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Solution to a heavyweight structural dilemma

With increasing energy bills forcing companies to keep a tight rein on costs m&e contractors are increasingly being asked to upgrade roof-based chiller plant. However, as Dan Rushton discovers, this task presents a host of challenges
It seems that a week doesn't go by at present without yet another story in the media focussing on escalating energy bills. This in turn is forcing cost-conscious businesses to review their own costs. As a result m&e contractors are increasingly being asked to upgrade existing air conditioning units and chillers to cut energy bills.

This combined with the fact that Part L is forcing an ever tighter grip on efficiency, means replacing plant on flat roofs is becoming an urgent priority for many firms. However, this is not a project that can be taken on lightly.

In the past contractors often craned in steel RSJs or cast traditional concrete plinths as supports for mounting heavy plant on roofs. However, over the years it has become apparent that these solutions are costly and time consuming, with concrete plinths requiring casting early in construction to ensure sufficient curing time before plant can be mounted. Also once cast they provide little or indeed no flexibility should the specification of the plant change or require re-siting at a later date.

Understanding the roof structure

Other factors that contractors need consider today include lighter roof construction which means greater care has to be taken regarding the weight of plant on roofs and the correct load dispersal. Contractors need an accurate understanding of an existing or new roof structure to ensure it can take the weight of the plant and support system. Meanwhile, using traditional penetrative support systems on flat roofs may also compromise roof integrity causing thermal bridging, possible leaks, or potentially affecting roof warranty requirements.

With traditional installation methods having a number of drawbacks when retrofitting cooling and/or heating equipment on flat roofs, clearly a cost effective, less intrusive, easy to install support alternative is required.

So what is the answer? Faced with the problems of mounting chillers and plant on flat roofs, manufacturers set out to provide an economical alternative. A solution to these installation headaches is to use non-penetrative, repeatable support structures mounted on anti-vibration mats. These structures are configurable in that they can be adjusted according to the demands of each individual project.

Ideal for both new build installations and retrofitting, non-penetrative systems are quick and easy to install ensuring project cost and time clarity, negating a need for time consuming and costly penetrations through the roof in the form of poured or pre-cast concrete plinths, steels or fabrications. Non-penetrative support systems thus have the advantage of maintaining the integrity of the roof membrane, therefore avoiding thermal bridging and the potential for moisture ingress.

Maintenance of both plant and roof is also made easier because supports raise the plant above the flat roof making it more accessible and creating space to run pipes beneath the units.

For example, Big Foot Systems has developed its Heavy Duty modular framework systems which are ideal for retrofitting heavier cooling plant on flat roofs. The Heavy Duty Range is non-penetrative, quick to install, robust and offers an array of solutions to support a variety of plant, such as air handling units and chillers, providing a cost and time effective alternative to on-site concrete constructions.

One of the most popular products in the HD range is the Big Foot HD Cube which is designed to support heavier services on flat roofs, making it ideal for the support of chillers.

These types of non-penetrative support systems, in the form of frameworks that are wider than the unit(s) they are supporting, afford a greater degree of protection against plant tipping ('the tipping moment'). To determine the correct frame for an installation, a supplier will typically run calculations that consider the cumulative weight of the supported unit(s), roof capability, whether the installation is wind protected or not and, where necessary, Met Office information relating to local peak wind speed.

These calculations establish the size of frame required to prevent plant tipping, and the number and size of foot bases in contact with the roof needed to ensure the cumulative weight is dispersed evenly over the roof area and is within workable loads of the roof build up (kn/m2).

Health and safety critical

Health and safety is always a high profile issue when working at height and any new support system must take this into account. Non-penetrative support systems have much to offer in this critical area.

These framework systems also meet the changing needs of a building since they are repeatable and can easily be moved or adapted to fit new requirements, allowing building owners to easily make further upgrades to plant as needed. It is also possible to adjust the supports laterally and longitudinally to suit any installation.

In some projects, limited space, existing upstands and lightweight roofs may all require custom support solutions rather than a standard framework approach.

With the aid of its technical surveyors and by using its ANSYS FEA software, Big Foot can offer custom-made, one-off solutions to meet the demanding environment present on some of today's roofs. The critical issues of accurate load bearings and dispersal are also considered at this stage.

As demand for new and retrofitted heavy plant installations on flat roofs continues to spiral upwards an easily installed system that will not damage a flat roof is well worthy of consideration. Not only do non-penetrative, repeatable support structures eliminate a whole range of problems associated with concrete supports but they also drastically improve day-to-day health and safety.

// The author is commercial manager at Big Foot Systems //
24 June 2013

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