Last edited 19 May 2016

Structural systems for offices


[edit] Introduction

The choice of the structural system for a building can be complex. It will be influenced by many factors including:

  1. Loadings.
  2. Frame and materials.
  3. Flexibility and future adaptability.
  4. Deflections and tolerances.
  5. Vibration.
  6. Robustness.
  7. Sustainability.
  8. Structural fire rating.

[edit] Loadings

There are two types of load, live and dead. A live load will be changeable or temporary whereas a dead load will be constant or permanent.

The brief and specification for live loading will influence a building’s capacity to adapt to a change in use from office to, for example, an auditorium, a restaurant or residential use. The preparation of loading plans to illustrate the specification for the dead and live loads will be useful in understanding the assumptions made in the original design when considering a change of building use or reuse of the foundations in the future.

[edit] Standard allowances for a live load

  • For a general area: A minimum of 2.5 kN/m2 for floors above ground floor and 3.0 kN/m2 at, or below, ground floor over approximately 95% of each potentially sub-lettable floor area (source: the NAD to BS EN 1991-1-1:2002).
  • The specification of a live load of 7.5 kN/m2 over 5% of the floor area improves the flexibility of the building to accommodate local storage areas.
  • For high loading areas, a loading of 7.5 kN/m2 over approximately 5% of each potentially sub-lettable floor area and not in primary circulation routes.

Historically, UK office buildings have been designed and marketed with live loadings significantly higher than the British Standard loading threshold of 2.5 kN/m2 (typically 3.5–4.0 kN/m2). Research has shown this to be an over provision. (Source: Structural floor loading and raised floor specification for office buildings: Stanhope Position Paper, January 2004.)

The specification of a reduced but appropriate live load contributes to savings in the volumes of materials in the frame and its foundations and, consequently, a saving in the cost of the structural frame and a reduced impact on the environment. However, this must be weighted against the fact that an increase in live load may increase the future flexibility and adaptability of the building and thereby potentially increase the life of the building.

[edit] Standard allowances for a dead load

  • For demountable partitions: 0.5–1.2 kN/m2 depending on the self-weight of the partition used (source: BS EN 1991-1-1:2002). Where the partition material is not known, a load of 1.0 kN/m2 should be used.
  • Raised floors, ceiling and building services equipment: 0.85 kN/m2

[edit] Frame and materials

When selecting a structure for both low and high-rise office buildings, a steel or concrete type structure is equally acceptable. The concrete may be conventionally reinforced or predominantly post-tensioned. It is not unusual for steel and reinforced concrete elements to combine in a single structural frame.

[edit] Steel

  • Relatively lightweight.
  • Greater depths.
  • Minimum depth solutions inefficient.
  • Good for longer spans but depth requirements imply combined structure and services zone.
  • More efficient on rectangular grids than on square grids.
  • Inherently good for holes and fixings (into soffit).
  • Framing of holes and openings straightforward.
  • Depth and weight may be governed by footfall dynamics.
  • Fire protection is an additional trade.

[edit] Concrete

  • Relatively heavy.
  • Shallower depths.
  • For modest spans, reinforced flat slabs give minimum depths.
  • Can be efficient on rectangular and square grids.
  • Holes and fixings can be accommodated but strategy must be considered early. Holes should avoid pre-stressing tensions.
  • Mass provides good inherent response to footfall vibration.
  • Fire protection is inherent.
  • No need for intumescent paints which can be harmful to the environment.
  • Exposed concrete soffits can allow the omission of ceilings and improve the thermal performance of the building.

The relative costs of steel and concrete solutions vary with building form and shape. Cost comparison between the two should be carried out on a project by project basis at an early stage of the design development process.

[edit] Timber

Timber is another suitable structural frame material, particularly for low-rise office buildings (two to four floors). It is also particularly suitable for floor structures, either as floor slab systems or as primary beams.

Timber can be used in combination with other materials, for example, glued, laminated timber beams with concrete slabs can be used to provide composite floors, or timber floors with steel or concrete columns and cores. Column grids will typically be slightly smaller than those for steel or concrete frames, but floor plates or spans of 12–15 m are readily achievable.

Timber building elements readily lend themselves to prefabrication and speedy erection.

If it is acquired from a renewable source, timber can reduce the overall environmental impact of a development. It can also be combined with concrete to provide thermal mass.

[edit] Sustainable materials

The increased use of forests and wood products makes an important contribution towards tackling the problem of climate change. Almost 50% of the world’s certified forests are found in the EU. Therefore, within the European market, most timber product categories have strong sustainability credentials.

A clear strategy for the sustainable use of structural materials should be developed. The prefabrication of components of the structure can improve the quality of those components and reduce waste of their constituent materials.

The Green Guide to Specification (BRE 2008) can be used to identify materials with the lowest embodied carbon impact. Timber structures often provide the most sustainable solution. However, the use of cement replacement and recycled or secondary aggregates in concrete mixes can significantly reduce the embodied carbon of concrete. Incorporating recycled materials such as ground granulated blast furnace slag (GGBFS) into concrete can help to reduce the carbon emissions and overall embodied energy of a building. See Sustainable materials for more information.

[edit] Flexibility and future adaptability

A clear strategy should be developed for building flexibility and future adaptability. Flexibility should be considered during three phases:

  • Design: Select geometry and performance criteria with flexibility in mind.
  • Construction: Fit-out design may be concurrent with shell and core construction and the design should be able to adapt quickly to minor changes such as abnormal loads, additional risers, small holes and fixings.
  • Operation: Modifications are likely to be minor, but it must be possible to form a larger hole. This may be a reasonably complex operation, but it is likely to be an infrequent event, say once in every 20 years.

Future adaptability should be considered, for example, new (service) openings, staircases, lifts, extended service risers and infill of any atria.

The building structure must stabilise the building. The stability of the structure can have a major influence on the design, and frequently utilises structural walls located within and around the service cores. Cores are increasingly constructed using slip forming techniques, which means that the core must be shaped to allow it to free stand prior to construction of the floors. These requirements necessitate careful co-ordination of the design.

[edit] Deflections and tolerances

The overall dimension of structural zones and all non-structural elements and finishes connected or applied to the building frame should be detailed to accommodate:

A clear set of interface requirements between the structure and those connecting elements which require specific movement or tolerance criteria should be identified. These are likely to cover elements such as:

To reduce the deflection of any structural element of a frame, whether it is in steel or concrete, its stiffness or rigidity has to be enhanced. This will result in strengthening of the section, increasing the size or weight of the construction material, and hence increasing its cost.

[edit] Vibration

The structure of a building can be subjected to a number of actions that can result in vibration. The primary effect tends to be the footfall-induced vibration of floors. However, there can also be secondary effects, such as structural-borne vibration arising from the proximity of a railway. Acceptance criteria are often subjective and will depend on the type and quality of office space being considered.

Footfall-induced vibration tends to be significant in lightweight, medium and long span floors. Particular care should be given to the performance criteria of dealer floors and other buildings with sensitive uses.

The effects of vibration may be mitigated by stiffening the floor, increasing mass or damping or, in the case of structural borne vibration, by isolation of the affected areas.

Structural slab vibration due to footfall impacts, plant items, lifts and escalators should be reviewed to ensure mid-span excitation is such that ‘adverse comment is not expected’ when assessed in accordance with BS 6472-1: 2008 Table 1, based on heavy trafficked floors with nominal live loads , i.e. as in electronic or paperless offices.

[edit] Robustness

The partial collapse of a block of flats at Ronan Point in 1968 was disproportionate to the event that caused it. This led to an amendment of the Building Regulations in 1970 to ensure that a degree of robustness was provided in buildings over five storeys in height. The principal requirement is the provision of effective vertical and horizontal ties so that alternate load paths may be mobilised in the event that the structure is damaged.

The Building Regulations were further amended in 2004 so that, in addition to the existing provisions, many offices will be required to be designed for all abnormal hazards that may be reasonably foreseen during the life of the building. These hazards are to be identified by a systematic risk assessment and may include, for example, vehicular impact or terrorist attacks.

[edit] Sustainability

Structure-related issues that currently award BREEAM credits include:

The refurbishment of an existing building has to be weighed against building a new office building, which may be better suited to modern requirements.

[edit] Resource efficiency

  • Reduce the amount of new materials through efficient design and the amount of waste during construction and operation by using prefabrication and efficient waste management.
  • Reuse or refurbish existing buildings, or reuse materials in new buildings.
  • Use recycled materials, for example for concrete structures maximise the amount of cement replacement and/or recycled or secondary aggregates.
  • Build for deconstruction to enable easy recovery of materials.

[edit] Embodied energy

The embodied energy of a material is the energy used in its processing and transport to site often measured through an environmental profiling system such as the Green Guide for Specification (BRE 2008). This is aimed at allowing materials to be selected with as low an embodied impact as possible. The use of materials with a low embodied impact will also contribute towards credits under BREEAM.

[edit] Operational energy

The structure is typically not used as part of the active cooling system (which accounts for a significant proportion of operational energy), however, there are ways that it can be. These include utilising the inherent thermal mass of the building and the use of energy piles.

[edit] Thermal mass

Standard low mass design such as ceiling finishes, services distribution, and lighting has to be weighed against the use of thermal mass, i.e. exposed slabs, which can save on operational energy or carbon, but may carry more embodied energy. Note: It is often not necessary to increase the concrete depth of slabs as in the vast majority of cases there is sufficient mass in even the slimmest of concrete-on-metal- deck slabs. The important issue is to expose the slab in order to enable the slab to absorb or emit heat.

[edit] Climate change

Climate change may influence the structure through increased wind loads, temperature movements and the requirement to accommodate the increasing risk of flooding. Water systems for capture or attenuation are being used more frequently and this influences support systems, especially roof structures. The structure also increasingly has to accommodate on-site renewable energy generation systems such as photovoltaics, large biomass boilers, or other loadings such as green roofs.

[edit] Flexibility

Lean design such as lower loadings or minimum weight design has to be balanced against flexibility when accommodating future requirements, including potential change of use and possibly higher load requirements.

[edit] Durability

Designing for long life and future flexibility, for example using durable materials and a solid structure, has to be weighted against designing for a short life and single use (embracing the temporary use of lightweight materials).

[edit] Fire rating

To comply with the building regulations and other statutory legislation, the fire protection systems and compartmentation requirements for a building must be considered and supporting documents should indicate the fire strategy.

The building structure can be protected inherently, for example, through element size and cover to reinforcement for reinforced concrete, or by applied finishes, such as paint systems for structural steelwork.

Fire engineering approaches can be used in some cases to reduce the statutory requirements to approval by the appropriate authorities.

This article was created by --The College of Estate Management in collaboration with the British Council for Offices. 15 March 2013.

[edit] Find out more

[edit] Related articles on Designing Buildings Wiki

[edit] External references

  • Further information on Specification for Offices can be found on the British Council of Offices website.
  • The NAD to BS EN 1991-1-1:2002.
  • BS EN 1991-1-1:2002.
  • The Green Guide to Specification (BRE 2008).
  • BS 6472-1: 2008 Table 1.