Anchor bolt

Column-to-foundation connection[1]


Anchor bolts are used to connect structural and non-structural elements to the concrete.[2] The connection is made by an assembling of different components such as: anchor bolts (also named fasteners), steel plates , stiffeners. Anchor bolts transfer different types of load: tension forces and shear forces.[3]A connection between structural elements can be represented by steel column attached to reinforced concrete foundation.[4] Whereas, a common case of non-structural element attached to a structural one is represented by the connection between a facade system and a reinforced concrete wall.[5]

Types

Type of anchors[1]

Cast-in-place

A cast-in-place anchor bolt

The simplest – and strongest – form of anchor bolt is cast-in-place, with its embedded end consisting of a standard hexagonal head bolt and washer, 90-bend, or some sort of forged or welded flange (see also Stud welding). The last are used in concrete-steel composite structures as shear connectors.[6] Other uses include anchoring machines to poured concrete floors[7] and buildings to their concrete foundations. Various typically disposable aids, mainly of plastic, are produced to secure and align cast-in-place anchors prior to concrete placement. Moreover their position must also be coordinated with the reinforcement layout.[3] Different types of cast-in-place anchors might be distinguished:[3]

  • Anchor channels: used in precast concrete connections.[8] The channel can be a hot-rolled or a cold-formed steel shape in which a T-shape screw is placed in order to transfer the load to the base material.
  • Headed Stud: consist of a steel plate with headed studs welded on (see also Threaded rod).
  • Threaded sleeves: consist of a tube with an internal thread which is anchored back into the concrete.

For all the type of the cast-in-place anchors, the load-transfer mechanisms is the mechanical interlock[3], i.e. the embedded part of the anchors in concrete transfers an the applied load (axial or shear) via bearing pressure at the contact zone. At failure conditions, the level of bearing pressure can be higher than 10 times the concrete compressive strength, if a pure tension force is transferred.[3] Cast-in-place type anchors are also utilized in masonry applications, placed in wet mortar joints during the laying of brick and cast blocks (CMUs).

Post-installed

Post-installed anchors can be installed in any position of hardened concrete after a drilling operation.[3] A distinction is made according to their principle of operation.

Mechanical Expansion anchors

A wedge anchor

The force-transfer mechanism is based on friction mechanical interlock guaranteed by expansion forces. They can be furtherly divided into two categories:[3]

  • torque controlled: the anchor is inserted into the hole and secured by applying a specified torque to the bolt head or nut with a torque wrench. A particular sub-category of this anchor is called wedge type. As shown in the figure, tightening the bolt results in a wedge being driven up against a sleeve, which expands it and causes it to compress against the material it is being fastened to.
  • displacement controlled: usually consist of an expansion sleeve and a conical expansion plug, whereby the sleeve is internally threaded to accept a threaded element.

Undercut anchors

The force-transfer mechanism is based on mechanical interlock. A special drilling operation allows to create a contact surface between the anchor head and the hole's wall where bearing stresses are exchanged.

Bonded anchors

The force-transfer mechanism is based on bond stresses provided by binding organic materials. Both Ribbed bars and threaded rods can be used and a change of the local bond mechanism can be appreciated experimentally. In ribbed bars the resistance is prevalently due to shear behavior of concrete between the ribs whereas for threaded rods friction prevails.(see also Anchorage in reinforced concrete).[9] Bonded anchors are also referred as adhesive anchors. [10] The anchoring material is an adhesive (also called mortar[3]) usually consisting of epoxy, polyester, or vinylester resins. [1] The performance of this anchor's types in terms of 'load-bearing capacity', especially under tension loads, is strictly related to the cleaning condition of the hole. Experimental results[3] showed that the reduction of the capacity is up to 60%. The same applies also for moisture condition of concrete, for wet concrete the reduction is of 20% using polyester resin. Other issues are represented by high temperature behavior [11] and creep response.[12]

Screw anchors

The force-transfer mechanism is based on concentrated bearing pressure exchange between the screw and concrete through the pitches.

Plastic anchors

Their force-transfer mechanism is similar to mechanical expansion anchors. A torque moment is applied to a screw which is inserted in a plastic sleeve. As the torque is applied the plastic expands the sleeve against the sides of the hole acting as expansion force.

Powder-Actuated anchors

They act transferring the forces via mechanical interlock. This fastening technology is used in steel-to-steel connection, for instance to connect cold-formed profiles. A screw is inserted into the base material via a gas actuated gas gun. The driving energy is usually provided by firing a combustible propellant in powder form.[13] The fastener's insertion provokes the plastic deformation of the base material which accommodates the fastener's head where the force transfer takes place.

Other Languages
dansk: Ankerbolt
Deutsch: Ankerschraube
eesti: Kiilankur
français: Tige d'ancrage
українська: Анкер
Tiếng Việt: Bulong neo