Horisontalstabilisering av småhus i trä: en jämförelse av dimensioneringsmetoder

Sammanfattning: In the last ten years, the number of building permits applied for houses with timber frame structure has more than doubled. Today, there is no general requirement to establish horizontal capacity, but is something that can be demanded and that many structural engineers have occasionally experienced as problematic. The design phase can then be time-consuming, where solutions for determining capacity against wind loads can become expensive and complicated, which does not always receive a positive response from production. Against this background, there is a general interest among both structural engineers and production to evaluate appropriate approaches to meet the interests of both parties. The purpose of the work is to establish a more optimal approach by examining the available calculation methods. The aim is to determine what strengths and weaknesses the methods entail and under what conditions the methods are suitable. The case study includes a literature study, survey study and calculations where a reference object is the basis for the analysis. The object in question is a long and narrow two-floor single family house that consists of a prefabricated wooden frame and is situated in Bunkeflostrand, south of Malmö. The area is close to the sea and is at times very exposed to wind. To meet the requests of future residents, the floor plan is open, and most openings are located on the gables, which leads to few stabilizing walls. As this type of house is becoming more common and the object in question may become a standard house for similar establishments, it is of interest to study. Through the survey study, information and experiences are gathered from structural engineers and house manufacturers to get a background on how the industry responds to the problem. The respondents' answers form a basis for some choices and assumptions in further analysis. In the thesis, different design methods are evaluated for calculating the shear resistance of a wooden frame. The methods that are analyzed and compared are Method A and Method B according to Eurocode 5, elastic design according to Gyproc's manual 7 and plastic design according to A. Girhammar and B. Källsner. A finite element method modeling is performed to investigate if and how the method can possibly be implemented when calculating an object's horizontal capacity. The analysis is divided into two parts which are referred to as Study 1 and Study 2. In the first part, the calculation methods are investigated based on the choice of board material and screw distance to reach a degree of utilization as close to 100% as possible. The different methods cost outcomes are compiled and compared. Furthermore, the shear resistances according to all methods are evaluated under the same assumption to get an idea of how the methods differ. In the second part, a number of changes in geometry are made to examine how the horizontal capacity of the reference object is affected. The results show that the plastic method with full anchoring gives approximately 20% higher shear capacity compared to the recommended method in Eurocode 5, Method A, as stabilizing boards above and below openings can be used for stabilization. As the choice of board material is the largest cost-driving factor, this gives approximately 30% lower cost than Method A. Lowering the reference object's height from 8.5 m to 7.0 m gives the most advantageous outcome as the wind load is reduced and approximately 50% higher degree of utilization is obtained compared to Study 1. The horizontal capacity regarding Method A, Method B, and elastic method is higher when full board widths are used. Plastic design with partial anchoring gives a higher shear capacity when continuous wall lengths are obtained since a plastic shear flow can be built up over a longer distance. Furthermore, plastic design is not recommended if openings are placed close to each other as the capacity in boards above and below openings must be greatly reduced.