Resisting forces and stresses

A useful adaptation is to place steel reinforcing bars inside concrete. Concrete resists compression well but is relatively weak in tension. The steel reinforcement carries tensile loads, helping the combined structure resist cracking and failure. This shows why choosing a material that is strong under one type of loading does not automatically make it suitable for every force.

Compression: resisting a squeeze

Compression occurs when forces push towards one another, tending to squash or shorten a material. Building columns and pillars carry compressive loads.

Chair legs provide another example. The sitter's weight pushes down through the chair, while the floor pushes upwards against the legs. These opposing forces compress each leg. A leg must carry this load without crushing or becoming unstable.

A composite panel can have a honeycomb core sandwiched between its outer faces to resist compression. The core supports the faces and helps prevent them being pushed together, while keeping the panel relatively lightweight. Its internal structure contributes to its performance, rather than relying only on a large mass of solid material.

Bending: stretching one side and compressing the other

Bending makes an object curve. A shelf or horizontal beam supported at both ends bends downwards when a load is placed on it. As it curves, its upper side becomes shorter and is compressed, while its lower side becomes longer and is in tension. Bending therefore involves both compression and tension within the same component.

Steel beams often have an I-shaped cross-section to resist bending. The broad upper and lower parts, called flanges, place material away from the centre of the beam, where it is effective at resisting bending. The connecting vertical web holds these parts apart. Changing the cross-sectional shape allows the beam to resist bending efficiently without making the entire section solid.

Bending can also be useful. Leaning against a chair backrest applies a bending load; a design may allow some controlled flex rather than requiring the backrest to remain completely rigid.

Torsion: resisting or transmitting a twist

Torsion is a twisting action. It occurs when one end of a component is turned while the other end resists, or when the ends are turned in opposite directions.

A screwdriver works with torsion: the handle is turned, and the shaft transmits the turning action to the screw. The shaft must resist excessive twisting while carrying that action.

A tower crane, by contrast, needs to resist unwanted twisting. Diagonal bracing helps stabilise its framework against torsion. Adding braces changes how forces pass through the structure, rather than simply increasing the thickness of every member.

Shear: resisting sliding or using it to cut

Shear occurs when opposing forces tend to make neighbouring parts of a material slide past each other. Unlike tension, which pulls along a component, shear acts across a section of it.

Scissors deliberately use shear. Their blades move past each other and apply opposing forces to the material between them, causing it to separate.

In a joint, shear can be unwanted. If connected parts try to slide relative to one another, the bolt holding them together can be sheared across its section. Bolts can be hardened to improve resistance. Positioning the smooth, unthreaded shank across the loaded joint also avoids placing the reduced section at the thread roots where shearing could occur.

Connecting force, shape and performance

Increasing a load's magnitude generally increases the demands on the component. Changing its direction can change the type of loading: a length of material pulled along its length experiences tension, but turning its ends relative to each other produces torsion.

An adaptation does not necessarily reduce the external load. An I-section still carries the applied load, but its shape helps it resist bending. Steel reinforcement and diagonal braces similarly change how a product carries forces. A sound design explanation therefore connects three things: the loading action, the relevant feature, and how that feature helps the product resist or use the force.

For a beam supported at both ends and bending downwards: top in compression, bottom in tension.

Match the adaptation to the force

• Tension: steel bars reinforce concrete and carry tensile loads.
• Compression: a honeycomb core supports the faces of a sandwich panel.
• Bending: an I-section positions material effectively to resist bending.
• Torsion: diagonal bracing helps a tower crane resist twisting.
• Shear: hardened bolts and an unthreaded shank across the joint improve resistance.

Resist or work with forces

Structures resist unwanted deformation; screwdrivers transmit torsion and scissors use shear to cut.

Consider both magnitude and direction. Changing a component's shape can improve resistance without reducing its external load.