4.1. Introduction

Cables and trusses are 2D or 3D structures composed of an assemblage of simple one-dimensional components which transfer only axial forces along their axis. Cables can carry only tensile forces, trusses can carry tensile and compressive forces.

This chunk defines cables and trusses. Cables are flexible structures that can only bear forces that pull on them (tensile forces), while trusses are more rigid and can bear forces that either push or pull (tensile and compressive forces). Axial forces are forces that act along the length of a member, meaning that cables and trusses are designed to react primarily to these types of forces.

Imagine a clothesline (a cable) that only holds your clothes up by pulling on them with tension. Now think of a bicycle frame (a truss), which uses both tension and compression to keep its shape and support a rider.

Cables tend to be flexible, and hence, they tend to oscillate and therefore must be stiffened. Trusses are extensively used for bridges, long span roofs, electric towers, and space structures.

Cables are known for their flexibility, which makes them susceptible to swaying or oscillating in the wind; hence, they require additional supports or stiffening methods to maintain stability. On the other hand, trusses are preferred in various construction applications, such as bridges or roofs, due to their strength and ability to span larger distances without sagging.

Think about a suspension bridge where the cables need to withstand the sway caused by strong winds. Engineers take care to design the system so that the cables don't oscillate excessively. The structure of the bridge, made up of trusses, allows it to carry heavy traffic without collapsing.

For trusses, it is assumed that

1. Bars are pin-connected
2. Joints are frictionless hinges
3. Loads are applied at the joints only.

Three key assumptions streamline truss analysis: first, that the bars are connected at the joints like pins, allowing rotation, which simplifies force calculations. Second, the joints act as frictionless hinges to allow free movement without resistance. Lastly, loads are considered to act at the joints, rather than along the bars, which simplifies structural analysis.

Imagine a mobile toy where different pieces connect at joints. The ease of movement at the joints represents frictionless connections. If weights (loads) are added to specific joints as you play, those areas are where the toy would need to be most stable, just like in a truss.

A truss would typically be composed of triangular elements with the bars on the upper chord under compression and those along the lower chord under tension.

Trusses are commonly made up of triangular shapes. This design is effective because, in a triangle, the structure is more stable when under different loads. The upper parts of the truss (the upper chord) experience compressive forces, meaning they are being pushed together, while the lower parts (the lower chord) experience tensile forces, meaning they are being pulled apart.

Think of a classic triangle you see in a bridge. If you imagine pushing down on the peak (the top part of a triangle), the base holds it together, just like how the bars of the truss work under the forces acting on them.

Fig. 4.1 illustrates some of the most common types of trusses. It can be easily determined that in a Pratt truss, the diagonal members are under tension, while in a Howe truss, they are in compression.

The text mentions common types of trusses, such as the Pratt and Howe trusses. Each type has its structural characteristics based on the arrangement of their members. In a Pratt truss, the diagonal members are designed to be tension members, making them suited for materials that resist tension effectively. In contrast, the Howe truss uses diagonal members that experience compression, which can be more favorable for heavier timber constructions.

Consider two types of trees: the tall and slender poplar tree represents the Pratt truss handling wind, while the stout oak represents the Howe truss that can handle heavier loads. Each structure uses its form to maintain stability and strength in the face of pressures.

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