Author: Bryan Carroll, managing director, Masonry Fixing Services Ltd
Modern fastening technology is becoming increasingly important in civil and structural engineering worldwide. Post installed fastening systems, which are installed in hardened concrete or masonry, have found widespread use in construction practice. Although millions of anchors are installed in concrete and masonry elements on construction sites around the world each year, the state of knowledge about this technology in practice can often be very poor.
This lack of knowledge is evident with regard to both the installer who is tasked to install the products and the designer who is tasked with selecting and specifying the products. In this article, I am going to explain some of the fundamental changes with regard to the design of connections between steel and concrete.
Heavy-duty fixing devices have been on the market since the early 1970s. In the early days, the products and the design of their resistance capacities was very basic. In the years that have passed since their introduction, a lot more development has taken place to enable very detailed designs to be carried out using modern sophisticated products and a very detailed design for the base-material capacity. This design method is known as the concrete capacity (CC) method.
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The basic design is the same since the very beginning. The resistance capacity in tension is based on the tensile strength of stress cones in the concrete. The early design method, known as the kappa system, was one where the tensile strength of the concrete was derived from the compressive strength and applied to a basic formula to determine the force required to pull a cone out of the concrete.
The size of these cones was a function of the embedment depth of the anchor. The cone was considered to breakout at circa 45 degrees. This meant that an anchor with 100mm setting depth, for example, would develop a cone with a diameter equal to twice the embedment depth of the anchor, or in this example 200mm. The published design resistance was based on a fully developed cone resisting the design action N. So, in order to achieve the published values for the anchor in tension, the anchors needed to be equal to or greater than twice their embedment depth away from the next anchor, or one times the embedment depth away from a free edge.
To deal with reduced capacities caused by anchors at closer centres, the anchor producers provided simple tables which gave reduction factors that the designer employed when the actual anchor spacing or edge distance was below the published value – in other words, when the theoretical cone at the surface overlapped cones from neighbouring anchors or free edges.
Consideration was also given to the concrete strength, due to the fact that the tensile strength of the cone was calculated from the compressive strength of the concrete. Therefore, the producers also provided a factor to take into account any change between the actual compressive strength and the compressive strength on which the published values are based. The required concrete thickness was generally twice the setting depth and each anchor had a fixed embedment depth.
The published values for edge distance and anchor spacing, while derived from the physical size of a stress cone in tension, were generally applied to anchors in shear as well. The safety factors used were global safety factors in the region of 4. All of this resulted in a simple but conservative design that led to conservative design resistance being used in somewhat restrictive conditions.
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As time progressed and more research data became available, a more refined design approach was developed. The CC method now forms the backbone for all steel to concrete connection design. Among other things, designers wanted to use anchors in concrete elements where the concrete thickness was less than twice the embedment depth of the anchor and anchor manufacturers produced anchors that could be set at more than one standard embedment depth. The concept of ‘cracked’ (tension zone) concrete was investigated and the behaviour of an anchor in a crack was researched in detail.
All of the research data was evaluated and included in the development of a single European design guide. The guidance was published by the European Organisation for Technical Approvals (a division of CEN, the European Committee for Standardisation) and presented in a European Technical Approval Guideline (ETAG 001), ‘Design of metal anchors for use in concrete’.
The new design approach differs greatly from the old method and includes the following additional proofs: