مکانیسم شکست برشی بلوک از گروه های بار محوری پیچ ها
Abstract
1- Introduction
2- Block shear model
3- Setting of model parameters
4- Experimental investigations
5- Results and discussion
6- Summary, conclusions & outlook
References
Abstract
Self-tapping screws are fasteners that are versatilely applicable in timber engineering. For the design of such screw connections, preferential axial-loading, all possible failure mechanisms have to be considered. Recently, in compact groups of axially-loaded screws the block shear failure mechanism, which has not been investigated so far, turned out to fail rather brittle at load levels lower than currently allowed. This failure mechanism is defined as failure of (rolling) shear and/or tension perpendicular to grain planes encompassing the group of screws. This failure mechanism was observed in groups given a number of different parameter settings, i.e. thread-fibre angles of 90° and 45°, glulam, structural timber and cross laminated timber and various group designs. This paper focuses on groups of axially-loaded screws in glulam and solid timber of Norway spruce (Picea abies) and inserted at a thread-fibre angle of 90°. Varying group sizes, loading and supporting distances and group designs, i.e. various penetration lengths lef and spacing in and perpendicular to grain, a1 and a2, respectively, are analysed by two different “push-pull”-test setups. To predict the block shear capacity and failure characteristics of such groups of screws and to separate this failure mechanism from other failure mechanisms, a mechanical-based block shear model was established. This parallel acting spring model considers load sharing and redistribution between concerned failure planes and depends on a number of material, geometrical and stress distribution parameters. To ensure a reasonable parameter setting, background and potential influencing parameters on each model parameter are discussed. In validation, the model shows overall good predictions of capacities, failure mechanisms and failure sequence for all test series involved. It turned out that the current regulations, comprising the definition of minimum spacing together with minimum edge and end distances, are not sufficient for controlling this three-dimensional block shear failure. In addition, the consideration of the number of screws in the group as well as the penetration length is required.
General comments
In contemporary timber engineering, dowel-type fasteners are differentiated in fasteners primary stressed in shear, e.g. dowels or nails, or axially in tension or compression, e.g. self-tapping screws or glued-in rods. Whereas in the first group the timber is primary stressed in compression, in the second group the timber is primary stressed in shear. Self-tapping screws are optimised for load-bearing purposes axial in tension, as addressed in this study, made of hardened steel and feature high resistance and stiffness but only minor plastic deformability until failure. The versatile possibilities how to apply them can be differentiated in ”active applications”, i.e. for connecting structural elements, and in ”passive applications”, i.e. for reinforcing structural elements; see Ringhofer et al. [1] and Ringhofer [2]. Active applications with several screws are realised often with outer steel plates, as exemplarily shown in Fig. 1. Differentiation can be made in (i) joints with steel plates constantly pressed on the timber surface, i.e. joints featuring a restricted deformability of the timber surface as shown in Fig. 1(b), named further as ”restricted joints”, and in (ii) joints with steel plates constantly taken-off from the timber surface, see Fig. 1(c), named further as “free joints”. We further concentrate on “free joints” of primary axially and in tension loaded self-tapping screws, thus possible positive effects on the resistance of restricted joints are excluded.