Author: Bryan Carroll, technical director, Masonry Fixing Services Ltd
In order that the major producers of anchors and anchoring systems respond to the increasing requirement for their products to satisfy the high demands of the Eurocodes and the design models of CEN EN 1992-4, they must constantly develop and improve their products and design aids. This has led to the development of sophisticated products and design software, the use of which can be most effective if the user understands the principles behind them. As with many aspects of design, the move from national standards to Eurocodes has resulted in familiar simplistic designs becoming comprehensive and technologically advanced standards. The design and use of anchors is no exception to this.
As a designer, it is important to understand the different anchor types so that you can ‘value engineer’ the connection. You must, however, not lose sight of the fact that fixings are a very small part of the overall value in any project. While the value can vary depending on the nature of the project, recent research suggests that all fixings on a project equate to about 0.5% of the overall cost of the project.
For this very reason, fixings are often treated in accordance with their monetary value and their value with regard to safety and structural integrity is often overlooked. If spending an extra few thousand euro is going to save a life or prevent extensive remedial works at a later date, then this needs to be considered.
Some of the main changes that have come about with anchors are that anchors must now function in either cracked or non-cracked concrete, chemical injection resin can now be installed to a depth of 20 times the bar diameter, fire and seismic design has been developed and chemically anchored post installed rebar can be designed to EC2.
Before I explain the different anchor types and how they function, I would like to mention the concept of cracked and non-cracked concrete. Many people I speak to in my working day are confused by the term ‘cracked concrete’, they think somehow it refers to broken or damaged concrete when in fact it refers to cracking in the tensile zone of concrete due to loading. These cracks open and close as the building member is loaded and unloaded.
Anchors located in these cracks have to be able to cope with this opening and closing by exhibiting a function commonly known as ‘follow-up expansion’ in the case of expansion anchors, or by having higher bond strengths in the case of bonded anchors. When cracked concrete is considered, the design calculation for concrete capacity will return a lower resistance capacity if compared to a design resistance for anchors in non-cracked concrete. The typical difference is 1.4.