The Bad Reichenhall tragedy has caused a great deal of discussion about wooden roofs. In the aftermath of this tragedy a lot of journalists and politicians have contacted us in order to gather information about wooden roofs. It therefore seems appropriate to look more closely into this subject. In the following we have prepared a detailed discussion of this subject and you will find a compilation of questions and answers with useful information. Should you wish to obtain additional information, please don't hesitate to contact us.
- How do we define a hall? How can we narrow down the discussion to the halls relevant to the subject?
- How many halls do we have in Germany?
- Is there any reason to believe that large span-length constructions are exposed to an increased risk if a single part of the supporting formwork fails or collapses?
- What is a supporting formwork?
- What do we always have to take into consideration concerning the construction of large span-lengths?
- Which building materials do we use for hall roofs?
- What are the advantages and disadvantages of the various building materials we use?
- What are the critical points we have to take into consideration with regard to the various building materials?
- How can we describe the risk potential of roofs as opposed to the risk potential of walls?
- Is it true that certain weather conditions pose a risk to the load-bearing capacity and structural rigidity of a building and if so, is there anything we can do about it?
- Why is the design and construction of the supporting formwork so important for the safety and quality of a building?
- Does the age of a construction pose a fundamental threat to the durability of a building?
- Can regular maintenance help to prevent problems and thus enhance the durability of a building?
- As far as public buildings are concerned, building codes do not require mandatory maintenance at regular intervals. Is there a reason for this negligence?
- Why do we use timber as a building material for hall-roofs?
- What are the advantages if we compare timber to other building materials?
- Is every kind of timber we use in Germany equally appropriate as building material?
- What do we know about the economic lifespan and durability of timber constructions?
- The roof of the EXPO-building in Hanover is a timber construction. Is there any risk that this roof will collapse in a few decades?
- How is it possible that straight and curved wooden members can be manufactured from wooden boards that have the size of a tree?
- Which methods do we use to join single wooden boards?
- What kind of glue do we use?
- How can we be sure that a certain kind of glue is appropriate? Are there any tests and if so, since when?
- How can we make sure that manufacturers of Laminated Timber comply with quality standards?
- Is there any kind of quality control for manufacturers of Laminated Timber in Germany that can make sure that no inappropriate procedures are applied in the industry?
- When it comes to the construction of public buildings, the building authorities are involved as well. Is it true that building authorities are better informed about and more sensible with regard to routine maintenance of public buildings?
In general, a hall can be described as a one-story building that can be used for many purposes. Halls can be used for playing sports, cultural events can take place in halls, and halls can be used for the production, sale or storage of goods. Although most halls are covered, there are also halls with open cover. commercial buildings and apartment houses cannot be defined as halls, but sometimes there are buildings with a rooftop hall, as for example tennis centers or shopping malls.
The ongoing discussion, however, is about halls such as riding halls, tennis centers and ice-rinks.
It can only be estimated how many halls there are in Germany. In a small town with a population of about 25.000 people, there are approximately 200 halls. Further to that the number of halls in residential areas is much smaller than the number of halls in industrial areas. As Germany has a population of about 80 million people, it can be estimated that there are about 600.000 halls in Germany. And when we consider the number of halls in Germany, we have to bear in mind that the discounter ALDI has at least 5.000 supermarkets: All of this points to the fact that there may be a lot more than 600.000 halls in Germany. And one must also remember that there are lots of production halls, transfer stations and freight halls in metropolitan areas.
The load-bearing capacity of a supporting formwork is enormous, even if the span length is only 10 to 13 meters. If a single part of the supporting formwork fails, the remaining parts will bear the weight.
When we speak of span-lengths larger than 15 meters, however, we have to bear in mind that the mode of construction is different for large span lengths: In case of a failure of one part of the supporting formwork, the adjoining structural members will be affected as well. It would appear, then, that this has been the case in Bad Reichenhall.
The term span-length refers to the gap between two supporting parts, as for example the gap between two columns. The ice-rink in Bad Reichenhall had a length of 80 meters with a span-length of 40 meters. The span-length always refers to the primary supporting formwork which gives stability to the whole structure.
A supporting formwork is a load-carrying structure. The most important aspect of a supporting formwork is its ability to transfer the self-weight of the roof and the weight of the snow into the ground.
In order to plan and construct buildings with large span-lengths, knowledge about the material properties and static calculations is absolutely necessary. Further to that it is necessary to calculate load factors such as wind and snow that may have an influence on the construction.
In general, reinforced concrete, ordinary steel, wood or hybrids of these materials are used as building material for hall roofs. As far as individual components are concerned, it is also possible to combine various materials as for example wood and concrete. A striking example of how various materials can be mixed together successfully within a supporting formwork is the ice-rink in Deggendorf.
Timber constructions are recognized as a safe and reliable form of construction because wood is a strong, lightweight material and it can be manufactured into new products easily. Therefore wood constructions are predestined for large span-lengths. In addition to this the smooth surface is soil-resisting and prevents polluted or saline air from harming the material. And one must also remember that the fire performance characteristics of timber constructions are also excellent because in case of fire, the wooden structural members develop a charcoal layer that reduces thermal conduction, combustibility and flame spread, so that the inner part of the cross-section is protected and the stability of the construction is not at risk.
Although the dimension of steel constructions might be rather small, the rigidity of steel constructions is impressive. Unfortunately, the fire resistance of steel constructions is not very good and due to open cross-sections, settled dust is a problem as well. And on the subject of corrosion it must be said that regular maintenance is required in order to prevent corrosion in an environment with saline air and/or a lot of humidity.
It is worth stating at this point that reinforced concrete constructions combine the advantages of concrete and the disadvantages of steel. Although concrete is a very strong material which can absorb a lot of pressure, steel is required in order to assume the tractive powers. But, as mentioned before, corrosion poses a serious problem for steel constructions. And particularly with regard to large span-length-constructions we have to bear in mind that the weight of reinforced concrete constructions is problematic as well, because the weight of reinforced concrete is five times as high as the weight of wood. So a considerable part of the load-carrying capacity of reinforced concrete is wasted to the enormous self-weight of this building material. As reinforced concrete as a building material for large span-length-constructions suffers from a clumsy appearance and is not very cost-effective, prestressed concrete is the building material of choice for span-lengths of more than 25 meters.
As far as the safety of the various building materials is concerned, there is not much difference between them. Each building material has advantages and disadvantages. This is why engineers and architects need to have expert knowledge about the material properties of each building material they use. Further to that profound knowledge about design calculations is required as well.
The first thing to point out is that the load-carrying potential of roofs with a large span-length is enormous, because the self-weight of the construction as well as the snow load has to be transferred to the walls of the building. It would hardly be an exaggeration to say that this exceptional kind of bending does increase the risk potential.
Indisputably, the dimensions of walls are different. In contrast to the higher altitude and the large span lengths of roof constructions, the walls of a building carry a much lower risk potential. The-ice rink in Bad Reichenhall had a roof with a span-length of 40 meters and the walls had a height of 8 to 10 meters. Undeniably, if parts of the wall construction collapse, the roof will collapse as well.
Durability and longevity are very important parameters of design and construction. The terms durability and longevity refer to the length of time a building continues to fully meet its intended end use. In general, wood frame construction is durable, but in building situations where there is an increased exposure to elevated moisture levels, preservative treatment and moisture control procedures are necessary. Further to that, wood can be seriously damaged by bio-deterioration if conditions exist that permit the development of wood-degrading organisms such as fungi and insects. As far as steel constructions are concerned, corrosion can harm the construction and moisture control is absolutely necessary in order to extend the lifespan of the building material. In order to protect a building and to further enhance the durability of the building material, it is necessary to use the right materials in the right locations, and to employ proper maintenance techniques.
In this context it is also important to note, that frost can harm a building, too: If water comes into a crack of a building, the freezing process turns water into ice, the ice is expands, the crack breaks up and the concrete- or wood construction is damaged.
Although heat in general does not harm building materials, fire safety is an important concern in all types of construction. In case of fire, steel- and reinforced concrete constructions collapse when the temperature reaches 300 to 400 degrees Celsius. In contrast to steel- and reinforced concrete constructions, the fire resistance of wood constructions is excellent. In case of fire, wooden structural members develop a charcoal layer which reduces thermal conduction, combustibility and flame spread. Thus, the inner part of the cross section is protected.
It would hardly be an exaggeration to say that expertise in the planning and construction process is of utmost importance when it comes to the realization of a building system.
Today, it is possible to design supporting formworks that can prevent the collapse of the whole hall construction if a single, individual component fails. So it would hardly be an exaggeration to state that an intelligent planning process with regard to the supporting formwork can result in a limitation of damage in case of a collapse. Unfortunately, this kind of planning process is not common practise yet, because it is very expensive. So when it comes to the realization of a construction, the strategy and planning process in many cases is influenced rather by economic decisions than by security purposes. Unfortunately, a cost-effective strategy seems to be more important than the limitation of damage in case of a tragedy.
As mentioned before, the term durability refers to the length of time that a building continues to fully meet its intended end use. In order to achieve this end, it is necessary to employ proper maintenance practices. This can only be achieved when a building is inhabited and someone looks after the building. If this is not the case and the building is more or less abandoned, it will decay in a few years. This applies to residential houses and to every other kind of building, too.
It goes without saying that regular maintenance can help to identify and remove any kind of damage. If the small damages are not taken care of at an early stage, it might already be too late to prevent massive consequential damage.
It must be realized that this is a question of politics, because the legislature follows the philosophy that the owner of a building or the operating authority is responsible for the maintenance of the respective building. In order to keep regulations at a low level, the legislature has refrained from signing regular maintenance into a law.
Today, there is a general tendency in the building industry put the owner of a building or the respective operating authority in charge of the building so that they have to assume responsibility during guarantee and construction time.
Authorities in Brussels are interested in reducing regulations in order to save money. As the authorities in the building industry are hard pressed for money as well, they highly appreciate this policy. As a consequence, expert jobs within the building authorities are kept vacant, public authorities are no longer responsible for public buildings, and the operating authorities have to assume the responsibility instead.
Timber has been used as a building material for large buildings, convention halls and residential houses for centuries. It would hardly be an exaggeration to say that timber has proved its worth as a building material long before steel- or concrete constructions even existed. In Germany, we have some outstanding examples concerning durability and longevity of timber constructions: Take for example the "Knochenhauer Amtshaus" in Hildesheim or the caravanserais along the historic Silk Road. And there are also numerous old town halls and half-timbered houses that have stood the test of time.
As far as modern buildings are concerned, the development of Laminated Timber has facilitated the use of timber in the construction industry because Laminated Timber can be manufactured to an almost limitless variety of straight and curved members. Furthermore, Laminated Timber offers the advantage to manufacture large structural members for roof constructions.
Timber has been used as the building material of choice for such a long time, that many people have begun to take its advantages for granted. First of all, timber is a strong, light-weight-material, it is easy to use and it is easy to manufacture into new products. Compared with other building materials, timber has several distinct advantages in size capability and architectural effects. This is why timber is often used where the structure of a building is left as an architectural feature. We can see, then, that timber offers artistic freedom without sacrificing requirements: The fire safety of this material is excellent and building codes recognize timber constructions as a safe and reliable form of construction.
This is definitely not the case. As far as halls with extraordinary large span-lengths are concerned, Laminated Timber or solid wood are used as the building material of choice. In most cases, spruce is the standard material that is used for timber construction in Europe. Unfortunately, spruce as a raw material is not very resistant against bio-deterioration. Therefore it is necessary to keep the building material dry. Dry or dehumidified timber can last for centuries.
With regard to the durability, larch and pine wood are less problematic than spruce, and oak as building material is only used as solid wood. Furthermore, oak is not very cost-effective and it is not allowed to use oak for the production of Laminated Timber.
Dry or dehumidified timber can last for centuries and today there are various examples of wooden constructions that have been built hundreds of years ago. Take for example the Kapellbrücke in Luzern, which has a 500 year old roof. Due to catastrophes and wars such as the Thirty Years War, many old buildings have been destroyed, but nevertheless, in the southern part of Germany you can still find numerous half-timbered houses, which have been built in the Middle Ages and still exist today.
Although the EXPO-roof with its on a 40 x 40 meters base area constructed parasols are an excellent example of modern structural engineering, it sure needs maintenance. If the EXPO-roof gets regular maintenance, it can last for centuries.
One of the key advantages of wood as a building material is the ease with which wooden structural members can be joined together. Single wooden boards can be connected by finger joints and many different kinds of lamellas can be produced during the manufacturing process. After having applied the respective adhesives, the wooden boards can be curved and twisted easily.
As this has been the common practice since the middle of the 19th century, the German Otto Hetzer, a famous pioneer in the industry, also used this method and further enhanced the development of this method. In the German town of Wuppertal, a sports hall, which has been built by Otto Hetzer, can still be visited today. And in Southampton, England, you can see a college with twisted members, which has been built in the year 1869.
It is interesting to note, however, that only after the Second World War Laminated Timber has had its breakthrough as the building material of choice. Half of the Laminated Timber that we use today is manufactured in the United States of America and the half of the Laminated Timber is manufactured in Europe. And most of the European Laminated Timber is produced in German-speaking countries.
Wooden boards are usually connected by finger-joints. After having applied the respective adhesives, the production of large structural elements is possible. The structural elements are called trusses, beams or girders.
With regard to the production of Laminated Timber, the bonding process is very important and adhesives play an important role in wood construction. The term adhesive in general defines a substance capable of holding materials together by surface attachment. Adhesives are used for the production of laminated products as a means of increasing the structural rigidity. Although glue belongs to the conceptual class of adhesives, the term glue is used for adhesives that contain water as solvent.
In the past, only glue was used as the proper bonding material. Otto Hetzer, for example, used glue that had been produced from bones. Nowadays, however, the industry uses adhesive joints that consist of glue plus hardener in order to ensure the most effective bonding. After having mixed together glue and hardener, the mixture hardens and the water disappears.
The latest developments during the last twenty years have been polyurethane solution adhesives and epoxy resin.
In Germany, it has been the common practice for decades that adhesives, which are used in the construction industry, have to comply with the norms and standards of the construction inspection authorities. As general approval of the adhesives is mandatory, representative samples are tested for adequacy of glue bond and all end joints are stress-tested to ensure that each joint exceeds the design requirements. Each member fabricated has a quality assurance record indicating glue bond test results, including glue spread rate, assembly time, curing conditions and curing time. It is standard practice that the respective glue has to be tested by the Federal Institute for Material Testing in Stuttgart. After that it has to be approved and authorized by the Federal Institute of Structural Engineering. Independent certification agencies as for example the Federal Institute for Materials Testing in Stuttgart ensure that the adhesives meet the requirements of the manufacturing industry.
In Germany there are approximately 40 companies that manufacture Laminated Timber. German manufacturers of Laminated Timber are required to be qualified according to certain standards. These standards set mandatory guidelines for equipment, manufacturing testing and record keeping procedures. So for manufacturers of Laminated Timber it is mandatory to provide a certificate that authorizes the respective company to manufacture glued Laminated Timber members.
In Germany, there are approximately 30 manufactures of Laminated Timber that are authorized to manufacture extra large span-lengths. Manufacturers that agree to submit themselves to strict construction supervision procedures will be granted the privilege of receiving the Quality Certificate according to RAL-Standards. All in all, Laminated Timber manufacturers in Germany comply with quality standards that have to be called extraordinary in the construction industry.
In Germany the manufacturers of Laminated Timber have to be in a position to demonstrate responsible manufacturing practices. And since 1996 the manufacturers of Laminated Timber have agreed to submit themselves to strict controls. Twice a year there are unannounced controls on a random basis by an independent institute.
Further to that manufacturers of Laminated Timber have to provide a certificate that they are allowed to manufacture structural members, and the validity of these certificates is controlled regularly be independent institutes as well.
When it comes to the construction of public buildings, the building authorities are involved as well. Is it true that building authorities are better informed about and more sensible with regard to routine maintenance of public buildings?
This is definitely not the case. When it comes to the construction of a public building, it is often the case that subventions or government grants can only provide financing for the construction process. After having paid for the construction of the building, there is not enough money left for employing sound maintenance practices on a regular basis. In many cases, public authorities only become more sensible concerning routine maintenance of public buildings in the aftermath of a tragedy.