Office building concept Q&A

Published 16 January 2020
Following the launch of our office building concept in November 2019, we received some interesting questions about renewable wooden office buildings and wood design and construction. Read this Q&A to find out what how our experts say on topics like sustainability, fire safety, and financial benefits of wooden office spaces.

What do you think are the major barriers in building with wood? What do you think should be done to overcome these barriers and further expand wood in construction?

We see that the main current barriers have to do with education and experience, as both have had a stronger focus set on mineral/based construction methods during the last century. But this is changing quickly and we see the result of decades of great research and education campaigns that make a strong case for timber as a construction material.

We see this reflected in current architectural trends, and especially young architects consider timber as a first-choice material and wish to use it in their projects. This is a promising development toward more sustainable construction - let’s keep it up! Sooner than we think, building with wood will be as standard as building with concrete or steel is today. The office building concept is our contribution to accelerating this change.

Does the concept provide fire safe solutions?

The concept provides guidance for the most relevant issues to keep in mind when planning an office building made with timber. There is a great lack of knowledge in the construction industry, and specifically in the planning sector, about fire safety in timber buildings, and this is our contribution to support the early decision makers. We would also recommend ensuring a competent fire safety engineer is part of the design team to ensure that specification and design meets local building regulations.

Beyond the design manual section on Fire (pages 30 to 33), you can read our documentation on fire resistance with our components. The documentation provides guidance and build ups to achieve the required fire resistance for your market.

Does the use of trees for construction damage the forest? Does it take long for a tree to grow?

When forests are managed sustainably, they grow faster than we harvest and use them. European forestry is semi-natural and based mainly on native tree species in their natural growth environments. The trees typically take 60-80 years to reach the size of a mature tree. Forest areas that are harvested, are always regenerated either by planting or sowing trees, or using natural seeding from the trees growing in the area.

This is one of the most important issues covered by the forest management certification and chain of custody certification. In simple words, it grows back faster than we harvest it.

Forest 

For example, despite the fact that the timber industry has been growing, the size of Swedish forests has doubled over the last 100 years. To learn more about how we work with sustainability at Stora Enso, visit our sustainability pages.

With your production being in Central Europe, is it not bad for the environment to produce building parts there and deliver them to construction sites all around Europe?

When looking at the carbon calculations, we take into account all emissions. This includes forestry and factory equipment as well as emissions from transport and the empty return of the truck (embodied carbon). With this taken into account, we are still carbon negative.

A good example is Ellerslie road in Glasgow, Scotland, where 906 tonnes of CO2 were removed by the timber used, 74 tonnes were created by production, and 75 tonnes created by transport, but overall, the project was carbon negative by 757 tons. This shows that even when you take into account production and transport, wooden construction is still very sustainable. There are also fewer lorries used in a mass timber building compared to, for example, concrete, where this is roughly 1/7th of the trucks required for the same building.  

We have manufacturing facilities in Finland, Sweden and Austria, and so project location also plays a factor when looking at logistics. For when we have to transport very large distances, we then also change to vessels, so in regards to places like Australia or the US, we minimize the route & emissions taken by road as much as possible.

Is it possible to build office projects with larger spans than the 7.5 to 9 metres recommended by the building concept?

There are definitively no technical limitations for office grid spans over 7.5 metres. In fact, our LVL and CLT rib panels can achieve longer spans, up to 9 or 10 metres (see page 23 of the design manual) quite comfortably, and larger spans are also technically possible.

What we have found, is that cost efficiency is at its best in the range of 6 to 9 metres. For example, our Study case 3 (page 72 of the manual) explores a 9 x 6 grid, which ends up being cost competitive to concrete. Larger spans are definitively possible and in these cases, it might also be interesting to study hybrid solutions with steel.

How can you protect a timber structure from rainwater- and snow penetration during erection in mid- and high risers?

This is quite a common concern, but experience shows it can be overcome with good planning.  Every site needs a moisture management plan and the key is to prevent standing water and avoid moisture ingress to the elements. This can be done by  letting water either drain or clear it out as quickly as possible to avoiding standing water. Then, during the follow-up trades, it is necessary to make sure that the timber is dry before covering it with the following layers of the build-ups. It is advisable to check the humidity content in the inside of the panel and not only on the surface, as the moisture content can vary through its thickness. End grain sealers are a good additional measure.

Long-standing experience demonstrated that, at least in Central and Western Europe, there is no need for a site tent. A couple of recent projects suggest that it is not needed in Finland either. In the Wood City Office Buildings, a prefabricated watertight envelope was installed following the erection of the structural frame, to shorten the period where the structure was exposed to weather as much as possible.

Is this concept suitable to design and build in seismic zones?

The basic principles for the design and efficiency of the frame should be as valid in a seismic zone, as in for example in London, where the case studies were located. But of course, as in any project, the lateral stabilization needs to be designed carefully by an experienced structural engineer, considering local conditions and codes. The considerations in the design manual’s section about bracing (page 26) might have to be rethought according to the specific local seismic requirements.

What are the fire resistance properties (REI) considered when carrying out the design of the study cases?

REI 60 for studies 1 and 3, with 4 and 3 storeys respectively. REI 90 for Study 2 with 7 storeys. It has been assumed that local regulations allow for exposed timber, and no active protection strategies such as sprinklers has been considered. Since the requirements can vary significantly in different countries, it will be necessary to adapt the solutions to local requirements and conditions. As in any project, these need to be designed by a competent fire safety engineer.

How high can you build with timber? And with this concept?

There is no known technical limit, and we have seen that new and built projects get higher and higher every year. Currently, the highest building in the world is Mjøstårnet in Norway, where we delivered CLT parts for the elevator and stair shafts. Cambridge University published an interesting study about even taller buildings.

In this concept, we have found that the sweet spot for timber from the cost efficiency perspective seems to be in mid-rise buildings, and this is why we approached these typologies with the case studies. However, we will definitively be seeing higher and higher buildings in the years to come.

Office building render by Scott Brownrigg 

Aside from the office buildings, is the beam to column system used for residential blocks as well?

In some markets we see the same type of Post and Beams types of frames used for residential construction. The main advantage is the flexibility in the planning by minimizing the number of structural walls and maximizing the use of dry walling. This allows flexible floorplans both in the initial stage and for any changes in the future. This is an interesting topic to study and develop further - Brock Commons at the University of British Columbia is a nice example.

How much lighter is a timber structure compared to a comparable concrete one?

Let´s approach this question from the material and then zoom out. Wood’s weight as a material is about 1/5 of reinforced concrete. A CLT panel would have a comparable thickness, therefore a similar weight ratio. An LVL floor panel would have a higher construction depth, but much less material. It would weigh less than 1/10 of a comparable reinforced concrete one.

If we compare the build ups (including gravel, insulation and screeds), the CLT-based option would weigh 40% of the concrete one, and the LVL Rib panel about 38%. In past projects, we have seen that the design load of a timber structure on the foundation is in the range of 15% to 20% less than a concrete one. In some cases, this can mean an important saving in foundations, or even building more m² when the loading capacity of the site is limited.

What specifically speeds up the construction process when using wood when compared to concrete or steel?

in one word: Industrialization! Wood-based elements can be delivered with a higher level of prefabrication than steel and concrete and are often easier to fix and erect on site. Because they are lighter, they can usually be prefabricated in larger parts, which can significantly increase the speed of erection. Time saved in the follow up trades can have a much higher impact on the overall construction time than the pure erection of the structure.

Many timber-based projects have been built using some steel beams. Why do you sometimes still use steel?

Every material has its advantages, and steel has some interesting ones too. As its stiffness is higher than that of wood, a steel beam is shallower than a comparable wooden one. This difference becomes more relevant in longer spans. Therefore, it can be an advantage and is relatively common to use steel beams in some parts of a project with long spans or exceptionally high loads.

In the concept, the timber option seems to be more expensive than concrete. Are you saying that concrete is cheaper?

This concept found out that all studied cost plans are in a range of +/- 3% of each other, in regards to construction costs alone. While it may be very slightly more expensive per m2 GIFA for construction costs, this is a very small difference when you take into account the other financial benefits of building this way. 

Building with this concept also offers several cost advantages outside of construction costs shown in the cost graph that make it more competitive from an overall perspective, for example:

  • Lower weight may also make larger GIFA projects possible on the same site, meaning that Gross Development Value (GDV) is higher for the same site.
  • Mass timber buildings are very attractive to the market currently, so higher rents and higher sale values are possible.
  • Speed of construction generates revenue faster and allows more projects to be completed in the same year.
  • Shorter loan periods mean lower interest costs and a greater ROI for the developer.
  • Predictable and consistent build periods mean delays to construction are less likely, so less financial risk.
  • Less remedial works needed due to advanced manufacturing and assembly, which allows for better quality assurance and reduces remedial works while also reducing the chances of costly delays on site

When you take these cost and financial factors into account, the differences in m2 rate are insignificant.

If you missed the launch of the Office building concept, you can watch the recording and find out more about the office buildings of the future.

Office buildings launch thumbnail

Office building concept launch

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