WOOD IS ALWAYS MOVING:

The natural phenomenon of shrinkage cracking

Fessurazione del legno

 

FOREWORD:

All wood tends to crack during the drying phase due to the loss of its liquids like the lymph or by under pressure treatment: this natural phenomenon is called hygroscopicity and no one can oppose to this.

THE CRACKS:

The term "crack" in the construction world is a symptom of pathology and announces possible adverse events (when you think of cracked concrete or iron) In the case of wood it is a completely different thing, and should therefore be defined as the natural phenomenon of "shrinkage cracking". The shrinkage cracks on beams, planks, posts do not compromise at all the static resistance of the element. The longitudinal separation of the wood fibers which produces the opening of the crack, does not involve significant changes on flexibility or the mechanical strength of the wood. Shrinkage cracking is nothing else than the natural signal indicating that the wood is well dried. Also the durability of the wood, if properly under pressure treated, is not reduced in the presence of shrinkage cracks.

 

legno-fuori-cuore

The sections of wood containing the spinal marrow (the "heart") do open up more often than sections without the marrow, technically called "OUT OF HEART". Most Legnolandia wood articles are made with out of heart wood. Many of them are even made with laminated out of heart wood in ​​square or cylindrical section, the top quality that is available on the market. Even these 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 minimum part of Legnolandia's articles derives from wood with marrow. Some examples are: fences, articles made of thick trunks or for some panel boards where the use of out of heart or laminated wood would absolutely be out of the market.

The poles "with heart" are also used in the base version of the Easy Adventure line, in order to maintain a more favorable price. Please note that these products can also provided with laminated cylindrical wood, of course at a higher price.

 

giochi in legno

Example of an article made ​​from trunks of wood "with heart", subject to shrinkage cracking.

 

AESTHETICS:

The aesthetic aspect definitely counts, but must be always associated with the material that the equipment is made of. A plastic or steel pipe has a pleasant aspect when it is well painted and has a smooth surface, the same analysis can not be done on a wooden element, as this is a hygroscopic material that is subject to repeated status changes depending on the humidity. Shrinkage cracking is a variation of the natural state that should not affect the aesthetic analysis but must justify the choice made ​​beforehand: that of choosing a natural and renewable material by cutting harmful CO2 emissions.

Laminated cylindrical post: excample for applied technology applied to limit shrinkage cracking

 

 

 

 

 

 

SAFETY IN WOODEN PLAYGROUNDS:

The need to reduce the cracks increases in products intended for children such as playground equipment. The safety regulations which are in force UNI-EN 1176 (safety of playground equipment in public areas) does only marginally treat the topic of cracks in the wood. The danger of cracking could exist only for in an improbable but possible entrapment of children's fingers. The rules state that the risk of entrapment of fingers exists between 8 and 25 mm but only if at more than 60cm height from the ground. At lower heights no measuring is made as standing on the ground allows the child to remove the fingers itself.


MAINTENANCE IN CASE OF CRACKS:

The shrinkage cracks are a natural phenomenon that may appear at a later time (for example in the occasion of the first exposure of the wood in a very hot and dry place). In this case, the technology applied in the construction of the product may not be sufficient to avoid natural withdrawl and on the surface of some elements cracks may occur, depending on the humidity of the air.

 

 This event should not be seen as a defect of the product or as a danger for the safety of the equipment. It is a simple natural effect that can be mitigated with some simple steps:

  • 1) If the user does not accept this cracking it is possible to fill it with wood putty. In case of further openings, however, the hold of the putty is not guaranteed. Parts of dried putty may also be unshackled by younger children and used in a dangerous manner. Legnolandia does not recommend filling with plaster or similar materials.
  • 2) The splinters that may occur close to the cracks (in particular on the edges of them) are easily to be eliminated with fine-grained sandpaper. If necessary, adjust with an impregnating primer for wood of appropriate color.
  • 3) The knots which sometimes detach from the wood that has been dried quickly, can be filled in with corresponding wood plugs (false knots), after drilling, plug insertion and sanding.

 


 

Gioco in legno lamellare

Example structure for playground constructed from 100% laminated wood.

 

CLASSIFICATION of shrinkage cracks:

The structural classification of wood, shown in the European technical standards CE, compare the size shrinkage cracks to the size of the cracked element. The example in the picture shows as described: these shrinkage cracks of a few millimeters are ALWAYS ALLOWED and not considered defects in the product but inevitable natural phenomenon of wood that is subject to external agents. In general these are the dimensions that can be observed on cylindrical wooden profiles (the cylindrical section is mostly subject to this phenomenon):

Post with heart Ø 10/14 cm: 

shrinkage cracks of up to 15 mm width non-linear progression and varying over tim.

Post out of heart Ø 10/14 cm:

shrinkage cracks with up to 6 mm width non-linear progression and varying over time.

Post out of heartn laminated Ø 10/14 cm:

shrinkage cracks with a of up to 4 mm width linear progression and varying over time.

 

 

 

CONCLUSIONS:

All wooden structures may sooner or later present some cracks, particularly if exposed to external agents. This does not mean that they are dangerous for users or of a limited duration. Wooden outdoor playground equiment and furniture shall be appreciated ignoring these "small natural defects." Otherwise it is preferable to chose polyethylene or steel articles from our catalogue, which certainly do not have cracks but are different from the harmony of wood in a natural environment. Wood is THE natural building material which is most loved throughout the world, it is always "on the move" and must be accepted with all its strengths and weaknesses.

 

 

 

 

 

 

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