Answer: Load in sub-truss: w'=5x10=50kN/m,M'max=(1/8)wL2=(1/8)x50x10x10=625kNm If 0=60,h=1.73m,it is the first choice for sub-truss structure. N=625/1.73=361kN,stress=N/A=361000/2500=144.5Mpa<200Mpa,OKl Check shear,Qmax=250kN,Ns=250/0.866=289kN,OK! For most economical purpose,0 can be changed as 51,h=1.25m,so that the maximal stress in sub-truss can reach 200Mpa,h=1.25m,N=500kN Load in main truss:P=w'x5=250kN, Mmax=7.5x0.5P+2.5xP=6.25P=6.25x250=1562.5kNm If 0=60.h=4.325m,it is the first choice for main truss structure. N=1562.5/4.325=361kN,Stress=-N/A=361000/2500=144Mpa,OK! Check shear,Qmax=500kN,Ns=500/0.866=577kN,Stress=N/A=231>200Mpa Fail! The tube with 50cm2 can be used in main truss with 0=32,h=1.563m N=1562.5/1.563=1000kN,Stress=-1000/5000=200Mpa,OK Qmax=500kN,Ns=500x1.89=945kN,Stress=N/A=189<200Mpa OK Weight of sub-truss=-8(10+8+10x1.6)x0.0025x7850=5.338T Weight of main truss==2(35+30+14x2.95)0.005x7850=8.344T Cost=(5.338+8.344)10000=136820RMBAnswer: Load in sub-truss: w’=5x10=50kN/m, M’max=(1/8)w’L²=(1/8)x50x10x10=625kNm If θ=60, h=1.73m, it is the first choice for sub-truss structure. N=625/1.73=361kN, stress=N/A=361000/2500=144.5Mpa<200Mpa, OK! Check shear, Qmax=250kN,Ns=250/0.866=289kN, OK! For most economical purpose, θ can be changed as 51, h=1.25m, so that the maximal stress in sub-truss can reach 200Mpa, h=1.25m, N=500kN Load in main truss: P=w’x5=250kN, Mmax=7.5x0.5P+2.5xP=6.25P=6.25x250=1562.5kNm If θ=60, h=4.325m, it is the first choice for main truss structure. N=1562.5/4.325=361kN, Stress=N/A=361000/2500=144Mpa, OK! Check shear, Qmax=500kN, Ns=500/0.866=577kN, Stress=N/A=231>200Mpa Fail! The tube with 50cm² can be used in main truss with θ=32, h=1.563m N=1562.5/1.563=1000kN, Stress=1000/5000=200Mpa, OK Qmax=500kN, Ns=500x1.89=945kN, Stress=N/A=189<200Mpa OK Weight of sub-truss=8(10+8+10x1.6)x0.0025x7850=5.338T Weight of main truss=2(35+30+14x2.95) 0.005x7850=8.344T Cost=( 5.338+8.344)10000=13 6820RMB