Multiple Bolts Research Articles

Tensile behavior of hybrid multi-bolted/bonded joints in composite laminates

In composite structures, the strength of a standard single-lap joint with multiple bolts at best matches the strength predicted by the standard open-hole tension (OHT) test, which is about 50% of the tensile strength of the unnotched material. Although bonded joints do not have such limitation, they carry other drawbacks. The advantages of bolting and bonding may be combined in hybrid/bonded-bolted (HBB) joints. This study investigates HBB joints using carbon and glass-fiber reinforced composites with up to three bolts. It is found that multi-bolt specimens with or without adhesive fail in net-tension at the outer bolts like in OHT tests. However, HBB joint is not anymore limited by the OHT strength. The addition of the adhesive increases the strength of a three bolts joints by 70% for cross-ply laminates and 30% for quasi-isotropic laminates. The synergy between the bolts and the adhesive in the HBB system is interpreted by the fact that the outer bolts limit peel stresses and concurrently, the adhesive reduces the stress concentration around the bolts. This is particularly important for the cross-ply configuration where the stress concentrations around the holes are high. Other features observed suggest that for multi-bolted HBB joint, only external bolts are needed. Such joint configuration combines the safety provided by the bolts and the efficient load transfer provided by the adhesive.

A bolt preload monitoring method based on the refocusing capability of virtual time reversal

Bolted joints are widely used in aerospace and civil structures. Nevertheless, bolts may easily lose preload because of inappropriate initial preloads or time-varying servicing loads. Hence, bolt loosening monitoring is important for ensuring the safety and reliability of the bolt-jointed structures. In this paper, a guided wave method based on virtual time reversal (VTR) and tightness index representing refocusing capability is developed for bolt preload monitoring and to mitigate the requirement of baseline. In VTR, a response signal acquired from the bolted structure in fully tightened condition is reversed and used as the excitation signal for the bolted structure in loosening states. On this basis, the ratio of the energy of the refocused wave packet in final received wave signal to the energy of the entire signal is defined as tightness index (TIE). In this way, the calculation of TIE does not need to compare with baseline acquired in fully tightened condition. The effects of roughness of contact surfaces on guided wave propagation and the refocusing capability are analyzed by a 2D FE model. The proposed method is also experimentally validated by bolt-jointed structures with single and multiple bolts. The experimental results show that TIE increased almost linearly with the decease of bolt preload and is effective at the early stage of bolt loosening. In addition, the effect of temperature on preload detection accuracy is weak. Furthermore, it is important to select a right frequency to ensure that the TIE is sensitive throughout the entire bolt torque range.

A Paradigm for the Analysis of Preloaded Bolted Joints

Abstract The purpose of this paper is to present a paradigm, or guide, to the analysis of preloaded bolted joints made using multiple bolts. Classical analysis methods are applied to the interaction of the joint elements subjected to combinations of both in-plane and out-of-plane loads and moments. The distribution of the external loads and moments within the preloaded joint is determined in relationship to individual bolts. An analysis of loads and stresses in individual bolts and dowels along with flange bending and thread shear in tapped or threaded holes is developed. The article brings together a number of concepts and links them into a practical design analysis process that is applicable for many cases of preloaded bolted joints and are adequate to demonstrate the structural integrity of each element of the joint. Interpretation of results, within the context of design standards, is provided. In some cases finite element methods may be more appropriate, and the methods discussed can be used in the validation process.

Finite Element Analysis of the Mechanical Behavior of Multibolted Joints Subjected to Shear Loads

When subjected to external forces, bolted joints behave in a complex manner especially in the case of the joints being clamped with multiple bolts. Friction type joints are widely used for the joints subjected to shear loads. Bearing type joints, which support the shear loads on the bolt cylindrical surface, are used less frequently, since its mechanical behavior is too complicated to accurately estimate the load capacity. In this study, mechanical behavior of the bearing type multibolted joints subjected to shear loads is analyzed by three-dimensional (3D) FEM. As a result of comprehensive calculations, it has been found that the shear load applied to bearing type joints distributes with a concave shape along the load direction, and a fair amount of the shear load is supported by friction forces as in the case of friction type joints. In addition, a simple method that calculates the shear load distribution using elementary theory of solid mechanics is proposed, which can estimate the shear load distribution with sufficient accuracy especially for the case of small friction coefficient.

Preload Monitoring of Bolted L-Shaped Lap Joints Using Virtual Time Reversal Method.

L-shaped bolt lap joints are commonly used in aerospace and civil structures. However, bolt joints are frequently subjected to loosening, and this has a significant effect on the safety and reliability of these structures. Therefore, bolt preload monitoring is very important, especially at the early stage of loosening. In this paper, a virtual time reversal guided wave method is presented to monitor preload of bolted L-shaped lap joints accurately and simply. In this method, a referenced reemitting signal (RRS) is extracted from the bolted structure in fully tightened condition. Then the RRS is utilized as the excitation signal for the bolted structure in loosening states, and the normalized peak amplitude of refocused wave packet is used as the tightness index (TIA). The proposed method is experimentally validated by L-shaped bolt joints with single and multiple bolts. Moreover, the selections of guided wave frequency and tightness index are also discussed. The results demonstrate that the relationship between TIA and bolt preload is linear. The detection sensitivity is improved significantly compared with time reversal (TR) method, particularly when bolt loosening is at its embryo stage. The results also show that TR method is an effective method for detection of the number of loosening bolts.

Discussion about effecting of stiffener in four bolts in a row end plate connection for long span and heavy load steel structures in Vietnam

Nowadays, steel structure industry in Vietnam is in strong development. The construction of steel structure becomes larger span and heavier load. The issue spawned a number of issues arise from optimizing connections. Typical of steel connections in prefabricated steel structure that is an end plate (face plate) bolted connection. When the connection carried a heavy load, then the number of bolts is required much more. Increasing the number of rows bolts will less effective because can still not enough strength requirements, the bolts in row near rotational center will level arm reduction, then it cannot carry heavy loads. The current solution is doing multiple bolts in a row. Current standards such as EN [1], AISC [2] are no specific guidelines for calculating the connection four bolts in a row that primarily assumes the way works like a T-stub of the two bolts a row. Some articles studied T-stub four bolts in a row [3], [4], [5], [6] by component method but it has some components which weren’t considered. In this paper, in order to provide a contribution to improve the T-stub four bolts in a row, the stiffener component in T-stub will be added and compared with T-stub without stiffener by the finite element model to demonstrate effectiveness in reducing stress and displacement of T-stub. It gives ideas for the economic design of four bolts in a row end plate connection in Vietnam for future.

An optimized virtual beam-based event-oriented algorithm for multiple fault localization in vibrating structures

This paper investigates a new multiple fault localization method using the virtual beam-based event-oriented algorithm. The virtual beams, represented by some sensor chains, are recently proposed to isolate the potential faults according to the distribution of sensors to certain regions and their associated nonlinear dynamic feature characteristics. To improve the accuracy of fault localization and isolate the multiple faults at more specific and detailed regions, in this paper, the virtual beam-based method is combined with a specially developed feature characterization algorithm named as the subtract on negative add on positive (SNAP). This newly proposed virtual beam-based subtract on negative add on positive (VB-SNAP) method is to take advantage of the virtual beam-based algorithm to narrow the search scope for the potential fault events using fewer sensors and adopt the event estimation strategy of the binary-based SNAP method to enhance the accuracy of fault localization by providing smaller and specific regions containing the potential faults. Two likelihood matrices are constructed according to the alarm states of sensors and the response of sensors from the virtual beams, respectively. Fault localization of the VB-SNAP is, therefore, simplified to estimate the overlapping cells obtained from those two likelihood matrices. The performance of the proposed virtual beam-based event-oriented algorithm is demonstrated through real experimental results. It is shown that the proposed VB-SNAP is superior to the original virtual beam algorithm and SNAP method alone through the localization of multiple bolts loosening in solar panel of a satellite-like structure.

Nonlinear Modeling and Identification of an Aluminum Honeycomb Panel with Multiple Bolts

This paper focuses on the nonlinear dynamics modeling and parameter identification of an Aluminum Honeycomb Panel (AHP) with multiple bolted joints. Finite element method using eight-node solid elements is exploited to model the panel and the bolted connection interface as a homogeneous, isotropic plate and as a thin layer of nonlinear elastic-plastic material, respectively. The material properties of a thin layer are defined by a bilinear elastic plastic model, which can describe the energy dissipation and softening phenomena in the bolted joints under nonlinear states. Experimental tests at low and high excitation levels are performed to reveal the dynamic characteristics of the bolted structure. In particular, the linear material parameters of the panel are identified via experimental tests at low excitation levels, whereas the nonlinear material parameters of the thin layer are updated by using the genetic algorithm to minimize the residual error between the measured and the simulation data at a high excitation level. It is demonstrated by comparing the frequency responses of the updated FEM and the experimental system that the thin layer of bilinear elastic-plastic material is very effective for modeling the nonlinear joint interface of the assembled structure with multiple bolts.

Experimental testing and simulation of bolted beam-column connections having thick extended endplates and multiple bolts per row

Retrofit of existing steel buildings often requires strengthening of the connection regions. One common connection, the bolted beam-column connection, is often strengthened in design using stiffened extended endplates, or with continuity plates welded between the column flanges. In a retrofit scenario, adding stiffeners to the endplate is difficult due to the concrete slab and metal deck, and excessive field welding of continuity plates may be uneconomical. Simplifying retrofit efforts, and for economy, connection strength may be improved by simply adding more bolts to the connection. Current code methods, broadly generalized to all connection configurations, are currently based on component experiments having only one bolt on either side of the column web. This study experimentally investigates strengthening of bolted beam-column connections, having no column web stiffeners, using more than one bolt on either side of the column web. Six full-scale bolted beam-column connections are tested, representing exterior beam-column connections (beams attached to only one column flange). Connections with both extended and flush endplates are considered. Two column sections (HE300A and HE300B) are tested along with HE300B beams creating both equal-column-beam, and weak-column strong-beam scenarios. Analytical simulations provide insight into local connection demands, and experimental results are compared with current code methods. The experiments indicate that closer inner-bolt spacing relative to the column web increases connection moment capacity but decreases rotation capacity (connection ductility) due to increased bolt prying forces from column flange distortions. The outer bolt of multiple-bolt-per-row configurations contributes very little to the connection resistance when column web stiffeners are not considered. With the exception of specimen T-3B which failed through bolt-thread shear after 0.02rad, all connections with multiple bolts per row still achieved rotations greater than 0.06rad. The Eurocode 3 component method and adapted Eurocode 3 procedures conservatively predicted the connection strength of each test specimen, including weak-column strong-beam assemblies, and accurately identified the initial connection limit states.

Behaviors of a cracked lapped joint under mixed mode loading

Fatigue failure of steel connections is a common failure mechanism, especially for structures which sustain heavy cyclic loads like steel bridges. In this paper, lapped bolted joints were modeled numerically to study the effect of a crack on the ultimate response of the joint. The site of crack initiation was located under different mixed mode loading in single and multiple bolts joints. The effects of axial to transverse loading ratio or load mixity (LM=Fx/Fy), friction coefficient (μ), and bolt diameter were analyzed. For a single bolt pin-joint, by increasing LM, the crack initiation site angle (γ) increased up to a certain value at which it became constant (γf) independent of LM. This value γf depended only on the coefficient of friction and the bolt hole diameter. Stress intensity factor and crack path of a propagated crack emanating from the predicted crack initiation site were analyzed in the lapped joints under either mode I or mixed mode loading. It was found that, for multiple-bolt joints, loaded with mixed mode loading, the crack path remained approximately horizontal like that for mode I loading. For pin joints, the crack path remained at the direction of the crack initiation. The numerical model developed was validated using existing experimental results for the initial stiffness of the bolted joint and using theoretical prediction of the stress intensity factor. A parametric study for different bolt diameters and numbers was developed to study the behavior of these connections under double and single side cracks. It was found that the crack advancement in a specific bolt hole may cause crack to initiate in other bolt holes, due to the increase of the stress concentration factor (SCF), Kt.

A study on the strength improvement of the multi-bolted joint

The joining of composite materials has traditionally been achieved by adhesive bonding or mechanical fastening. Mechanically fastened composite joints are free from surface treatments, and they are unaffected by the service temperature, humidity and other environmental conditions. Most aircraft bolt joints consist of multiple bolts, and they may share the load unequally due to the relatively brittle nature of the composites. In this paper, a new method for improving the strength of the multi-bolted composite joint was evaluated. Multi-bolted composite joints with five bolts were manufactured and tested. The differences of displacement between the bolts were calculated by finite element analysis by applying a uniform load to each bolt of the multi-bolted joint. When these values were used for the positive clearances of the multi-bolted joints, it was experimentally demonstrated that the load acted upon all of the bolts uniformly compared to the multi-bolted joint without the clearance. Additionally, the failure strength of the joint specimen was improved by more than 13.1–21.8%.

Functional outcome of displaced intra-articular calcaneal fractures

The purpose of this study is to assess the clinical results of a minimally invasive treatment featured the concept of internal compression, including an anatomic plate and multiple compression bolts compared with open reduction and internal fixation for displaced intra-articular calcaneal fractures (DIACFs). We retrospectively analyzed 329 patients (383 feet) who were identified from trauma inpatient database in our hospital for DIACFs from January 2004 to December 2009. Of them, 148 patients (170 feet) were treated with open reduction and internal fixation (OR group), which involved using a traditional L-shaped extended lateral approach, and fractures were fixed by plate and screws from January 2004 to December 2006; 181 patients (213 feet) were treated with a minimally invasive approach featured the concept of calcaneal internal compression (CIC group), which was achieved by an anatomic plate and multiple compression bolts through a small lateral incision from January 2007 to December 2009. Postoperative complications were recorded. During follow-up, pain and functional outcome were evaluated with the American Orthopaedic Foot and Ankle Society (AOFAS) scores and compared between the two groups. Subsequent subtalar arthrodesis and early implant removal were performed when indicated. Routine hardware removal was scheduled for all patients at 1-year follow-up. There were no significant differences in sex, age, and fracture classification (Sanders classification) between the two groups. Wound healing complications were 4 of 213 (1.88%) in CIC group and 20 of 170 (11.76%) in OR group. Subtalar arthrodesis had to be performed in one case in OR group. Four cases in CIC group and four cases in OR group had the hardware removed earlier due to complications. The average time after surgery to start weight-bearing exercise is 5.64 weeks in CIC group and 9.38 weeks in OR group (p < 0.001). The mean AOFAS score is higher in CIC group than in OR group, although the difference is not statistically significant (87.53 vs. 84.95; p = 0.191). The overall results according to the AOFAS scoring system were good or excellent in 185 of 213 (86.85%) in CIC group and 144 of 170 (84.71%) in OR group. The subjective portion of the AOFAS survey answered by patients showed statistically significant difference in activity limitation and walking surface score (7.31 vs. 7.02 and 3.72 vs. 3.42; p < 0.05) but not in pain and walking distance between the two groups (32.72 vs. 32.29 and 4.37 vs. 4.42; p > 0.05). The study results suggest that this minimally invasive approach featured the concept of the calcaneal internal compression can achieve functional outcome as good as, if not better than the open techniques. It is proved to be an effective alternative treatment for DIACFs. Therapeutic study, level IV.

Load–deflection behavior of thin-walled plates with multiple bolts in shearing

This paper presents a numerical and analytical study of the behavior of thin-walled plates connected by multiple bolts under shear. A validated finite element model has been used to perform a comprehensive set of parametric studies to investigate the effects of different design parameters on the connected plate behavior, including initial stiffness, ultimate resistance and maximum deformation at the ultimate resistance. The design parameters include edge and end distances, bolt spacing, number of bolts along and perpendicular to the loading direction and elongation limit of steel. It has been found that the existing methods for calculating the stiffness and ultimate resistance are directly applicable. The main focus of this paper is the maximum plate deformation at the ultimate plate resistance. Based on the parametric study results, it has been found that strain distributions around the bolt holes for different failure modes, as proposed by the authors for plate connected by a single bolt, are still applicable. However, it is necessary to modify the plate dimension ranges within which the different strain distributions apply to recognize the difference in failure modes for plates with different bolt arrangements.

Comparison of Single- and Two-Bolted LVL Perpendicular-to-Grain Connections. I: National Design Specification for Wood Construction Equations

AbstractLittle research is available for perpendicular-to-grain connections in wood using multiple bolts. Experimental testing of single- and two-bolt connections was compared with predicted values from General Dowel Equations for Calculating Lateral Connection Values Technical Report 12. Two different laminated veneer lumber materials were tested with variable loaded-edge distances, and variable bolt sizes in both single- and two-bolt connections. Splitting failures, and crushing of main member and formation of a single plastic moment in the bolt were observed. Connection resistance increased as loaded-edge distance and number of bolts increased. Most design factors of safety representing the ratio of experimental strength to lateral design values were controlled by shear design provisions. Design factors of safety were greater at smaller loaded-edge distance with single bolt compared to other configurations. Further investigation of the design shear provisions may be needed. Yield theory overpredicted t...

Experimental investigations on row shear and splitting in bolted connections

The paper presents an experimental investigation of brittle failures in bolted timber connections loaded parallel to grain. Tests were conducted on steel–wood–steel connections with a very low slenderness ratio. Connections with a single bolt and connections with multiple bolts in a single row were tested. Brittle failures in the considered type of connections in general occur due to a combination of shear stresses and perpendicular-to-grain tensile stresses. Some already available simple analytical models for determination of shear stresses and perpendicular-to-grain tensile stresses are examined and compared with the experimental results.

せん断荷重を受ける多数ボルト締結体の力学特性の評価

When subjected to external forces, bolted joints behave in a complex manner especially in the case of the joints being clamped with multiple bolts. Friction type joints are widely used for the joints subjected to shear loads. Bearing type joints, which support the shear load on the surface of bolt body, are used less frequently, since its mechanical behavior is too complicated to accurately estimate the load capacity. In this study, mechanical behavior of multi-bolted joints subjected to shear loads is comprehensively analyzed by three-dimensional FEM. Load distribution patterns of the bearing type joint are compared to those of the friction type joint. It has been found that the shear load, which is applied to the bearing type joint, distributes with a concave shape along the load direction as the number of bolts is increased and a fair amount of the shear load is supported by friction. Additionally, a simple method that calculates the shear load distribution is proposed using elementary theory of solid mechanics, which can estimate the shear load distribution with sufficient accuracy for the case of small friction coefficient.

Development of super high-strength bolts with tensile strengths of 1600 to 2000 N/mm2

High-strength bolts with much higher strengths are effective in bolted joints of high-strength steels to reduce the number of bolts for both labor saving and more rapid construction. This project is directed at developing super high-strength bolts with a 1600 to 2000 N/mm2 ultimate strength class as the joining parts intended for steel plates with an 800 N/mm2 ultimate strength class. This paper reports the results of the investigation of the resistance to the delayed fracture of high-strength bolts, prototype and shear tests of individual bolts, and the FEM analysis of a joint including multiple bolts aligned in a line.

Effects of Rock Bolting on Stress Distribution around Tunnel Using the Elastoplastic Model

ABSTRACT To ensure the stability of a tunnel during construction, rock bolts are usually installed, which affects the stress distribution around the tunnel. Therefore, it is necessary to study the effects of rock bolting on the stress distribution around the tunnel. In this article, the effects of rock bolting on the stress distribution around the tunnel, including the position and orientation of bolts, the overburden depths, and the bolt lengths, are simulated using the ANSYS software with an elastoplastic model. The effect of multiple bolts of 2 m and 1 m lengths on the stress distribution in the roof and on the lateral sides of a tunnel and at different overburden depths is considered. An important finding is that the tensile stress region that is very dangerous for rock in the bottom of the tunnel grows rapidly with increasing overburden depths when rock bolts are installed only in the roof or on the lateral sides of a tunnel. The determination of the length of the rock bolt used around a tunnel is dependent on the loads and the integrity of the rock mass around the tunnel. In addition, rock bolting around the tunnel can obviously reduce the coefficients and the size of the region of stress concentration, especially when installed in high-stress areas. This fact is very important and essential for the design of tunnels and ensures engineering safety in tunnel engineering.

Combined in-plane and through-the-thickness analysis for failure prediction of bolted composite joints

Although two-dimensional methods provide accurate predictions of contact stresses and bolt load distribution in bolted composite joints with multiple bolts, they fail to capture the effect of thickness on the strength prediction. This study presents an analysis method to account for the variation of stresses in the thickness direction by augmenting a two-dimensional analysis with a one-dimensional through-the-thickness analysis. The two-dimensional in-plane solution method is based on the combined complex potential and variational formulation. The through-the-thickness analysis is based on the model utilizing a beam on an elastic foundation. The bolt, represented as a short beam while accounting for bending and shear deformations, rests on springs, where the spring coefficients represent the resistance of the composite laminate to bolt deformation. The combined in-plane and through-the-thickness analysis produces the bolt/hole displacement in the thickness direction, as well as the stress state in each ply. The initial ply failure predicted by applying the average stress criterion is followed by a simple progressive failure. Application of the model is demonstrated by considering single- and double-lap joints of metal plates bolted to composite laminates.

Experimental study on multiple-bolt steel-to-timber tension joints

An experimental investigation on steel-to-timber joints arranged with multiple bolts was carried out, both in the case of load parallel and perpendicular to the timber grain. The joints were made with timber side members and steel main member; high strength bolts were used for fasteners. The experimental results allowed determining the stiffness in service, the slip at failure and the reference loads (proportional limit, yielding, ultimate). A descreasing ductility of joints was observed at the increase of the number of bolts. With the above characteristic loads the bolt effectiveness of multiple joint loaded parallel to grain was evaluated and compared with that obtained from some analytical predictions found in the literature. It was evidenced that the provisions of Eurocode N.5 provide values for the bolt effectiveness quite closer to experimental reslts. The results of joints loaded perpendicular to the grain were compared with some analytical predictions; significant differences were noted with all analytical predictions considered.