What lies beneath: Colin ledsome CEng FIED explains how a better understanding of the structure underlying all solid objects can help across the whole spectrum of design.
A dress designer considers the ways materials will drape and move and how they can be cut and sewn to produce a desirable result for the wearer; but they are defining shapes of flexible materials and the relationships between them. All designers of products, for whatever underlying reasons, spend their time defining solid objects in some way. The skill of design is, at heart, based on an understanding of the way solid materials behave and an ability to use that understanding to produce a desirable result. What makes a solid into a solid is its ability to resist attempts to deform it, ie, its structure. Thus all product and engineering designers are, in effect, designers of structure.
This may seem trivial, but the point here is to suggest that if any designer of physical objects has a better understanding of structural behaviour they will produce better designs, whatever the main reasons for their work. As we grow up, we gain an innate feel for the strength and stiffness of everyday objects when we bump into them, climb on them, or see them supporting a heavy weight. We find from experience how hard or soft, how sturdy or fragile, how robust or breakable things are. We learn how different materials react to the way we treat them, how warm or cold, rough or smooth they feel. We know more about structure than most of us realise. We use that knowledge in all our interactions with the objects around us.
The first thing to realise, to better understand structural behaviour, is that any form of loading leads to deformation. A solid material resists deformation by generating internal forces to counter it. The more it deforms, the more it resists, until a balance is reached. Nothing is rigid; there are just different degrees of stiffness. The internal forces are in proportion to the amount of deformation. For most structural materials this relationship is described as 'linearly elastic', which means that if the load increases by a small amount, the deformation increases by the same proportion, and when the load is reduced again, the object returns to its original shape. Not all materials have a linear behaviour, but at modest loadings will still behave elastically, returning to their original shape.
This load and deflection interaction is what we call stiffness. Chunkier structures are stiffer than slimmer structures of the same shape and material. Each material type has a different inherent stiffness measured linearly by the Modulus of Elasticity, also known as Young's Modulus. Some, like wood or fibreglass, have different stiffnesses in different directions. Where there are several routes for a load to pass through a structure, it will divide in proportion to the stiffnesses of the paths available. This is a useful model to help design structure. It can also help to diagnose problems. Let's consider a few real cases.
Velcro is a very useful way of connecting things. It consists of two tapes, one covered in tiny hooks and the other tiny loops. These are fastened to the surfaces to be connected and simply pressed together. Although small pieces can be putted apart, larger parts need to be peeled, which means that at least one of the parts must be flexible. Velcro is used extensively in clothing, often with elasticated materials, which is where a problem arises because of the difference in stiffness between the elastic material and the stiffer Velcro tape.
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The reaction to the tension in that waistband is a pressure distributed round your waist; although which is load and which reaction depends on whether you are fastening your belt or having a heavy lunch. A familiarity with structural behaviour can reveal occasions where the designer can take advantage of differences in stiffness.
An engineer friend of mine wanted to replace a Eight garden gate with a heavier one. The strap hinges were firmly cemented into the gatepost, so he decided to use them again, but was a bit worried that they might not take the extra weight.
With this type of hinge arrangement all the weight is carried on one hinge. Although there are two potential toad paths, any slight difference in the distance between the hinges on the gate and gatepost transfers the load to only one. As Figure 2 shows, if the straps are further apart than the supports, all the load is carried by the bottom hinge, and if closer, then on the top one. (Both hinges carry horizontal loads to resist the offset load of the gate.) The problem is that whichever hinge is carrying the weight, its deflection is unlikely to close the gap in the other hinge. Even if the distances were exactly the same when fitted, a slight change in temperature or humidity (with a wooden gate) would unevenly expand or contract the spacing. My friend weighed the new gate. He selected a spring which would compress to a convenient height under a load equal to half the weight. He then positioned the straps on the gate closer together by the height of the compressed spring.
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This arrangement meant that the weight was shared about equally between the hinges. Any slight change in spacing would only change the load distribution by a few per cent. Reducing the stiffness of one of the load paths allowed a more favourable distribution of the load.
Few structures provide a simple straight load path between where the load is applied and where it is reacted. Whenever a load changes direction, some compensating pattern of force is inevitably introduced. The simplest expression of this happens in a truss structure. This consists of a network of individual members, which each carry only axial loads, connected by pins or similar joints, which cannot carry bending loads. These can be two or three dimensional structures.
At each joint, a toad in one member has to change direction and divide into components to be routed along the other members. These new loadings can only balance each other, if they completely compensate for the direction changes.A simple bridge truss can illustrate this. The load on the bridge has been concentrated at the lower pins by the local roadway structure and flows along the various members to be reacted at the end support points.
Note that this results in major loads in the horizontal top and bottom members, even though the loading and reactions are vertical.
In three dimensions, the ultimate truss structure is the geodetic truss (as seen in the new King's Cross Station roof). RJ Mitchell introduced these structures on the legendary Spitfire and they were developed further by Barnes Wallis on bombers such as the Wellington. They had the great advantage of being damage tolerant in that, even with large holes in the structure from anti-aircraft fire, the truss could re-route loading around the damage and stay in one piece.
For structures with the capacity to resist bending, a more continuous change of load flow operates. Consider a simple bridge beam carrying the same loading as the truss in Figure 5.
Here the load is spread along the top of the beam itself. Load flows down into the web of the beam and is transferred sideways as `shear' until it reaches the supports at the ends. The distortion induced by this sideways flow is resisted by a build up of tension and compression loads in the flanges, in a similar way to the loads in the top and bottom members of the truss bridge above. In practice, additional stiffeners would be needed in the web to spread loading down into the web.
Loads come in many forms, from a 'body' force such as the weight of an object, where the load is spread over the whole object, to the pressure of a load distributed over a surface, to the force applied by a spring or the pressure of a finger on an area small enough to be considered as a point. Loads are not all forces. Some are the rotational loads usually referred to as torques or moments. These loads result in either a balancing pattern of reactions at the supports for a static case, or an acceleration in proportion to any unsupported forces or torques. This is described by Newton's third law of motion, 'action and reaction are equal and opposite', which applies for all directions and rotations.
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Creating a sensible mind model of the flow of forces between the toad application and the reaction of the supports, or resulting accelerations, is the basis for designing the structure of an object. This may not be totally intuitive. It's easy for a designer to generate a problem simply by having an over-simplified structural model in their head. Consider the following familiar object, a rotary clothes after (see above). The original design work is not available, but we can draw some conclusions from looking at the structure of one of the support frames supporting the weight of the clothing.
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It is easy to imagine the original designer having something like this picture in mind, when considering the design details. In particular, you can see that the member A-B is in tension no matter how much washing is on the lines. Thus the main member C-D is designed with a larger section to resist bending, and A-B is a simple tension rod, as can be seen in Figure 8. However, if you examine any broken airers, in most of them A-B is buckled. It has failed in compression not tension. To see why, we must take a larger view of the whole structure. Consider just one of the clothes lines:
Here we can see that the Loadings applied to the support frames are nearer horizontal than vertical. What does this do to the force diagram from Figure 9?
Now we can see that A-B is in compression and a buckling failure becomes understandable. Some recent versions of the rotary airer have replaced the tension rod design with a channel section. This not only resists compression better than a rod but allows the parts to nest together when the airer is folded.
This example shows the importance of care in forming a structural mind model. All structures are three-dimensional and must be modelled that way. The more detailed the thinking behind the structure, the better it can be designed.
These examples are intended to show how a better understanding of the structure underlying all solid objects can help across the whole spectrum of design. Designs can be lighter and more efficient; details can be more robust and longer lasting. I hope that I have also shown that it is not difficult to understand structural behaviour without getting tangled in the complexities of analytical equations. A better appreciation of the role of the structure of materials should be a key part of all design education.
(Some of the diagrams used were originally drawn for the Engineering Design Teaching Aids series, published in the Design Council's Engineering Design Education magazine during the 1980s.)
The skill of design is, at heart, based on an understanding of the way solid materials behave
Few structures provide a simple straight load path between where the load is applied and where it is reacted
In three dimensions, the ultimate truss structure is the geodetic truss
Create a sensible mind model of the flow of forces between the load application and the reaction of the supports
The New Science of Strong Materials or Why You Don't Fall Through the Floor, J E Gordon and P Ball, Penguin. Structures: or Why Things Don't Fall Down, J E Gordon, Penguin.
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|Title Annotation:||Structural design|
|Date:||Jul 1, 2012|
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