An engineer designing a bridge has twin obligations, to his client to use his money wisely, and tosociety to produce a structure that will enhance the built environment. In fact, beauty inengineering design has its roots in the tension that exists between designing for economy anddesigning for appearance.
Economy in this context is not simply saving money; it is a concept of rationality and frugality. Itis fundamental to engineering design that the designer is constantly planning how he can savematerials, and how he can make the construction process simpler, even if many of thesedesign decisions in isolation would not register on the overall balance sheet of a project.An example of this tension between appearance and economy is given by the design of anaccessramp to a high level bridge, Figure 1.1. The main bridge consists of a trapezoidal boxsection, 2.4 m deep, allowing it to span 60 m or more.
The access ramp must climb fromground level to merge with the main structure. At the point of merger, the ramp has thesame depth and shape as the main bridge. However, the 2.4 m depth would be out of scale for adeckclose to ground level. Consequently, the ramp is given a depth that gradually reduces to0.7 m as it approaches the ground, with the spans shortening correspondingly. This is clearlynot the most economical choice, as the formwork for the downstand webs of the ramp willbe continually changing.
In order to mitigate this additional cost of formwork, the geometry ofthe ramp deck may be de ned by keeping the length of the web shutters constant and equalto those of the main bridge, but changing their angle. Thus if the ramp is built span-by-span, theside shutters of the webs may be re-used for each span. This is an intellectual concept basedon an attempt to rationalise the construction method and save cost, which gives rise to adistinctive appearance. Finally this appearance must be judged on its own merits.
When an engineer designs, whether it is the overall concept of a bridge or an individualmember, he rst must understand the structural behaviour, and then seek rationality and economy. Thesearch will usually leave him many options, which allows him to make choices concerningthe appearance of the structure.
A very simple example is the design of the bridge pier carrying a single bearing, Figure 1.2. Thepier is subjected to a vertical load and to a horizontal load at the top which produces abending moment that increases linearly to a maximum at the base of the pier, Figure 1.2 (a).The size of the pier at the top will be limited by the size of the bridge bearing, while at thebottom it will be governed by the combined effect of the compression force and the bendingmoment. The engineer has a choice between, for instance:a prismatic column of a generous size that allows minimum reinforcement to be used throughout, Figure 1.2 (b);a smaller prismatic column that needs minimum reinforcement at the top, but heavyreinforcement at the base, Figure 1.2 (c);
a column that is as small as possible at the top and tapers uniformly to the bottom, Figure 1.2 (d);a column which is as small as possible at the top and whose width then varies such that theminimum reinforcement may be used throughout, Figure 1.2 (e);some combination of any of these.
His choice will be informed by other aspects of the project, for instance:the number of similar columns in the project;the range of heights of such columns;the need for variations on the basic column size to cater, for instance, for bridge expansionjoints, anchor piers or different length spans;the need for a family of columns to cater for other bridges forming part of the same project;the architectural context of the bridge.As the engineer considers the economy of the various choices to be made, he will most probably ndthat several options have costs that are within the margin of estimating error. Consequently, althoughthe search for economy is at the heart of his design, it cannot be used as an alternativeto aesthetic judgement; the engineer must choose the shape he considers is best in allthe circumstances.
Once he has made his basic choices, he then has to rene his design, both for economy and appearance; small changes of shape can greatly affect the appearance, as may be seen incomparing the options for a column of varying width shown in Figure 1.3. Reinforced concretedetailing considerations may also suggest minor dimensional changes, to give a rationalarrangement of bars, or to make best use of standard bar lengths and minimise waste.What an engineering designer cannot do and retain the integrity of his design is to yinthe face of rationality and economy, and design a heavily loaded column that, for instance,tapers towards the bottom, Figure 1.2 (f), creating an articial problem that then needsto be solved by misdirected engineering ingenuity. This is true even if the additional cost as compared with a rational design is negligible.
There is no reason that the column should not be decorated, with corners cut off, the sidesfaceted, Figure 1.4, or with ribs or other decorative nish, Figure 1.5 (7.15.4), as long asthe cost of this decoration is reasonable in the context of the project. Some aspects of suchdecoration may be functional, for instance to reduce the apparent bulk of the column bychanging the way light re ects off it or to control water runs to improve its weathering, whilesome may be just to make it more attractive.
Engineering design is thus driven by the simultaneous consideration of rationality, economy andappearance. Designing economically alone is not enough. There is no automatic linkagebetween economy and beauty; aesthetic judgement is required at every step of a design.Engineers have been known to put their faith in the idea that if they design honestly, andre ect in their structure the ow of forces, the result will inevitably be aestheticallysatisfactory, or even beautiful: the idea that 'form follows function'. Unfortunately,this is not suf cient. Within the con nes of honesty and economy, the engineer is left with a wide choice, which requiresaesthetic judgement.
A useful analogy is to consider the design of the human face,which is well de ned by its function, but which gives rise to an in nite number of outcomes.If bridge designers are not condent of their aesthetic ability, they should request theassistance of an architect, who should be involved from the earliest stages of the design. Ifthey are lucky, they will nd one who understands the special quality of engineering design,and who does not take over the project with his own, nonengineeringtaste.
An engineer designing a bridge has twin obligations, to his client to use his money wisely, and tosociety to produce a structure that will enhance the built environment. In fact, beauty inengineering design has its roots in the tension that exists between designing for economy anddesigning for appearance.
Economy in this context is not simply saving money; it is a concept of rationality and frugality. Itis fundamental to engineering design that the designer is constantly planning how he can savematerials, and how he can make the construction process simpler, even if many of thesedesign decisions in isolation would not register on the overall balance sheet of a project.An example of this tension between appearance and economy is given by the design of anaccessramp to a high level bridge, Figure 1.1. The main bridge consists of a trapezoidal boxsection, 2.4 m deep, allowing it to span 60 m or more.
The access ramp must climb fromground level to merge with the main structure. At the point of merger, the ramp has thesame depth and shape as the main bridge. However, the 2.4 m depth would be out of scale for adeckclose to ground level. Consequently, the ramp is given a depth that gradually reduces to0.7 m as it approaches the ground, with the spans shortening correspondingly. This is clearlynot the most economical choice, as the formwork for the downstand webs of the ramp willbe continually changing.
In order to mitigate this additional cost of formwork, the geometry ofthe ramp deck may be de ned by keeping the length of the web shutters constant and equalto those of the main bridge, but changing their angle. Thus if the ramp is built span-by-span, theside shutters of the webs may be re-used for each span. This is an intellectual concept basedon an attempt to rationalise the construction method and save cost, which gives rise to adistinctive appearance. Finally this appearance must be judged on its own merits.
When an engineer designs, whether it is the overall concept of a bridge or an individualmember, he rst must understand the structural behaviour, and then seek rationality and economy. Thesearch will usually leave him many options, which allows him to make choices concerningthe appearance of the structure.
A very simple example is the design of the bridge pier carrying a single bearing, Figure 1.2. Thepier is subjected to a vertical load and to a horizontal load at the top which produces abending moment that increases linearly to a maximum at the base of the pier, Figure 1.2 (a).The size of the pier at the top will be limited by the size of the bridge bearing, while at thebottom it will be governed by the combined effect of the compression force and the bendingmoment. The engineer has a choice between, for instance:a prismatic column of a generous size that allows minimum reinforcement to be used throughout, Figure 1.2 (b);a smaller prismatic column that needs minimum reinforcement at the top, but heavyreinforcement at the base, Figure 1.2 (c);
a column that is as small as possible at the top and tapers uniformly to the bottom, Figure 1.2 (d);a column which is as small as possible at the top and whose width then varies such that theminimum reinforcement may be used throughout, Figure 1.2 (e);some combination of any of these.
His choice will be informed by other aspects of the project, for instance:the number of similar columns in the project;the range of heights of such columns;the need for variations on the basic column size to cater, for instance, for bridge expansionjoints, anchor piers or different length spans;the need for a family of columns to cater for other bridges forming part of the same project;the architectural context of the bridge.As the engineer considers the economy of the various choices to be made, he will most probably ndthat several options have costs that are within the margin of estimating error. Consequently, althoughthe search for economy is at the heart of his design, it cannot be used as an alternativeto aesthetic judgement; the engineer must choose the shape he considers is best in allthe circumstances.
Once he has made his basic choices, he then has to rene his design, both for economy and appearance; small changes of shape can greatly affect the appearance, as may be seen incomparing the options for a column of varying width shown in Figure 1.3. Reinforced concretedetailing considerations may also suggest minor dimensional changes, to give a rationalarrangement of bars, or to make best use of standard bar lengths and minimise waste.What an engineering designer cannot do and retain the integrity of his design is to yinthe face of rationality and economy, and design a heavily loaded column that, for instance,tapers towards the bottom, Figure 1.2 (f), creating an articial problem that then needsto be solved by misdirected engineering ingenuity. This is true even if the additional cost as compared with a rational design is negligible.
There is no reason that the column should not be decorated, with corners cut off, the sidesfaceted, Figure 1.4, or with ribs or other decorative nish, Figure 1.5 (7.15.4), as long asthe cost of this decoration is reasonable in the context of the project. Some aspects of suchdecoration may be functional, for instance to reduce the apparent bulk of the column bychanging the way light re ects off it or to control water runs to improve its weathering, whilesome may be just to make it more attractive.
Engineering design is thus driven by the simultaneous consideration of rationality, economy andappearance. Designing economically alone is not enough. There is no automatic linkagebetween economy and beauty; aesthetic judgement is required at every step of a design.Engineers have been known to put their faith in the idea that if they design honestly, andre ect in their structure the ow of forces, the result will inevitably be aestheticallysatisfactory, or even beautiful: the idea that 'form follows function'. Unfortunately,this is not suf cient. Within the con nes of honesty and economy, the engineer is left with a wide choice, which requiresaesthetic judgement.
A useful analogy is to consider the design of the human face,which is well de ned by its function, but which gives rise to an in nite number of outcomes.If bridge designers are not condent of their aesthetic ability, they should request theassistance of an architect, who should be involved from the earliest stages of the design. Ifthey are lucky, they will nd one who understands the special quality of engineering design,and who does not take over the project with his own, nonengineeringtaste.
Such collaboration can be very creative, but success depends rstly onthe engineer being skilled and condentin the technical domain, and secondly in thearchitect having a genuine interest and feeling for engineering structures. Even engineerswho have condencein their aesthetic judgement can nd collaborationwitha talented architect very creative, with the architect questioning the engineer'schoices,and proposing different ways of seeing the design.
Such collaboration can be very creative, but success depends rstly onthe engineer being skilled and condentin the technical domain, and secondly in thearchitect having a genuine interest and feeling for engineering structures. Even engineerswho have condencein their aesthetic judgement can nd collaborationwitha talented architect very creative, with the architect questioning the engineer'schoices,and proposing different ways of seeing the design.
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