Stay compliant with Eurocode 2: always consider cracked concrete when designing concrete anchor fastenings

The new Eurocode 2 Part 4 (EN 1992-4) for the design of fastenings for use in concrete has already been published in many EU countries. The long-awaited introduction of the standard, which sees anchor design in Europe being promoted from guideline (ETAG 001 Annex C) to code, is expected to profoundly shape the way the industry approaches anchor base plate applications, undoubtedly further encouraging design engineers to approach anchor specification with the quality it deserves.

When it comes to safety in concrete anchoring, the base material condition with regard to cracks plays a critical role, and here we see two provisions (4.5 on project specification and 4.7 on the determination of concrete condition) in the new part of Eurocode 2 that address this topic:

In the rest of this article, we’ll share some helpful insights on how to determine this base material condition, why you should be very cautious when considering uncracked concrete, how to (very) easily run a calculation determining the presence of cracks and how to select the right anchor systems.

Should I consider my concrete cracked or uncracked for anchor fastening design?

EN 1992-4 provides the following verification formula for in provision 4.7 (2): 

Under this condition, the stress in the concrete induced by external loads must be in constant compression status. For applications on a compressed wall, pre-stressed concrete may be considered as uncracked, but this must be checked carefully. Moreover, as per EN 1992-4 9.2.2 and D.1 (2), fasteners under seismic conditions or exposed to fire should have a European Technical Product Specification for use in cracked concrete. To establish the base material as uncracked concrete under all circumstances is obviously quite cumbersome, so to stay compliant with the Eurocode we recommend always considering cracked concrete for your concrete anchor fastening design.

But… why is cracked concrete so important for concrete anchor fastening design?

Cracks are fairly common in reinforced concrete structures, which is why reinforcement is important in the first place. If we look in more detail, we realize the crack occurring in the reinforced concrete structure will most likely intersect with the rebar, which will start to offer its tensile resistance to the system once there is a crack.

Cracks are common and have no effect on a concrete beam’s load resistance.

Conversely, the anchor fastener will most likely be in the position of the crack. As a result, the stress distribution inside the concrete will be changed, and resistance will be lower. 

Anchors tend to become dislodged more easily when cracks occur

if they are not qualified for this base material condition.

According to EN 1992-4, concrete cone resistance is 30% lower than in uncracked concrete. More importantly, anchor behavior may be unpredictable without prequalification: bonded fasteners may lose bonding resistance, for example, and mechanical anchors designed for uncracked concrete may become dislodged under very low tensile loads.

Cracks change the stress distribution inside the concrete

when compared with an uncracked condition.

Pull-out resistance will be also influenced by cracks.

How to design anchors in cracked concrete conditions

Simply select cracked concrete under the base material label in our software! PROFIS Engineering will automatically display only those anchors suitable for cracked concrete and consider the cracked concrete design requirements in your calculation. If you haven’t used PROFIS Engineering before, check out how the new software provides precise verification of all the components of a joint according to Eurocodes and Eurocode-based engineering principles, combining anchor design provisions with a powerful CBFEM finite element calculation for the design of the base plate, welds, stiffeners, profile and concrete basement, and helping you assess the rigidity of the base plate.

Crack-proof anchor design in PROFIS Engineering

can be easily performed by selecting the respective concrete condition.

Which anchor to use

When design cracked concrete, we can stay compliant with EN 1992-4, but it is more demanding on anchor resistance. Hilti offers a wide range of anchors that perform extremely well in cracked conditions (e.g. HST3, HIT-HY 200-R V3). Generally, you can easily check in the ETA document of the anchor system whether the anchor is suitable for cracked concrete (and, of course, don’t forget to run the design accordingly).

If you have always designed base plate connections following the ETAG Annex C calculation method and want to know more about the major changes introduced with EN 1992-4, you might find the following article helpful. Finally, if you want to know more about cracked concrete from a physical perspective, find out more here.