Understanding Seismic Design and AS5216:2021 - What Is An Annular Gap?

10 November 2023
Understanding Seismic Design and AS5216:2021 - What Is An Annular Gap?

When anchors are installed within a connection which consists of more than two anchors, there are likely small clearance gaps which is present between the base plate hole and anchor element for tolerance. This gap is called the annular gap within AS5216:2021 and other anchoring standards. The annular gap which is intended for clearance during installation can result in an uneven distribution of loads across the anchors when loaded in shear. 

When anchor connections greater than 2 anchors are loaded in shear, some anchor elements may not be in contact with the edge of the drilled hole within the baseplate and therefore not take up any shear forces. This concentrates all the shear forces to be distributed to s smaller portion of anchors within the connection. As an example, an anchor connection of 6 may be designed to take the shear loads evenly but the shear forces may only be resisted by 2 anchors in the connection.

When these connections are close to an edge, annular gaps can drastically reduce the shear capacity of the connection. Unless selected otherwise, the ICCONS DesignPro software will assume that annular gaps are present for connections larger than 2 anchors as a conservative effort and assume the anchors close to an edge will take all the shear loads. Resulting in a smaller concrete edge capacity than if all the anchors were to take the shear loads evenly.

Annular gaps are also important in seismic design of anchors. As part of the AS5216:2021 design process for seismic actions, anchor connections with annular gaps require a reduction factor of αgap = 0.5 to be applied.

Annular gaps can be filled with high strength epoxy anchors such as our ICCONS BIS-PE Gen 3 and BIS-E to ensure even distribution of loads. These chemicals can be used alongside filling washers to ensure that the anchor is sufficiently rigid, it also ensures the shear loads are distributed evenly on the anchor element.

Want to know more about AS5216:2021 and 100 year design life? Click here!

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