4. STRUCTURAL DESIGN OF LVL STRUCTURES The structural design of LVL structures is similar in purpose and principle to any load-bearing wood-based structure: to verify that the structure fulfils its strength, serviceability and other structural requirements. The basis of limit state design is to verify, based on the partial safety coefficient method, that the design value of an action Ed is smaller than the design value of the resistance Rd of a section, structural member or connection: Ed ≤ Rd where Ed is the design value of the effect of actions such as internal force, moment or a vector representing several internal forces or moments Rd is the design value of the corresponding resistance. 4.1 BASIS OF STRUCTURAL DESIGN The Eurocode design system has been used for this purpose in Europe since 2010 with country-specific adjustment factors that are defined in national annexes. For timber construction the important parts of the Eurocode system are: EN 1990 Eurocode 0 Basis of structural design EN 1991 Eurocode 1 Actions on structures EN 1993 Eurocode 3 Design of steel structures EN 1995 Eurocode 5 Design of timber structures Eurocodes are limit state design codes that have two main limit states: Ultimate limit state (ULS) and serviceability limit state (SLS). In ultimate limit state (ULS) design the requirement is to verify that a structure has adequate safety against failure during its whole designed service life. What is regarded adequate is defined in the relevant building regulations. Serviceability limit state (SLS) design evaluates whether the structure is fit for purpose. In most cases, the building regulations do not define exact limit values for this evaluation. They provide recommendations for the maximum level of deformation and, e.g., human-induced floor vibration, but it is ultimately up to the contractor and the client to agree what is acceptable. The risk of structural failure is depending on the probability that the expected actions are exceeded and the probability that resistance of the structure is lower than calculated in ULS design. Normally it can be presumed that the actions and the resistance of a structure are random variables. When their distribution functions are known, it is possible to calculate the risk of failure by the methods of probability theories. These methods, however are usually too complicated for structural design in practice, but they can be used for code calibration when the requirements are defined for building regulations and in comparison between different construction materials. They also have an important role e.g. in verification of the safety level of simplified design methods, such as the partial safety factor method used in Eurocodes 29, 30. The primary purpose of the design rules for load-bearing structures is to prevent failures that can lead to the risk of human injury and to ensure buildings function properly according to their intended use. Building codes such as the Eurocodes provide approved verification methods for verifying that the requirements are fulfilled. They also present the loads and loading combinations that the structures must resist. The Eurocodes have common parts that are applied in all countries in connection with national annexes. The annexes specify country-specific safety factors and parameters that take into consideration climate and geology specific conditions. In addition, national choices are made in some alternative verification methods or adjustment factors within them. The general design requirements according to the Eurocodes are 29, 30: • Choice of structural system, structural design and construction work must be done by sufficiently qualified and experienced persons. • Work must be supervised adequately and quality assurance implemented throughout the construction process from design offices and factories to workshops and building sites. • Structures must be made of construction materials and products defined in the Eurocodes or their harmonized standards or in other harmonized technical specifications. • Buildings must be adequately and regularly maintained throughout their design working life. • Buildings must be used for the purpose for which they have been designed The next generation of Eurocodes is currently under development and scheduled to be ready for use after some years. 4.1.1 Actions (Loads) Actions are loads that cause, e.g., bending moment, shear and axial stresses and deformation of the structures. The determining actions and their combinations for each structure are evaluated based on the type, magnitude and duration of the action at the most unfavourable locations of the structure. Different load combinations are determined as load cases. The load combinations consist of the main loads combined with reduced values of other loads that may be acting at the same time. Reduction LVL Handbook Europe 113
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