Design a single angle strut:

Design a single angle strut of effective length 2.5 m to carry a compressive force of 70 kN. Use steel with f_y = 250 MPa.

Solution

Try an equal angle strut

(a) Choose ISA 80 × 80 × 8,

Area = 12.21 cm² , r_min = 1.55 cm

∴  λ = 2500/15.5  = 161

Corresponding value of σ_ac = 41 MPa (from Table 2).

∴ P allowed = (0.8 × 41) × 1221 N = 40 kN which is too low.

(b) Retry with ISA 100 × 100 × 10, Area = 19.03 cm²,

r_min  = 1.94 cm.

λ = 2500 /19.4= 129

Corresponding value of σ_ac = 57 MPa

∴ P allowed = (0.8 × 57) × 1903 N = 86.7 kN which is too high

than the given load of 70 kN.

(c) Hence a third trial is made with ISA 100 × 100 × 8 for which the area is 15.4 cm² or r_min is 1.94 cm as λ remains the same as in case (b) above. σ_ac = 57 MPa and axial load allowed = 0.8 × 57 × 1540 N = 70.2 kN which is ok.