Structures
Bibliotheek TU Delft
IIIIIIIIHI
•
Prof.lr. A.J. Hogeslag
Prof. Di pl.-Ing. J.N.J.A. Vambersky Prof.Dr.lr. J.C. Wal raven
Ir. W.A. de Bruijn Marjo van der Schaaf Anneke Kool
of Concrete
Structures
Proceedings of the International Seminar Delft 1990
Delft, The Netherlands
October, 25-26, 1990.
Edited by:
AJ. Hogeslag
J.N.J.A Vambersky
J.C. Wa/raven
Published and distributed by:
Delft University Press Stevinweg 1
2628 CN Delft The Netherlands (0)15 -783254
by order of:
Delft Precast Concrete Institute Faculty of Civil Engineering Delft University of Technology
Cover:
Bloemenveiling Westland
Waco-Liesbosch Beton BV, Utrecht, The Netherlands
CIP-GEGEVENS KONINKLIJKE BIBLIOTHEEK, DEN HAAG Prefabrication
Prefabrication of concrete structures : proceedings of the International Seminar 1990 Delft, The Netherlands, October, 25-26, 1990 led. by A.J. Hogeslag, J.N.J.A. Vambersky, J.C. Walraven. - Delft: Delft University Press. - 111.
ISBN 90-6275-612-3
SISO 693.5 UDC 624.04.012.3 NUGI 841 Trefw.: betonconstructies.
Copyright © 1990 by Delft University Press
No part of this book may be reproduced in any form by print, photo-print, microfilm or any other means without written permission from Delft University Press
Foreword 5
General Introduction 7
SESSION 1 - DESIGN ÀSPECTS 13
General Design Philosophy 15
Prof.Dipl.-Ing. J.N.J.À. Varnbersky, Delft University of Technology / Corsrnit Consulting Engineers
Stability of Precast Concrete Structures 29
Prof.lr. À.J. Hogeslag, Delft University of Technology
Precast Concrete Cores and Shear Walls 41
Ing. J.P. Strarnan, Delft University of Technology
SESSION 2 - PRECÀST CONCRETE FÀCÀDES 55
Building Physics and Facade Design 57
Prof.lr. C. den Ouden, Delft University of Technology / EGM Engineering
Double Skin Facades 69
Prof.lr. À.J. Hogeslag, Delft University of Technology
Prestressed Thin Wal led Concrete Facades 81
Mr. À. Suikka, M.Sc. Civ.Eng., Partek Concrete Industry
Thin Walled Fibre-reinforced Concrete Facades 89
Ing. À. Gerritse, HBG, Dept. S&O
Surf ace and Surface Treatrnent 103
Ir. À. van Àcker, Partek Ergon NV
SESSION 3 - CODES ÀND CÀLCULATION PRINCIPLES 115
Prefabrication and Eurocode 1992 117
Ir. À. van Àcker, Partek Ergon NV
Bond and Ànchorage of Strands 129
Composite Action of Floor Elements 155 Prof.Dr.lr. J.C. Wal raven , Delft University of Technology
Mortar Joints Loaded in Compression 167
Prof.Dipl.-Ing. J.N.J.A. Vambersky, Delft University of Technology / Corsmit Consulting Engineers
SESSION 4 - PRODUCTION 181
Production of Wall Elements 183
Ir. H.W. Bennenk, Schokbeton BV
High strength Concrete 187
Prof.Dr.lr. J.C. Walraven, Delft University of Technology
RECEIVED AFTERWARDS 201
Diaphragm Action 203
Nowadays, high requirements are imposed on the quality of structures and building processes. On the other hand the last decade has shown a drama tic increase of man-hour costs especially in the building industry. Owners, developers and construction companies are demanding a more man-hour efficient way of building.
Designers are being required to meet ever more stringent requirements concerning the efficiency of construction to cut down its own spiraling costs.
The heavy labour on the construction site, which very of ten takes place under wind, rain and frost conditions is criticized by the unions and the health organizations. In this respect prefabrication of concrete structures offers a number of advantages, which deserve a due consideration.
Prefabrication of structural members stands for high quality with regard to strength, stiffness and durability. It offers a wide variation in spans, shapes and colors. Manufacture and erection of prefabricated concrete structures can occur in short periods, with a small crew of qualified people, who can work in more favourable working circumstances.
Designing and building in prefabricated concrete however requires other concepts and strategies than applied for in-situ concrete, not only with regard to strength and stability but also with regard to organizational aspects. Only when this is realized, the way is open to successful applications.
The papers presented at this seminar are dealing as weil with the fundamental principles of prefabrication as with the recent developments in structural design, code provisions production techniques and products. As the field of prefabrication is very large, actually too large to be sufficiently covered by a seminar with a time span of only one and a half day, the subjects dealt with were this time limited to only those related to buildings.
The seminar is intended to give a survey of concepts which are experienced and known to be appropriate for actual building practice, to show new trends and developments and to open the way to face new challenges in structural engineering.
Prof.Dipl.-lng. J.N.J.A. Vambersky
GENERAL INTRODUCTION
Ir. A. Van Acker
Partek Concrete International, Commission on Prefabrication
Chairman of the FIP
At the start of this seminar on Prefabrication of Concrete Structures, it might be interesting to analyze the essence of concrete prefabrication in Western Europe today, and to outline the challenge for the future.
When analyzing the market of precast concrete structures, i t is not an easy matter to get precise figures about
quantities, output, market penetration etc., since
statistics hide many completely different products under the same heading: from street furni ture to bridge beams, and from roof tiles to large floor units.
For this reason we have based our analysis on the amount of hollow core floor elements produced in each country. Related to the number of m2 per inhabitant, i t gives a good representation of the present situation of the precast construct ion. m2 pro capita 0.70 Finland (O.65) 0.60 0.50 0.40 Holland (0.36) 0.)0 Sweden (0. JO) 0.20 0.10 Norway (O.l2) Oenmark (0.15) BelqiulII (0.08) Italy (0.06) UK (0.05) France (0.03)
0.00 L -_ __ __ _ - " ' =n:L.JlJL..IllJllll--.5.ll.il.lJlcJ..Q.O,..g.O.Q!0lL
Annual production of hollow core floors pro capita in different european countries
As you can see, the differences are enormous : from nearly 0.7 m2 to zero. Which other observations can we make:
- Smaller countries have a relative much higher production than the large ones.
Nordic countries are clearly using more precast floors and structures than the more southern ones. The climate is cer tainly an influencing factor. However, the market share in Holland, and to a lower degree in Belgium, illustrates that this can't be the only decisive parameter.
- Finland is the absolute fore runner in spite of the fact that their precast industry started much later than in most other countries.
Historically, the West European prefab industry has grown out of many small private companies. One of their characteristics was that they were fighting each other on small local markets. Af ter the war some larger prefab companies appeared, but the majority of the firms still remains small and medium size.
In comparison, the steel industry or cast insitu construction the real competitors of the prefab industry -contain small companies, as well as a large number of big companies and multi national Groups. Their policy and strategy is far broader than the local market. In addition they have astrong foothold in all existing national and international committees to defend their common interests.
These rather opposite attitudes of both precasters and their real competitors have had direct consequences on the penetration of precast concrete in the construction activity. In the fol1owing some aspects are analyzed.
1. In smaller countries, the impact of Prefab Companies and Federations on the mark et has been much larger than in bigger countries.
A typical example in respect to Standardization is Germany: DIN standards are rather conservative, and hence less favourable for precast construction, the latter being developed much later than cast insitu constructions . Unfavourable limi tations are imposed on maximum floor loads, steel stresses for prestressing, detensioning stresses, lateral load distribution, etc. All this resul ts in higher costs for the same product than in other countries.
Eurocodes are certainly going to improve the situation. In fact already now we see a positive influence on e.g. type approvals.
2. Another factor is the chauvinism of the bigger countries. They don/t feel inclined to look at evolutions abroad, since they are convinced of their own savoir-faire. Small countries on the contrary, are much more open and interested in what is going on abroad. They constantly try to take advantage of all possible evolutions which may improve their mark et position.
3. The dominance of big contractors over the prefab industry is much heavier in large countries than in the smaller ones. Precasting is not being considered as a specif ic
construction technique, with its own design and
execution philosophy, but rather as a complement to cast insitu structures.
As a direct consequence, precasting is not used
optimally, e.g. with respect to modulation, connections,
vertical bracing structures etc. , and thus less
economical, slower and more difficult to industrialize than i t could be. In this way, one is running in a vicious circle, because their is no real motivation to exploit precasting to its maximum.
The complete opposite has happened in Finland, which explains to a great extend, the success of precast structures over there. Already from tradition, through
its wooden houses, Finland has been closer to
prefabrication than other countries. In addition, af ter the war reconstruction has started much later in Finland than in most other West European countries. In order to decrease delays, all important contractors decided to work with precasting. At the same time, a national study group worked out a precast construction system based on the interchangeability of the different components used.
Architects and consultants were informed about the
system and made their projects accordingly. The system has already been improved during the seventies, and a complete new building system, named TAT has been worked out during the last years.
4. Another parameter which played an important role in the development and the use of precast concrete is research.
During i ts introduction on the market every new
technology has to be supported by research in order to get the necessary credibility from the users.
This is also true for prefabrication, and has been proven by countries such as Finland, the Scandinavian
countries, The Netherlands, Belgium, etc, where
Technical Institutes, Federations and precast companies have spent large efforts in research and development of precast concrete structures. Technical universities from
Delft, Göteborg and Finland have played a leading role in this respect and should be followed also in other countries.
5. Construct ion , and especially housing habits are playing a role in the spread of precast concrete. The act of building has a st rong country-linked character. For example, when comparing Holland to Belgium we see completely different housing styles: housing estates with mostly the same houses in Holland, build by the public authority, against highly individualistic houses in Belgium, build by the tenant.
Politics are certainly playing a role in all this.
Indeed, countries with socialistic governments are
constructing far more social housing projects than liberal governments.
It is obvious that precasting has only a chance in the first case. Indeed, nearly 50% of the total Dutch precast hollow core floor market is for housing, whereas in Belgium a much smaller percentage goes into that segment.
Unknown ,unloved is one of the conclusions of the previous statements. The intention of this seminar to do something about this. The different themes are dealing with all
aspects of prefabrication, ranging from fundamental
principles to recent developments in structural design, code provisions and product i on techniques.
The challenge for the future
Among different construct ion materials and tèchniques, precast concrete obviously disposes of a great potential with respect to the future. The question is what to do in order to meet this challenge.
A first target is to acquire the image of a modern progressive construction technique, with numerous advantages for the user wi th respect to quali ty, industrialization, environment etc.
In fact, the term "Precasting" should be reserved only for products made in fixed, weIl controlled plants. It should be a sort of trade mark, enabling to distinguish ourself from light prefabrication or other kinds of so cal led in situ prefabrication. The latter being a complementary execution technique to cast in situ construction , rather than real precasting.
Another objective is research and development. There should
be a continuous effort to keep our industry ahead of
competition from steel and cast insitu construction. We see
two different domains: fundamental research and applied
research.
Fundamental research is clearly the domain of universities. It should deal with basic research, both with respect to new materials and to calculation models and methods.
A good example is fibre reinforced concrete. I'm still
wondering why, in the past, universitïes have been so little interested in glass fibre concrete. still it was, and is an interesting new material with a lot of potential, but also a lot of open questions with respect to material technology. It is obvious that the classical methods of design and calculation for mass concrete do not apply for thin walled structures, because of the relati vely large importance of
stress concentrations due to material properties,
dilatations, element configuration etc. I'm convinced th at a lot of problems could have been avoided if more basic research would have been done in the past.
There is of cause the problem of finance at universities: since budgets are rather limited investments in expertise and basic research need to be allocated carefully. Some people suggest th at the financial means should come from the precasting industry. However i t is quite obvious that the industry is only interested in those institutes where basic experience in precast techniques is al ready available.
The continuous improvement and development of new products,
construction systems and product ion techniques, is the
domain of the precasting industry itself. This doesn't mean
that universities and technical institutes should be
excluded from this work - on the contrary they should assist
the precaster in all possible aspects but the
responsibility belongs to the industry.
A third point of interest is Standardization. One should not underestimate the influence of the coming international
Design Codes, Product Standards and 9ther regulations.
Precasting has still to make up arrears compared to cast insitu concrete, for example in the domain of composite action between precast elements mutually, or between precast elements and other materiaIs. Also the inherent high fire resistance of weIl designed precast structures hasn't been fully recognized. Future codes should enable us to benefit more on safety margins, higher concrete quality, etc.
Finally I would like to plead for more courses on precast construction at universities and technical institutes. Design and building in precast concrete requires other concepts and strategies than applied for insitu concrete, not only with regard to strength and stability but also with re gard to organizational aspects. Future architects and engineers should become familiar wi th these techniques so that they are not hesi tating any more to design precast constructions, because of a lack of knowledge, which unfortunately, is of ten the case now.
Uni vers i ties and Technical Insti tutes in the Nordic countries and also in The Netherlands have understood the need for education and basic research on precast concrete construction. In most of the other countries a lot has still to be done in this domain. The Delft Precast Concrete Institute is leading the way, not only through their intensive teaching and research programme, but also through the organization of this Seminar. I wish to congratulate and to thank the organizing committee for this initiative.
SESSION 1
GENERAL DESIGN PHILOSOPHY
Prof.Dipl.-ing. J.N.J.A. Vambersky, Delft University of Technology/ Corsmit ConsuIting Engineers The Netherlands.
1. INTRODUCTION
The dramatic increase of manhour costs during the past several years has impacted heavily on the building industry. Owners, developers and construction companies are demanding more manhour efficient way of building.
Designers are being required to meet ever more stringent requirements concerning the efficiency of construction to cut down its ever spiraling costs.
The heavy labour on the construction site, which very of ten takes place under wind, rain and frost conditions is criticised by the uni ons and health organisations.
These are some of the reasons why today more and more owners, developers and construction companies are choosing precast concrete for their building projects.
In an industrialized proces of manufacturing of precast concrete elements the actual work consists of relatively simple repeated handlings, so that it is possible to employ a great deal of unskilled and semi-skilled labour and still produce high quality products. Only a relatively small supervisory staff of foremen and specialists is necessary. This property, together with
- short construct ion time
- easy to reach high concrete quality - low sensitivity to weather conditions
- reduced manpower on site and the above dramatic increase of manhours costs have led to a significant application of precast concrete elements within the construction industry all over the world.
The impressive expansion of this technique, ho wever, does not always show the same expansion of the "know-how" in the field of design and detailling.
It is all too often forgotten, that a good building or other construction constructed from precast concrete elements should, from the very beginning, be designed as a construction made of precast concrete elements.
Instead of that all too of ten the design of a traditional building is merely adapted to a precast concrete, which is not the right approach.
The prefabrication has its own design approach which has to be respected to achieve the fulI profit which the prefabrication offers. This design approach - or design philosophy - is explained further in this article in particular for the precast concrete buildings.
2. DESIGN
Every construction system has its own specifics, which more or Ie ss influence the lay-out, storey height, stability, statical system etc.
To achieve an optimum design and an optimum structure a design should, from the very outset, respect these specifics and particular demands of the structure and system aimed at.
Hence, it is very important when designing a precast concrete structure, to realize that the best result is arrived at if the structure is designed as a precast concrete structure from the very outset and is not merely adapted from the traditional cast in situ or masonry design to a pre cast one.
Not respecting this may cause unnecessary faults and problems during the fabrication and construction of the elements and during the service life of the whole building. This is a very important point which, strangely enough, is still very of ten neglected. It should be realized, that it is a good initial pre cast concrete design which to a great extent determines whether the advantages of precast concrete shall fully develop and so contribute to cutting down the construction time and costs and ensuring better quality, as weIl as cutting down life cycle costs by assuring adaptability and openess to current and future demands or not.
Designers should, therefore, bear in mind the possibilities, restrictions, advantages and disadvantages of pre cast concrete, including its detailing, manufacture, transport, erection and serviceability stages before coming to a final design in precst concrete. As a rule modular co-ordination should be used throughout the building in every design, and also necessary tolerances should be carefully considered.
~ Detailing
All successful ideas, formulas and inventions are mostly based on strikingly simple solutions. This is generally valid, but even more for design and detailing of pre cast concrete structures.
As time saving is one of the most important advantages of precast concrete, care should be taken that the details used correspond with the principles of precast concrete design and fit into the short construction time.
Too elaborate or vulnerable details should be as much as possible avoided.
A good design in precast concrete should use as simple details as possible, the simplicity of the details determining all advantages of precast concrete.
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Where possible standardized units, elements and systems should be used, and non-standard solutions and details should be avoided wherever possible.
This also applies to the grid line distances, storey heights, stair step dimenstions, etc. Precast concrete elements should be as large as possible, bearing in mind limitations of manufacture, transport and erection and crane-lifting capacity, to re duce erection handlings.
Precast concrete columns should preferably be made as one single unit over two or more storeys to reduce the time taken in manufacture and erection, as weU as the number of connections. To keep the detailing of the column/beam connections as simple as possible and to permit the use of columns in one piece over two storeys or more, the column/beam connection should preferably be realized by simply supporting the beams by corbels (corbels forming part of the columns).
Cast in situ structural concrete topping should be used as little as possible, since it is a disturbing element in the erection process consuming time (and money).
The connections between elements should be as simple as possible and in no way attempts should be undertaken to make the connections similar to the cast in situ ones.
2.2. Structural connections Criteria:
Precast concrete connections must meet a variety of design, performance and other criteria. Not all connections are required to meet the same criteria. Some of the items discussed in this chapter are self-evident. Other requirements may not be so obvious and may require special consideration or specification. by the designer or owner of the structure.
The usual criteria are as follows:
a. Codes and Standards statical calculations and dimensioning
The statical calculations and dimensioning of the connections must comply with the relevant national codes and standards.
b. Strength
A connection must have the strength to re sist the forces to which it will be subjected during its lifetime. Some of these forces are apparent, caused by dead and live gravity loads, wind, earthquake, and soB or water pressure. Others are not so obvious and are frequently overlooked. These are the forces caused by restraint of volume changes in the members and those required to maintain stability. Volume changes are caused by temperature change, creep and shrinkage of the
elements. Instability can be caused by eccentric loading (intentional or unintentional), as weil as lateralloads from wind and earthquake. Very of ten, when not properly looked at, measures taken to resist instability will tend to aggravate the forces caused by volume changes, and vice-versa.
In addition to loads or forces that may be anticipated the Engineer may choose to provide additional capacity to resist some "unanticipated", "accidental" or "abnormal" loads. These loads could include foundation settlement, explosion, aircraft or vehicle collision or others. These "abnormal" loads may be accommodated in connections by a capacity for overload, by ductility within the connection, or by redundancy (alternate load paths) in the total structure or within the connection.
The connection strength can be categorized by the types of stress that may be induced. These include:
- compressive - tensile - flexural - shear - torsional
lt is obvious, that different connections will have a different degree of resistance to the different types of stresses as specified above.
Many connections will have a high degree of resistance to one type of stress, but little or no resistance to another. In many cases it may be unnecessary, or even undesirable to provide a high capability to resist certain types of stresses.
c. Volume change influences
The combined shortening effects of creep, shrinkage and temperature drop can cause severe stresses on precast, concrete members and their connections, if the connections restrain move ment. These stresses must be considered in the design, but it is usually far better if the connection will allow some movement to take place, thus relieving the stresses.
d. Ductility
Ductility is usually defined as the ability to accommodate relatively large deformations without failure. In structural materials, ductility is usually measured by the amount of deformation that occurs between first yield and ultimate failure. Ductility in building frames is usually associated with moment resistance. This is particularly true in designing for earthquake forces, which is where concerns over
ductility are usually expressed. In concrete members with moment-resisting connections, the flexural tension is normally resisted by steel components, either reinforcing bars or structural steel members. First yield occurs in this steel component, and final failure may be from rupture of the steel, crushing of the concrete, or a failure of the connection of the steel to the concrete. Ductility of the connections should be always considered together with the total structural concept of the structure.
e. Durability
Evidence of poor durability is usually exhibited by corrosion of exposed steel elements, or by cracking and spalling of concrete. Connections which will be exposed to weather should have steel elements adequately covered with concrete, or should be painted or galvanized. If not, sufficient non corrosive connection materials should be used.
All to weather or other aggressive environment exposed connections should be periodically inspected and maintained. Most precast concrete elements are of high quality, and flexural cracking is seldom a serious problem, provided tensile stresses are kept within code limits. However, local cracking or spalling can occur wh en improper details result in restraint of move ment or stress concentrations.
e.1. Concrete encasement of exposed connection steel
The preferred method of protecting exposed steel connection elements is to cover them with concrete, mortar or grout. Mix proportions, aggregate size and application procedures will vary with the size, location, and orientation of the element to be covered. Mixes containing chlorides should be avoided. ' Patches in architectural panels and others that will be permanently exposed to view will of ten not be accepted. Anchoring the concrete or grout to the relatively large steel surfaces is a problem that is of ten overlooked. Larger elements such as steel haunches can be wrapped with mesh or wire. For recessed plates and similar elements, connections such as those shown in fig. 3 can be used.
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f. Fire resistance
Many precast concrete connections are not vulnerable to the effect of fire and require no special treatment. For example, the bearings between slabs and beams do not generally require special fire protection. If the slabs or tees rest on elastomeric pads or ot her combustibie materiais, protection of the pads is not generally needed because deterioration of the pads will not cause collaps. Af ter a fire, the pads could be replaced. Connections in which weakening by fire would jeopardize the structures stability should be protected to the same degree as that required for the structural frame. For example, an exposed steel bracket supporting a beam may be weakened enough by a fire to cause the beam to collapse. Such a bracket should be protected. The amount of protection depends on (a) the stress-strenght ratio in the steel at the time of the fire and (b) the intensity and duration of the fire. Connections which require a fire resistance rating will usually have steel elements encased in concrete. Other methods of fire protection include enclosing with gypsum wallboard, coating with fire protecting coating, or spraying with fire protection material. There is evidence that exposed steell
hardware used in connections is less susceptible to fire-related strength reduction than other exposed steel members. This is because the concrete elements provide a "heat sink", which draws off the heat and reduces the temperature of the steel. Research on this sUbject is recommended.
g. Fabrication simplicity
Maximum economy of precast concrete construction is achieved when connection details are kept as simple as possible, consistent with adequate performance and ease of erection. Furthermore, complex connections are more difficult to design, to make and to con trol and will of ten result in poor fitting in the field. This can contribute to slow erection and less satisfactory performance. Underneath follows a list of items to consider during the design in order to improve fabrication simplicity. In many cases, some of these items must be compromised in order for the connection to serve its intended function, or other functional reasons.
g.l Avoid congestion
The area of the member in which the connection is made frequently requires large amounts of additional reinforcing steel, embedded plates, inserts, blockouts, etc. It is not unusual for so many items to be concentrated into one area that there is then only very little room left for concrete, not to mention for concrete to be placed properly.
In this respect it should be remembered that reinforcing bars and prestressing strands, which appear as lines on drawings, have real cross-sectional dimensions (which are larger than the nominal dimension because of the deformations); a fa ct to be considered in the design phase. Reinforcing bar bends require minimum radii, which can cause fit problems and leave some regions unreinforced. If congestion is suspected, it is helpful to draw large scale details of the area in question. In some cases, it may be economical to increase the member sizes just to avoid congestion.
g.2 Avoid penetration of forms
Since most precast, prestressed concrete members are cast in steel forms, projections which require cutting through the forms are difficult and costly to place. Where possible, these projections shouJd be limited to the top of the member as cast. Even this inhibits finishing of the top surface, especially on deck members or double tees and hollow-core sJabs.
g.3 Minimize embedded items
Items which are embedded in the member, such as inserts, plates, reglets, etc., require plant labor to locate precisely and attach securely. Therefore, these items should be kept to a minimum. This applies especially to the items embedded in the top surface.
g.4 Reduce post-stripping work
A plant casting operation is most efficient when the product can be taken directly to the storage area immediately after it is stripped from the form. Any operations which are required af ter stripping and before placement at the job site, such as special cleaning or finishing, or welding on projecting hardware, should be avoided, whenever possible. These operations require additional handling, extra work space, and added labor, of ten with skilled trades. The trade-off between penetration of forms and post-stripping work will sometimes need to be evaluated.
g.5 Use standard items
Wherever possible hardware items such as inserts, studs, steel shapes, etc., should be standard items that are readily available, preferably from more than one supplier. Custom fabricated or very specialized proprietary items add cost and may cause delays. It also simplifies fabrication if similar items on a product or project are standardized as to size and shape. There is also Ie ss chance of error. The same principle applies to reinforcing bars, embedded plates, studs, etc.
g.6 Avoid non-standard tolerances
Dimensional tolerances which are specified to be more rigid than industry standards, are difficult to achieve.
Connections which require close-fitting parts without provision for adjustment should be avoided.
g.7 Use repetitious details
It is very desirabie to repeat details as much as possible. Similar details should be identical, even if it may result in a slight overdesign. Once workmen are familiar with a detail, it is easier to repeat it than to learn a new one. Also, it will require fewer form set-ups and improve scheduling.
g.S Be aware of material limitations
Examples of this are the radius requirements for bending reinforcing bars,
.standard lengths for certain sizes of inserts, etc.
g.9 Allowalternates
Very of ten, a precast concrete manufacturer will prefer certain details over others. The producer should be allowed to use alternative methods or materiais, provided the design requirements are met. Allowing alternate solutions will of ten result in the most economical and best performing connection.
h. Loading conditions during erection
During erection loading conditions can occure, which induce stresses, as weIl in the precast concrete units as in the connections, which are higher then those under the service conditions. When designing the connections due consideration have to be paid to these effects unless special measures are taken during the erection - such as temporary supports etc. - to prevent such situations.
i. Erection simplicity
Much of the advantage of precast concrete construction is due to the possibility of fast erection of the structure. To fully realize this benefit, and to keep the costs within reasonable limits, field connections should be kept simpie. In order to fulfill the design requirements, it is sometimes necessary to compromise fabrication and erection simplicity. Underneath follows a list of items that should be considered during the design and detailing of the connections in this respect.
i.l Plan for the shortest possible hoist hook-up time
Hoisting the precast pieces is usually the most expensive and time-cri tic al process of the erection.
Connections should be designed so, that the unit can be lifted, set, and unhooked in the shortest possible time.
Before the hoist can be unhooked, the precast piece must be stabie and in its fin al position.
Stabilityof the precast concrete elements:
Some shapes of precast units such as double tees and hollow-core si abs are inherently stabie and require no additional connections before releasing the crane. Others, such as columns, deep beams, wall panels and single tees
usually require some supplemental shoring, quying, or fastening before the hoist can be unhooked.
Preplanning for the fewest, quickest, and safest possible operations that must be performed before releasing the hoi st, will greatly facilitate erection. Bearing pads, shims, or other devices upon which the piece is to be set, should be placed ahead of hoisting. Loose hardware that is required for the connection should be immediately available for quick aUachment. In some cases, it may be necessary to provide temporary fasteners or leveling de vi ces, with the permanent connection made af ter the hoi st is released. For example, if the permanent connection requires field welding, grouting, dry packing, or cast-in-place concrete, erection bolts, pins, or shims can be used. These temporary devices must be given careful aUention to assure that they wil! hold the piece in its proper position during the placement of all pieces that are erected before the final connection is made.
Stability of the total structure:
It is important, that in every stage of the erection the stability of the
structure as a whole is assured. If not, additional measures have to be taken. The type of connections used may play a decissive role in this.
i.2 Provide for adjustment
A certain amount of field adjustment at the connections is always necessary. Normal fabrication tolerances preclude the possibility of a perfect fit in the field.
i.3 Provide accessibility
Connections should be planned so, that they are accessible to the workman. The type of equipment necessary for making the connection should be considered. Operations which require working under a deck in an overhead position should be avoided, especially for weId ing. Alternatives to anything tha t requires temporary scaffolding should be considered. Room to place wrenches on nuts and swing them should be provided for bolts etc.
i.4 Standardize connection types
All connections which serve similar functions within the building should be
standardized as much as possible. As workmen become familiar with the
procedures required to make the connection, productivity is enhanced, and there is less chance for error.
Some types of connections require skilled craftsmen to accomplish, for example, welding and post-tensioning.
The fewer of these skilled trad es required, the more economical the connection will beo
i.5 Standardize Si:t.l;~ of components
Whenever possible, such things as field bolts, loose angles, etc., should be of common size for all connections. This re duces the chance for error, and the time required to search for the proper item.
i.G Use connections that are not weather-sensitive
Such materials as grout, drypack, cast-in-place concrete, and epoxies need special provisions to be placed in cold weather. Welding is slower when the am bie nt temperature is low. If the connections are designed so that these processes must be completed before erection can continue, costly delays may result.
i.7 Use connections that are not susceptible to damage in handling.
Reinforcing bars, steel plates, dowels, and bolts th at project from the precast piece will of ten be damaged in handling, requiring repair to make them fit, especially if they are of small diameter or thickness. Threads on projecting bolts should be protected from damage and rust.
i.B Allowalternates
As with fabrication, the precast concrete manufacturer or erector may prefer certain details or procedures not anticipated by the designer. Allowing alternate solutions will of ten resuIt in more economical and better performing connections.
j. Internal and extemal transport and storage
Oue consideration have to be given to the fact, whether the shape and the dimensions of the chosen connection details can cause problems during the transport and storage of the elements. This as well with respect to the damage of the connection
-
liS
with respect to the required space (economy) etc.k. Economy
The costs of the connection itself depend on the magnitude of the forces to be carried over and the repetition (number of the same connections) involved. For the economicaly justified choice it is however important to consider also the influence which the connection has on the total cost of the prefabricated structure as a whoie. The direct costs of the connection should be weighed against the costs of the element manufacturing, storage, transport, erection and finishing.
I. Appearance
When the precast members are exposed, the appearance of the connection is of ten an important consideration. It is sometimes necessary to compromise fabrication and erection simplicity, and hence, increase the cost, to provide a satisfactory appearance.
2.3. Standardization
Standardization has been always a hot item in the precast concrete industry. There are enough examples known from differènt countries, where standardization has been carried out to such an extent that stereotype buildings with the same appearance and character over the whole country were the result. This so far carried standardization has very of ten and understandably, lead to a certain aversion against prefabrication. This is ofcourse a wrong approach.
In general, when architecture and the building structure are optimized for each building - such as always should be the case - the component "standard" should be limited as to allow wide applicability.
This means that precast concrete production plants should be as versatile as possible in order to guarantee continuity of production and the standardization should be pursued for details, cross-sections connections, base-type products and systems, taking modular coordination into account, rather than 100% standardization of components let stay the 100% standardization of the whole building.
3. SAFETY
Stability and safety are necessary structural considerations in prefabrication. In cast in situ traditional buildings, up to a certain height the stability is mostly assured without applying special provisions, but in precast concrete the stability and safety of the structure should alsways be considered, even if the height is very smal!.
Also the possibility of progressive collapse should be carefully studied and, where necessary, provision should be made to prevent this or at least to reduce the risk to an acceptable minimum.
Progressive collapse is a chain-reaction failure, which causes extensive damage or total collapse of a badly designed and/or badly detailed structure. Basically, it is the propagation of a massive failure, which radiates out from initial damage caused to a relatively small portion of a structure.
lt is initiated by loading conditions not generally considered in the design - so called
accidental loading.
Accidental loading conditions and effects which can be structurally significant include: errors in design and construction; local overloading; service system (gas) explosions; ground vehicular and aircraft collisions; tornadoes, flooding; bomb explosions; fire loads; and foundation settlements.
To reduce the risk of progressive collapse three approaches are commonly employed, of ten in combination:
(a) reducing the risk of accidental loading
(b) preventing the propagation of a possible initial failure
(c) designing the structure to withstand accidental loading.
Generally spoken properly designed and detailed structures, with proper attention paid to continuity, structural integrity and anchorages and, most important, ductile performance of reinforced concrete members and connections, re sist in general accidental loading without turning into a mechanism af ter the failure of the first link.
BIBLIOGRAPHY
1. STUPRé (uitgave Betonvereniging), "Constructieve verbindingen van
geprefabriceerde betonelementen", Zoetermeer, The Netherlands 1976
2. L.O. MARTIN and W.J. KORKOSH, "Connections for Precast Prestressed Concrete
Buildings including earthquake resistance", P.C.I. March 1982
3. FIP Recommendations "Design of Multi-Storey Precast Concrete Structures"
STABILITY OF PRECAST CONCRETE STRUCTURES
Prof. ir. A.J. Hogeslag
Delft University of Technology, Department of Civil Engineering, Concrete Structures Corsmit Consulting Engineer, Rijswijk
SOME REMARKS ABOUT DESIGN AND PRELIMINARY CALCULATIONS.
I.INTRODUCTION
Short and to the point: To satisfy the requirements as to stability and rigidity of prefabricated bearing structures the partly restrained column seems to be the most suitable solution.
By "restrained column" is meant the whole sequence of elements from the real column, via the shearwall to the core (a system of composed walis), which all can be seen statically as a partly restrained column.
This type of construction can easily be analysed by the approximate method, shown in the figure.
c
c=oo Fig. 1 1 1 1 = + -FE FE 1,
FE,
2EI + I = moment of inertia of uncracked cross section
g, c = buckling length (1 storey g, c 2 g" > 5 storeys g,c
FVd
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C FE,2=
i
(column) 2e FE,
2=
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1,12 g,)n ~ Ó n-l' 0 n ~ 10 o.k. 10 > n > 5 acceptable n < 5 improve design n < 1 incorrect 2. FRAMES
The monolithic frame does not fit weil in our design-philosophy of prefabricated
structures. To -enable a quick erection, the connections between the elements must be simple. Moment resistant frames for lateral load resistance need connections and unfortunately the most effective and best performing moment-connections usuaily employ some cast-in-place concrete, which is laborious and thus not in line with the speed of erection.
111
I 1
Fig. 2
An effective way of providing simpier connections is to design members with the connections at a certain distance from the junction of the beam and the column, preferably at the points of zero bending moments.
However in general the members made in th is way are complicated and difficult to handle in connection with bracing and variations in dimensions and other measures.
floors
?
Fig. 3 / brace I ~ I1
element lieverarm 0= hingeAn acceptable solution is the ll-shaped element, possibly combined with a hinged beam.
1T frame
Fig. 4
But still by removing the points of zero bending moment from the middle of the column to the boHom the rigidity of the frame decreases. On the other hand a possible moment-connection by means of threaded bars is succeptible to dimensional variation. Altogether the moment resistant frame is to our opinion a less attractive and out of date structure.
An exception is the external bearing framework, obtained by joining efficiently rigid units together, to which I will return later.
3. PARTLY RESTRAINED COLUMN
For buildings with a limited number of storeys ( 1 to 4 storeys or up to about 12 m height) the simplest form of moment-resisting is the column fixed in the base. The horizontal framing members are connected to the columns with connections, which are assumed pinned. Of course the foundation must be able to withstand the restraining moments due to fixed connection.
be such that the safe bearing pressure will not be exceeded.
If piles are used at least three piles are necessary under the pile cap, unie ss beams in two directions are applied.
. mI rnl ~;:L---...J~~
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-~.I ! ... Fig. 5 I I-Two principal methods for restraining precast columns in the foundation are: connection in a foundation-socketj
connection by protruding bars.
Fig. 6
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The connection with steel base-plate and extended anchors (a rea I steel-connections) is attractive from the point of view of erection, but quite costly.
The connection in a foundation-socket is mostly applied if no piles are used. In Germany and Belgium these sockets are prefabricated and as a half standard product put on the market.
If a pile foundation is used the socket will be too laborious, employs much material and therefore is Ie ss attractive because of the costs.
In The Netherlands the application of foundation on piles dominates and is the foundation socket Ie ss popular. MosUy the connection by extended bars is used, in spite of the disadvantages of the wet connection, the need of bracing of the columns and a large dimensional accuracy. Also the cross-sectional area of the columns is larger in connection with the required spa ce for the recesses.
The aim is to transfer the lateral loads as much as possible to all columns. Then there is hardly any difference between the loads on the columns so that Ie ss column-types will do, or even one type will do without a waste of material.
r~
n columsFig. 7
To transmit horizontal forces the floor- and/or roof slabs must possess a certain amount of rigidity within their own plain. Next speaker will deal with that topic. It implies also contradictionary demands on the beam-column connection, because on the one side the build-up forces caused by the restraint of shrinkage, creep and temperature-change must be prevented and on the other hand the lateral wind forces must be transferred.
The solution can be found on the ground of the characteristic differences between these two loads: the slow proces of shrinkage etc. opposite to the fast developing lateral forces. The recess around the threaded bars in the connection can therefore be filled up with bitumen: plastic for slowly build-up forces, but rigid for fast build-up forces. Of course each column must be checked with regard to its own inclination and geometrical imperfections.
The loads are usually distributed to the columns in proportion to the flexural column-stiffness (depending on cross-sectional area, available normal force and rotational stiffness of the foundation). "Exact analysis" has become increasingly complex and adds considerably to the time and cost of engineering. Uniformity in the design and a regular basic grid of the columns can simplify the method of analysis to a large extent.
otter erec! ion to pour bdumen threaded bar 11 II onch~~.~ __ ~-'.I fostener Fig. 8 column
This type of structure is very ductile because each column provides his own stability.
Besides, by the simple connections, the structure can be regarded as suitable for
demountable design. The provision of the stability, as defined before, is not limited to
single-storey (industrial) buildings, as it will appear from the following examples:
Extension of the flower-auction-buildings at Aalsmeer and Honselersdijk.
Some data:
Column grid: about 16 x 20 m
Cross-sectional area columns:
1,2 x 1,2 m2
Loads on floors and roof:
10 kN/m2
4. SllEAR WALLS AND CORES
For multy-storey buildings, higher than ca 12 m, the most commonly adopted method of ensuring stability is by means of shear walls or rigid cores in combination with horizontally rigid floor slabs.
Minimum: three walls without a theoretical common intersection point of the axis are required or a torsion-stiff box-shaped core, combined with rigid floor slabs.
o
Fig. 10
In many cases it is taken as a basis of the structural design that the core resists the entire horizontal loading and thus completely ensures the required stability. In that case the columns can be detailed as members hinged at top and base. The bending moments developing at the connections are small, so that simple detailing of the connections is possible. The statical model of the total structure can be as shown in the figure.
Fig. 11
This sche me justifies using a si mple method of analysis. To determ ine the second order effects the core will be se en as a partly restrained column.
The procedure of the calculations is the same as already discussed before for the single column. The location of the stability elements in the plan has to be as effective as possible. In this choice the structural arguments must have priority.
In practice it is of ten taken for granted that walls of stairwells and elevators are used as shearwells and cores. In principle that is wrong, because the location of stairwells and elevators is determined on the ground of serviceability requirements of the
building. In my opinion the stability structures must be located on the most
structurally-favourabie position in the plan. Perhaps, with a little adaptation, either structural or architectural a combination of stairwells, elevatorshafts or wet cells can
be possible.
For the distribution of lateral forces it is assumed, that each floor can be modeUed as an infinite rigid deep beam, elastically supported by the shear waUs. As already has been said, the required diaphram action of the floor wil! be discussed in the next lecture.
In general a building loaded by lateral forces wil! translate as weU as rotate.
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Fig. 12Starting from the supposition mentioned and from linear elastic behaviour, the rotation point (the centre of gravity of the flexural stiffness EI of the stability elements) can be determined.
In principle the horizontal displacement of each point of the plan is known and with that the contribution of each element. Depending on the amount of stabilizing elements and the way of determination of the flexural stiffness, the calculations can be complex by which a computer is necessary or can be a simple short manual calculation. By applyil'lg the latter the flexural stiffness of the elements has to be of the same magnitude, as far as a statically indetermined situation is concerned (i.e. minimum EI not less than 8096 of the maximum EI), because of the fact that by the second order effect redistribution of forces occurs; the stiffer elements are loaded higher than calculated with a linear elastic method.
In the preliminary design phase, linear elastic calculations to establish the load distribution wil! do, af ter that for each separate element the second order excentricities have to be determined. If the additional moments, caused by these excentricities, are large, the sum of the moments can be redistributed among the stabilizing elements. In this calculation the use of a fictive EI is necessary, taking into account the non-linear behaviour of the element and also the rotation-stiffness. An example is shown in the figure.
Z 1. Wind: 1:~ . • H 1 Total on wall i: H 2 3 e z b , Fig. 13 H.e 1.. b. TorslOn: . 1 1 2 . H.e 1:(1. .b. ) 1 1 1 b. H. 1.. ( - + ' 1 -2• e) 1 Lli 1:(1 .• b. ) 1 1 n b. - 8.-Z 1 1
The stabilizing element can be a single wall, or a composition of coupled walls or even a box-shaped element. If the bearing structure is prefabricated it is obvious to prefabricate the stabilizing structures too.
---1
D
Fig. 14H
For mid-rise buildings the application of prefabricated elements for shear walls and cores is, from the point of view of costs, attractive. Because of the limitation of the composing elements by transport restrictions or crane capacity, the problem of joints is introduced. Mr. Straman will discuss this later. Of course it is possible to make a monolithic structure by means of welding re bars.
If it is necessary to increase the flexural stiffness of the cracked cross-section of an element a lot of reinforcement is needed and that means an expensive solution. A better solution is to avoid tension in the cross-section, so that no ties between the elements are needed, the elements keep uncracked and rigid. The most effective way is increasing the structural dead load on the elements. A good design with a sensible location of the stabilizing elements will contribute to it to a large extent. However
of ten the most favourable constructive solution is not possible by other requirements.
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Fig. 15
In the figure examples are shown of favourable, less favourable and incorrect locations. A practical choice to irlcrease the vertical force is the use of post-tensioning.
5. LOAD BEARING EXTERNAL WALL-UNITS
An important development is the load-bearing external wall. On account of strength, sound-proofing and heat-economy in general a lot of material is used for these elements. By efficiently joining these units together, a rigid structural framework is obtained, which is able to transmit the horizontal loads to the foundation.
These elements of the external walls are of ten one storey high with a width of up to 5,4 or 7,2 m. The floor spans usually between the longitudinal facades.
In some cases, at a ratio between the length and the width of the building < 4, the facade structure at both ends can cooperate with the longitudinal facades. In such a case a building structure is stabilized by an "external core", composed of strongly connected facade elements. The norm al force diagram at the base as a result of lateralloads perpendicular to the longitudinal facade is shown in the figure.
~
A
11
IJ
UJllD"'"
"""ïl!!!Jl
Fig. 16By shear lack there is no linear distribution of the normal forces. The calculation of this type is rather time-consuming. For preliminary design purposes a simple calculation method has been developed to deal with this problem. The calculation is based on a experiment by mind by which the facade has been modelled as a series of equal rectangular units horizontally coupled by a shear force connection. A working-out is given in the appendix. For a 10-storey building some results are shown in the figure.
By the horizontal loads tension as weU as pressure forces exist. As the end facade concerned the ten sion forces are only compensated by the own weight of the elements. However, with astrong connection between the facades, a part of the floor loads, in the first instance carried by the longitudinal facade, will be transmitted to the end facades. By this the tension forces are leveled, at a right length-width ratio of the building. The effective part of the longitudinal facade amount to about half the length
. of the end facade.
The effective width will increase if the height of the building increases. With regard to the check of the stability the first adoption can be, that all columns (the vertical parts of the units) in the longitudinal fa ca de are equally loaded. The calculation of the total building is reduced to a single fixed column. In the case of instability the lowest
l l
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endfacade long i ludinal element
focade
Fig. 18
columns are S-shaped curved. The columns of the end facades are supposed to be rocker members. The column located on the leeside of the building is loaded with the largest normal force; its contribution to the stability is small, which will be compensated by the other, less loaded columns. Partial buckling is in general no problem because the buckling-length is l/2, as the remaining columns prevent the horizontal displacement.
PRECAST CONCRETE CORES AND SHEAR WALLS
Ing. J.P. Straman
Delft University of Technology Department of Civil Engineering Concrete Structures
INTRODUCTION
For housing as weil as for appartment and office buildings pre cast concrete elements have found significant application.
To keep the detailing of the connections as simple as possible, beams and floor slabs are usually simply supported.
If buildings have more than two or three storeys, the stability is of ten provided by walls or cores, which already can be present for other (functional) considerations. In the circumstances it is obvious to prefabricate the stabilizing walls and cores also. However prefabrication of shear walls is not widely developed. To increase the application, the following items are important:
.ll
Economic connectionsBecause the dimensions of prefabricated elements are limited by transport restrictions and by the capacity of the crane, the shear walls consist of elements, coupled together by connections.
Development of economical connections, sufficient strong and stiff, is necessary.
~ Knowledge of the load deformation response of the connection
The structural behaviour of precast panel systems is mainly influenced by joints. Different stresses occur in the joints depending on their location within the plane and on the existing supporting conditions.
In the case of stability structures deformations are very important. From literature it has been found that experimental research usually is based on knowledge concerning the strength properties and not on the stiffness properties of the connections.
~ Knowledge of the influence of joints on the overall behaviour of the wallor co re By lack of suitable design models it is not possible to establish the extra stresses and extra deformations of the prefabricated structure, caused by the presence of the joints.
The three above mentioned items lead to an investigation, carried out by Delft University of Technology.
The following topics will be dealt with:
1) Forces in the connections 2) Types of connections
3) Influence of joints on the structural behaviour of the entire system 4) Design methods
5) Conclusions
1. FORCES IN THE CONNECTIONS
Besides a part of the vertical loads, stabilizing structures carry over the horizontal loads to the foundation.
The elements of the prefabricated walIon core are normally one storey high, so that the horizontal joints are placed level with the floors. The location of the vertical joints depends on different considerations, su eh as:
as much as possible equal elements;
influence on the structural behaviour of the entire system; crane capacity and transport restrietions (fig. 1).
1.1 Horizontal joints
The loads cause stresses in the horizontal joints. When possible, the structure must be designed in such a manner, th at tensile stresses are avoided. Because there are already many solutions available for the connection of the horizontal joints, the investigations have been concentrated on the vertical joints, especially on their stiffness properties. 1.2 Vertical joints q. t f I IJ I1 1 hotizonlal joint s 1 ... .,rticol joinlos Fig. 1
The vertical joints are mainly loaded by shear forces. These forces are carried over by means of in-situ concrete joints or by welded steel plates, cast in the elements. By
