Survey of Fire Incidents in Multi-Story Timber Buildings 1996–2023

The study aims to increase knowledge about modern timber construction related to fire by collecting statistics on modern timber buildings, fire incidents, and analysing the data. MSB’s fire incidents for apartments in buildings over two stories in modern timber buildings are compared with fire incidents for all apartment buildings. This is to investigate if the number of fire incidents differs for buildings with timber frames compared to all apartment buildings. Modern timber buildings are defined as those where the majority of the vertical load-bearing parts are made of wood, are more than two stories high, and built after the change Swedish building code in 1994. The definition of apartment buildings includes special housing, such as student housing and care homes. A total of 37,243 apartments built with modern timber construction have been identified. By the end of 2023, there were 3,003,861 apartments in apartment buildings nationwide (SCB, 2025). SCB’s definition includes residential buildings with three or more apartments, including corridor-access houses, and buildings with one or two stories.

All fire incidents in apartment buildings and row, semi-detached, and terraced houses reported by the country’s rescue services to MSB from 1996 to 2023 are included in the study. A total of 93,458 fire incidents for all apartment buildings were reported to MSB during this period. Assuming incidents in the total stock of apartments are equivalent to those in apartment buildings with timber frames, there would statistically have been 354 fire incidents in apartment buildings with timber frames. MSB’s data identified 166 such incidents, indicating a lower occurrence of fire incidents in apartments in timber-framed buildings than in the total population. From 1996 to 2023, seven fire incidents spread beyond the initial fire cell, affecting the entire or parts of the building outside the initial fire cell. This is lower than the statistically expected nine incidents based on the total housing stock. The lower number can be partly explained by the relatively new construction of the studied buildings, but other factors may also play a role.

Denna rapport presenterar RISE utvecklingsstrategi för att driva träindustrin mot en mer hållbar och cirkulär framtid. Trä, som ett förnybart och koldioxidlagrande material, har stor potential att spela en central roll i omställningen till ett klimatpositivt samhälle.

Strategin bygger på framtidsbilder för tio utvecklingsområden som beskriver hur träindustrin kan utvecklas för att möta samhällets ökande krav på hållbarhet, resurseffektivitet och klimatpåverkan. De tio framtidsbilderna täcker in olika aspekter av träindustrins potential, från klimatneutralt byggande till ökad biologisk mångfald och långlivade produkter. För att uppnå dessa framtidsbilder har vi identifierat 30 strategiska aktiviteter inom trävärdekedjans olika delar.

I denna rapport presenteras resultaten från ett omfattande samarbetsprojekt mellan RISE och träindustrin under 2023-4. Syftet med projektet har varit att noggrant analysera och förstå de mest framträdande trenderna som påverkar träindustrin, med ett särskilt fokus på de tre perspektiven: klimatavtryck, cirkularitet och konkurrenskraft. Med tanke på de stora förändringar som sker inom digitalisering, miljö och hållbarhet, har projektet framhävt framsyn som ett avgörande område och arbetssätt för industrin.

Within the project Dyna-TTB, vibrational tests have been conducted on eight high-rise timber buildings, in Europe. A main objective of the project is to gain knowledge about damping in timber buildings to assist in predicting the accelerations, at the top of a building, due to wind-induced vibrations. One of the buildings is Eken (the oak) in Mariestad in Sweden. That building is seven stories tall, thus questionable as a tall timber building, yet an interesting test object. The building structure is made up of glue laminated timber beams and columns stabilized with glulam trusses. Forced vibration were conducted on Eken with the aim to estimate the building’s dynamic properties from test data. Estimates of the eigenfrequencies, mode shapes, and their scalings are useful both in the calculations of wind-induced vibrations and to calibrate numerical models. However, the most important outcome is estimates of the modal damping values. The damping impacts the acceleration and thus the serviceability of the building, and at the same time, it is very hard to model damping. So, during the design phase, one must rely on previous test data (of which very few exist for taller timber buildings) and rule of thumbs. It is therefore important to gain knowledge about the damping for timber buildings in order to enable good designs of future and taller timber buildings.

Thirty years of building with wood Sweden has a long tradition of building in wood. Today’s tall wooden buildings, often higher than two stories, are based on completely different technical solutions to the ones used historically. Modern wood construction is rooted in extensive international and national research and development – not on a single national tradition. It began simultaneously across Europe, as the rule change was part of an EU directive. Modern wood construction is also characterized by a high level of industrial prefabrication and short construction times, while modern IT is widely used in the planning and production process. Altogether, these factors entail a very different process from concept to completion, to the one used in the traditional construction industry. A whole host of factors determine the value of a building. It must be fit for purpose, sustainable and attractive. It should be eco-friendly and able to be erected and managed at a reasonable cost. Five different organizations have now been interviewed about their experiences and about the property management. All the interviewees are very positive to build, use and manage wooden buildings. Experience they have gathered by themselves and in cooperation with others give them the confidence to keep building with wood. They can meet every present and predicted demand by customers and governmental rule or regulation. The setbacks are few and easy to solve and interviewees can’t find any reason to not build in wood.

Wind-induced dynamic excitation is a governing design action determining size and shape of modern Tall Timber Buildings (TTBs). The wind actions generate dynamic loading, causing discomfort or annoyance for occupants due to the perceived horizontal sway, i.e. vibration serviceability problem. Although some TTBs have been instrumented and measured to estimate their key dynamic properties (eigenfrequencies, mode shapes and damping), no systematic evaluation of dynamic performance pertinent to wind loading had been performed for the new and evolving construction technologies used in TTBs. The DynaTTB project, funded by the ForestValue research program, mixed on site measurements on existing buildings excited by mass inertia shakers (forced vibration) and/or the wind loads (ambient vibration), for identification of the structural system, with laboratory identification of building elements mechanical features, coupled with numerical modelling of timber structures. The goal is to identify and quantify the causes of vibration energy dissipation in modern TTBs and provide key elements to finite element models. This paper presents an overview of the results of the project and the proposed Guidelines for design of TTBs in relation to their dynamic properties.

Although building systems made of cross-laminated timber (CLT) have become common in Sweden in the past 20 years and they have developed rapidly during the same period, steps remain to be taken to simplify the assembly of such systems, especially at construction sites. Current construction methods, however, remain labour-intensive and thus show room for improvement.

This paper describes a novel connection for the assembly of building elements made of CLT. Simple and inexpensive, the connection is fairly insensitive to manufacturing tolerances and enables rapid, more efficient construction than the connections for CLT structures currently used. Test results show the excellent strength and stiffness of the connection, which also allows the replacement of numerous fasteners, including nails and screws, with only a single steel rod. 

Forced vibration tests have been conducted on the seven-storey timber building Eken in Mariestad in Sweden. The main objective is to estimate the building’s dynamic properties from test data. The eigenfrequencies, mode shapes and their scaling are useful to calibrate numerical models. However, the most important outcomes are the estimates of the modal damping values. The reason is that the damping impacts the acceleration, and thus the serviceability of the building, and at the same time, it is very hard to model damping. So, during the design phase, one must rely on previous test data (of which very few exist for taller timber buildings) or rule of thumbs. It is therefore important to gain knowledge about the damping for timber buildings in order to enable good designs of future and taller timber buildings. The test data shows that the modal damping is roughly equal to 2% of the critical viscous ones for the eigenmodes extracted. The test campaign on Eken is made as a part of the project Dyna-TTB in which vibrational tests have been performed on eight high-rise timber buildings, in Europe, of which Eken is one.

The use of structural timber in residential buildings has increased considerably in Sweden during the last decade. Concomitantly, there has been an emphasis on augmenting the level of prefabrication for such structures. A simultaneous need for education imparted in universities exists, and this paper contributes to this need by outlining a design example of a residential timber building with simple geometry using prefabricated volumetric elements. The serviceability limit state as well as the ultimate limit state have been defined and studied for some critical design situations. The results indicate, for example, that the degree of utilisation in the studied building system for compression perpendicular to the bottom rail in the first storey and buckling of the studs are in the same range - 91 % to 97 % - for the walls separating apartments.

Today there is no standard for strength grading and CE-marking sawn timber of Birch grown in Sweden, which makes it impossible to use this material as structural timber. Internationally, there are standards for visual grading of hardwood in several European countries. However, these standards may not be used to classify Birch timber grown in Sweden without having been tested and verified to meet the standard's requirements. In a project (BizWOOD) run by RISE, 600 pieces of sawn birch wood from Sweden is visually graded and strength tested. The project aims to result in grading rules for birch, either by using international standards or where international standards will be used as references for the identification of defects that will be included in the grading model. This paper summarizes parts of the BizWOOD project and includes 1) description of sampling and timber origin, 2) visual grading according to international standards, 3) strength testing of the material, 4) evaluation of mechanical characteristics, 5) evaluation of grading techniques.