Civil Engineering Reference
In-Depth Information
Chapter 17
Design of concrete elements
Owen Brooker
Technical Director, Modulus Structural Engineering Ltd., UK
doi: 10.1680/mosd.41448.0293
CONTENTS
17.1 Introduction
293
17.2 System selection
293
17.3 Preliminary sizing
296
This chapter gives an overview of the design of concrete framed buildings. It will explain the
factors to consider in the selection of a particular floor type, including reasons why concrete
may be selected ahead of other materials. A number of floor system options are introduced
representing the most popular systems in use for situations where labour costs are more
significant than material costs. Once a particular system has been selected, there is guidance
on how to determine the preliminary sizes for both the floors and the columns. The chapter
also explains how to carry out the detailed design of elements for the phenomena covered in
Eurocode 2, including flexure, shear and deflection. It explains how to interpret the code for
typical elements, providing derived equations and design aids that are not given in the code
itself. There is also guidance on determining anchorage and lap lengths.
17.4 Stability
296
17.5 Detailed design
296
17.6 Conclusions
308
17.7 References
308
range of influences so that the engineer can make a rational
assessment. As this chapter is focused on concrete, the first
consideration is why concrete should be used.
17.1 Introduction
Concrete is the most widely used construction material in the
world: only water is consumed more widely per head of popu-
lation than concrete. Concrete has been widely used in con-
struction from the mid-nineteenth century, but it was not until
the turn of the twentieth century that reinforcement was used to
enable concrete to be used as a flexural element. Initially rein-
forced concrete was a proprietary product with several systems
available, but during the twentieth century Codes of Practice
were introduced which set out the safe use of reinforced con-
crete for general use.
This chapter will explain why concrete is used widely in
building construction and explain the advantages and design
considerations for a variety of floor systems.
The detailed design in the chapter will be based on BS
EN1992-1-1 (BSI, 2004), which will be referred to as
Eurocode 2. This is the current concrete design standard for
the whole of Europe, and increasingly countries outside of
Europe. As part of the Eurocode suite of design standards it is
considered to be the most advanced concrete standard in the
world. However, preliminary sizing rules and general guid-
ance on the use of concrete should be applicable whichever
design standard is used.
The other point to note is that the chapter is written with
northern European markets in mind. In this region the costs
of labour and formwork are high when compared to material
costs. It is, therefore, recognised that this may not suit some
worldwide markets and local conditions should be considered
when proposing initial options.
17.2.1 Why use concrete?
Concrete is a versatile material, whose raw materials are found
throughout the world and therefore in many situations it is the
first choice building material. However, the reasons for using
concrete go deeper than just easy availability and some of the
benefits are explained below.
17.2.1.1 Economy
For the vast majority of building projects, economy is the key
driver. The material costs for concrete are generally low com-
pared to other materials, but other factors also have an influ-
ence: labour and in particular formwork costs can be up to half
the cost of a concrete frame. However, concrete is still an eco-
nomical material having a market share of more than 50% in
most countries, the UK being a notable exception.
17.2.1.2 Programme
In the UK there is a perception that concrete framed buildings
are slow to construct. However, this is not necessarily the case.
An
in situ
concrete frame construction may take longer than
other materials, but overall construction times are compar-
able. This is because the lead time can be shorter and because
follow-on trades can follow more closely behind the concrete
works. Further details can be found for various building types
in a series of studies by The Concrete Centre (2007, 2008a,
2008b).
Pre-fabrication can also significantly reduce programme
times and there are many ways in which concrete elements can
be pre-fabricated, including precast columns, twin-wall pan-
els, hollowcore units and lattice girder slabs.
Programme times can also be improved by good detailing
and consideration of buildability.
17.2 System selection
The choice of structural system will be often be determined on
the balance of different influences; each construction project is
unique and will have a different set of influences. The engineer
along with the design team and client will have to assess those
options and the benefits. This section is intended to explain the
