Post-installed Anchor Design in New Zealand

Currently, post-installed anchor design in concrete is regulated in New Zealand by NZS 3101:2006 Amd3:2017. NZS 3101 does not however cover concrete filled metal deck applications. For reinforced concrete, NZS 3101 Clause 17.5.5 delegates all post-installed anchor design to EOTA TR 045 “Design of Metal Anchors for Use in Concrete Under Seismic Actions”. In 2018, EOTA TR 045 has been superseded by EN 1992-4:2018 (Eurocode 2, Part 4), therefore, post-installed anchor design in New Zealand shall follow the seismic provisions provided by Eurocode 2, Part 4 (which standard also excludes concrete filled metal deck applications). For seismic design, two seismic performance categories are defined in Eurocode 2, Part 4 for post-installed anchors, C1 and C2.

Post-installed anchor qualification in reinforced concrete

NZS 3101 Clause 17.5.5 requires that post-installed anchors shall pass the prequalification testing stipulated in ETAG 001, Annex E, “Assessment of Metal Anchors Under Seismic Action”. In 2016, ETAG 001, Annex E has been superseded by EOTA TR 049, therefore, post-installed anchor assessment in New Zealand shall follow the simulated seismic testing protocols provided by EOTA TR 049 (which document excludes concrete filled metal deck applications). For seismic assessment, the post-installed anchors are installed into reinforced concrete flat slabs that are pre-cracked according to the instructions given in EOTA TR 049. The crack widths are controlled throughout the complete assessment process. The holes for the post-installed anchors are drilled into the pre-formed cracks and the anchors are installed into these holes. As such, it is achieved that there is a crack running at the axis of the post-installed anchor. For seismic performance category C1 qualification the controlled crack width is w = 0.5 mm, both for tension and shear testing. The load protocols are cyclic decreasing amplitude load patterns, very similarly to low cycle fatigue testing. The crack widths are not changing during the qualification testing. For seismic performance category C2 qualification the controlled crack width depends on the type of testing and can be constant w = 0.5 mm or w = 0.8 mm, or the crack width is cyclically opened and closed in the w = 0 to 0.8 mm range. The load protocols are cyclic increasing amplitude load patterns, that is widely used for simulated seismic testing of structures. In tension load assessment, the first two-thirds of the load cycles are performed at constant w = 0.5 mm crack width and the last one-third of the load cycles are performed at constant w = 0.8 mm crack width. In shear load assessment, the load cycling is performed at constant w = 0.8 mm crack width. In crack cycling assessment, at two different pre-determined tension load levels, the cracks are cyclically opened and closed in the w = 0 to 0.8 mm range- following an increasing amplitude protocol.

Seismic C1 and C2 performance categories in Eurocode

Eurocode 2, Part 4 gives recommendations for safety related applications in which the failure of post-installed anchors may result in collapse or partial collapse of the structure, cause risk to human life or lead to significant economic loss. In this context, the standard covers the design of both structural and non-structural connections. In accordance with Eurocode 2, Part 4, the concrete in the region of the post-installed anchors shall be assumed to be cracked when determining design resistances unless it is demonstrated that the concrete remains uncracked during the seismic event. Post-installed anchors are not allowed to be designed in critical regions of concrete elements where concrete spalling or yielding of the reinforcement might occur during seismic events as e.g., in plastic hinge zones. Displacement of the post-installed anchors shall be also accounted for in the seismic design. In accordance with Eurocode 2, Part 4, the selection of post-installed anchors depends on the seismicity level of the construction site of interest, the Importance Class of the building (that is equivalent to the Importance Level (IL) defined in AS/NZS 1170.0:2002) and, in some cases, the application type of the connection. Eurocode 2, Part 4 describes the seismicity level of a construction site of interest with the parameter ag∙S (see Clauses 3.2.1 and 3.2.2 of EN 1998-1:2004, Eurocode 8, Part 1, for the meaning of ag and S). Eurocode 8, Part 1 defines ag∙S as the spectral acceleration at T=0 period, on the elastic horizontal ground acceleration response spectrum (elastic response spectrum), i.e. Sc(T=0) = ag∙S that is equivalent to the elastic site hazard spectrum C(T) at T=0 period in accordance with NZS 1170.5:2004 Eq. 3.1(1). Eurocode 2, Part 4 sets the limits for the parameter ag∙S for type A ground (see EN 1998–1:2004, 3.2.1), that can be either Class A (strong rock) or Class B (rock) in accordance with NZS 1170.5:2004 clause 3.1.3. If ag∙S > 0.1g then seismic performance category C2 qualified anchors are required for all IL2 buildings and above, for both structural and non-structural connections.

Seismic C1 and C2 performance categories in New Zealand

Based on the above, it can be demonstrated that for NZS 1170.5:2004 class A & B rocks the parameter ag∙S translates to ag∙S =Sc(T=0) ≈ Ch(T=0)∙Z∙R∙N(T,D) in accordance with NZS 1170.5:2004 Eq. 3.1(1).

In New Zealand the minimum value for the elastic site hazard spectrum at T=0 period for class A & B rocks ag∙S ≈ Ch(T=0)∙Z∙R∙N(T,D) is 0.246 g (except for Northland, where 0.189 g) in accordance with NZS 1170.5:2004 Eq. 3.1(1), for IL2 (R=1.0) and N(T,D)=1.0, for 50 years Design working life acc. to AS/NZS 1170.0:2002 Table 3.3. Therefore, in New Zealand, all seismic post-installed anchor design in reinforced concrete requires seismic performance category C2 qualified anchors, for both structural and non-structural connections without exception.

Reach out to your local field engineer or send us an email at NZEngineers@hilti.com if you would like to discuss this in detail.