Timber structures: sustainability Timber structures in building refurbishment

The approach to timber structures in building refurbishment

The retention, repair and retrofit of timber building structures has now become a priority to preserve the embodied carbon stored in the building fabric. James Walker describes the skills required.

Velvet Mill, Lister Hills, Bradford. Photo: Urban Splash / Joel Chester Fildes

“Timber is an excellent material choice for the vertical extension of buildings made from all major construction materials due to its high strength-to-weight ratio, which helps to minimise additional load on the foundations.”

The lowest carbon building is one that has already been built. Whether maintaining a structure or refurbishing a building, when there is timber involved, appropriate skills and expertise are required to avoid condemning structural elements because of damage, decay or insufficient strength.

The approach

There are guiding principles of conservation for the restoration and refurbishment of historic buildings,1,2 but a new set of principles are required for a net zero future where carbon release is minimised and building life is maximised:

1. Do the minimum intervention possible to achieve the client’s objectives.

2. Question and revise the client’s objectives as knowledge about the building is gained.

3. Maintain existing load paths and magnitudes where possible.

4. Reuse existing materials where possible or source new materials with the lowest environmental impact.

5. Prioritise the design solution and details likely to have the longest life.

6. Construction details should be reversible to facilitate future adaptation or deconstruction.

Timber is an excellent material choice for the vertical extension of buildings made from all major construction materials due to its high strength-to-weight ratio, which helps to minimise additional load on the foundations.

One of the drivers for building refurbishment will be the retrofit of new technologies to improve the operational energy performance of buildings. Even relatively new buildings are likely to require additional insulation (pay careful attention to the building physics) and more efficient mechanical and electrical systems (ideally in accessible zones separate from the structure). >>

Any building refurbishment project should adopt the following workflow, which may be iterative:

1. Desktop study 2. Survey 3. Appraisal 4. Design integration 5. Modify, repair and reinforce.

Survey

To assist future designers, we must record the design information of buildings, ideally within the fabric of the building itself to mitigate against the loss of electronic information.

A traditional 2D measured survey may be appropriate for simple interventions. New technology can survey a building in 3D using laser acquired point cloud measurements, supplied as a 3D CAD model (Figure 1). This allows designers to integrate the new building components with the existing building and exploit the benefits of Building Information Modelling (BIM) and Modern Methods of Construction (MMC).

A structural survey is essential to identify the existing structural members and assess their condition. A complete understanding of the building structure is unlikely without opening-up works with follow-on surveys, product/species identification, testing and/or visual grading.3 Timber is particularly susceptible to moisture-related biological attack, and areas of rot and decay should be identified and repaired as necessary.

Appraisal

The building surveyor should be familiar with the common methods of timber construction as used over the past 40 years and the myriad of timber technologies available.4 See Table 1.

Figure 1: 3D CAD model under construction from laser surveyed point cloud data. Source: James Walker

Structural member

Sawn softwood walls, roofs and joists

Engineered joists

What to assess

The strength properties for sawn softwood timber can be conservatively assumed to be C16,5 but C24 can often be demonstrated by in-situ grading. A powerful magnet can help locate the fixings through the dry lining and determine the centres of the studs.

The bending strength and stiffness of timber I-joist and metal web joist floors can be derived based on the area of the flanges and the distance from the neutral axis. The shear strength and stiffness of I-joists can be approximated from the thickness, depth and material of the web, while for metal web joists a truss analogy can be used with strength limited by the anchorage capacity of the metal ‘web’ and stiffness is based on the slippage of the connection.

Glued laminated timber (glulam)

Lamimated veneer lumber (LVL)

Crosslaminated timber (CLT)

Trussed rafters Glulam beams are difficult to visually grade since they may be a ‘combined’ grade where the outer laminations are a higher grade than the central laminations. Glulam columns are normally ‘homogeneous’ grade to ensure uniform buckling behaviour. For preliminary design, assuming a C24 solid section is a good starting point.

Strength and stiffness depend on whether the veneers are all unidirectional or contain some cross-grain veneers – look at the end/edge or take a small core sample. Most LVL is manufactured from softwood, but beech LVL is darker in colour.6 For preliminary design, assuming a C27 solid section is a good starting point.

Generally manufactured from C24 lamellas of different thickness and comes in a variety of different layups. A core sample, in an area of low stress, can identify the layup for analysis using the γ-method for mechanically jointed beams.7

Extremely efficient in their use of material and difficult to adapt without significant intervention. Specialist software including appropriate nail plate models is required for checking.

Racking boards Look out for oriented strand board or other structural racking boards in walls, but remember that nonwood-based racking boards are now commonly used to satisfy fire requirements.

Where it is not possible to assess the strength of a structural member, an alternative approach is to calculate the load supported by the structural element and ensure that the new loading arrangement does not increase the internal stresses on the member. If stresses exceed the limits of the structural members, reinforcement may be required. However, Eurocode 5 and other modern design codes are primarily written for new buildings and alternative means of demonstrating suitable reliability may be appropriate for existing buildings.8

Repairs and reinforcements

Modern self-tapping timber screws and bonded plates and rods can be used to repair and reinforce existing timber elements. Design rules will be included in the next version of Eurocode 5. In the meantime, design methods can be found in academic papers9,10 and screw manufacturers’ European Technical Assessments (ETA).11 Adhesives are particularly good for repairs where the original strength and stiffness of the structural member needs to be reinstated (Figure 2 ), but glued elements should be designed to be easily deconstructed at end of life. Adhesive repairs should be carried out by a specialist contractor.

Figure 2: Example of an adhesive repair in the moisture-damaged tension zone of a CLT floor panel. Photo: James Walker

Conclusion

Building designers need to adapt their skills to ensure existing building stock can be continually updated, refurbished and renewed in a net zero carbon landscape. Timber is the perfect material for building extensions made of any material, but to modify an existing timber requires good knowledge of standard building practices and the growing range of timber products. n

About the author

James Walker MEng MSc CEng MICE MIStructE Technical Director Milner Associates

Further information

For more information, see Timber in refurbishment, BM TRADA, 2010, available from the BM TRADA bookshop www.bmtrada. com/bookshop or visit www.trada.co.uk/sustainability

References

1. Yeomans, D., Repair of Historic Timber Structures, second edition, ICE Publishing, 2020

2. Lawrence, A., and Ross, P., Appraisal and Repair of Timber

Structures, second edition, ICE Publishing, 2020

3. BS 4978:2007+A2:2017 Visual strength grading of softwood.

Specification, BSI

4. Structural Timber Engineering Bulletin 2: Engineered wood products and an introduction to timber structural systems,

Structural Timber Association, 2014

5. BS EN 338:2016 Structural timber. Strength classes, BSI

6. www.trada.co.uk/wood-species

7. BS EN 1995-1-1:2004+A2:2014 Eurocode 5: Design of timber structures – General. Common rules and rules for buildings, BSI

8. Macchioni, N. et al., ‘Guidelines for the on-site assessment of historic timber structures’, in International Journal of

Architectural Heritage, 2013

9. Dietsch, P. and Brandner, R., ‘Self-tapping screws and threaded rods as reinforcement for structural timber elements – a state-of-the-art report’, in Construction and

Building Materials, 2015

10. Branco, J., Dietsch, P. and Tannert, T., Reinforcement of Timber Elements in Existing Structures, Springer, 2021

11. ETA-11/0030, Rotho Blaas Self-tapping screws and threaded rods, ETA Denmark, 2020