Insights
All wood tends to crack during the drying phase due to the loss of internal liquids composed of sap or autoclave treatment: this natural phenomenon is called hygroscopicity and no one can counteract it.
The term ‘crack’ in the construction world is a symptom of pathology and heralds possible negative events (think of cracked concrete or cracked iron), in the case of wood it is an entirely different matter, it would therefore be appropriate to define the natural phenomenon of shrinkage with the term ‘shrinkage crack’. Shrinkage cracks on beams, planks, posts, do not affect the static strength of the element in the slightest. The longitudinal detachment of the wood fibres, which open up to produce the crack, does not lead to significant changes in either the bending or the shear stress of the element. Shrinkage cracking is nothing more than the natural signal that the wood is well dried. The durability of wood, if properly autoclaved, is also not reduced in the presence of shrinkage cracks.
Wood sections containing pith crack more than sections without pith, which are technically called ‘OUT OF HEART’. Almost all of the wooden elements that make up Legnolandia’s articles are made of out-of-heart wood. Many of them are even made of LAMELLAR WOOD OUT OF HEART, with a square and cylindrical section, the highest quality possible on the market. Even such elements, of the highest quality, are nevertheless subject to cracks of various thicknesses and lengths, insignificant from an aesthetic and functional point of view.
For technical, economic and functional reasons, a small proportion of Legnolandia articles are made from pith wood. Some examples are: fences, articles obtained from large-section logs, some notice boards for which the use of heartwood or lamellar wood would be absolutely out of the market.
Aesthetic appearance certainly matters, but it must always be associated with the material that makes up the object. A plastic or steel pipe has an aesthetically pleasing appearance when it is well painted and has a smooth surface, the same analysis cannot be made on a wooden element which is a hygroscopic material subject to continuous state variations depending on humidity.
Shrinkage cracking is a natural state variation that should not influence the aesthetic analysis but should motivate the choice made a priori: that of having chosen a natural and renewable material while reducing harmful CO2 emissions.
The need to reduce cracking increases in products intended for children such as playground equipment. The current safety standard UNI-EN1176:2017 (Safety of playground equipment in public areas) does not recognise any danger in wood cracks. The danger of cracks could only arise from difficult but possible entrapment of children’s fingers. The standards stipulate that the space at risk of finger entrapment is between 8 and 25 mm, but only if measured at a height above the ground of more than 60 cm. At lower heights, the measurement is omitted because the ground support of the feet allows the child to pull the fingers out of an opening independently.
Shrinkage cracks are natural phenomena that may appear later (e.g. when the wood is first exposed to a very hot and dry environment). In this case, the technology applied in the construction of the product may not be sufficient to avoid natural shrinkage, and cracks may occur on the surface of some elements depending on the humidity of the air.
This event should not be seen as a product defect or as a danger to the safety of the equipment. It is a simple natural effect that can be mitigated with a few simple steps:
The structural classification of wood, indicated in the European CE technical standards, contemplates the analysis of shrinkage cracks, which are ALWAYS PERMITTED by parameterising them according to the size of the cracked element. The example in the photo to the side well represents what is described above: these are shrinkage cracks of the width of a few millimetres, which should not be considered a product defect but an inevitable natural phenomenon of wood subjected to external agents.
Generally speaking, these are the dimensions that can be observed on wooden profiles, taking into consideration the cylindrical section, which is the most susceptible to the phenomenon:
Ø 10/14 cm core post:
shrinkage cracks with a width of up to 15 mm twisted and time-varying trend.
Pile outside heart Ø 10/14 cm:
shrinkage cracks with a width of up to 6 mm twisted and time-varying course.
Ø 10/14 cm lamellar off-core post:
Shrinkage cracks with an amplitude of up to 4 mm twisted and time-varying course. Larger cross-sections result in greater tolerances.
All wooden structures may show some cracks sooner or later, particularly when exposed to external agents, but this does not mean that they are dangerous for users or that they have a limited lifespan. Wooden toys, furniture and tools should be appreciated by ignoring these ‘small natural defects’. Otherwise, it is preferable to opt for the numerous articles in our catalogue made of polyethylene or steel, which certainly do not have cracks, but which approach the naturalness of a well-equipped space in harmony with the environment. Wood is the most natural and beloved building material in the world, it is always ‘in motion’ and must be accepted with merits and flaws.