What is drywall anchors?
Earthquakes are unavoidable events which may have far reaching effects, from economic impact due to structural failure of impaired service to human casualties. To effectively design our infrastructure for our conditions it is important to fully understand how components of a structure, and the structure itself will behave in the event of an earthquake. The proper design and qualification of the connection mechanisms between structural and non-structural elements is one area where the underlying principles are considerably different when subjected to seismic conditions.
To understand what is happening to a post-installed anchor in concrete when subjected to seismic loads we need to consider what is happening during an earthquake. The ground is subjected to multiaxial cyclic acceleration. By breaking down how this affects the loads, the concrete base material, and by considering dynamic behaviour within the concrete we can build up a realistic design model to ensure safety of design.
Concrete anchors
Seismic design and assessment of post-installed fasteners is now covered in Australian Standard AS 5216:2021 (Design of post-installed and cast-in fastenings in concrete). Earthquake (seismic) actions are considered in the National Construction Code in the design and construction requirements for buildings and structures and design is in accordance with AS 1170.4. Australian Standard for the design of post-installed and cast-in fastenings in concrete, AS 5216 was updated in July 2021. AS 5216:2021 now includes the following additions:
- Design of fastenings under seismic actions
- Design of post-installed reinforcing bar connections
- Design of redundant non-structural fasteners
- Design of anchor channel loaded in shear in the longitudinal direction
Appendix F of AS 5216:2021 outlines the design and pre-qualification requirements for post-installed fasteners under seismic actions. Pre-qualification of fasteners for seismic actions are categorized under seismic performance categories C1 and C2 and determined in accordance with EOTA TR049. Design of fasteners under seismic actions shall be conducted in accordance with AEFAC Technical Note TN-10.
What is seismic category C1 and C2?
Fasteners are evaluated for their performance under two seismic categories C1 and C2.
Performance category C1 - tests fasteners under pulsating tension load and alternating shear load while taking into account the effect of concrete cracking, for C1 a maximum crack width of 0.5mm is considered in the test program.
Performance category C2 – considers fastener reference tests to failure as well as tests under pulsating tension load and alternating shear load while taking into account the effect of concrete cracking, for C2 a maximum crack width of 0.8mm is considered and cycled from 0.1 to 0.8mm. C2 is a severe seismic test program and difficult for many post-installed fasteners to qualify against.
Fasteners seismically qualified under category C1 contain seismic tension and shear loads while fasteners qualified under category C2 contain seismic tension, shear and displacement information. This information is typically included in the fastener ETA (European Technical Assessment) document. Design of post-installed fasteners under seismic conditions in Europe is performed in accordance with EN 1992-4:2018.
Seismic testing summary C1 and C2
