Structural Design Principles

Structural Balance

Structural balance considers the forces in furniture to be in equilibrium. When forces are not in equilibrium, cantilevers can fail, shelves can sag, and furniture can tip over. Furniture must be able to withstand lateral forces (forces applied from the side), shear forces (internal forces working in parallel but opposite directions), and moment forces (rotational forces applied to joints).

Structural forces inspire design ideas and are an important consideration in the concep­tion of form. For example, rocking chairs give the user control of back-and-forth movement and rely on the weight and motion of the user, the form of the chair, and the center of gravity of both the user and the chair to function properly. Rocking back and forth also creates significant dynamic forces, stressing both the form and joinery of the rocker, therefore structural bracing is to be considered with this type of design.

Dimensional Limits

When a person is sitting in a chair, the tilting point should be located directly beneath the center point of gravity for the chair and body combined. The prin­ciple of balance is an important aspect to consider and achieve in the design for any human body support.

In cases where furniture is precariously tall and narrow or where the center of gravity lies beyond the tipping point, it may become necessary to attach furniture to a floor, wall, or ceiling in order to maintain structural stability. There are dimen­sional limits to the horizontal span of shelving before deflection occurs and imbal­ance appears. Shelving systems, bed frames, music stands, cabinet doors, and speaking podiums all depend on basic engineering principles to maintain structural balance and function safely.

Joinery Investigation

Joining is the stage where your project starts coming together through the making of joints for the various parts of your workpiece. Different joints are stronger in different circumstances, so it is important to use joints that are appropriate for the purpose of your piece. Where joints are wisely selected, and created with care, accuracy and precision, the piece will be strong and future-proof.

There is a wide variety of joints that you can create using the machines, tools and jigs available in the MSD Machine Workshop.

To read about the different types of joints and determine what would be best for your project, jump to this page:

Structural Bracing

A bracing system is a secondary but essential part of designing and constructing a piece of furniture. Bracing serves to stabilise the furniture structure, to distribute downward, lateral, moment and shear forces of load effects and to provide support to joinery.

Three Legs

A three-legged project will not wobble, regardless of slight differences in leg length, because the weight will always be distributed across all three. So consider this option for stools or small tables.

Cross-bracing

Cross bracing is usually seen with two diagonal supports placed in an X shaped manner; these support compression and tension forces. With different forces, one brace will be under tension while the other is being compressed. It helps make structures stand sturdier and resist lateral forces. Cross bracing can be applied to any rectangular frame structure, such as chairs and bookshelves.

Corner Blocks

A triangular wood block is used for added strength, in concealed structure under table tops, under seat pans, inside cases and at points of stress on upholstered furniture frames. Corner blocks provide diagonal and triangulation support to help prevent twisting and provide extra support to the joint.

Stretchers / Spindles

A stretcher is a horizontal support element of a table, chair or other item of furniture tying vertical elements of the piece together. There are numerous styles of the stretcher including circumferential, double and spindle design. This term is sometimes referred to as a stretcher beam. A very common pattern for chairs has each front leg connected to the back by the lateral stretchers, which in turn are connected by a medial stretcher.

Aprons / Rails / Skirt

An apron, is a wooden panel that connects the surface and legs of a table, desk, or sideboard that sits on legs. Some wooden side chairs could have aprons, although most chair legs are attached to the seat. An apron is placed at right angles to the underside of the top of a table, sideboard, desk, or seat of a chair and extends between the tops of the legs.

The apron's main purpose is to provide structural strength and support, but it also sometimes adds a decorative touch if visible.