NEW EUROPEAN REGULATIONS FOR THE USE OF ANCHORS IN SEISMIC AREAS
The seismic design requirements for anchors are provided by the EOTA TR045 (until EN 1992-4 publication). This document refers to the seismic assessment of anchors included in the Annex E of ETAG 001, which implies the differentiation of use into seismic performance categories C1 and C2.
Seismic performance category C1 provides anchor capacities only in terms of resistances at ultimate limit state, while seismic performance category C2, the most severe, provides anchor capacities in terms of both resistances at ultimate limit state and displacements at damage limitation state and ultimate limit state. The Table relates the seismic requirements to the seismicity level and building importance class.
|Seismicity levela||Importance Class acc. to EN 1998-1:2004, 4.2.5|
|Very lowb||ag S ≤ 0,05 g||No additional requirement|
|Lowb||0,05 g < ag S ≤ 0,10 g||C1||C1d or C2e||C2|
|> Low||ag S > 0,10 g||C1||C2|
a – The values defining the seismicity levels are may be found in the National Annex of EN 1988-1.
b – Definition according to EN 1998-1:2004, 3.2.1.
c – ag design ground acceleration on Type A ground (EN 1998-1:2004, 3.2.1),
S – Soil factor (see e.g. EN 1998-1:2004, 3.2.2).
d – C1 for Type ‘B’ connections (see TR045 §5.1) for fixings of non-structural elements to structures
e – C2 for Type ‘A’ connections (see TR045 § 5.1) for fixings structural elements to structures
Many European Regions are affected by the hazard of earthquakes occurrence. Seismic shaking can cause danger to human life as to functionality and physical integrity of the built environment.
Seismic hazard map of Europe of events with 10% probability of exceedance in 50 years.
Seismic problem in constructions affects both structural and non-structural components. Up to last decades the main aspect taken into account by civil engineering has been the structural dynamics in order to first avoid the building collapse. This problem has been tackled developing refined design analyses, nowadays included in current regulations, and finding new solutions, such as isolators and dampers. Lately the study has shifted to the response to earthquakes of non-structural elements and building content.
Earthquakes represent one of the most dangerous events which can have hugely detrimental effects on structures. The construction of a building can oppose the horizontal seismic actions with a fragile or ductile behaviour depending on restrain and boundary conditions. Connections between elements are fundamental points of energy dissipation in a general efficacious aseismic behaviour.
Considering recent strong seismic events (L’Aquila 2009, Chile 2010, Christchurch 2011, Tohoku 2011, Emilia 2012) the damage to non-structural components caused huge economic losses and long downtime for the recovery of building functionality.
Multiple aspects affect the seismic behaviour of a structure and thus also the dynamic response of equipment; regional seismicity, proximity to an active fault, local soil conditions and dynamic characteristics of the building are only some of these. Nevertheless, several further and important factors influence only the seismic response of non-structural elements. Among all the location of the element within the building, the dynamic characteristics of the non-structural element and its bracing to the structure as well as its geometrical configuration and its connection system are some of the most relevant.
The overall behaviour of non-structural component can be better understood if the different class of effects, arising during an earthquake, are considered. These groups are listed in the following.
- Inertial Effects – The elements subjected to this phenomenon can be also defined as “acceleration sensitive”. This response can cause overturning or sliding of elements depending on their restraint conditions (e.g. unrestrained cabinets, freestanding bookshelves, racks, emergency power generating equipment).
- Building Distortions – This effect as well as the inter-story drift has a large influence on “displacement sensitive” non-structural components (e.g. windows, internal and external partitions, other items tightly locked into the structure).
- Building Separation – Pounding (impact among buildings deforming in different ways) of closely spaced adjacent buildings may occur during earthquakes. This can cause damages to both acceleration-sensitive and drift-sensitive non-structural elements crossing the separation (e.g. parapets, cornices on the facades, piping, fire sprinkler lines, HVAC ducts, partitions and flooring).
- Non-structural Interaction – Non-structural components may interact when they share the same space such as in a ceiling plenum or pipe chase. These systems may have different shapes, sizes and dynamic characteristics, as well as different bracing requirements, may vibrate differently from one another, thus causing dynamic interaction (e.g. sprinkler distribution lines interacting with ceiling, adjacent pipes, suspended mechanical equipment swings and impacts a NC).
For each class of effects a different solution can be identified to avoid damages or failure of non-structural components and this directly reflects also on the anchoring system to be adopted. For these reasons post-installed anchors should be selected and designed carefully in order to ensure a good and reliable behaviour under seismic actions.
ITW RESEARCH COLLABORATION
The research programme “Seismic Application of Fastening” developed by ITW Construction Products Italy with the Department of Civil, Environmental and Architectural Engineering at University of Padova has dealt with the topic of seismic behaviour of fastening. In particular the first part of the research project has been dedicated to the study of post-installed anchors for non-structural elements through shaking table tests.
The realization of the experimental campaign allowed a knowledge on the dynamic performance of selected products to be considerably deepen. Indeed the most critical and common applications in which anchors are installed were under study. Among all the outcomes the research provided an help for the development of new products as well as investigating new applicative fields uncovered by regulations.
The testing setup and the structural units were designed in order to reproduce the effects induced by an earthquake on non-structural components located inside buildings, such as mechanical or medical equipment. The tests were performed at the Laboratory of Structural Dynamics and Vibration Monitoring of the ENEA Research Centre (Rome). The programme focused on the application of anchors on two widespread base materials, namely concrete elements and masonry infill walls. The used concrete class is C25/30 and the infill walls consisted of masonry panels built with hollow bricks Poroton®. The fastening specimens installed in concrete were tested in both non-cracked and cracked conditions. On the surface of masonry panels a layer of plaster of about 1cm was applied in order to simulate a real condition.