Anchors & Anchor Bolts
The connection of steel members to concrete is a common structural feature with applications in both highway and building construction. A typical steel-to-concrete connection includes the following: a steel attachment consisting of a base plate welded to the attached member, the anchor bolts that actually do the connecting, and an embedment of the anchors into the concrete.
Concrete Anchor Bolt
1. Concrete anchors are an essential design component for fastening structural items to reinforced concrete or masonry. Anchor systems are typically used to connect two parts of a structure or to connect a non-structural item to the structure. There are two types of anchors: Post-installed and Cast-in-place anchors.
1.1 Cast-in-place anchors are placed in position before concrete is cast. A cast-in-place anchor can be a headed bolt of standard structural steel, placed with its head in the concrete. It can also be a standard threaded rod and a hexagonal nut, with the nut end embedded in concrete. Finally, it can be a bar bent at one end and threaded at the other end, with the bent end placed in concrete. A headed cast-in-place anchor depends on a mechanical interlock at the bolt head for load transfer. Some bonds may also exist between the anchor shank and surrounding concrete. Cast-in anchors also include hooked bolts (J- or L-bolt), and cast-in-place anchors (such as inserts).
1.2 Post-installed anchors are installed in existing concrete or masonry structures. They are widely used in repair and strengthening work, as well as in new construction, due to advances in drilling technology, and to the flexibility of installation that they offer. There are many different types of post-installed anchors, classified according to their load-transfer mechanisms. Post-installed anchors include expansion anchors, undercut anchors, screws, and adhesive anchors. There are two types of post-installed anchors, mechanical and adhesive.
1.2.1 Mechanical post-installed anchors are inserted into pre-drilled holes and then expand once in the concrete.
220.127.116.11 Expansion anchors – An expansion anchor consists of an anchor shank with a conical wedge and expansion element at the bottom end. The spreading element is expanded by the conical wedge during installation and throughout the life of the anchor. The spreading element is forced against the concrete wall of the hole as the wedge is pulled by tension on the anchor shank. The external load is transferred by the frictional resistance from the conical wedge to the spreading element, and from the spreading element to the surrounding concrete. Depending on the relative diameters of the bolt and the drilled hole, expansion anchors are classified as either bolt-type or sleeve-type anchors. For a bolt-type anchor, the nominal diameter of the drilled hole equals that of the anchor bolt. For a sleeve-type anchor, the nominal diameter of the drilled hole equals that of the sleeve encasing the bolt. MASON wedge anchor is the most common bolt-type anchor.
18.104.22.168 Undercut anchors – An undercut anchor is installed in a hole in the base material that is locally widened (undercut). The undercut hole accommodates the expansion elements of the anchor, expanded during installation. Undercut anchors mainly rely on bearing to transfer tension load.
1.2.2 Adhesive post-installed anchors are attached to the concrete using some type of adhesive product to bond the anchor to the concrete surface. Steel elements for adhesive anchors include threaded rods, deformed reinforcing bars, or internally threaded steel sleeves with external deformations.
2. Definition of Embedment Depth – Anchors are commonly identified by a nominal embedment depth, used primarily to indicate the required hole depth. The effective embedment depth of an anchor is the distance between the concrete surface and the bearing portion of the anchor head.
- Base Materials – The concrete structures shall be of normal weight concrete with grades ranging from C20/25 to C50/60 (i.e. characteristic cube strength of concrete ranging from 25N/mm2 to 60N/mm2). For other concrete grades, depending on the type of anchor bolts, the base concrete may be cracked or non-cracked. Cracks exist under service load conditions and in the tension zone of concrete and will affect the resistance of the anchor bolt. Non-cracked concrete may be assumed where the tensile stress in the concrete is smaller than the tensile strength of the concrete and there will be no cracking occurring (i.e compression zone). As a conservative approach, cracked concrete shall be assumed for anchor bolt design if the condition of concrete is not known.
Other base material types are also in common practice.
Lightweight concrete, Autoclaved Aerated Concrete AAC, Aerated Lightweight Concrete ALC, Hollow Concrete, Masonry, Brickwall, and Drywall, etc. The characteristic resistances of anchor bolts in the above base materials shall be determined by laboratory tests/ manufacturer in-house tests or based on engineering judgments. It is advised to consult the MASON Technical Team for the design of anchor bolts in the above base materials.
- Anchor Material and Corrosion – Anchor bolts are often used in various applications, from dry indoor conditions to harsh industrial conditions where additional protection against steel corrosion is needed. MASON Anchor bolts are available with alternative protective coatings. Standard coating options and their corrosion resistance for MASON anchor bolts are described below.
4.1 Zinc plated anchor bolts provide the least rust resistance and are the least costly but are not recommended for outdoor use. These anchors are excellent for indoor use or in a location where moisture is not present.
4.2 Hot-dipped galvanized anchor bolts provide a medium level of corrosion resistance. They are also suitable for use outdoors or in humid environments.
4.3 Stainless steel anchor bolts in the 304 and 305 designations provide excellent corrosion resistance and work well in most corrosive environments. The 316 stainless steel provides the greatest corrosion resistance in most environments.
- Connection Terminology – Attachments (structural or mechanical elements) that are attached into concrete (or masonry) structures using MASON anchors can be subject to various types of loading. Loads on the attachments are transferred into the base concrete through anchors as concentrated loads, by friction, mechanical interlock, bond, or a combination of these mechanisms. Many types of anchors are currently used. The load-transfer mechanisms of anchors determine their performance characteristics.
- Anchor Design – Anchors in concrete are typically designed for Tension and Shear. Tension refers to when stress forces are applied to a surface using right angles, or perpendicular to the surface. Shear stress is when forces are applied parallel to the surface. Both tensile and shear failure has the potential to happen if the force exceeds the amount that the concrete is rated to handle.
Mostly, MASON concrete anchor designs were covered by ETAG 001 developed by EOTA (European Organisation for Technical Assessment), including 2 key annexes – C and E. For the benefit of the design engineer, all these design guidelines are now merged under the new design document EN 1992-4 (Design of fastenings for use in concrete), which is part 4 of the Eurocode 2 (Design of concrete structures). EN 1992-4 describes the design of various fastening systems in one document for Cast-in fasteners, Post-installed mechanical fasteners, and Post-installed bonded fasteners. Therefore, designers can use the design provisions according to EN 1992-4 with corresponding MASON Product ETAs for the verification of fastening systems.
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 close 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 facet of the concrete can lead to spalling damage. A rule of thumb is that an anchor should generally have a minimum of 12x the diameter of anchor-spacing to an adjacent anchor or any concrete edge (check with manufacturer specifications for your use for the actual values to be used). Also, the concrete should have a minimum thickness of 1.5x the depth of embedment of the anchor.
Depending on the type of anchor, the strength of the anchor steel, the strength of the surrounding concrete embedment, and sometimes also on the condition of the drilled hole during installation, an anchor can exhibit different failure modes, each with a corresponding failure capacity.
The failure modes of anchor bolt under tension forces include Steel failure, Pull-out failure, Concrete cone failure, Splitting failure. The failure modes of mechanical anchor under shear forces include Steel failure, Concrete edge failure, Concrete pry-out failure. It is important to predict the failure mode of an anchor bolt or a bolt group which governs the resistance as it depends on a couple of factors such as the magnitude and direction of forces, anchor bolt grade, concrete condition and grade, embedment depth, edge distance, bolt spacing, etc. Therefore, it is necessary to calculate the resistance of each failure mode. MASON will provide all the values of design resistance under different failure modes of a single anchor bolt and design guidelines in a design manual. Design engineers only need to follow the design manual to calculate the ultimate design resistance of an anchor bolt group. However, very often, these design guidelines are simplified and only applicable to simple bolt configurations such as double bolts.
In various types of construction, it is common to attach mechanical and structural components to structures. This is accomplished using embedded anchors, through which tension and shear forces are transferred into the base concrete. To safely and reasonably design such connections, it is important to use high-quality materials from a reputable source to ensure various combinations of loading and conditions, and the effects on anchor behavior caused by different conditions of the base concrete. Fasten Enterprises provides a reliable fixing system with high-quality anchor materials complied with ETA European Design Code and Material Specification Standard along with reliable and well-proven technical support and services.