5,8 larc,1,9,7,18.5 ! arcs for cable ... larc,5,9,7,18.5 lsel,s,line,,1,6,1 lsel,a,line,,7,9 lsel,a,line,,11,13 latt,1,,1,,9,,1 lsel,s,line,,10 latt,1,,1,,1,,1 lsel,all esize,1.0 lmesh,1,13,1 ! mesh truss with Ibeam sections nsel,none lsel,s,line,,14,15 lesize,all,,,24,0.25 latt,2,2,2,,7 lsel,all lmesh,14,15 ! mesh cables mat,3 type,3 real,3 *get,ncount,node,0,count ncurr=0 *do,i,1,ncount ncurr=ndnext(ncurr) e,ncurr ! create nodetosurface contact *enddo nsel,none 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) n2=node(6.0,0.5, 0.1) mat,3 type,4 real,3 tshap,cyli e,npilot,n2 ! rigid target cylinder (pulley) tshape,pilo e,npilot ! pilot node element nsel,all esel,all mat,5 type,5 real,5 secnum,5 ntruss=node(6.0,0.0,0.0) e,ntruss,npilot ! pulley support ... ! e,ntruss,n2 /pnum,type,1 /num,1 eplot fini /solu antype,static outres,all,all nlgeom,on ! large displacement solution nfix=node(0,0,0) d,nfix,all,0.0 ! fix left end of truss nroll=node(12,0,0) d,nroll,uy,0.0 ! allow right end of truss to roll d,all,uz,0.0 ! make problem 2D ... d,all,rotx,0.0 d,all,roty,0.0 time,1.0 nsubst,5,100,2 solve ! establish initial strain in cable save time,2.0 nsubst,1,10,1 f,node(3,3,0),fy,50000.0 ! downward concentrated loads ... f,node(6,3,0),fy,50000.0 f,node(9,3,0),fy,50000.0 acel,0.0,9.8,0.0 ! add gravitational load, m/sec^2 solve save fini /post1 /edge,,1 set,1,last /title, Bottom Beams in Compression from Cable Preload (Initial Strain) plnsol,s,x /wait,3 set,2,last /title, Bottom Beam Compression Reduced form Additional Loading ... plnsol,s,x /eof . 

