With developments in design technology, the scope of timber construction has expanded exponentially over the past 20 years from private houses to schools, apartment blocks, long-span industrial buildings and now increasingly large city-centre office buildings. ‘Engineered’ timber products – formed by laminating (gluing) timber together to form larger, stronger and more robust components – have enabled a paradigm shift in the use and perception of wood. With improved technology and research we are now able to safely address issues which plague traditional timber construction such as fire, acoustics, vibrations and long spans.
The opportunity to build with timber has a profound impact on the planet. The materials specified by structural engineers account for 11% of global green house gas emissions, and while steel and concrete come with large carbon footprints, timber construction actually locks in carbon and can help to reverse the negative impact. Consultants and clients with a real interest in sustainability will need to seriously consider timber on future projects in order to help achieve the UKs ambitious net zero targets. Andy Heyne’s article The Future is Wood provides a compelling narrative on the future of timber in the construction industry.
Engineered timber construction is in many respects fundamentally different from the concrete and steel structures to which the industry has become accustomed. Early engagement with clients and design teams is critical in order to help better understand how a timber solution could impact the various nuances of a project. With decades of experience working with engineered timber, we at HTS are happy to share our experience with collaborators, and to go beyond our engineering silo to support cost consultants, architects and mechanical engineers with solutions that we’ve learnt along the way. The sections below describe key considerations which we think should be considered early on in the design process.
A pure timber structure – often formed with glulam beams, glulam columns and cross-laminated timber (CLT) floor panels – cannot effectively span quite as far as a steel frame and tends to be economically limited to grids of around seven by nine metres. The structural floor depth will also tend to be deeper than a steel or concrete alternative, resulting in a net increase in the building height. Pure timber solutions tend to work most efficiently together with underfloor air handling, thus avoiding the need to run ducts below the structural beams. If these constraints can’t be accommodated on your site, they can generally all be overcome with a ‘hybrid’ structure using steel beams and timber floor slabs as a viable, but somewhat environmentally compromised alternative to the pure timber solution.
While current trends suggest that this may not be the case in the future, timber structures do currently come at a cost premium when compared to equivalent concrete buildings. This is further exacerbated by the additional floor height mentioned in the previous section, the additional build-up required for acoustic performance and an increase in insurance premiums. This can however be offset against the enhanced speed of construction and weight of the building which will provide a benefit to foundations and should also be considered holistically together with the potential increase in value associated with the timber aesthetic. We have seen timber construction realised as measurable value on several schemes. For example, on one Westminster office project, only the floor with an exposed timber soffit was successfully let. Over the years we have been able to establish a range of typical cost rates for timber construction projects, but these should be considered cautiously with an eye on factors such as € to £ exchange rates, supply chain, temporary works and design responsibility and we recommend that cost consultants approach contractors early to attain up to date cost estimates.
Timber structures do not provide the inherent acoustic performance of concrete structures, and generally need further build-ups to achieve industry standards such as BCO and Approved Document E. We have recently carried out a host of acoustic tests on completed timber office buildings, and are collaborating with acoustic consultants to better understand the real noise transmission in timber buildings. Generally we are finding that BCO can be achieved with minimal additional intervention to a typical CLT floor thickness. We nevertheless recommend that the stringent BCO criteria are assessed on a project-by-project basis, as some projects can benefit significantly from slight reductions to the requirements.
Rightly, and partially because of the Grenfell tragedy, fire safety engineering has developed significantly in recent years. We have seen an enhanced focus on issues such as fire spread to adjoining buildings, construction-phase management and burn-out verification. At HTS we have developed a good understanding of the types of issues which may arise as a consequence of these considerations, and are continuing to deliver tall timber buildings while helping to address these concerns. This has included working closely with specialist fire consultants and even partaking in full-scale fire tests for more unusual building layouts. Guidance is now readily available which will help you establish whether your project would be considered high risk, and we recommend that a suitable fire consultant is appointed early in the design process were this to be the case.
Bituminous membranes, fire stopping to mechanical services and riser walls are examples of auxiliary products which historically haven’t been sufficiently tested for timber buildings. Today this is no longer the case, and products exist with warranties specifically for timber substrates. This does however limit availability and supplier base which is worth considering at the onset of a project particularly in regard to a potential cost uplift.
Timber can be considered the most traditional construction material – versatile, lightweight and readily available in nature. Its timely resurgence through modern engineering technology, places it at the heart of construction’s net zero future, making it viable even on those schemes where it would historically have been discounted. It is our responsibility as designers to promote and broaden its safe use with potential added value to clients, end users and the planet alike. Although timber might not be the right choice on every scheme, we need to start giving it serious consideration and educate the industry in order to give timber a fighting chance.