Avoid foreseeable risks as reasonably practicable, including specific details on the removal of any temporary works once construction is complete.
Coordinate with the permanent works engineer and principal contractors to discuss the effects of any temporary work loading and possible disturbances during the construction of the permanent structure.
Temporary works are most commonly the contractor's responsibility.
The cost of any temporary works is generally included in the build-up of the tender. Temporary works will often be taken from site to site and re-used and it is important to consider the components robustness when approaching the design.
Similarly with seismic and accidental load design, a local failure within the temporary works should not be disproportionate from its cause and initiate a global collapse of the structure.
Scaffolding includes providing a temporary safe working platform for:
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It is formed from individual tubes and joints or, proprietary components
There are two main types of scaffolding:
Freestanding scaffolds - Independent towers
Independent tied scaffolds - Independent towers tied to an adjacent structure
The most common piece of equipment in scaffolding is the scaffold tube. The tube generally comes in two thicknesses, 3.2mm and 4mm outer diameters. The tubes are galvanised due to their applications externally 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, the tube fitting, which is far lower than the actual tensile resistance of the tube.
Scaffolding is designed for its self-weight, ie. the weight of the boards, tubes, guardrails, toeboards etc. and imposed loads including wind.
The imposed load applied to the scaffolding depends on its use.
Four classes of loading are available:
Service Class 1 - 0.75 kN/m2 - Inspection and very light duty access
Service Class 2 - 1.50 kN/m2 - Light duty such as painting and cleaning
Service Class 3 - 2.00 kN/m2 - General building work, brickwork, etc.
Service Class 4 - 3.00 kN/m2 - Heavy duty such as masonry and heavy cladding
The wind load applied to the scaffolding will change depending on whether sheeting or debris nets are required. The magnitude of the wind load will alter the required capacity of the ties and may affect their frequency.
By tying scaffolding to a building it uses the permanent structure for stability. 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 beforehand.
Scaffolding is also braced laterally to stiffen it using façade and ledger bracing. Workmanship and inspection is vital for the erection and dismantling of scaffolding and must be undertaken by competent personal under supervision.
Façade retention involves supporting existing façades or party walls for renovation and has become more popular due to the repair and maintenance work required to grade 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 modern day structural requirements.
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 are often structures in their own right and play a major role in the financial viability of a project. From the outset, the design team should address the importance of the retention as a critical element and careful feasibility studies should be carried out to meet the general requirements.
A thorough understanding of the existing building is vital including its age, the overall structural form, neighbouring property details, details of connections between the façade and the existing internal structure and existing foundation sizes. The nature of the site constraints that may affect the location and design of the temporary works are also important.
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Types of retention include:
Scaffolding, suitable for low level facades between 3 and 4 storeys, with sufficient space at their base for installation.
Proprietary retention, involving props, ties and bracing suitable for higher facades as the general quantity of components are reduced.
Fabricated steelwork, when cost of hiring proprietary equipment over long periods of time out weigh the cost of fabricating the steel, often combinations of fabricated and proprietary retention systems are utilised.
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 possibility of potential movement can be reduced and deflection is limited. The overall stability of the system must be maintained in all directions taking into consideration wind loads and impact loads.
The scheme must also resist the overturning moment as well as moments generated by eccentric dead load, kentledges are incorporated in the design to counteract these.
Tower cranes are usually supplied on hire and the customer is 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 temporary. Sometimes the crane base will be incorporated into the permanent structure to save on cost and time.
Loads are given in two forms, 'in service' loads, the crane is functioning and wind speeds are restricted and 'out of service' loads, the crane is not being used and maximum wind speed may occur.
The location of the crane is carefully selected in order to provide a maximum working radius and when two cranes are being used on the same site mast heights and job lengths must be considered so that neither crane 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 downwards.
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. Where possible a structural fill can be compacted and used to support a crane with the load spreading through layers of track support at 45° in to the soil strata below.
When loads from the crane increase, reinforced concrete foundations may be required. This can involve a series of reinforced concrete beams used to support line loads as a result of the crane loading.
When ground conditions are particularly bad piled foundations may be required. Careful design is required to ensure the reinforcement at the top of the pile top does not cause problems for positioning the mast base section of the crane.
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:
Type 1 - Aluminium support legs with aluminium frames assembled into falsework systems eg. Ischebeck Titan, SGB GASS or PERI MultiProp.
Type 2 - Individual aluminium or steel props, including either timber header beams or proprietary panels - eg. PERI Multiflex or Doka Eurex Systems
Type 3 - Heavier steel falsework - eg. RMD Kwikform System Shoring or A-Plant Acrow Props
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 multiple times. Falsework capacities are provided by the manufactures and permanent, imposed and environmental loads are all accounted for in their design.
As with general construction, stability is often identified as the main cause of collapse. BS 5975 (BSI, 2011; clause 18.104.22.168) recommends that all falsework is designed for 2.5% of the vertical load acting horizontally as a tolerance for workmanship during erection.
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Workmanship and inspections play key roles in the design and installation of falsework requiring a need for attention to detail.
Formwork is the term used for the temporary mould into which concrete is poured and formed. Traditional formwork is fabricated using timber but can also be constructed from steel, glass fibre reinforced plastics and other materials.
Timber formwork is normally constructed on site out of timber and plywood. It is easy to produce although time consuming for larger structures. It is used when the labour costs are lower than the cost of producing re-usable formwork out of alternative materials such as steel or plastic.
Re-usable plastic formwork is utilised for quick pours of concrete. The formwork is assembled either from interlocking panels or a modular system and is used for relatively simple concrete structures. It is not as versatile as timber formwork due to the prefabrication requirements and best suited for lost cost, mass housing schemes.
Stay-in-place structural formwork is assembled on site usually from prefabricated fibre-reinforced plastic. It is used for concrete columns and piers and stays in place acting as permanent axial and shear reinforcement for the member. It also acts as resistance against environmental damage to both the concrete and rebar.
Proprietary systems are used to support vertical formwork while the concrete cures, consisting of a series of tubes and ties. When selecting the formwork the type of concrete and temperature of the pour are important factors to consider as they both impact the pressure exerted.
Once the concrete has gained sufficient strength the formwork can be striked. A minimum value of 5N/mm2 is recommended in all cases when striking vertical formwork so not to damage the permanent concrete in the process.
Workmanship and checking provide not only a high standard of concrete, but also appearance. The degree of success in achieving the required result often solely depends on the skill and expertise of both the formwork and concrete pour.
A trench is defined as an excavation when its length greatly exceeds its depth. Shallow trenches are usually considered to be less than 6m deep and deep trenches greater than 6m. Depending on the dimensions of the trench, excavation can be either carried out by hand or using a mechanical digger 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. When trenches reach depths greater than relatively shallow, a trench support scheme may be required.
The original type of trenching involved using timber to support horizontal and or vertical soil loads and 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 is easy to handle and transport.
Suitable for low risk situations in stable dry ground and can be placed in a pre-excavated trench 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.
Most adaptable of the systems and are most commonly used due to their ability of retaining poorer soil conditions. 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 would buckle under large vertical loads.
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