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Projects and Features

Integrating Services and Structure

July 1, 2002 by NSC2 in Projects and Features

Proposed office developments must demonstrate minimum impact on the environment and maximum flexibility for a long sustainable life. Steel shows the way with integrated construction. Michael Hawes reports.

Planning and architectural restrictions generate competition between structure and services for the available space. Structural zones are squeezed up resulting in heavier, inefficient structural solutions, whilst the service engineer is forced to use inefficient, high aspect ratio rectangular ducting. The use of Cellular beams solves both problems with consequential environmental advantages.

Fig 1: Integrating services allows longer spans without increasing construction depth. (High aspect ratio rectangular ducting is replaced by highly efficient circular ducts)

The beams are designed compositely in office floor construction with a smaller upper tee compared to the lower tee to ensure the maximum efficiency of steel use in both structure and ducting.

The Cellular beams take up the full available ceiling zone, enabling the spans to be doubled without increasing the construction depth. Ducting is then integrated within the beam depth (fig. 1).

This paper explores the many opportunities offered by Cellular beams to reduce the environmental impact of construction in the short, medium and longer-term.

Improving the Building Grid

Cellular beams are replacing restrictive short span grids with more flexible long span layout without increasing construction depth or cost. This has been demonstrated on many projects and independently verified by the UK Department of the Environment Transport and the Regions sponsored report [1], which concluded… “The most economic long span system was the Cellular beam scheme, which was equivalent in cost to the shorter span composite scheme….”

Changing the grid to long span without increasing the construction depth offers many environmental advantages.

When considering the environmental impact we need to evaluate all stages of the building life, and to quantify the true impact and legacy of our decision to build in a particular form. The stages are: –

  • Life expectancy of the structure and whether the building layout will ensure maximum operational use before demolition becomes the only option
  • Environmental impact of the proposed structure on the construction process and costs
  • Environmental impact of maintaining the building during its life
  • Reusability of the building and its individual components
  • The legacy to the environment after demolition

Extending Life Expectancy

Short span floor layouts limit flexibility in use. Positions of corridors and walkways are dictated by the column positions and much floor space becomes unusable. The options for office layout are limited and lower utilisation of floor space results. By contrast, doubling the span and removing the internal columns provides complete flexibility of space. Desk and office layouts can be designed for optimum efficiency, however many times the tasks and needs of the occupants may change during the building life.

Short span floors with unwanted internal columns will become increasingly difficult to lease, whereas clear span office open floor layouts increase the number of potential users and ensure maximum building occupancy and longer building life.

The most environmentally damaging action is to demolish one building in order to replace it with another, simply because it no longer meets current needs.

Reducing Environmental Impact of Construction – Structure

The effect of using a long span office layout in comparison to a short span one is dramatic. A single span of 18 metre layout offers a dramatic saving over a two bay 9m grid. A typical 9 m grid contains nine beams, whilst the clear span equivalent contains only five beams. This is a 44% reduction in beams, without considering the reduction in columns.

The work of a fabricator is all in the connections, cutting to length, notching, drilling, punching and welding of end plates and fittings, with multiple handling of each element through from stockyard to painting and delivery. Every part of the frame construction process is significantly reduced by going clear span. With 44% fewer beams there is a matching reduction in energy expended on fabrication.

Fig 2: Twice the area can be erected in the same time

Erection time is not governed by the member length, but the time needed to pick, swing and temporarily bolt each member into place. A clear span layout will be erected in half the time of a short span arrangement (fig. 2).

The number of foundations will also be reduced and this can have striking benefits on sites with contaminated or poor ground conditions. With low-rise buildings it is often possible to remove the internal twin pile caps and span to the outer piles, without changing the size of the remaining piles. This offers significant environmental savings both immediate and long term. Many inner city sites with poor ground conditions are littered with piles from previous structures. Unwanted old piles cause many problems, restricting the options for future use. Contaminating the ground with more piles than necessary is costly and poor use of resources.

When sites are contaminated, all risks are directly proportional to the number of times penetrations need to be made into the ground. Clear span schemes minimise the risk and, where possible, leave low level contamination undisturbed.

Reducing Environmental Impact of Construction – Servicing

Changing from rectangular to circular ducting offers a number of benefits. It reduces the volume of materials consumed and more than halves the installation time. Circular ducting is more rigid, allowing the use of thinner gauge material. The circular ducting also allows higher flow rates without inducing noise borne pollution from “drumming”.

Table 1: Comparison of equivalent rectangular to circular duct

Replacing a 600 x 350 mm duct with a 500 mm circular one saves nearly 33% in duct material. This ignores the additional savings from production wastage, as circular ducts are usually made from continuous strip spiral wound with minimal waste, whilst fabricated rectangular ducts are cut and bent from flat sheets with higher wastage levels (table 1).

Installation time is more than halved, as only a single drop rod is needed rather than double hangers for rectangular ducts. Circular ducts with push fit seals guarantee minimum leakage and they can be installed in a fraction of the time taken to apply sealant or tighten multiple bolt flanges on rectangular ducts. (When access is limited, sealing is never adequately completed). See references [3] and [4] for detailed savings.

Reducing Environmental Impact of Maintenance – Sick Building Syndrome

Much has been written on the causes of SBS and its effects on occupational heath. Poor air quality in buildings is the main issue. Over time, ducting becomes contaminated with dust and fungus. Regular inspection and cleaning of ducting is required to maintain a healthy environment for the occupants. Rectangular ducts are prone to health problems.

They require lower air velocities to avoid noise pollution from drumming, and stagnant airflow at the corners leads to early deposits of dust. Circular ducts with higher velocities and minimum frictional drag stay cleaner for longer. Rotating brushes attached to a vacuum extraction unit provide fast, effective cleaning inside circular ducting. In comparison, rectangular ducts cannot be so efficiently cleaned.

Table 2: Impact of replacing rectangular ductwork with circular in a Belgian office building (Initial leakage of rectangular took many person-hours to reduce to final leakage hours, but circular push fit ducts required no supplementary work to outperform rectangular.) [ref. 3] (p. 78)

Fig 3: Cumulative life-cycle Carbon Dioxide. (See reference [2])

Fig 4: Cumulative life-cycle energy. (See reference [2])

Fig 5: Future proof for servicing needs, and office layout

Air Tightness

A European Commission funded report on energy conservation in buildings [3] addressed the significant losses caused by air leakage from ducting. Quality circular ducting is manufactured complete with push fit seals usually guaranteed to category D. The Report concluded, “The order of magnitude of energy savings that can be achieved by using air-tight ductwork in Europe is probably in the region of 1 to 10 TWh/year.” (p. 105). Field tests in Belgium showed how dependant rectangular ducts are on installation quality controls, when compared to push fit circular ducting (table 2).

Life Cycle Assessment

In-depth analysis was undertaken by the Steel Construction Institute, UK, ref. [2] looking at short and long span options in both concrete and steel. This concluded that variations in embodied energy and CO2 from one form of structure to another are insignificant when compared to the energy in use and CO2 released in running the building.

The highest initial embodied energy for the structures considered was 9.1 GJ/m2 and 810 kg/m² CO2. This included all aspects of obtaining, delivering and erecting the building on site. Of this, the embodied energy from steel frame and columns was only 2.5 GJ/m² with carbon dioxide at 220 kg/m².

Comparing these figures to the total life cycle costs from reference [2] (figs 3 & 4) shows just how insignificant the structure cost (thin black line at base) is compared to the running costs. The Cellular option costs less to build the structure and install the ducting. It also yields both short and long term environmental and commercial savings.

Avoiding Demolition

Choosing to demolish one building to replace it with another, simply because the old structure no longer meets current needs, is the worst environmental option.

Using a clear span, flexible, Cellular beam structure maximises opportunities for office use, whilst providing space for expanding servicing needs. This avoids the need to make that final choice (fig 5).

Conclusions

Proposed office developments must demonstrate minimum impact on the environment and maximum flexibility for a long sustainable life. Steel shows the way with integrated construction.

Improving office-building stock by using an open clear span, instead of inflexible short span frames with a forest of internal columns, ensures maximum usable building life, with optimum efficient utilisation of floor space. Selecting a form of construction that promotes the use of circular ducting has the added benefit of reducing CO2 emissions during the extended building life.

Cellular beams can achieve all of this without increasing construction depth or cost. Michael Hawes is Technical Director of Westok Structural Services.

This paper was presented at the “Steel in Sustainability” conference in Luxembourg, May 2002.

© 2002 International Iron and Steel Institute

References

  1. Mckenna, P.D. & Lawson, R.M. (P166) Design of Steel Framed Buildings for Service Integration. Published by The Steel Construction Institute, sponsored by UK Department of the Environment, Transport and the Regions.
  2. Eaton, K.J. & Amato, A. (P182) A Comparative Environmental Life Cycle Assessment of Modern Office Buildings. Published by The Steel Construction Institute.
  3. Carrié, F.R, Andersson, J , Wouter, P., Improving ductwork. A time for tighter air distribution systems. European Commission Directorate General XVII for Energy group Energy Conservation in Buildings and Commercial Systems Program report.
  4. Lindab Ventilation AB. Ductwork turns full circle. A designers guide to the benefits of circular ductwork systems. Published by Scandiaconsult Box 35 S-164 93 Kista, Sweden.
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