Category CONCRETE FORMWORK

FORMWORK ECONOMY AND SIGNIFICANCE

Formwork is the largest cost component for a typical multistory reinforced concrete building. Formwork cost accounts for 40 to 60 percent of the cost of the concrete frame and for approximately 10 percent of the total building cost. Figure 1.1a, b presents a breakdown of different cost categories for conventional concrete slab and wall formwork. A large proportion of the cost of conven­tional formwork is related to formwork labor costs. Significant cost saving could be achieved by reducing labor costs.

Formwork costs are not the only significant component of
the formwork life cycle. Other important aspects of the formwork operation include speed, safety, and quality.

Formwork System

A formwork system is defined as ‘‘the total system of support for freshly placed concrete including the mold or sheathing which contacts the concrete as well as supporting members, hardware, and necessary bracing.’’ Formwork system development has paral­leled the growth of concrete construction throughout the twenti­eth century. As concrete has come of age and been assigned in­creasingly significant structural tasks, formwork builders have had to keep pace. Form designers and builders are becoming increas­ingly aware of the need to keep abreast of technological advance­ments in other materials fields in order to develop creative innova­tions that are required to maintain quality and economy in the face of new formwork challenges.

Formwork was once built in place, used once, and subse­quently wrecked. The trend today, however, is toward increasing prefabrication, assembly in large units, erection by mechanical means, and continuing reuse of forms. These developments are in keeping with the increasing mechanization of production in con­struction sites and other fields.

CONCRETE FORMWORK

Подпись: Copyright © Marcel Dekker, Inc. All rights reserved.The construction of a concrete building requires formwork to sup­port the slabs (horizontal formwork) as well as columns and walls (vertical formwork). The terms concrete formwork and concrete form carry the same meaning and are used interchangeably in this book. Formwork is defined as a temporary structure whose pur­pose is to provide support and containment for fresh concrete until it can support itself. It molds the concrete to the desired shape and size, and controls its position and alignment. Concrete forms are engineered structures that are required to support loads such as fresh concrete, construction materials, equipment, workers, var-

ious impacts, and sometimes wind. The forms must support all the applied loads without collapse or excessive deflection.

CONCRETE CONSTRUCTION

A quality reinforced concrete structure offers many advantages over structures made with other building materials. Concrete is a durable material that reduces building maintenance costs and provides a longer service life. A concrete structure will reduce en­ergy usage because of its mass and high resistance to thermal interchange. The use of concrete will lower insurance costs by virtue of its high resistance to fire. Buildings made of concrete are also more secure against theft and vandalism. Concrete floors and walls reduce the transfer of noise, yielding a quieter environment and happier occupants. Reinforced concrete possesses consider­able strength for resisting seismic and wind loads. These factors and others make the selection of reinforced concrete an economi­cal alternative.

Preface

Formwork development has paralleled the growth of concrete construc­tion throughout the 20th century. In the last several decades formwork technology has become increasingly important in reducing overall costs, since the structural frame constitutes a large portion of the cost of a form­work system.

This book has three objectives. The first is to provide technical descriptions and evaluations of ten formwork systems that are currently used in concrete construction. The second is to serve as a tool to assist contractors in selecting the optimal formwork system. The third is to present the design criteria for conventional formwork for slabs and walls using the stress and the stress modification factors provided by the Na­tional Design Specifications (NDS) and the American Plywood Associa­tion (APA).

Подпись:Following a comprehensive introductory chapter, five types of form­work systems for concrete slabs are presented in chapters 2-5. These are conventional wood forms, conventional metal forms, flying forms, the column-mounted shoring system, and tunnel forms. The last four chap-

ters describe five types of formwork systems for concrete columns and walls: conventional wood forms, ganged forms, jump forms, slip forms, and self-raising forms. Particular consideration is given to topics such as system components, typical work cycles, productivity, and the advan­tages and disadvantages associated with the use of various systems.

The selection of a formwork system is a critical decision with very serious implications. Due consideration must be given to such factors as the system’s productivity, safety, durability, and many other variables that may be specific to the site or job at hand. Chapters 5 and 9 provide a comparative analysis of forming systems for horizontal and vertical con­crete work to facilitate the selection of the optimal forming system.

Existing formwork design literature is inconsistent with the design criteria for wood provided by the NDS and the APA. Chapters 3 and 7 provide a systematic approach for formwork design using the criteria of the American Concrete Institute committee 347-94, the NDS, and the APA. For international readers, metric conversion is provided in the Ap­pendix.

This book is directed mainly toward construction management, construction engineering and management students, and concrete con­tractors. It may also serve as a useful text for a graduate course on con­crete formwork, and should be useful for practicing engineers, architects, and researchers.

Подпись: Copyright © Marcel Dekker, Inc. All rights reserved.

Awad S. Hanna