24 November 2021
With climate targets and deadlines closing in, adapting and retrofitting buildings is a brilliant way of achieving low-impact developments. But how can this be achieved if the existing building does not have the required layout, enough net floor area or is generally an unknown?
The fact is existing buildings often have built-in excess capacity. With thorough research to find the original drawings and investigations to expose and test structure on site, it is possible to adapt and extend to suit a new market, rather than demolish and start again.
Refurbishment brings challenges:
- Strengthening existing foundations is difficult and costly.
- Existing superstructure may not be laid out in an optimum arrangement.
- The buildability in and around an existing building and site, especially when some areas are occupied, can create logistical and viability issues.
Engineered timber has solutions to many of these issues. Its high strength-to-weight ratio means that it has the capacity to support the required loads while adding much less weight to the existing structure and its foundations than other materials. Strengthening and work in the ground are therefore significantly reduced. Its off-site factory production, and minimal weight, are also ideal if you need the surrounding existing building to remain in use during construction, or if there is any below-ground infrastructure such as rail tunnels or services – the use of engineered timber can help reduce the programme time and minimise risk and disruption.
Older buildings usually have a smaller grid than new-build offices and these grids are often ideal for timber. Cross-laminated timber (CLT) floors require a certain thickness for fire and acoustic reasons, so why not make that thickness work at capacity, not wasting any material, rather than adding thick acoustic build-ups and other heavy materials to achieve this afterwards? CLT can often span further than standard office secondary grids, which in turn reduces the amount of beams in the soffit and often then tying in well with an existing 4.5m or 7.5m column grid.
Existing buildings come in all shapes and sizes, and different engineered timber products can suit various constructions.
Reinforced concrete buildings often have built-in excess capacity and are relatively simple to extend, but internal alterations are often more difficult due to the monolithic nature of the slabs. Timber frame, with glued laminated timber (glulam) or laminated veneer lumber (LVL) beams and columns, can be anchored into the existing structure.
With an existing steel building it is very simple to extend with a hybrid steel and CLT solution. This can also work well with a glulam frame, as the baseplates and connecting plates are often also made from steel. If the building dates from the first part of the 20th Century it may have rivets on the existing beams and columns, which should be surveyed prior to detailing and production of the connections. Steel frames don’t always have the excess capacity required, but are much easier to strengthen using new steel plates and members.
CLT walls can be used to extend masonry wall structures in the same form, but with a more lightweight material. The connection between the two walls, whether on top or to the side, should be given due consideration, not only in terms of transferring loads but also with regard to aesthetics, access (if a party or external wall) and, not least, fire and acoustic requirements.
Everything must be surveyed, at various stages – early, post-strip out, post-demolition – and as the design develops, a rolling programme of investigations and site measures is required. Existing structures will not only have different tolerances but may also have moved over time, and engineered timber is precise. The design team should consider this from the early stages to take out any differences between the two materials and ensure the extension is buildable and aesthetically pleasing. Standard panel sizes should also be considered within the parameters of the existing structure; how can minimal waste and the positions of joints work with the aesthetic of the existing space?
Designers must be mindful of the connections they are proposing to the existing structure from the start of the process. Connections should be designed early on by the consultants, or with early input from the fabricators, to ensure that the architect and client understand the aesthetic nature of the connections and the sub-contractor can investigate their buildability at pricing stage.
Existing buildings may have low floor-to-floor zones so careful consideration of the grid and therefore member depth, along with the building services solution, is required when extending laterally. The timber may not be placed at the same level as the existing adjacent slab for the best solution, but how will that appear visually and is there a good detail to address the junction of the two?
When extending any building laterally or vertically the fire rating and performance of the structure will need to be addressed. A fire risk assessment may be appropriate to ensure that the extension does not create a higher risk than in the original building. Timber is a combustible material and therefore a self-extinguishment analysis by a fire engineer may be the best approach to obtain an appropriate fire design period, along with agreement of details between compartments and at junctions with the existing structure.
Acoustic separation is a challenge with CLT; its lightweight nature means that it has little mass to stop noise transfer. This must be addressed early on, as the acoustic build-up can add to the floor thickness and therefore increase the floor-to-floor height of the building. In some situations it can also cancel out the weight saving gained by using CLT in the first place, and detrimentally increase the strengthening required to the existing structure below. Products and details are available that provide a relatively heavy finish over a raised floor, which can increase the performance. There are no building regulations for acoustic separation between office floors and so clients should understand what they are requesting and its impact on the carbon footprint of the building.
Waterproofing details, temporary for the build and proposed for the design, should be considered carefully. If the existing building is occupied at lower floor levels or in part, a temporary roof may be appropriate. A detailed water management plan should be provided by the main contractor to manage the water by all trades until the building is completely watertight. A temporary water- shedding and drainage solution must be in place during construction which ensures that water is routed away from areas that may be susceptible to ponding or other surface wetting so there is no water damage to the timber. Water damage must be prevented at joints, for example by taping over and sealing them in such a way as to ensure no leakage into the centre of a member or panel, and that colour changes develop evenly over time across the surface.
Retrofit with engineered timber is the future, but the design team must be willing to survey and develop details as they go.
About the author
Kelly Harrison, Associate Director of Whitby Wood and TRADA Board Member.
12 January 2022
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