Tensile Behaviour of Galvanised Grade 8.8 Bolt Assemblies in Fire

Abstract

In structural fire engineering, the importance of bolt assemblies is often overlooked. Connection design uses the temperature-dependent bolt strength-reduction factors prescribed in Eurocode 3, despite the existence of two distinct failure modes under tension; necking of the bolt shank, and thread-stripping. While literature exists to predict failure modes at ambient temperature, there is no method for failure mode prediction for elevated temperatures where ductility is critical to avoid collapse. Galvanised M20 structural bolt assemblies and bolt material from a single batch have been tested under tension at a range of temperatures and strain-rates typical of those experienced in fire. Turned-down bolt test data produced stress-strain curves characteristic of different microstructures at ambient temperature, despite a tempered-martensitic microstructure being specified in the standards. The failure modes of bolt assemblies were found to be dependent on the as-received microstructure at ambient temperature. At elevated temperatures, however, only thread-stripping was observed.Keywords: Bolt assemblies, ductile and brittle failure, thread stripping, microstructure INTRODUCTIONThis paper examines the tensile behaviour of bolt material, in the form of turned-down bolts, and bolt assemblies, at a range of temperatures and strain-rates. A single batch of galvanised M20 structural bolting assemblies, in accordance with BS EN 15048 [1], were chosen, as they are commonly used in UK construction. The bolts were Grade 8.8, while the pairing nuts were of Property Class 10. The bolt assemblies were donated by a UK distributor; which had imported the components, carried out quality assurance testing and applied its own manufacturers mark. This is common practice in the UK, where all structural bolts are currently imported due to the high cost of raw materials [2]. According to ISO 898-1 "A distributor who distributes fasteners that are marked with his (or her) own identification mark shall be considered to be the manufacturer" [3], which makes the original overseas manufacturer untraceable.The strength reduction factors provided in Table D.1 of Eurocode 3 [4] came directly from the results of testing carried out during the 1990s on bolt assemblies manufactured in the UK [5,6]. At the time at which this research was carried out 'structural' bolting assemblies did not exist. All nuts and bolts could be purchased individually and interchangeably. Quality assurance testing, therefore, did not include the mechanical testing of the assembly as a whole. Geometrical differences also exist between the bolts tested by Kirby [5,6] and those discussed in this paper. Kirby tested assemblies of looser thread tolerance class combinations, known as 7H8g, as specified in BS 4190 [7]. However, the assemblies tested in this paper are of the tighter tolerance class 6AZ6g, as specified in . The equivalent tight tolerance class for uncoated assemblies is 6H6g, as specified in ISO 4017 [10] for fully-threaded bolts or ISO 4014 [11] for par...

A unified rheological model for modelling steel behaviour in fire conditions

“…This attempts to test the performance of model R4, using its previously calibrated constitutive components, in re-creating the strain output over temperatures between 500-700°C. The comparison with the results of stationary coupon tests in Figure 7(a, b) shows very good prediction of the stress-strain outputs of the tests from studies [17] and [20], indicating the applicability of the constitutive material components of model R4 for different steel grades; in particular, the damper c 1 , which models strain-rate effects, seems applicable to steel grades S275, S355 and the bolt steel grade 8.8. Some discrepancy in predicting the yield strengths at 550 and 600°C is observed in Figure 7(c) from Renner's study.…”

“…Boko's study is based on high-temperature strain-rate tests of steel coupons at 0.0002s -1 . Figure 7(b) compares the model predictions with the study by Bull et al [20], which reports medium-speed strain-rate tests on bolts of Grade 8.8. Figure 7(c) shows a further comparison against the study by Renner [18] for medium-strain-rate coupon tests of steel of Grade S275.…”

“…A study conducted by Bull et al [20] involved high-temperature constant-strain-rate tests on M20 grade 8.8 bolts. Strain rates ranging from 0.002-0.02 min -1 and temperatures from 550-700°C were analysed within the study, which was used for verification of the unified model in order to test its capabilities outside the range of contemporary steel grades.…”

Behaviour of stainless and high strength steel bolt assemblies at elevated temperatures — A review

“…Only austenitic types were studied [15,16] Fully-threaded bolts [7] Type of thread: coarse and fine All reported studies considered coarse types only Nut styles: thin nuts, regular nuts and high nuts…”

“…Design codes attempt to encapsulate the deterioration of bolt assemblies at elevated temperatures by means of so-called reduction factors, which give the ratio of the shear or tensile strength of the bolt assembly at elevated temperatures to their respective values at ambient temperatures. These are principally based upon experimental results and a limited number of numerical investigations [7][8][9][10][11][12][13][14][15][16]. At present however, there is no consensus on the exact conditions of the testing methods, nor the particularities of the numerical models employed and hence the reliability of the available reduction factors.…”