Prevent Pitfalls with Proper Propping!
The days of sending the apprentice down to the yard to fetch a brace of Skyhooks ought to be long gone by now, but I am often reminded that this might not always be the case.
In UK building practice it is normal for the design of the permanent works to be provided by the project architect and engineer, and the contractor is responsible for procuring and executing the temporary works design. This will include propping and shoring, plus any other measures needed to ensure the building remains safe and stable throughout all phases of the works. This gives the contractor the freedom to choose the methods that suit his preferred method of operations, but it does not excuse him from giving proper consideration to the design of the temporary works.
Naturally, this type of work is covered by the Health and Safety at Work Act (HSAW) and the Construction, Design and Management Regulations (CDM) 2015.
Design of temporary works is normally in accordance with BS5975-2008 but it must be noted that the way in which the design loads are handled is quite different in this standard than with the limit-state design method (Eurocode or British Standard) used in the permanent works.
If the designer of the permanent works does not state the unfactored dead and imposed loads on the part of the structure that is to be supported, then you must ask him to provide the information to ensure that the propping system is adequate for the task.
Design forces on the structure are categorised into permanent and variable actions or dead and imposed loads, depending on whether Eurocodes or British Standards were used for design, plus accidental forces. These may not be the same in the temporary case as in the permanent case; for example a part-completed structure may have a reduced permanent load, and if this is used to counter a destabilising load this may create a stability problem. Snow on a roof might not be an issue during the period of the works, but wind loads should not be ignored, even if the probability of a Force 9 storm is less than over the lifetime of the building.
Imposed loads on floors need to be considered – a floor that is propped ought not to be used for its normal domestic or office duty, but it is likely to experience construction activity including stored materials and dynamic loads due to items being moved around.
Scaffolding is an important part of any construction project and the design of most simple scaffolds is covered by the NASC Technical Guide TG20, which covers scaffolds up to two working lifts that are tied to the permanent structure for stability. Just about any other type of scaffold, particularly those which are used to support or restrain parts of the works, require a bespoke design by a competent person.
Your scaffolding contractor will either have their own designer or will appoint a specialist engineer; but it is your responsibility to ensure that the scaffold designer is informed of all the circumstances of the project, in particular the ground conditions and any hazards such as buried services, abnormal loads and so on.
Besides scaffolding, temporary works includes falsework such as propping to support incomplete structures, shoring and façade retention to provide stability, formwork to support concrete casting, and ground support to allow safe excavation. Cranes and their foundations are also part of the temporary works, and possibly also temporary roads and bridges for construction traffic. Many of these will require engineering design by a competent person, and the law demands that the client and contractor ensure that appropriate measures are put in place to design and specify the means by which the works are executed.
Ordinary propping tasks such as supporting a wall to allow insertion of a lintel do not ordinarily require specialist design, unless the opening is unusually wide (over 4 metres) or if heavily loaded beams bear on the supported wall.
Needling and propping is the preferred method for this, using telescopic “Acrow” BS4074 props and needles of timber or steel punched through the wall at intervals of 900 to 1200mm. Typically a 100×100 steel box section or 125×125 C16 softwood can be used as needles to support a house wall with props 1 metre apart. The needles will flex more than allowable for permanent works, but the adjustment of the props will compensate for this. If in doubt, consult an engineer for guidance.
Strongboy heads are undoubtedly convenient and avoid the inevitable situation where punching needle beams into the wall means having to remove a bath and destroying a newly-tiled wall! However, these have a safe working load of only 340kg which is much less than that of the Acrow prop.
This means that they should only be used for situations where the supported wall will arch over the opening so that the Strongboy only needs to support the triangle of masonry below the arch. The opening will need to be less than 2 metres wide with sufficient width ( more than 600mm) of wall either side to buttress the arch, and no openings to interrupt the arch above.
Both inner and outer leaves of a cavity wall need to be supported directly; it is not safe to use the floor joists to support the inner leaf because there is usually a gap between the top of each joist and the next course of masonry above, which leaves the skirting boards to transfer the full weight of the wall to the props via the floor – think about it!
This last phrase sums up temporary works design quite neatly – the whole ethos of the CDM Regulations is to ensure that all aspects of the construction process have been considered by a competent person and this is the most effective way of ensuring safety throughout the project.
And just to show the exception that proves the rule, this is one site I visited where the contractor “got away with it” – fortunately the buildings on either side provided lateral restraint and the roof and floor were already old and distorted and disguised the distortion that subsequently occurred.
If you need guidance or have any doubts about your ability to support a building safely, do contact a structural engineer with experience in temporary works for your size of project.
Charles Tallack is a Chartered Member of the Institution of Structural Engineers and the Institution of Mechanical Engineers and is the principal of Structural Engineers Cambridge Limited, serving small to medium-sized construction projects in the South-East of England.