Slip Modulus Research Articles

Experimental and numerical analysis on timber-concrete connections with glued reinforcing bars

Shear connectors have a great influence on the mechanical behavior of timber-concrete composite structures, interfering with the stiffness and strength of the structural elements. This paper presents experimental and numerical studies about the mechanical behavior of timber-concrete composite structures by evaluating two types of connectors: one formed by an inclined rebar and another formed by the association of an inclined rebar with a triangular notch. The rebars used were ribbed-type with yield strength equal to 500 MPa, and their fixation on timber was made by gluing with epoxy resin. The experimental determination of the strength, the slip modulus and the failure modes were made by means of push-out tests. Subsequently, a numerical evaluation using ABAQUS was carried out in order to develop a three-dimensional numerical model that represented these types of connections, considering the nonlinearities of the materials, the orthotropy of timber and the interaction between the components. It was observed that the notch connection showed greater strength and stiffness when compared to the connection without notch. The comparison between the numerical and experimental results showed that the numerical model was able to predict, with good approximation, the failure load and the slip modulus of the connection without notch, while the stiffness of the numerical model proved to be higher than that observed in the experimental analysis for the notch connection.

Effect of Interlayer and Inclined Screw Arrangements on the Load-Bearing Capacity of Timber-Concrete Composite Connections

The solution of timber-to-concrete composite (TCC) floors represents a well-established construction technique, which is consistently used for both the retrofitting of existing timber floors and the realization of new diaphragms. The success of TCC floors relies on the intrinsic effectiveness in increasing both the in-plane (for lateral loads) and the out-of-plane (for gravity loads) performance of existing timber floors. As a widespread retrofit intervention, it is common to use existing floorboards as a permanent formwork for the concrete pouring. Rather few research studies of literature, in this regard, highlighted an overall reduction of load capacity and slip modulus due to the presence of such an interposed interlayer. In this regard, the present paper focuses on the use of screws as efficient mechanical connectors and analyses different configurations and inclination angles for their arrangement. This main goal is achieved by performing parametric Finite Element (FE) numerical analyses, validated on previous experimental tests, in order to specifically investigate the influence of the in-between interlayer, as well as the role of friction phenomena and the influence of the test setup and experimental protocol to achieve the basic mechanical performance indicators.

Shear behavior of notched connection for glubam-geopolymer concrete composite structures: Experimental investigation

The glubam (glue-laminated bamboo)-geopolymer concrete composite (BGCC) structure is a possible way to achieve sustainable construction due to its combination of renewable resources and industrial waste. This study combined glubam and geopolymer concrete in composite structures and investigated the shear behavior of BGCC structures with notched connections. Four groups of push-out tests were designed to evaluate the influence of the number of notches and screws on the slip modulus and shear capacity. The results showed that the composite structures with notched connections failed first due to shear cracking at the interface notch. The double-notch specimens increased the shear capacity by 54% compared to single-notch specimens. The shearing bearing capacity rose by 35% on average as a screw increased in a single notch. The ductility and slip modulus were influenced primarily by the screws, with each extra screw in a single notch increasing the slip modulus by 21% in each stage. Based on the test results, a modified formula was proposed to predict the shear-bearing capacity of notched connections in BGCC structures. This study provides comparison data for further studies in the long-term behavior of BGCC structures.

Behavior of horizontal steel plate + studs connectors in a glulam-UHPC composite system: Experiment and analysis

Compared with conventional timber–concrete composite structures, glued laminated timber (glulam)–ultrahigh–performance concrete (UHPC) composite (GUCC) structures can remarkably reduce the deadweight and have small long-term deflection, good durability, and improved span capacity. Thus, this work designed a horizontal steel plate + stud (HS) connector with high stiffness and high shear capacity to ensure a reliable connection in the designed glulam–UHPC composite system and then investigated the structural behavior of the HS connector in the GUCC. In total, fifteen push–out specimens (in five groups with three replicates of each group) were fabricated and conducted a static load test. Effect of size of steel plate, the height-diameter ratio of stud, size of slab, and slab type (UHPC or NC) on the load-slip rules, shear stiffness, failure mode, and the shear capacity of HS connectors in GUCC were evaluated experimentally. The experimental results show that the average slip modulus and shear capacity of the HS connection ranges from 134.9 to 165.6 kN/mm and from 167.6 to 200.3 kN respectively. The primary failure mode of the structure comprises the shear failure of the glulam along the grain and the studs. Lastly, the empirical formula for calculating the load–slip relationship and the design formula for the shear capacity of the HS connection of GUCC systems are proposed. A comparison between the theoretical and experimental results reveals that the developed method has reasonable accuracy and can be used to guide the design of GUCC structures.

Experimental investigation of headed studs in steel-ultra-high performance concrete (UHPC) composite sections

Featuring high compressive and tensile strengths, ultra-high performance concrete (UHPC) is an efficient and durable alternative for conventional concrete in steel–concrete composite sections. However, few experimental studies to date have involved large diameter (≥ 25 mm) headed studs, UHPC with compressive strengths larger than 130 MPa, and relatively low strength but cost-effective UHPC. This study investigates the shear performance of steel headed studs embedded in UHPC through pushout tests with 24 specimens. Specimens included headed studs with diameters of 16 mm, 19 mm, 22 mm, and 25 mm, and either UHPC with compressive strength of 152.5 MPa or a cost-effective UHPC with a strength of 85.7 MPa (named as HPC hereafter). The results showed that smaller-diameter specimens (16 or 19 mm) failed with stud shank fracture, while the larger-diameter specimens (22 or 25 mm) failed at welding lines. Compared to HPC, UHPC did not notably increase the performance of headed shear strength. The load response consisted of a linear stage, a softening stage, and a post-peak stage lasting up to a slip of 10 mm or greater, with high variability in post-peak response among specimens with identical parameters. A global database of 120 push-out test specimens from this study and references was established, covering concrete strengths from 23.5 MPa to 152 MPa. The formulae given by AASHTO LRFD Bridge Design Specification, Eurocode-4, and Chinese code GB50017-2003 overestimated the shear capacity of headed studs by 52 %, 5 % and 53 %, respectively. The equation for the slip modulus slightly overestimated the slip modulus of headed studs. In conclusion, this study broadened the ranges of concrete strength and diameter of headed shear studs, and revealed that the existing equations of predicting shear capacity and slip modulus need improvement.

Experimental Study of Aluminium-Timber Composite Bolted Connections Strengthened with Toothed Plates.

This paper presents the first experimental study of the load-slip behaviour of aluminium-timber composite bolted connections reinforced with toothed plates. The effectiveness of the strengthening was evaluated in laboratory push-out tests. The push-out test samples consisted of laminated veneer lumber panels, aluminium alloy I-beams, and bolts (grade 8.8 10 mm × 125 mm and 12 mm × 135 mm bolts, grade 5.8 10 mm × 125 mm and 12 mm × 135 mm bolts). A group of 16 specimens had toothed plates as additional reinforcement, while 16 specimens had no reinforcement. The impact of the bolt diameter (10 and 12 mm) and bolt grade (5.8 and 8.8) on the behaviour of the connections was also analysed. The values of the ultimate load and the slip modulus for the bolted connections with grade 8.8 10 mm and 12 mm bolts and with grade 5.8 12 mm bolts reinforced by toothed-plate connectors were comparable to the values for the non-reinforced connections. This was because, in the case of grade 8.8 10 mm × 125 mm and 12 mm × 135 mm bolts and grade 5.8 12 mm × 135 mm bolts, the laminated veneer lumber (LVL) slabs split both in the reinforced and non-reinforced connections. The toothed-plate connectors reduced timber destruction in the bearing zones in the LVL slabs. However, they did not protect the LVL slabs against splitting. Therefore, the impact of the toothed plate connectors on the stiffness and strength of the bolted connections with grade 8.8 10 mm and 12 mm bolts and with grade 5.8 12 mm bolts analysed in this paper was found to be negligible. In the case of grade 5.8 10 mm bolts, the LVL slabs did not split. The mean slip modulus k0.6 of the connections with grade 5.8 10 mm bolts reinforced with toothed plate connectors was 2.9 times higher than that of the non-reinforced connections. However, the strength of the connections with grade 5.8 10 mm bolts was 1.2 times lower after reinforcing. This was because the shanks of the bolts were sheared faster in the reinforced connections than in the non-reinforced connections as a result of the bolt shanks being under the bearing pressure of the aluminium flange, the LVL slab, and the toothed-plate flange. This situation did not occur for the remaining connections because they had a higher strength (grade 8.8 bolts) or a larger diameter (12 mm), and their bolts were less prone to cutting off. The investigated load–slip curves of the reinforced bolted connections can be used for designing and numerical modelling of aluminium-timber composite beams with this type of connection.

Shear performances of shallow notch-screw connections for timber-concrete composite (TCC) floors

Shallow notch-screw connections, which showed potential superior slip moduli and load-carrying capacity compared with the traditional screwed connections and can be employed in the timber-concrete composite (TCC) floors were examined in this study. Eight groups of shallow notch-screw connections were designed to perform the push-out tests, in which the arrangement of screws, the heavy timber types, and the width of the shallow notch were considered. The depth of the notch was uniformly 15 mm. The vertical screws and the cross inclined screws were separately selected as the reinforcement for the shallow notch connections. The common heavy timber panels, including nail laminated timber (NLT), glulam, and cross laminated timber (CLT), were adopted. The width of the shallow notch tested included 100 and 200 mm. The experimental results showed that the shallow notch connections underwent ductile failure. The effects of testing factors on the shear strength, slip moduli, and ductility were discussed. The design proposals about the slip moduli of the shallow notches using each timber panel types were proposed, aiming to provide guidance for the application of the TCC floors with shallow notches.

Pull Out Resistance of Glued in Rod Parallel to Grain in Laminated Bamboo

Research on hollow cross section beams and columns made of laminated bamboo has been carried out. So that the two elements can be assembled into a building structure, it is necessary to do connection. Glued in rod is a new connecting tool whose mechanical properties still need to be studied. For this reasons this research was conducted. To achieve this objective, an experiment was conducted with the independent variables (1) the distance between the two edges, (2) the diameter of the threaded rod, and (3) the embedded length. The results showed that (1) the greater the distance between the two edges, the greater the pull out strength of the glued in rod, (2) the larger the diameter of the threaded rod, the greater the pull out of the glued in rod, (3) the greater the embedded length of the threaded rod, the stronger the pull out of glued in rod, (4) the value of the slip modulus is affected by variations in diameter and embedded length but is not affected by the distance between the two edges. The greatest pull out strength, which is 34.85kN, is achieved at a glued in rod diameter of 12mm with an edge distance of 4.5d and a embedded length of 6d, while the lowest value is 14.38kN lies in steel rods with a diameter of 8mm with a embedded length of 5d and a distance of two edges is 2.5d. The highest slip modulus or the highest stiffness value of 9.75kN/mm is achieved at a diameter of 12 mm with an embedded length of 6d and a distance of two edges of 3.50d. The smallest slip modulus value of 3.75kN/mm was achieved at a diameter variation of 8 mm with an embedded length of 5d and a distance of two edges 2.5d

Thermo-mechanical analysis on shear behavior of grooved connectors for glulam-concrete composite beams under fire

This research analyzed the effects of fire duration, groove length, groove (beam) width, groove depth, screw diameter, and screw penetration length on the shear behavior of grooved connectors for glulam-concrete composite beams under fire. Three-dimensional non-linear finite element (FE) models were used to evaluate the thermo-mechanical behavior of grooved connectors under ISO fire. The model was confirmed by comparing load-slip curves with experimental results. Experimental and numerical results showed that when the failure mechanism shifted from the shear plane failure of the concrete groove to compression failure of the wood in front of the groove, the influence of groove length on shear capacity of connectors was no longer significant. Parametric analysis indicated that with the rise of groove depth, the shear capacity and slip modulus of connectors rose progressively. However, when the failure mode shifted from compression failure of wood in front of the groove to the shear plane failure of the concrete groove, the groove depth had no significant influence on the shear capacity of connectors. The groove (beam) width had a substantial influence on the shear capacity and slip modulus of grooved connectors under fire, whereas it was not the case for screw diameter and screw penetration length.

A serviceability investigation of dowel-type timber connections featuring single softwood dowels

Dowel-type connections are common in timber engineering, but current design codes are largely based on empiricism and are oversimplified. This inhibits the optimised use of connections, which is essential for the design of economically efficient timber structures. This study investigates the use of 3D computational modelling to predict the slip modulus (a key measure of stiffness) of single and double shear, dowel-type connections featuring a single softwood dowel. Initial modelling was conducted on parallel- and perpendicular-to-grain connections to establish a suitable mesh size and to validate the model against experimental work. The slip modulus in angled orientations was then investigated, a relationship given no consideration in design codes. The results show that the current design codes greatly overestimate the slip modulus in both single and double shear connections involving timber dowels. In comparison, the models in this study are more than twice as accurate at predicting slip modulus. Furthermore, slip modulus was shown to vary sinusoidally as the grain-to-grain angle changes between the parallel and perpendicular orientations. Differences between model and experimental values can be attributed to uncertainty in the mechanical properties of the timber in the experiments, the assumption of uniform properties for timber in each principal direction in the models, and the inherent variability of timber which affects experimental results.

Shear performance of assembled shear connectors for timber–concrete composite beams

This paper presents three types of assembled shear connectors consisting of a steel component anchored to timber by screws and glued-in rods (GiRs) for timber-concrete composite (TCC) beams. Single-shear tests were carried out to evaluate the shear performance of the proposed shear connectors. The test results showed that the anchors bent obviously and finally failed in “single-hinge” and “double-hinge” yield modes under lateral load. The timber surrounding the anchors was obviously crushed, and cracks in concrete were also observed during testing. The shear connectors with I-shaped steel anchored by GiRs exhibited the highest shear capacity of 158.92 kN. The folded steel plate shear connectors with GiRs and screws showed the highest slip modulus (38.41 kN/mm), and the highest ductility with an average coefficient of 29.6, respectively. In general, the shear connectors anchored with GiRs showed better shear performance compared to the screwed shear connectors. Finally, numerical simulation results indicated that the simulated load–slip curves were generally in good agreement with the test curves. The anchor diameter and strength and the frictional coefficient between anchors and the wall of the holes in timber have significantly influenced the shear performance of the proposed shear connectors.

Shear performances of hybrid notch-screw connections for timber-concrete composite structures

This paper presents the push-out experimental results of hybrid notch-screw (HNS) connections for timber-concrete composite structures. A total of 7 groups of specimens were designed and tested. The experimental parameters included the loading constraint conditions (i.e., the test specimens were loaded either in local compression or in uniform compression), shapes of notches in the wood, screw number in notch, notch width, and the inclusion of a self-tapping screw reinforcement for timber or not. The experimental results were discussed in terms of failure modes, ultimate strength, slip moduli, and ductility. The yield strengths and ductility factors were determined based on the load-slip curves according to existing standards. The experimental results showed that both the shear timber width and the self-tapping screw reinforcement played important roles in terms of the ultimate strengths, ductility, and deformability. Rectangular notched connections with screw reinforcements displayed timber shear failure coupled with brittle failure. With the trapezoidal notch, the ductility of the connections improved, coupled with a decrease in the slip modulus. The self-tapping screw reinforcement for shear timber could greatly improve the ductility performance of the HNS connections. The slip modulus models for the connection with vertical deep notches were provided, which were in agreement with the experimental results.

Numerical Investigation of Connection Performance of Timber-Concrete Composite Slabs with Inclined Self-Tapping Screws under High Temperature

The timber-concrete composite (TCC) slabs have become a preferred choice of floor systems in modern multi story timber buildings. This TCC slab consisted of timber and a concrete slab which were commonly connected together with inclined self-tapping screws (STSs). To more accurately predict the fire performance of TCC slabs, the mechanical behavior of TCC connections under high temperature was investigated by numerical simulation in this study. The interface slip of TCC connections was simulated by a proposed Finite Element (FE) model at room temperature, and different diameter and penetration length screws were considered. The effectiveness of this FE model was validated by comparing with the existing experimental results. Furthermore, the sequentially coupling thermal stress analyses of this model were conducted, and the relationship between the reduction coefficient of connection performance and the effective penetration length of screws was summarized. This study gave the fitting expressions for the reduction coefficient of slip modulus and joint strength. Finally, the numerical investigations of the fire performance of TCC slabs considering the char fall-off of Cross Laminated Timber (CLT) were performed to verify the effectiveness of the proposed reduction law. Comparing the fire-resistance time with experimental results showed deviation of the proposed model was −14.02%.

Study on shear performance of notched connections for glulam-concrete composite beams under fire

Shear connection guarantees that the timber beam and concrete slab of a composite beam interact effectively. Shear connection performance markedly affects the fire resistance of timber-concrete composite beams. Shear connection experiments under fire are uncommon in comparison to those conducted at ambient temperature, and there have been few previous investigations on the shear performance of notched connections under fire. Six specimens were sheared at ambient temperature and ten specimens were sheared at elevated temperature in this study to determine the shear behavior of notched connections exposed to ISO fire. The length of the notch connection and the duration of fire exposure are two factors that may influence the shear behavior of the connections. With the increase of fire time, the shear bearing capacity and slip modulus of notched connections decreased. As notch length increased from 150 mm to 250 mm, the failure modes of shear specimens under fire changed from shear failure of concrete in the notch to compression failure of timber near the notch. Experimental results and finite element thermal analysis results were used to establish an analytical model for the shear bearing capacity of notched connections for glulam-concrete beams under ISO fire exposure.

Experimental and theoretical investigation of long-term performance of steel-timber composite beams

The structural performance of the steel-timber composite floors under sustained long-term loads was investigated by laboratory tests over a period of 22 months. The long-term deflections, end-slips, and strain distributions over the cross-section were measured and reported for six beams with a loading span of 5.8 m. Effects of different shear connectors, arrangement of the timber panels and timber slab segmentations and the magnitude of applied loads were studied. The timber slab segmentation had a detrimental effect on the short- and long-term stiffness of the tested beams. The long-term effects (after 22 months) led to a maximum 19% increase in the midspan deflections. Furthermore, the moisture transport, variation of mechanical properties with respect to moisture content, long-term compressive deformation of the panels and long-term slip moduli of the shear connectors were investigated to provide a thorough understating of the long-term behaviour of the steel-timber composite system. A simplified analytical model for estimating the flexural stiffness of the steel-timber composite beams was proposed and employed to estimate an overall creep deformation factor for the composite beams. The creep deformation factor of the tested beams was in the range of 0.29–0.39 depending on the shear connector type and timber slab orientation.

Analytical modelling of cold-formed steel-to-timber connections with inclined screws

An analytical model that can describe the load-slip response of cold-formed steel-to-timber connections, based on three components: timber embedment, screw bending and the axial pull-through, is presented. The model takes into consideration the screw inclination and the damage to the timber caused during the screw drilling process. The model was validated against available push-out test results, where it was shown that the model can accurately describe the ultimate load, the slip at ultimate load, and the two slip moduli, ks and ks,m, with mean model-to-test ratios of 1.01, 1.03, 1.09 and 0.96, respectively. The observed failure modes were also accurately predicted. Comparisons between the test data and the predictions of the EN 1995-1-1 design expressions showed that the Eurocode underestimated the ultimate loads by around 50% on average and overestimated the slip moduli by around 500% on average; the proposed model therefore offers a significant improvement over current design provisions.

Shear capacity of timber-to-timber connections using wooden nails

ABSTRACT With increasing environmental concerns, the building sector must rethink the selection of building materials not only for structural members but also for connections. Densified wooden nails might become an alternative to metallic fasteners, at least in applications with lower structural requirements. However, their structural behaviour in timber-to-timber connections is not yet systematically studied. This paper presents a series of 90 shear tests to explore the shear capacity and slip modulus of timber-to-timber connections with wooden nails. Specimens with different nail dimensions, different nail orientations, and different nail arrangements were investigated. A typical three-member push-out test setup was adopted where the wooden nails were oriented perpendicular or inclined to the shear plane. Specimens with inclined nails exposed to shear and tensile forces showed a higher shear resistance due to the activated tensile capacity of the wooden nails. Failure of the wooden nails was predominantly observed when the nails were loaded in tension. This means that the interface between the wooden nails and the timber did not fail. Based on the results from the shear tests, an analytical model was developed to predict the load bearing capacity of timber-to-timber connections exposed to bending stresses. The analytical model was validated with experimental investigations.

Evaluation of parameters influencing the load-deformation behaviour of connections with laterally loaded dowel-type fasteners

ABSTRACT Connections made with laterally loaded dowel-type fasteners are important details in timber structures. According to Eurocode 5, their load-carrying capacity can be calculated with the so-called European Yield Model (EYM) and simplified rules for the determination of slip-moduli are given. The slip modulus is given as a mean value for the serviceability limit state and a simple reduction of slip modulus in the ultimate limit state is given in addition. Despite these simple regulations, it is well known that connections with dowel-type fasteners show a considerable non-linear load-deformation behaviour with different degrees of ductility. This ductility can enable the load redistribution in complex and statically undetermined structures and allow to achieve higher capacities compared to linear elastic design. In the paper, the deformation behaviour of connections with laterally loaded dowels dowel-type fasteners is studied based on more than 750 test results of bolted connections. The parameters influencing the slip-modulus, ductility ratio, and ultimate deformation are evaluated. It is focused on the effects of these parameters and the resulting variability in deformation behaviour. Recommendations are given on how different levels of ductility and deformation capacity can be achieved in dependency of the spacing of fasteners and other geometrical parameters.

Timber-to-timber and steel-to-timber screw connections: Derivation of the slip modulus via beam on elastic foundation model

The aim of this paper is to propose formulations for the slip modulus prediction of timber-to-timber connections (TTC) and steel-to-timber connections (STC) with inclined screws and possible interlayers. The beam on elastic foundation model, previously developed for timber-to-concrete connections, was extended to consider the flexibility of both media where the screw is inserted. Since a significant influence of the fastener diameter on the foundation modulus was observed in tests, an interpolating formula correlating the foundation modulus with timber density and the fastener diameter was derived. The exact solution of the timber-to-timber analytical model was found to agree well with experimental results for total and double thread screws. A parametric study was undertaken to prove that connections with inclined screws have significantly higher slip modulus and lower degradation of performance as the diameter decreases or the thickness of the intermediate layer increases compared to connections with screws perpendicular to the sliding plane. Furthermore, the slip modulus of inclined screws was found to be limited by the weakest timber layer. Closed form expressions for the prediction of the slip modulus were derived by interpolation for the most important cases of technical interest. These formulas can be proposed for the implementations in codes of practice such as the Eurocode 5, since simplified formulas of the slip modulus are currently missing for connections with inclined fasteners and interlayers.

Effect of Rod Diameter and Adhesive Thickness to the Pull-out Strength of Threaded Steel Rod Glued in Laminated Bamboo

The purpose of this study was to determine the effect of threaded steel rod diameter and adhesive thickness on pull-out strength, shear strength, slip modulus, and damage patterns of glued-in rod embedded in laminated bamboo. The shape of the specimen is a cube with the dimensions of 100mm × 100mm × 100mm made of laminated Asian Bamboo (Dendrocalamus asper). The threaded steel rod diameter consists of three sizes, namely 8mm, 10mm, and 12mm. Each threaded steel rod is embedded in laminated bamboo to a 40mm length. The epoxy brand Sikadur 372 is used as an adhesive between laminated bamboo and threaded steel rods with three variations of adhesive thickness, namely 2mm, 3mm, and 4mm. Each variation of the test object was repeated five times. The results showed that the rod diameter and adhesive thickness affect the pull-out strength, shear strength, and slip modulus.