Holding Down Bolt Specification Guide
Holding down (HD) bolts anchor steel columns to concrete foundations. They resist uplift from wind loading and, in moment-resisting bases, transfer bending forces into the substructure. The choice of HD bolt type, size, and embedment depth depends on the frame configuration, base plate detail, and whether the bolts need to be cast in before the concrete pour or fixed afterwards.
This reference covers the three main HD bolt types used in UK steel construction, standard sizes from M20 to M36, embedment depths per BS 7419 and manufacturer data, pull-out capacities, chemical anchor options, and typical base plate combinations per BS EN 1993-1-8.
Types of Holding Down Bolt
There are three main approaches to anchoring steel columns into concrete foundations. Cast-in bolts (J-bolts and headed bolts) are positioned before the concrete is poured, while chemical anchors are drilled and bonded into hardened concrete after the pour.
Bolt Boxes & Cone Bolts
Bolt boxes (sometimes called waxed cardboard bolt boxes or bolt cones) are used during the concrete pour to create a pocket around each cast-in HD bolt. The pocket gives you adjustment room to align the bolts with the base plate holes after the concrete has set. Standard bolt boxes are waxed cardboard tubes, typically 150mm to 200mm diameter, filled with sand or left hollow and grouted after alignment. They are essential on most portal frame foundations because getting bolt positions accurate to within 2-3mm on a building site is extremely difficult without them.
Cone bolts are a variation where the embedded end has a conical anchor head rather than a hook or flat washer plate. The cone shape provides mechanical interlock with the concrete, giving reliable pull-out resistance without relying on bond alone. They are less common than J-bolts or headed bolts in standard portal frame work, but are used where high tensile loads are expected.
Standard HD Bolt Sizes
HD bolts for structural steelwork are typically M20, M24, M30, or M36. The table below shows the key dimensions for these sizes. Grade 4.6 is standard for most pinned bases; grade 8.8 is used where higher tension capacity is needed (moment bases, high wind uplift).
| Bolt Size | Shank Dia. (mm) | Clearance Hole (mm) | Head Across Flats (mm) | Tensile Stress Area (mm²) |
|---|
Embedment Depths by Bolt Type
Embedment depth varies significantly depending on the bolt type. Cast-in headed bolts need less depth than J-bolts because the washer plate provides more reliable mechanical anchorage. Chemical anchors have the shallowest embedment but require the concrete to be fully cured and holes to be properly cleaned.
| Bolt Size | J-Bolt Min. Embedment (mm) | Headed Bolt Min. Embedment (mm) | Chemical Anchor Typical (mm) | Headed Bolt Washer Plate |
|---|
Embedment measured from the face of the foundation to the bottom of the bolt. Actual depth depends on concrete strength, edge distances, and bolt spacing. Always verify with the structural engineer's foundation drawing.
Indicative Pull-Out Capacity (Cast-in Headed Bolts)
The table below shows approximate tension capacities for cast-in headed bolts in C25/30 concrete. These are indicative values for single bolts with no edge effects. In bolt groups, the concrete cone overlaps and individual bolt capacity reduces.
| Bolt Size | Grade 4.6 Tension (kN) | Grade 8.8 Tension (kN) | Notes |
|---|
Values assume single bolt, adequate embedment, no edge effects. Design to BS EN 1992-4 for anchorage in concrete. For bolt groups, reduce capacity for concrete cone overlap.
Chemical Anchor Systems
Chemical anchors (resin anchors) bond threaded rod into drilled holes in hardened concrete. They are the go-to solution for retrofit work, remedial fixes where cast-in bolts are misaligned, or situations where the concrete is poured before the steelwork design is finalised. The two main manufacturers used in UK structural work are Hilti and Fischer, both of which provide free design software for calculating anchor capacities to ETA (European Technical Assessment) standards.
Hilti Systems
| Product | Type | Typical Use |
|---|
Fischer Systems
| Product | Type | Typical Use |
|---|
Indicative Tension Capacities (Hilti HIT-RE 500 V3)
Approximate values for single anchors in C25/30 uncracked concrete, no edge effects. Always use the manufacturer's design software for actual project design.
| Rod Size | Embedment (mm) | Tension Capacity (kN) |
|---|
Cure Times
Resin anchors cannot be loaded until the resin has fully cured. Cure time is heavily temperature-dependent, which is particularly important on UK sites during winter months. The values below are approximate for hybrid resins; pure epoxy resins are slower.
| Temperature | Approximate Cure Time | Notes |
|---|
Installation Requirements
Standard Base Plate & HD Bolt Combinations
The table below shows typical base plate configurations for simple (pinned) column bases per BS EN 1993-1-8. The plate size, thickness, and HD bolt arrangement are matched to the column serial size. For moment-resisting bases, larger plates with more bolts are needed, and these should be designed by the structural engineer to the specific loading requirements.
| Column Size | Base Plate (mm) | Plate Thickness (mm) | HD Bolt Spec | Bolt Pattern |
|---|
Configurations shown are for simple (pinned) bases per BS EN 1993-1-8. Moment-resisting bases require specific design with additional bolts positioned outside the column flanges.
SSC stocks M20 to M36 holding down bolts, bolt boxes, plate washers, and can supply complete base plate kits cut and drilled to your specification. View HD bolt products