What is temporary works:
Temporary works is defined as parts of the works that allow or enable construction of, protect, support or provide access to, the permanent works eg. falsework, formwork, scaffolding, shoring, excavation support, façade retention, crane support etc.
In most of the cases the temporary works are removed after use hence terminology “temporary” is in place. On some other occasions temporary works can be incorporated into the permanent works – eg. haul road foundations and crane or piling platforms may be used for hardstanding or road foundations.
Despite its seemingly ancillary nature, temporary works play crucial role in the whole construction process. It creates link between existing structure and methodology leading to safe, on time, cost-effective and environmentally friendly project completion.
Looking at statistics - over 70% of site accidents are related to temporary works. As far as costs are concerned on average 35% of contract costs are temporary works which is leading to the conclusion that special consideration should be provided to temporary works design and construction of it on site.
Time of removal of formwork:
Structural Member OPC (Ordinary Portland Cement) Rapid Hardening Cement
Beam sides, walls & columns 2-3 days 2 days
Slab (Vertical supports remain intact) 4 days 3 days
Slab (Complete formwork removal) 10 days 5 days
Beams (Removal of sheeting, props remain intact) 8 days 5 days
Beams & Arches (complete formwork removal up to 6m span) 14days 5-8 days
Beams & Arches (complete formwork removal more than 6m span) 21days 8-10 days
Times are presented for guidance only and are subject to check & approval of engineer on site. The time at which striking of formwork can be carried out should be carefully controlled. Formwork should not be removed until the concrete has gained sufficient strength, is self supporting and can resist any damage caused by striking.
Temporary works design:
The design concept behind temporary works differs from that of permanent works. The main differences include:
- Temporary works is usually highly stressed up 90% of its capacity over short period of time.
- The designed components are often planned to be reused throughout the cycle of concrete pours or to follow required work sequence.
- The temporary works design can be very sensitive to changes and need to take into account assembly tolerances when erected on site.
- The formwork often relies on its own self-weight for stability. It is rarely fixed down to firm foundations so transfer of side loads / overall stability in particular need to be considered at every stage during the construction.
To ensure these differences are clearly identified and right balance, ie. the economy of solution and safety of all people involved, is achieved the temporary works design should be prepared by individuals who have a knowledge not only of engineering design principles but also of the many temporary works systems available on the market.
Categories of temporary works design checks:
Category Scope Independence
0 Restricted to standard solutions Site issue - by another member of site team or design team
1 Simple design (falsework top restrtaint is not assumed) By another member of design team
2 More complex design incl. structural steelwork, By individual not involved in design and not consulted by
foundations, top restrained falsework etc. the temporary works designer
3 Complex or innovative design By another organisation
Temporary works definitions (in accordance with BS 5975 Code of Practice for temporary works procedures and the permissible stress design of Falsework):
Formwork vs Falsework:
Formwork - A structure, usually temporary, but in some cases wholly or partly permanent, used to contain poured concrete to mould it to required dimensions and support it until it is able to support itself
Falsework - Any temporary structure used to support permanent structure while it is not self-supporting
Backpropping vs repropping:
Backpropping – Propping installed at levels below the slab that supports the falsework in order to distribute the load on the uppermost slab to suitable supports, such as lower slabs or the foundations.
Repropping – A system used during construction operations in which the original supports to a member being cast are removed and replaced in a planned sequence
Scaffolding – A temporary structure that provides access, or from which persons work, or that is used to support material, plant or equipment.
Formwork Design minimum requirements:
A good formwork design should meet following criteria:
1. Proposed system / solution should be strong enough to withstand imposed concrete pressure, dead and live loads.
2. Designed formwork should be rigid enough to retain its shape during concrete pour as well as accordingly propped & braced to withstand wind loads at every stage of use.
3. All joints should be detailed in a manner that will prevent formwork from grout loss.
4. Selection of components should allow for removal of various parts in order to reuse materials to follow desired sequence.
5. Proposed shutter elements should be as light as possible to allow for manhandling.
6. Adequate firm, flat base to formwork should be provided.
To ensure safety along with best economy of the solution all these points should be met as a bare minimum during every formwork design preparation.
Shoring is the method of providing temporary support (shores) to unsafe structure, excavation when in danger of collapse or during repairs or alterations. Shoring may be vertical (dead shores), horizontal (flying shores) or angled (raking shores).
Dead shores - a system providing temporary support when opening in the wall are to be made. Typically it consists of Needles, Props, Bracings and Sole Plates to transfer wall vertical loads.
Flying shores - a system of providing temporary support to the party walls of the two buildings where the intermediate building is to be pulled down and rebuilt.
Raking shores – a system of providing temporary support through inclined members known as rakers. Typically it consists of Walers, Cleats, Rakers, Bracings, Sole Plates to transfer angle axial loads to suitable foundations / thrust blocks.
Traditional Timber Formwork vs Proprietary Formwork Systems:
Proprietary formwork systems are built out of prefabricated panels with a metal frame, usually steel or aluminium, and covered on other side with material having the wanted surface structure (steel, aluminum, timber, etc.).
Comparing both types of formwork - Proprietary formwork systems are stronger and can be installed and dismantled with greater ease and speed than timber forms. Because they are more water resistant, and do not absorb moisture from concrete, they are more durable hence can be reused for longer period of time as far as formwork life span is concerned.
Traditional timber formwork still have a wide application where the labour costs are lower than the costs for procuring reusable formwork. It is also the most flexible type of formwork, so even where panel systems are in use, complicated sections may require timber formwork design to suit the more awkward wall shapes, openings etc.
Single Sided Formwork Design - Selection of solutions:
Variety of single sided formwork solutions can be proposed depending on actual site conditions, available access, wall geometry etc. The most economical option is fixing formwork to the existing wall. This however very often is not possible and more complicated alternatives need to be sought after.
Cantilevered formwork design – up to 2m
Walls up to 2m high can be cantilevered from previous pour. Cantilevering can also be used on tall walls when successive pouring in short lifts is acceptable. This solution is also highly recommended when access outside the wall area becomes restricted.
Design with raking struts – up to 3.5m
The most common solution includes raking struts where top strut is set at angle to the face to resist the force of the concrete. This results in uplift force on the formwork that needs to be restrained by suitable fixing into the previously cast base. The magnitude of the uplift force can be significant, hence max 3.5m pour heights are normally recommended. For walls higher than 3.5m concrete pressure need to be considerably reduced which leads to slower rate of rise and compromised practicality of the solution.
Formwork design with proprietary Support Frames – up to 8.0m
A Support Frame formwork solution relies on ties fixed at 45degree angle into the previously constructed base to provide lateral and vertical (uplift) restraint. The back of the frame is supported by the base slab to prevent frames from overturning. Created vertical force at the back of the frame as well as anchor loads should be indicated by Temporary Works designer to ensure adequacy of the existing slab that frame is relying on. It may be necessary to thicken the base slab locally, include additional reinforcement or specify minimum concrete strength that base needs to achieve before the walls are cast.
Formwork design with two parallel walls propped against each other horizontally and constructed simultaneously.
This method of construction has significant advantage as no uplifting forces are created during concrete pour which can be very beneficial when fixing to the base slab is not allowed. At the same time the solution should be avoided wherever possible as it proves difficult to ensure that concrete for both walls is placed in even layers. Failure in meeting this requirement can result in unbalanced forces, instability and overturning of the adjacent form.