SEED Guides Selecting Structural Configuration < >

Structural shapes are essentially linear, planar, or spatial, although of varying geometry. Each of these categories may be divided into a range of increasingly complex shapes. Any of which may be in the form of a solid, truss, or frame or some combination of them. It should be emphasised that all structures are three dimensionally solid, whatever their basic shape. One of the important results of this is the Poisson's ratio effect in solid materials, which measures the strain which occurs perpendicular to the main direction of loading. This causes stress and changes the shape of the structure in those directions where there are no applied loadings or reactions. Thus a short tube being crushed may actually fail in tension due to its expanding periphery.

Linear Structures

• A straight linear structure is compact in section about one long straight axis. Columns, and simple beams are typical examples.
• An arced linear structure has its long axis bent to follow some smooth two dimensional curve. Arches, suspension bridge cables, and complete rings fall into this category. Loading is usually in the plane of curvature but may be across it as in curved beams.
• A 2-D cranked linear structure has a series of straight or smoothly curved sections joined by steps or angles. All of the sections lie in the same plane, such as a simple portal frame in a building.
• A 3-D curved or convoluted linear structure may lie on, or be offset from a single axis, as in a helical spring, or have a totally three dimensional configuration, like the structure of a bent tube chair.

Planar Structures

• A planar structure may be thin, thick, or made from a thin sheet supported by a regular or irregular series of stiffeners or beams. It may have the open structure of a grillage or interconnected series of members in the same plane acting in a plate-like manner, or be a plate with holes for access or to lighten the structure.
• A flat "plate" is an important basic structural shape. Floors, walls, and even circuit boards are examples. It may also be a plane truss or frame in any of a wide range of configurations, either carrying loads mainly within its plane in a totally 2-D situation or loaded transverse to the plane and spanning between supports. Polygons of members, usually in rectangles, surrounding shear panels are specifically used for in-plane loading.
• An arched shell or plate is curved in one direction. These may be part cylindrical, have a more complex curve, or be corrugated. Where the curve is pronounced relative to the thickness of the plate, the lateral effects are significant and the structure is more properly spatial.
• A folded plane is a series of flat segments joined along straight parallel bend lines. While having much in common with arched structures, there are often high stresses near the bends.
• A compound curved shell may be as simple as a part spherical dome or as complex as a hyperbolic roof. Shells with multiple fold lines and convoluted curves should be considered as spatial structures. (Pressure vessels are a special case of curved shells which will be dealt with in another unit.)

Spatial Structures

• Spatial structures may be made up of a series of similar planar frames or trusses connected by spanning members. For much of the loading, each frame may be considered separately as a planar structure with relatively low loading perpendicular to those frames. Many single storey portal frame buildings and similar structures come into this category, but care should be taken where strong interactions between frames are necessary. These are sometimes referred to as 21/2-D structures.
• 3-D trusses are the simplest fully three dimensional structures. They were used extensively during the early part of the industrial revolution for bridges, roof trusses, towers, and much more, taking advantage of the materials then available, but now tend to be used mainly for smaller structures.
• 3-D frames make use of the bending strength of materials and the moment carrying ability of joints in all directions. The introduction of structural steels and reinforced concrete early in the twentieth century made the reliable use of frames possible. Portal frames are used extensively in buildings. Some structures may be trusses in some planes and frames in others.
• 3-D panelled structures are typically a series of open or closed rectangular boxes with thin shear panels surrounded by linear members.
• 3-D continuous structures are generally more solid, whether assembled or cut from solid.