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The term 'Temporary works' is a very generic meaning, which can be related to a wide range of processes and mechanisms on construction sites; such as the use of tower cranes to sheet piling to site cabins. This report will focus mainly on the formwork and falsework involved in the construction of reinforced concrete flat slabs. This comes from site experience, where it has become apparent that few people are fully aware of the technical requirements of the temporary works being constructed on site. By a better understanding of the way in which temporary works interact with the structure in its construction phase will greatly increase the quality, efficiency and overall safety of a concrete structure.
A large amount of scientific experimentation and investigation was carried out by a temporary works consultant, Eur Ing P. F. Pallett at the European Concrete Building Project (ECBP) where a best practice guide; "Guide to Flat Slab Formwork and Falsework" was produced. Much useful data and advice on the procurement, design, implementation and management of formwork can be applied to site-specific situations from this guide looking at every stage involved in Flat Slab production. This report is based largely on the knowledge obtained from the experimental work from the above guide, however looks specifically at the issues related to the use of Table Systems. This type of system was used effectively on a Galliford Try site for the Corby Cube in Northamptonshire. Therefore, some of the issues raised are related to specific site issues experienced on this project.
The use of flat slabs can be a very cost effective solution as opposed to structural framing systems, hence the continued use in buildings of today. There are many designed types of flat slab; however, they can only be categorized under two basic groups; one way or two way spanning slabs. This section also looks at some of the characteristics of a variety of slab designs.
The selection of falsework can be difficult, as there are a wide variety of methods and suppliers. Throughout the years falsework methods have changed from the use of timber framework to modern day aluminum table systems. The development of aluminium systems has lead to the increased efficiency in concrete structures, largely from the availability of repetition. As there is an abundance of different falsework options it is necessary to understand each system; consider the advantages, disadvantages and be aware of the suitability and processes involved for the construction of the structure in hand. These considerations would be important at tender and design stage to have control of the costs involved and the feasibility of construction.
Weather Conditions: In general it is difficult to predict weather conditions; however, certain areas have higher likelihood of extreme weather, such as highlands and northern regions. This will generally force the use of stronger more rigid systems, such as steel skeletal systems, which will be more resistant to tougher weather conditions.
Aluminium systems are most commonly used by contractors, due to their wide availability and the general advancement in the systems of today. They comprise of large props of 100-150mm diameter with long lengths, connected together by frames and supported by cross-bracing. A stiff head is formed by long length primary beams, on which, secondary beams are placed and then the required falsework paneling (usually plywood decking). Due to the rigid nature of the head, the tables can be handled at 12m lengths, although it is possible to handle greater lengths.
Tables are most effective when used in repetitive usage, as once they are made up they can be used again and again. Flying out table forms can become an issues if it passes outside the site, and building boundaries - therefore requires essential planning considerations. Tables are likely to be constructed on site, out of position and then craned into position. This will increase efficiency; however, is largely dependent on the space available onsite.
Flying form systems are similar to table systems, however, are much larger and comprise of long deep steel or aluminium trusses, as opposed to vertical props. This enables the falsework to be much larger due to being stiffer and stronger and allows for the units to fly out of the building for use in repetition. Above the main truss sections are secondary beams with provide additional strength and accommodates either a plywood or waffle formwork.
Flying form systems become economical after 10 uses, therefore are suitable if used for flat slabs, however this most include adequate space for the units to fly out of the building and back again. Clearances from 12 to 25m from the building line are suggested for this method of falsework due to the difficulty in moving large constructed falsework. It would be important to employ a trained and experienced foreman, whom may provide input into the planning of the construction procedure as there are a number of variables to be considered on site. Careful planning and designing of falsework can produce cycle times of as little as 4days.
In theory, a 25% margin is recommended, which is the percentage of characteristic strength that is used to make a estimate of striking time. The striking time is based on 'When will the insitu concrete have the required strength, to hold its self weight and take additional loading?' A mean strength is used to assess the strength of concrete which incorporates the margin specified for the structure:
Theoretical striking times are all based on cube strength which may not have been subjected to the same conditions as insitu concrete. Aspects such as compaction and moisture condition will affect the cured strength. This is what makes striking of concrete structures somewhat unpredictable and challenging for site operatives to ensure the best results.