Last edited 28 Jun 2021

Temporary works for construction


[edit] Introduction

Construction works are works a contractor undertakes for an employer, consisting of the construction itself, goods, materials and services to be supplied, and the liabilities, obligations and risks to be taken by that contractor.

Temporary works are described in BS 5975:2019, Code of practice for temporary works procedures and the permissible stress design of falsework, as 'providing an "engineered solution" that is used to support or protect either an existing structure or the permanent works during construction, or to support an item of plant or equipment, or the vertical sides or side-slopes of an excavation during construction operations on site or to provide access. It is used to control stability, strength, deflection, fatigue, geotechnical effects and hydraulic effects within defined limits.' It might or might not remain in place at the completion of the works.

Temporary works designers have the same designer duties as permanent works designers and must be sufficiently competent in their specific field.

The designers should avoid foreseeable risks as reasonably practicable, including risks in relation to the removal of any temporary works once construction is complete. They should also coordinate with permanent works designers and principal contractors to discuss the effects of any temporary work loading and possible disturbances during the construction of the permanent structure.

[edit] Types of temporary works

Temporary works might include:

[edit] Scaffolding

Scaffolding provides a temporary safe working platform for:

There are two main types of scaffolding:

  • Freestanding scaffolds, such as independent towers,
  • Independent tied scaffolds, such as independent towers tied to an adjacent structure.

The most common piece of structure used in scaffolding is the scaffold tube. The tube generally comes in two thicknesses, 3.2mm or 4mm. The tubes are galvanised due to their exposure to the elements and axial capacity loads are given either ‘as new’ or ‘used.’ Capacities of tubes used in tension are usually limited by the safe slip load capacity of the coupler, which is far lower than the actual tensile resistance of the tube.

Scaffolding is designed support its own self-weight, ie. the weight of the boards, tubes, guardrails, toeboards etc. and imposed loads such as wind. The imposed load applied to the scaffolding depends on its use. The wind load applied to scaffolding will change depending on whether sheeting or debris nets are used.

Four classes of loading are available:

When scaffolding is tied to a building it uses the permanent structure of the building to provide stability. Scaffold sheeting protects employees and the structure from rain and wind and is attached with scaffold ties or alternatively scaffold netting is used to prevent debris from hurting those on lower levels. The selection of tie positions should be tested and checked before use and the suitability of the permanent structures composition to carry the ties should be analysed. Foam scaffold tubes are a popular accessory for protecting pedestrians at lower levels.

Scaffolding is also braced laterally using façade and ledger bracing.

Workmanship and inspection is vital for the erection and dismantling of scaffolding and must be undertaken by competent personnel under supervision.

[edit] Façade retention

Façade retention involves supporting existing façades or party walls for renovation and is often used for works to listed buildings. By retaining the façade, the overall look of a building is preserved while new internal floor structures and layouts can be constructed to meet the needs of modern occupants.

A shoring retention scheme is generally required to support the front façade while construction of the new internal layout takes place. Once construction of the internal structure is complete, the existing façade can be connected to it.

The temporary works involved in façade retention can be significant structures in their own right and play a major role in assessing the financial viability of a project. From the outset, the design team should address the importance of the retention as a critical element of the project and careful feasibility studies should be carried out to to assess its viability and likely costs.

A thorough understanding of the existing building is vital including its age, the overall structural form, the structure of neighbouring properties, details of connections between the façade and the existing internal structure and existing foundation sizes. Site constraints (such as available space) may also affect the location and design of the temporary works.

Types of retention include:

The support system must be stiff enough to prevent excessive movement, which could cause cracking to the façade. By pre-loading the façade with a series of flat jacks the likelihood of movement can be reduced and deflection limited. The overall stability of the system must be maintained in all directions taking into consideration wind loads and impact loads. The system must also resist the overturning moment as well as moments generated by eccentric dead load. Kentledges can be incorporated into the design to counteract these moments.

[edit] Crane supports

Tower cranes are usually supplied on a hire basis, with the client being responsible for the design and construction of the base upon which the crane will be erected. Details of loading are provided by the crane supplier and the base is most commonly designed as a temporary structure, although sometimes a crane base will be incorporated into the permanent structure to save on cost and time.

Loads are given in two forms, ‘in serviceloads, where the crane is functioning and wind speeds are restricted (ie cranes will not operate at high wind speeds), and ‘out of serviceloads, where the crane is not being used but maximum wind speeds may occur.

The location for a crane should be carefully selected to provide a maximum working radius, and when two cranes are being used on the same site mast heights and jib lengths must be considered so that they do not clash.

Cranes are typically structured around two rails at their base between 4.5m-10m apart with wheels in each corner. Cranes are not normally tied down, so sufficient kentledge must be provided to ensure vertical loading from the crane passes through the rails and into the foundation. The foundation is designed so that the unfactored loading from the crane and the unfactored loading from the foundation itself create a bearing pressure which is less than the allowable bearing pressure of the soil.

Various foundation types can be selected depending on the ground conditions:

[edit] Falsework

Falsework involves a temporary structure used to support other permanent structures until they can support themselves. There are three main types of systems used for falsework. These include:

The design philosophy behind falsework differs from that of permanent works. They are highly stressed, usually to 90% of their capacity over short periods of time and involve reusable components. Props are rarely tied down and rely on their self-weight and supported load for lateral stability.

The design of the falsework must make allowances for erection tolerances and take into account that the components are re-used many times.

Falsework capacities are provided by the manufactures and permanent, imposed and environmental loads must all be taken account of in their design.

As with general construction, stability is often identified as the main cause of collapse. To ensure the lateral stability of general falsework structures, including beam grillages, BS 5975: 2019 (BSI, 2019, Clause [IP address hidden]) recommends that they should be designed to be able to resist, at each phase of construction, the applied vertical loads (W) and a horizontal disturbing force FH which is the greater of:

a) 2.5% of the applied vertical loads (i.e. 2.5%W) considered as acting at the points of contact between the vertical loads and the supporting falsework; or

b) the forces that can result from erection tolerances (normally taken as 1% of the applied vertical load (i.e. 1%W), refer to 19.2.4) plus the sum of other imposed loads …’.

Workmanship and inspections play key roles in the design and installation of falsework, as attention to detail is particularly important.

[edit] Formwork

Formwork is the term used for a temporary mould into which concrete is poured and formed. Traditional formwork is fabricated using timber, but it can also be constructed from steel, glass fibre reinforced plastics and other materials.

When selecting formwork, the type of concrete and temperature of the pour are important considerations as they both effect the pressure exerted.

Once the concrete has gained sufficient strength the formwork can be struck (removed). A minimum value of 5 N/mm2 is recommended in all cases when striking vertical formwork as so not to damage the permanent concrete in the process.

High quality workmanship and inspection are necessary to ensure a high standard and appearance of the resulting concrete structure.

[edit] Trench support

A trench is defined as an excavation when its length greatly exceeds its depth. Shallow trenches are usually considered to be less than 6 m deep and deep trenches greater than 6m. Depending on the dimensions of a trench, excavation can either be carried out by hand or using a mechanical digger. Trenches are commonly required to allow services, pipelines or foundations to be laid.

Water ingress into the trench is often a major issue and ground water table locations and soil strata should be investigated before any extensive excavation takes place.

Over short periods of time for relatively shallow depths most soil types will stand almost vertically without any problems. However, trenches other than those which are relatively shallow may require a trench support scheme.

[edit] Timber supports

Historically, trenching involved using timber to support horizontal and vertical soil loads and this technique is still used today. Timber trenching is generally used for low risk, narrow trenches, shafts or headings. The timber solutions require good workmanship and are reasonably labour-intensive, however they are versatile and the equipment required is easy to handle and transport.

[edit] Trench boxes

Trench boxes are suitable for low-risk situations in stable, dry ground and can be placed in pre-excavated trenches or installed using the ‘dig and push’ technique. The system requires at least two struts at each panel for stability which must be considered when access is required for construction work or piping.

[edit] Trench sheets

Trench sheets are the most adaptable of the systems available, and are most commonly used to retain poorer soil. They can support deeper trenches with larger surcharges and provide a continuous support. They require multiple levels of strut support and the slenderness of the sheets can often limit the depth of the trench as they are installed by light machinery and could buckle under large vertical loads.

[edit] Related articles on Designing Buildings Wiki

[edit] External references


BS5975 is now dated 2019, BS5975:2019.

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