The design height of the mitigation structure is determined by comparing the design bounce height (action height) with the resistance height of the structure. Various partial factors of safety are applied in order to compensate for uncertainties in the modeling results.
The design bounce height hE,d is based on the characteristic bounce height determined by rockfall analysis results, which are to be carried out by state-of-the-art techniques, and the product of a partial factor of safety according to the consequence class. The 95th fractile of bounce height distribution for the design block generated from simulations is used for the characteristic bounce height (hE,k), measured perpendicular to the slope and taken from the top of the design block. Note: many simulation programs output the center of mass height, which would necessitate the addition of 1/2 the block diameter. The partial factor of safety (ᾱ1) is defined as per the table below.
hE,d = hE,k ∙ ᾱ1
hE,d : design height value
hE,k : characteristic modeled bounce height value that is equal to h95[design block]
ᾱ1 : partial factor of safety as per table.
The design resistance height hR,d is based on the available commercial characteristic height of a structure (hR,k) according to ETAG 27 guidelines or EAD 340059-00-0106 reduced by a partial factor of safety according to the consequence class. Note: hR,k may only be up to 0.5 m taller than the as tested height of the structure for systems tested below 4 m high and up to 1 m taller for systems tested at 4 m or higher. Systems may never be below the as tested height. The partial factor of safety (ᾱ2) is defined as per the table below.
hR,d = hR,k ∕ ᾱ2
hR,d : design resistance height (height of fence)
hR,k : characteristic value of fence height according to product approval (ETAG 27 or EAD 340059-00-0106)
ᾱ2 : partial factor of safety as per table.
For the verification of a fence height the design bounce height must be smaller than or equal to the design resistance height as follows:
hE,d ≤ hR,d
EOTA (2012) ETAG 27: Guideline for European Technical Approval of Falling Rock Kits. Brussels, Belgium.
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