Structures for Architects

Bryan J.B. Gauld

• Publisher: Routledge
• Year: 2014
• ISBN:
  9781317892120
• Edition: 3
• Topic: Architecture
• Subtopic: Architecture General

Demand from building control officials for structural calculations - even for very simple projects - means that today's architects must have a thorough understanding of everyday structural concepts. Structures for Architects satisfies the need for a basic introduction to the structural problems encountered by the architect, surveyor and builder. This third edition reflects advances in recent techniques and refers to current Building Regulations and Codes of Practice.Students of architecture, building and surveying at degree, diploma or professional (RIBA, RICS, CIOB) examination level will find this book a valuable course text. Professionals in these fields who must perform structural calculations to satisfy building control authorities will also find it a useful handbook.

Chapter 1

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INTRODUCTION TO BUILDING STRUCTURES

The essence of this book is to establish the sizes of structural elements within a building when the elements are made from timber, steel, masonry and concrete. At the same time the overall concept of stability, form and function are covered to demonstrate how good design is related to good structural decisions.

What is meant by structural decisions and structural design is the process of arriving at a suitable system to support a form or shape and to prevent it from collapsing. The support system is called the structure and the structural elements are those individual parts of the structure which help to support the form.

Before the reader works through the simple calculations found in later chapters, fundamentals of building support systems are explored using examples from Nature. This will help to generate an understanding and feeling for structural principles which can be readily applied to the most complex and the most simple problems, and a realisation that good structural design is related to common sense rather than complex mathematical equations.

1.1 Structural requirements of form and function

Much can be learnt from the structural form and function found in Nature. Everywhere we see the phenomena of adaptation producing mechanical fitness to cope with some particular function. Outward forms and inner structures have a perfect comprehension confirming that Nature makes all things with a purpose.

To understand these principles, it is necessary to examine the mechanical properties used by Nature. The materials will have to be strong enough to resist all the natural forces that may be imposed during a lifetime. These forces will take many forms but in simple engineering terms there are two kinds of strengths; strength to resist compression and strength to resist tension. Compression members need to be stiff so that they do not buckle, while tension members require no stiffness but only strength to resist the pull. The structure of a bone is stiff and meets the compression requirement, while the tendon of a muscle is excellent in resisting tension.

Figure 1.1 Stability of a tree

Running parallel to strength is the requirement for stability. The natural form must have balance and internal stiffness. The roots of a tree prevent the tree from falling over (Fig. 1.1). The roots extend even further than the branches, thus providing a sound base for resisting the wind on a windy day when all the branches are moving to and fro. The tree also has a certain amount of internal stiffness, preventing the trunk and branches being flattened by the wind. In fact, many trees are very cunning as they shed their leaves during the windy months of the year thereby reducing their resistance to the wind.

With animals, their legs provide stability by being placed in such a way as to procure balance during running, walking or standing still. The internal stiffness is achieved by a combination of bones and muscles and to a certain extent, the tautness of the skin. The inner functions have to be protected and not be deformed so that the outward form must be stiff enough to maintain its form and not collapse when exposed to any stress. For example, the rib cage of any animal must be stiff enough to prevent the lungs from being crushed, and the skull must be stiff enough to protect the brain.

Man-made structures have to obey the same fundamental natural laws. They must have enough strength to resist compression and tension and have enough stiffness to be stable both externally and internally. The building must not be blown away, overturned, collapse during a storm, or fall to bits when overcrowded. If it is subjected to earthquake tremors, hopefully it will survive without any loss of life. In short, the building should meet all the basic requirements, and in particular all the basic structural requirements.

To delve more deeply into structural requirements, it is necessary to ask the question, ‘what are buildings for?’ If this is answered in a very simple way, it could be said that a building encloses a space and protects this space from the natural elements. The fabric which wraps around the building provides the protection. This can be in the form of bricks, blocks, concrete, timber, steel, reinforced plastics, glass, etc. Then, if the materials to be used as the fabric have good structural properties, then three fundamental questions are raised:

(a) Should the building fabric be self supporting?

(b) Should the fabric be supported by an independent framed structure?

(c) Should a combination of (a) and (b) be used?

The structural requirements for each approach are different, and have to be studied, although the fundamental ones of strength, function and stability are the same.

Figure 1.2 Cross-section of rabbit (internal frame structure)

Figure 1.3 Cross-section of a tortoise (external support structure)

At this point, it is appropriate to return to examples from Nature. The examples of a rabbit and a tortoise shown in Figs 1.2 and 1.3 are good examples to illustrate the differences between a framed structure and a fabric supporting structure, questions (a) and (b). The rabbit is the example of a form or shape having an internal framed support system, while the tortoise is the example of a support system generated by the stiffness of the fabric.

The rabbit has a fur coat or outer fabric to contain all the internal functions and to keep it warm. The vital organs are protected by the rib cage which in turn is supported by the spine which in turn is supported by the legs. The brain, the most sensitive of the organs, is well protected by the skull. The head itself is supported by the neck in the same way as a cantilevered beam supports a load requiring considerable strength where the neck meets the main part of the body. This strength is achieved by a combination of bones slotted together in the form of the spine and muscles which wrap around the bones to keep everything together. The design of a whole skeleton of the rabbit is sensitive to the demand of strength at every point. The skeleton constantly changes in shape and size, only using the amount and size of bones and muscle which are really necessary, but at the same time allowing freedom of movement. It is a computerised design to the very highest standard.

All this struction support keeps the rabbit well clear of the ground and allows it to carry out all its functions of running, hopping, sitting, eating, etc. The tortoise on the other hand is close to the ground and does not have a great deal of movement. He has a shell which protects him from his enemies and a soft head which normally protrudes, but when threatened can be very sharply tucked away under his shell.

The shell which protects the tortoise also acts as the main support system for the whole body including all the vital organs. The legs and neck are connected to the shell and the whole design is in keeping with the nature of the tortoise, renowned for its slow progress, but well protected against the outside world. The structure is very strong and stiff providing an excellent internal space where heart and lungs can operate without fear of external forces. The strength generated to support all this is all in the shell with local stiffeners being incorporated at points in the shell where extra strength is required. Thus, the shell fulfils both the functions of frame and fabric. However, all this is at the cost of freedom of movement, which in the case of the tortoise does not matter, as this lack of movement is one of the parameters of his design.

With building structures, the same comparisons can be made between buildings supported by a frame and those supported by the fabric. A frame tends to be more flexible and be able to accommodate movement, while a fabric supporting structure tends to be rigid and unable to accommodate movement without causing some damage to the structure.

Having examined the structural strength characteristics of a tortoise and a rabbit, the next poin...