Holding-Down Bolt Grades — BS 3692 & BS EN 15048

UK holding-down bolts are typically Grade 4.6, 8.8, or 10.9 to BS 3692 (ISO metric precision hexagon bolts). For base plates, Grade 8.8 is standard.

Bolt Tension Capacity — BS EN 1993-1-8 Table 3.4

The tension capacity of a single holding-down bolt:

P_t = 0.8 × p_t × A_t

Where p_t is the bolt material strength from BS EN 1993-1-8 Table 3.4, and A_t is the tensile stress area.

Bolt Shear Capacity

For base plates with shear keys or friction grip, the bolt shear capacity:

P_s = p_s × A_s

Where p_s is the shear strength (BS EN 1993-1-8 Table 3.4), A_s is the shear area (shank area if shear plane passes through unthreaded portion). Where shear plane passes through threads: use A_t × 0.6.

For Grade 8.8 M24 bolt with shear plane in shank: P_s = 0.6 × 800 × 353 / 1000 = 169.4 kN (threaded) or P_s = 0.6 × 800 × 452 / 1000 = 217 kN (unthreaded shank, 24 mm diameter).

Concrete Pull-Out — BS EN 1992-1-1 / EN 1992-4

BS EN 1993-1-8 references BS EN 1992-1-1 for concrete anchorage. For a cast-in holding-down bolt with anchor plate, the concrete pull-out capacity is:

N_Rd,c = k × f_ctm × π × h_ef² (per EN 1992-4)

Where h_ef is the effective embedment depth. For Grade C30/37 concrete (f_ck = 30 MPa):

f_ctm = 0.3 × f_ck^(2/3) = 0.3 × 30^(2/3) = 2.90 MPa.

For a single M24 bolt with 300 mm embedment (h_ef = 300 mm): N_Rd,c = 0.9 × 2.90 × π × 300² / 1000 = 737 kN (ultimate, approximate — partial safety factors apply).

In practice, the concrete cone capacity is shared among bolt groups. The reduced group capacity with overlapping cones must be checked per EN 1992-4 Clause 6.2.

Worked Example — Column Base Plate

Problem: UK steel column 203x203x52 UC with factored axial compression N_Ed = 850 kN and base moment M_Ed = 45 kN·m. Design the holding-down bolts. Base plate 350 x 350 x 25 mm. Grade C30/37 concrete. S275 base plate.

Step 1 — Bolt Forces:

Eccentricity e = M_Ed / N_Ed = 45 / 850 = 0.053 m = 53 mm.

Base plate width b = 350 mm, effective bearing width under the plate: Assume compression block at one edge. Lever arm between bolt centre and compression centroid ≈ 280 mm.

Tension per bolt pair: T = (M_Ed - N_Ed × a') / z where a' = half plate depth minus compression block depth.

Simplified: T = M_Ed / z - N_Ed / 2 (conservatively ignoring compression recovery). T = 45 / 0.28 - 850 / 2 = 160.7 kN — this negative value indicates no net tension. The full base is in compression. Bolts are nominally stressed.

Step 2 — Re-check with Uplift Scenario:

If wind uplift produces N_Ed = 120 kN tension + M_Ed = 20 kN·m: T = N_Ed / n_b + M_Ed / z = 120 / 4 + 20 / (2 × 0.28) = 30 + 35.7 = 65.7 kN per bolt.

Use Grade 8.8 M24 bolts: P_t = 158.1 kN > 65.7 kN. OK.

Step 3 — Concrete Bearing Check (BS EN 1993-1-8 Clause 6.2.5):

Bearing pressure under base plate: sigma = N_Ed / A_eff. For C30/37 concrete, design bearing strength = 0.6 × f_cu = 0.6 × 37 = 22.2 MPa (BS 8110). sigma = 850 × 10³ / (350 × 350) = 6.94 MPa < 22.2 MPa. OK.

Step 4 — Edge Distance:

Minimum edge distance per BS EN 1993-1-8: Bolt holes for holding-down bolts should be at least 1.5d from the plate edge. For M24 bolts (26 mm clearance hole): 1.5 × 26 = 39 mm. Actual edge distance 60 mm. OK.

Selected: 4 × M24 Grade 8.8 holding-down bolts, 350 × 350 × 25 mm S275 base plate on C30/37 concrete with grout pad.

Combined Shear and Tension � BS EN 1993-1-8 Table 3.4

Holding-down bolts subject to combined shear and tension must satisfy:

(F_v,Ed / F_v,Rd) + (F_t,Ed / (1.4 � F_t,Rd)) <= 1.0

This linear interaction is more conservative than the elliptical interaction used for ordinary bolts. The 1.4 amplification on tension capacity accounts for the reduced ductility of holding-down bolts under combined loading, as the concrete pull-out cone can be compromised by shear cracking.

Worked check: For a Grade 8.8 M30 holding-down bolt with V_f = 40 kN shear and T_f = 95 kN tension: F_v,Rd = 0.6 � 800 � 561 / (1.25 � 1000) = 215.4 kN (threads excluded � grouted shank). F_t,Rd = 0.9 � 800 � 561 / (1.25 � 1000) = 323.1 kN.

Interaction: (40 / 215.4) + (95 / (1.4 � 323.1)) = 0.186 + 0.210 = 0.396. OK.

Where shear is high and tension is moderate, the shear term often governs. For base plates with significant lateral loads, shear keys or shear lugs are preferred to eliminate bolt shear entirely and maximise available tension capacity.

For post-installed anchors in existing concrete, BS 8539 requires manufacturer-specific interaction curves based on ETA (European Technical Assessment) data, which may use a 5/3-power interaction per EN 1992-4 rather than the linear BS EN 1993-1-8 form.

Grout Pads and Shear Keys � UK Practice

Grout pad requirements (BS EN 1993-1-8 Clause 4.13):

• Minimum grout thickness: 25 mm for base plates in building structures. 50 mm preferred for columns over 2000 kN axial load.
• Grout strength: minimum Grade C30/37, matching or exceeding the foundation concrete strength.
• Non-shrink proprietary grout (e.g., SikaGrout, Fosroc Conbextra) must be used. Site-mixed sand/cement grout is not permitted for structural base plates per BS EN 1993-1-8.
• Grout must extend minimum 50 mm beyond the base plate perimeter on all sides to prevent edge spalling under compression.

Shear key design: When base shear exceeds the friction capacity of the grout interface (mu � N_Ed, typically mu = 0.3 for grout-to-steel per BS EN 1993-1-8 Clause 6.2.2), a shear key is required:

• Shear key: typically a hot-rolled flat or UB section welded to the underside of the base plate, cast into a recess in the foundation and grouted in place.
• Bearing stress on concrete: sigma_b = V_Ed / (b_key � d_key) <= 0.6 � f_cd (BS EN 1992-1-1).
• Key embedment into foundation: minimum 150 mm, with reinforcement around the key recess to prevent edge breakout.

For a 350 kN shear on a base plate with N_Ed = 850 kN: friction = 0.3 � 850 = 255 kN < 350 kN. Provide shear key. Use 150�30 mm flat 250 mm long, bearing = 350,000 / (30 � 250) = 46.7 MPa � this exceeds concrete bearing. Increase to 200�40 mm: bearing = 350,000 / (40 � 200) = 43.8 MPa. For C30/37, f_cd = 0.85 � 30 / 1.5 = 17.0 MPa. Even this is too high � the concrete bearing governs.

Alternative: use a shear lug fabricated from a short length of 152x152x23 UC section, cast into a reinforced pocket: bearing area = 152 � 200 = 30,400 mm�. Stress = 350,000 / 30,400 = 11.5 MPa < 17.0 MPa. OK. Provide #10 hairpin bars around the pocket to transfer shear into the foundation.

Related Pages

• UK Base Plate Design — BS EN 1993-1-8
• UK Bolt Capacity Tables — BS EN 1993-1-8
• UK Bolt Hole Sizes — BS EN 1993-1-8 Clearance
• UK Combined Loading — BS EN 1993-1-8 Interaction
• UK Column Design — BS EN 1993-1-8
• UK Connection Design — Overview
• Base Plate & Anchor Calculator — Free Tool

Try Our Free Calculator

Check your UK anchor bolt design with our free Base Plate & Anchor Calculator — full derivation, code clause references, instant results.

Open Base Plate & Anchor Calculator →

Educational reference only. Verify against current SCI P398 and BS EN 1992-4. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent Chartered Engineer verification.