Glued laminated timber

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Glued laminated timber
Glued beams with a nail connection to a steel beam

Under glulam (briefly BS-wood or BSH , often referred to as laminated wood or Leimbalken called) one understands at least three board layers and in the same direction of the grain glued wood. They are mainly used in timber engineering , i.e. for static loads. Binder glulam be as laminated header or laminated beams , respectively.

As strand lumber glulam beams are called, which were made of fine veneer strips.

If vertical board layers are glued or nailed to flat construction elements for the production of walls and ceilings, then these are called board stacks . If several flat board layers are glued crosswise to form flat construction elements, one speaks of cross laminated timber .

Manufacturing

Wooden boards are technically dried, planed and then finger-jointed lengthways to form so-called lamellas. These board lamellas are then glued into squared timber of various dimensions and then planed again. In between, the wood moisture is measured with an electronic wood moisture measuring device, which in turn is checked by the kiln sample.

Production is solid wood used. As a rule, glued laminated timber is always made from one type of wood . It usually find spruce - fir - pine - larch - or Douglas fir timbers use. Other softwoods are rather uncommon. With the exception of poplar, which is common in France, hardwoods are currently rarely used for load-bearing purposes, as they are not permitted under building law in most European countries. Investigations with oak and beech wood , however, show the basic suitability when observing the boundary conditions for production and use.

Glulam manufacture

The adhesive bonds of BSH must be made with special care. The manufacturers of glued products must have a gluing permit (proof of the suitability of load-bearing wooden components) which is comparable to the welding proof in steel construction. Requirements include suitable and heatable production rooms as well as specialist staff and extensive experience in the field of gluing. BSH products are subject to constant internal monitoring and regular external monitoring.

The glue joints are very thin and the proportion of glue in the finished product is less than 1 percent. Approved adhesives that comply with the European standard EN 301 must be used. Polycondensation adhesives such as melamine resin and phenol-resorcinol resin adhesives , which are also known as glues , as well as polyurethane adhesives from the group of polyaddition adhesives are usually used . Only polyurethane adhesives are formaldehyde free; the modified melamine resins and phenol-resorcinol resins contain formaldehyde. Because of the small proportion of joints and because adhesives that are particularly low in formaldehyde are used, their expected indoor air concentrations are, however, well below the limit values ​​of the formaldehyde directive.

In practice, modified melamine resins are usually used for the finger joints, and polyurethane adhesives for the gluing of the lamellas.

The production takes place in different qualities, both in terms of appearance and structure.

In general, standard goods are currently being produced. Glued laminated timber can also be used to produce a wide variety of wooden parts, including multi-axis curved parts.

To avoid deformation, the last layer is glued the other way around so that the core side (right side) faces outwards.

qualities

Statically, the glulam is divided into the classes GL24h, GL24c, GL28h, GL28c, GL32h, GL32c, GL36h and GL36c according to the new DIN 1052 (December 2008). GL stands for "Glued Laminated Timber". The following number indicates the permissible characteristic bending stress in N / mm². The c or h stands for "combined" or "homogeneous" glued laminated timber, whereby combined means that lamellas with a higher strength class are used in the highly stressed outer areas and the lamellas in the inner areas consist of lower quality material. Glued laminated timber is called homogeneous when the entire structure consists of high-strength lamellas. The decisive factor here is whether transverse tensile stresses are decisive. If nothing is specified, c (combined) is considered required.

With regard to the surface, a distinction is made between industrial, visual and readout quality. According to VOB / C visual quality is to be delivered as standard , unless otherwise agreed. Industrial quality is used when there are no requirements for the optical surface quality, e.g. B. with concealed installation.

rating

advantages

Complex glue frame construction

Since glued laminated timber is made from dried wood and has a multi-layer structure, cracking occurs to a much lesser extent than with solid wood . Thus, glued beams are ideal for open roof structures in residential buildings or for other purposes where the inevitable cracking of solid wood is undesirable.

Since the strength of wood, unlike that of steel, does not depend on the temperature and the wood burns slowly from the outside in the event of a fire, whereby it forms a protective carbon layer on the surface, the load-bearing capacity can possibly be maintained longer than z. B. with steel girders .

Since glued laminated timber is made from pre-sorted timber that has been freed from defects, load capacities can be achieved that cannot be achieved with solid timber of the same cross-section.

Larger cross-sections are possible than with solid wood; the only restrictions on the dimensions of glulam are the size of the planing machines, the production rooms or for architectural reasons.

The possibility to bend the individual lamellas before gluing allows the production of attractive carrier shapes. In addition to the beams with a single curve, double-curved and twisted shapes are also possible.

Compared to steel and prestressed concrete structures, glulam offers advantages in terms of weight and chemical resistance at a similar price (in terms of load-bearing capacity).

disadvantage

The disadvantage compared to steel is the lower resistance to changing moisture and the need for larger dimensions. Wood can be mechanically destroyed more quickly than durable steel constructions, so that it is susceptible to machining or abrasive processes. Wood, even if it is glued, can be attacked by pests.

See also

  • At the beginning of the 20th century, Otto Hetzer helped glued wood construction achieve its industrial breakthrough with his inventions.
  • Hetzerhalle

literature

Web links

Individual evidence

  1. ↑ see the article en: Brettstapel in the English Wikipedia
  2. ↑ http://www.brettschichtholz.de/publish/5a07db68_20ed_76a6_2c281fc33d4723a6.cfm