Code-check of preloaded bolts according to Indian standards

Slip resistance

Slip resistance of preloaded bolt is checked according to IS 800, Cl. 10.4.3:

$V_{sf} \le V_{dsf}$

where:

• $V_{dsf} = V_{nsf} / \gamma_{mf}$ – design shear capacity of a bolt as governed by slip for friction type connection
• $V_{nsf} = \mu_f n_e K_h F_0$ – nominal shear capacity of a bolt as governed by slip for friction type connection
• $\mu_f$ – coefficient of friction (slip factor) as specified in IS 800, Table 20; editable in Code setup
• $n_e = 1$ – number of effective interfaces offering frictional resistance to slip; each shear plane is checked separately
• $K_h$ – factor for bolt holes; $K_h = 1.0$ for fasteners in standard holes, $K_h = 0.85$ for fasteners in oversized and short slotted holes, $K_h = 0.7$ for fasteners in long slotted holes
• $\gamma_{mf}$ – partial safety factor for bolts – friction type – IS 800, Table 5, $\gamma_{mf}=1.10$ if slip resistance is designed at service load, $\gamma_{mf}= 1.25$ if slip resistance is designed at ultimate load; editable in Code setup
• $F_0 = A_n f_0$ – minimum bolt tension (proof load) at installation
• $A_n$ – net tensile stress area of the bolt
• $f_0 = 0.7 f_{ub}$ – proof stress

Capacity after slipping (IS 800, Cl. 10.4.4) should be checked by switching bolt type from friction to bearing – tension/shear interaction for design capacity at ultimate load.

Tension capacity of bolts

A bolt subjected to a factored tensile force is checked according to IS 800, Cl. 10.3.5:

$T_f \le T_{df}$

where:

• $T_{df} = T_{nf} / \gamma_{mf}$ – design tensile capacity of the friction bolt
• $T_{nf} = \min \{ 0.9 f_{ub} A_n, \, f_{yb} A_s (\gamma_{mf} / \gamma_{m0}) \}$ – nominal tensile capacity of the friction bolt
• $f_{ub}$ – ultimate tensile strength of the bolt
• $f_{yb}$ – yield strength of the bolt
• $A_n$ – net tensile stress area of the bolt
• $A_s$ – cross-section area at the shank
• $\gamma_{mf}$ – partial safety factor for bolts – friction type – IS 800, Table 5, $\gamma_{mf}=1.10$ if slip resistance is designed at service load, $\gamma_{mf}= 1.25$ if slip resistance is designed at ultimate load; editable in Code setup
• $\gamma_{m0} = 1.1$ – partial safety factor for resistance governed by yielding – IS 800, Table 5; editable in Code setup

Prying forces are determined by finite element analysis and are included in the tensile force.

Friction bolt subjected to combined shear and tension

A bolt required to resist both design shear force and design tensile force at the same time shall according to IS 800, Cl. 10.3.6 satisfy:

$\left( \frac{V_{sf}}{V_{df}} \right)^2 + \left( \frac{T_{f}}{T_{df}} \right)^2 \le 1.0$

where:

• $V_{sf}$ – applied factored shear at design load
• $V_{df}$ – design shear strength
• $T_f$ – externally applied factored tension at design load
• $T_{df}$ – design tension strength