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UNDERSTANDING ANCHORING SYSTEMS

UNDERSTANDING ANCHORING SYSTEMS

In order for major producers of anchors and anchoring systems respond to the increasing need for their products to satisfy the high demands of the international codes and the design models of CEN EN 1992-4, they must continually develop and improve their products. This has resulted in the development of cosmopolitan products, the use of which can be most effective if the user understands the principles behind them. The move from national standards to international codes has resulted in well-known simplistic designs becoming comprehensive and technologically advanced standards.

It is important to understand the different anchor types so that you can ‘value engineer’ the connection. However, you mustn’t lose sight of the fact that fixings are a very small part of the overall value in any project. While the value can fluctuate subject to 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 neglected. If spending an extra few thousand bucks is going to save a life or prevent extensive corrective works at a later date, then this needs to be considered.

Many people are often confused by the term ‘cracked concrete’, they assume that it somehow refers to broken or damaged concrete when in actuality 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.

When an anchor is installed into concrete, there is an area surrounding the anchor called a cone of influence in which the anchor is affecting and it is affected by. When two anchors are spaced too closely to one another and/or too closely to an edge, the anchor’s cone of influence reduces or becomes interfered with. When this occurs the anchor’s tension and shear capacities, which are obtained from test data, are significantly reduced. Most manufacturers provide reductions for tension and shear capacities for these limitations as these are common occurrences in the field.

As embedment increases, the anchor’s cone of influence increases and there will be an increase in tension and shear capacities. However, embedding an anchor too close to the opposite face of the concrete can lead to spalling damage. A rule of thumb is that an anchor should have a minimum of 12x the diameter of anchor spacing to an adjacent anchor or to any concrete edge. Also, the concrete should have a minimum thickness of 1.5x the depth of embedment.

 

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