Seismic design: how do you design a seismic-resistant fastening?

In modern construction, especially in seismically active areas, ensuring the safety and stability of structures through the proper use of post-installed anchors in concrete is essential.

In Europe, the design of these fixings under seismic conditions is carried out following Eurocode 2, Part 4 “Design of concrete structures. Design of fastenings for use in concrete.” In the standard, two categories are established for the seismic calculation of fixings: C1 and C2.

This document focuses on these categories C1 and C2, describing their applications and requirements according to geographical location, building type, ground type and structure type.

All anchors must be previously evaluated for operation under seismic actions in one or both of these categories. The corresponding performance levels are specified in the relevant European Technical Assessments (ETAs).

Seismic design: seismic categories C1 and C2

In Europe, the seismic categories for mechanical anchors in concrete are mainly defined according to the seismic actions anticipated and the specific conditions of the project. These categories help to determine design specifications and the selection of appropriate anchors in order to guarantee the strength and durability of structures under seismic loads.

• Category C1: Applies to situations with moderate seismic demands. Anchors must be able to withstand seismic loads that are not extremely high, but still require special considerations to ensure structural integrity.
• Category C2: Applies to situations with serious seismic demands. Anchors in this category must be able to withstand significant seismic loads and therefore require more stringent specifications and rigorous testing to ensure their performance.

Factors influencing seismic classification

Geographical location

In Europe, the geographical location has a significant influence on the seismic classification due to the variability of seismic activity in different regions.

There are specific national seismic risk maps depending on the geographical location of the works to be carried out. This risk is represented as a_g (the design ground acceleration on type A ground). For example:

• Regions of high seismicity: Countries such as Italy, Greece and Turkey have areas of high seismic activity. In these areas, category C2 is frequently required for most applications due to the increased likelihood of severe earthquakes.
• Regions of low or moderate seismicity: Countries like Germany, France and the United Kingdom have areas with a lower seismic activity. Here, category C1 may be enough for many applications, although in specific areas of high seismicity within these countries, category C2 may also be necessary.

Building type

Buildings are classified into different categories according to their importance and use, which influences the seismic anchorage requirements:

• Type I buildings: Buildings with a low operational importance and low risk for people. Category C1 is sufficient.
• Type II buildings: Commercial, residential and office buildings. Category C1 is usually sufficient, although in areas of high seismicity C2 may be required.
• Type III buildings: Public infrastructure such as schools and theatres. Category C2 is preferred due to the need to ensure post-earthquake functionality.
• Type IV buildings: Critical buildings such as nuclear plants, hospitals and emergency operations centres. Category C2 is always required due to the severe consequences of a structural failure.

Ground type

Ground type influences the amplification of seismic waves and therefore the choice of seismic category:

• Rigid ground (A): Causes less amplification of seismic waves. The use of category C1 can be permitted in many situations.
• Medium ground (B and C): Causes a moderate amplification of seismic waves. Depending on the severity of local seismicity, category C1 or C2 may be required.
• Soft ground (D and E): High amplification of seismic waves. Here, the use of category C2 is usually required to ensure structural stability.

A “ground factor” is established for each type of ground (S).

Type of structure

Elements of a structure are classified as structural or non-structural, which determines the requirements for anchors:

• Structural elements: These include essential elements such as columns and beams. They require higher strength and therefore category C2 is generally necessary to ensure stability under seismic loads.
• Non-structural elements: These include elements such as interior partitions and façade systems, whose failure does not compromise the structural stability of the construction. Category C1 may be enough, although in areas of high seismicity, in type IV buildings, category C2 may be required.

Selection of fixing design under seismic categories C1 or C2

Annex C of Eurocode 2, Part 4, indicates the seismic design criterion according to the level of seismicity of the area, the importance of the construction, the ground type and the element to be fixed: