Dagens byggregler avseende hållfasthet för brandutsatta konstruktioner vilar starkt på brandpåverkan av standardbrandkurvan. Denna brandkurva representerar dock ej en naturlig brand särskilt väl. EKS (Boverkets tillämpning av europeeiska konstruktionsregler) ger även möjlighet för att använda naturliga brandförlopp för att utvärdera en byggnadsdels hållfasthet. Brandteknisk forskning har påvisat att det finns metoder som kvantitativt kan jämföra olika brandförlopps påverkan på byggnadsdelar med varandra, dessa metoder har utvecklats genom historien av brandteknisk forskning.  Den första modellen presenterades av S. H. Ingberg (1928) elva år efter att standardbrandkurvan som vi känner den idag (EN 13501-2, 2016) fått fäste inom den brandtekniska vetenskapen. Ingberg formulerade en hypotes baserad på ett stort antal försök på fullskaliga bränder där ansatsen var att en brands kvantifierade brandpåverkan kunde identifieras som arean som för en viss studerad tid begränsas i höjdled av brandens temperatur i överkant, och i underkant av en baslinjetemperatur som bestäms av den bärande konstruktionens materialegenskaper. Ingbergs ansats baserade sig på faktiska bränder i jämförelse med standardbrandkurvan (Ingberg, 1928). Denna teori har ifrågasatts då Ingberg i sina försök ej tog hänsyn till brandrummens olika ventilationsförutsättningar.   För att undersöka hur bärande byggnadsdelar dimensionerade med standardbrand står sig jämfört med två metoder med naturliga brandförlopp görs en kvantitativ analys under ansatsen att de naturliga bränderna har samma brandpåverkan i enlighet med Ingbergs teori om lika areor. Det genomförs beräkningar för fullständiga brandförlopp med 800 samt 1600 MJ/m2 i dimensionerande brandbelastning. Totalt 258 fall med olika parametrar och för tolv av dessa fall analyseras resultaten nogrannare. För att avgöra om en brand är på osäker sida jämfört med normenliga kravnivåer görs två jämförelser i tidsdomänen mot tidskravet för standardbrand och en jämförelse i temperaturdomänen genom att analysera ett ståltvärsnitts temperatur mot dess kritiska temperatur.   Av de 12 närmare studerade bränderna överskrids nominella krav i sex av dem. De bränder som ger upphov till överskridna krav är bränder i rum med låg tillgång till ventilation och ytskikt med låg termisk tröghet. Resultaten är desamma för den lägre och den högre brandbelastningen.  Modellen med lika areor bedöms däremot ej vara komplett då metoden ej kan appliceras på vilken temperaturdata som helst då underliggande faktorer som kännedom om ventilation och bränsle kan neglegeras. Vidare bedöms det finnas risker med metoder då brandpåverkan riskerar att dölja kännedom om avgivna strålningsnivåer från branden som kan ge upphov till stora värmeflöden till bärande byggnadsdelar. / Today's building regulations regarding strength for fire-exposed structures rest heavily on the fire impact of the standard fire curve. However, this fire curve does not represent a natural fire very well. Eks (The National Board of Housing and Urban Development's application of European design rules) also provides the opportunity to use natural fire processes to evaluate the strength of a building part. Research in fire technology has shown that there are methods that can quantitatively compare the severity of different fires on load bearing structures with each other, these methods have been developed through the history of fire technical research.  The first model was presented by S. H. Ingberg (1928) eleven years after the standard fire curve as we know it today (EN 13501-2, 2016) gained a foothold in fire safety science. Ingberg formulated a hypothesis based on a large number of experiments on full-scale fires where the approach was that the quantified fire impact of a fire could be identified over time as the area limited at the top of the fire temperature, and at the lower edge of a baseline temperature determined by the material properties of the load bearing structure. Ingberg's approach was based on actual fires compared to the standard fire curve  (Ingberg, 1928). This theory has been questioned since Ingberg's experiments did not take into account the different ventilation conditions of the fire rooms.   In order to investigate how load bearing structures designed with the standard fire curve compares to two methods with natural fire processes, a quantitative analysis is carried out under the assumption that the natural fires have the same fire impact in accordance with Ingberg's theory of equal areas. Calculations are carried out for natural fires with design fire loads 800 and 1600 MJ/m2. A total of 258 cases with different parameters and for twelve of these cases the results are analysed more closely. To determine whether a fire is on uncertain side compared to normative requirement levels, two comparisons are made in the time domain against the time requirement for standard fire and one comparison in the temperature domain by comparing the temperature of a steel cross-section against its critical temperature.   Of the twelwe closer studied fires, six of them exceeded nominal requirements. The fires that exceeded requirements are fires with low access to ventilation and rooms with linings of low thermal interia. The results are the same for the lower and higher fire loads.  However, the model with equal areas is not considered complete as the method cannot be applied to any given temperature data as underlying factors such as knowledge of ventilation and fuel can be ignored. Furthermore, it is considered that there are risks with the methods as the equal fire severity risks concealing knowledge of radiation levels emitted from the fire that can give rise to large heat flows to the load bearing structure.

Purpose: The purpose of this study has been to investigate the essentials of being able to incorporate high-strength steels (460 MPa and beyond) for structural elements in buildings. As of late, structural steels with a yield point of 355 MPa have been considered standard and have been for the past decade. One of the problems that occur with an increased yield point, is that deflection of structural elements increases, as the Young’s modulus does not increase with increasing yield point. Welding, stability, behavior during fire, and fatigue are also subjects of interest. Method: The study was conducted through several courses of action: a literature review covering the latest research of high-strength steels within the sought-after area of interest, followed by calculations of a truss resting on two columns, being subject to bending moment and compressive force, in both 355 MPa and 700 MPa, in order to review the differences that occur and how they can be counteracted. Lastly, interviews were carried out, where structural engineers gave their thoughts and experiences on the matter at hand. Results: The results show that welding is one of the largest hurdles with being able to utilize high-strength structural steels, though there are newer, more promising methods of welding which can be used, such as electron beam welding. Regarding structural integrity and buckling of structural elements, high-strength steel can be used for trusses, where the structural members are mainly being pulled, opposed to being subject to compressive force. This was shown with the performed calculations, during the interviews, and by the literature overview. Conclusions: The general conclusions of the study is that for welding, further research, education, and training is required for all concerned parts, such as the structural engineers and the on-site welders, which will increase the knowledge regarding how welding of high-strength steels should be performed, but also raise awareness about newer and more modern methods. Fire behavior for high-strength steels are a higher risk factor that should be treated and executed with higher degrees of caution by engineers. Reduction factors for fire affected steel construction elements should be corrected to fit the behavior for high-strength steels as well, as they differ from the current Eurocode 3 for lower class steels. Problems with instability can be counteracted by utilizing the steel in pulled structural members, such as trusses and struts. Lastly, for high-strength steels to be used more widely, structural engineers and manufacturers need to work together for any of the two to profit, as low production rates are costly.