Predicting post-fire mechanical properties of grade 8.8 and 10.9 steel bolts Hesamoddin Ketabdari, Amir Saedi Daryan ⁎, Nemat Hassani Faculty of Civil, Water and Environmental Engineering, Shahid Beheshti University, Tehran, Iran abstract article info Article history: Received 5 November 2018 Received in revised form 3 August 2019 Accepted 14 August 2019 Available online xxxx Structural fire safety is one of the primary considerations in the design of high-rise buildings where steel is often a popular material for structural members selection. Therefore, predicting post-fire mechanical properties of steel bolts as a crucial element in steel structures is highly valuable. In this paper, the behavior of the High-Strength Steel Bolts (HSSB), after exposing to fire, is investigated and practical equations for the mechanical properties, in- cluding the ultimate strength, the yield strength, and the modulus of elasticity, are proposed as well. Accordingly, Grade 8.8 and Grade 10.9 steel bolts are employed in a variety of sizes, from M6 to M24, experiencing six different target temperatures. After natural cooling, a tensile test is applied to all the bolts, the corresponding stress-strain curves are derived, and finally all required data for each specimen are obtained by means of these curves. Results from these curves indicate that at 400 °C or fewer temperatures, more than 80% of the mechanical properties are recovered. Between 400 °C and 500 °C, the features began to reduce, however, above 500 °C, a sudden drop was noticeable. Besides, by using both the obtained data and the Gene Expression Programming (GEP) as a branch of Genetic Algorithm, equations for mechanical properties of HSSB are derived. The validation results indicate that the relative error of GEP-based models is less than 10%. All in all, the minimum error in the GEP-based models demonstrates favorable equations for post-fire mechanical properties of HSSB and also better predictions than the traditional models. © 2019 Elsevier Ltd. All rights reserved. Keywords: Steel bolts Mechanical properties Post-fire Gene expression programming Practical equation 1. Introduction Connections are one of the most important structural elements in steel structures. Therefore, having appropriate connections can cause decent structural performance. Furthermore, any failure in the connec- tions can endanger the unity of the whole structure and finally leads to its collapse. It is worth mentioning that fire incidents have been one of the most critical factors in the failure of steel structures [1–5]. Upon surfing a few of existing studies about the different steel grades, it is noticeable that the mechanical properties are the most cru- cial factors in post-fire states. By consideration of both the heating and the cooling phase, Outinen and Makelainen [6] studied the mechanical properties of S355 cold-formed steel. Qiang et al. [7,8], using experimental studies, estimated the mechanical properties of the high-strength structural steel S460, S690, and S960 after exposure to the temperatures above 1000 degrees and being cooled down. Gunalan et al. [9] studied the residual strength of G300, G500, and G550 cold- formed steels, and reported that residual strength decreases at temper- atures above 300 °C. Wang et al. [10] stated that the residual mechanical properties of high-strength Q460 steel, including the yield strength, the ultimate strength, and the residual elasticity modulus, are considerably reduced after being exposed to the temperatures above 600 °C. Steel bolted connections are the most important fasteners, and the role of the bolts are highly valued. On the other hand, a few studies about their mechanical properties exist in the literature [11–15]. The cooling phase, nevertheless, has not been considered except by few. Kirby [16], using M20 bolts, investigated the behavior of grade 8.8 steel bolts at the temperatures higher than 800 °C, and proposed the strength reduction factors as well. Hanus et al. [17] utilized the M12 grade 8.8 steel bolts in a heating-cooling cycle to investigate steel bolts behavior under tensile and shearing tests. Lou et al. [18] investi- gated the behavior of the M20 steel bolts with grade 45 steel and MnTiB20 material after the fire and used both water and natural cooling in their heating-cooling cycle. Kodur et al. [19] examined the mechani- cal features of grade A490 and A325 high-strength steel bolts in a range of temperatures between 20 °C to 1000 °C. Maraveas et al. [20] used a few existing studies and proposed equations for steel bolts stiffness de- terioration, although, the differences between each experimental condi- tions make these equations less reliable. Saedi Daryan and Ketabdari [14] investigated the failure modes and strength properties of steel bolts with different materials. Results indicated that the use of alloy steel such as JIS G4053 SMC435 can enhance the residual strength of the bolts. Journal of Constructional Steel Research 162 (2019) 105735 ⁎ Corresponding author at: Shahid Beheshti University, Tehran, Iran. E-mail address: amir_saedi_d@yahoo.com (A. Saedi Daryan). https://doi.org/10.1016/j.jcsr.2019.105735 0143-974X/© 2019 Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect Journal of Constructional Steel Research