In construction and engineering projects of any scale where concrete plays a major role, choosing the right fasteners and anchoring solutions is vital for structural stability and safety. Anchoring solutions, ranging from simple mechanical and chemical anchors to larger hold-down rods, are chosen for their strength, dependability, and brisk installation. Where matters get difficult is selecting the right fastener for the job.
This comprehensive guide on hold-down bolts delves deeper into fasteners used in cast-in-place anchoring, typical applications, the major types and material grades, and things to look for when buying.
Hold Down Bolt Basics
Anchoring bolts used to attach structural elements or heavy machinery and equipment to existing or pre-poured foundations go by different names –hold down bolts, foundation bolts, and anchor rods (to name a few). They’re usually found in industrial and construction applications, for instance, when anchoring steel frames in concrete slabs, or column base plates to concrete flooring. The fasteners are inconspicuous in finished builds but contribute to the stability and durability of the attached elements, and provide a lasting connection, ensuring attached elements remain firmly in place.
When embedded to required depths, tightened to desired torque and sourced in the correct types and sizes, anchor bolts prevent movement which could lead to cracking and structure damage. Moreover, they help in even weight distribution, removing the risk of structural failure. This is done by effectively transferring loads to the foundation, and withstanding external stresses.
Major Types
The bolts come in two basic shapes – straight or bent – each with its own intended use and merits. They can be fully threaded and made of all-thread rods, partially threaded, threaded on either end or consist of threaded concrete rebar. Design differences, like bolts with forge hex or square heads, or swedge rods also have their uses. Let’s take a closer look:
L/J-shaped Hold Down Bolts
These are some of the most common types and are used to secure and hold heavy support structures or machinery, such as electrical poles or mobile phone towers. They have threaded ends to maximise load-bearing and can be secured with a hex nut and washer. The bolts can be sourced in varying lengths, diameters and material grades to suit varying anchoring projects. Other common uses include supporting beams, ledges and plates, attaching signage, and wall embedding when supporting load-bearing and non-load-bearing walls.
Threaded Rods with Plate Anchors
Plate-type anchor bolts are threaded at either end, with one end consisting of a plate washer, either welded to the bolt or with a tack-welded nut inside the concrete. These types of hold down bolts are often used to anchor structures to foundations, such as traffic sign poles, or load-bearing elements like columns.
Hex/Square Head Bolts
These have heavy-duty hex or square heads at one end, enabling the bolt to be permanently embedded into the foundation. They’re frequently used in securing steel columns, attaching light poles, and other non-structural elements, including heavy machinery.
Swedge Bolts
Anchors and rods with indentations in the shanks go by the name of swedge bolts. They’re mainly used in applications with limited footing space for embedding, usually highway work, and connecting piers or girders. The indentations are pressed during production and can cover shafts on multiple sides to prevent pull-out resistance and ensure stability.
Choosing What’s Right for Your Application
Since these fasteners are faced with high load-bearing and external forces, they need to be of the correct size, optioned in materials with high tensile strength, and installed to the specified torque to work as intended. Choosing the right hold-down bolt is based on the specifics of the application – including what’s being anchored to the concrete, at what depth, embedment and material requirements, and correct alignment.
Lengths and Diameters
Hold-down bolts are some of the largest fasteners you’ll encounter. Sizes for Australian purposes are stated in metric units. Overall lengths can reach over 800mm, and width diameters (and that of the threading) 36mm or more. Threading length is also specified and for bent L and J-bolts, as is the leg distance, to ensure higher pull-out resistance. Size foundation bolts according to structural needs that ensure maximum stability and anchoring strength.
Materials, Coatings and Standards
Bolts must comply with Australian Standard AS 1252:1996, ensuring dimensional requirements and nut and washer spacing, and AS 4291.1: 2015 for material requirements. Both are based on international ISO standards for metric fasteners.
The fastener material is graded. For hold-down bolts, this starts with 4.6 steel – a mild, low-carbon steel with tensile strengths averaging 400 MPa (megapascals) and yield strengths of 240 MPa. This is suitable for non-structural anchoring applications. Move up to 8.8 steel, with medium carbon content and tensile strength of 800 MPa for structural anchoring or 10.9-grade steel with 1000 Mpa for heavy-duty industrial and construction purposes.
Other steel grades are used for specific applications. 5.8 low-carbon steel, for instance, is preferred where ductility and resistance to deformation under higher stress levels (such as heat fluctuations) are basic requirements.
To ensure bolts are protected, specifically against corrosion and wear, source them with the correct coatings. Zinc-plating, galvanising, and black passivation are the most common coating procedures. Galvanised bolts have higher corrosion resistance, making them suitable for outdoor uses, while the addition of black oxides also increases overall strength and elasticity, leading to lower wear.
Installation
Stability, weight and load distribution and overall structural integrity depend on correct alignment and installation. Longer bolts can be tensioned to ensure they protrude to the desired embedment depth and distribute loads over a larger area. This reduces the stresses on the fasteners and the concrete, leading to a more stable connection.
To the same end, ensure the correct use of nuts. To provide optimal load transfer, use welded or embedded plates when anchoring non-structural elements (machine/equipment frames and bases). Each application requires choosing anchor bolts of the correct length and diameter, anchoring them to the prescribed depth and torque, and using terminations where feasible for the highest durability and stability.