Design Method for Strut-Beam Connection in Hinged Frames

Sammanfattning: Glulam is almost exclusively the chosen material of timber frame structures. Of those, three-hinged (three-pin) portal frames are incomparably the most common type. Being both statically determinate and stable against horizontal forces in its own plane offer both practical (basic constructive details) and economical benefits. The design of the haunch allows for various solutions: it can be curved with continuous laminates, finger jointed, jointed with steel dowels and slotted-in plates, or built-up with a strut. The form of the frame derives from the main load’s force line. The most appropriate forms for large spans are curved or built-up haunches, as they fulfil both functional and aesthetic aspects. Three-pin portal frames are suitable for spans up to 30-40 meters, being the limiting factor the transport feasability of the frame’s halves. In recent years a handful of three-hinged structures with built-up haunches have collapsed, leaving behind a need to analyse and study the stresses and resistance of this structures. For obvious reasons it is specially interesting to research and clarify the fracture risk on the built-up haunches as well as to map the stresses created by the inner frame leg through compression to the lower edge of the frame rafter. Today there is only one general method used to design built-up haunches, present in both the Glulam Handbook and the German Institute for Standardization, DIN, which is based on established practice. Both standards share similar simplifications and assumtions, giving surprisingly little importance to the shear stress that occurs in the contact area of the built-up haunch. The aim of this master’s thesis was to put together a tool, in form of a diagram, to help design the built-up haunch, with particular emphasis on the contact area between the inner frame leg and the frame rafter, and the subsequent shear stresses this contact creates on the rafter. To achieve this result a sample frame rafter was calculated using 2D frame software and generic loads and materials. The section forces obtained were then used to create a FEM model of the built-up haunch. This FEM model provided a clearer understanding of the behaviour of the frame leg-frame rafter’s contact area as well as yielding a map of the shear stresses present in the joint. Finally with the use of Linear Elastic Fracture Mechanics and the mean stress criterion a diagram-tool was created. This report yielded two results that may be deemed of special interest: • Both the Glulam Handbook and the German DIN standard relegate the shear stress in the notch as merely a design checkout, never a design factor. • Both the Glulam Handbook and the DIN completely disregard the size effects in the capacity of birdmouth joints. The fracture mechanics calculations presented in this report provide prove of strong size effects, so that current design standards significantly overestimate the capacity of great-sized built-up haunches. This conclusion could be important in practical terms, and should be further investigated. Experimental tests should also be inlcuded in future research.