How Strong are these Bolts?

how to test deflection

The deflection is one of the most important measurements of your tree house. Andrew, one of our builders, shows you how to do it.

Making sure that the bolts are strong enough to match the design load of your treehouse is one of the most important safety measures you can take when constructing your treehouse. There is no set amount of weight a bolt can hold, as the type of tree, the bending moment, bolt placement, and weather, all affect its deflection. This is our method for testing the strength of our treehouse attachment bolts.

testing deflection

  1. First, we place our hydraulic ram under our TAB to ensure it is plum, and that perpendicular force is being applied to the bolt, preventing slippage.
  2. Then, we make sure the dial indicator is placed securely on the tree, and make sure it is directly in line with our hydraulic ram.
  3. Next, we zero all our measuring devices before applying ANY pressure to the bolt.
  4. Then, we may apply pressure by pumping the jack on the hydraulic ram and watch as the reading changes!
  5. When the force gauge reaches our desired pressure, we take the reading of the dial indicator to determine the amount of deflection at that distance from the tree. (We also take into consideration that when setting up, the further out on the bolt that we apply force, the more the bolt will deflect. We adjust our design accordingly before installing the bolt.)
  6. Lastly, after we release the pressure on the hydraulic ram, we wait a minute and take a reading of the permanent deflection where the tree has been deformed and will not return to its pre-test state.


What does all this mean? The more load you place on any bolt, the more likely it is to crush the wood fibers beneath the bolt. We are testing the crushing strength of the tree.

Although trees are resilient, we test the initial and permanent deflection. This is because trees are elastic, and they will support a temporary load initially, and when that load is removed, it will spring back to somewhere near the deflection from normal loads, showing us the permanent deflection.

We test for the permanent deflection because we want to avoid stressing a bolt to failure. Failure occurs when there is a permanent crushing of the wood fibers beneath a bolt. We feel that if we load a tree within its elastic limits, the tree will spring back to where it was.

What are these limits, you ask? In varying circumstances, our bolts have tested between 500 and 15,000 pounds. This may not be the same range for you, as it depends on your individual situation, and that’s why you have to test them.

Those are the basics, but feel free to contact us if you have any questions!

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