Requirements for glued solid timber constructions

For all materials used in construction, individual manufacturing, performance and quality requirements are set. To ensure the quality of a GSTC product, the company must adhere to all requirements of construction manufacturing and raw materials, end-product and quality control determined by the standard LVS EN 14080 "Timber structures – Glued laminated timber – Requirements".

In accordance with the standard LVS EN 1194, "Timber structures – Glued laminated timber – Strength classes and determination of characteristic values", GSTC is dividend into four strength classes, starting from the weakest: 1st class – GL 24c; 2nd class – GL 28c; 3rd class – GL 32c and 4th class – GL 36c.

Testing of GSTC static bending strength can be done using special devices (in the image above – a two point static bending strength testing device).

Similarly to GSTC, several strength classes have been set for saw-timber. In accordance with LVS EN 338 "Structural timber – Strength classes", depending on the nominal strength in static bending, it is dividend into 12 classes (C14 to C50) for softwood and 6 classes (D30 to D70) for hardwood. Virtually all saw-timber of softwood grown in Latvia under normal conditions, without substantial faults, is of classes C14 – C24, however it is possible to manufacture GSTC of any strength class.

GSTC is divided into two types:

1. Homogenous GSTC – constructions that are made by bonding lamellas of similar strength, of single or several wood species.
2. Combined GSTC – constructions that are made by bonding lamellas of different strength, of single or several wood species.

The symbol after the GSTC strength class indicates for what type of construction materials the respective requirements are set; e.g., GL 32h is homogenous GSTC with nominal strength of 32 N*mm-2.

For the combined type GSTC, at least 2 higher strength class lamellas are placed in the tension or compression zones of the construction – in the outer edges – and lower-quality materials are used in the middle. Such manipulations do not affect the construction’s bearing capacity in static bending, and allow utilizing lower-quality materials more effectively.

The conformity of raw materials (lamellas) and GSTC to these strength classes is tested by special devices. It is done by the manufacturer or another competent authority. For testing of lamellas and GSTC, the standard LVS EN 408 is used (in the image – solid timber constructions, bound in the wide side, using lamellas of similar species and strength).

For GSTC quality control, the company utilizes the standard LVS EN 385:2002 "Finger jointed structural timber – Performance requirements and minimum production requirements", which determines the requirements for fingerjointed connections (in the image below – a fingerjointed connection for manufacturing of a fingerjointed lamella), as well as the minimum requirements for glued finger-joint milling and bonding.

The Standard determines that fingerjointed connections are classified based on static bending tests on the narrow side. This classification is determined by the lamella manufacturer, as self-declaration, showing bending strength value or class for the corresponding batch of products. If it is required to prove the strength of a finger-jointed connection to the purchaser of fingerjointed constructions, the company can do it by carrying out appropriate tests in accordance with the standard LVS EN 408:2003 "Timber structures – Structural timber and glued laminated timber – Determination of some physical and mechanical properties". In Latvia, classification of fingerjointed lamellas by static bending strength is not being done, but such classification often exists in countries to which these materials or GSTC are being exported. If GSTC is being exported to Germany, and if the client requests it, the fingerjointed lamellas are classified by the standard DIN 68140 – 1. Part 1 of this Standard determines the required fingerjoint size, possible fingerjoint milling directions, technologic requirements of pressing, applicable adhesives, quality of raw materials, nominal strength of finger-jointed connections, as well as quality control.

Sorting lamellas by the classification determined in DIN 68140 – 1, it is possible to choose the supplier in accordance with the required lamella usage. If the company manufactures GSTC of lamellas, which are made by bonding the wide sides, it is possible to estimate the construction strength class. According to DIN 68140 – 1 p. 5.4, lamellas by nominal bending strength are classified in compliance with the strength classes determined by DIN 4074-1 (see table).