Dowel-type Fasteners Research Articles

Nonlinear dynamic study on existing masonry-infilled concrete frames retrofitted with timber panels

This study investigates the dynamic performance of a timber-based seismic retrofit for existing reinforced concrete (RC) buildings, named RC-TP. The core of the proposed retrofit intervention is the replacement of the existing masonry infills with structural timber panels that are connected to the concrete elements using metal dowel-type fasteners and a timber subframe. The unreinforced frames were designed according to codes in force in the 60 s and 70 s, considering exclusively gravity loads to simulate a condition common to a large part of the built heritage across Europe. The seismic performance of single-storey single-bay frames before and after the application of the RC-TP retrofit was assessed via nonlinear simulations, for which the incremental dynamic analysis method was adopted. The results of the extensive dynamic study show the effectiveness of the RC-TP retrofit in increasing the seismic acceleration resisted by the frames and their base shear capacity and in promoting a more ductile failure mechanism. Some general considerations helpful in guiding the implementation of the retrofit system were also drawn.

Experimental Investigation of Full Hole Embedment Behavior of Bamboo Scrimber with Dowel-Type Fasteners

A comprehensive understanding of the embedment behavior is of great importance in the design of contemporary bamboo constructions with connections utilizing dowel-type fasteners. The objective of this research was to assess the embedment behavior of bamboo scrimber using full-hole embedment tests. To investigate the effect of the loading angle and bolt diameter, a series of tests were performed using bolts of varying diameters (16 mm, 18 mm, and 20 mm) and loading angles (0° to 90°, with an increment of 15°). The experimental results demonstrated that the loading angle has a considerable influence on the embedment behavior. As the loading angle was increased, the failure mode underwent a change from a brittle failure mode, which was dominated by shear mechanisms, to a ductile failure mode, which was dominated by fiber crushing. The yield and ultimate embedment strengths showed an M-shaped response to changes in the loading angle, with the lowest values being 0°, 45°, and 90°. The bolt diameter was found to have no impact on the failure mode of the specimen. However, an increase in bolt diameter resulted in a reduction in the embedment strength when the specimen was loaded at 90°.

Rotational performance of spliced glulam joints with slotted-in-steel-plates and dowel-type fasteners: Experimental investigation and analytical approach

Due to the environmental friendliness and good architectural aesthetic, large-span timber structures are increasingly attracting more attentions from both academia and industry. Spliced joints are critical connections in large-span glulam members. In this research, the rotational performance of spliced glulam joints with slotted-in-steel-plates were studied. Four full-scale specimens with three different configurations were designed. Cyclic loading test was carried out to obtain the load resisting capacity of the specimens, and finite element models were developed and verified by the test results. The multiple fasteners in the joint exacerbated the convergence difficulties caused by the non-linearity of interactions, thus the key points for convergence performance of the model were also discussed. The stress distribution and the contact-free displacement measurement were obtained to investigate the load resisting mechanism of the joints. A series of calculation formulas for the ultimate strength of the joints were then deduced. The formulas were further verified by the tests and finite element modeling. For most cases, the formulas can well predict the bearing capacity. However, when the numbers of rows and columns are odd, the calculation will underestimate the strengths. Therefore, the results were amplified with a configuration-related parameter under this condition. The parameter was then simplified as a constant coefficient for easier calculation. The modified results constrained the errors within 10 %. The presented research in this paper aims to benefit the engineering design and of spliced joints for large-span glulam members, which may help to support the development of large-span timber structures.

Exploring the relationship between embedment behaviour in cross-laminated timber and its layers: An experimental study

Cross-laminated timber (CLT) has become popular as a construction material and the design of CLT connections is typically based on empirical equations with an average embedment strength of the wood-based product. The experimental study presented herein aims at enhancing the understanding of the relationship between the CLT embedment behaviour and the single layers it is composed of. Non-linear embedment stress–displacement relationships were measured in 244 full-hole and half-hole embedment tests on structural timber boards as well as on CLT produced of parts of the same boards. Three different fasteners, namely a steel dowel with a diameter of 12 mm and the threaded part of screws with a nominal diameter of 6.5 mm and 10 mm, were studied. The design of the experiments allowed to validate a simple mechanical model with parallel springs for the CLT, which showed good agreement with experiments. In addition to a validation of the non-linear spring model, this unique experimental data-set is further exploited in a comparison with empirical equations and for the derivation of a non-linear embedment ratio, between parallel and perpendicular to the grain embedment stresses, over the displacement. Knowledge of the relationship between the non-linear embedment behaviour of the single layers and the CLT element can be further exploited in the engineering design and numerical modelling of CLT connections with dowel-type fasteners.

Load-bearing capacity of slender dowel-type fasteners in Timber-Concrete Composite connections

Load-bearing capacity of slender dowel-type fasteners of round ring-shank nails and U-shaped connectors (U-conn:s) of square cross-sectional nail thread used as connections in Timber-Concrete Composite structures have been studied, experimentally and with two different theoretical models. Tensile strength and withdrawal capacity of fasteners and embedment strength of timber were tested and evaluated. The parameters were used for calculation of the plastic hinge location in the fastener on the timber side and the load-bearing capacity of the connections. For this, both the European Yield Model known from the Eurocode 5, as well as a model where the load-bearing capacity is determined from mechanical equilibrium of fasteners in deformed state were used. Double shear push-out tests were carried out to determine the load-carrying capacity of the connections. The calculated values are compared to the experimental results. The models showed similar results for the U-conn:s regarding both plastic hinge location and load-bearing capacity. For the nails, all models show good correlation with experimental values, however, the model derived from the deformed state predicted both the plastic hinge location and the load-carrying capacity more accurately.

Influence of the joining technique on the structural behaviour of hybrid timber-steel cross-sections

ABSTRACT Timber as construction material has a long tradition and is increasingly coming into focus as a renewable material in the planning and construction of buildings. Nevertheless, there are mechanical limitations, especially for multi-storey buildings or large spans. In order to enable large spans with small cross-sections, steel and timber can be combined in a hybrid cross-section for bending beams. In this case, the performance of the hybrid component decisively depends on the bond of the individual cross-sections. The aim of this publication is to compare and investigate different joining techniques for an efficient bond between timber and steel. Small-scale specimens were used to investigate different bonding methods: dowel-type fasteners, punched metal plate fasteners (PMPF) and adhesive bonds between timber and steel. Adhesive bonded specimens exhibited stiff but brittle behaviour. The adhesive bond remained intact in all tests, while shear failure could be detected in the timber. Adhesive bonds exhibited 93 times the stiffness of dowel fasteners and 5.5 times the stiffness of PMPF. The load carrying capacity increased by approximately 80% compared to dowel-type fasteners and 30% compared to PMPF, demonstrating the superior suitability of adhesive bonds for high performance timber-steel hybrid sections.

Experimental and analytical evaluation of semi-rigid timber connection with screwed-in threaded rods and steel coupling part

A major challenge for mid and high-rise timber buildings is fulfilling serviceability requirements. Moment-resisting timber frames with semi-rigid moment-resisting connections as the primary lateral load-resisting system can be an alternative to buildings with wind bracing or shear walls, allowing for more free space, enhanced building performance, and architectural flexibility. Moreover, introducing semi-rigid timber connections can improve the performance of floors against human-induced vibrations and potentially decrease the material used. The performance of moment-resisting frames depends mainly on the properties of their beam-to-column connections. The paper presents an experimental investigation of semi-rigid moment-resisting connections with long screwed-in threaded rods and a steel coupling part. Long threaded rods (i.e., screwed-in threaded rods with wood screw threads) feature high axial stiffness and resistance and allow for immediate load transfer without initial slip. Due to these properties, threaded rods are preferred over conventional dowel-type fasteners such as bolts and dowels. Five full-scale moment-resisting timber connections were tested under cyclic and monotonic loading to investigate moment resistance, rotational stiffness, and energy dissipation. The stiffness obtained by cyclic and monotonic tests was compared with numerical and analytical methods. The beam-to-column moment-resisting connections demonstrated a high moment resistance of 92.2 kNm and rotational stiffness of 8049 kNm/rad without initial slip. The experimental results and analytical calculations of rotational stiffness differed only by 5 %, whereas the finite element model exhibited a discrepancy of approximately 35 %. The equivalent viscous damping was determined based on cyclic load tests at service load levels and was found to be in the range of 5.5 %–10.0 %. Finally, a parametric study is presented to discuss the effect of semi-rigid connections on the performance of the timber floors and material savings. The increase in rotational stiffness of the beam-to-column connections could lead to up to 13 % material savings.

Embedment strength and foundation modulus at an angle to the grain for fully threaded bolts

ABSTRACT As the main parameter, embedment strength is used to assess the load-carrying capacity of timber joints with dowel-type fasteners and foundation modulus is used to assess their stiffness. The embedment properties, i.e. embedment strength and foundation modulus, are system properties, which depend not only upon the material properties but also upon the load-to-grain angle and surface conditions of fasteners. The formulae for the embedment strengths of dowels and bolts in the design standards of timber structures are almost exclusively derived from the embedment tests on smooth dowels, with the surface conditions different from those of alternative fully threaded bolts in timber structures. Moreover, there are hardly any formulae for determining the foundation modulus. In this study, the embedment tests at different angles to the grain were carried out using the fully threaded bolts with different diameters in oversized holes as they would be used in practice. The load-to-grain angle had statistically significant effects on these embedment properties. The empirical formulae for the embedment strength and foundation modulus were derived. The embedment tests on fully threaded bolts and smooth dowels in tight-fitting holes were also performed to explore the effects of bolt hole sizes and surface conditions.

Beam-on-Foundation modelling of dowel-type single fastener connections in cross laminated timber

In this study, a numerical model that aimed to predict the strength and stiffness, of a single laterally loaded mechanical fastener in steel-to-cross laminated timber (CLT) connections using a Beam-on-Foundation (BoF) model, is presented. The BoF model was used to predict the ductile failure modes of steel-to-CLT connections and considered the layered structure of CLT. When compared to the European Yield Model (EYM), the BoF model was found to be advantageous as it not only predicted the strength but also the stiffness of the connections. A comparison of the slip modulus and strength from BoF model simulations of experimental tests carried out on steel-to-CLT connections showed good agreement with the experimental results. Using the BoF model, the influence of; (i) density, (ii) deck layer orientation, (iii) embedment behaviour, (iv) dowel diameter, (v) steel plate thickness, and (vi) embedment length, on steel-to-CLT connections was investigated in a parameter study. The results presented herein highlight the benefit of the BoF model that considers CLT layer specific embedment behaviour in determining the shear capacity and stiffness of steel-to-CLT connections.

Evaluation of dowel bearing strength of fungal-decayed cross-laminated timber

ABSTRACT Dowel bearing strength of three cross-laminated timber (CLT) species (Douglas-fir, Norway spruce, spruce pine fir) were periodically assessed through 40 weeks of exposure to two brown rot fungi species (Gloeophyllum trabeum and Rhodonia placenta). Time-dependent regression models were developed to describe the relationship between level of fungal exposure and dowel bearing strength of CLT. Obtained dowel bearing strength values were used to predict the capacity of floor-to-wall CLT connection systems. Predicted strength values were compared with capacity of actual connection systems which were subjected to similar biodeterioration treatments. Significant reductions in the dowel bearing strength of CLT were observed 30 weeks after fungal inoculation. Similar patterns of degradation were observed for both fungi in all three CLT species, but R. placenta was slightly more aggressive than G. trabeum, causing a 16.2 MPa reduction in dowel bearing strength of Douglas-fir after 40 weeks of exposure. Connection capacities estimated using National Design Specification yield models for dowel type fasteners and the dowel bearing strength of fungal-damaged CLT were consistent with observed properties and were within 85% of actual capacity of connections tested, especially in early stages of decay.

Behaviour of small-scale glulam timber connections with slotted-in steel plates and dowel-type fasteners reinforced with self-tapping screws

Braced frames are a common lateral force-resisting system in mass timber structures. In timber braced frames, timber-steel dowel connections are relied upon to provide ductility and energy dissipation capacity under earthquake loads. However, the ductility and energy dissipation capacity of these connections can be limited by the onset of a brittle failure mechanism (e.g., row-shear or group tear-out) prior to significant dowel yielding. This paper presents an experimental campaign investigating the structural performance of timber-steel dowelled connections reinforced with self-tapping screws. Twelve connections were tested; with 8 tested under monotonic loading and 4 tested under a cyclic loading regime. The tested connections included two different fastener diameters of 11 mm and 16 mm, with one or two internal steel plates, both with and without reinforcing screws designed to prevent brittle shear failure. The results demonstrate that the reinforcing screws are most effective for improving the ductility and energy dissipation for connections with similar design values of dowel yield and row shear capacities. In addition, the increases in ductility and energy dissipation with reinforcement were greatest for connections with high slenderness (defined as the ratio of dowel diameter to timber thickness) dowels. There was little improvement in the ductility of connections whose design capacity was dominated by row shear, although there was improvement in the energy dissipation of the connections. Overall, results of this study demonstrate the potential for using self-tapping screws to reinforce steel dowel connections and improve the seismic performance of mass timber braced frames.

Simulation of Load–Slip Capacity of Timber–Concrete Connections with Dowel-Type Fasteners

Quality assessment of stiffness and load-carrying capacity of composite connections is of great importance when it comes to designing timber–concrete composite structures. The new European regulation intended explicitly for timber–concrete structures has made a significant contribution to this field, considering that until today there was no adequate design standard. Due to the proposed general expressions for determining the stiffness and load-carrying capacity of composite connections made with dowel-type fasteners, which are incapable of describing most of the commonly applied fasteners, engineering, and scientific practice remained deprived of a quality assessment of the essential mechanical properties of the connection. In order to overcome this problem, this paper proposes a numerical model of the connection suitable for determining the whole load–slip curve, allowing it to estimate the stiffness and load-carrying capacity of the connection. The model was developed by considering the non-linear behavior of timber and fasteners, which is determined through simple experimental tests. For the numerical model validation, experimental tests were carried out at the level of the applied materials and on the models of the composite connection. Through numerical simulations, analysis of obtained results, and comparison with experimental values, it can be confirmed that it is possible to simulate the pronounced non-linear behavior of the timber–concrete connection using the proposed model. The estimated values of stiffness and load-carrying capacity are in agreement with the conducted experimental testing. At the same time, the deviations are much less than the ones obtained from recommendations given by the new regulation. Additionally, apart from evaluating the value and the simulation of the complete curve, it is possible to determine local effects, such as the crushing depth in timber and concrete, the fastener’s rotation, and the participation of forces in the final capacity of the connection.

Embedment behavior of dowel-type fasteners in birch plywood: Influence of load-to-face grain angle, test set-up, fastener diameter, and acetylation

Birch plywood has superior mechanical properties compared with plywood made from most softwood species, which makes it suitable for structural application. Plywood is also more environmentally friendly, cost-efficient, and less prefabrication demanding than steel plates. For a proper design of birch plywood as joint plates in timber-timber connections, the embedment behavior of mechanical connectors into plywood needs to be properly investigated. In this study, the authors performed embedment tests of dowel connectors into birch plywood specimens under five different angles from parallel to perpendicular to the face grain with a step size of 22.5°. Dowel connectors with three different diameters were utilized to study the influence of dowel diameter on embedment strength and stiffness. Besides, test series using self-drilling screws and acetylated birch plywood specimens were conducted to study the effect of both fastener type and acetylation. Moreover, full-hole and half-hole test results were compared. The embedment strengths were calculated analytically according to different term definitions. The test results were then compared with the analytical results based on formulas reported in the literature, e.g., Eurocode 5 and other design standards.

Embedment strength of dowel-type fasteners for the case of cross-laminated timber plane side insertion

The strength property of dowel-type connections in cross-laminated timber (CLT) is dependent on the embedment strength of the adopted dowels. In this study, eight groups with 164 CLT cubic specimens were tested based on the half-hole embedment test configuration to address the effects of the potential factors on the embedment strength. In the embedment tests, the axial direction of the smooth dowels was perpendicular to the plane face of the CLT cubic specimens (i.e., in the case of CLT plane side insertion). The considered influencing factors included the loading angle θ, the ratio between the thickness of the transverse layers to the thickness of CLT (RTrLay), and the gap between the longitudinal laminas within the embedment area. The experimental results were further utilized to verify the current empirical equations that could predict the embedment strength. Results show that the embedment strength of the dowels in the case of CLT plane side insertion is enhanced by 19.96–68.40 % when changing the θ from 90 degree to 0 degree. When the gap between the longitudinal laminas exists in the embedment area, the distinction between the embedment strength measured through the θ of 0 degree and that measured through the θ of 90 degree can be weakened. When the θ is 90 degree, the gap existing in the embedment area has almost no effect on the CLT embedment strength. In contrast, when the adopted θ is switched to 0 degree, the gap existing in the embedment area can weaken the CLT embedment strength by up to 37.11 %. When no gap exists in the embedment area, the embedment strength declines with an increase of the RTrLay; whereas when a gap between the longitudinal laminas exists in the embedment area, a growing trend of the embedment strength is illustrated with the increasing RTrLay.

Composite Beams Made of Waste Wood-Particle Boards, Fastened to Solid Timber Frame by Dowel-Type Fasteners

To increase the sustainability of prefabricated timber buildings and constructions, composite timber beams with "box" cross-sections were developed in collaboration with an industry partner. They were constructed from a solid timber frame and from webs made of residual waste wood-particle boards from prefabricated timber buildings production. The developed beams' design concepts presented in this paper were governed by architectural features of prefabricated timber buildings, geometrical limitations, available production technology, and structural demand related to various possible applications. The paper presents the results of experimental bending tests of six variations of the developed composite timber beams constructed by mechanical fasteners only. The developed design concept of composite timber beams without adhesives is beneficial compared to glued beams in terms of design for deconstruction and lower VOC emissions. The tests were conducted to study the influence of the following parameters on the beams' mechanical behavior: (i) web material (oriented strand boards (OSBs) vs. cement-particle boards); (ii) the influence of beam timber frame design (flanges and web stiffeners vs. flanges, web stiffeners, and compressive diagonals), and (iii) the influence of stiffener-flange joint design. Besides the beams' load-bearing capacities, their linear and non-linear stiffness characteristics were the main research interest. While adding compressive timber diagonals did not prove to significantly increase the stiffness of the beams in the case of cement-particle board webs, it increased their load-bearing capacity by enabling the failure of flanges instead of prior webs and stiffener-flange joints failure. For beams with OSB webs, failure of the bottom flange was achieved already with the "basic" timber frame design, but timber diagonals proved beneficial to increase the stiffness characteristics. Finally, mechanical characteristics of the developed beams needed in structural design for their application are provided together with further development guidelines.

Dowel–bearing behavior of bamboo scrimber for bolted–type joint

Dowel-bearing behavior is one of the key material characteristics in the design of laterally loaded joints with dowel-type fasteners and provides its strength and stability of structures. In this study, the dowel-bearing behavior of bolted joints on bamboo scrimber were investigated by analyzing the effect of specimen size, bolt diameter and loading direction on the dowel-bearing behavior. The estimation formulas of dowel-bearing strengths of bamboo scrimber for engineering utilization were developed based on conducting on 16-group experiments comprising a total of 80 half-hole samples. The results indicate that the initial stiffness was affected by the bolt diameter, sample thickness and loading direction. Dowel-bearing strength was only affected by bolt diameter and it decreased gradually with the increase in bolt diameter, irrespective of specimen sizes and loading direction. The failure mechanisms of the samples loaded in parallel- and perpendicular-to-grain were completely different. The existing empirical expressions was suitable for bamboo scrimber loaded in parallel-to-grain when larger bolts were used, but can under-estimate the dowel-bearing strengths in perpendicular-to-grain direction. Finally, semi-empirical regression-based formulas as functions of compressive strength and bolt diameter were proposed, which exhibited good agreement with the experimental dowel-bearing strength. The findings will be useful for designing safe and efficient bolted joints to promote more widespread use of bamboo scrimber in construction, as well as supplying basic data for systematic development of test and design standards for bamboo structures.

Elastoplastic Analysis of Timber Connections with Dowel-Type Fasteners at Ambient and High Temperatures

Three-dimensional elastoplastic finite element (FE) analysis models considering a damage zone and cleavage were developed, including plastic behavior of embedment tests parallel and perpendicular to the grain at ambient and high temperatures, in order to study the embedment behavior of timber. In a numerical analysis of compression tests to define material properties of timber, the post-yield behavior of the X-axis in the axial direction was larger than the test values when the post-yield slope of the test result were set to the plastic modulus. The reason was that the stress on the Y-axis and Z-axis increased after the yield point; thus, the equivalent yield stress decreased. In the numerical analysis of the embedment tests, the effect of the damage zone must be considered in the analytical models because the initial stiffness of the numerical results in compression parallel to the grain was considerably higher than that of the test results when the damage zone was not considered. In the numerical analysis of the embedment tests in compression perpendicular to the grain, the two models reproduced the stiffness and behavior after the yield point of the test results. The model that considered cleaving showed the stress distribution after cleavage.

Numerical and Experimental Analysis of the Load-Carrying Capacity of a Timber Semi-Rigid Dowel-Type Connection.

The paper deals with the analysis of the load-carrying capacity of a timber semi-rigid connection created from a system of two stands and a rung. The connection was made from glued laminated timber with metal mechanical dowel-type fasteners. Not only a common combination of bolts and dowels, but also fully threaded screws were used for the connection. The aim of the research and its motivation was to replace these commonly used fasteners with more modern ones, to shorten and simplify the assembly time, and to improve the load-carrying capacity of this type of connection. Each of these two types of connections was loaded statically, with a slow increase in force until failure. The paper presents results of the experimental testing. Three specimens were made and tested for each type of the connection. Experimental results were subsequently compared with numerical models. The achieved results were also compared with the assumption according to the currently valid standard. The results indicate that a connection using fully threaded screws provides a better load-carrying capacity.

Experimental and theoretical study on mechanical performance of dowel joints with inserted steel plates constructed of laminated bamboo lumber

Laminated bamboo lumber (LBL) is a promising material in engineering structures due to its excellent mechanical properties and eco-friendly characteristic. LBL joints comprised of dowel-type fasteners and inserted steel plates are introduced to study the mechanical performance based on the ASTM-D5652-15. A total of 39 joints in 13 groups were carried out to investigate the effects of dowel diameter, thickness of specimens, spacing in a row and dowel number on the stiffness and strength of the joints. Three typical yield modes of the dowels were observed, which accorded with Johansen yield theory. The theoretical results of bearing capacity calculated by rigid-plastic model and elastic-plastic model were larger than experimental results, and the difference in both models was in the range of 9.1%–11.33%. By introducing the calibration coefficient and effective dowel number, the calculation method of bearing capacity was provided and proven to conform well to the experimental results. It can provide the reference to calculate bearing capacity of with inserted steel plates constructed of laminated bamboo lumber.

Numerical and Experimental Analysis of the Rotational Stiffness of a Timber Semi-Rigid Dowel-Type Connection

The paper deals with the analysis of the rotational stiffness of a semirigid connection created from a system of two stands and a rung. The connection was made from glued laminated timber with metal mechanical dowel-type fasteners. Not only a common combination of bolts and dowels but also fully threaded screws were used for the connection. The aim of the research and its motivation was to replace commonly used fasteners with more modern ones, to shorten and simplify the assembly time, and to improve the load-carrying capacity of this type of connection. Each of these two types of connection was loaded to the level of 60%, 80%, and 100% of the ultimate limit state value. Subsequently, the rotational stiffness was determined for each load level after five loading and unloading cycles. This paper presents the results and comparison of the experimental testing and the numerical modeling. The obtained results were also compared with the assumption according to the currently valid standard.