Tall Timber Sustainable Construction post Hackitt by Swedish Wood

Organised by Swedish Wood and held at the IStructE HQ in London, the day-long workshop comprised four guest speakers who discussed topics that influence the drive for, as well as relate to the practicalities of, increased building using CLT (Cross Laminated Timber) as the primary structural material.

The guest speakers and topics discussed were as follows;

1) The Importance of Sustainable Construction

Professor Gideon Henderson (University of Oxford)

2) Building High in Sweden

Robert Scmitz (White Arkitekter)

3) Structural Implications for Tall CLT

Andrew Lawrence (Arup) and Alan Dowdall (Ramboll UK)

4) UK CLT Projects and the Way Ahead

Kirsten Haggard, Senior Associate, Waugh Thistleton Architects

1) The importance of Sustainable Construction by Professor Gideon Henderson

Professor Gideon Henderson discussed the environmental facts as a primary influencer for changing the way we build currently, as well as the need to extract carbon from the atmosphere.

He presented statistics on the rising trend in human emmissions of CO2, the resulting rise in atmospheric CO2, and the subsequent impact this is having on increasing global temperature.

We looked at the Paris Agreement (COP21) and its targets for future global temperature and greenhouse gas (GHG) emissions, including to limit temperture increases to substaially less than 2 degrees C with a target of 1.5 degrees C, and the aim for the UK to produce 'net zero' emissions by 2050. We looked at the various ways of achieving this, not only by the generation of green energy and reducing activities that create emissions such as burning fossil fuels and building activities such as cement production, but also by removing carbon from the atmosphere, or Greenhouse Gas Removal (GGR). All of these contributors will be necessary to achieve the UK's net 0 by 2050 target.

GGR Methods and BECCS

We looked at GGR methods, taking a particularly close look at BECCS (Bioenergy with Carbon Capture and Storage). This is a major contributor of GGR and particularly relevant to the building industry since timber is a form of biomass which can capture and store the carbon. This process happens during photosynthesis, where a trees's leaves absorb light energy and carbon dioxide, which then remains intergral to the tree without being re released back into the atmosphere. This therefore constitues a negative emissions technology, or in other words the process removes carbon from the atmosphere and stores it long term within the timber.

Biomass as Large-scale Long-term Carbon Storage

Once the carbon is stored in the growing tree, cutting the wood to use for building creates the opportunity to plant more tress to capture and store even more carbon. It is important that mature trees are cut to make way for new trees to be planted, since trees no longer absorb carbon once they reach maturity.

Carbon sequestaion and displacement

We looked at the opportunites for combined carbon sequestration and displacement across various indiustries. It can be seen that using timber frame construction in place of more common masonry construction presents the greatest opportunities for reducing carbon in the atmosphere across all of the industries considered.

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2) Building High in Sweden by Robert Scmitz (White Arkitekter)

White Arkitekter is a Swedish architectural practice with offices in Sweden, Denmark, Norway and the UK.
The company won a competition to design a new cultural centre in the Swedish city of Skellefteå to be known as the Skellefteå Cultural Centre. Lead project architect Robert Schmitz presented the plans for the centre, which demonstrated the scope of engineered timber technology at its current stage of development.

At 76 meters high the centre will be the tallest timber frame structure in Nordic countries. It has 19 floors over which a museum, art gallery, concert hall, library and a four-star hotel will be distributed. The height of the building will allow views of the countryside and forests around the city from the upper floors. Timber was the appropriate choice of material for the centre not just for reasons of sustainability, but also due the city's long tradition of timber production and construction.

The cultural spaces inside the centre are designed for maximum flexibility with open planned space achieved by moveable walls. The centre is enveloped in glass to allow visitors and passers by not just to see the timber structure, but also the behind the scenes work of designers and curators in creating stage sets for upcoming productions and exhibitions for example.

Alongside CLT, the centre utilises Glulam engineered timber technology for the timber frame pillars and beams. The construction is actually a hybrid in terms of materials; the hotel modules will be constructed of prefab CLT modules stacked on top of one another and reinforced by concrete slabs to create the tower, whereas in lower parts of the centre exposed steel and timber trusses can be seen on the ceiling. These steel and Glulam hybrid trusses have the capacity to span large distances to allow large open plan spaces within the cultural centre. The floors in the cultural centre are constructed using an HBV-system (Holz-Beton-Verbund) which helps to improve acoustics between floors.

The building will be encased in structural glazing to allow the timber structure to be visible from outside. Due to the nature of timber, it is possible to conceal joints within the members which adds to the aesthetic of the building. Other benefits to using timber frame construction include speed of construction and being able to source the timber locally further reducing the carbon footprint. The centre will also exhibit a green roof which will contribute to thermal insulation, noise blocking, biodiversity, and rainwater absorption.

Construction of the centre began in 2017 and is due for completion in 2020.

Briefly we looked at other tall timber frame structures worldwide. The tallest proposed timber framed structure worldwide will be the W350 in Tokyo, which once completed will stand at 350 meters high.

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3) Structural Implications for Tall CLT Andrew Lawrence (Arup) and Alan Dowdall (Ramboll UK)

Andrew Lawrence from Arup discussed fire resistance concerns for buildings constructed using CLT. He emphasised in particular the need to carry out rigorous tests on this relatively new technology and that threats to safety become greater as buildings become taller. He addressed the need to update the current standards and building regulations that were written at a time when more traditional building methods were dominant.

Preventing collapse is essential, particularty in taller buildings where evacuation times are longer and impact to surrounding area is greater in the event of a collapse. Compartmentalisation of CLT buildings prevents the spread of fire and therefore lessens risk to the overall strcutural integrity as well as to loss of life.

We looked at different methods for the prevention of fire spread. These include methods of fully encapsulating the structure so that once the contents of the compartment are burnt, the fire is unable to spread to the structural elements and therefore self-extinguishes leaving the structure intact.

Another method of fire protection is to use heat resistant glues in the CLT, in which case some members of the building can be left as exposed CLT. As long as other members in contact with those exposed are encapsulated, fires are likley to self-extinguish once contents are burned. This method of fire protection needs more rigorous testing to be proved reliable.

Using softening and therefore non-heat resistant glues in the CLT is likley to cause floors to collapse in a fire if they are unencapsulated. Encapsulated members will remain unaffected.

Another option is to cover an unencapsulated CLT structure in heat resistant plasterboard. This however will likley cause fires that burn longer, since once the plasterboard is burned off the CLT becomes gradually more exposed. This will result in the likley eventual collapse of the building.

Where there is less risk to loss of life, more timber can be exposed.

Where there is higher risk of loss of life, all or most timber must be encapsulated for protection against fire, with detailed design carried out to determine which and how many timbers may remain exposed. The building must be compartmentalised so that fires extinguish locally once contents in that region have burned out, thus preventing the spread of fire throughtout the building and ommiting risk of collapse

It is safe to use CLT for external walls if properly detailed, however, a robust design validation test is required to ensure the safetry of designs.

Rigarous testing is required to derive how real CLT structures behave in fire. Designers must account for the effects of glue softening and char fall off (or delamination), and how these result in more damage to the structure than may have been anticipated. The use of heat resistant glue will limit this. All test data must be published to ensure all possible anomalies are accounted for in the design process.

Compartmentalisation methods already in common use must be tested in conjunction with CLT buildings. These must be tested to prove effectiveness and results must be published as above. Tests must be extend to the use of CLT in conjunction with other materials since this is often the case in reality.

Tests must also be carried out on external wall build up which include elements such as insulation and cladding. It must be observed how these composite wall constructions behave in a real fire situation under load. Similar situaions with floor build up must be tested. The effectiveness of compartmentalisation must be tested and behaviour of a fire travelling thoughtout a building must also be studied.

In Summary, it is essential that tests are carried out on how CLT and CLT hybrid structures behave in real fire situations, and that results are accessible globally as part of an industry collabortaion. A major incident would not just cause loss of life, but damage the whole timber frame and CLT industry. This could have a wider negative environmental impact in a lost opportunity for sustainable building.

Alan Dowdall discussed some of the practical structural challenges associated with CLT buildings, namely stability and fire protection.

Using Dalston works as a case study, we discussed the structural elements that contribute to stability in CLT structures, these being connection, bending, shear and podium stiffness.

In terms of connection stiffness, there are various methods of connecting CLT members to other CLT or non CLT members such as metal plates or steel threaded rods glued into the CLT. Due to the easily machined nature of CLT, concealed joints are achievable in certain situations which improve the overall aesthetic of the structure.

Unlike the timber in timber framed structures, CLT has an inherent fire resistance which, unlike steel, allows it to remain structurally stable when subject to high temperatures. 

This fire resistance is due to ‘charring’. During a fire, the surface of the CLT burns which creates a black layer of char on the surface. This becomes an insulating layer, preventing the heating, and therfore ignition of the remaining timber core. The panel is able to continue its structural function during exposure to fires in excess of 400 degrees C.

Encapsulation can provide an additional layer of fire resistance. As above, in some cases this may fail with the effect of increasing the length of burn time.

As timber buildings become taller, new regulations are required.

It is now legal requirement that perimeter wall cladding and other non timber wall build up mateials must be of Euroclass A1 or A2 in terms of fire resistance, and that timber is of Euroclass D.

Where the CLT panels connect to the Steel Framing System (SFS), encapsulation of the steel structure be continuous through this junction to avoid the steel framing elements being exposed to heat during a fire.

We looked at the typical SFS makeup used in conjunction with CLT.

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4) UK CLT Projects and the Way Ahead Kirsten Haggard

Kirsten presented some of the background information on building with CLT as well as some of the opportunities from an architectural point of view. Topics included carbon emissions, challenges, opportunities and some associated issues such as legislation. We then looked at some of current CLT projects designed by Waugh Thistleton Architects.

We were presented with the facts on carbon emissions generated by production of traditional non-timber building materials. Concrete and steel production is responsible for 10% of all carbon emissions.

We looked at the cycle of CLT production and some of it's advantages. CLT is a clean and sustainable construction material and during its production cycle, some waste timber products from construction can be returned to the factory to be used for further CLT production. Any carbon produced during combustion of waste material can be absorbed by the trees grown to produce it.

Comparing traditional non-timber construction to timber construction, the latter is quicker, uses materials which are lighter, requires fewer deliveries and fewer construction personel.

We discussed some of the legislation associated with CLT.

The ban of certain cladding must be taken into account when considering the cladding for CLT structures.

Some of the challenges include an anticpated government ban on combustibles including CLT in high rise buildings, along with other challenges such as insurance fears.

We looked at how CLT performs in fire, how the timber members may be protected and how to prevent the spread of fire throughout the building.

Using a case study we looked at some of the practicalities and advantages of building with CLT. CLT members can be delivered to site in panels ready-made to the correct size and bolted together on site.

We went on to further discuss on and offsite assembly methods. CLT panels can be assemebled on site, likewise for hybid construction using timber and non-timber members. There is also the opportunity to construct the buildings using indiviudal CLT modules. These modules are assembled off-site, delivered to site ready built then assembled with other modules onsite to create the building.

We were presented with some statistics to illustrate the benefits of CLT in comparison to traditional non-timber building methods.

We looked at the opportunity for creating framed systems of stackable modules, therefore removing the need for external CLT.

Through case studies we looked at some examples of hybrid projects built partly using CLT.

We looked at some of the practicalities and capabilities of building using CLT including the use of CLT floor plates, Glulam beams and columns and the opportunity to create bespoke as well as concealed joints.

Using Cambridge Heath as a case study, we looked at some of the features of recent CLT buildings. This included example types of render, using exposed CLT to create features within the building and the trend for biophillia in buildings.

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