In steel structures, bolted connections made of austenitic and duplex stainless steel according to EN 1993-1-4 are predominantly designed and executed as non-preloaded category A (pure shear) bearing type connections, category D (pure) tension connections or as category A+D bolted connections subjected to combined tension and shear loading. The current European design rules were initially developed for carbon steel bolts and specified in EN 1993-1-8. However, the load bearing behaviour of austenitic and duplex stainless steel bolts is significantly affected by their specific material properties, such as nonlinearity, pronounced strain hardening and a transformation of austenitic to forming induced martensitic material in the tensile cross-section originating from the thread rolling process with a larger hardness and higher strength in the border of the cross-section of the threads. These specific material properties must already be considered in the design of these bolted connections. Therefore, it was crucial to address this normative gap through comprehensive investigations.

The load bearing behaviour of austenitic and duplex stainless steel bolts, as well as bolting assemblies, has been systematically examined through a combination of experimental and numerical investigations, complemented by reliability analyses. These comprehensive studies, conducted as part of the German IGF research project FOSTA P 1386/IGF-No. 20651 N, have been further extended through additional independent research at the Institute for Metal and Lightweight Structures of the University of Duisburg-Essen, Germany. The investigations comprised pure tension, single-shear and combined tension and shear loading scenarios. Finally, new and more economical "Ease of Use" design rules were proposed. These include an increase of the tension resistance F_t,Rd of stainless steel bolts by k₂ = 1.0 (except countersunk bolts) harmonised with EN ISO 3506-1 regarding the minimum ultimate tensile load F_mf, new (higher) shear coefficients a for the shear resistance F_v,Rd and new elliptical interaction equations for the unthreaded and threaded bolt part in the shear plane of stainless steel bolts. The proposed design rules were statistically verified in reliability analyses according to EN 1990, Annex D against y_M2 = 1.25 for connections, and implemented in the revision of EN 1993-1-4, FprEN 1993-1-4, and the renewed German national technical approval/General construction technique permit abZ/aBG Z-30.3-6:2022.

In this thesis, the background of the new proposed design rules for stainless steel bolts was outlined, discussed in the context of different international design concepts, and extended by supplementary experimental and numerical investigations into the tension and single-/double-shear resistance of bolts and bolting assemblies. The correlation between tensile strength and shear strength was analysed in direct and indirect multi-step pathway analyses, and the steel grade and bolt property class were identified as key parameters.

Furthermore, extended statistical evaluations according to EN 1990, Annex D were performed and complemented by distribution analyses to prove the resulting partial safety factors y_M* against the Eurocode safety requirements. Here, new assessment criteria were developed. Moreover, the conservative European design regulations of EN 1993-1-8 regarding the tension/shear carbon steel bolted connections were statistically re-evaluated since numerous recent experimental studies on carbon steel bolts have emerged but require further validation through statistical analysis. They were complemented by own experimental studies into the tension, single/double-shear and interaction resistances of carbon steel bolts. Combined with a thorough literature review, these findings were integrated into renewed reliability analyses to investigate the suitability of the existing design approaches. The culmination of the research efforts led finally to the proposal of new and more economical design rules for carbon steel bolts, which are harmonised with the proposed new design rules for austenitic and duplex stainless steel bolts according to FprEN 1993-1-4 and in line with the normative requirements of EN ISO 898-1.

In conclusion, the newly proposed design rules for structural stainless and carbon steel bolts demonstrate their suitability in accurately describing the bolt tension, shear and interaction behaviour. These proposed rules offer engineers a coherent and “Ease of Use” design concept that fully aligns with the safety requirements of the Eurocode. Additionally, new annexes were proposed as amendments for future revisions of EN ISO 3506-1 and EN ISO 898-1, dealing with simplified test procedures for the shear and interaction resistance of stainless and carbon steel bolts.