Design Solutions and Innovations in Temporary Structures

What is it about?

Temporary structures are defined as any structure that is used during the construction, rehabilitation and retrofit of buildings and bridges, or any other type of permanent structure. Temporary structures can be divided into various types, being falsework, formwork, scaffolds and shoring systems the most common. Bridge construction equipment (BCE) is a special type of temporary structures. A scaffold is that structure erected for the maintenance or alterations of an existing building or bridge. It is usually attached or tied to the permanent structure and for this reason is often called an access scaffold. Formwork is used to provide a containing shape into which concrete is poured to obtain the required shape and strength. Falsework structures are spatial framed systems that are used to support buildings and bridges. BCE are highly specialised systems and as the name implies are only used during the construction of bridges. This book will primarily deal with scaffolds, falsework and BCEs with only brief mentions of shoring and formwork.

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Why is it important?

Temporary structures have a major role in the execution time, cost, quality, durability, safety, efficiency, utility and aesthetics of any construction project. Past reviews conducted by several authors regarding the performance of temporary structures have shown that although these structures are in common use they frequently fail due to poor design or management. A correct choice, good planning, designing and operation of temporary structures are keys for the success of every construction project. This book will enable graduate students, educators, researchers, designers, producers, contractors, buildings and bridge owners, and managers of temporary structures to become aware of the breadth of the subject and the latest methods of analysis, design and management. The book commences with an overview of the research into temporary structures since 1970 as modern computer technology has enabled the design of the structures to change from traditional designs based on effective lengths and elastic analysis procedures to designs based on advanced nonlinear numerical analyses and methods. However, temporary structures design is often considered secondary to the design of permanent works and simplifications are often made without proper justification with regard to design actions and their effects on the structure, but also with respect to the interaction of various elements in the structural system. Often, to properly account for the latter factors more accurate analyses are necessary resulting in alternative design solutions. In addition, the requirements concerning the foundations upon which the structure is to be erected are commonly ignored. Past forensic studies have found recurrent deficiencies about foundations, lateral stability, design errors, design details and materials, to which can be added planning errors (e.g. insufficient inspection plans). When assisting in developing the design manual for the UK National Access and Scaffolding Confederation (NASC) there was considerable opposition from practitioners who found it hard to understand that their previous assumption in an access scaffold of considering that the effective length of the columns were equal to the lift height was in many cases incorrect and unconservative. This and other common misconceptions are an important reason for this book which details the most recent and relevant research into temporary structures emphasising modern knowledge of structural behaviour. In addition, many designers of permanent works do not always consider the problems arising in the erection or maintenance of their structures, leaving the problems to the contractors and sub-contractors employed to manage the construction. The prime objective of this book is to enable all practitioners in the temporary structures industry to gain an understanding of safe design and practice, and modern methods of analysis. The authors are based in Europe but since they are aware that engineers may work in many countries during their lives, have looked at practices worldwide in order to make this book relevant to as many people as possible . For example, when design codes are discussed and compared in detail in Chapter 6, not only European codes are presented but also codes developed in the USA, Canada, Australia and Hong Kong for a range of materials from steel and aluminium to bamboo. Sadly, many changes in design procedures are only made after major disasters occur. For example, the collapse of a falsework structure for a bridge over the river Loddon, near Reading, UK, in 1972 with the loss of three lives led to the UK Government forming an advisory committee on falsework which in 1975 published the Bragg report. This report led to the suggestion that supervisors called Temporary Works Coordinators be appointed to preside over temporary structures projects. It was first codified into the UK code BS 5975 in 1982 and only referred to falsework. However, later revisions included all temporary structures. The lessons that were documented in the Bragg report about poor site control and management still occur today as documented by many research studies. Further information on the causes of temporary structures failures can be found in Chapter 7, where collapses and failures are discussed and in Chapter 8, where detailed analyses of the management of temporary structures projects are presented with strong recommendations for the correct procedures to reduce and potentially eliminate the majority of failures. A second important objective of this book is to enable new researchers into temporary structures and advanced practitioners to gain knowledge of the latest methods developed for analysis. The book therefore presents the reader with knowledge of new methods of analysis which enable the force distributions, material’s stresses and strains to be calculated with improved accuracies. In particular the results of experiments conducted by André which led to improved models for column-to-column connections in Cuplok® systems (a typical proprietary scaffold and falsework system) and detailed models of extensible props by Ms Feng in 1994 are presented for the first time. Models developed by Beale for access scaffolds which have proved efficient and which were used to develop load tables for the UK NASC are also presented. Improved models for the moment-rotation curves of connections between vertical and horizontal scaffold/falsework members are discussed with recommendations on the best model presented. Modern risk analysis procedures applied to these structures and new ideas developed by André are also presented for the first time. A common assumption made in temporary structures design is that design factors used for permanent structures can be reduced because temporary structures are only in use for a limited period before being dismantled. However, Fyall, for example, when discussing the Milton Keynes scaffold collapse argued that wind distributions should be assumed to be taken as those that occur using a 100-year return cycle as opposed to the conventional limited 5-year or 25-year return periods specified in codes as the structures are more likely to have lower factors of structural stability and hence additional wind will cause failures with potential disproportionate consequences. Therefore, in this book a whole Chapter is devoted to risk analysis and its implication to the structural safety of temporary structures, including an application example. As will be discussed in the final two Chapters of the book, the management of a temporary structure project is crucial to the safe and successful outcome of any construction project. A significant percentage of the causes of failures and collapses can be shown to have occurred due to inadequate supervision and overview of the design and construction process of temporary structures. When analysing the causes of the Milton Keynes collapse, it was found that the scaffold structure was not built according to its design, changes were made to the scaffold without reference to the design drawings which were not on site, the scaffold was overloaded and crucially no inspections had been made by the supervisor in charge of the project for several months. These inspections would have made the likelihood of collapse smaller and the risk of collapse acceptable.