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WIND LOADS ON GREENING ELEMENTS
 
     
     
     

WIND FORCES ACTING ON TREES ON TERRACES OF BUILDINGS
        

      
       Example: 'Bosco verticale' of Stefano Boeri/Mailand  

If trees are integrated into the facades of buildings, they must meet the same safety requirements as building parts, such as technical facade elements, balconies, windbreaks, roofs, etc.  

This means that integrated trees have to withstand the same wind speeds as the building or parts of it. The layout speeds (gust speeds) are based on meteorological observations that have been carried out over decades. They are based on a return period of 50 years (highest gust speeds within 50 years).
 

WIND LOAD ON A TREE AT GROUND LEVEL AND ON A HIGH-RISE TERRACE

The following example shows the significant difference between the wind forces on a tree (here: Pinus Nigra - black pine), planted on level ground in the free, undisturbed flow and in a planter on the terrace of a high-rise building (assumed height: 50 m):

Ground level (without building):
Height of centroid of area (projection of crown) above ground: 4.0 m
Projection area of the crown: 5 m²
Lever arm: 4.0 m
Gust velocity: typ. 27.58 m / s
Amplification factor (no building, free-standing tree): 1.0
Layout velocity: 27.58 m / s
Horizontal wind force: 1.02 kN
Torque at the tree foot: 4.99 kNm
Minimum trunk diameter * (without safety factor): 12.59 cm 		  Terrace of a high-rise building:
Height of centroid of area (projection of crown) above ground: 50.0 m
Projection area of the crown: 5.0 m²
Lever arm 4.0 m
Gust velocity: typ. 39.98 m / s
Amplification factor (wind gain on the building): 1.5
Specific layout velocity: 59.97 m / s
Horizontal wind force: 4.83 kN
Torque at the tree foot: 23.59 kNm
Minimum trunk diameter * (without safety factor): 21.12 cm 		 

 
* Required trunk diameter against trunk breakage is set approximately constant with the height.
  Here, for example, the trunk diameter at a height of 1 m can be used.
 
As you can see, the identical tree (or large shrub) experiences almost five times the wind load! It is therefore absolutely necessary for the installation of planters with trees/large bushes in the facade of high-rise buildings to take into account in advance the local wind forces depending on the tree geometry, the height above ground, the wind zone, the terrain category, the aerodynamic exposure on the building and, if necessary, the neighboring buildings. Only then, a decision can be made whether a planter with tree is in a safe position. Taking into account the weights of tree and of planter/substrate/soil, it can be assessed, whether mechanical safety measures are required.  

 
WIND SHELTER BY TREES, TREE BELTS/BELTS ON MOUNDS 

Wind shelter belts are used to calm surfaces in agriculture and industry. In order to protect larger areas from wind, the shelter belts are often planted on base walls. 		 

 
   
     
© Engineering Buro Prof. Dr.-Ing. Bodo Ruck, Karlsruhe, Germany
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