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An introduction to the development,
An introduction to the development,
benefits, design and construction of
benefits, design and construction of
in-situ prestressed suspended floors
in-situ prestressed suspended floors
A
An introduction to the development,
An introduction to the development,
benefits, design and construction of
benefits, design and construction of
in-situ prestressed suspended floors
in-situ prestressed suspended floors
A
FOREWORD
FOREWORD
This publication was commissioned by the Reinforced This publication was commissioned by the Reinforced Concrete Council, which was set up to promote better Concrete Council, which was set up to promote better knowledge and understanding of reinforced concrete knowledge and understanding of reinforced concrete design and building technology.
design and building technology.
Its members are Co-Steel Sheerness plc and Allied Steel Its members are Co-Steel Sheerness plc and Allied Steel &
& Wire, representing the major suppliers of reinforcingWire, representing the major suppliers of reinforcing steel in the UK, and the British Cement Association, steel in the UK, and the British Cement Association, representing the major manufacturers of Portland cement representing the major manufacturers of Portland cement in the UK.
in the UK. The
The aann dd
author, Mark Stevenson, is an Associate with Gifford author, Mark Stevenson, is an Associate with Gifford Partners.
Partners.
ACKNOWLEDGEMENTS
ACKNOWLEDGEMENTS
The Reinforced Concrete Council and the author are The Reinforced Concrete Council and the author are grateful to
grateful to SamiSami Khan, PeterKhan, Peter Mathew,Mathew, David Ramsey andDavid Ramsey and all those who provided comments on the draft of this all those who provided comments on the draft of this publication, and for the photographic material supplied publication, and for the photographic material supplied b y P S C F r e y s s i n e t L i m i t e d . T h a n k s a r e a l s o b y P S C F r e y s s i n e t L i m i t e d . T h a n k s a r e a l s o extended to Alan Tovev of Tecnicom for his work in extended to Alan Tovev of Tecnicom for his work in co-ordinating this publication.
co-ordinating this publication.
ABOUT THIS PUBLICATION
ABOUT THIS PUBLICATION
This publication aims to de-mystify the techniques of This publication aims to de-mystify the techniques of post- post-tensioned concrete floor construction in multi-storey tensioned concrete floor construction in multi-storey b u i l d
b u i l di n g s , i n g s , w h i l s t l o o k i n g a t t h e w h i l s t l o o k i n g a t t h e e c o n o me c o n o m i c s a n di c s a n d practicability of construction. It is intended for two broad practicability of construction. It is intended for two broad categories of reader.
categories of reader.
Firstly, it presents an introduction to the benefits, Firstly, it presents an introduction to the benefits, constraints, principles and techniques for non-technical constraints, principles and techniques for non-technical professionals.
professionals. These people will wish to understand theThese people will wish to understand the broad issues of an essentially simple technique with which broad issues of an essentially simple technique with which they may not be entirely familiar, without being swamped they may not be entirely familiar, without being swamped by equati
by equations. ons. Sections 1 Sections 1 and 3 pand 3 provide the rovide the briefestbriefest executive overview, whilst reading Sections
executive overview, whilst reading Sections 11 to 4 plus 7to 4 plus 7
will give a broader picture. will give a broader picture.
Secondly, it can be used as a more technical review for Secondly, it can be used as a more technical review for those who, in addition to the above issues, wish to explore those who, in addition to the above issues, wish to explore further the principles of design and construction. In this further the principles of design and construction. In this case all sections should prove useful.
case all sections should prove useful.
This publication is not intended to be a full technical This publication is not intended to be a full technical reference book for design, but it does address many of the reference book for design, but it does address many of the issues often omitted from such works.
issues often omitted from such works.
97.347 97.347 P u b l i s h e d P u b l i s h e d b y t h e b y t h e B r i t i s hB r i t i s h C e m e n t A s s o c i a t i o n o n C e m e n t A s s o c i a t i o n o n b e h a l f b e h a l f First published First published 19941994 i n d u s t r y i n d u s t r y s p o n s o r s s p o n s o r s o f o f t h et h e R e i n f o r c e d C o n c r e t e R e i n f o r c e d C o n c r e t e C o u n c i l .C o u n c i l . ISBN 0 7210 1481 ISBN 0 7210 1481 XX Price Group D
Price Group D British Cement AssociationBritish Cement Association Century House, Telford Avenue Century House, Telford Avenue Crowthorne, Berkshire
Crowthorne, Berkshire RGllRGll 6YS6YS Brit
British Cemenish Cementt AssociatiAssociationon 19941994 TeTell (0(034344)4) 767626267676 FaFaxx (0(034344)4) 767612121414
of the of the
All advice or information from the British Cement Association is intended
All advice or information from the British Cement Association is intended forfor those who will evaluate the significance and limitations of its contents and takethose who will evaluate the significance and limitations of its contents and take responsibility for its use and application. No liability (including that for negligence) for any loss resulting from such
responsibility for its use and application. No liability (including that for negligence) for any loss resulting from such adviceadvice or informationor information ISISaccepted. Readersaccepted. Readers
should note that all BCA publications are subject to revision from time to time and should therefore ensure that they are in possession of the latest version. should note that all BCA publications are subject to revision from time to time and should therefore ensure that they are in possession of the latest version.
CONTENTS
CONTENTS
INTRODUCTION
INTRODUCTION
DEVELOPMENT AND
DEVELOPMENT AND
APPLICATIONS
APPLICATIONS
Principles of prestressing Principles of prestressing Development of post-tensioning Development of post-tensioningBEN
BENEF
EFIT
ITS O
S O F
F
POST-POST-TENSIONED CONSTRUCTION
TENSIONED CONSTRUCTION
RANGE AND SELECTION OF
RANGE AND SELECTION OF
FLOORS
FLOORS
Forms of construction Forms of construction Typical properties Typical properties Selection criteria Selection criteriaComparison with other floor types Comparison with other floor types
DESIGN CONSIDERATIONS
DESIGN CONSIDERATIONS
Theory
Theory and and design design 1010 Restraint Restraint Holes Holes
CONSTRUCTION
CONSTRUCTION
Aspects of construction Aspects of constructionDEMOLITION AND
DEMOLITION AND
STRUCTURAL ALTERATIONS
STRUCTURAL ALTERATIONS
Demolition Demolition Structural alterations Structural alterationsREFERENCES
REFERENCES
22
66
77
7 7 8 8 8 8 9 9 1 1 00 13 131144
13 13 17 17 17 17 17 17 1 9 1 9be
be properly properly evaluated evaluated while conswhile cons ideringidering familiar techniques.
familiar techniques.
o t h e r m o r e o t h e r m o r e
Over the last 20 years, many buildings with post-tensioned Over the last 20 years, many buildings with post-tensioned floor slabs have been successfully constructed in the USA, floor slabs have been successfully constructed in the USA, South East Asia, Australia and the rest of Europe, yet it South East Asia, Australia and the rest of Europe, yet it took the construction boom of the late 1980s, with took the construction boom of the late 1980s, with corresponding increases in both steelwork prices and corresponding increases in both steelwork prices and delivery times,
delivery times, to gento generate serate signifignificant icant interinterest inest in post- post-tensioned floor construction in the UK.
tensioned floor construction in the UK.
T h e p u r p o s e o f t h i s p u b l i c a t i o n i s t o w i d e n t h e T h e p u r p o s e o f t h i s p u b l i c a t i o n i s t o w i d e n t h e understanding of post-tensioned floor construction and understanding of post-tensioned floor construction and show the considerable benefits and opportunities it offers show the considerable benefits and opportunities it offers to both the developer and designer. These include:
to both the developer and designer. These include: Rapid construction
Rapid construction Economy
Economy
Maximum design flexibility Maximum design flexibility Minimum storey heights Minimum storey heights Minimum number of columns Minimum number of columns Optimum clear spans
Optimum clear spans
Joint-free, crack-free construction Joint-free, crack-free construction Controlled deflections.
Controlled deflections.
Post-tensioned floors may be totally of in-situ concrete or Post-tensioned floors may be totally of in-situ concrete or a hybrid of in-situ and precast concrete. Either may be a hybrid of in-situ and precast concrete. Either may be prestressed or a combination of prestressed and reinforced. prestressed or a combination of prestressed and reinforced. They can be designed as two-way spanning flat slabs, They can be designed as two-way spanning flat slabs, one-way spanning ribbed slabs, or as banded beam and one-way spanning ribbed slabs, or as banded beam and slab construction. Flat slabs are supported, without the slab construction. Flat slabs are supported, without the use of beams, by columns with or without column heads. use of beams, by columns with or without column heads. They may be solid or may have recesses formed in the soffit They may be solid or may have recesses formed in the soffit to create a series of ribs running in two directions (waffle to create a series of ribs running in two directions (waffle or coffered slab). All these post-tensioned floors are further or coffered slab). All these post-tensioned floors are further described in Section 4, and shown in Figures 4.1
described in Section 4, and shown in Figures 4.1 toto 4.4.4.4. This
This publicatipublication on alsoalso aimaims ts to do dispispel el the the mymy tensioned
tensioned concrete slabs by showing that:concrete slabs by showing that:
ths about ths about post- post-They are not ‘unexploded bombs’
They are not ‘unexploded bombs’ The floors can be demolished safely The floors can be demolished safely
Local failure does not lead to total collapse Local failure does not lead to total collapse Holes can be cut in slabs at a later date Holes can be cut in slabs at a later date The design is not necessarily complicated The design is not necessarily complicated
They are compatible with fast-track construction They are compatible with fast-track construction They do not require the use of high-strength concrete They do not require the use of high-strength concrete Th
Th ee formwork formwork does not carry any of the prestressingdoes not carry any of the prestressing forces.
forces.
Post-tensioned concrete can satisfy all of the above Post-tensioned concrete can satisfy all of the above requirements,
requirements, and for this reason it is commonly usedand for this reason it is commonly used throughout most of the developed world.
throughout most of the developed world.
There are a number of publications that highlight the There are a number of publications that highlight the suitability of in-situ reinforced concrete frames for both suitability of in-situ reinforced concrete frames for both e c o n o m y a n d s p e e d o f c o n s t r u c t i o n i n h i g h - r i s e e c o n o m y a n d s p e e d o f c o n s t r u c t i o n i n h i g h - r i s e buildings”,
buildings”, The use of post-tensioned concrete slabsThe use of post-tensioned concrete slabs complements such a frame and helps maximise the benefits complements such a frame and helps maximise the benefits from both economy and speed. An example of a prestigious from both economy and speed. An example of a prestigious city centre development using this form of construction is city centre development using this form of construction is Exchange Tower in
Exchange Tower in
Engineers have a responsibility to their clients to consider Engineers have a responsibility to their clients to consider all available construction methods. Post-tensioned concrete all available construction methods. Post-tensioned concrete will not always be the most suitable, but it should at least will not always be the most suitable, but it should at least
D E V E L O P M E N T
D E V E L O P M E N T
A
A N
N D
D
A P P L I C A T I O N S
A P P L I C A T I O N S
The practice of prestressing can be traced backThe practice of prestressing can be traced back asas far as 440far as 440 BC, when the Greeks reduced bending stresses and tensions BC, when the Greeks reduced bending stresses and tensions in the hulls of their fighting galleys by prcstrcssing them in the hulls of their fighting galleys by prcstrcssing them with tensioned ropes.
with tensioned ropes. A further example,
A further example, a n d o n e w h i c h d e m o n s t r a t e s t h ea n d o n e w h i c h d e m o n s t r a t e s t h e simplicity of prestressing, is
simplicity of prestressing, is aa traditional timber barreltraditional timber barrel where the tension in the steel hoops effectively compresses where the tension in the steel hoops effectively compresses the staves together to enhance both strength and stability. the staves together to enhance both strength and stability. One of the simplest examples of prestrcssing is that of One of the simplest examples of prestrcssing is that of trying to lift a
trying to lift a row of books as row of books as illustrated in illustrated in FigFigurure e 2. 2. 11. . ToTo lift the books it is necessary to push them together, ie to lift the books it is necessary to push them together, ie to apply a pre-compression to the row. This increases the apply a pre-compression to the row. This increases the resistance to slip between the books so that
resistance to slip between the books so that theythey can bccan bc lifted.
lifted.
Figure 2.1 Lifting a row of books Figure 2.1 Lifting a row of books
This example also demonstrates one of the common This example also demonstrates one of the common principles shared by most applications of prcstrcssing. principles shared by most applications of prcstrcssing. There is generally a deficiency which can be offset by an There is generally a deficiency which can be offset by an efficiency which can be easily exploited. In the case of the efficiency which can be easily exploited. In the case of the books there is no grip between the books but the books can books there is no grip between the books but the books can withstand compression loads which can be easily applied. withstand compression loads which can be easily applied. A simple definition of prestressing
A simple definition of prestressing the example of the books, is:
the example of the books, is:
,, which relates well towhich relates well to
Principles of prestressing
Principles of prestressing
Concrete has a low tensile strength but is strong in Concrete has a low tensile strength but is strong in compression: by pre-compressing a concrete element, so compression: by pre-compressing a concrete element, so that when flexing under applied loads it still remains in that when flexing under applied loads it still remains in compression, a more efficient design of the structure can compression, a more efficient design of the structure can be achieved. The basic principles of prestressed concrete be achieved. The basic principles of prestressed concrete are given in
are given in 2.2, and further information on the2.2, and further information on the 2,2, 3).3).
Docklands( Docklands(
Figure Figure
development of prestressing and prestressing materials is development of prestressing and prestressing materials is given in Reference 5.
given in Reference 5.
Under an applied load, a prestressed beam will bend, Under an applied load, a prestressed beam will bend, reducing the built-in compression stresses; when the load reducing the built-in compression stresses; when the load is removed, the prestressing force causes the beam to is removed, the prestressing force causes the beam to return to its original condition, illustrating the resilience return to its original condition, illustrating the resilience of prestressed concrete. Furthermore, tests have shown of prestressed concrete. Furthermore, tests have shown that a virtually unlimited number of such reversals of the that a virtually unlimited number of such reversals of the loading can be carried out without affecting the beam’s loading can be carried out without affecting the beam’s ability to carry its working load or impairing its ultimate ability to carry its working load or impairing its ultimate load
load capacicapacity.ty. In otheIn other words prr words prestestresressinsing endog endows thews the beam with a high degree of resistance to fatigue.
beam with a high degree of resistance to fatigue. It is indicated in
It is indicated in FigureFigure 2.2 that if, at working load, the2.2 that if, at working load, the tensile stresses due to load do not exceed the prestress, the tensile stresses due to load do not exceed the prestress, the concrete will not crack in the tension zone but, if the concrete will not crack in the tension zone but, if the working load is exceeded and the tensile stresses overcome working load is exceeded and the tensile stresses overcome the prestress, cracks will appear. However, even after a the prestress, cracks will appear. However, even after a beam has been loaded to beyond its working load, and well beam has been loaded to beyond its working load, and well towards its ultimate capacity, removal of the load results towards its ultimate capacity, removal of the load results in complete closing of the cracks and they do not reappear in complete closing of the cracks and they do not reappear under working load.
under working load.
There are two methods of applying prestress to a concrete There are two methods of applying prestress to a concrete member. These are:
member. These are:
1)
1) By pretensioningBy pretensioning -- where the concrete is placed aroundwhere the concrete is placed around previously stressed tendons. As the concrete hardens, previously stressed tendons. As the concrete hardens, it grips the stressed tendons and when it has obtained it grips the stressed tendons and when it has obtained sufficient strength the tendons are released, thus sufficient strength the tendons are released, thus transferring the forces to the concrete. Considerable transferring the forces to the concrete. Considerable force is required to stress the tendons, so pretensioning force is required to stress the tendons, so pretensioning is principally used for precast concrete where the forces is principally used for precast concrete where the forces can be restrained by fixed abutments located at each can be restrained by fixed abutments located at each end of the stressing bed, or carried by specially stiffened end of the stressing bed, or carried by specially stiffened moulds.
moulds.
2) By post-tensioning
2) By post-tensioning -- where the concrete is placed aroundwhere the concrete is placed around sheaths or ducts containing unstressed tendons. Once sheaths or ducts containing unstressed tendons. Once the concrete has gained sufficient strength the tendons the concrete has gained sufficient strength the tendons are stressed against the concrete and locked off by are stressed against the concrete and locked off by special anchor grips. In this system, which is the one special anchor grips. In this system, which is the one employed for the floors described in this publication, employed for the floors described in this publication, all tendon forces are transmitted directly to the concrete. all tendon forces are transmitted directly to the concrete. Since no stresses are applied to the formwork, this Since no stresses are applied to the formwork, this enables conventional
enables conventional formwork formwork to be used.to be used.
Development of
Development of post-
post-tensioning
tensioning
The invention of prestressed concrete is often accredited to The invention of prestressed concrete is often accredited to Eugene Freyssinet who developed the first practical Eugene Freyssinet who developed the first practical post-tensioning system in 1939. The majdrity of the early post-tensioning system in 1939. The majdrity of the early applications were in the design of bridge structures. The applications were in the design of bridge structures. The systems were developed around the use of multi-wire systems were developed around the use of multi-wire tendons located in large ducts cast into the concrete section, tendons located in large ducts cast into the concrete section, and fixed at each end by anchorages. They were stressed by and fixed at each end by anchorages. They were stressed by jacking from either one or both ends, and then the tendons jacking from either one or both ends, and then the tendons were grouted within the duct. This is generally referred to were grouted within the duct. This is generally referred to as a bonded system as
as a bonded system as thethe grouting bonds the tendon alonggrouting bonds the tendon along the length of the section.
the length of the section.
The bonding is similar to the way in which bars are bonded The bonding is similar to the way in which bars are bonded in reinforced concrete. After grouting is complete there is in reinforced concrete. After grouting is complete there is
Prestressed concrete can most easily be
Prestressed concrete can most easily be defineddefined asas precompressedprecompressed concrete.
concrete. ThisThis means that means that a a compressive stress is compressive stress is put into aput into a concrete member before it begins its working life, and is
concrete member before it begins its working life, and is positionedpositioned to be in areas where tensile stresses would otherwise develop under to be in areas where tensile stresses would otherwise develop under working load. Why are we concerned with tensile stresses? For the working load. Why are we concerned with tensile stresses? For the simple reason that, although concrete is strong in compression, it simple reason that, although concrete is strong in compression, it is weak in tension.
is weak in tension.
Consider a beam of plain concrete carrying a load. Consider a beam of plain concrete carrying a load.
As the load increases, the beam deflects slightly and then falls As the load increases, the beam deflects slightly and then falls abruptly. Under load, the stresses in the beam will be
abruptly. Under load, the stresses in the beam will be compressivecompressive in the top, but tensile in the bottom. We can expect the beam to in the top, but tensile in the bottom. We can expect the beam to crack at the bottom and break, even with a relatively small load, crack at the bottom and break, even with a relatively small load, because of concrete’s low tensile strength. There are two ways of because of concrete’s low tensile strength. There are two ways of countering this low tensile strength -by using reinforcement or by countering this low tensile strength -by using reinforcement or by prestressing.
prestressing.
In reinforced concrete, reinforcement in the form of steel bars is In reinforced concrete, reinforcement in the form of steel bars is placed in areas where tensile stresses will develop under load. The placed in areas where tensile stresses will develop under load. The reinforcement absorbs all the tension and, by limiting the stress reinforcement absorbs all the tension and, by limiting the stress inin this reinforcement, the cracking of the concrete is kept this reinforcement, the cracking of the concrete is kept withinwithin acceptable limits.
acceptable limits.
IIQQprestressed concrete, compressive stresses introduced into areasprestressed concrete, compressive stresses introduced into areas where tensile stresses develop under load will resist or annul these where tensile stresses develop under load will resist or annul these tensile stresses. So the concrete now behaves as if it had a high tensile stresses. So the concrete now behaves as if it had a high tensile strength of its own and, provided the tensile stresses do not tensile strength of its own and, provided the tensile stresses do not exceed the precompression stresses, cracking cannot occur in the exceed the precompression stresses, cracking cannot occur in the bottom of the beam. The precompression stresses can also be bottom of the beam. The precompression stresses can also be designed to overcome the diagonal tension stresses. The normal designed to overcome the diagonal tension stresses. The normal procedure is to design to eliminate cracking at working loads. procedure is to design to eliminate cracking at working loads.
11
However, bending is only one of the conditions involved: there is However, bending is only one of the conditions involved: there is also shear. Vertical and horizontal shear forces are set up within also shear. Vertical and horizontal shear forces are set up within a beam and these will cause diagonal tension and diagonal a beam and these will cause diagonal tension and diagonal compression stresses of equal intensity. As concrete is weak in compression stresses of equal intensity. As concrete is weak in tension, cracks in a reinforced concrete beam will occur where tension, cracks in a reinforced concrete beam will occur where these diagonal tension stresses are high, usually near the support. these diagonal tension stresses are high, usually near the support. In prestressed concrete, the precompression stresses can also be In prestressed concrete, the precompression stresses can also be designed to overcome these tension stresses.
designed to overcome these tension stresses.
Figure 2.2
no longer any reliance on the anchorage to transfer the no longer any reliance on the anchorage to transfer the precompression
precompression into the section. Another commoninto the section. Another common application is in segmental construction
application is in segmental construction (Figure 2.3)(Figure 2.3) wherewhere precast concrete bridge sections are prestressed together precast concrete bridge sections are prestressed together with steel cables or bars, developing the simple idea of with steel cables or bars, developing the simple idea of compressing the row of books.
compressing the row of books.
Figure
Figure 2.3 Segmental construction in bridge decks2.3 Segmental construction in bridge decks
Applications in building have always existed in the design Applications in building have always existed in the design of large span beams supporting heavy loadings, but the of large span beams supporting heavy loadings, but the systems were not suitable for prestressing floor slabs, systems were not suitable for prestressing floor slabs, which cannot accommodate either the large ducts or which cannot accommodate either the large ducts or anchorages.
anchorages.
T h e m o r e r e c e n t d e v e l o p m e n t o f p o s t - t e n s i o n i n g T h e m o r e r e c e n t d e v e l o p m e n t o f p o s t - t e n s i o n i n g specifically for in-situ floor slab construction has resulted specifically for in-situ floor slab construction has resulted in two systems: (1) bonded construction and (2) unbonded in two systems: (1) bonded construction and (2) unbonded construction.
construction.
Figure 2.4
Figure 2.4 Bonded tendons in beamBonded tendons in beam With the bonded system
With the bonded system (Figure 2.4)(Figure 2.4) the prestressingthe prestressing tendons run through small continuous flattened ducts tendons run through small continuous flattened ducts which are grouted after the tendons are stressed. which are grouted after the tendons are stressed. ThTh ee system has been used successfully in general floor slab system has been used successfully in general floor slab construction and is often used for specialist applications. construction and is often used for specialist applications. Cost-effective designs can be achieved by this system, the Cost-effective designs can be achieved by this system, the principal features of which are given below.
principal features of which are given below.
The most efficient prestress design is when the The most efficient prestress design is when the prestressing tendon is positioned eccentrically in the prestressing tendon is positioned eccentrically in the concrete section on a curved profile or deflected from concrete section on a curved profile or deflected from a straight line. The size of the duct used in a bonded a straight line. The size of the duct used in a bonded system and the minimum cover that must be provided system and the minimum cover that must be provided may control the maximum eccentricity that can be may control the maximum eccentricity that can be achieved.
achieved.
The ducts are formed from spirally-wound or
The ducts are formed from spirally-wound or seam- seam-folded galvanised metal strip. The limit on the curvature folded galvanised metal strip. The limit on the curvature
or profile that can be achieved with the prestressing or profile that can be achieved with the prestressing tendons is dependent on the flexibility of the ducts. tendons is dependent on the flexibility of the ducts. The ducts have to be grouted after stressing, which The ducts have to be grouted after stressing, which introduces a further trade into the construction process. introduces a further trade into the construction process. To offset these features in floor slab applications, To offset these features in floor slab applications, prestressing using unbonded systems Figure 2.5) was prestressing using unbonded systems Figure 2.5) was developed in the USA in the 1960s.
developed in the USA in the 1960s.
Figure 2.5
Figure 2.5 Unbonded tendons in a floor slabUnbonded tendons in a floor slab
From the early days of prestressed concrete bridge design, From the early days of prestressed concrete bridge design, the benefits of not bonding the tendons to the concrete the benefits of not bonding the tendons to the concrete structure have been appreciated, and bridges
structure have been appreciated, and bridges havehave beenbeen c o n s t r u c t e d w i t h u n b o n d e d t e n d o n s r u n n i n g b o t h c o n s t r u c t e d w i t h u n b o n d e d t e n d o n s r u n n i n g b o t h internally and externally to the structure. Systems for internally and externally to the structure. Systems for use on bridge structures were again based on large use on bridge structures were again based on large multi-strand tendons. The main benefits to the bridge multi-strand tendons. The main benefits to the bridge designer were that tendons were replaceable should designer were that tendons were replaceable should corrosion occur and, if external, could be inspected
corrosion occur and, if external, could be inspected asas partpart of routine maintenance.
of routine maintenance.
In an unbonded system the tendon is not grouted and In an unbonded system the tendon is not grouted and remains free to move independently of
remains free to move independently of thethe concrete. Thisconcrete. This has no effect on the serviceability design or performance of has no effect on the serviceability design or performance of a structure under normal working conditions.
a structure under normal working conditions. 1t1t does,does, however, change both the design theory and structural however, change both the design theory and structural performance
performance at the ultimate limit state, which is precededat the ultimate limit state, which is preceded by larger deflections with fewer, but larger, associated by larger deflections with fewer, but larger, associated cracks than with an equivalent bonded system. Thus, with cracks than with an equivalent bonded system. Thus, with an unbonded system there are obvious visual indications an unbonded system there are obvious visual indications that something is wrong well before failure occurs. that something is wrong well before failure occurs. The economic advantages of an unbondcd system
The economic advantages of an unbondcd system in floorin floor
slab construction were identified, and systems were slab construction were identified, and systems were developed in the 1960s specifically for this application. developed in the 1960s specifically for this application. Tendons used today are typically either 12.9 mm or Tendons used today are typically either 12.9 mm or 15.7 mm strands, coated in grease, within
15.7 mm strands, coated in grease, within aa protectiveprotective sheath. They are cast into the concrete slab with small sheath. They are cast into the concrete slab with small (typically 130 mm x 70 mm) anchorages fixed to each (typically 130 mm x 70 mm) anchorages fixed to each end. After concreting, and when the concrete has obtained end. After concreting, and when the concrete has obtained a specified compressive strength, the tendon is stressed a specified compressive strength, the tendon is stressed very simply using a small hand-held jack, completing the very simply using a small hand-held jack, completing the post-tensioning operation.
post-tensioning operation.
The particular features of an unbonded system are: The particular features of an unbonded system are:
Tendons can be located close to the surface of the Tendons can be located close to the surface of the concrete to maximise the eccentricity (Figure 2.6). concrete to maximise the eccentricity (Figure 2.6). Tendons are flexible and can be easily fixed to different Tendons are flexible and can be easily fixed to different profiles. They can be displaced locally around holes, profiles. They can be displaced locally around holes, (Figure
(Figure 2.7), and to accommodate changes in slab shape2.7), and to accommodate changes in slab shape
(Figure 2.8). (Figure 2.8).
The stressing operation is simple, and with no grouting, The stressing operation is simple, and with no grouting, is suited to rapid construction methods.
is suited to rapid construction methods.
The use of unbonded tendons permits an effective and The use of unbonded tendons permits an effective and competitive multi-storey construction to be carried out in competitive multi-storey construction to be carried out in post-tensioned concrete.
post-tensioned concrete.
B o
B on d en d edd t e n d o nt e n d o n Cover
Cover--5050 mm m mim minimumnimum
Unbonded tendon Unbonded tendon Cover
Cover --TypicallyTypically 3030--40 mm40 mm
in the UK since the 1960s and some interest was generated in the UK since the 1960s and some interest was generated during the early 1970s. However, it took the construction during the early 1970s. However, it took the construction boom of the 1980s to generate significant interest in
boom of the 1980s to generate significant interest in post- post-tensioning of floors and the many schemes now completed tensioning of floors and the many schemes now completed have demonstrated to clients, architects and engineers the have demonstrated to clients, architects and engineers the many advantages of the system when used in multi-storey many advantages of the system when used in multi-storey construction.
construction. FigureFigure 2.9(a)2.9(a) toto (e)(e) s h o w s e x a m p l e s o f s h o w s e x a m p l e s o f buildings using post-tensioned floors.
buildings using post-tensioned floors.
Figure
Figure 2.9(a)2.9(a) The Pavilion, London Docklands:The Pavilion, London Docklands: post- post-tensioned flat slab
tensioned flat slab
Figure 2.7
Figure 2.7 Flexibility of tendons allows displacementFlexibility of tendons allows displacement around holes
around holes
Figure 2.9(b)
Figure 2.9(b) Exchange Tower, London Docklands:Exchange Tower, London Docklands: post- post-tensioned ribbed slabs
tensioned ribbed slabs
Figure 2.8
Figure 2.8 Tendons displaced to accommodate changesTendons displaced to accommodate changes in slab shape
in slab shape
The bonded and unbonded systems provide a range of The bonded and unbonded systems provide a range of post-tensioning methods available to the designer of post-tensioning methods available to the designer of buildings. In some instances the most economical design buildings. In some instances the most economical design can be achieved with a post-tensioned floor slab, using can be achieved with a post-tensioned floor slab, using unbonded tendons, supported on long-span post-tensioned unbonded tendons, supported on long-span post-tensioned b e a m s w i t h b o n d e d t e n d o n s . P o s t - t e n s i o n e d f l o o r b e a m s w i t h b o n d e d t e n d o n s . P o s t - t e n s i o n e d f l o o r construction can also be combined with conventional construction can also be combined with conventional reinforced concrete slabs to extend the range of concrete reinforced concrete slabs to extend the range of concrete floor options.
floor options.
The benefits of post-tensioned construction for floors were The benefits of post-tensioned construction for floors were quickly appreciated in the USA and many other parts of quickly appreciated in the USA and many other parts of the developed world, where its use in multi-storey the developed world, where its use in multi-storey construction is widespread. Systems have been available construction is widespread. Systems have been available
Figure 2.9(c)
Figure 2.9(c) Office block,Office block, HemelHemel Hempstead:Hempstead: post- post-tensioned construction tensioned construction -CL -CL
----
____---e-e
---e-e
Figure2.9(e)
Figure2.9(e) Windmill Hill, Swindon: post-tensionedWindmill Hill, Swindon: post-tensioned waffle slab waffle slab
BENEFITS OF
BENEFITS OF
POST-TENSIONED
POST-TENSIONED
CONSTRUCTION
CONSTRUCTION
The following are just a few of the many benefits to be The following are just a few of the many benefits to be gained from using post-tensioned floor construction. gained from using post-tensioned floor construction. Long spans reduce the number of columns and foundations, Long spans reduce the number of columns and foundations, providing increased flexibility for internal planning, and providing increased flexibility for internal planning, and maximising the available letting space of a floor.maximising the available letting space of a floor.
Minimum floor thickness
Minimum floor thickness maximises the ceiling zonemaximises the ceiling zone available for horizontal services, minimises the selfweight available for horizontal services, minimises the selfweight and foundation loads, and keeps down the overall height and foundation loads, and keeps down the overall height of the building.
of the building.
Minimum storey height is achieved, as the need for deep Minimum storey height is achieved, as the need for deep downstands is reduced, and slab thicknesses are kept to a downstands is reduced, and slab thicknesses are kept to a minimum. As a result, the storey height of a concrete minimum. As a result, the storey height of a concrete building can be less than that of a steel-framed building by building can be less than that of a steel-framed building by as much as 300 mm per floor. This can give an extra storey as much as 300 mm per floor. This can give an extra storey in a ten-storey building. Alternatively, it minimises the in a ten-storey building. Alternatively, it minimises the exterior surface area to be enclosed, as well as the vertical exterior surface area to be enclosed, as well as the vertical runs of mechanical and electrical systems. The reduced runs of mechanical and electrical systems. The reduced building volume (Figure 3.1) will save on cladding costs building volume (Figure 3.1) will save on cladding costs and may reduce
and may reduce runningrunning costs of HVAC equipment.costs of HVAC equipment.
Deflection of the slab can be controlled
Deflection of the slab can be controlled enabling longerenabling longer spans to be constructed with a minimum depth of spans to be constructed with a minimum depth of construction.
construction.
Concrete
Concrete buildingbuilding is up tois up to 3.03.0mm lowerlowerthanthan thethe steelsteel alternativealternative
10-storey
10-storey buildingbuilding Figure 3.2
Figure 3.2 Effect of post-tensioned construction onEffect of post-tensioned construction on building height and volume
building height and volume
Crack-free construction
Crack-free construction is provided by designing the wholeis provided by designing the whole slab to be in compression under normal working loads. slab to be in compression under normal working loads. Appropriate details may also be incorporated to reduce Appropriate details may also be incorporated to reduce the effects of restraint, which may otherwise lead to the effects of restraint, which may otherwise lead to cracking (see
cracking (see
Restraint
Restraint
in Section 5).in Section 5). This crack-freeThis crack-free construction is often exploited in car parks with concrete construction is often exploited in car parks with concrete surfaces exposed to an aggresive environment.surfaces exposed to an aggresive environment.
Large area pours
Large area pours should be adopted on all concrete floorsshould be adopted on all concrete floors in order to reduce the number of pours and increase in order to reduce the number of pours and increase construction speed and efficiency
construction speed and efficiency ( F i g u r e( F i g u r e 3.2).3.2). WithWith prestressed floors, when the concrete has reached a strength prestressed floors, when the concrete has reached a strength of typically 12.5
of typically 12.5 N/mm2,N/mm2, part of the prestressing force canpart of the prestressing force can be applied to control shrinkage cracking and thus further be applied to control shrinkage cracking and thus further aid larger area pours.
aid larger area pours.
by
by volumevolume ofof concreteconcrete
Typical pour size Typical pour size limitedlimited
Figure 3.2
Figure 3.2 Large area pours for increased efficiencyLarge area pours for increased efficiency
Rapid construction
Rapid construction is readily achieved in multi-storcyis readily achieved in multi-storcy buildings as prestressing leads to less congested slab buildings as prestressing leads to less congested slab construction
construction (Figure(Figure 3.3). Prefabrication of tendons reduces3.3). Prefabrication of tendons reduces fixing time and early stressing enables forms to be stripped fixing time and early stressing enables forms to be stripped quickly and moved to the next floor.
quickly and moved to the next floor.
Flexibility of layout
Flexibility of layout can be achieved as the design methodscan be achieved as the design methods can cope with irregular grids, and tendons can easily be can cope with irregular grids, and tendons can easily be deflected horizontally to suit the building’s geometry or to deflected horizontally to suit the building’s geometry or to allow for openings in slabs.
allow for openings in slabs.
Future flexibility is provided as knockout zones can be Future flexibility is provided as knockout zones can be identified for future service penetrations. Tried and tested identified for future service penetrations. Tried and tested methods are available to enable large openings for stairs, methods are available to enable large openings for stairs, escalators and other features to be formed subsequently escalators and other features to be formed subsequently (see Section 7). (see Section 7). _ - _ - _ - _ - _ - _ _ - _ - _ - _ - _ - _ oncrctc oncrctc 1-1 1-1
Figure 3.3
Figure 3.3 Minimising the amount of site-fixedMinimising the amount of site-fixed reinforcement speeds up construction
reinforcement speeds up construction
RANGE AND
RANGE AND
SELECTION OF
SELECTION OF
FLOORS
FLOORS
Forms of construction
Forms of construction
For most multi-storey buildings there is a suitable concrete For most multi-storey buildings there is a suitable concrete f r a m i n g s y s t e m .
f r a m i n g s y s t e m . F o r F o r s p a n s s p a n s g r e a t e r t h a n g r e a t e r t h a n 6 . 0 6 . 0 m ,m , post-tensioned slabs start to become cost-effective, post-tensioned slabs start to become cost-effective, and can be used alone or combined with reinforced concrete and can be used alone or combined with reinforced concrete to provide a complementary range of in-situ concrete floor to provide a complementary range of in-situ concrete floor options. The three main forms of construction are given options. The three main forms of construction are given below.
below.
Solid flat slab
Solid flat slab
Spans: 6 m to 13 m Spans: 6 m to 13 m
An efficient post-tensioned design can be achieved with An efficient post-tensioned design can be achieved with a solid flat slab (Figure 4.1), which is ideally suited to a solid flat slab (Figure 4.1), which is ideally suited to multi-storey construction where there is a regular column multi-storey construction where there is a regular column grid. These are sometimes referred to as flat plate slabs. grid. These are sometimes referred to as flat plate slabs. The benefits of a solid flat slab are the flush soffit The benefits of a solid flat slab are the flush soffit and minimum construction depth, which are suited to and minimum construction depth, which are suited to rapid construction methods. These provide the maximum rapid construction methods. These provide the maximum flexibility for horizontal service distribution and keep slab flexibility for horizontal service distribution and keep slab weight low and building height down to a minimum. weight low and building height down to a minimum. The depth of a flat slab is usually controlled by deflection The depth of a flat slab is usually controlled by deflection requirements or by the punching shear capacity around requirements or by the punching shear capacity around t h e c
t h e co l u m no l u m n. . P o s t - t e n s i o n i n g P o s t - t e n s i o n i n g i m p r o v e s i m p r o v e s c o n t r o l c o n t r o l o f o f deflections and enhances shear capacity. The latter can be deflections and enhances shear capacity. The latter can be increased further by introducing steel shearheads within increased further by introducing steel shearheads within the slab depth (Figure 4.1 (a)), column heads
the slab depth (Figure 4.1 (a)), column heads (Figure(Figure 4.1 (b)),4.1 (b)), or drop panels (Figure 4.1
or drop panels (Figure 4.1 (c)).(c)).
Beam and slab
Beam and slab
Spans: beams 8 m to 20 m, slabs 7 to 10 m Spans: beams 8 m to 20 m, slabs 7 to 10 m
In modern construction, where there is generally a In modern construction, where there is generally a requirement to minimise depth, the use of wide, sha requirement to minimise depth, the use of wide, sha llowllow band beams
band beams (Figure 4.2)(Figure 4.2) is common. The beams, which areis common. The beams, which are either reinforced or post-tensioned, support the one-way either reinforced or post-tensioned, support the one-way spanning slab and transfer loads to the columns.
spanning slab and transfer loads to the columns.
Structural steel shearheads Structural steel shearheads or reinforcement assemblies or reinforcement assemblies can be used for increased can be used for increased shear capacity
shear capacity
\ \ C o l uC o l u m m nn
No drop or column head No drop or column head
(a) Solid flat slab (a) Solid flat slab
Column head
Column head no dropno drop
(b) Solid flat slab with column heads (b) Solid flat slab with column heads
Drop panel Drop panel
(c) Solid flat slab with drop panel (c) Solid flat slab with drop panel
Figure 4.1
Figure 4.2 Band beam and slab Figure 4.2 Band beam and slab
Ribbed slab
Ribbed slab
Spans: 8 m to 18 m Spans: 8 m to 18 m
For longer spans the weight of a solid slab adds to both For longer spans the weight of a solid slab adds to both the frame and foundation costs. By using a ribbed slab, the frame and foundation costs. By using a ribbed slab, (Figure 4.3) which reduces the selfweight, large spans (Figure 4.3) which reduces the selfweight, large spans can be economically constructed. The one-way spanning can be economically constructed. The one-way spanning ribbed slab provides a very adaptable structure able to ribbed slab provides a very adaptable structure able to accommodate openings. As with beam and slab floors, the accommodate openings. As with beam and slab floors, the ribs can either span between band beams formed within ribs can either span between band beams formed within the depth of the slab or between more traditional the depth of the slab or between more traditional downstand beams. For long two-way spans, waffle slabs downstand beams. For long two-way spans, waffle slabs (Figure 4.4) give a very material-efficient option capable of (Figure 4.4) give a very material-efficient option capable of supporting high loads.
supporting high loads.
Figure 4.3 Ribbed slab Figure 4.3 Ribbed slab
Figure 4.4 Waffle slab Figure 4.4 Waffle slab
Typical properties
Typical properties
The typical ranges of spans for the various forms of The typical ranges of spans for the various forms of c o n s t r u c t i o n a r e g i v e n i n
c o n s t r u c t i o n a r e g i v e n i n Figure 4 . 5 a n d t h e i rFigure 4 . 5 a n d t h e i r corresponding slab and beam depths in Table 4.1. Table corresponding slab and beam depths in Table 4.1. Table 4.2 gives the corresponding material quantities for 4.2 gives the corresponding material quantities for thethe more common arrangements. The depths and quantities more common arrangements. The depths and quantities given are for an imposed loading of 5
given are for an imposed loading of 5 kN/m2,kN/m2, but can bebut can be reduced when this is lowered. They will also depend on reduced when this is lowered. They will also depend on such things as number of panels in each direction, fire such things as number of panels in each direction, fire rating, durability and vibration requirements. Although rating, durability and vibration requirements. Although post-tensioned floors are usually thinner and lighter than post-tensioned floors are usually thinner and lighter than reinforced floors, vibration is not normally a problem and reinforced floors, vibration is not normally a problem and is typically catered for by controlling the span/depth is typically catered for by controlling the span/depth ratio. ratio. Reinforced concrete Reinforced concrete Prestressed concrete Prestressed concrete
** For office loading of 5For office loading of 5 kN/m2kN/m2 Figure
Figure 4.5 Typical economical spans with different types4.5 Typical economical spans with different types of slab
of slab
Table
Table 4.14.1 Typical slab and beam depths*Typical slab and beam depths*
SLAB TYPE SLAB TYPE
Solid flat slab Solid flat slab
Solid flat slab Solid flat slab with dropped with dropped panels panels
SLAB SLAB
SLAB SLAB BEAM BEAM SPAN/ SPAN/ SPAN SPAN D E P T H D E P T H D E P T HD E P T H D E P T H D E P T H m m mm mm mm mm RATIORATIO 6.0 6.0 200 200 --- 11 /30 /30 8.0 8.0 250 250 --- 1/321/32 8.0 8.0 2222 55 --- 1/361/36 12.0 12.0 300 300 --- 1/401/40 One-way
One-way slab slab 6.06.0 1515 00 300300 l/40/20l/40/20
with band with band 8.0 8.0 200 200 375375 l/40/21l/40/21 beams beams 12.012.0 300 300 550550 l/40/22l/40/22 One-way slab One-way slab 6.06.0 1717 55 375375 l/34/16l/34/16 with narrow with narrow 8.0 8.0 225 225 500500 l/36/16l/36/16 beams beams 12.012.0 3232 55 750750 l/37/16l/37/16 Ribbed
Ribbed slab slab 8.0 8.0 300 300 --- 11 /27 /27 12.0
12.0 450 450 --- 11 /27 /27 15.0
15.0 5757 55 --- 1 / 2 61 / 2 6 *Based on an imposed loading of 5
*Based on an imposed loading of 5 kN/m2.kN/m2.
S
Selection cr
election criteria
iteria
When faced with selecting the most economic frame for When faced with selecting the most economic frame for any particular building, it is not just the straight cost of any particular building, it is not just the straight cost of the frame element which is compared. The effect of the the frame element which is compared. The effect of the frame on the cost of other building provisions is often frame on the cost of other building provisions is often the governing criterion. The most important of these are the governing criterion. The most important of these are the external cladding, flexibility for future use and, in the external cladding, flexibility for future use and, in highly serviced buildings, the services distribution both highly serviced buildings, the services distribution both horizontally and vertically through the building.
horizontally and vertically through the building. 1.1.
Table 4.2 Material quantities* Table 4.2 Material quantities*
SLAB SLAB TYPE TYPE SLAB SLAB SPAN SPAN mm mm SLAB TENDON
SLAB TENDON REBARREBAR DE
DEPTH PTH DENSITY DENSITYDENSITY DENSITY
mm tendons mm tendons pp ee rr kg/m2kg/m2 metre width metre width Flat slab Flat slab One-way One-way slab with slab with band beam band beam 6.0 200 6.0 200 1.41.4 8.58.5 8.0 8.0 250250 2.32.3 12.012.0 6.0 6.0 150150 3.23.2 6.56.5 8.0 8.0 200200 3.03.0 7.57.5 Ribbed Ribbed 8.08.0 300300 2.5 2.5 10.010.0 slab** slab** 12.012.0 450450 2.52.5 13.013.0 15.0 15.0 575 575 3.83.8 17.517.5 *Based on an imposed load of 5
*Based on an imposed load of 5 kN/m2.kN/m2.
The tendon density is based on 15.7 mm diameter super The tendon density is based on 15.7 mm diameter super strand with a guaranteed ultimate tensile strength of 265 strand with a guaranteed ultimate tensile strength of 265 kN.kN. The rebar density is based on high yield bar,
The rebar density is based on high yield bar, N/mm2.N/mm2. Each
Each way way **Quantities **Quantities per per rib rib depend depend on on rib rib spacing.spacing.
Distribution of services
Distribution of services
The key to the successful design of most buildings is to The key to the successful design of most buildings is to co-ordinate the design across all disciplines in the team. Of co-ordinate the design across all disciplines in the team. Of fundamental importance is the co-ordination of services fundamental importance is the co-ordination of services and structure to ensure that clashes with beams, columns, and structure to ensure that clashes with beams, columns, etc do not happen, and that a practical discipline for both etc do not happen, and that a practical discipline for both primary and secondary service holes is established and primary and secondary service holes is established and adhered to. It is desirable, therefore, to construct a frame adhered to. It is desirable, therefore, to construct a frame which provides the least hindrance to the distribution of which provides the least hindrance to the distribution of services.
services.
The other pressure on the design team is to keep to The other pressure on the design team is to keep to a minimum the depth of the ceiling void used for a minimum the depth of the ceiling void used for distribution, in order to keep down the overall height of distribution, in order to keep down the overall height of the building and to minimise cladding costs.
the building and to minimise cladding costs.
It can be appreciated that the ideal floor slab to satisfy the It can be appreciated that the ideal floor slab to satisfy the above criteria must be the thinnest possible and with a above criteria must be the thinnest possible and with a flush soffit, hence the reason that post-tensioned floor flush soffit, hence the reason that post-tensioned floor construction is so attractive in multi-storey construction. construction is so attractive in multi-storey construction.
Flexibility of use
Flexibility of use
For office construction, flexibility is mostly concerned For office construction, flexibility is mostly concerned with likely future changes in the internal space planning. with likely future changes in the internal space planning. In many cases these do not substantially affect the In many cases these do not substantially affect the structure. Core areas, primary services distribution and structure. Core areas, primary services distribution and other major items usually remain fixed, although some other major items usually remain fixed, although some additional holes for minor services may be required additional holes for minor services may be required subs
subs equeeque ntlyntly . . On On the othe other hather hand, apnd, applications plications such such asas retail or health care require a higher degree of flexibility retail or health care require a higher degree of flexibility for changes in services, and these should be considered at for changes in services, and these should be considered at the design stage.
the design stage.
Regardless of construction type, forming large holes in Regardless of construction type, forming large holes in any existing structure is not straightforward. In
any existing structure is not straightforward. In post- post-tensioned design,
tensioned design, careful consideration is necessarycareful consideration is necessary before breaking out any openings in an existing slab and before breaking out any openings in an existing slab and this is discussed later in Section 7. Smaller holes seldom this is discussed later in Section 7. Smaller holes seldom present problems as they may be readily formed between present problems as they may be readily formed between ribs or grouped tendons. The positions of the tendons can ribs or grouped tendons. The positions of the tendons can be marked on the slab’s soffit to aid identification for be marked on the slab’s soffit to aid identification for future openings (see Section 6).
future openings (see Section 6).
Cladding
Cladding
The depth of the overall floor construction has a direct The depth of the overall floor construction has a direct effect on the cost of the external cladding, which often effect on the cost of the external cladding, which often costs more than the frame. This is particularly relevant in costs more than the frame. This is particularly relevant in multi-storey construction where a few centimetres saved multi-storey construction where a few centimetres saved at each floor can show a significant overall cost saving. at each floor can show a significant overall cost saving.
Comparison with other floor
Comparison with other floor
types
types
Figure 4.6
Figure 4.6 (a-d)(a-d) shows a comparison of various floorshows a comparison of various floor designs carried out at the scheme stage for a multi-storey designs carried out at the scheme stage for a multi-storey hospital structure. The planning grid is 6.6 m square and hospital structure. The planning grid is 6.6 m square and the floors have been designed for a total imposed load the floors have been designed for a total imposed load (dead and live) of 5.5
(dead and live) of 5.5 kN/m2.kN/m2.
Reinforcement estimate Reinforcement estimate Tendons
Tendons -- 2.1 kg/m each2.1 kg/m each wawa yy
Rebnr
Rebnr -- 12 kg/m’12 kg/m’
(a) Post-tensioned flat slab (a) Post-tensioned flat slab
275 275 Reinforcement estimate Reinforcement estimate Rebar Rebar --3030kk g / m ?g / m ? (b)
(b) Reinforced flat slabReinforced flat slab
Structural topping Structural topping
\
\ Fire Fire protectiprotecti onon required to beam required to beam Precast unit
Precast unit
(c) Steel beam with precast units (c) Steel beam with precast units
Metal decking permanent Metal decking permanent formwork
formwork with concretewith concrete t o p p i n g
t o p p i n g
Fire protection required to beam Fire protection required to beam
(d) Steel beam with metal deck composite floor (d) Steel beam with metal deck composite floor
fY=460 fY=460
