Hilti S-BT screw-in carbon steel and stainless steel threaded studs are widely used as fasteners to attach architectural, structural, mechanical, electrical and similar systems to base materials, fastening of gratings and as fasteners for electrical grounding and bonding of equipment.

When S-BT HL studs are installed into steel elements, predrilling of holes is required. The influence of powder-actuated fasteners, drilled holes and self-drilling screws (drilled holes with a tapped thread) on the static stress-strain characteristics of construction steel have been systematically analyzed in [1]. Tensile tests were conducted using steel coupons into which various types of powder-actuated fasteners, drilled holes and self-drilling screws were driven at various spacings. Table 1 provides an overview of the test parameters and specimens. The “weakening” of the base material by the installation of the S-BT HL studs is comparable to the impact of the self-drilling screws or the drilled holes because the diameter of the predrilled holes are in the same range.



Figure 1 and Figure 2 show examples of stress-strain curves for construction steel of high and low tensile strength. The influence of the drilled holes and the self-drilling screws is visible, however the presence of the drilled holes and self-drilling screws in the steel does not change the fundamental loadbearing characteristics of the steel in terms of its elasticity and plasticity. They don’t cause embrittlement of the base material, i.e. the gross sectional area of all test specimens yielded plastically before subsequent strain hardening. Strains at maximum load were between 5% and 10%. 



The maximum permissible values for the ratio AGross/ANet for S235 and S355, up to which the reduction for holes can be neglected, can be calculated with formula 6.6 and 6.7 in DIN EN 1993-1-1:2010. Table 2 provides an overview of the maximum permissible (limiting) values. The corresponding ratios for typical American construction steel in accordance with AISC Steel Construction Manual are also given.



With ASTM A36, ASTM A572 Grade 50, S235 and S275, with typical density of fasteners, the weakening effect of holes made by S-BT HL studs is still clearly below the maximum permissible values according to the codes (ref. to Table 2). Explicit verification of the loading capacity of the net section is thus not required.