Structures & Materials - Cranes and Steel Cable

[codz30]

Homework Statement

A crane consists of a crane arm which is held in place and moved by strong steel cables.
The crane arm is a truss structure, which is a rigid framework of quite thin steel beams welded together, forming many triangles.
The cables are made of steel and they are not one piece but consist of many thin strands of steel plaited together, in a way similar to most ropes. The cables carry very heavy loads but fortunatley they are almost perfectly elastic.

i) What is one reason that a truss structure like this is nearly as strong as a solid beam of about the same size?
ii) Why is the cable made of thin strands plaited into one thick cable stronger than a cable made of solid steel?
iii) Explain the meaning of 'elastic' with reference to the cable.

The Attempt at a Solution

i) The shape of the structurem using triangles, spreads out the load. Triangles are strong structures.

ii) Because the steel cables share the load better.

iii) It means the cables can spring back into shape.

I don't really think my explanations are correct, but it's the best I can think of.

 

[LowlyPion]

What about the cross sectional area for a beam? What about the weight of trussed lattice versus say a solid beam?

http://en.wikipedia.org/wiki/Beam_(structure )

Elastic suggests that it stretches uniformly depending on load doesn't it? Like a spring?

 

[nlightner]

Your explanations are on the right track, but let me provide some additional insights:

i) A truss structure is nearly as strong as a solid beam because it distributes and redirects the forces acting on it through the use of triangular shapes. These shapes are inherently strong and stable, allowing the structure to support heavy loads without collapsing. Additionally, the use of thin steel beams in a truss structure allows for a lightweight yet strong design, making it an efficient choice for cranes.

ii) The cable made of thin strands plaited into one thick cable is stronger than a cable made of solid steel because of the principle of load sharing. When a load is applied to the cable, the individual strands work together to distribute the load evenly, preventing any one strand from bearing the full weight. This increases the overall strength and durability of the cable.

iii) In this context, "elastic" refers to the ability of the steel cable to stretch and deform under a load, but then return to its original shape once the load is removed. This is important for cranes as they often need to lift and lower heavy loads repeatedly. The elastic nature of the cable allows it to withstand these repeated stresses without becoming permanently deformed or breaking.