Structural dimensions

The layer of structural dimensions brings to the fore the consequences of structural function on the structural form of an element. In structural engineer-ing, these consequences are determined by the internal forces of the element:

for a given structural material the required structural dimensions of an element can be calculated from the values of the internal forces.

These structural dimensions are determined by the most demanding structural functions and thus imply a filtering of functions: only those functions responsible for the final dimensions are to be brought to the fore.

This relation between function and form can be turned around: a structural form (in a specific material) is capable of performing certain structural functions (sometimes more than the one for which it is designed). Structural form then determines its possible functions.

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There are five major types of structural dimensions to be considered:

- Tension through axial transfer of force - Compression through axial transfer of force - Bending through parallel transfer of force - Torsion through axial transfer of moment - Bending through parallel transfer of moment

The characteristics of structural dimensions are directly derived from the characteristics of structural functions, except for axial transfer of force, which is divided into tension and compression. A distinction is made between tension and compression because structural dimensions can be smaller for tension (e.g. a cable) than for compression (e.g. a column) for the same load, since tensile members do not have to withstand buckling.

The symbol used for compression is two arrows pointing at each other. In order to express tension, two arrows pointing away from each other are used.

(In the lower figures, the conceptual element on which the layer is applied is that of a flat surface.)

The symbol used for bending through parallel transfer of force is I , indicated at one end of the structural axis. (This structural function is performed by a cantilever beam.) The symbol I indicates the position of maximum bending moment, where the material form requires the most height (if width is kept constant). It is drawn on the side of the structural axis where tension due to bending occurs. A distinction is made between tension in the upper side ( I on top of the axis) and the lower side ( I under the axis), because of the relation with the material form: the part of an element in ten-sion has no buckling

problems and can be slender compared to the part in compression.

In order to achieve a static balance for an element performing a parallel transfer of force, a ‘stabilizing’ moment or couple of forces is required. This couple of forces is placed at the side of the element where maximum bending moment occurs, as indicated with the symbol I . These stabilizing forces then respond to the maximum bending moment.

7. A structural language for conceptual design

137 Figure 7-13. Axial (tension/compression) and parallel (bending) transfer of force.

The symbol used for axial transfer of moment is a spiral. It indicates tension stresses that occur in the element due to the internal torsion along the axis of the element.

When a parallel transfer of moment occurs in a structural element, it is described in engineering sciences through a constant value for bending moment along the axis of the element. The symbols used here are two I on both ends of the structural axis. The symbols are placed on the side of the axis where tension through bending occurs. It indicates a continuous bending moment that the element has to withstand over its full length, and also the constant height of the material form.

The symbol used for axial transfer of moment is a spiral. It indicates tensile stresses that occur in the element due to the internal torsion along the axis of the element.

When a parallel transfer of moment occurs in a structural element, it is de-scribed in engineering science by a constant value for bending moment along the axis of the element. The symbols used here are two I ’s on both ends of the structural axis. The symbols are placed on the side of the axis where tension through bending occurs. This indicates a continuous bending moment that the element has to withstand over its full length, and also the constant height of the material form.

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Figure 7-14. Axial (torsion) and parallel (constant bending) transfer of moment.

This layer of structural dimensions relates within the engineering sciences to the calculated dimensions a structural element is required to possess (for a specific material) in order to perform certain structural functions. These calculations are performed using knowledge of the internal forces, which in turn relates to the layer of structural function.

The layer of structural dimensions only gives an indication (and not precise values) of the consequences of these structural functions on structural form, and is not related to any structural material. Through structural insight alone (i.e. without calculations), an engineer filters out redundant structural

functions and brings to the fore those functions that are vital to the structural dimensions of his or her design proposal. (Most engineers are trained in this filtering procedure of taking into account only the most important elements in an investigation.)

The rougher, more abstract structural information of this layer allows us to proceed more swiftly during design meetings, since time is not lost on extensive calculations.

This layer that brings to the fore dimensions of structural elements, relating closely to architectural design, as form is key in architecture. Through the layer’s filtering of structural information, the consequences of a structural design proposal on structural form are made explicit to architects. This filtering of information, however, might at times be achieved at the cost of a more transparent structural story.

7. A structural language for conceptual design

139 The layer of structural dimensions provides a link between a more intuitive understanding of structural behaviour through form, and a more theoretical understanding through internal forces. The former understanding is fed through everyday experiential encounters with structural forms; the latter is developed in engineering science.

In collaboration, the structural dimensions layer enables an engineer to express the design characteristics of his or her design proposal through the consequences for structural dimensions. This provides a more intuitive

understanding of the structural behaviour of his or her design proposition than is afforded by the structural function layer (though the two layers are closely related).

The structural dimensions layer also contains information on the conse-quences for the dimensions of an element when its general size is altered. For example, changing the length of a linear element that has the characteristic of axially transferring load under tension (e.g. a tie) will have no influence on its cross section. But when this characteristic is an axial transfer of load under compression (e.g. a column), making the element longer will require a larger cross section to counter the buckling problem.

The type of structural connection between elements is also expressed in this layer. For example, when a connection consists of two structural elements that both require bending moment height at their connecting sides, this connection will have to be stiff in bending.