Examples of the Design of Reinforced Concrete Buildings to BS8110

C.E. Reynolds

• Publisher: CRC Press
• Year: 2017
• ISBN:
  9781351989961
• Edition: 4
• Topic: Architecture
• Subtopic: Architecture Methods & Materials

The latest edition of this well-known book makes available to structural design engineers a wealth of practical advice on effective design of concrete structures. It covers the complete range of concrete elements and includes numerous data sheets, charts and examples to help the designer. It is fully updated in line with the relevant British Standards and Codes of Practice.

Part One

Design of Reinforced Concrete Buildings

Chapter 1

Introduction to limit-state theory

More than twenty years have elapsed since the appearance of the preliminary version of CP110, the first Code of Practice for concrete wholly based on limit-state principles, caused shock waves to pass through the structural engineering profession. Yet for the generation of designers who have come into the profession since 1969 it is probably difficult to comprehend what all the fuss was about, since the principle on which CP110 was conceived, the so-called limit-state method of design, is in many respects simply a reworking and extension of principles that had already been embodied in such codes for more than thirty years. As early as 1924 George Manning (ref. 1) had suggested that, because of the discrepancies between the behaviour predicted by elastic analysis and that occurring in practice, the only logical theory to employ for reinforced concrete design was one based on the conditions existing in an actual structure when it had just reached its ultimate load. In 1934 the Code of Practice published by the Department of Scientific and Industrial Research (DSIR) permitted axially loaded columns to be designed by summing the individual resistances of the concrete and the reinforcement (i.e. ignoring the differences between the strains in the two adjacent materials). However, to conform to the basis adopted in the rest of the document, suitable factors of safety were incorporated solely by specifying low permissible material stresses, rather than working on actual ultimate loads and strengths.

A further step in this direction came with the appearance of the 1957 version of CP114 (the predecessor to CP110), where the concept of load-factor design was specifically stated and now extended to beams, slabs and eccentrically loaded columns. However, as before, design by elastic-strain (i.e. modular-ratio) principles was permitted in the same Code. Therefore it was thought necessary, to avoid any possibility of confusion arising due to the use of working (i.e. service) loads and strengths, and ultimate loads and strengths in the same document, to modify the load-factor method in such a way that the calculations were undertaken in terms of working loads and stresses. Unfortunately such an approach has led to some confusion in the minds of those using the Code as to exactly what their calculations were predicting.

The implementation of the limit-state design method presented in CP 110 avoids such confusion. In addition it extends the logic of load-factor design, by permitting the relative uncertainty by which each individual type of load and material strength can be assessed to be considered individually, instead of needing to adopt a single global factor of safety to cater for all the possible uncertainties. As pointed out in the introduction to BS8110, an immediately apparent advantage of such a procedure occurs when a critical situation is brought about by a combination of loads, such that one load is at its maximum while the other is at a minimum. This happens, for example, where vertical load on a frame is combined with lateral wind forces. In such a case the greatest likelihood of overturning is when the least vertical load is combined with the greatest wind force. However, the use of a single global loading factor causes both loads to be increased.

Since CP110 was first published in 1972, there has been a gradual acceptance by the majority of engineers of the principles embodied in this document. When BS8110 appeared in 1985, it contained no basic changes in principle from its predecessor, although many minor modifications were introduced and it was considerably rewritten. BS8110 states that the redrafting and alterations were made in the light of experience of the practical convenience in using CP110, and that they were also undertaken to meet the criticisms of engineers preferring the form of CP114. Although going some way to achieve this aim, the rewording and rearrangement have sometimes introduced confusion as to whether a particular change is merely cosmetic or whether it indicates a definite change of policy.

Fortunately, there are two publications that help to resolve some of these doubts. As with CP110, the authors of the Code have produced a Handbook to British Standard BS8110:1985, which explains in detail the basis of many Code requirements; on later pages this is referred to as the Code Handbook for brevity. In addition Arthur Allen, who has for many years lectured on Cement and Concrete Association design courses dealing with CP110 and BS8110 and who has had long and detailed discussions with the BS8110 authors, has produced an invaluable book entitled Reinforced Concrete Design to BS8110 – Simply Explained, to which the present author is greatly indebted; it is referred to here as Allen.

Two other publications should be mentioned. In October 1985 a joint committee formed by the Institutions of Civil and Structural Engineers published the Manual for the Design of Reinforced Concrete Building Structures, which deals with those aspects of BS8110 of chief interest to reinforced concrete designers and detailers. The advice contained in this document, which generally but not always corresponds to the Code requirements, is presented concisely in a different form from that in BS8110 (and one clearly favoured by many designers); elsewhere in this book it is referred to as the Joint Institutions Design Manual. The Standard Method of Detailing Structural Concrete (ref. 2) is the product of another joint committee, this time of the Institution of Structural...