Glued laminated timber

Under glulam (short glulam or BSH, formerly often called laminated or Leimbalken ) refers to at least three board positions and in the same direction of the grain glued wood. They are used mainly for timber constructions, ie at a high static load. Binder made ​​of glulam are called laminated header or trusses.

Production

Wood planks are kiln-dried, planed and then joined lengthwise by means of so-called Finger fins. These laminations are then glued to square timber of various dimensions and finally planed again. In between the moisture content is measured with an electronic moisture meter, which in turn is checked by the oven dry method.

For the production of solid wood is used. As a rule, a glue-laminated timber is always made ​​from a type of wood. It usually find spruce, fir, pine, larch or Douglas fir wood use. Other softwoods are rather uncommon. Hardwoods are common in France, with the exception of poplar for load-bearing applications currently hardly used because they are not permitted construction law in most European countries. But studies with oak and beech wood to show the suitability obeying the boundary condition for the manufacture and use.

The adhesive joints of BSH must be made ​​with special care. Manufacturers of glued products must have a " glue approval " (proof of eligibility load bearing timber structures ), which is the " sweat proof " in steel construction comparable. Requirements include appropriate and heated production premises and a professional staff and extensive experience in the field of adhesive bonding. BSH products are subject to continuous self-monitoring and regular external monitoring.

The adhesive joints are very thin and the adhesive portion of the finished product is less than 1 percent. It must be tested adhesives are used, the ÖNORM EN 301. Are commonly used as polycondensation of melamine resin and phenolic Resorcinharzklebstoffe, also referred to as glues and polyurethane adhesives from the group of polyaddition. Only polyurethane adhesives are formaldehyde-free; the modified melamine resins and phenol - resorcinol resins contain formaldehyde. Be used because of the small joints share and especially since low-formaldehyde adhesives, but their expected ambient air concentrations are well below the limits of formaldehyde Directive.

In practice, modified melamine resins are used for the finger joints normally used for gluing the slats and polyurethane adhesives.

The production takes place in different qualities both optical as well as in static terms.

It is just generally produced standard products. With glulam, however, can also be a variety, also produce mehrachsial curved wood parts.

In order to minimize deformation, the last layer is the other way round affixed, so that the core side shows (right side) to the outside.

Qualities

Static BSH is divided into classes GL24h, GL24c, GL28h, GL28c, GL32h, GL32c, GL36h and GL36c according to the new DIN 1052 ( December 2008). GL stands for " Glued Laminated Timber" = glulam. The following figure indicates the allowable characteristic bending stress in N / mm ². The c and h stands for " combiniertes " or "homogeneous " glulam. Where combines means that in the highly stressed outer regions slats are used with a higher strength class and the slats in the inner regions consist of material of lower quality. Homogeneous Glulam is called when the entire structure of lamellae of the high strength is. The key here is whether Querzugsspannungen shall prevail. If not specified, c (combination ) is considered required.

As to the surface is made between industrial, visual and reading quality. By default, in accordance with VOB / C visual quality to deliver, unless otherwise agreed. Industrial grade is used, when no requests are made to the optical surface condition, such as a concealed mounting.

Assessment

Benefits

Since glulam is made ​​of dried wood and has a multilayer structure, is cracking place to a much lesser extent than solid wood. Thus, glulam beams are ideal for open roof constructions in residential buildings or for other purposes where the inevitable cracking of solid wood is undesirable.

Since the strength of wood, in contrast to that of the steel does not depend on the temperature and the timber slowly burns off in the event of fire from the outside, forming on the surface of a protective carbon layer, the load capacity can be maintained, if appropriate, more than, for example, in steel beams.

Since glulam from pre-sorted and freed from defects wood is made to carrying capacities can be achieved which can not be achieved with solid wood of the same cross-section.

There are larger cross sections are possible than with solid wood, have restrictions on the dimensions of BSH exist only in the size of the planing machines, production rooms or architectural reasons.

The ability to bend the individual plates before gluing, allows the production of responsive support forms. In addition to the straps with simple curvature and double-curved and twisted shapes are possible.

Compared with reinforced and prestressed concrete structures BSH offers at similar price (based on the carrying capacity ) benefits in terms of weight and chemical resistance.

Disadvantages

Disadvantage compared to steel is the lower resistance to changing humidity and the need for greater dimensions. Wood can be destroyed mechanically faster than in accordance with durable steel structures, so that a susceptibility to chipping or abbrasive operations there. Wood, even if it is glued, attacked by pathogens.