Double height slabs are common in commercial buildings and large public areas, as high ceilings are appropriate for these crowded places. Although desirable from an architectural stand point, forming these slabs which are typically 5-6m high, can be an additional headache for the contractor.
Many contractors who normally use a flex system on their typical height floors choose to stick with same approach except with a longer prop, rather than opting for a load bearing scaffold or proprietary tower system. The price of buying or renting a prop is obviously going to be cheaper than a tower, right?
The price for buying or renting the equipment is only part of the overall cost of erecting a slab deck. It is generally true that, in pure equipment cost, flex will be cheaper than towers, but is this a false economy?
For one thing erecting 5.5m flex system is not as easy as it sounds. With the weight of a 5.5m euro prop over 35kg, an average construction worker will struggle to lift it into an upright position. When upright, given its height, it is quite unstable even with tripod. Assuming its up and stable, the nut is 3m high and a relatively tall man is 1.75m making it very difficult for workers to adjust the height. Once the prop is standing, the next challenge is to get H20 installed which in practical terms requires a scaffolding to do it safely. If this is sounding less like such a good option, wait until it needs to be stripped down.
Because of the height, and high center of gravity relative to the height of the worker, taking down 5.5m props in a controlled manner is close to impossible, and where workers attempt it, they generally let go at some point to avoid strain or injury. Because of its weight, when the fully extended prop hits the ground, the inner tube often bends to an extent that the prop will not close and so is damaged beyond repair.
Think back to that notion that scaffold or stacking tower was an expensive way to form double height deck…not so clear cut. Factor in difficulty in using prop, stability, safety and damage cost and the picture looks very different.