Point Load Test Using 1" PVC

The dome had been constructed some 4 months prior. The mega hubs were all prototype ABS models made to specifications. The dome was fastened together using 1 ½” 8/32 Oval Head stainless steel machine bolts and matching nuts.

Platforms were built of ¾” marine plywood 2’X4’ dimension.  Holes were drilled in the four corners and ½” eyebolts were secured with matching nuts. Chains were attached to the four eyebolts with stainless S-hooks and connected to a heavy duty carabiner, which itself was connected to a heavier chain draped over the hexagonal hubs at the 5^th level from the ground (2 rings out from the center point).

4 platforms were secured and paper boxes weighing 52 lbs. Each one was successively added to the platforms. First, one box was placed on all 4 platforms, then 2 boxes on each, then 3, then 4. 

At the 4^th box of paper, representing 208 lbs (in addition to the chains and platforms), one of the connectors failed. The failure occurred in multiple places around the same hexagonal hub.  This caused that connector to break free of 4 of the 6 struts it was connecting, and the platform with its weight load dropped to the ground.

One consideration that could have led to a premature failure is the nature of the chain wrapped around the hub – this could have put significant pressure on certain points that the chain itself was touching.  The use of a nylon strap might lead to more evenly applied pressure on all surfaces of the hub. 

A follow-up test will be conducted on the mega hub dome when it is rebuilt using 1 ½” schedule 40 PVC struts as opposed to the 1” PVC used in this test.