Strength of axially loaded screw joints in wooden structures exposed to fire : An overview of existing knowledge and proposal for test method

Sammanfattning: Today, distinct approaches are required when designing timber structures and their joints in the event of fire. Considering that the steel fasteners have temperature dependent properties and that the surrounding wooden construction is also affected by the fire due to occurrence of charring during fire exposure, these types of joints are complicated to design.  This study includes an overview of existing knowledge in the subject, where a literature survey has been conducted to enlighten current design procedures, previous research, and available test methods for axially loaded screw joints in wooden structures exposed to fire. The literature survey has been supplemented with an interview survey, in which six respondents’ knowledge in the subject and desires for future studies have played the main role.  The results from this study clearly indicates that the existing knowledge regarding axially loaded screw joints in wooden structures exposed to fire is insufficient. Today’s standards for the design of these types of joints are incomplete with respect to the referred joint arrangement and there are no explicit test methods for determination of the strength of these joints under fire exposure. A few previous studies have been carried out and the results from these have proven to be difficult to interpret since the experimental set-up was not consistent between the fire tests. This complicates the process of determining which factor has the greatest impact on the strength of the joint and the main underlying causes generating a collapse of the structure. The interview survey further strengthens the perception, based on the literature survey, that there is a lack of knowledge in the subject.  By mapping the existing knowledge in the subject it has been possible to identify deficiencies in current information and, finally, creating and proposing a test method for future tests of axially loaded screw joints in wooden structures exposed to fire. The proposed test method is influenced by existing test methods and design conditions for structures at normal temperatures. The development of this test method has its primary purpose in the ambition that the knowledge of axially loaded screw joints in fire exposed wooden structures can be expanded.