High Strength Steel Connections Research Articles

Net section tension capacity of high strength steel single shear bolted connections

Single shear bolted connections under tension usually exhibit out-of-plane deformation due to the inherent load eccentricity, resulting in significant stress concentration around the bolt holes and a stress gradient through the plate thickness. This effect is particularly pronounced in high-strength steel (HSS) connections due to the relatively lower ductility and tensile-to-yield strength ratio of HSS. However, it remains unclear whether this issue can lead to premature fracture in high strain regions before effective stress redistribution develops across the net section, subsequently reducing the ultimate load capacity of the HSS connections. To examine the above design issue, an experimental and numerical study on single shear bolted connections was conducted and reported in this paper. A total of 25 connections were tested, among which 17 were made of HSS (Q690 and Q960), and 8 were made of Q345 mild steel (MS). Based on a validated finite element (FE) model, a comprehensive numerical analysis was carried out to interpret the structural behaviour of the specimens and explore the influence of various design parameters on the ultimate tension capacity of single shear bolted connections. The findings revealed that, despite the existence of out-of-plane deformation in the steel plate, all connections (either MS or HSS) investigated in this study reached the theoretical net section resistance (the product of the net section area and the ultimate tensile strength). This is because (1) the relatively low ductility and tensile-to-yield strengths ratio of HSS materials are still sufficient to allow an effective stress redistribution across both the transverse net section and the plate thickness, and (2) the biaxial stress state around the bolt holes enhances the tension capacity of the net section. Finally, a statistical evaluation of the combined test and numerical data showed that the existing design equations for predicting the net section tension capacity of HSS (specified in EN 1993-1-12:2007 and prEN 1993-1-1:2020) can be safely applied to the design of HSS single shear bolted connections, even in scenarios where the tensile-to-yield strengths ratio of HSS is as low as 1.06.

A macro prediction model for butt-welded (U)HSS connections with softened HAZ

This paper reports an investigation on the tensile resistance of fully-penetrated butt-welded connections of (ultra-)high-strength steels considering the strength softening of the heat-affected zone (HAZ) and the strengthening due to the transverse constraint. Firstly, a parametric study is carried out to analyse the tensile behaviour of fully-penetrated butt-welded S700 connections using validated finite element (FE) models. Based on the FE results, the effect of the investigated parameters on the ultimate resistance of the connections is quantified. Consequently, a Macro prediction model is proposed, neglecting the detail of HAZ inhomogeneous microstructures but considering five design parameters, which are the width and thickness of the specimen, the width of HAZ, the softening ratio, and the matching ratio. Finally, the proposed Macro model is validated against the available experimental data of fully-penetrated butt-welded (ultra-)high-strength steel connections in literature. The results show that the Macro model is proven successful in accurately quantifying the effect of HAZ, concerning the strength softening and the transverse constraint strengthening, on the tensile resistance of fully-penetrated butt-welded (ultra-)high-strength steel connections. It is also found that 157 and 96 out of 185 experiments are predicted within a 5% deviation range using the Macro model and prEN 1993-1-8:2021, respectively, proving that the Macro model is more consistent with the experimental data compared to the prediction of prEN 1993-1-8:2021.

Experimental study on high-strength Q460 steel extended end-plate connections at elevated temperatures

This paper presents an experimental study on high-strength Q460 steel extended end-plate connections at ambient and elevated temperatures. Ambient temperature tests were conducted on connections with 8 and 12 mm end-plates to investigate the ambient temperature structural response, including the initial rotational stiffness, plastic flexural resistance and ultimate rotation. The results show that high-strength steel connection with 12 mm end-plates significantly improves mechanical properties compared to that with 8 mm end-plate with an increase of 22% at least while not influencing the failure modes. Then, the experimental results were compared with the theoretical predictions of Eurocode 3. There are reasonable agreements between these two methods on plastic resistance, while the calculation by Eurocode 3 on initial rotational stiffness exhibits much higher results. In addition, the temperature distribution of the connection specimens and furnace air, rotation-temperature characteristics, and failure modes of the connections in transient tests were obtained to study the influence of different end-plate thicknesses. The 12 mm end-plate effectively improves the connection's rotation capacity at elevated temperatures with an increase of 29% compared to 8 mm end-plate while influencing the critical temperature and fire endurance lightly. Besides, the fire performance results were compared and discussed between mild steel and high-strength steel connections. Finally, a rotation-temperature relationship model was proposed to predict the fire behavior of high-strength steel end-plate connections under transient conditions and validated against the experimental data with considerable accuracy.

Experimental and numerical study on high-strength steel flange-welded web-bolted connections under fire conditions

Experimental and numerical study on high-strength steel (HSS) flange-welded web-bolted connections at ambient temperature and under fire conditions are conducted in this study to reveal their fire behaviors. Eurocode 3 Part 1–8, AISC360–16, and GB50017–2017 are compared with the experimental results to verify the applicability of the current specifications on HSS connections. Furthermore, the finite element model validated by experimental results can reasonably simulate the behaviors of HSS flange-welded web-bolted connections under fire conditions and at ambient temperature. At ambient temperature, the provision of Eurocode 3 Part 1–8 on the flexural capacity of the connections seems rather conservative, while those of GB50017–2017 and AISC360–16 are unsafe. Improving the weld quality is significant to avoid weld failure of HSS flange-welded web-bolted connections. In addition, appropriately matching the steel grade and thicknesses of the web, as well as the flange of beam–column components should be considered. The elastic design method and the plastic hinge relocation mechanism should be discussed sufficiently in future optimization design. This study can be employed for performing performance-based optimization analysis and fire-resistance design of HSS flange-welded web-bolted connections.

Effect of bolt pre-tension on the bearing behavior of high strength steel connections

Bolt pre-tension is regularly used in the practical case of bearing-type connection. Due to confinement of lap plates, complicate triaxial stress field can be observed near bolt hole, which may affect bearing behavior of the whole connection. Currently, investigations on the related issues are limited. To fill this research gap, this paper presents a combined experimental and numerical study on high strength steel single-bolt connections with bolt pre-tension. Results reveal several meaningful recognitions on the bearing behavior of connection with bolt pre-tension. Firstly, the effect of bolt pre-tension on the final tearout failure mode is negligible. Secondly, distribution of triaxial stress field initiates with a circular pattern and gradually shrinks to a localized pattern between shear fracture lines at ultimate resistance of the connection. Thirdly, out-plane confinement will limit the piling-up of plate material in front of bolt, which will reduce the related bearing resistance. However, increasing bolt grade is still beneficial to the whole connection due to a trade-off between ultimate resistance of the connection and related bearing resistance. Comparison with current code is further conducted. It is found that current Eurocode3 has adequate design safety for connection with bolt pre-tension while extra safety is needed for current AISC 360-16.

Bearing behavior of multi-bolt high strength steel connections

This paper presents an experimental and numerical investigation on the bearing behavior of multi-bolt high strength steel connections in single bolt line. Three grades of high strength steel with nominal yield stress of 550 MPa, 690 MPa and 890 MPa were used to fabricate thirty lap connections with two or three bolts. It is found that the deformation capability of high strength steel plate near bolt hole is sufficient to meet the deformation demand of compatibility in multi-bolt connection even with an intended bolt hole misalignment of 2 mm for M24 high strength bolt. The comparison with Eurocode3 and AISC 360-16 is conducted. When no partial or safety factors are considered, the original formulae of Eurocode3 already has sufficient safety while AISC 360-16 gives unsafe predictions. Numerical models were established and verified by test results. With the obtained results, the full-range distribution of internal load among different bolts can be divided into four stages. Features for each stage are summarized and the effect of bolt hole misalignment on the distribution pattern is evaluated. Numerical analysis results indicate that, behavior of an individual bolt in multi-bolt connection may be influenced by surrounding bolts. The formation of critical yielding band between different bolts may explain this influence. Based on the experimental and numerical analyses, this paper proposed a new ultimate bearing resistance model for multi-bolt connections. The proposed model is capable of describing the nonlinearity in the ultimate bearing resistance of multi-bolt connection and achieves improved predicting accuracy compared to classical models.

Influence of weld details on fracture behavior of connections using high-strength steel

Based on post-earthquake research, brittle fracture of steel structures typically occurs near connection weld seams. The study proposes a new welded structure to improve the fracture resistance of connections. The fracture toughness of steel may decrease with increases in steel strength, and thus the fracture behavior of connections using high strength steel (HSS) was studied. Local welded specimens with different design details were tested under quasi-static loads, a finite element model was established, and initial cracks were assumed to analyze the fracture behavior of three types of local welded structures. Based on fracture mechanics, mode I elastic stress intensity factor (KI) and inelastic J-integral (JI) at the crack tip were calculated to predict the failure modes of HSS connections with different local welded structures. The experimental results were consistent with the expected results predicted via fracture mechanics, and the fracture of the specimen with the new design detail involving a backing strip welded at both sides occurred at a location away from the weld seam. The results indicated that the new design detail of local welded structure improves the fracture behavior of HSS connections.

Research advances in HSS structures at Tsinghua University and codification of the design specification

AbstractIn Memoriam of Prof. Dr. Bernt JohanssonOwing to the advantages in economy, environmental protection and energy efficiency, high‐strength steel (HSS) has represented an important trend in the development of steel structures and has been adopted in numerous building and bridge structures. This article summarizes engineering applications for HSS structures and comprehensively reviews the recent research advances in HSS structures at Tsinghua University. Investigations were conducted into the behaviour and performance of HSS materials, sections, members, connections and structural systems. Based on the research results, the mechanical behaviour of HSS structures could be improved compared with ordinary strength steel structures. In order to update the current design codes so that they are applicable for HSS structures, a new specification in China, the ”Standard for Design of High‐Strength Steel Structures“, is undergoing codification, with Tsinghua University as the chief editorial institute. The outline of the specification, as well as the new design methods included in it, is introduced in this paper.

Bearing strength at bolt holes in connections with large end distance and bolt pitch

The paper presents test results of one and two bolt connections with very large end distances that failed in bolt bearing, where very high bearing forces were recorded. The purpose of the tests was to investigate the behaviour of the plate in bearing that was not limited by shear, splitting, bolt or net cross-section failure, but with tearing of the material in front of the bolt. The tests showed that the bearing capacity increased insignificantly as the width of the plate increased. Numerical simulations show that bearing resistance is in cases of large end distances significantly influenced by friction forces. They develop because the bolt head and nut, together with the lap plates, constrain the increase of the plate thickness related to plastic deformation. The paper deals with the formation of the stress-strain field as well as the influence of the inner bolt determined by the numerical simulations. The resistances obtained from the tests agree very well with the bearing strength model presented recently by Može and Beg (2014). The coefficient of the bearing strength model that considers the influence of the inner bolt is calibrated on the basis of the presented test results as well as on high strength steel connections with several bolts. The presented bearing strength model was adopted in the final draft of the second generation of EN 1993-1-8.

01.08: Bolted bearing connection with high strength steel and grade 12.9 bolt

ABSTRACTThis paper presents an experimental program investigating the behavior of high strength steel connections consisting of one bolt in double shear. A total of 15 bolted connections fabricated from three grades of high strength steel with the nominal yield strengths of 550 MPa, 690 MPa and 890 MPa were tested. The effects of end distance and steel grade on the bolt bearing behavior were evaluated. The bearing resistances of the bolt on various high strength plates were determined and compared with respect to the tensile strength of the plates. The bolt hole elongation due to bolt bearing on high strength steel plate was measured and its implication on the plate bearing resistance was discussed. The test results were compared with Eurocode 3 and AISC 360‐10 predictions and it was found that Eurocode 3 could be used conservatively to predict the bolt bearing resistance on high strength steel with nominal yield strength up to 890 MPa whereas AISC 360‐10 method tends to overestimate the bearing resistance of the bolted connection. A regression analysis was performed based on the test data and those from the literature so that a more general method was proposed to predict the bolt bearing resistance on normal strength and high strength steel plate.

Behavior of single bolt bearing on high strength steel plate

This paper presents an experimental program investigating the behavior of high strength steel connections consisting of one bolt in double shear. A total of 24 bolted connections fabricated from three grades of high strength steel with the nominal yield strengths of 550MPa, 690MPa and 890MPa were tested. The effects of end distance, edge distance and steel grade on the bolt bearing behavior were evaluated. The bolt hole elongation due to bolt bearing on high strength steel plate was measured and its implication on the plate bearing resistance was discussed. The test results were compared with Eurocode 3 and AISC 360-10 predictions and it was found that Eurocode 3 could be used conservatively to predict the bolt bearing resistance on high strength steel with nominal yield strength up to 890MPa whereas AISC 360-10 method tends to overestimate the bearing resistance of the bolted connection. A regression analysis was performed based on the test data and those from the literature so that a more general method was proposed to predict the bolt bearing resistance on normal strength and high strength steel plate. Splitting failure was observed as a transitional failure mode between tearout failure and net cross-section failure. Splitting failure showed a lower resistance than the bolted connection with tear-out failure, therefore, a reduction factor was proposed to improve the prediction. The upper and lower boundaries of end distance to edge distance ratio for splitting failure were theoretically derived and experimentally verified.

Reliabilities of distances describing bolt placement for high strength steel connections

In the bolted connections, bolt placements are generally described and are generally made in the direction of design effects and in the perpendicular direction to design effects. In these both directions, the reliability of the distance of bolts to the edges of connection plate and the distance of bolts to each other is investigated for high strength steel connections built up with high strength bolts in this study. For this purpose, simple SL (bearing type shear connection) and SLP (bearing type shear connection for body-fit bolts) type steel connections with St 52 grade steel plates with 8 different thicknesses and with 8.8D grade high strength bolts (HV) were constituted and analyzed under H (Dead Loads+Live Loads+Snow Loads+Roof Loads) and HZ (H Loads+Wind Loads+Earthquake Loads) loadings. Geometric properties, material properties and design actions were taken as random variables. Monte Carlo Simulation method was used to compute failure risk and the first order second moment method was used to determine the reliability indexes of those different distances describing the placement of bolts. Results obtained from computations have been presented in graphics and in a Table. Then, they were compared with some values proposed by some structural codes. Finally, new equations were constituted for minimum and maximum values of distances describing bolt placement by regression analyses performed on those results.

A Study on Deformation Capacity of High Strength Steel Beam-to-Column Connections According to Welding Detail at Beam End

For high-strength steel, it is difficult to be applied to flexible structural member because it have high yield ratio and low basic material`s toughness. One of the great problems when using high-strength steel connections is the brittle fracture at the end of the beam member in common with general mild steel connections. In the cases of mild steel connections, it has be developed that special moment frame connection details by reinforcing structural member or improvement of welding access hole. But, it is incomplete at yet about applicability estimation of high-strength steel connections. This study is the initial step research for the applicability estimation of beam-to-column connections being applied to developed high-strength steel, HSA800. And, it studied about structural performance of the high-strength steel connections according to the details of welding access hole through full-scale test and analytical method.

Investigation of high strength steel connections with several bolts in double shear

A series of 26 tests on tension splices with three or four bolts positioned in the direction of bearing force are presented. Steel grade S690 was used for the fabrication of connection plates. The tests were also numerically simulated to obtain the distribution of forces between bolts. Tests on similar connections made of high strength steel were gathered from the literature. These tests were also numerically simulated. The numerical results agreed with experimental data very well, and were used to evaluate the bearing resistance formula according to EN 1993-1-8.