_Article_  
**Influence of the Thermal Insulation Type and Thickness on the Structure Mechanical Response Under Fire Conditions**  
**[PERSON]\({}^{\copyright\), [PERSON] \({}^{\ast}\) and [PERSON]**  
Faculty of Civil Engineering and Architecture, Kielce University of Technology, 25-314 Kielce, Poland  
\({}^{\ast}\) Correspondence: [EMAIL_ADDRESS]; Tel.: +48-41-34-24-803  
Received: 15 May 2019; Accepted: 23 June 2019; Published: 27 June 2019  
**Featured Application: The problems analysed in the study are extremely important from the point of view of the fire safety of steel structures. In technical approvals, manufacturers of fire-proofing materials usually provide tables in which the necessary insulation thicknesses are listed, depending on the type of material used, heating conditions, section factor, and the required fire resistance class. The manufactures of fireproofing materials recommend that insulation of a constant thickness be used for all elements of the truss. This approach seems highly uneconomical. An alternative solution has been proposed in the paper. This proposal determines the optimal necessary insulation thickness for individual bars of the truss, based on additional static-strength calculations. This article describes the original approach to modelling protected steel structures against fire. The advantage of the proposed solution is a more optimal (economical) method of fire insulation for steel lattice girders.**  
**Abstract: The concept of fire safety covers an extremely vast scope of issues. To ensure an adequate fire safety level, it is necessary to combine research and actions in several fields, such as the mathematical, physical, or numerical modelling of a fire phenomenon. Another problem is to design different types of fire protection, including alarm systems, sprinkler systems, and also roads and evacuation systems, in a manner that ensures maximum safety for the building's users. A vital issue is the analysis of the static-strength response of the structure under fire conditions. This study, concerned with such analyses, is limited to steel truss structures. In technical approvals, manufacturers of fire-proofing materials do not account for the character of the performance of individual structural members. The components in compression need thicker insulation than those in tension. This phenomenon is related to the fact that under fire conditions, the flexural buckling coefficient in compressed members is abruptly reduced with an increase in temperature. In turn, this increase in temperature leads to a fast reduction in resistance. In addition, members in tension have much higher resistance than those in compression in the basic design situation, i.e., at the instant of t = 0 min. Consequently, even a considerable decrease in the resistance of tension members is not as dangerous as that of compression members. Therefore, due to the nature of the performance of individual elements, fire-proofing insulation of every steel structure should be computationally verified. Additionally, in this paper, the influence of the type of fire insulation on the mechanical response of the structure was investigated. Calculations were carried out for different types of sprayed-on insulation, and also for contour and box insulation panels. The graphs show the behaviour of the elastic modulus, the yield point, and the resistance of the elements in the successive minutes of the fire for the different methods of fire protection used. The best results were obtained for vermiculite and gypsum spray.**  
**Keywords: fire insulation; fire analysis; steel structures; sprayed-on insulation; fire-proofing boards**