Tension splice for 250x16 and 250x8 flats

Data: factored tension 400 kN; main flats 250×16 and 250×8; two cover plates 6 mm each ($2\times6=12>8$ mm); M20 HSFG bolts class 8.8 ($f_{ub}=800$ N/mm²); $d_0=22$ mm; $f_y=250$, $f_u=410$ N/mm². Bolts are in double shear ($n_e=2$). The thinner flat (8 mm) governs the plate checks. The flats differ in thickness by $16-8=8$ mm, so an 8 mm packing plate is placed on one face of the thinner flat to let the cover plates bear evenly.

Plate checks (common)

• Gross section of 8 mm flat: $T_{dg}=\dfrac{250\times8\times250}{1.10}=454.5$ kN $>400$ ✓
• Net section (2 bolts in a line): $T_{dn}=\dfrac{0.9(250-2\times22)\times8\times410}{1.25}=486.5$ kN $>400$ ✓
• Covers: $2\times6\times250$: $T_{dg}=681.8$ kN ✓

(i) No slip at ultimate load (slip-critical, $\gamma_{mf}=1.25$)

• $F_o=0.7f_{ub}A_{nb}=0.7\times800\times245\times10^{-3}=137.2$ kN
• $V_{nsf}=\dfrac{\mu_fn_eK_hF_o}{\gamma_{mf}}=\dfrac{0.48\times2\times1.0\times137.2}{1.25}=105.4$ kN per bolt ($\mu_f=0.48$, sand-blasted)
• $n=\dfrac{400}{105.4}=3.8$ → 4 bolts on each side (2 rows of 2), capacity $421.5$ kN.

(ii) Slip permitted at ultimate load (bearing type)

• Shear (double shear, one plane in thread): $V_{dsb}=\dfrac{800}{\sqrt3\times1.25}(245+314)\times10^{-3}=206.6$ kN; packing thicker than 6 mm: $\beta_{pkg}=1-0.0125\times8=0.90$ → $185.9$ kN
• Bearing on 8 mm flat ($e=40$, $p=60$): $k_b=0.606$; $V_{dpb}=\dfrac{2.5k_bdtf_u}{1.25}=79.5$ kN → bolt value $79.5$ kN
• $n=\dfrac{400}{79.5}=5.03$ → 6 bolts on each side (2 rows of 3), capacity $477.1$ kN.
• On the 16 mm flat side the bolts bear on the two cover plates (12 mm): $V_{dpb}=119.3$ kN, so $400/119.3=3.35$ → 4 bolts would be enough there; 6 bolts are used on both sides for symmetry.

Detailing

Pitch 60 mm ($\ge2.5d$), gauge 100 mm (edge distance 75 mm), end distance 40 mm; cover plates 250 mm wide, about 290 mm long in case (i) and 410 mm long in case (ii); 8 mm packing under one cover plate over the length of the thinner flat. Surface preparation (clean, sand-blasted faying surfaces) is required in case (i).

Answer: (i) 4 bolts M20 (8.8) on each side; (ii) 6 bolts M20 (8.8) on each side; 8 mm packing on the thinner flat.