At the heart of each post is an energy-absorbing element and deformable member that holds the post module perpendicular to its base plate. The post has been designed as a detachable module from the base plate, both having separate waterproof sealing barriers. In some cases of post deployment, the detachable module can be removed and replaced without the need to disturb the base plate and weathering detail, which is highly advantageous on a flat roof installations.
In the event of a fall the forces involved will cause the deformable member, designed to resist static loading, to release the absorbing element reducing the forces back to the roof and critically saving the user(s).
Under static loading conditions the deformable member will start to release the energy-absorbing element at around 4kN. Dynamically, the peak force back to the post is below 6kN.
During the arrest of a falling mass the energy-absorbing element within the post orientates in the same plane as the base plate. Although there will be load exerted in a tensile (Tl) direction the predominance would be for shear (Sl) loading through the fasteners holding the base plate to the roof system.
From the graph below we know that the Peak force, Fp = 6kN for 200kg free falling mass and the attachment point height from base plate, Ap = 50mm after deployment.
The whole sequence from initial impact through to peak force (dt) is typically around 0.5 seconds. After this point the mass will bounce, with this bounce decaying until rest. The data shown was taken from a calibrated 12kN drop test being captured on 1kHz instrumentation.
Using the position of the furthest most fastener from the direction of deployment to the front edge of the base plate, here Df = 410mm we can now determine the tensile load (Tl) and the shear load (Sl) in the fasteners as:
Sl = Peak load / Number of fasteners: 6kN / 8 = 0.75kN per fastener
Tl = Peak load x Attachment point height / Df: 6kN x 50mm / 410mm = 0.73kN
It will generally be the case that the furthest most fasteners from the leading edge will be subjected to a greater tensile load (Tlr), as such we can conclude:
Tlr = Tl / Number of furthest fasteners: 0.73kN / 4 = 0.183kN per fastener
Considerations to remember are the length of the base plate used and the number of fasteners holding the base plate to the roof system.