The Different Types Of Decking Methods

Tableforms are constructed as individual frames with the table top made up on the ground to eliminate any working at height issues. Handrails may be installed at this point also. The tops are then crane lifted onto the frames in the correct position. Tableforms are by far the most economical system of formwork where repetition ensues, but the initial construction is time consuming.

The two panel based systems in this study are erected slightly differently. Skydeck’s legs and main beams are erected from the ground, with the panels placed from a mobile platform such as a mini tower or MEWPS. Topec is erected by building a starting frame of legs and gates onto which the first panel is placed. Each subsequent leg is used to raise the fitted panel into position. Panel systems are ideally suited to enclosed areas such as rooms or straight slabs between drop beams.

There is no system ideally suited to all applications.

The five proprietary systems

The five falsework/formwork systems studied in this report consist of three beam based systems and two panel based systems:

Peri Multiprop/GT24 Girder system is constructed of Peri multiprop legs, MRK gates of varying sizes, fork heads and GT24 timber beams of varying sizes. There are multiple different sizes of MRK gates and GT24 Girders. This provides versatility in usage, but confusion in ordering, construction and off-hiring. MRK gates may be set anywhere over the length of the outer or inner leg (which remain as a single unit) as per design. A property not available on any other system. Any fixing of the GT24 Girders is done by nailing or bolting of special brackets.

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SGB GASS system constitutes outer legs of three different sizes and separate inner legs. Ledger frames of three main sizes that can be positioned anywhere along the length of the outer leg, providing a very solid frame. T225 aluminium primary beams are fixed to the top of the leg by a “T”bolt. T150 or S165 aluminium secondary beams are fixed to primaries where required by use of “T” bolt based “A-clamps. There are also auxiliary components to aid in awkward designs.

O’Shea Alshor Plus system provides outer legs in 500mm increments from 1m to 4m in length. They may also be spliced together with spigots to obtain any required length. The bracing frames may only be fitted in 500mm increments along the length of the outer leg. This reduces the versatility of the system. Although the ease of fitting the frames does make for a very speedy process. Primary and secondary beams are as SGB GASS.

Peri Skydeck is a panel based system that primarily consists of three main elements. Multiprops with fitted dropheads are inserted into the ends of main beams and used to erect them to a horizontal position. Panels are then fitted from a raised platform and the process repeats. A simple process, but the Peri stipulate that all panels be clipped and extra bracing be used to counteract lateral loads. Walkways are possible only by use of longer main beams and walkway panels.

SGB Topec, the final system, is also panel based, but has no main beam. One side of the panel is inserted into the claws of two heads and raised with an extendable tool. The next leg is then inserted from beneath. This process repeats in two directions until the required area is covered. Definitely the simplest of the systems within the study, makes no provisions for walkways or cantilevered slabs.

Project design

The generic problem solving approach I chose to use in this report is the journey approach. The first paragraph in each chapter describes one or more specific problems associated with that chapter heading. This is followed by an ideal objective in relation to those same problems. The final paragraph identifies where each proprietary system either meets or falls short of these objectives. Any alternative improvements resulting from the report are also included at the end of the relevant chapter. The conclusion summarises the benefits of each system as well as improvements that can be made. The fact is also highlighted that no system would ever be ideal for all applications.

Integrated safety

Some of the main risks involved in the erection and striking of falsework come under the Work at Height regulations 2005. “The purpose of these regulations is to reduce the number of deaths and injuries resulting from falls […]” (CITB 2009, C1, p1). Other risks come under the Manual Handling Operations regulations 1992. Their purpose is to reduce the risk of “musculo-skeletal injuries […] brought about by manual handling activities in certain industries” (CITB 2009, A6, p7). The falsework designer has a legal duty under CDM regulations to produce a design that complies with the above regulations.

To comply with the WAH regs, a design must accommodate safe access to all components, regardless of their location and height above ground level.To comply with the MH regs, all components must be <=25kgs or stipulate mechanical handling methods.

All of the systems in this case study, with the exception of Skydeck and Topec, provide ancillary items such as access platforms, integral ladders and handrail attachments that can be incorporated into the falsework. These are chargeable, so contractors only hire a minimal amount. These then need to be removed and re-fitted for each section of falsework, thus increasing the amount of manhandling and man-hours required for erection. A pre-handrailed pedestrian platform may be used as a quicker alternative where clearance is available between the top of the frame and the underside of the beams. This can be moved along in a single unit by hand to the next work area. Due to the design of Alshore Plus, this method is not possible. This system is generally designed with extension jacks at the bottom and frames fixed close to the primary beam, thus reducing the clearance between the top of the frame and beams. Skydeck is erected from the ground, but panels need to be placed from a platform. This is a safe process, but has the additional hire of an access tower. Topec is erected in its entirety from the ground, meaning only infills and handrails are installed from a height.

Structural Stability

During the erection process, any falsework must be able to support its own weight and that of any necessarily imposed load, at any given stage.

There are two types of load a deck endures. Vertical load is imposed by anything laid on the deck including operatives, material and the pouring of concrete. The heaviest of these being concrete, has a mass of 2200kg/m3 approx. The thickness of the slab determines the actual load. The loading capabilities of each system is given in appendix 1. Lateral load is imposed by anything moving on the deck. This can be operatives, initial loading of materials, and the placing of concrete. This is normally calculated at 2.5% of the vertical load.

The most stable form of falsework is that which is made up of legs and frames. Extra frames provide for extra stability. Skydeck and Topec are only framed at their starting point and around certain columns. All other legs are otherwise freestanding. Skydeck incorporates extra bracing into their design, but this adds to extra handling and another sub-process. To support heavier slabs, it is possible to reduce the centres of each element so as to increase the loading capacity per m2.

Speed of erection

The number of crane-hours, man-hours and sub-processes involved has a direct effect on cost in both monetary and program terms.

SGB claim that by using Topec, average erection times of 0.22 hours/m2 of decking is achievable. Tableforms can achieve similar results but initial construction takes longer.

Both Alshore Plus and Peri Multiprop incorporate a measuring scale into their extendable jacks. This provides for faster initial setup of levels. This also speeds up the process of re-installing tableforms. SGB GASS and Peri Multiprop provide for greater versatility in the positioning of frames anywhere on the length of the leg. Gates on Alshore Plus can only be positioned in 500mm increments, thus proving problematic in all but the simplest designs. Peri Skydeck, although fast to erect within enclosures and in straight lines, is not well suited to odd shaped buildings. Topec comprises of only two main components, requires minimal craneage and in the most part, can be erected entirely from the ground.

Minimal components

Systems with large numbers of components employ more sub-processes during erection/striking. There is also a greater risk of components being lost or broken, resulting in counter charges by the hirer.

All of the proprietary systems in this study use very few main components, but ancillary components are not mentioned. These still require fitting and are charged for within the hire.

Clips and fixings make up the majority of components in all of the systems. Reductions in these mean faster erection/striking times and better production. Peri, as a rule do not take permanent structural elements into account when designing in lateral load capabilities. As a result, they tend to incorporate extra bracing into both their Multiprop and Skydeck designs. Peri Multiprop also require all GT24 girders to be fixed at each end with special clips and double-headed nails to prevent them falling over. A-clamps are used by both Alashore Plus and GASS to fix secondary to primary beams. These are tightened with a spanner, so access must be achieved from underneath when used in strike and erect applications. A future possibility for this is to design a new type of clamp that can be fitted and removed by use of an extendable hand tool from the ground.

Integrated Walkway

Most applications require a perimeter walkway around a slab edge. This must be incorporated into the falsework/formwork.

A minimum walkway of between 800mm to 1200mm is recommended around the perimeter of an open deck. Its installation should require no additional types of equipment or change in process than that of the erection of the slab supporting deck.

All of the systems using primary and secondary beams provide for natural cantilevers that can provide walkways. Beams may be designed to cantilever 30% of their overall length. Longer beams, when used toward slab edges provide for bigger walkways. The panel systems are less suited to walkways. Peri Skydeck can design walkways through use of extra components. No such option is available for Topec .

Simplistic infills

The deck must be cut in and sealed around any permanent structural elements protruding through it. This must be a simple and speedy process.

Ideally, infill pieces should be pre-fabricated and only require dropping in.

This may be achievable when using tableforms on repetitive structures if great care is taken. Skydeck and Topec systems provide infill supports that can be incorporated into the deck. With tableforms or standard beam systems, the primary and secondary beams can be laid out in such a way that any infills just require cutting in.

Compatibility as Tableforms

Tableforms are pre-fabricated areas of complete falsework decking that are re-used multiple times in the same project. They strike as one piece from the slab soffit, and are manoeuvrable to their next position of use.

Tableforms are only useful if they cover a large square metreage and require little or no infills. Ideally, each grid between columns/walls will be completely covered by one tableform. They must be safely manoeuvrable from their striking position to their next position of use.

The three systems that may be used as tableforms are very similar in structure. They are all made up of legs, gates and beams. The only difference between them is how they are fixed together. Multiprop use legs/jacks in the same unit, so jacks cannot fall out while being crane lifted. The GT24 beams are all nailed with special clips. This is a slow process. Gass and Alshore Plus require extra banding between the jack and the main body of the tableform. All beams are fixed with clips that are quick to install with a battery powered socket wrench.

Safety of striking

Striking must be carried out from beneath. As a result, elevated elements such as secondary and primary beams and their associated clips may be difficult to reach from the ground. This introduces “working at height” issues that need to be overcome. Consideration must also be taken on the amount of labour required to strike a particular area as regards manual handling.

It should be possible to perform all striking activities from the ground.

The same systems of platforms must be employed as for safe in-situ erection. Again, the use of a type of clamp that can be fitted and removed by use of an extendable hand tool from the ground would reduce the need for such platforms. Tableforms, as they remain intact, can be dropped from their loaded position and moved without the need to work at height.

Speedy striking

As with erection, the number of man-hours and additional equipment required to strike an area of decking has a direct effect on the cost of the process.

Jacks/legs should be easily lowered when fully loaded. High level members should be safely removable from the ground.

The main body of a Skydeck application uses drop heads that can be dropped by 65mm with a single blow. This lowers the main beam, but not the leg itself, which remains as back propping. Topec employ a similar method, but the minimal number of components used in Topec mean that once the initial load is released, it is very fast to strike. It is possible to drop Alshore Plus jacks by 12mm with a single hammer blow. Here, the whole leg drops. This is not possible with any of the other beam based systems,

Rapid stacking and clearing from site

The quantity and physical characteristics of falsework elements determines how quickly and easily they can be cleaned and stacked for off-hire. T225 and Aluma beams take up a lot of space when stacked. They also come in many lengths, so many part bundles are likely to be required for any one work area.

There should be minimal sizes of each component type. This will reduce the number of incomplete stacks around the striking area, thus maintain a tidier and safer work area. All components are likely to get splashed with wet concrete, so it is imperative that they possess a coating that is easily cleaned for off-hire or next use.

Both panel systems provide for easy stacking and cleaning of their components. All are designed to be easily man-handled. Multiprop legs incorporate the jack into the outer leg, thus requiring fewer bundles.

Conclusion

Program constraints and actual cost per m2 are the two main factors considered when choosing a proprietary falsework design. Although each application is different, a design can be produced to suit any application using any system. However, this report shows that each application will have a system appropriate to it. Where repetitive multi-storey blocks may be accommodated by the simplicity and easy-action striking of the Alshore Plus design, more intricate one-off slabs may be achieved by the versatility and rigidness of GASS. The GASS system could be improved however, by use of the integrated measuring tape used by Alshore Plus and Multiprop.

Another advantage of the Multiprop leg becomes apparent when used in tableforms. The inner jack is permanently locked within the outer leg, so there is never a risk of it falling free while the tableform is being crane lifted. This also provides for easier stacking at the end of their use.

When using the GASS or Multiprop systems for strike and erect applications, the cumbersome integrated platforms can be replaced by a 6m X 0.6m handrailed platform. This provides simple and safe access to beams, clips and A-clamps during erection and striking.

The A-clamps themselves would benefit from a re-design that would enable them to be tightened and released from the ground, by use of an extendable hand tool.

Areas not easily accessible by crane and slabs enveloped by walls or beams such as multi-storey basements and lift/service cores, would benefit from the safety and simplicity of the Topec design. It has been proven in suitable applications, to be the fastest strike and erect system available.