Practical Yield Line Design





1.0 Introduction

This publication

The aim of this publication is to (re-) introduce practical designers to the use of Yield Line Design. The intention is to give an overall appreciation of the method and comprehensive design guidance on its application to the design of some common structural elements. It assumes that the user has sufficient experience to recognise possible failure patterns and situations where further investigation is required. The basic principles of Yield Line theory are explained and its application as a versatile method for the design and assessment of reinforced concrete slabs is demonstrated. Theory is followed by practical examples and the accompanying commentary gives insights into the years of experience brought to bear by the main author, Gerard Kennedy. The publication is intended as a designer’s aid and not an academic paper. It commits to paper a

practical approach to the use of Yield Line for the design of concrete slabs. It gives guidance on how to tackle less simple problems, such as the design of flat slabs, rafts, refurbishment and slab-beam systems. Whilst the publication covers the design of common elements, it is an introduction, not a comprehensive handbook: in more exacting circumstances, designers are advised to consult more specialist literature.

The examples are practical ones that may be followed, but should not be extended too far without reference to more specialist literature. Yield Line Theory challenges designers to use judgement and not to rely solely on computer analysis and design. Once grasped, Yield Line Theory is exceedingly easy to put into practice and everyone in the procurement chain benefits. Simple design leads to simple details that are fast to detail and fast to fix. Current initiatives such as Egan

[4] and partnering, etc, should challenge designers to revisit and re-evaluate the technique.

1.1 The essentials

1.1.1 What is Yield Line Design?

Yield Line Design is a well-founded method of designing reinforced concrete slabs, and similar types of elements. It uses Yield Line Theory to investigate failure mechanisms at the ultimate limit state. The theory is based on the principle that:

work done in yield lines rotating = work done in loads moving

Two of the most popular methods of application are the ‘Work Method’ and the use of standard formulae. This publication explains these two methods and illustrates how they may be used in the practical and economic design of reinforced concrete slabs such as flat slabs, raft foundations and refurbishments

1.1.2 What are the advantages of Yield Line Design?

Yield Line Design has the advantages of: - Economy - Simplicity and - Versatility

Yield Line Design leads to slabs that are quick and easy to design, and are quick and easy to construct. There is no need to resort to computer for analysis or design. The resulting slabs are thin and have very low amounts of reinforcement in very regular arrangements. The reinforcement is therefore easy to detail and easy to fix and the slabs are very quick to construct. Above all, Yield Line Design generates very economic concrete slabs, because it considers features at the ultimate limit state.


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