Behavior of High-Strength Grade 10.9 Bolts under Fire Conditions

Abstract

In the past decades, structural fire design has changed essentially. In the past, the structural integrity of single elements was mostly proven by load tables that were based on fire tests. The introduction of the “hot” Eurocodes (Parts 1–2) offers the possibility to assess the resistance of even complex structural systems under fire conditions. For this purpose, a calculation can be made according to the Eurocodes taking into account the relevant material behavior, which is mainly known for structural steel and concrete elements. The material behavior is defined in the Eurocodes by stress–strain curves, depending on the temperature. Even for connections, design rules are given. The resistance of bolts in fire can easily be calculated using global reduction factors that depend on the member temperature. The global reduction factors for bolts are valid at present for all grades. Tests showed that either differing alloying materials or varying treatment methods can lead to a relevant change in the ultimate strength. These two points have an important influence on the production process of zinc-coated high-strength grade 10.9 bolts (f _u = 1000 N/mm²). On account of this, tension tests on specimen and zinc-coated bolt assemblies have been conducted to assess the behavior under fire conditions. The results, on one hand, provide the possibility to verify the reduction factors for grade 10.9 bolts given in the Eurocode and, on the other hand, lead to a more precise understanding of high-strength bolt behavior under fire conditions. This paper describes the state of the art, the tests, and the present results of numerical analysis.