Cover page

Table of Contents

Title page

Copyright page

Preface

Notation

CHAPTER 1 Precast Concepts, History and Design Philosophy

1.1 A Historical Note on the Development of Precast Frames

1.2 The Scope for Prefabricated Buildings

1.3 Current Attitudes towards Precast Concrete Structures

1.4 Recent Trends in Design, and a New Definition for Precast Concrete

1.5 Precast Superstructure Simply Explained

1.6 Precast Design Concepts

CHAPTER 2 Procurement and Documentation

2.1 Initial Considerations for the Design Team

2.2 Design Procurement

2.3 Construction Matters

2.4 Codes of Practice, Design Manuals, Textbooks and Technical Literature

2.5 Definitions

CHAPTER 3 Architectural and Framing Considerations

3.1 Frame and Component Selection

3.2 Component Selection

3.3 Special Features

3.4 Balconies

CHAPTER 4 Design of Skeletal Structures

4.1 Basis for the Design

4.2 Materials

4.3 Structural Design

4.4 Columns Subjected to Gravity Loads

4.5 Staircases

CHAPTER 5 Design of Precast Floors Used in Precast Frames

5.1 Flooring Options

5.2 Hollow-core Slabs

5.3 Double-Tee Slabs

5.4 Composite Plank Floor

5.5 Precast Beam-and-Plank Flooring

5.6 Design Calculations

CHAPTER 6 Composite Construction

6.1 Introduction

6.2 Texture of Precast Concrete Surfaces

6.3 Calculation of Stresses at the Interface

6.4 Losses and Differential Shrinkage Effects

6.5 Composite Floors

6.6 Economic Comparison of Composite and Non-composite Hollow-core Floors

6.7 Composite Beams

CHAPTER 7 Design of Connections and Joints

7.1 Development of Connections

7.2 Design Brief

7.3 Joints and Connections

7.4 Criteria for Joints and Connections

7.5 Types of Joint

7.6 Bearings and Bearing Stresses

7.7 Connections

7.8 Design of Specific Connections in Skeletal Frames

7.9 Beam-to-Column and Beam-to-Wall Connections

7.10 Column Insert Design

7.11 Connections to Columns on Concrete Ledges

7.12 Beam-to-Beam Connections

7.13 Column Splices

7.14 Column Base Connections

CHAPTER 8 Designing for Horizontal Load

8.1 Introduction

8.2 Distribution of Horizontal Load

8.3 Horizontal Diaphragm Action in Precast Concrete Floors without Structural Toppings

8.4 Diaphragm Action in Composite Floors with Structural Toppings

8.5 Horizontal Forces due to Volumetric Changes in Precast Concrete

8.6 Vertical Load Transfer

8.7 Methods of Bracing Structures

CHAPTER 9 Structural Integrity and the Design for Accidental Loading

9.1 Precast Frame Integrity – The Vital Issue

9.2 Ductile Frame Design

9.3 Background to the Present Requirements

9.4 Categorisation of Buildings

9.5 The Fully Tied Solution

9.6 Catenary Systems in Precast Construction

CHAPTER 10 Site Practice and Temporary Stability

10.1 The Effects of Construction Techniques on Design

10.2 Designing for Pitching and Lifting

10.3 Temporary Frame Stability

10.4 On-Site Connections

10.5 Erection Procedure

10.6 In situ Concrete

10.7 Handover

References

Index

Title page

Preface

Of all the major forms of multi-storey building construction, structural precast concrete is perhaps understood by the fewest practitioners. This is a significant ‘blind spot’ in that part of the building profession associated with the design and construction of large or small multi-storey precast and prestressed concrete frames. This is due mainly to two particular factors:

Consequently, the trainee structural designer is rarely exposed to the virtues of using precast concrete in this way. Opportunities to study the basic concepts adopted in the design, manufacturing and site erection stages are not often made available to the vast majority of trainees.

Even where precast concrete is accepted as a viable alternative form of construction to e.g. steelwork for medium to high-rise structures, or to insitu concrete for some of the more complex shaped buildings, or to masonry for low-rise work, it is often considered only at a late stage in the planning process. In these situations, precast concrete is then often restricted to the substitution of components carrying their own locally-induced stresses. The economic advantage of the precast components also carrying global stresses is lost in the urgency to commence construction. Indeed, precast component design has long been considered as having a secondary role to the main structural work. Only more recently have precast designers been challenged to validate the fundamental principles they are using, and to give clients confidence in precast concrete design solutions for entire structures.

To meet ever-increasing building specifications, precast manufacturing companies have considerably refined the design of their product. They have formed highly effective product associations dealing with not only the marketing and manufacturing of the product, but also with technical matters. These include common design solutions, research initiatives, education, unified design approaches, and, importantly, the encouragement of a wider appreciation of precast structures in the professional design office. Even so, the structural and architectural complexity of some of the more recent precast frames has widened the gap between precast designers and the rest of the profession. The latter have limited sources for guidance on how the former are working. Satisfying codes of practice and the building regulations plays only a minor role in the total package; there is so much more, as this book shows.

Nowadays, the use of precast reinforced and prestressed concrete for multi-storey framed buildings is widely regarded as an economic, structurally sound and architecturally versatile building method. Design concepts have evolved to satisfy a wide range of commercial and industrial building needs. ‘Precast concrete frames’ is a term which is now synonymous with high quality, strength, stability, durability and robustness. Design is carried out to the highest standard of exactness within the concrete industry and yet the knowhow, for the reasons given above, remains essentially within the precast industry itself.

Precast concrete buildings do not behave in the same way as cast-in situ ones. The components which make up the completed precast structure are subjected to different forces and movements from the concrete in the monolithic structure. It is necessary to understand where these physical effects come from, where they go to, and how they are transferred through the structure.

Consequently, this book aims to disseminate understanding of the disparate procedures involved in precast structural design, from drawing office practice to explaining the reasons for some of the more intricate operations performed by precast contractors on site. The principal focus is upon on skeletal-frame type structures, the most extensively used form of precast structural concrete. They are defined as frameworks consisting essentially of beams, columns, slabs and a small number of shear walls.

From the structural and architectural viewpoints, skeletal frames are the most demanding of all precast structures. They contain the smallest quantity of structural concrete per unit volume. The precast components can be coordinated into the architectural façade, both internally and externally, to meet the social, economic and ecological demands that are now required. Ever greater accuracy, quality control, and on-site construction efficiency are being demanded and achieved. The construction industry is turning to high-specification prefabricated concrete for its advancement, using ‘factory engineered’ precasting techniques.

The chapters in this book have been arranged so that different parts of the design process can be either isolated (for example in the cases of precast flooring, or of connections), without the reader necessarily referring to the overall frame design, or read sequentially to realise the entire design. Chapters 1 to 3 present an overview of the subject in a non-technical way. Chapters 4 to 9 describe, in detail, the design procedures that would be carried out in a precast manufacturing company’s design office. Chapter 10 describes the relevant site construction methods. Numerous examples have been used to demonstrate the application of design rules, many of which are not code-dependent.

There are many aspects to the design of precast skeletal frames that have evolved through the natural development of precast frame design since the 1950s. One aim of this book is to update and coordinate this information for the future. Historically, the precast concrete industry considered many of its design techniques commercially sensitive, particularly those for connector design, and was consequently criticised by developers and consultants. More information is now freely available since the expiry of many patents of ideas. One of the main purposes of the first edition was to bring together in a coherent manner, for the benefit of everyone, the widely varied design methods used in the industry. The second edition aims to extend that process in the context of continually developing technology and the introduction of Europe-wide design requirements embodied in the Eurocodes. It also demonstrates the trend towards greater, often fully serviced, spatial precast components.

Precast concrete designs are not entirely code-dependent, but the primary recommendations are in accordance with Eurocode 2 (BS EN 1992-1-1) and its predecessor BS 8110. Where the design procedures from the two codes differ, they are explained. Where major differences occur, or accumulate in design examples, the text is presented in two parallel columns with the Eurocode version in the left column and the BS 8110 text in the right column. When minor textual differences occur for the application of the two codes, Eurocode 2 forms the basic text, with the alternative BS text within braces or curly brackets thus: {to BS 8110}. It may help the reader to know that the authors have retained braces exclusively for this purpose, leaving the use of round brackets for the two contextually differentiable functions of parenthesis or mathematical grouping, and square brackets for references.

The combination of a broad overview, background research, and detailed analysis, the references to the familiar British Standards and the new Eurocodes, and an extensive range of illustrations together combine to offer a valuable resource for both undergraduate and practising engineers in the field of precast concrete.

The authors are indebted to the following individuals and companies for their personal assistance and corporate help in the preparation of this book: Beresford Flooring Ltd (Derby, UK), Bison Manufacturing (Swadlincote, UK), British Precast Concrete Federation and Precast Flooring Federation (Leicester, UK), CERIB (Epernon, France), Composites Ltd (formerly Com­posite Structures, Eastleigh, UK), Corsmit Consulting Engineers (Rijswijk, Netherlands), The Concrete Centre (formerly British Cement Association, Camberley, UK), Creagh Concrete (Antrim, UK), Ergon (formerly Partek, Lier, Belgium), Andrew T. Curd & Partners (USA), Federation International du Beton (fib), Gruppo Centro Nord (Cerano, Italy), Hume Industries Berhad (Malaysia), The New Civil Engineer (London), Peikko Finland Oy (Lahti, Finland), Precast Concrete Structures Ltd (Gloucester, UK), SCC Ltd (Stockport, UK), Spenncon AS (Sandvika, Norway), Spiroll Precast Services Ltd (Derby, UK), Strängbetong AB (Stockholm, Sweden), T&A Prefabricados (Igarrasu, Brazil), Tarmac Building Products Ltd (Ettingshall, UK), Trent Concrete Ltd (Nottingham, UK), University of California (San Diego, USA), Waycon (Plymouth, UK) and Mr Arnold van Acker (fib Commission member).

Notation

Latin upper-case letters
AAccidental action
ACross-sectional area
AbCross-sectional area of bolts
AbstArea of bursting reinforcement
AcCross-sectional area of concrete
AcGross cross-sectional area of hollow-core slabs
Ac(net)Net cross-sectional area of hollow-core slabs
AdArea of diagonal reinforcement
AfCross-sectional area of flange
AhdArea of diaphragm reinforcement
AkContact area in castellated joint
Ap, ApsArea of a prestressing tendon or tendons
AsCross-sectional area of tension reinforcement
AsArea of compression reinforcement
AsArea of longitudinal reinforcement in top of boot of beam
AscTotal area of reinforcement in column
AshArea of horizontal reinforcement
AshvArea of horizontal punching shear reinforcement
As,minMinimum cross-sectional area of reinforcement
AsvArea of shear reinforcement
AswCross-sectional area of shear reinforcement; area of reinforcing bars welded to plate
BBreadth of void in slab; breadth of building; breadth of foundation
CCompressive force
DDiameter of mandrel; depth of pocket in foundation; depth of floor diaphragm; depth of hcu
DEdFatigue damage factor
EEffect of action; Young’s modulus of elasticity
E′Equivalent Young’s modulus in precast in situ joint
EcTangent modulus of elasticity of normal weight concrete at a stress of σc = 0
EcYoung’s modulus of in situ concrete
Ec(28)Tangent modulus of elasticity of normal weight concrete at 28 days
Ec,effEffective modulus of elasticity of concrete
EcdDesign value of modulus of elasticity of concrete
EciYoung’s modulus of concrete at transfer
EcmSecant modulus of elasticity of concrete
Ec(0)Tangent modulus of elasticity of normal weight concrete at a stress of σc = 0
Ec(t)Tangent modulus of elasticity of normal weight concrete at time t
EiYoung’s modulus of infill concrete
EpDesign value of modulus of elasticity of prestressing steel
EsDesign value of modulus of elasticity of reinforcing steel
EIBending stiffness
EQUStatic equilibrium
FAction; force
FbtUltimate tensile force in bars at start of bends
FcCompressive force in concrete
FcRUltimate compressive resistance force
FdDesign value of an action; tensile force in diagonal reinforcing bars
FhTensile force in horizontal reinforcing bars
FkCharacteristic value of an action
FsTensile force in reinforcing bars
FtNotional tensile force in stability ties
FtTensile force in stability ties
FtRUltimate tensile resistance force
FvShear force to one side of interface (composite construction)
GShear modulus
GkCharacteristic permanent action
HTotal height of building; length of foundation; horizontal force
HbstBursting force
HvHorizontal resistance of infill wall
ISecond moment of area of concrete section
IcTransformed second moment of area of composite section
IcoCompound second moment of area
KStress factor M/fck b d2 {M/fcu b d2}; shrinkage factor
KbFlexural stiffness of connections between members
KsShear stiffness of connections between members
KtBond length parameter
LLength; span; length of void in slab; length of building base plate overhang distance
L′Clear opening between columns; length of wall
LeEffective length
LsClamping length
LsbBond length
L1Distance from face of column to load
L2, L3Length of stress block (insert design)
L4Length of embedment of insert
MBending moment
M′Moment of resistance based on strength of concrete = 0.206 fck b d2 {0.156 fcu b d2}
MaddAdditional bending moment due to deflection = Nau
MbBending moment due to shrinkage
McBalancing bending moment due to shrinkage
MEdDesign value of the applied internal bending moment
MhBending moment in floor diaphragm
Mmax, MminMaximum and minimum bending moment
MnetMmax-Mmin
MoDecompression bending moment
MRMoment of resistance
MsrServiceability moment of resistance
MRd {Mur}Ultimate moment of resistance
M1, M2, M3Strength of connections in frames
NAxial force; number of strands/wires/bolts
NEdDesign value of the applied axial force (tension or compression)
PPrestressing force
P0Initial force at the active end of the tendon immediately after stressing
Ppo {Pf}Prestressing force after all losses
Ppi {Pi}Initial prestressing force
PpmiPrestressing force at installation
Ppm0 {Pt}Prestressing force at transfer
PrPrestressing force after initial relaxation
PtPrestressing force at transfer
QkCharacteristic variable action
QfatCharacteristic fatigue load
RResistance; prop force
RaRoughness factor
RyDiagonal resistance of infill wall
SInternal forces and moments; first moment of area; plastic section modulus
ScFirst moment of area to one side of interface
SLSServiceability limit state
TTorsional moment; tension force
TEdDesign value of the applied torsional moment
ULSUltimate limit state
VShear force; reaction force
VcoShear resistance in flexurally uncracked prestressed section
VcrShear resistance in flexurally cracked prestressed section
VdShear force in dowel
VEdDesign value of the applied shear force
VhHorizontal shear force
VrIncreased shear capacity due to additional reinforcement (insert design)
VRd,c {Vco}Shear resistance in flexurally uncracked prestressed section
VRd,cr {Vcr}Shear resistance in flexurally cracked prestressed section
VRd,CFUltimate shear capacity (compression field theory)
VuhUltimate horizontal shear force
WSelf weight of column
WkCharacteristic wind load
WuUltimate wind load
XDistance to neutral axis; stress block depth factor
ZElastic section modulus
Zb, ZtElastic section modulus at extreme bottom and top fibres
Zb,co Zt,coCompound elastic section modulus at extreme bottom and top fibres
ZcElastic cracked concrete section modulus
ZuElastic uncracked concrete section modulus
Latin lower-case letters
aDistance; lever arm distance; distance to wall from shear centre; geometrical data
ΔaDeviation for geometrical data
a′Distance from compression face to point where crack width is calculated
abClear distance between bars; cover to inside face of bars
acrDistance from nearest bar to point where crack width is calculated
aeffEffective bearing length at corbel
afTangent of coefficient of friction, i.e. μ = tan αf
auSway deflection
avLever arm distance to shear force
bOverall width of a cross-section, or actual flange width in a T- or L-beam
be, beffEffective breadth
blLength of bearing
bpBreadth of bearing
btBreadth of section at centroid of steel in tension
bvBreadth of shear section or shear web
bwWidth of the web on T-, I- or L-beams
cCover distance; distance to centre of bar
cminMinimum distance to bar in tension
cwCrack width
dDiameter; depth; effective depth of a cross-section to tension steel; depth of web in steel sections
d′Effective depth to compression steel
d″Effective depth to tension steel in boot of beam
dfEffective depth to edge of foundation pocket
dgLargest nominal maximum aggregate size
dhEffective depth of half joint
dnDepth to centroid of compression zone
eEccentricity
e′Effective eccentricity; lack of verticality
ei {eadd}Second order eccentricity
enetNet eccentricity
exMinimum eccentricity (infill wall)
fbUltimate bearing stress; limiting flexural compressive stress in concrete
fbcBottom fibre stress due to prestress after losses
fbciBottom fibre stress due to prestress at transfer
fbdUltimate bond stress
fbpdUltimate bond stress within the anchorage length
fbptBond stress within the transmission length
fcCompressive strength of bearing material
fcEffective compressive strength of precast in situ concrete joint
fccPrestress at centroid of tendons after losses
fcciPrestress at centroid of tendons at transfer
fcdDesign value of concrete compressive strength
fciCharacteristic compressive cube strength of concrete at transfer
fckCharacteristic compressive cylinder strength of concrete at 28 days
fcmMean value of concrete cylinder compressive strength
fcpPrestress at centroidal axis (taken as positive)
fcpxPrestress at centroidal axis at distance x from end of section
fctLimiting flexural tensile stress in concrete
fctkCharacteristic axial tensile strength of concrete
fctmMean value of axial tensile strength of concrete
fcuCharacteristic compressive cube strength of concrete
fcuCharacteristic compressive cube strength of infill concrete, ditto at lifting
fcylCharacteristic compressive cylinder strength of concrete
fkCharacteristic compressive strength of brickwork
fpTensile strength of prestressing steel
fpbDesign tensile stress in tendons/wires
fpeFinal prestress in tendons/wires after losses
fpi {fi}Initial prestress in tendons
fpkCharacteristic tensile strength of prestressing steel
fpm0 spm0Prestress at transfer
fpo spo spm∞ {fpe}Final prestress in tendons/wires after losses
fp0,10.1% proof-stress of prestressing steel
fp0,1kCharacteristic 0.1% proof-stress of prestressing steel
f0,2kCharacteristic 0.2% proof-stress of reinforcement
fpuCharacteristic strength of prestressing tendons/wires
fRdUltimate bearing stress
fsStress in reinforcing steel bars
ftTensile strength of reinforcement; limiting direct (splitting) tensile stress in concrete
ftcTop fibre stress due to prestress after losses
ftiTop fibre stress due to prestress at transfer
ftkCharacteristic tensile strength of reinforcement
fvCharacteristic shear strength of brickwork
fyYield strength of reinforcement
fybCharacteristic strength of bolts
fydDesign yield strength of reinforcement
fykCharacteristic yield strength of reinforcement
fykw {pweld}Strength of weld material
fyvCharacteristic strength of reinforcing steel links/stirrups
fywdDesign yield of shear reinforcement
fywk {fyv}Characteristic strength of reinforcing steel links/stirrups
gkCharacteristic uniformly distributed dead load
hHeight; floor-to-floor height; overall depth of section
h′Clear floor-to-floor height; reduced depth at half joint
haggNominal size of aggregate
hsDepth of slab in composite construction
iRadius of gyration
kCoefficient; factor; core distance = I/(0.5 hA)
ksShear stiffness in joints
kbFlexural stiffness in joints
kjRotational stiffness in joints
lLength; span; distance between column-to-column centres
lbBearing length
lbdDesign anchorage length
leEffective length
l0Clear height between restraints
lbpd {lp}Prestress anchorage {development} length
lpt1Prestress transmission length, lower bound
lpt2 {lt}Prestress transmission length, upper bound
lrDistance between columns or walls (stability ties)
ltPrestress transmission length
lwLength of weldment
lxPenetration of starter bar into hole
lzDistance between positions of zero bending moment
mMass; distance from centre of starter bar to holding down bolt (base plate); modular ratio
maddAdditional bending moment due to deflection = NEdei
msModular ratio (of elastic moduli) at service
muModular ratio (of strength) at ultimate
nNumber of columns in one plane frame, number of bars in tension zone of wall; number of storeys
pbBearing strength of steel plate
pweldStrength of weld material
pyk {py}Strength of steel plate
qPressure (key elements)
qkCharacteristic uniformly distributed live load
rRadius; radius of gyration; bend radius of reinforcing bar
1/rCurvature at a particular section
sSpacing of reinforcing bars
tThickness; time being considered; temperature range
teffEffective thickness
t0The age of concrete at the time of loading
uPerimeter of concrete cross-section, having area Ac; perimeter distance
u, v, wComponents of the displacement of a point
vThickness of in situ infill; vEdi {v} ultimate shear stress
vaveAverage interface shear stress
vcDesign concrete shear stress
vhDesign interface shear stress
vRdi {vu}Ultimate shear stress resistance
vtDesign torsion stress
wLength of steel plate; uniformly distributed load; breadth of compressive strut
w′Diagonal length of infill shear wall
wkCharacteristic uniformly distributed wind pressure
xNeutral axis depth; dimension of stirrup in boot of beam; distance to centroid of stabilising system
x, y, zCoordinates
xse Average crack spacing
ypoHalf bearing breadth (bp/2)
y0Half section breadth (b/2)
zLever arm of internal forces
Greek upper-case letters
ΔSecond order deflection; deformation; construction tolerance distance
ΦReinforcing bar or dowel diameter; ductility factor
Greek lower-case letters
αAngle; ratio; ratio Zr/Zb; coefficient of thermal expansion; characteristic contact length in infill wall
αcRatio of sum of column stiffness to beam stiffness
αcminMinimum value of αca
α1Ratio of distance to shear plane/transmission length lx/lpt2
βAngle; ratio; coefficient
γPartial factor
γAPartial factor for accidental actions, A
γCPartial factor for concrete
γFPartial factor for actions, F
γF,fatPartial factor for fatigue actions
γC,fatPartial factor for fatigue of concrete
γGPartial factor for permanent actions, G
γMPartial factor for a material property, taking account of uncertainties in the material property itself, in geometric deviation and in the design model used
γPPartial factor for actions associated with prestressing, P
γQPartial factor for variable actions, Q
γSPartial factor for reinforcing or prestressing steel
γS,fatPartial factor for reinforcing or prestressing steel under fatigue loading
γfPartial factor for actions without taking account of model uncertainties
γgPartial factor for permanent actions without taking account of model uncertainties
γmPartial factors for a material property, taking account only of uncertainties in the material property
δIncrement/redistribution ratio; deflection; shear slip
εStrain
εbFree shrinkage strain in precast beam or slab
εbbFree shrinkage strain in bottom of precast beam or slab
εbtFree shrinkage strain in top of precast beam or slab
εcCompressive strain in the concrete
εc1Compressive strain in the concrete at the peak stress fc
εcuUltimate compressive strain in the concrete
εfStrain of reinforcement or prestressing steel at maximum load
εmFree shrinkage strain in in situ concrete flange or topping
εsAverage strain at level where crack width is calculated
εshSteel strain; relative shrinkage strain εfεbt
εuShrinkage strain
εuStrain at the level where crack width is calculated
εukCharacteristic strain of reinforcement or prestressing steel at maximum load
ζReduction factor/distribution coefficient
ηTotal losses in prestressing force; force reduction factors
θAngle; slope of infill wall
λSlenderness ratio; relative stiffness parameter; joint deformability
μCoefficient of friction; degree of prestress force Pi/P
νPoisson’s ratio; strength reduction factor for concrete cracked in shear
ξRatio of bond strength of prestressing and reinforcing steel; bursting coefficient; prestress loss due to elastic shortening
ρStress; reinforcement ratio = A/bd; oven-dry density of concrete in kg/m3
ρ1000Value of relaxation loss (in %), at 1000 hours after tensioning and at a mean temperature of 20°C
ρlReinforcement ratio for longitudinal reinforcement
ρwReinforcement ratio for shear reinforcement
σcCompressive stress in the concrete
σcpCompressive stress in the concrete from axial load or prestressing
σcpSpalling stress
σcuCompressive stress in the concrete at the ultimate compressive strain, εcu
τTorsional shear stress; shear stress
ϕDiameter of a reinforcing bar or of a prestressing duct; rotation; diameter
ϕnEquivalent diameter of a bundle of reinforcing bars
ϕ(t,t0)Creep coefficient, defining creep between times t and t0, related to elastic deformation at 28 days
ϕ (∞,t0)Final value of creep coefficient
ψFactor defining representative values of variable actions
ψ0Factor for combination values
ψ1Factor for frequent values
ψ2Factor for quasi-permanent values

CHAPTER 1

Precast Concepts, History and Design Philosophy

The background to the relevance of precast concrete as a modern construction method for multi-storey buildings is described. The design method is summarised.

1.1 A Historical Note on the Development of Precast Frames

Precast concrete is not a new idea. William H. Lascelles (1832–85) of Exeter, England devised a system of precasting concrete wall panels, 3 ft × 2 ft × 1 inch thick, strengthened by forged, 1/8 inch-square iron bars. The cost was 3d (£0.01) per ft2. Afterwards, the notion of ‘pre-casting’ concrete for major structural purposes began in the late nineteenth century, when its most obvious application – to span over areas with difficult access – began with the use of flooring joists. François Hennebique (1842–1921) first introduced precast concrete into a cast-in situ flour mill in France, where the self-weight of the prefabricated units was limited to the lifting capacity of two strong men! White [1.1] and Morris [1.2] give good historical accounts of these early developments.

The first precast and reinforced concrete (rc) frame in Britain was Weaver’s Mill in Swansea. In referring to the photograph of the building, shown in Figure 1.1, a historical note states: … the large building was part of the flour mill complex of Weaver and Co. The firm established themselves at the North Dock basin in 1895–6, and caused the large ferro-concrete mill to be built in 1897–98. It was constructed on the system devised by a Frenchman, F. Hennebique, the local architect being H. C. Portsmouth …  At this time Louis Gustave Mouchel (1852–1908, founder of the Mouchel Group) was chosen to be Hennebique’s UK agent. Mouchel used a mix of cast-in situ and prefabricated concrete for a range of concrete framed structures, building at the rate of 10 per year for the next 12 years.

Figure 1.1 Weaver’s Mill, Swansea – the first precast concrete skeletal frame in the United Kingdom, constructed in 1897–98

(courtesy of Swansea City Archives).

c01f001

The structure was a beam-and-column skeletal frame, generally of seven storeys in height, with floor and beams spans of about 20 feet. The building has since been demolished owing to changes in land utilisation, but as a major precast and reinforced concrete construction it pre-dates the majority of early precast frames by about 40 years.

Bachmann and Steinle [1.3] note that the first trials in structural precast components took place around 1900, for example at Coignet’s casino building in Biarritz in 1891, and Hennebique and Züblin’s signalman’s lodge in 1896, a complete three-dimensional cellular structure weighing about 11 tons [1.3].

During the First World War storehouses for various military purposes were prefabricated using rc walls and shells. Later, the 1930s saw expansions by companies such as Bison, Trent Concrete and Girling, with establishments positioned close to aggregate reserves in the Thames and Trent Valley basins. The reason why precast concrete came into being in the first place varies from country to country. One of the main reasons was that availability of structural timber became more limited. Some countries, notably the Soviet Union, Scandinavia and others in northern Continental Europe, which together possess more than one-third of the world’s timber resources but experience long and cold winters, regarded its development as a major part of their indigenous national economy. Structural steelwork was not a major competitor at the time outside the United States, since it was batch-processed and thus relatively more expensive.

During the next 25 years developments in precast frame systems, prestressed concrete (psc) long-span rafters (up to 70 feet), and precast cladding increased the precasters’ market share to around 15 per cent in the industrial, commercial and domestic sectors. Influential articles in such journals as the Engineering News Record encouraged some companies to begin producing prestressed floor slabs, and in order to provide a comprehensive service by which to market the floors these companies diversified into frames. In 1960 the number of precast companies manufacturing major structural components in Britain was around thirty. Today it is about eight.

Early structural systems were rather cumbersome compared with the slim-line components used in modern construction. Structural zones of up to 36 inches, giving rise to span/depth ratios of less than 9, were used in favour of more optimised precasting techniques and designs. This could have been called the ‘heavy’ period, as shown in C. Glover’s now classic handbook Structural Precast Concrete [1.4]. Some of the concepts shown by Glover are still practised today and one cannot resist the thought that the new generation of precast concrete designers should take heed of books such as this. It is also difficult to avoid making comparisons with the ‘lighter’ precast period that was to follow in the 1980s, when the saving on total building height could, in some instances, be as much as 100 to 150 mm per floor.

Attempts to standardise precast building systems in Britain led to the development of the National Building Frame (NBF) and, later, the Public Building Frame (PBF). The real initiative in developing these systems was entrenched more in central policy from the then Ministry of Public Building and Works than by the precasting engineers of the building industry. The NBF was designed to provide: … a flexible and economical system of standardised concrete framing for buildings up to six storeys in height. It comprises a small number of different precast components produced from a few standard moulds [1.5].

The consumer for the PBF was the Department of Environment, for use within the public sector’s expanding building programme of the 1960s. Unlike the NBF, which was controlled by licence, the PBF was available without patent restrictions to any designer. The structural models were simple and economical: simply supported, long-span, prestressed concrete slabs up to 20 inches deep were half recessed into beams of equal depth. By controlling the main variables, such as loading (3+1 kN/m2 superimposed was used throughout), concrete strength and reinforcement quantities, limiting spans were computed against structural floor depths. Figures 1.2 and 1.3 show some of the details of these frames. Diamant [1.6] records the international development of industrialised buildings between the early 1950s and 1964. During this period the authoritative Eastern European work by Mokk [1.7] was translated into English, and with it the documentation of precast concrete had begun.

Figure 1.2 Typical structural details for the National Building Frame [1.4].

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Figure 1.3 Floors used in the National Building Frame [1.4].

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Unfortunately, the modular design philosophy was reflected in the façade architecture. The results were predictable, exemplified at Highbury Technical College in Portsmouth (now a part of the University of Portsmouth) shown in Figure 1.4. The precast industry has found it difficult to dispel the legacy of such architectural brutalism.

Figure 1.4 Precast construction of the 1960s using the National Building Frame, The building is Highbury College, now part of the University of Portsmouth

(courtesy of Costain Building Products).

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Following the demise of the NBF and PBF, precast frame design evolved towards more of a client-based concept. Standard frame systems gave way to the incorporation of standardised components into bespoke solutions. The result, shown in Figure 1.5 of Western House (1990), Surrey Docks (1990) and Merchant’s House (1991), established the route to the versatile precast concrete concepts of the present day.

Figure 1.5 Examples of precast construction of the 1980s. (a) Western House, Swindon

(courtesy Trent Concrete Ltd);

(b) Surrey Docks, London

(courtesy Crendon Structures);

(c) Merchant House, London [1.10].

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In the mid-1980s, the enormous demands on the British construction industry led developers to look elsewhere for building products, as the demands on the British precast industry far exceeded its capacity. Individual frame and cladding companies (with annual turnovers of between £1 m and £3 m) were being asked to tender for projects that were singularly equal to or greater in value than their annual turnovers. Programmes were unreasonably tight and it seemed that the lessons learned from mass-market-led production techniques of the 1960s had gone unheeded. One solution was to turn to Northern Europe, where the larger structural concrete prefabricators were able to cope with these demands. Concrete prefabricated in Belgium was duly transported to the London Docklands project, shown in Figure 1.6.

Figure 1.6 An early example of quality architecturally detailed precast concrete imported from Belgium. (a) Overall impression. (b) Architectural detail.

(Courtesy of A. Van Acker, TU Ghent.)

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In making a comparison of developments in Europe and North America, Nilson [1.8] states: Over the past 30 years, developments of prestressed concrete in Europe and in the United States have taken place along quite different lines. In Europe, where the ratio of labor cost to material cost has been relatively low, innovative one-of-a-kind projects were economically feasible. … In the U.S. the demand for skilled on-site building labor often exceeded the supply, so economic conditions favored the greatest possible standardisation of construction … 

North America’s production capabilities are an order of magnitude greater than those of Europe. Figure 1.7 shows the construction of a 30-storey, 5000-room hotel and leisure complex in Las Vegas. The conditions to which Nilson refers are changing. The gap between labour and material costs in Europe is now closer to that of North America. At the same time, progressively lower oil and transportation costs into the early twenty-first century made it feasible to manufacture components virtually anywhere in the world and transport them to regions of high construction demand. Recent indications are that increasingly scarce energy resources and narrowing pay differentials will reverse this trend.

Figure 1.7 MGM Hotel and Casino at Las Vegas, US constructed in 1992

(courtesy of A. T. Curd).

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While the market share for complete precast concrete frames has remained constant in the UK, the development of high-strength concrete for columns and the use of innovative shallow prestressed concrete beams, together with speed of construction to rival that of steelwork, has led to successful tower buildings in Northern Europe, particularly in Belgium and Holland. The twin-tower building ‘galaxy’ in Brussels, Figure 1.8, is such an example. With column sizes of 600 mm diameter (cast in two-storey heights using 95 N/mm2 concrete strength) and beam and floor spans up to 9.2 m × 405 mm depth, construction rates achieved 2 storeys in 8 working days [1.9].

Figure 1.8 36-storey precast skeletal tower buildings in Brussels

(courtesy of Ergon, Belgium).

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Changes to the way in which the construction industry should operate in a ‘zero-waste and zero-defects’ environment were given in the 1998 Egan Report [1.11]. The report called for sustained improvement targets: 10 per cent in capital construction costs; 10 per cent in construction time; 20 per cent of defects, and a 20 per cent increase in predictability. Further, the report goes on: … The industry must design projects for ease of construction, making maximum use of standard components and processes. Although the reports did not use the term ‘prefabrication’, to many people that is what ‘predictability’ and ‘standard components’ mean. The precast concrete industry is ideally placed to accommodate these higher demands by using experienced design teams and skilled labour in a quality-controlled environment to produce high-specification components. Figure 1.9 illustrates this in the repetitive use of granite cast spandrel beams and columns to form a building in the convoluted shape of a shell. Since 2000, high-quality architectural finishes have been more widely adopted for the exposed structural components, as illustrated in the integrated structure in Figure 1.10. The requirement for off-site fabrication will continue to increase as the rapid growth in management contracting, with its desire for reduced on-site processes and high-quality workmanship, will favour controlled prefabrication methods. The past five years have witnessed extensive developments in student accommodation – for example in Figure 1.11, where some 600 rooms were constructed using precast wall and floor systems in just over eight months, and were completed for occupancy within 18 months of site possession in 2009.

Figure 1.9 Granite aggregates in spandrel beams and columns, polished to reflect the Australian sunshine at No. 1 Spring Street, Melbourne, 1990.

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Figure 1.10 Asticus Building, London, 2010. Precast cruciforms form (a) the exterior structure, and (b) architectural finish.

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Figure 1.11 Student accommodation buildings at the University of West of England, Bristol. Precasted by Buchan, UK.

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1.2 The Scope for Prefabricated Buildings

1.2.1 Modularisation and Standardisation

The precast industry is still struggling to overcome the misconceptions of modular precast concrete buildings. This is not surprising, as many texts refer to: … the design of a precast concrete structure on a modular grid. The grid should preferably have a basic module of 0.6 m … [1.12]. The Continental Europeans introduced the phrase ‘modular coordination’, which meant the interdependent arrangement of dimensions, based on a primary value accepted as a module [1.13]. This dimension was 30 cm horizontally and 10 cm vertically. Moreover, the storey height in precast concrete apartment buildings was fixed at 280 cm, with the horizontal grid dimension on a 30 cm incremental scale between 270 cm and 540 cm. Strict observance of these rules facilitated the optimum assembly of prefabricated structures – in other words, all prefabricated buildings looked the same. And they were nearly all boxes – ‘People are getting tired of this shoe-box architecture’, said Harry Seidler, OBE, architect for the nautilus-shaped building in Figure 1.9.

There is a clear distinction between ‘modular coordination’ and ‘standardisation’. The precast industry deplores the former and encourages the latter. What is the difference and how can this be?

Modularisation offers zero flexibility off the modular grid. The end product is evident in the comparison of the two buildings adjacent to Vauxhall Bridge in London, shown in Figure 1.12. Interior architectural freedom is possible only in the adoption of module quantities and configuration, and one cannot escape the geometrical dominance and lack of individuality of the older building on the left of the photograph. Exterior façades may of course be varied indefinitely, as in the ‘twin façade’ system shown in Figure 1.13, but that requires a full precast perimeter wall. In skeletal frames; one need go no further than the adoption of families of modular precast concrete components to obtain the optimum solution for any building, within reasonable limits.

Figure 1.12 Examples of past and present use of precast concrete, Vauxhall Bridge, London.

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Figure 1.13 Principle of the twin facade system: (a) diagrammatic representation and (b) offices, Brussels.

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Industrial modularised buildings were introduced in Europe in the 1950s, during the mass construction period following the Second World War. The problems in the architectural and social environment brought a re-emergence of traditional methods, and closer control on design and factory production. This has inevitably led to a new philosophy in what is called the ‘modulated hierarchical building system’ [1.14], which aims at the subdivision of a building into:

Precast modular frame manufacturers have been able to synthesise these requirements through continuous development of improved products and creative use of limited ranges of precast concrete products.

Standardisation is quite different from modularisation. It refers to the manner in which a set of predetermined components are used and connected. The buildings shown in Figures 1.14 to 1.16 were constructed using more or less the same family of standardised components. (The Reinforced Concrete Council has published case studies on precast frames, e.g. [1.10], where this may also be appreciated.) By adjusting beam depths, column lengths, wall positions, etc. the same components in any of these buildings could have been used to make a completely different structure. This is not possible with the modular system vision of the 1960s.

Figure 1.14 Structural ‘grey’ precast frame at Nottingham, UK (1995).

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Figure 1.15 Commercial office development

(courtesy of Crendon Structure, 1988).

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Figure 1.16 Visual structural units with a polished concrete finish

(courtesy of Trent Concrete Ltd, 1992).

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The four basic types of precast concrete structure are:

(1) The portal frame, Figure 1.17, consisting of columns and roof rafters or beams, provides large and adaptable ground-floor space. Portal frame structures are used for single-storey retail, warehousing, and industrial manufacturing facilities.
(2) The crosswall frame, Figures 1.18 and 1.19, consisting of solid or voided vertical wall and horizontal slab units only. Here the structural walls serve as shear walls to resist lateral forces that increase with height, and also as acoustic and thermal partitioning. However, the walls interrupt the internal space and reduce functional flexibility. Wall frame structures are used extensively for multi-storey hotels, retail units, hospitals, housing, and partitioned offices.
(3) Volumetric, or cellular, structures such as that in Figure 1.20 are developments of wall frames in which a number of the walls and floors are constructed together as units. These units are suitable for high levels of factory installation of finishes and services, e.g. complete bathrooms, plumbing and wiring. Cellular units are then assembled to provide grouped, individual facilities such as student accommodation, hotel rooms or prisons.
(4) Skeletal structures, Figure 1.21, consisting of columns, beams and slabs for low- to medium-rise buildings, with a small number of walls for high-rise. Skeletal frames are used chiefly for commercial offices and car parks, where both clear spans and multiple storeys are required.