How to Determine the Load Capacity of Tree Branches

Trees have developed characteristics that permit them to bear heavy loads, but the load-bearing capacity of a limb is compromised by decay and cracking throughout the tree. Professionals, such as arborists, rely on a visual inspection of the roots, stem and branches, looking for evidence of structural weakness, before applying the weight of a person and equipment to the tree. External defects signal that the tree may be damaged internally, and may fail under a load. Assessing the potential for failure is a matter of considering other factors as well, such as the overall structural condition of the tree.

Binoculars
Mallet

Instructions

- 1
  Check the ground around the roots of the tree for cracks in the soil, and signs the flare of the stem -- trunk -- is raised from its position, indicating tipping and instability. Look for severed roots and root decay around the tree, within a radius of 1 to 1 1/2 times the diameter of the stem.
- 2
  Examine the base of the tree for signs of decay fungi, especially fruiting bodies at the base of the stem. Look at the condition of the crown of the tree, and note any poor or wilted appearance.
- 3
  Study the stem of the tree, looking for dead bark, growth depressions, crack formations, and fruiting bodies of decay organisms. Note the formation of wood tissue around the opening of a cavity, as a sign that loss of strength has been partly restored.
- 4
  Take note of fresh cracks on the stem -- they're more hazardous than older ones showing the growth of wood over the wound. Look for a radial crack that spans from one side of the stem to the other, indicating reduced load capacity.
- 5
  Use the binoculars to scan the wood for evidence of woodpeckers or wood-boring insects. Check for cracks in the junction between two branches -- the crotch -- caused by failure under a load. Examine branches for other signs of mechanical failure under a load that might be caused by decay, such as split forks and cracks.
- 6
  Look for branches bending upwards or downwards, dead wood, signs of old branches breaking off, and healthy branches attached to a trunk that's rotted or cracked. Tap branches with thin-shelled bark with the mallet -- a dull, hollow sound indicates structural weakness.
- 7
  Consider the initial strength of the tree and its form and development, to evaluate how much strength is lost in the defects. Take into account signs of compensating growth and wound-wood, or repair tissue.
- 8
  Review the tree species for general characteristics and reputation for load-bearing capacity or structural weakness. Consider the position of the tree and the effect on its strength -- a tall tree on the edge of a clearing may be more susceptible to failure.

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