Anchoring bolts are heavy-duty bolts used to connect structural and non-structural elements to concrete. They differ in design, size, shape, materials, and of course, application. Anchoring bolts used to attach structural elements are referred to by several names – hold down bolts, foundation bolts, anchors rods, ground bolts and steel frame anchor rods since they are most commonly used in industrial and construction applications to secure steel frames to concrete pads, or steel column base plates to concrete flooring.
Where are Hold Down Bolts Used?
A hold down bolt finds scores of uses. They are found in anchoring applications in steel-framed buildings where they are required to hold down steel columns to the concrete foundation. Bolts here are usually pre-installed before the pouring of concrete, but post-installed hold down bolts are also common.
Hold down bolts are also used in steel and concrete bridges, in securing structural railings and approach slabs. Swedged bolts are used along with bridge bearings to increase roll-out resistance.
Other uses include fixturing road and railway traffic poles and signs, light poles and water storage tanks. Industrial uses comprise securing heavy industrial equipment and machinery to concrete foundations in wastewater treatment, oil refineries and chemical facilities. In addition, hold bolts are found in substations and high voltage transmission lines, and large-scale solar installations.
Materials and Grades
Hold down bolts are manufactured from a range of materials, to different grades and meet Australian construction standards. Steel and steel alloys are the most common materials, but differ in pressure ratings or grades. Mild steel is used in light-duty anchoring, whereas tempered and treated alloy steel is preferred in heavy-duty application. Treatment includes zinc plating to prevent corrosion, and hot-dip galvanisation for optimal corrosion resistance. Stainless steel bolts are preferred for high pressure and high-temperature settings, in either G304 or G316 grade.
Aluminium, nickel, brass and nylon bolts are only used in limited structural applications.
Hold Down Bolts Types
There are two basic types of hold down bolts when it comes to shape – straight and bent. Each type has several different configurations.
Straight bolts can be fully threaded (all-thread rod), threaded on either end, swedged rod, threaded rebar, and rods with forged hex or square heads. All thread rod is often used in chemical anchoring with epoxy in pre-installed anchors, or with nuts and plates on one or either end in post-installed anchors. Rods with threading on both ends are used with tack weld nuts on both ends, and with plates to increase stability. Bolts with forged heads increase pull out resistance, and with the headed end embedded into the foundation. Swedged bolts have threading above the foundation, but have indentations (or ‘swedges’) pressed into the rod to increase pull out resistance. These are commonly found in securing traffic signs and poles. Threaded rebar is used in securing larger steel structures.
Hold down rods with a bent section can be in an L or J bend, with the bent portion installed in the foundation, and the threading on the opposite end taking a hex nut and washer with or without a plate. The bend ensures increased holding strength in the foundation. Like a straight hold down bolt, these too are available in different grades of steel and coatings. Similar are Eye and U foundation bolts.
How are Hold Down Bolts Made?
A hold-down rod is made by cutting a steel round bar to the desired length either by shearing or saw cutting. The ends are then chamfered to allow for threading. Threading can be along the whole length, at one end, or both ends and to a specified length. Cut threading removes steel at equal (and often coarser) thread pitch, while roll threading involves dies and extrudes excess material to form precise (and fine) thread pitch. Headed bolts are formed when the unthreaded part is induction heated, then placed in an upsetter where a plunger forms the head. Hold down bolts may have a welded nut, or an anchored steel plate in the threaded part and are secured by nuts.
Securing the bolt is also achieved by press forming indentations in swedged bolts to better hold epoxy, concrete and grout. Plates used with foundation bolts are square or round, and often installed on the end installed in the concrete foundation. Anchoring templates can hold multiple hold down rods (depending on the application) with square plates sheared and punched from a flat bar, and round plates laser cut.
The bolts can undergo galvanising for better corrosion resistance in outdoor applications they are often used. First, they are cleaned with a solution of acid to remove impurities, washed in water and then immersed in molten zinc at 450°C. Final inspection of the bolts ensures that excess zinc is removed and they can take nuts. Other treatment processes, like manganese phosphating and cadmium plating increase hardness and resistance to saltwater respectively in harsh and underwater settings.
Hold down bolts are sold in different diameters and lengths. The diameter also states the threading width, with common sizes ranging from M6 (6mm) to M32 (32mm). Customers can also specify the length of the threading, and in bent bolts the leg distance.