A feel for the numbers

Published:  02 June, 2011

Rules of Thumb, BSRIA, Glenn Hawkins
The power or rules of thumb — Glenn Hawkins.

Every engineer needs a feel for figures to enable him to sketch out designs and check that figures in designs are realistic. What they won’t have is the breadth and depth of figures that have been brought together by BSRIA — as Glenn Hawkins explains.

Just imagine how useful it would be if someone took the time to research the latest design, cost and energy performance guidance for the built environment, consulted extensively with leading manufacturers and construction project teams about space and weight requirements for buildings-services plant — and then presented it all in a set of easy-to-read charts and tables. This is exactly what BSRIA has done in order to produce the latest version of its immensely popular ‘Rules of thumb’ document.

The origin of the term rule of thumb is unclear. It is an expression that has been attributed to a diverse range of sources dating back hundreds of years. These include woodworkers using their thumbs as measurement devices, millers assessing the coarseness of ground flour by rubbing it between a thumb and forefinger, brewers testing the temperature of fermenting beers and farmers sowing seeds or setting plants at an approximate soil depth.

Whatever the application, a rule of thumb can be considered as a general principle or means of estimation derived from practice and experience, rather than precise theory. It represents a method for broad application that is not intended to be accurate for every situation. However, a rule of thumb can be easily learned and applied, which means that it can be extremely useful for approximately calculating a value, setting outline targets, rapidly comparing different options or quickly evaluating a building’s performance.

Furthermore, rules of thumb can be employed throughout the project delivery process to sense-check precise calculations and emerging building layouts, rapidly perform what-if scenarios for different design options, validate installation and commissioning works, and quickly analyse energy, maintenance and utilities costs. They also provide a simple means of verifying the work of junior construction professionals.

For example, at the inception stage of an office development its project team could quickly establish that over 70% of the life-cycle cost of an office building is associated with running costs. This may influence the strategy for project delivery and result in the provision of a post-occupancy aftercare service to evaluate and improve building performance.

During feasibility studies, this project team will need to establish if the new office development will be able to be integrated into the existing utilities infrastructure. The rule of thumb for electricity consumption in an air-conditioned office of 87 W/m2 of gross internal area can be employed to quickly accomplish this, for example. Also, the construction cost guidance of £360 m2 for a city-centre shell and core office development could also be used to help this project team configure its initial cost plan.

Rules of Thumb, BSRIA, Glenn Hawkins
Without consulting manufacturers’ literature a rule of thumb makes it simple to allocate the right amount of space for a pair of air-cooled condensers for an air-conditioning system — and there are many similar charts in BSRIA’s guidance.

As a design progresses from concept design through to technical design, every project team needs to ensure that appropriate space is provided for the installation, operation, maintenance and decommissioning of building-services plant. This vital interaction between the services, structural and architectural disciplines is made significantly easier when a project team can quickly establish that a 1000 kVA diesel generator requires an installation area of 35 m2 and an installation height of 3.6 m, for example. In a similar manner, the ability to rapidly determine that this generator will have an operating weight of 7000 kg will help ensure that structural-design calculations make appropriate provision for principal building-services plant.

During the appointment of specialist trade contractors, rules-of-thumb guidance showing the breakdown of total installation cost into the cost for each building-services system can be employed to provide a reality check of the project cost plan and tender returns. For example, on a secondary-school project, 29% of the overall building-services installation cost is associated with the electrical installation and 9% is associated with water installations.

After handover, the guidance provided about energy consumption, CO2 emissions, energy costs and maintenance costs can be used to help compare client’s requirements, design intent and actual performance outcomes. The benchmarks for CO2 emissions in a teaching hospital are 46.5 kg/m2 for electricity and 83.2 kg/m2 for natural gas, for example. The energy cost benchmarks for this hospital are £7.7/m2 for electricity and £10.5/m2 for gas, and its maintenance cost benchmark is £7.7/m2

The latest version of the BSRIA ‘Rules of thumb’ guide has been produced in a durable, spiral-bound, A4-sized format. This means that it can be easily and regularly used on each construction professional’s desk. Space has also been provided on each page to enable all users to make their own notes and therefore create a personal reference document based on their own experience.

Glenn Hawkins is a senior consultant and author of the fifth edition of ‘Rules of thumb — guidelines for building services’.

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