How can we capture the experience gained on one project, to improve delivery of the next? In this article, Thomas Boothby discusses the true meaning of empirical design and how all parties involved in construction identify, categorise and reuse their experience with buildings.
A better understanding of the time-honoured method of empirical design could benefit anyone practicing engineering by allowing quick visualization of a proposed project for immediate assessment by more modern methods, and by conceding some value in assessment to the previous history of a project.
For an experienced practitioner, empirical design means drawing on personal experience with building structures to be able to determine approximate size and configuration for the next project. For an intern engineer or architect, or for a building manager, this experience is gained by critical evaluation of every building and every project that they encounter.
Learning from experience
Often, in senior engineers, the accumulation of experience makes a practitioner capable of intelligent empirical decisions in the preliminary layout and sizing of a building project. Graduate engineers may need a more organised presentation of the sizing of structural elements based on experience. Any other professional associated with building may benefit from being able to size the elements of a building structure intelligently.
The word ‘empirical’ derives from the Greek word for ‘experience.’ Empirical design, as applied to the structural design of buildings, is the application of proven sizes, proportions, materials, and assemblies. The basis of empirical design is previous experience, without regard for any systematic theory.
There are two primary benefits to the application of empirical design. The first, is the ability to make expedient decisions on layout and sizing (and to communicate these decisions to the rest of the design team) prior to undertaking a detailed structural design.
The second, in the area of historic preservation, is that by understanding the method by which most historic buildings were designed, it becomes less necessary to impose a modern structural outlook on these structures in order to trust their safety.
History of Empirical Design
Buildings have been designed and built by empirical design for most of the history of building, up to the nineteenth century, when some rational forms of design began to appear.
The temples of ancient Greece and Rome were built according to proportioning rules that defined the size of all of the elements on the basis of the column diameter. (In an Ionic temple, the columns are spaced 2 column diameters apart, the architrave has a depth of 1/12 of the column height, the frieze ¾ the height of the architrave, etc.)
Other monumental buildings, such as the Pantheon, the Colosseum, or the Constantinian basilica of St. Peter’s, were worked out according to similar proportional rules.
The structural design of the roof of the Pantheon contains a number of empirically-based strategies for minimizing the horizontal thrust of the roof, such as attenuating the dome towards the top, the total removal of material at the crown of the dome for the open oculus, and the addition of weight at the supports of the dome.
To our knowledge, the design of medieval buildings is the application of geometrical constructions and geometrical ratios to building design. Such ratios not only generated the form of the plan and elevations, but gave some assurance of structural safety.
Similarly, the buildings of the Renaissance are based on the development of harmonious proportions that simultaneously satisfy the eye, and create stable, enduring structures. It is likely that these two guiding principles are fundamentally the same.
How the building professions can use empirical design
Empirical rules can be very useful for contemporary architectural and engineering practice. In the USA, for instance, the reinforced concrete building code (ACI 318) prescribes span/depth ratios for beams and slabs.
Minimum sizes of approximately 450 mm for reinforced concrete columns are well-understood by practitioners, primarily to ensure that concrete can be placed at the bottom of the column form without segregation, although there is no theoretical basis for this size.
On the other hand the application of standard practices for sizing structures can lead to much quicker preliminary designs.
Such designs, of course, have to be refined and checked by a qualified engineer, but it may be useful to know for instance that a steel office building normally has bays of approximately 9000 mm square and that infill beams and girders have a depth of 1/20 of the bay size.
Getting acquainted with empirical design
In the end, this is the meaning of empirical design: that all parties involved in construction identify and categorize and reuse their experience with buildings. When all the parties involved in building design recognize the meaning of empirical design, they will understand which aspects of their decisions are rational, and which are founded on experience.
A respect for the value of experience will increase our confidence in experience as a basis for design.
In examining our own work, or the work of structural designers through history, we can recognise the merit of such methods, and begin to recognise the merit of this approach in our own work.
Written by Thomas Boothby, Empirical Structural Design for Architects, Engineers and Builders
, provides an explanation of empirical design as a practical means of making preliminary structural design decisions.