Article on DOKA published in the magazine Concrete Frame Construction in November 2016. www.concrete.org.uk
UK, November 3, 2016.- Contemporary architecture is dominated by ever higher buildings and ever more complex geometries. The success of such projects depends to a large part on successful optimisation of the construction process. In the field of high-rise construction, the decision for the appropriate climbing system has a huge impact on time and cost. As a result, diligent operations scheduling is of crucial importance in order to find the optimal formwork solution. Factors such as cycle time, construction method, type of reinforcement and site equipment must be taken into account during the planning phase.
Not all high-rises are alike
High-rises differ not only in terms of their outside appearance or architecture but also in terms of structural design, building materials and construction methods. However, many have common features, such as one or more cores built with cast-in-place concrete for developing systems that ensure accessibility. For this reason, in most cases it makes sense to use a climbing system. The market offers many climbing systems that differ in terms of technical function and price.
It makes sense to invest in a higher-quality climbing system if it improves construction processes and results in savings of manpower, cost and time. Observing cycle times when constructing a storey plays an important role in staying on schedule. High- rise cores are frequently built in a four- to five-day cycle.
The term ‘climbing formwork’ describes the combination of shaping wall formwork with a load-transferring primary working platform, comprising a climbing scaffold or a climbing platform. Anchor parts with load capacity predefined by the manufacturer allow for safely suspending the climbing system on the preceding casting section at any height. Minimum concrete strength must be observed diligently because the climbing formwork is always attached to the most recent wall section. It is important to carefully check each suspension point to ensure the concrete can absorb the forces introduced – possibly by way of additional reinforcement that might be required.
Normally, rising and suspended platforms – for reinforcing, pouring and operating the climbing system, and reworking the concrete and dismantling the suspension points – are installed above and below the main working platform. Climbing formwork is lifted up to the next casting section either by crane or independent of a crane by way of hydraulic cylinders, guided or not guided on the structure. Depending on the type of building, climbing systems can also be used at the edge of the slab for producing façade walls or columns, vertically transporting formwork material or in the form of a fully enclosed screen system to protect the site crew from falling and from the weather.
Large-area and framed systems
The climbing scaffold can be combined with large-area formwork and with framed formwork systems as well.The distinguishing feature of large-area formwork is that it can be adapted to allow for customisation with any ground plan, in terms of panel size, fresh concrete pressure and number of form-tie points. On the other hand, there are framed formwork systems consisting of standard panels in various sizes.They are assembled in accordance with the kit principle and adapted to the shape of the structural element to be created out of concrete.When building shafts have a well- thought-out formwork design, this may result in saving hours of work, due to the often large number of similar casting sections and repetitive operations.
Here ‘thinking of stripping while making plans for forming’ may turn out to be a crucial advantage. With inside corner areas in particular, it is vital to ensure easy separation of formwork from concrete and to create the most spacious stripping distance possible for efficient working conditions. At the same time, it is beneficial to keep the number of form-tie points to a minimum.Tie-rod systems thatcan be operated from one side may result in significant time savings.
Generally speaking, it is important to use durable formwork sheets when producing a large number of casting sections. Ordinarily these are high-quality multi-ply formwork sheets coated with phenolic resin or plastic but sheets made of steel are used as well. Changing sheets is a time-consuming operation best avoided or deliberately scheduled in the construction workflow. In addition to its own weight and that of the formwork, the climbing scaffold must also transfer live and wind loads into the casting section already completed. For this reason, securely anchoring the climbing scaffold is of enormous importance.The climbing system can be lifted as soon as the section poured last has achieved sufficient strength.
When using a crane for lifting, it will lift the climbing scaffold and formwork to the next section as one unit, therefore crane and load capacity must be planned accordingly. Here a distinction is made between systems that are ‘guided on the building’ and those that are ‘not guided on the building’. During lifting, climbing formwork not guided on the building is completely detached from the structure and suspended freely from the crane. Even at low wind speeds the area exposed to and resisting wind may lead to problems. The process of lifting individual units creates temporary fall-hazard locations that must be secured accordingly. Systems that require cranes for lifting are normally used for buildings of up to 20 casting sections.
The advantage of guided climbing systems is that they remain connected to the building during the lifting process as well.The aspect of safety while lifting climbing systems must be considered during formwork planning. The average wind speed rises proportionally to the height above ground. In order to reduce the risk of downtime, systems that can be lifted even at higher wind speeds are used in this case. Currently the limit is at 72km/h wind speed. Steel profiles secured in climbing shoes attached to the structure ensure that the system is guided on the building.
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Crane-independent climbing systems guided on the building avoid some of the open fall- hazard locations because several climbing units are lifted simultaneously.The preferred way of moving these climbing systems is through portable hydraulic cylinders and units. A single hydraulic unit can supply power for several hydraulic cylinders at once. When crane capacity is available, some types of climbing systems are quickly lifted as well by means of a crane.
The principal difference to simple crane- independent climbing systems guided on the building is the all-hydraulic equipment of climbing units. They allow for safely climbing large-platform gangs, such as all platforms on the outside of a core, in a single lifting procedure without open fall-hazard locations. Such systems cannot be lifted via crane.The hydraulic lifting process requires two important steps. First the climbing profiles in the climbing shoes anchored on the building are raised by hydraulic cylinders up to the next section. In the second step, the climbing scaffolds are pushed upward along the climbing profiles by the same hydraulic cylinders.This type of climbing formwork is extremely versatile and also allows for climbing inclines, radii and bends. In addition to high-rise cores, these systems are also used with piers and pylons. Systems with 5 and 10 tonnes lifting capacity per climbing unit are established in the market.
Platform systems are formwork machines geared especially to building high-rise cores. The primary component is an enclosed forming and work platform for safe working conditions and protection from weather, even at lofty heights. It can accommodate the entire site equipment, consisting of formwork, reinforcement steel for daily work, material container and concrete placing boom for a building core. Long- stroke hydraulic cylinders raise the platform in a single lift to the next casting section without a crane.The formwork is suspended from a girder grille, like a curtain, for easy forming and stripping.The larger the area of formwork that can be positioned below the pouring and reinforcement platform, the more economical the system. Structural and customer requirements, especially installation of reinforcement or mounting parts, play an important role in this regard.
Everything must be in sync
The method of construction is greatly influenced by the building’s structural design with corresponding reinforcement layout. Here a distinction is made between the ‘core formed ahead’ and the ‘slab and walls cast in one pour’ construction methods. Ordinarily, subcontractors carry out reinforcement work piece by piece. It is important to gear the climbing system to the requirements and in doing so influence the workflow positively. Vertical scaffold systems start off with simple pouring platforms on the formwork.The disadvantage here is that as soon as the formwork is stripped, these platforms can no longer be used for installing the reinforcement. In contrast, pouring and reinforcement platforms that are independent of the formwork allow for optimal reinforcement installation even during the stripping process. In this way, forming work is separated from reinforcement work, thereby allowing for simultaneous work on several levels.To accommodate taller reinforcement bars in advance, several rising reinforcement platforms can be arranged vertically.
In the event of thicker walls and increased degree of reinforcement, planning for reinforcement work to be carried out from two sides makes sense as well. The higher the building, the less economical the concrete installation by way of crane and bucket.As the building height increases, so do lifting and sinking times, and also the time the crane is tied up. Installation capacity can be increased by using concrete pumps with various types of placement systems and large booms for efficient installation of concrete. Combining the concrete placement system with an automatic climbing system also provides the opportunity for lifting the system withouta crane – separately or together with the remaining automatic climbing formwork, as required.Another important consideration is the confounding factors resulting from changed building geometries and related modifications to the formwork and replacement of worn sheets.
Work for producing staircases/stair landings must be included too. For this reason it is important to have the foresight to address these issues during planning. As mentioned initially, a high-rise does not consist solely of a core. It also includes façade and floor slab through to the installation of the façade panels. It is important to consider the synergy here as well.
Depending on the ideas of the customer, there are many different requirements for climbing formwork. Not every climbing system is suitable for every building. Likewise, the building height does not immediately determine the best system. Therefore, when developing a formwork concept it is very important to consider from the very beginning the various customer requirements as well as framework conditions related to construction operations. With each and every climbing project the following steps are strongly recommended: close co-operation with the formwork supplier, using its experience and expertise; and an overall economic concept in terms of engineering, construction workflow and budget to be provided early in the project development stage.
Climbing Formwork: https://www.doka.com/uk/solutions/Climbing-Formwork