# steel beam type and size for a clothesline

I have four posts in a square formation. Posts are also square. I want to bolt a beam between two posts on the left and a beam between two posts on the right, and than run some wire between the beams. Beams will be 2.8m long and distance between them will also be 2.8m. They will be 2m above ground. I was thinking a M10 or M12 bolt at each end of the beam. If the wire is right in the middle of beams and is holding 50kg (number taken from a commercial clothesline) what type of beam will be able to support this?

I was considering an angle 50x50x2mm because I can attach it flat to the square post or a 40x20x2mm SHS beam.

P.S. adding a picture as suggested. Black are posts, blue are beams, red is the wire.

• Rule zero of good engineering questions: draw a diagram! Commented Jun 30, 2020 at 7:25
• In Africa they use 2 poles with a T at the top to which are connected 4 lines, while at the base an angled beam resists inward deflection of the pole at an angle of 45°. All connecting are welded. Commented Jun 30, 2020 at 22:11

It depends on the material and slack of the wire.

A wire strong enough and pulled tight can impose large tension forces on the beam.

Say the wire is pulled straight and under 50kg deflects only 1cm.

The tension force or horizontal component of its reaction at beam will be

Fx = 280/2(1)50 = 7000 kg.

Which is impractically too large. And causes a huge moment for the columns as well.

If you heave some slack on the wire horizontal tension will decrease proportionally. Like if you have 2 cm slack tension becomes half 3500 kg.

But this stress is the governing stress not the vertical component of the wire loads.

• That does seem rather large, yet I see clotheslines everywhere even made out of smaller angles than I was considering. How is it possible? Commented Jun 30, 2020 at 22:05