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CROSS Safety Report

Permanent formwork to slabs

Report ID: 215 Published: 1 January 2011 Region: CROSS-UK

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Overview

Concerns are raised over deflection issues that are not being considered when using permanent formwork.

Key Learning Outcomes

For civil and structural design engineers:

  • Be aware that the deflection of permanent formwork under the self weight of the concrete needs to be properly considered

  • If there are any uncertainties seek advice from the permanent formwork manufacturer

  • It should be clearly stated on the drawings if temporary propping is required and whether slabs are to be laid to level or to a constant thickness

  • Helpful guidance can be found in the Steel Construction Institute (SCI) publication P300

For construction professionals:

  • If you are unsure of the deflection allowances or propping requirements of permanent formwork, seek clarification from the design engineer

For permanent formwork suppliers:

  • The deflection of permanent formwork should be taken into account when specifying the sheeting

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The advantage of permanent formwork to slabs is that it can remove the need for temporary falsework and formwork. However, designers of structural frames usually leave the specification of the sheeting to the contractor and do not specify what propping is required when the concrete is placed.

Self weight deflection

In turn, the people responsible for procurement within the contractor go out for quotes to specialist suppliers of the sheeting. The suppliers typically do not know the deflections of the steel frame under dead load so assume zero deflection and specify sheets over one, two or three spans. There is no feedback to the designer of the proposed sheeting and the designer does not usually check that the sheeting is adequate when deflections of the structure are taken into account.

Consequences of excessive deflection

Typically, the structural steel will deflect by 40-70 mm under the dead load of the concrete, resulting in an increase to the depth of the concrete slab. The consequences of this include the following:

  • Permanent formwork could be excessively loaded (e.g. a slab with a nominal thickness of 130 mm would become 195mm thick and increase loading by 50%)

  • Increased stress induced in steel beams before composite action is achieved (possibly close to permissible stress even before live load is applied)

  • Deflection of steel beams becomes a permanent feature of the structure, limiting space available for services

The ideal procurement method for permanent formwork is well defined (e.g. Composite slabs and beams using steel decking: Best Practice for design and construction. SCI Publication P300) but this is not followed in practice and the method does not work well within the UK construction industry.

There are several potential solutions:

  • Follow the sequence detailed in SCI publication P300

  • The consulting engineer should specify sheeting based on his knowledge of the structure

  • Suppliers of permanent formwork should take deflection into account when specifying the sheeting

With pressure to keep fees and prices low there is a real risk of this issue not being adequately addressed by either consultants or contractors.

Expert Panel Comments

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This report echoes a recurring theme and illustrates one of the most significant problems within the industry, i.e. the apportionment of tasks according to commercial expediency, rather than according to appropriate risk management.

Issues cause by deflection

There have been many reports of excessive concrete depth due to deflection of support structures. Such deflection causes at least two problems: firstly the cost of the extra concrete and secondly a safety issue in that the extra weight of concrete carried has to be subtracted from the system’s capacity to carry live load.

The issue becomes particularly acute for long span (composite) floors and beams since the absolute value of system sag can then be large in comparison with the nominal slab depth thus increasing dead load significantly.

There is often uncertainty over whether slabs should be put into nominal depth or to level: if the latter there is this risk of ‘ponding’ and excessive concrete thickness. Overall, one designer should be in charge of the whole process and there should be a site presence so that there is no divided responsibility for assigning overall structural capacity.

There is often uncertainty over whether slabs should be put into nominal depth or to level: if the latter there is this risk of ‘ponding’ and excessive concrete thickness

Limiting risks

Designers may be able to limit risks by considering the following issues and communicating their intent on drawings:

  • Being satisfied that a reasonable permanent formwork solution exists
  • Saying whether steel beam design relies on composite action with the slab
  • Saying whether propping is required
  • Providing the deflection criteria
  • Indicating whether slabs are to be laid to level or to a constant thickness
  • Showing anticipated deflections so that propping the beams, or the beams and decking, or pre-cambering the beams, can be considered

It is however perfectly practical to pour to a given thickness using screed rails. This may not fit with modern laser levelling techniques but does avoid the problems described. It is then a specification and co-ordination issue as the following trades will need to deal with the deflection which becomes a tolerance issue.

This solution avoids the additional weight and additional material use. At any stage in the design process the designer should be satisfied that the design at that stage of development can be safely (and economically) built, and that information needed by those who follow is provided.

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