Precast concrete prefabrication for Affordable Housing...
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Prefabrication for affordable housing. Stateof-art report. fib bulletin 60 Book · August 2011
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Prefabrication for affordable housing State-of-art report prepared by Task Group 6.7
August 2011
Subject to priorities defined by the Technical Council and the Presidium, the results of fib’s work in Commissions and Task Groups are published in a continuously numbered series of technical publications called 'Bulletins'. The following categories are used: category Technical Report State-of-Art Report Manual, Guide (to good practice) or Recommendation Model Code
minimum approval procedure required prior to publication approved by a Task Group and the Chairpersons of the Commission approved by a Commission approved by the Technical Council of fib approved by the General Assembly of fib
Any publication not having met the above requirements will be clearly identified as preliminary draft.
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This Bulletin N° 60 was approved as an fib state-of-art report by Commission 6 in October 2010.
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This report was drafted by fib Task Group 6.7, Affordable housing:
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David Fernández-Ordóñez (Prefabricados Castelo, Spain, Convener), Antoni Cladera Bohigas (Univ. Of Balearic Islands, Spain), Barry Crisp (Crisp Consultants, Australia), Bruno Della Bella (Gruppo Centro Nord, Italy), Iria Doniak (ABCIC, Brazil), Jaime Fernández Gómez (Intemac, Spain), Holger Karutz (CPI, Germany), Diane Laliberte (BPDL Precast Concrete International, Canada), Marco Menegotto (Italy), Julian Salas (Ministerio de Ciencia y Tecnologia, Spain), Javier Angel Ramírez (Univ. Politecnica de Madrid, Spain), Spyros Tsoukantas (Greece)
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Corresponding Members: Cliff Billington (J&P Building Systems, UK), Andrzej Cholewicki (Building Research Institute, Poland), Thomas D´Arcy (The Consulting Engineers Group, USA), Paulo E. Fonseca de Campos (Brasil), Antonello Gasperi (Italy), Ravindra Gettu (Indian Institute of Technology Madras, India), Subbaiya Kanappan (Larsen & Toubro, India), Luciano Marcaccioli (Officine Piccini, Italy), Pablo Moñino (Prefabricados Castelo, Spain), Shirish Patel (Shirish Patel & Associates, India), José Adolfo Peña (OTIP, Venezuela), Sthaladipti Saha (Larsen & Toubro, India), Arne Skjelle (Construction Products Association, Norway)
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Figures in Chapter 5, “Examples of housing systems”, were drafted by. J.A. Ramirez.
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© fédération internationale du béton (fib), 2011
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Although the International Federation for Structural Concrete fib – fédération internationale du béton – does its best to ensure that any information given is accurate, no liability or responsibility of any kind (including liability for negligence) is accepted in this respect by the organisation, its members, servants or agents.
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All rights reserved. No part of this publication may be reproduced, modified, translated, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without prior written permission. First published in 2011 by the International Federation for Structural Concrete (fib) Postal address: Case Postale 88, CH-1015 Lausanne, Switzerland Street address: Federal Institute of Technology Lausanne – EPFL, Section Génie Civil Tel +41 21 693 2747 • Fax +41 21 693 6245
[email protected] • www.fib-international.org ISSN 1562-3610 ISBN 978-2-88394-100-7 Printed by DCC Document Competence Center Siegmar Kästl e.K., Germany
Foreword
AC H E
The need for housing has increased significantly during the last decades all over the world. It is felt particularly in countries where the population growth rate is high and the economy is developing fast; but everywhere people are shifting from country land to towns, where housing in neighbourhoods often becomes critical. Apart from problems of adaptation of people to different lifestyle and of urban planning, a difficulty may rise even in meeting the demand for buildings.
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Also, the necessity for a great production of houses in a limited time appears, when reconstruction urges after disasters or renewals for inadequacy, together with updated quality requirements.
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Large projects always face cost and time constraints. Local conditions may be rather variable, with respect to physical, social and market environments. Thus, minimising cost and time of construction, while maximising quantity and quality of product, may lead to different solutions. The concept of “affordable”, meaning compatibility of demand and means, is well understood as such everywhere, although its practical application may be much different from place to place.
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Prefabrication, with its adaptability and quality consciousness, may offer valid, speedy, cost efficient and sustainable solutions in these instances. fib Commission 6, Prefabrication, is well aware of this and had the idea of sharing information on this issue, which materialised in 2003 when a very successful workshop was held in Chennai, India. The contribution of experts from many countries gave rise to quite interesting mutual information and comparisons.
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Then, Task Group 6.7 was started, with the aim of collecting the experience from around the world on affordable housing built with precast structural concrete. Its work is concluding with this “State-of-art Report”, offering an overview of housing systems as well as information on their features. A document of this kind was not available before; this report is therefore deemed to be of great interest and a source of ideas for all those who have to confront similar problems.
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Commission 6 is very grateful for this result to David Fernàndez Ordòñez, who has led the Task Group with determination, up to this successful accomplishment.
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Marco Menegotto Chairman of fib Commission 6, Prefabrication
fib Bulletin 60: Prefabrication for affordable housing
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Contents Foreword
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Introduction
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Scope
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Historical review
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General features
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AC H E
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4.1 Requirements for housing
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4.4 Production, transportation and erection process
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Production Transportation Erection
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4.6 Services and installations
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Examples of housing systems
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Bibliography
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4.5 Waterproofing and insulation
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4.4.1 4.4.2 4.4.3
General Structural systems Selection of a structural precast system Frame systems Wall systems Floor systems Stairs
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4.3 Precast structural systems 4.3.1 4.3.2 4.3.3 4.3.4 4.3.5 4.3.6 4.3.7
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Precast concrete / concrete Ferrocement Autoclaved aerated concrete Wood Steel Polystyrene Plaster Mortar/grout Fly ash Soil / Soil-cement mixture Composites
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4.2.1 4.2.2 4.2.3 4.2.4 4.2.5 4.2.6 4.2.7 4.2.8 4.2.9 4.2.10 4.2.11
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4.2 Materials
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fib Bulletin 60: Prefabrication for affordable housing
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Introduction
AC H E
There is a great need for housing in the world. This need has increased significantly during the last decades due to the fact that the greatest population growth has occurred in the outskirts of cities in developing countries, where the distribution of wealth is increasingly polarized. There is a large amount of human capital but a lack of technology and knowledge, as well as training for designers, contractors and other specialists in the construction industry. In addition, financing and the cost of land is a great problem for the construction of houses around the world.
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UNESCO states that the right to housing, the right to shelter for individuals and families, is a condition of citizenship. Also Human Rights Watch states that every person has a right to an adequate level of living, and that housing is within this right.
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In developing countries there is a need for shelter that can guarantee safety against extreme natural or climatic conditions, but these affordable shelters have also to comply with other needs like waterproofing, insulation and installations or sanitation facilities. These needs vary strongly from one part of the world to other. Also financing, road networks, water supply and infrastructure vary from one part of the world to others.
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In developed industrialized countries, housing is normally considered a product. In developing countries housing becomes a process in which the owner is often also the builder. This process starts with land reclaiming and can last five to fifteen years. Many times people live in a house that is not finished but in a continuous process of building. In this construction process there is collaboration between different agents like administration, co-operative organizations and builders.
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With industrialization and prefabrication it is intended to build mass production elements or to optimise the design of materials for building construction. There is much experience in the industrialization of construction systems around the world, but it is unknown to other parts, and many times even to neighbouring areas. We must understand industrialization as the result of applying technologies either to production (process technology) or to the product (product technology).
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It is of primary importance to take into account the construction possibilities in a given area. This means that production equipment, erection equipment – like mechanic or manual cranes and transportation, both trucks and mobile elements, and infrastructures such as roads – are different in each area. These differences impose the available solutions for each area.
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Also the economics of the solution change from area to area because in developing countries costs are different from those in developed countries. In many cases the cost of materials, equipment and technology is higher in developing countries but the cost of manpower is lower. Therefore in many occasions it is necessary to look for straightforward technology and elements of a size that can be produced, moved and erected by manpower. The technology adopted for housing components should be such that the production and erection technology can be adjusted to suit the level of skills and handling facilities available under metropolitan, urban and rural conditions. The structural systems and components selected should ensure minimum use of materials with maximum structural advantage.
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Industrialization is interesting as a way to achieve harmony between construction and industry. The technology of developed countries can be applied to the local possibilities of developing countries. Production procedures exist in the industry but they have to be adapted to the specificity of each area. Specific parts added to industrial components will form the industrialized autonomous system.
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Housing can be considered both as a whole and as a combination of functional elements. Affordability of housing is not simply a matter of more economical construction technologies. What dominates the total cost is more often the price of land, or of services, and what finally determines affordability is long-term financing of housing and whether it can be made available to the income groups that are expected to move into the housing.
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The methodology for affordable housing therefore has to be less sophisticated, involving less capital investment.
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There are four key challenges to be overcome in the shortage of affordable housing facing developing countries, namely: lack of resources, insufficient funds, shortage of skills, and time constraints. The construction industry needs to make greater use of prefabrication in undertaking projects in order to overcome the shortage of specialized manpower.
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There are several levels of industrialization, from closed industrial systems to a partial use of industrial components or open systems that allow a large freedom of design and construction.
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There are two directions of technological transfer: vertically, from theory to practice; and horizontally, from one industry segment to another.
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The simple adaptation of western ideas for materials or processes does not in general support housing solutions for low-income parts of society. It is of vital importance to use designs that are appropriate to the economic, social, cultural and natural conditions of each community. It is of primary importance to stop the outflow of currency to purchase machinery and raw material for local production. Standards and rules coming directly from developed world make housing unavailable in third world and developing countries.
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The following principles were put forward by J. Salas at the United Nations Conference on Human Settlements (“Habitat I”), held in 1976 in Vancouver, Canada:
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Concrete is a material with a lot of advantages to be used in affordable housing: it is durable, as it needs little maintenance, has good thermal inertia, can be used both as structural and finishing material, and is not sensitive to organic attack. It also has some disadvantages, such as higher cost in developing countries compared to developed countries, and also a possible lack of materials, mainly cement or admixtures. Today recognizing diversity is a primary need for developing processes in the industry of production and erection of elements for construction.
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1 Introduction
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Scope
The concept of “affordable” or “low-cost” housing can take on rather different meanings. Although globalization affects the way of life of all people in some way, economic social and functional requirements of goods are so different around the world that useful low-cost items in one context may be easily unaffordable or unsuitable in other ones.
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However, given a particular context, the concept has a clear meaning and it is well understood. Housing is a primary need of humans. Building houses is an important activity everywhere and controlling its overall cost appears to be a major issue in any context.
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The need for houses at a more or less low cost – and in the shortest amount of time, which also saves costs – may concern high-rate urbanization, rural areas to be upgraded, workers’ settlements in remote regions, rebuilding dwellings destroyed by disasters such as earthquakes, floods or wars, and up to holiday resorts and leisure dwellings.
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Prefabrication of structural concrete has become so versatile that it can offer valid solutions in any situation and at any scale, ranging from simple houses to sophisticated architectures, and provides products ranging from elements for self-construction up to large turn-the-key projects that require significant investments and heavy equipment; always for an “affordable” cost, relatively speaking, and practically without alternatives.
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The scope of this State of Art Report is to provide an overview of what is being done and can be done by prefabrication in the field of housing, under the most varied conditions.
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By showing the main features of a number of construction systems, although not entering into the details of the solutions, it aims to make possible a comprehensive comparison, which should help in learning, exchanging and developing ideas on how to better meet the housing needs everywhere, at sustainable cost.
Historical review
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Prefabrication, intended as fabrication of large units to be subsequently assembled into a structure, dates back in prehistory. In the early 19th century, the industrial revolution had an immeasurable influence on architecture and prefabrication. All design was affected by the common use of new materials such as steel and glass. Design changes were fundamental in some cases and gave rise to new styles that were solidly based on the concept of industry.
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The post World War I era in Europe saw a major increase in the industrialization of building. Due to the destruction of existing buildings and the lack of new construction during the intra-war years, there was an acute demand for economical and simple building systems. World War II concluded with another housing crisis both in the United States and Europe. Though United States territories had not seen any action, there was a need for housing due to the number of returning soldiers who quickly started families. A population explosion accompanied the end of the war. Once again, prefabrication was used to meet the demand for housing. This was the time when industrial prefabrication of structural concrete developed and expanded to take a prominent part into the construction market.
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Prefabrication is one of the ways to industrialize the construction process, but not the only one. Prefabrication with large panels and closed systems is not much used at present, even if it took a large part on the development of housing after World War II. We can distinguish three main periods in the evolution of prefabrication for housing during the second half of the 20th century:
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1950-1970: massiveness, euphoria and business. Closed systems based on large panels were dominant in the East Europe, and also very important in today’s European Community countries. These systems imposed rigid constraints because of the economy, speed of construction and limitations to architecture.
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The constraints were: • need for minimum of several thousands of housing units grouped together; • very rigid projects, with very little formal variations to minimize the different elements; • blocks of units set in a linear way as long as possible so there was no need to change crane tracks; • minimum spans and heights to comply with transportation; deck slabs of room size; • little or no flexibility for the redistribution in plan due to the use of massive panels, even non-load-bearing, so most could be built in the factory.
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Industrialization was, for the designer, a matter of economy of construction and the system was a strong constraint to architecture. To change any parameter of the system would imply losing competitively in the market. Camus said once when asked about thermal bridges: “I sell too much, I do not have time to improve”.
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1970-1985: crisis and confusion. Prefabrication with large elements tried to get out of the maze in which it was trapped, by providing more flexibility and variation in its products. The market changed in the EU from demand-led to offer-led. Then quality became the key point. Some panel systems changed, offering variety and quality with good response to small demands; others looked for ways to export, and many disappeared.
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The basis for what was called “open prefabrication” was set, with several compatible components.
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Technology applied to component production adapted very well to the crisis, and even with higher costs they could be more easily adapted to smaller and different works. Also components could adapt very easily to the new growing market of individual and low-rise housing in Europe.
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A dramatic reduction in the size of construction works penalized closed concrete systems and favoured component construction. Also components could be easily adapted to new and fast normative changes. Since 1985: other concepts in prefabrication, subtle prefabrication. Most elements in housing are now component construction. Large element construction has practically disappeared New prefabrication techniques and new designers get involved in small and large projects with excellent results. Now different kinds of individual elements are built using the highest
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3 Historical review
possible amount of automation to adapt to more and more individualized demands. The use of modern industrialization concepts that are common in the automotive industry is more and more common in the prefabrication of housing. It is of the highest importance to use all the experience of the last century in Europe to develop new ways of construction for affordable housing in all parts of the world.
General features
AC H E
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The right to housing, the right to shelter for individuals and families, is a condition of citizenship. However to define the requirements for shelter are different, depending on many diverse aspects like geographical situation, climate, seismic risk, economy, and social.
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The aim is to design and build a shelter that is consistent with the income of the future users. Regarding this aspect, evolutive systems that allow growth from very simple constructions are especially interesting.
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Housing is a basic need and like any basic human need will be constantly in demand. The concept of liveability of a house is usually framed in terms of activities in a house. The human requirements for space differ widely depending upon the geographic location and the climatic conditions of the site and upon the socio-economic and cultural standards of the population.
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Certain principles – like planning a good environment with adequate light and air, orientation, protection against noise, dust and local hazards – must be observed and minimum specifications must be followed without violating code rules for foundations, superstructure, plastering, painting, doors, windows and roofs.
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Standardisation, modular co-ordination and typification of building design are essential. Rationalisation of the dimensions of building components and the finished structure has a major influence on the planning of buildings. This results in optimum use of space, increased productivity and efficient use of building materials. Such a rationalisation is achieved by dimensional co-ordination.
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Geographical situation, and the climate which goes along with it, is relevant for the needs in these kinds of buildings. Areas with tropical weather require water tightness before other needs. In these areas special attention has to be applied to connections between structural members and also panels when used. Also the way in which water is removed from the building is relevant.
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To meet minimum health standards, certain household services and facilities are required. These include water supply, sanitary means for the disposal of household wastes including domestic sewage, facilities for washing clothes and cleaning household utensils, for bathing, for storage, preparation, cooking and consumption of food, and for storage and safeguarding of personal property. Due to the difficulty of air conditioning climate installations, at least in the beginning, ventilation is a key factor when building affordable housing. Therefore proper windows in the walls, protected by trusses, as well as roof ventilation to create natural ventilation, are relevant.
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In some circumstances the need for permanent housing has to take into account the stability of the structure against animal attack, rot or other biological hazards. In these situations the use of concrete leads to solid structures that resist natural deterioration.
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Seismic action is a significant factor when building housing. There have been many disasters due to the lack of properly constructed houses in areas affected by earthquakes. It is very important to follow some simple construction recommendations when building houses in seismic areas. Some relevant ones are to tie different parts of foundations and walls or structural parts of the structure, so that a local failure does not lead to an absolute ruin, or let the structure have deformations without leading to collapse. Some simple rules can be applied depending the type of structure that is used, e.g. wood, masonry, concrete, precast or in situ, steel or some combination of them. Many times sufficient rigidity is easily obtained in low rise buildings without the need for ductility. Also construction techniques with light materials are interesting in seismic areas because they reduce the seismic forces.
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Access to the working site might also be relevant when deciding what system is to be used. Often transportation and lifting capabilities are defining the length and weight of the elements. Therefore in many cases it is only possible to use elements that can be lifted by two persons.
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Housing is a basic necessity as well as, being a vital part of the construction sector, an important factor in the economy. As such, housing acts as a major contributor to employment and income generation and helps people both directly and indirectly in their socio-economic development. Economy is an aspect that is often limiting for a population to be able even to reach housing. Therefore sometimes the ability to obtain enough available funds is a key factor to start building a house. Another relevant factor is the availability of land. It is no wonder that in many situations, housing starts on land that is not used in any other way and later on is occupied with self-construction of housing. At first this starts without any common works, roads, water, sewers or other infrastructures, which later self-develops as the community does.
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There are some kinds of help available to finance affordable housing, some from local governments, normally for the purchase of building materials. Some non-profit organizations have taken part in specific projects with funding and technical help, which has been very useful locally. Private financing organizations have not taken part except for some recent micro credits activity, which extends quite quickly.
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Social aspects are relevant when deciding both the internal and external organization of the building. Depending on social conventions, there will be, for example, more separated rooms besides kitchen or a community room, or other distribution of internal space, as for example in Indian communities where the bathrooms are not inside the house, and it is considered a relevant characteristic to have a courtyard within the house. Some other cultures tend to organize social life outside the house in social or communitarian buildings.
4.1
Requirements for housing
“Affordable housing” must comply with the same requirements of safety, use, health and energy saving as normal buildings. The codes of different countries deal with this subject, or when no national codes exist, it is possible to use International Standards, taking into account the limitations due to the economical design.
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4 General features
A possible classification of requirements to comply with could be as follows:
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• Basic requirements for structural safety, which aim to ensure that the building has an adequate structural behaviour against possible actions during its service life and construction. To comply with these requirements, buildings are to be designed and built according to the applicable standards and codes, and must have the same level of safety as other similar constructions. Any reduction in safety is not allowed due to economic reasons. These requirements are divided in two groups: − strength, stability and robustness to avoid inadmissible risks; − service life capacity for normal use, with regard to deflections, cracking and vibrations.
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• Basic requirements for safety in the case of fire, which consist of reducing the risks of user damage due to an accidental fire. For this purpose it is necessary to comply with the following requirements: − interior propagation – limiting the risk of fire spreading to other places of the same building; − exterior propagation – limiting the risk of fire spreading outside of the building, − evacuation of users; − possibility to install fire-extinguisher equipment, even with individual kits; − possibility for access and intervention by the fire brigade; − structural strength for a determined period during the fire.
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• Basic requirements for safety of use, which implies that the users won’t be hurt as a result of the characteristics of the design, construction or maintenance. For this purpose the following requirements must be observed: − safety against the risk of falling, due to slippery floors, holes, staircases or level changes; − safety against the risk of impact, being trapped or imprisoned; − safety against the risk caused by inadequate lighting; − safety against the risk caused by high occupation conditions; − safety against the risk of drowning; − safety against the risk caused by moving vehicles; − safety against the risk caused by the action of rays of sunlight.
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• Basic requirements for health and protection, which consist of reducing to acceptable limits the risk that users, while making normal use of the building and installations, suffer illness or injury. Also the building must not be deteriorated or deteriorate the environment. For this purpose, the following requirements must be fulfilled: − protection against water and dampness; − collection and evacuation of wastes; − quality of interior air; − supply of drinking water; − evacuation of sewage. • Basic requirements for protection against noise, i.e. to limit inside the building disturbances created by noise. For this purpose, it is necessary to comply with the relevant acoustic protection limits, for noise produced both inside and outside the building.
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• Basic requirements for energy saving. The buildings must also comply with some requirements for the thermal insulation of walls and facades, air permeability and thermal bridges. The risk of dampness due to condensation must be avoided. Heating installations, where they exist, must be suitable to give thermal comfort while avoiding energy waste.
4.2
Materials
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This chapter provides general background information about various materials that can be used for affordable housing. Restricting ourselves to the attached catalogue of example housing systems, we focus on materials that are used in it.
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Looking at construction materials for affordable housing, one can divide their properties roughly into physical and environmental properties. Physical properties can be structural properties or building physics properties. Interesting environmental properties are durability, wind and weather resistance.
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It is necessary to know about both the physical and the environmental properties of the materials used in order to be able to erect a proper housing structure.
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Foundations should be made of very durable material. Depending on the location, they have to be water resistant. The compression strength of foundations necessarily needs to be able to withstand the sum of the loads of the housing structure, including dead load and live load. Although one can erect low cost housings even without a ground floor slab, it is recommendable to take it into account. The ground floor slab should protect the inhabitants from temperature variations, and its compression strength should be high enough to resist the live load.
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The walls of the buildings need to have enough compression strength to bear the vertical loads. In addition, they should protect the inhabitants from wind and rain, and they can also have heat insulation properties. If a structural framework is used, the wall panels need only to fulfil the above-mentioned inhabitant protection requirements.
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The floor decks above the ground floor have to carry the loads of the upper level, if any. If their function is mainly the roofing of the building, they should – besides their structural function – be weatherproof.
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Besides the structural properties, further important aspects of the materials to be considered are thermal insulation and fire resistance. Less important for affordable housing, but to be taken into account for more comfortable housing, are properties like sound insulation, etc. 4.2.1
Precast concrete / concrete
Precast concrete and concrete in general can be used for all kind of structural elements for buildings. The main advantages of concrete for affordable housing are very good compression strength, high durability and fire resistance. Prestressed concrete slabs and hollow core slabs can carry high live loads at a low dead weight. Lightweight concrete increases thermal insulation properties. Fibre reinforcement with polypropylene or steel fibres increases the ductility of the material. Concrete, especially precast concrete, can be used for linear structural elements like beams and columns, for foundations, and for 2D elements like walls 8
4 General features
and floors. At least structural concrete elements subject to stresses other than compression should be reinforced with steel. Concrete blocks can be used to erect solid walls. Mortar/grout is mostly used for the stacking of concrete blocks. 4.2.2
Ferrocement
4.2.3
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Ferrocement a highly versatile form of reinforced concrete made of cement mortar and wire mesh reinforcement. Due to the same components, ferrocement possesses similar strength and serviceability to reinforced concrete. Autoclaved aerated concrete
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Autoclaved aerated concrete (AAC) is a lightweight building material with excellent thermal insulation properties. AAC consists of 80% air and 20% solid material. Raw materials needed for its production are sand, cement and/or lime as binders, and water. To create the pore structure of AAC, small quantities of aluminium are used. The production process of AAC is highly automated using autoclaves.
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AAC provides good compressive strength even though the specific weight of AAC is relatively low. Fire protection is similar to concrete. AAC is available as reinforced AAC, as well, providing better bending strength than without reinforcement.
Wood
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AAC is available in the form of blocks, which are very suitable for self-construction, and of large wall panels for automated construction processes.
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Wood is mainly used for linear structural elements/structural framework, suitable for compression, tension and bending forces. Load bearing floors can be realized with wooden beams, but mostly this construction requires additional topping with planks. Wooden planks can also be used for the ground floor when there is no danger of water/moisture. Due to the low dead weight, walls and floors made of wooden planks are easy to erect.
Steel
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Wood is an orthotropic material, as its properties are dependent on the direction. One has to differentiate between loading in parallel and orthogonal to the fibres of the wood. Without protection, wood has low fire resistance, and also low durability in comparison to concrete.
D
O
C
U
Load bearing steel structures are not included in the attached catalogue. Thin steel plates can be used to protect the outer walls of the buildings, since they are weatherproof. They can of course also be used for the roof closure. Steel generally has to be protected to avoid corrosion. With suitable coatings, steel can be very durable, fire resistant and weatherproof. Thermal and sound insulation properties are not particularly good, compared with concrete, for example. 4.2.6
Polystyrene
Polystyrene cannot be used as load bearing material, but the thermal insulation properties are very good compared with other materials mentioned. If possible, one should combine a
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load bearing structure, heat insulation consisting of polystyrene and a weatherproof outer layer. 4.2.7
Plaster
Plaster can be used to provide outer walls with weather resistance. This might be necessary in case the walls were not erected with proper materials, having many joints in between. Plaster can help to close these joints, and can of course be used for decorative needs as well. 4.2.8
Mortar/grout
D
Fly ash blocks
S
4.2.9
E
AC H E
Mortar/grout is necessary to glue concrete blocks, especially for walls, together. It should not be taken into account as load bearing material itself. It needs to be combined with suitable blocks for wall structures. Another possibility is to use grout to close vertical joints.
Soil / soil-cement mixture
M
4.2.10
IE
M BR
O
Fly ash blocks consist of around 90% fly ash, 5% lime and 5% gypsum, as well. With an acceptable compression strength, fly ash blocks provide good insulation properties at a low dead weight.
Material properties
U
C O D
Young’s modulus [N/mm²]
Thermal conductivity λ [W/mK] 2.3 0.13-1.6
1.200-2.500
2.5−10
0.25−1.0
0.09
500-1.000
E┴ = 250-1200 E║ = 8000-17000 210.000 1-11 1.000-5.000 5.000-35.000 – 0.5-50
σ┴ = 0 σ║ = 4-15 240 0.2-0.5 0.3-0.5 1-5 – 0
0.09-0.24
7.800 15-30 700-1.800 700-2.000 1.000-1.400 1.400-2.200
σ┴ = 2-8 σ║ = 6-20 240 0.01-0.07 2-5 1-50 5-6 1-2
SO
Tensile strength [N/mm²] 1.6-5.2 0.8-4.1
A
TO
PA
R
300-1.000
Steel Polystyrene Plaster Mortar/grout Fly ash blocks Soil
4.2.11
25.800-45.200 850-32.000
Compressive strength [N/mm²] 12-100 12-60
2.400 400-2.000
M EN
Concrete Lightweight concrete Autoclaved aerated concrete Wood
Density [kg/m³]
U
Properties
EX C
Table 4.2-1:
LU
SI V
O
D
E
Some systems of the attached catalogue propose the use of soil, or soil-cement mixture as load bearing material. Although this is possible in principle, one has to take into account that the physical properties of this material are not as constant as those of other materials. Blocks made of soil-cement mixture might be used for wall structures with low demands. To protect these walls from weather influences, they should be coated, e.g. with plaster.
50 0.03-0.04 0.25-1.0 0.21-1.6 1.2-1.6 1.5-2.0
Composites
Fibre reinforced plastics (FRP) laminates: FRP are used for the manufacture of prefabricated, portable and modular buildings as well as for exterior cladding panels that can simulate masonry or stone. Typical application items are doors, as well, that might be insulated in between two layers of FRP.
10
4 General features
Fibre reinforced cements: FRC are used mainly for exterior cladding panels, that can also simulate other materials. Typical is glass-fiber reinforced cement (GFRC).
4.3
Precast structural systems
4.3.1
General
AC H E
Natural fibre composites: Reinforcement fibres made of jute, sisal, coconut fibre, banana fibre, etc., are cheap and their use does not affect the environment. They can be used as for reinforcement for prefabricated products that need low bending capacity, e.g. roof tiles or others.
O
S
D
E
As mentioned in previous chapters, when speaking about “affordable housing”, it is of primary importance to take into account the available technological level and construction possibilities in the given area.
M
IE
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Also the economic level of each country or area of the country should not be neglected, since it might be (and this is generally the rule)in developing countries) that the cost of materials, equipment and technology is much higher than manpower.
SI V
O
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Speaking now about reinforced concrete prefabrication, production and erection equipments (cranes, etc.) as well as suitable infrastructure (roads, etc.) and transportation means (tracks, etc.) are needed.
EX C
LU
Thus it might be that, to achieve affordable housing, full reinforced concrete prefabrication is not the best solution everywhere.
SO
Nevertheless, structures made with precast concrete have many advantages, such as strength in both static and dynamic loading, good response to strong winds or earthquake actions, durability, fire resistance, low sensitivity to organic attack, good thermal inertia, etc.
TO
PA
R
A
U
Also it should be kept in mind that prefabrication based on reinforced concrete offers a lot of advantages for affordable housing, such as speed of erection, simplicity according to demand, and waterproofing. Generally speaking it has suitable quality, including all that cannot always be provided without the high level of industrialization of prefabrication.
C
U
M EN
Affordable housing is in most cases also based on simplicity. In this respect, buildings with simple layouts, e.g. buildings with orthogonal plans, are ideal for precasting because they exhibit a degree of regularity and repetition in their structural grid, spans, and member size that lead inevitably to economical solutions.
D
O
In what follows an attempt is made to present briefly the main features of representative precast reinforced concrete structural systems, as well as the main principles of their behaviour under static and dynamic loading. Intentionally, no formulas are presented in this chapter, since it is a matter of specific design. 4.3.2
Structural systems
A large number of technical solutions and systems may be identified in the precast industry for reinforced concrete precast buildings. Nevertheless, they all belong to a rather limited number of basic structural systems, of which the design principles are more or less identical.
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By structural system, we mean a proper arrangement of vertical and horizontal bearing members, suitably connected between them to be capable to resist any kind of vertical and lateral loads.
AC H E
Depending on the type of bearing elements that constitute such a structural system, we may distinguish systems made by: i) linear elements (columns – beams), ii) walls, iii) combination of linear and wall elements, iv) cells (precast monolithic room cell systems).
D
Selection of a structural precast system
S
4.3.3
E
The above (i-iii) structural systems are completed with a number of complementary precast elements for the realization of floors, roof, staircases and facades.
IE
M BR
O
Although a lot of “closed” different precast systems are found in the precast market, the choice of a specific structural system depends mainly on the use of the building, the soil conditions, the seismicity and the technological level of the area.
O
D
E
M
Frame systems are the most common type of precast systems and are found everywhere around the world, since they permit a high degree of flexibility for the function of the final building, and also give a high degree of freedom to the architect.
EX C
LU
SI V
Multi-storey frame systems are mainly used for apartment buildings, commercial buildings, offices, car parks, etc. For these structures, particular attention should be paid to the selection of the type of frame system – hinged (Fig. 4.3-1a,b) or moment resisting (Fig. 4.3-1c) – and its lateral stability needs. Normally moment resisting frame systems are not used on affordable housing buildings due to the complexity of ensuring “emulative behaviour”.
M EN
TO
PA
R
A
U
SO
Single-storey frame systems are normally industrial buildings or warehouses. These types of buildings, see Fig. 4.3-2, are characterized by their large degree of repetition and large spans (say up to 30 m or more). The frame normally comprises two (or more) columns with moment fixed connections at the foundation and free supported mostly sloped prestressed roof beams. The distance between portal frames is governed by the span of the roof beam, on which roof elements are directly supported. They are normally not included in the catalogue of precast affordable housing systems but their concepts and design features can be used within more simplified systems in affordable housing.
D
O
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U
Wall systems, see Fig. 4.3-15, are mostly used for apartment buildings but also for individual housing or for prisons, hotels or bungalow construction. The surface of the elements is mostly smooth on both sides and ready for painting or wallpapering. In some cases in wall systems most of the walls are load bearing in one or the other direction, depending on the height of the building and the seismicity of the area. Nevertheless, the trend is to arrange load-bearing walls only in one direction of the building and to use light partition walls in the other, which is much recommended for non-seismic areas or for areas of low seismicity. There are sophisticated systems for housing or office buildings not suitable for affordable housing but other more simple systems are frequently used in affordable housing due that the precast panels are used both for structural and for façade or interior partition purposes.
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4 General features
AC H E O
S
D
E
a)
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c)
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E
b)
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A
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Fig. 4.3-1: Hinged frame system: a) structural scheme, and (b) sketch of the arrangement of the members [41a] c) Structural scheme of a moment resisting frame system [37]
Fig. 4.3-2 Typical single storey industrial building
Monolithic room cell systems are sometimes used for individual housing, with a proper arrangement one after and/or above the other, but also they may be used for parts of buildings, e.g. for bathrooms, kitchen blocks, garage boxes, etc. The main advantage of such systems lies in the speed of construction and the industrialization of manufacture since the
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finishing and equipment of the cells can be completely done at the precasting plant. Nevertheless transport problems and lack of flexibility in the layout of the project should be taken into account when using room cell systems. They have been widely used in closed systems for large amounts of houses to be erected in a very short time.
4.3.4
AC H E
All systems depend, for the distribution of lateral loads (from wind or earthquakes), on the diaphragm action of the roof and floor systems. To this end, independently of the type of the floor units, the diaphragm action of the entire floor has to be secured. This can be achieved through adequate connections between the floor units, with or without the use of cast in-situ concrete topping, and by a proper horizontal tying system. Frame systems
D
E
4.3.4.1 General
M BR
O
S
Different solutions have been developed in the precast industry when frame systems are used. Generally their type varies according to the height and use of the building and the seismicity of the area.
O
D
E
M
IE
According to the way in which beams and columns are connected, two main categories of frame systems may be identified: − hinged beam-column frame systems with cantilever action of the columns, − moment resisting beam-column frame systems.
SI V
4.3.4.2 Beam-column hinged frames
EX C
LU
Beam–column hinged frames usually consist of one-piece columns (along the whole height of the building) and simply supported beams with the vertical load path provided through direct support on corbels or on the top of the columns, depending on the number of the storeys.
D
O
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M EN
TO
PA
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A
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SO
The columns are fixed to the foundation with moment resisting connections (Fig. 4.3-1c). In Fig. 4.3-3 and Fig. 4.3-1a, the static scheme of such frames is presented.
Fig. 4.3-3: Beam-column hinged frames
14
4 General features
Such frame systems are widely used mostly in low-rise buildings because they are relatively simple to build and also economical, thus some of affordable housing systems have adopted this kind of structure in part. Usually in the frame direction the hinged connection is made by the use of two parallel dowels which are properly anchored in the body of the column (or in a corbel) and make the connection of the beam against horizontal shear actions by means of in-situ grout, which surrounds (and activates) the dowels and fills the slots at the beam ends.
AC H E
These two parallel dowels, mobilize also a certain protection against lateral overturning of the beams, which can be inducted by the seismic action, by mobilizing a lateral moment resistance of the connection.
U
M EN
TO
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A
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LU
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In Fig. 4.3-4 and Fig. 4.3-5 typical details of a hinged beam-column connections are shown schematically
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Fig. 4.3-4: Typical arrangement of a hinged beam-column connection on the top of a column (in this sketch, the secondary beam serves also for water flow) [42]
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AC H E E D S O M BR IE M E D
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Fig. 4.3-5: Individual precast members according to Fig. 4.3-4
LU
To ensure structural stability:
TO
PA
R
A
U
SO
EX C
i) Cantilever action of the columns may be assumed for buildings up to three levels but not in areas of high seismicity. In this respect the columns are continuous (one piece) for the full height of the structure and are fixed to the foundations by moment-resisting connections to act as cantilevers when exposed to horizontal loading due to wind forces or earthquakes (Fig. 4.3-6). To this end column-to-foundations connections should be properly designed. In seismic areas, columns should be designed and detailed not only for strength but also for ductility. It is possible to achieve sufficient ductility in this kind of structure by providing enough confinement reinforcement within a length of two depths of the column at the base.
D
O
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M EN
ii) Wherever the height of the building is more than 3 or 4 floor levels, and always in areas of higher seismicity, shear walls or boxes made by precast concrete or cast-in-place concrete should be preferred to secure the lateral stability and stiffness of the structure.
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4 General features
AC H E E D S
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Fig. 4.3-6: Schematic presentation of deformations and bending movement distributions in an unbraced three storey high hinged frame structure due to lateral loads [31]
SI V
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M
In Fig. 4.3-7 an example of a stabilizing system in braced frames is presented schematically together with indication of the flow of internal reactions due to horizontal loading in x and y directions. In such cases the horizontal “floor-diaphragm” system as well as the vertical “stabilizing system”, should be carefully designed.
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A
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LU
Particular attention should be paid also in the arrangement of the walls into the structure to achieve balanced resistance to horizontal forces. In Fig. 4.3-8 an example is given of how the torsion induced by an eccentric position of a core should be balanced by suitable arrangement of shear walls.
Fig. 4.3-7: Example of stabilizing system in braced frames [31]
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AC H E E D S O
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M BR
Fig. 4.3-8: Shear walls are needed to balance the torsion induced by the eccentric position of the core [30]
E
4.3.4.3 Moment – resisting beam – column frame systems
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LU
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Precast moment-resisting frames are constructed by a set of precast elements (columns and beams) or precast units of different forms suitably connected together to form a stable frame system (Fig. 4.3-9) able to meet the requirements of a corresponding monolithic one. These kinds of structures are more complex than isostatic ones and are seldom used in precast affordable housing systems. In any case the concepts and details used in their development are useful for affordable housing systems in high seismic areas.
Fig. 4.3-9 Schematic presentation of deformations and movement distributions in a momentresisting beam-column frame system [31]
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4 General features
For precast moment resisting frame systems usually the “emulative design” is used. That is, these systems are designed to closely emulate the response of conventional cast–in–place reinforced or prestressed construction, in terms of stiffness, strength and, in earthquake areas, in terms of ductility capacity and energy dissipation characteristics.
IE
M BR
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AC H E
Moment resisting frames are arranged to provide lateral force resistance in one or two ways, see Fig. 4.3-10.
O
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Fig.4.3-10: Classification of precast concrete moment-resisting systems according to the in place lateral force resisting mechanisms [41]
LU
SI V
Such frames, with or without additional lateral resisting systems such as shear walls, may be used for low or high-rise buildings independently of the seismicity of area.
SO
EX C
As a rule precast moment-resisting frames are complex to build, especially for the connections to achieve continuity at the supports, are costly and demand a rather high technological level to build. For these reasons, they are not much used for affordable housing.
U
4.3.4.4 Structural integrity of frame systems
M EN
TO
PA
R
A
In all cases of frame systems (and, generally speaking, in all precast systems), a suitable interaction of the bearing elements of the system (beams – columns – walls) with the floor system should be secured to ensure the structural integrity of the total structure. To this end, a set of adequate connections (contributing to force transfer continuity and ductility) should be provided between all structural parts of the systems. This is usually obtained through a threedimensional network of ties (Fig. 4.3-11).
D
O
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U
Ties are continuous tensile elements consisting of reinforcement bars (or sometimes tendons) placed in cast in-situ infill strips, sleeves or joints between precast elements, in longitudinal, transversal and vertical directions. Their role is not only to transfer normal forces (between units) originating from wind and other loading (e.g. earthquakes), but also to give additional strength and safety to the structure to withstand to a certain extent accidental loading, gas explosions, collisions, etc.
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Types of ties in skeletal frames [31]
E
M
Fig. 4.3-11:
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4.3.4.5 Column to foundation connections of frame systems
EX C
LU
SI V
Precast frame buildings without shear walls, and especially when only beam-columnhinged frames are used, depend on the moment capacity of the column base to resist lateral loads. The ability of a footing to resist moments is dependent on the type of the column-tofoundation connection and on the rotational characteristics of the base.
PA
R
A
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SO
The most common type is the pocket foundation, which is highly recommended on good soils, independent of the seismic area. The pocket should be wide enough to enable a good concrete filling around and below the column. It is very advisable in seismic areas to use pockets with keyed surfaces. Pockets may be prefabricated or cast by in situ concrete. In this case the concrete grade should be at least C30/37.
M EN
TO
In Fig. 4.3-12 a typical pocket foundation is presented schematically, together with a simplified model of the force transfer (4.3-12b). In Fig. 4.3-13 an example is presented, with a possible arrangement of the pocket reinforcements.
D
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In some cases column to foundation connections are realized by means of steel base plates with external (see Fig. 4.3-14a) or internal (see Fig. 4.3-14b) anchor bolts.
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4 General features
AC H E E D S O M BR IE
b)
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a)
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Fig. 4.3-12: a. Pocket foundation b. Simplified model for the force transfer in a pocket foundation [37]
Fig.4.3-13: Pocket foundation with smooth surfaces and possible arrangement of reinforcements fib Bulletin 60: Prefabrication for affordable housing
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Wall systems
SI V
4.3.5
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Fig 4.3-14: Steel base plates with external or internal anchor bolts [34]
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A wall system is composed of load bearing walls with floors and roofs made of solid panels, hollow core units, floor plank systems, etc. Depending on the arrangement of the bearing walls in the ground plan and on the serviceability needs of the building, non-bearing panels (plain or of sandwich type) are also used as partition or façade walls (see Fig. 4.3-15).
Fig. 4.3-15: Illustration of an integral wall system
22
4 General features
The type of a prefabricated building is best described by the arrangement of the load bearing structural units. Depending on the orientation of the main load bearing walls relative to the long axis of the building, we can distinguish:
EX C
LU
SI V
O
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E
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IE
M BR
O
S
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E
AC H E
• cross-wall systems, where load bearing walls run at right angles to the long axis of the building (Fig. 4.3-16b); • long-wall systems, where the load bearing walls are placed longitudinally, parallel to the main axis of the building (Fig. 4.3-16a); • two-way span systems, where the supporting members run both longitudinally and transversely (Fig. 4.3-16c).
R
A
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SO
Fig. 4.3-16: Arrangement of load bearing walls in buildings: (a) cross – wall system, b) long – wall system, (c) two-way span system
TO
PA
Wall systems generally provide good horizontal stability, ensured by means of cantilever action in walls (and/or cores).
O
C
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M EN
The acting horizontal loading is distributed (through the diaphragm action of floors) over the different walls and cores proportionally to their respective stiffness. When walls have rather large openings, for example for doors, it should be checked whether the part of the wall above the door opening could contribute.
D
The different storey height superposed wall panels have to be connected to each other in such a way that the total wall can function as a cantilever. In Fig. 4.3-17 the response of such units in vertical and horizontal loading is presented schematically. The prevailing actions in connections between individual walls units are: • shear forces in the vertical joints (Fig. 4.3-17); • compressive forces in the horizontal joints, accompanied by shear forces and bending moments (Fig. 4.3-18);
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AC H E E D S O M BR
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Fig. 4.3-17: Load deformations and shear forces in wall structures
Fig. 4.3-18: Schematic presentation of actions on horizontal joints. [41a].
Shear connections, from the point of view of strength and ductility, are mostly of wet keyed type, with connecting reinforcement by mean of loops (Fig. 4.3-19). In Fig. 4.3-20 some typical shear connections in horizontal section are presented, in which lateral loops across the joint and longitudinal reinforcement along the joint are shown.
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4 General features
AC H E
lj = length of the vertical joint between floors t = thickness of the precast panel tj = middle thickness of the joint concrete ao = slope of the key (45o÷60o recommended values) n = number of keys along lj a = thickness of the key inside the panel (1.5÷3cm) b = width of the joint, b ≥ t ho = length of the key inside the precast panel h1 = distance between keys
S
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n ⋅ h o = density of the keys (≤ 0.5) lj
M
IE
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Astr = transverse reinforcement (loops) Asl = longitudinal reinforcement lb = proper anchorage of the loops main reinforcement of the connection and of the precast panels
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Fig. 4.3-19: Typical shear connection [41a]
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additional local reinforcement
Fig. 4.3-20: Horizontal sections on typical shear connections
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AC H E
In order to ensure structural integrity it is also necessary to realise a three-dimensional coherence between the different elements. To this end, vertical and horizontal connections between the panels themselves as well as between panels and floor units should be properly designed. Also adequate tie-reinforcements should be provided in all directions (Fig. 4.3-21).
SI V
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Fig. 4.3-21: Schematic location of ties in spine wall structure [33]
LU
According to the cross section of the walls we may distinguish between the following types of load-bearing walls: Plain walls, Fig. 4.3-22 and Fig. 4.3-23, are the most commonly used in affordable housing, even with small thickness.
•
Sandwich walls, Fig. 4.3-24, composed of two concrete layers and one of insulating material. Such walls are mostly external and, in case they having bearing function, their internal concrete layer is provided with suitable thickness (≥ 150 mm) and reinforcement. Not normally used in affordable housing due to their complexity and cost.
•
Double walls, Fig. 4.3-25, composed of two precast elements, suitably reinforced and connected to each other during production. Such elements are placed adjacent to each other, and, after positioning of the slab elements and the additional reinforcement, in situ concrete is poured above the slabs and between both precast layers. In this way the final structure is similar in behaviour to monolithic reinforced concrete structures. In Fig. 4.3-26 some typical connections between double walls are shown schematically, and in Fig. 4.3-27 typical connections to their foundations are presented.
D
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•
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4 General features
AC H E E
TO
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Fig. 4.3-22: Plain wall with door opening
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Fig. 4.3-23: Plain wall with large window opening in which connecting and assembly reinforcement are shown
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Fig. 4.3-24: Bearing wall of sandwich type with openings
Fig. 4.3-25: Typical double wall system [38]
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4 General features
AC H E TO
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Fig. 4.3-26: Typical double wall connections in horizontal cross sections
M EN
a) bearing wall of bearing sandwich type
b) double wall
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Fig. 4.3-27: Typical details of connections between walls and their foundations
4.3.6
Floor systems
There are a variety of precast floor systems, which may be used for any type of precast structures but are also suitable for monolithic ones.
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The most common types for dwelling are the following. prestressed hollow-core floors (with or without structural topping)
•
double Tee ╥ or Π slab floors
•
massive slab floors
•
plank floors (reinforced or prestressed)
•
beam and block floors
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•
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The main structural requirements of floors are span load bearing, transverse load distribution of concentrated loadings, and distribution of horizontal actions by in-plane diaphragm action, as well as the ability to handle the effects of accidental actions affecting it or its supporting structures. In addition, depending on their use, floors can also have to fulfil other requirements such as acoustic insulation, fire resistance, etc.
M EN
TO
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From the above floor systems, the first three may be used also without cast in place concrete topping depending on the live loads and the span of the floor elements. Principally topping deals with the diaphragmatic performance of the floor system, and with better load transfer between the precast members of the floor. Plank floors are always used with additional in situ concrete, while beam and block floors are used by in situ concrete filling usually combined with concrete topping.
D
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Whenever structural topping will not be used, particular attention should be paid on the detailing of the connections of the adjacent individual precast members due to the tendency for different vertical deformations and to shear stresses along the connection arising from the diaphragm action of the whole floor. Fig. 4.3-28 shows a schematic presentation of actions on horizontal joints between floor precast members, diaphragmatic actions included. Fig. 4.3-29 presents a simplified model of flow of actions for joints of precast floor members. Fig. 4.3-30 shows a schematic presentation of the mechanism for lateral load distribution for floors made by hollow-coreslabs.
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4 General features
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b)
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a)
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Fig. 4.3-28: Schematic presentations of actions on horizontal joints between floor precast members [37]
c)
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Fig. 4.3-29: Simplified model of flow of actions (a and b) [37] Principal stresses inside and around the connection (c)
Fig. 4.3-30: Schematic presentation of the mechanism for lateral load distribution in H-C-S floors. [31]
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Plank floors are always composite and the precast pre-slabs contain the main slab reinforcement together with possible connecting reinforcement with the cast in-situ concrete topping. In Fig. 4.3-31, a cross section of a composite plank floor is presented schematically, in which typical reinforcement is also shown.
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Fig. 4.3-31: Schematic presentation of a plank floor
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Beam and block floors are made by rather shortly spaced (0.4 – 0.8 m) precast joists, prefabricated infill block usually of ceramic material or other, and in-situ concrete filling usually combined with an integral concrete topping. These kinds of floors are used with a high variety of mixed construction in affordable housing.
Fig. 4.3-32: Example of a typical beam-block floor. In-situ concrete filling and integral concrete topping are not shown. [39]
Prestressed hollow-core-slab units are manufactured using either long line extrusion or slip form process to have longitudinal cores of which the main purpose is to reduce the weight of the floor. They are used for every type of buildings such as apartment buildings, hospitals, schools, shopping centres, and suit for rather large spans of the floors. They are characterized by their favourable cost/efficiency ratio and the fast erection. Normally small kind of hollow core slabs for small spans, either reinforced or prestressed are used in some mixed affordable housing systems. 32
4 General features
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The edges of the units are suitably profiled to ensure vertical (sometimes also horizontal) shear transfer across the grouted joint between adjacent units (see Fig. 4.3-33).
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Extruded elements
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Fig. 4.3-33: Typical hollow core slab cross-sections [30]
In order to achieve structural integrity, floor systems consisting of individual precast concrete units should be tied together by a tying system to form an entity, either with or without a cast in-situ structural topping over the whole precast floor surface. In Fig. 4.3-34 some tie arrangements in a hollow core floor are presented schematically.
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Fig. 4.3-34: Horizontal ties in hollow-core floors [31]
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In order to ensure the diaphragm action of a precast floor system, which is needed for the transmission of the horizontal loads from wind, earthquakes, etc., to the structural system of the structure, tensile, compression and shear forces (created from the diaphragm action of the floor), should be carefully estimated and covered with suitable reinforcement accordingly.
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To this end, design models (arch-and-tie or strut-and-tie) should be used according to the type and arrangement of the vertical resisting members of the structural system, as well as of the type and arrangement of the floor units.
Stairs
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In Fig. 4.3-35 some possible models are presented, based on the arch-and-tie scheme.
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Precast concrete stairs are rather industrialized products with high degree of finishing ranging from smooth as cast to polished concrete. Compared to in-situ solutions they are particularly cost effective, especially in the case of a reasonable amount of repetition, as in the case of multi-storey buildings. Different types of stairs may be built according to the different needs of each particular case.
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4 General features
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a) Analogous plate girder
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b) Force distribution in floor diaphragm
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Fig. 4.3-35: Simplified models of diaphragm action based on the principles of the “horizontal deep beam analogy” [30]
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Fig. 4.3-36: Typical stair construction using precast stair members [36]
Production, transportation and erection process
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4.4
Production
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In developing countries, the production, transportation and erection process play a significant role in the expansion of local prefabrication technologies. It is necessary to take into account the fact that roads could significantly limit the possibility of transportation and it will therefore be necessary to adapt production and transportation to this limitation. Moreover, the components to be manufactured should be limited, at the beginning, to those that can be easily erected on the work site without assistance of heavy equipment. In fact, in some countries, the prefabrication approach has been difficult to introduce due, in part, to a shortage of equipment needed for casting, curing, transportation, and lifting of modules [43].
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4.4.1.1 General aspects of production
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Precast elements are generally constructed under factory conditions, as the quality of the work is easier to control than on a construction site. However, in some contexts, it will be also possible to look at in-situ prefabrication.
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For a well-established prefabrication industry, the production unit mainly calls for: • large scale use of machines; • large scale use of factory produced standardized building components; • co-ordination of management leading to efficient planning, programming, and control of projects; • continuous research in design and production systems.
The main essentials and advantages of prefabricated production will be: • avoiding waste; • standardization of repetitive work; • creating a uniquely custom product; • reduction of resource idleness; • reduction of average waiting time; • reduction of time between deliveries of finished products; 36
4 General features
• • • • •
reduction of variability; decrease in time for processing parts to traverse the system; decrease in costs; reducing inventor; increase in production rate.
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The reduction of waiting time and resource idleness can be overcome by ensuring that crews move in parallel for erection and grouting activities. Decrease in time for processing parts is assured by using the innovative methods developed herein and by assuring that crews are occupied through low resource idleness. Collectively, the above ensure high production rate, and cost reduction occurs due to the economies of mass construction.
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Production units require storage of raw material such as cement, steel, aggregates, lime, timber, etc., and hence extensive storage is done in stockpiles, yards, silos and warehouses. A machinery yard is also necessary to house the various cranes, concreting and material handling equipment. Adequate stacking area for the finished products such as wall panels, floor panels, etc., is required, as they have to be maintained in stock. Thus storage arrangement becomes one of the primary functions of the production. Production should be planned in such a way that the panels have aged some days before they are erected.
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Next in importance are industrial sheds, wherein the prefabrication company will manufacture the various building components. These industrial sheds may have gantries and other such facilities. The administrative wing, sales and service wings, design offices, training wing, the computer centre and production control offices are some of the other units which will be required. Railway siding, wide roads, water and power supply and security arrangements are also to be provided.
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The transportation wing is another important area for the transportation of the finished products to the building sites. A large vehicle parking lot in front of the industrial estate can also be envisaged.
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Modern methods of housing manufacture and erection, utilizing principles such as computer integrated manufacturing (CIM), flexible manufacturing systems (FMS), and lean manufacturing coupled with innovative ideas can hope to erect houses at fast speed and high quality. The balance between site and factory processes and the optimum level of prefabrication for housing designs can be analysed using software tools such as DSM (Dependency Structure Analysis), a system analysis tool for the investigation of interactions and interdependencies between elements in a complex system. Used on the architecture of the product, DSM determines possible integrative components, in other words, it seeks potential for employing larger factory pre-fabricated modules. Used on the assembly process, DSM creates optimised task sequences that can be fed into planning tools to determine the critical path for the assembly process. 4.4.1.2 Special light elements The production of light elements that can be moved and erected by few non-specialized people, without the help of machinery, may play an important role in developing countries. There exist some examples in the technical literature [29], [43]: • blocks for walls, • panels for walls, • ferro-cements wall panels, fib Bulletin 60: Prefabrication for affordable housing
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pre-cast lintels, ferro-cement over-hangs with lintels, cantilever stairs, joists and plank system, RCC precast door and window frames, ferro-cement shutters for door and windows, structural columns, ferro-cement water tanks.
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4.4.1.3 Floor units
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The production techniques vary according to the type of the floor unit to be produced. The techniques also vary depending on whether they are reinforced or pretensioned.
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Solid slabs are mostly produced in strips of 1.2 to 2.4 m width when they are cast on a long line, but are made as room size panels when they are individually cast using timber or steel. Cored slabs too are produced in a similar manner as solid slab. The cores can be formed, in both the case of reinforced and pretensioned floor units, by means of polyurethane foam which is bundled in polythene sheets or by using rigid cardboard pipes.
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Hollow-core units are produced without core forms. Ribbed floor units may be in the form of a single or a double tee, or a channel section. Single as well as double tee floors units are produced by using rigid steel formwork.
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4.4.1.4 Wall panels
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There are two types of wall panels used in prefabricated buildings: external cladding walls and internal partition walls. These walls maybe load bearing or partition walls. The production technique for producing external wall panels is different from that of partition walls.
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The external wall is normally of a sandwiched type incorporating the thermal insulating material in between its two vertical faces. The panels are cast horizontally using tilt-up moulds. At first, a thin layer of concrete forming the external, non-structural layer is cast. Over this is placed the insulating material such as polyurethane foam in the form of thin strips. Stainless steel ties connecting the external non-structural concrete layer to the internal structural layer are then inserted and the structural layer of concrete is then laid to the required thickness. The external concrete layer is adequately reinforced with wire mesh to take up the thermal stresses. Transportation
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The size of the panels and slabs are limited by the transportation used to deliver them. By limiting the lengths of the panels and slabs to 12 meters and the widths to 3 meters, standard trailers can be used, assuming there are good road conditions. Larger size panels and slabs may be used but special permitting and routing to the construction site may be needed, adding to the cost of construction. Smaller size panels will be recommended if the road conditions are not acceptable or/and it may not be taken into account the help of cranes.
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Coordinating delivery so that the panels arrive as they are erected adds to efficiency, but because of the flexibility of this system this is not critical. 4.4.3
Erection
In developing contexts, as it has been previously commented, the erection process will be a key point for the design of prefabricated elements. The maximum weight that can be manipulated without cranes could be around 100 – 160 kg [43].
Waterproofing and insulation
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In other circumstances, tower cranes and boom cranes are generally employed, depending upon the weight, dimensions and erection height for the handling the erection work. It is desirable to simplify site erection by reducing the number of parts that are required to fulfil a whole building again this will increase the weight and dimensions of the part, so a balance between the number of elements and the dimension should be made to have economy in construction.
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A shelter’s “thermal envelope” separates outside conditions from inside conditions. This envelope consists of the components of all six sides of the house: the four walls, roof, and foundation. The roof is the most important part of the home to insulate in all climates [42]. In hot weather, the sun beats directly on the roof. Even though heat moves in all directions by radiation, in cold weather, heat loss out of the roof or attic is a particular problem because hot air rises. Wall insulation is far more important in a cold climate than a hot one. Floor insulation is very important in areas where the ground freezes.
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At the same time, adequate ventilation shall be provided to maintain a healthy environment inside the house and to limit the risk of transmission of diseases. Ventilation should be maximised in hot-climates to reduce inside temperature, and minimised in cold-climates to retain heat within the shelter.
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Affordable housing should take local climatic conditions into account and be designed to minimise the use of energy. Insulation of roof and external walls is important to minimise energy demand and provide internal comfort for the occupants. Climate variations should have an important impact on planning and designing affordable houses. In next paragraphs, key points for hot dry climate, hot wet climate (tropical areas) and cold weather will be highlighted [43].
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In hot dry climates, shade from the sun is a basic concern. Several key design considerations should be taken into account: • small windows to prevent high solar gain during the day and heat loss at night; • position doors and windows away from prevailing winds, as they can be really hot; • thermal mass in buildings should be ensured by constructing thick walls and insulating roofs (by means of thermal mass or creating an air chamber). In wet climates, draining water will be the principal consideration, therefore: • construction in sites with slope to provide adequate surface drainage; • roof with sufficient pitch for water drainage. Drains connect to reservoir to harvest rainwater;
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roof overhang to protect walls and openings from water during rainy seasons and from sun in hot weather; compacted plinth, with raised floors to protect from flooding; provide sufficient openings (with small windows) for good ventilation and air convection.
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In cold climates, heaters are an essential part of the heating strategy for a shelter. Once the room has been heated, it is important to ensure that the heat does not escape. For that reason, some key design considerations should be considered: • use of materials with high thermal mass and added insulation; • strong roof to resist heavy snow loads; • small window will prevent heat loss, but ventilation is always necessary to prevent respiratory diseases; • divide large rooms into several small ones.
Services and installations
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In any case, all cultures have developed adequate and affordable housing solutions; if these are used as a starting-point, appropriate housing is easier and cheaper to provide. Participation of the beneficiaries, even with respect to waterproofing and insulation, will be important, as the final design should be in accordance to their beliefs and habits. For example, in some countries like Indonesia, where torrential rains are common, it is usual to raise floors to avoid water floods. Some inhabitants believe that a house at floor level facilitates the access of evil spirits, and if houses are not elevated, the habitants could even reject the new constructions.
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Ideally, prefabricated construction should comprise of a number of pre-engineered panelled or sectioned building elements and units designed and prefabricated to include all of the basic services such as wiring, plumbing, and ductwork.
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Affordable housing when initially constructed may not always include such services. If this is the case the selected construction system should allow for future installation and integration of services.
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Affordable housing should take local climatic conditions into account and be designed to minimise the use of energy. Insulation of all external walls and roof is important to minimise energy demand and provide internal comfort for the occupants.
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Where possible the orientation of the house should reflect the climatic conditions. Elevations facing the sun should be designed to reflect heat in summer and gain heat in winter. This can be accomplished by appropriate configuration of windows and roof overhangs. Through-ventilation should be provided to take advantage of cooling winds. Water supply is vital in all housing and this can be provided by installation of a water tank to collect rainwater from the roof. Water distribution can be collected direct from the tank or piped by gravity or pumped systems to within the house. Economy in plumbing services can be attained by the use of single stack system for plumbing. This is a one-pipe system in which the wastes from both the kitchen and toilet are carried out of the building in a single stack. The service stack itself serves the purpose of a ventilation pipe and eliminates the need for a separate ventilation stack. It embodies the 40
4 General features
merits of both the conventional two-pipe system and the modern one-pipe system. The use of this system gives a savings of about 30 % in the plumbing work. For economy in wiring, electricity can be distributed using a ring circuit system in place of conventional multi-circuit systems.
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The prefabricated panels should be designed so as to provide basic services of storage in the form of cupboards. Properly ventilated lofts at around 2.1 m level for a width of about 0.6 m will be useful in bedroom, dining hall and kitchen. Special attention must be given to the design of kitchens and bathrooms, as their relative position in the house responds, very often, to local beliefs or climate facts.
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In India, for example, some affordable houses were designed following western standards: small bathroom between two bedrooms, dinning room – sitting room with a south oriented veranda and a kitchen in the northwest corner. The beneficiaries, mostly in rural areas, did not use the bathroom, which was turned into a small storage room. The veranda, one of the most used spaces in the Indian culture, was always empty, and they did not use the kitchen, as they continued cooking outside the house. After some research, the project leaders learnt that most Indian houses have the bathroom attached to the house, but with an independent entrance.
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They also realized that the kitchen should be oriented to east, as many Hindu people perform a ritual in that direction before cooking. Finally, the best orientation for a veranda in a really hot area was north. As this small real life example highlights, the observation of the local constructions and the participation of the beneficiaries in design, would have helped to build a more accepted and useful construction.
Examples of housing systems
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A large number of industrialized housing systems that are used in several parts of the world have been studied. Among all these systems, some have been selected for presentation in this bulletin.
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The systems were selected depending on several basic ideas. They all are industrialized systems that are used in their respective environments for affordable housing. The systems were also selected because they have at least some important part that was built with precast concrete elements. There are systems with at least precast foundations, walls, columns, beams or decks.
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The part of the world where the systems are used is relevant when selecting systems. In this case, more systems from America were selected due to the fact that more systems have been developed and their information was available. Systems from other parts of the word were not so easily available.
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Another characteristic for choosing a system is that there is an all around catalogue of different types of systems. Some aspects that were studied are: country
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material − concrete − mixed
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structural system − wall − frame − mixed
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climate − dry − wet − flooding − medium
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seismicity − high − low − none
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construction − contractor − self built − both
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incremental construction − yes, no
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handling capacity −
