Current Design Equations Research Articles

Numerical study on the stiffness and ultimate strength of cold-formed steel C-shaped connectors under pull-out failure

ABSTRACT In this paper, cold-formed steel C-shaped connectors, as one of the most commonly used components in steel wall/framing systems, were investigated under the pull-out failure modes using a detailed numerical study. Finite element models were validated using the data obtained from an experimental program; and the results were compared with the current design equations. Finally, a new pull-out strength equation was proposed based on the critical parameters affecting the behaviour of C-shaped connectors. In addition, initial bending stiffness of C-shaped connectors was studied using a simplified analytical approach; and the results obtained from the proposed new stiffness equation were compared with the initial slope of experimental and numerical force-displacement curves. These comparisons confirm the accuracy of the new strength and stiffness equations in order to apply to the analytical methods such as component-based method in future works.

Design of thin steel battens subject to pull-through failures

Localized pull-through failures of cold-formed steel roof battens have often been observed at the roof batten to purlin/rafter/truss screw connections during storms and cyclones. Recent research studies have therefore proposed improved design capacity equations by undertaking both experimental studies and finite element analyses. However, they have not investigated the effects of very thin steel roof and ceiling battens (<0.50 mm), safety lip features and purlin flexibility on the localized pull-through failures. Hence this study was aimed at investigating the effects of all such practical design issues and to assess the suitability of current design equations. For this purpose, a series of static and fatigue pull-through failure tests of roof and ceiling battens was undertaken under simulated static and cyclic wind uplift loads. This paper presents the details and results obtained from this study including suitable design recommendations. It has shown that the recently developed pull-through capacity equations can be used for very thin steel battens. Higher capacity reduction factors are recommended for use with these design equations for battens made of Australian cold-formed steels used in this study. The use of safety lip features did not affect the static pull-through capacities, but it improved the fatigue performance of roof battens. Finally, the suitability of using low strength steel battens instead of high strength steel battens was evaluated.

A novel machine learning model to predict the moment capacity of cold-formed steel channel beams with edge-stiffened and un-stiffened web holes

A novel machine learning model, eXtreme Gradient Boosting (XGBoost), was used for the purpose of evaluating the moment capacity of cold-formed steel (CFS) channel beams with edge-stiffened web holes subject to bending. A total of 1620 data points were generated for training the XGBoost model, using an elasto-plastic finite element model which was validated against 12 sets of test data taken from the existing literature. The R2 score of XGBoost predictions for the moment capacity was around 99%. The performance of current design equations was evaluated through the comparison of their results against those obtained from the XGBoost model. The moment capacities obtained from the XGBoost testing dataset were also compared with that determined from the existing design equations for un-stiffened holes (USH) and edge-stiffened holes (ESH). The moment capacities determined from the current design equations for USH and ESH were found to be excessively conservative by 38.3%, and unconservative by 36.2% on average, respectively. Therefore, new design equations were proposed based on the results of parametric study using the XGBoost model. In the detailed parametric analysis, the effects of web depth, section thickness, and beam length on the moment capacity of channel beams (CFSCB) with ESH were considered. From the results of XGBoost outputs, the absolute percentage error of new design equations for that based on the strengths of unperforated CFSCB was 8.78%, and for that based on the strengths of CFSCB with USH, the absolute percentage error was 13.7%. Additionally, a reliability analysis was performed to evaluate the accuracy of the proposed equations that were used to predict the moment capacity of CFS channel beams with ESH subject to bending. The reliability indices of all the proposed equations were greater than 2.5 which can be reliable as per the guidelines of AISI.

DSM design of fixed-ended slender built-up back-to-back cold-formed steel compression members

Built-up back-to-back cold-formed steel channel (BC) sections have gained increased application as compression members in building structures. Although many researchers have studied the behaviour and design of BC compression members, the design equations in the current standards are still inadequate for them, while the new design equations proposed in recent studies are inconsistent and lead to predictions with varying accuracy. This study focuses on slender BC members with fixed ends, which are sensitive to local-global interaction or global buckling. Many tests of fixed-ended slender channel and BC compression members, which failed mostly by local-global interaction buckling, were conducted first, followed by developing validated finite element models. The models were then used in a parametric study of BC compression members with varying sections, screw arrangements and lengths, to investigate their failures by local-global interaction and global buckling. The outcomes from tests and finite element analyses provided improved understanding of the behaviour of BC members, and were used to compare their compression capacities against twice the individual member capacities, and show the limitations of the current design equations available in the standards and research publications. Finally, reliable design guidelines were proposed using the Direct Strength Method (DSM).

Unified design equations for web crippling failure of cold-formed ferritic stainless steel unlipped channel-sections with web holes

This paper addresses the design of cold-formed ferritic stainless steel unlipped channel-sections with offset web holes and fastened flanges subject to web crippling under one-flange load scenarios. The results of a total of 18 new experimental tests, not reported previously, are presented; these were conducted on ferritic 1.4016 grade unlipped channel-sections under both interior- and end-one-flange loadings. The results are used to validate a non-linear quasi-static finite element model to simulate the behaviour of such sections. A comprehensive parametric study including a total of 576 FE models is then undertaken to determine the web crippling strengths for unlipped channel-sections with different web heights, web thicknesses and location of web holes under both interior- and end-one-flange loadings. In addition, the experimental and FE results are compared against strengths predicted in accordance with the American Iron and Steel Institute (AISI S100-16) for cold-formed carbon steel plain lipped channel-sections as well as the equations proposed in previous studies for stainless steel plain lipped channel-sections. It is found that the current design equations are unreliable and unconservative to use for cold-formed ferritic stainless steel unlipped channel-sections by as much as 22%. To address this issue, based on the results of this study, two new reliable web crippling strength reduction factor equations are proposed for such unlipped channel-sections. The proposed equations should prove useful for practical design of cold-formed ferritic stainless steel unlipped channel-sections with offset web holes and fastened flanges. For practical design application, the equations are then unified by combining the proposed equations with existing equations from the literature to allow a unified design equation to be developed for ferritic stainless steel unlipped channel-sections with centred and offset web holes and with both flanges fastened and unfastened to the bearing plates under one- and two-flange load scenarios.

Shear bearing capacity of Self-drilling screw group connections of CFS sheets

Self-drilling screws are commonly used as connectors in cold-formed steel (CFS) buildings as they can be drilled easily through thin sheet steels, allowing on-site assembly. The screw connections in the project all appear in the form of screw group connections. Studies have shown that the shear bearing capacity of a screw group connection of CFS sheets is not simply the shear bearing capacity of a single screw connection multiplied by the number of screws. But instead, there is a reduction effect of the screw group connection, called the Group Effect. However, there is no design provision for the shear bearing capacity of the screw group connection in the current design specifications, AISI S100-16w/S1-18, AS/NZS-4600:2018, EN1993-1–3, and GB50018-2002. Based on the authors' and other researchers' tests collected by authors, this paper verifies and analyzes the four existing design equations for the connection strength of the screw group connections that consider the Group Effect. The results show that the four current design equations cannot cover the test data that list in the paper because the existing four design equations all have their respective scopes and limitations, such as the screw spacing and the strength of steel sheetings, etc. For that, the paper proposed a new design equation that can calculate the nominal shear capacity of self-drilling screw group connections, including high-strength steel such as G550 CFS sheets and extensive screw spacing connections. The authors' new test of twelve screw group connections and the other 237 tests verified the new equation. The unsafe rate for the new design equation is 4.42% among the 249 connections test, and the new design equation has high reliability and broader application scope.

Numerical study on the bearing behaviour and design of aluminium sub-heads with removable beads in façade systems

A window wall is a lightweight external wall composed of glass and aluminium, which does not carry any gravity loads from the building except for its own weight. Wind load is transferred to the main structural system through the connections of sub-frames (consisting of sub-heads and sub-sills) with slabs. An aluminium sub-head with a removable bead (ASHWRB) is a common type of sub-head consisting of two parts: a base and a bead. Under wind load, the bead flange is directly loaded by the mullion and undergoes bearing failure due to its long length. This paper presents a detailed numerical study on the bearing capacities of the ASHWRB sections subject to wind loading. For this purpose, finite element models were simulated using ABAQUS/CAE and validated with our previously conducted experimental work in terms of ultimate loads, load-displacement curves, and failure modes. Validated finite element models of the ASHWRB sections were then used for an extensive parametric study covering a wide range of thicknesses, flange widths , bearing widths, and engagement lengths. Notably, the ultimate loads obtained from the parametric study are not product nor manufacturer specific, whereas, the data obtained from experimental study is specific to the manufacturer products. The acquired bearing database was used for a detailed assessment of the consistency and reliability of the previously proposed design rules by the authors (DRA) based on the experimental tests as well as currently available cold-formed steel design rules (DRSs). It was found that the bearing capacities determined by both DRA and DRSS are unreliable for predicting the bearing capacities of the ASHWRB sections in the window wall. Thus, modifications were made to DRA and DRSS. Consequently, new design rules were proposed in this paper (DRA P ), which accurately predicted the bearing capacities and correlated very well with the numerical results. • Numerically investigated the bearing behaviour of aluminium sub-heads with removable beads in façade systems. • Considered various section geometries, bearing widths and, loading and boundary conditions. • The finite element models were validated against the previous test results and extensive parametric study were performed. • Current cold-formed steel design equations were not suitable for aluminium sub-heads with removable beads in window walls. • New design rules were proposed for aluminum sub-heads with removable beads based on the numerical results.

Experimental and numerical study on net section resistance of high strength steel staggered bolted connections

This paper investigates the structural behaviour and ultimate strength of high strength steel (HSS) staggered bolted connections under tension. A total of 33 HSS (Q690 and Q960) and 15 Q345 staggered bolted connection specimens were tested. All specimens failed in net section fracture through the staggered path. The predicted net section resistance based on the s2/4g rule was achieved by most of the HSS specimens. For the specimens with a small bolt pitch to gauge (s/g) ratio, the connection efficiency (i.e., the ratio between the test ultimate load and the calculated net section resistance) exceeded 1.0 and decreased gradually with an increasing s/g ratio. Compared with the mild steel (MS) specimens, in general the HSS specimens have lower connection efficiency and overall deformation capability. Subsequently, the structural behaviour of the test specimens was investigated using finite element (FE) analysis to further interpret the test observations. The results of the test and the FE analysis indicated that for the HSS specimens, effective stress redistribution along the staggered net section was observed although the ductility and the ratio of tensile strength to yield strength (fu/fy) of HSS material are relatively lower than those of the MS material. In addition, it was also observed that the biaxial state of stress that existed in the plate area between the staggered bolt holes was influenced by the s/g ratio, which would affect the ultimate strength of the test specimens. A reliability analysis of the HSS specimens test results was carried out to re-evaluate the partial factor adopted in the design equation of the net section resistance of HSS (up to S700) in EN 1993-1-12. The analysis results of the limited test data showed that the net section resistance of HSS staggered bolted connections was predicted conservatively by the current design equation with a partial factor of 1.25.

DSM design of CFS columns subject to distortional buckling at elevated temperatures

Cold-formed steel (CFS) columns are subject to distortional buckling unlike hot-rolled steel columns. Many experimental and numerical investigations have been conducted on the distortional buckling behaviour and capacity of CFS columns at ambient and elevated temperatures. However, constitutive laws of CFS at ambient and elevated temperatures are different with varying levels of nonlinear stress-strain characteristics. Thus, the ultimate distortional buckling capacities predicted by the elevated temperature design rules can be unconservative when the ambient temperature design equations are used with elevated temperature mechanical properties. However, no study has fully incorporated the effects of nonlinear stress-strain characteristics of CFS at elevated temperatures. Therefore, this research study investigated the effects of nonlinear elevated temperature stress-strain characteristics on the distortional buckling capacities of CFS columns using three parameters, namely, nonlinearity, strain hardening and yield strength to Young's modulus ratio, through a numerical parametric study. A comparison of ultimate capacities from the parametric study with those predicted by the current AS/NZS 4600 direct strength method (DSM) showed that the current design equations cannot accurately capture the effects of nonlinear elevated temperature stress-strain characteristics. Consequently, modified DSM equations were proposed to accurately predict the distortional buckling capacities of CFS columns at elevated temperatures.

Bearing behaviour of aluminium sub-heads with removable beads in façade systems

Occupying a substantial proportion of the overall building expense, building envelopes are the subject of ongoing research and improvement in both aesthetic and structural aspects. The aluminium window wall frame, consisting of vertical members (mullions) and horizontal members (head and sill transoms), is placed between sub-heads at the top and sub-sills at the bottom of the wall. These window walls are designed to carry lateral wind loads. Of particular importance to design stability is the bearing behaviour of aluminium sub-heads. The bearing behaviour and capacities of C-shaped sub-heads were recently investigated through a detailed experimental study at Griffith University for the development of strength prediction equations. To allow for easier assembly of façade panels, a kind of sub-head section known as sub-head with removable bead is used in aluminium window wall system. Two parts of this sub-head are connected together without any external mechanisms. The present research places emphasis on assessing the bearing behaviour of aluminium sub-heads with removable beads through comprehensive experimental testing. A total of 36 tests were conducted using six section geometries, two engagement lengths, and three bearing widths. The bearing strengths obtained from tests of aluminium sub-heads with removable beads were compared with the bearing capacities predicted using the design rules developed by the authors for conventional C-shaped aluminium sub-heads and available cold-formed steel design provisions (AISI S240 2015, TI 809-07, and SSMA 2000 specifications). As a result of the investigation, the current design equations were found to be unreliable for estimating the bearing capacity of aluminium sub-head sections with removable beads. Hence, new design expressions have been developed which accurately predict the strengths of aluminium sub-heads with removable beads under out-of-plane loads. Furthermore, the influence of the removable beads on the bearing behaviour and capacity is discussed in detail.

Behaviour of cold-formed steel channels bent about the minor axis

The Direct Strength Method (DSM) in AISI S100 and AS/NZS 4600 is a newly developed alternative design method to the Effective Width Method in calculating the design moment capacity of cold-formed steel channels. The DSM uses elastic buckling stresses with an appropriate strength curve to calculate the design moment capacity. However, the current DSM design equation has been calibrated for channels bent about the major axis and is known to be conservative for channels bent about the minor axis with the web in compression. In order to extend the DSM for channels bent about the minor axis, it is necessary to prescribe and calibrate a DSM design equation for minor axis bending. This paper aims at deriving a new DSM design equation for cold-formed steel channels bent about the minor axis with the web in compression. For this purpose, the behaviour of 33 cold-formed steel channels bent about the minor axis inducing local buckling in the web are investigated using elastic finite strip buckling analysis and nonlinear finite element analysis. The cross-sectional dimensions such as thickness, web and flange widths are varied to cover a wide range of section slenderness. Deformed shapes and stress distributions at ultimate moment for various channels of different slenderness are described. Due to the considerable conservatism the current DSM design equation exhibited in comparison to the nonlinear finite element analysis results, a new DSM design equation for cold-formed steel channels bent about the minor axis is proposed.

Numerical modeling on thin-walled cold-formed steel clip angles subjected to pull-out failures

Pull-out failures are one of the common failures in load bearing cold-formed steel clip angles causing catastrophic damages, where plate and screw fastener are dislocated. Thus, a detailed experimental study was first conducted to investigate the pull-out failures in cold-formed steel clip angles under tensile loading conditions. Then, since experimental tests are time-consuming and expensive, the ability of finite element modeling in the ABAQUS software environment was used to predict the pull-out behavior of the clip angles. The force–displacement behavior, ultimate strengths, and failure modes were compared with those obtained from experimental tests to validate the finite element models. The results were compared with those obtained from current design equations. Then, new design equations associated with numerical and both numerical and experimental results were provided by considering all geometrical and mechanical parameters to accurately predict the pull-out failure loads of the clip angles.

Contribution of Shear Reinforcements and Concrete to the Shear Capacity of Interfaces between Concretes Cast at Different Times

Currently, shear transfer models have diverse assumptions about the contribution of shear reinforcements and concrete to the shear capacity of interfaces between concretes cast at different times (that is, cold joints). Furthermore, the knowledge about the effect of high-strength shear reinforcements (HSSR) having a nominal yield strength exceeding 414 MPa (60 ksi), is still limited. This study aims to determine the extent to which shear reinforcements and concrete contribute to the shear capacity of cold joints and explore the effect of HSSR on shear transfer behavior. To this end, nine cold joint push-off specimens were tested to investigate the effects of yield strength of shear reinforcement (YSSR) and concrete compressive strength (CCS) on the interface shear strength and stiffness. In addition, code provisions evaluation was used to identify the degree of concrete contribution, based on a database of 108 cold joint push-off test results established in this study. Test results indicate that the increase of YSSR (rising from 400 MPa to 600 MPa) has no evident effect on the shear strength and stiffness of cold joints, whereas the increase of CCS can significantly increase them. Moreover, when CCS rises from 20 MPa to 50 MPa, the interface shear capacity of push-off specimens increases by approximately 110%, i.e., from 103.5 kN to 217.5 kN. Hence, concrete has a considerable contribution, whereas the contribution of shear reinforcements is not as great as expected. Code provisions evaluation results also indicate that concrete considerably contributes to the shear capacity in the form of cohesion. On the basis of these findings and current design equations, a shear capacity equation, which accounts for the effect of CCS and the limited participation of shear reinforcements, is proposed. This equation is found to provide reasonable prediction of cold joint shear capacity, especially for the cold joint composed of high-strength concrete.

Block shear capacity in cold-formed lean duplex stainless steel double-shear bolted connections

Since stainless steel has many advantages such as excellent durability, ductility and corrosion resistance, the use of stainless steel has been increased in a variety of structural members and industrial fields. Even if stainless steels are included as structural materials of the Korean Design Standard, the corresponding design standards for stainless steel structural members are not specified in Korea. Recently, lean duplex stainless steel, STS329FLD with less nickel (reduced to 0.5%~1.5%) has been developed as a substitute for austenitic stainless steel (8%~10.5% nickel in STS304) and included in the Korean Industrial Standards. In this paper, experimental studies on double-shear bolted connections fabricated with the lean duplex stainless steel were conducted and the validation of finite element analysis model was verified in the field of ultimate strength and fracture mode through the comparison with test results. Parametric studies were also performed with extended end distances and edge distances. Analysis strengths were compared with block shear predictions by the current design equations and suggested equations by previous researchers. A modified block shear strength equation was recommended considering the material properties of lean duplex stainless steel and the shear stress distribution in the critical shear section of block shear. Moreover, additional tests for seven specimens were performed and the feasibility of the recommended block shear equations in this study was also verified.

Design of cold-formed steel columns subject to local buckling at elevated temperatures

Cold-formed steel (CFS) is one of the popular materials in low and mid-rise building construction. However, CFS member capacities reduce at elevated temperatures as their yield strength and Young's modulus decrease with increasing temperature. In addition, CFS exhibits nonlinear stress-strain characteristics at elevated temperatures, which is significantly different to the ambient temperature stress-strain characteristics. Although several studies have been conducted on the elevated temperature local buckling capacities of CFS columns, they have not considered the effects of nonlinear stress-strain characteristics such as the nonlinearity between proportional limit stress and yield strength, varying yield-strength to Young's modulus ratio and strain hardening between yield and ultimate strengths. Thus, this research study investigated their effects in detail, and proposed modified design equations for the elevated temperature local buckling capacities of CFS columns based on the effective width and direct strength methods given in AS/NZS 4600, since the current design equations give unsafe predictions in many cases. The modified design equations incorporating the effects of elevated temperature nonlinear stress-strain characteristics were developed using the local buckling capacities of CFS columns obtained from 740 finite element models of lipped channel sections. A simplified design method is proposed to determine the compression capacities of commonly used Australian open CFS sections at elevated temperatures.

Characterizing debonding strain in sand-lightweight high strength concrete T beams strengthened with CFRP sheets

The premature debonding of Fiber Reinforced Polymer (FRP) sheets has been well studied and understood for normal weight concrete. This type of failure mode can be averted by keeping the maximum FRP strain below the debonding strain of FRP as specified by ACI 440.2R-17. However, the debonding strain in lightweight concrete has not been addressed or evaluated yet. Accordingly, this paper investigates the debonding strain in lightweight concrete using full-scale reinforced concrete T beams strengthened with different layers of carbon fiber reinforced polymer (CFRP). These beams were prepared and strengthened in two series. The first series included three T beams that were cast with normal weight concrete. The second series consisted of another group of three T beams using lightweight concrete. Both series had identical geometry, reinforcement, and span lengths. Furthermore, they had almost identical concrete properties and the same FRP material to effectively examine the deboning strain difference in both types of concrete. The debonding strains of the lightweight and normal weight concrete are compared and the reduction factor is addressed in light of the current design equation in ACI 440.2R-17.

Experiments and numerical modelling of cold-formed steel beams under bending and torsion

Cold-formed steel C- and Z-section beams are widely used in light gauge steel building systems as flexural members. In their applications as flexural members, mono-symmetric C-sections and point-symmetric Z-sections are often subjected to transverse loads eccentric to the shear centre, leading to combined bending and torsion actions. However, research and design methods for cold-formed steel beams subjected to combined bending and torsion are limited. Hence this research has investigated the structural behaviour, strength and design of cold-formed steel C- and Z-section beams under the action of combined bending and torsion. Twenty-four tests were conducted on simply-supported beams subjected to a mid-span eccentric load. Two C-sections and two Z-sections were used in the tests with two different spans, each with three loading-eccentricities. A special test set-up was developed and used to simulate the different loading-eccentricities and to provide accurate boundary conditions. Numerical models of tested beams were then developed using ANSYS and nonlinear finite element analyses including the effects of large deformation and material yielding were performed. The numerical results agreed well with the test results in terms of ultimate strengths, failures modes and load-displacement curves. Finally the results from tests were compared with predictions from the current design equation for bending and torsion. This paper presents this investigation of cold-formed steel beams subjected to combined bending and torsion, and the results.

Machine learning-based prediction of CFST columns using gradient tree boosting algorithm

Among recent artificial intelligence techniques, machine learning (ML) has gained significant attention during the past decade as an emerging topic in civil and structural engineering. This paper presents an efficient and powerful machine learning-based framework for strength predicting of concrete filled steel tubular (CFST) columns under concentric loading. The proposed framework was based on the gradient tree boosting (GTB) algorithm which isone of the most powerful ML techniques for developing predictive models. A comprehensive database of over 1,000 tests on circular CFST columns was also collected from the open literature to serve as training and testing purposes of the developed framework. The efficiency of the proposed framework was demonstrated by comparing its performance with that obtained from other ML methods such as random forest (RF), support vector machines (SVM), decision tree (DT) and deep learning (DL). The accuracy of the developed predictive model was also verified with the current design equations from modern codes of practice as well as existing ML-based predictive models.

Reliability considerations of modern design codes for CFST columns

Concrete filled steel tubular (CFST) columns have been increasingly used in tall buildings and bridges due to offering excellent structural and economic benefits. Current design codes for such columns exhibit certain limits in terms of material strengths and section slenderness. This paper aims to evaluate the reliability and the applicability of the current design equations in American code AISC 360–16, European code EC 4 and Australian/New Zealand code ASNZS 2327 for the design of the columns beyond their material and slenderness limits. A comprehensive database with over 3200 tests was collected to develop the statistics of the model errors for different types of columns. Monte Carlo and subset simulation techniques were developed based on Markov Chain Monte Carlo algorithms, to accurately and efficiently predict the reliability index of structures with small failure probability because they account for all uncertainties in material and geometric properties, loads and model errors. The results from the reliability analysis indicate that the reliability index of the concentric column designed by three considered codes is much higher than that of the eccentric column (i.e. beam-column). The results from a parametric study suggest that all three codes can be safely extended to the design of columns beyond the current material and section slenderness code limits.

A study of net-section resistance of high strength steel bolted connections

This article presents an experimental and numerical investigation on the net section resistance of high strength steel (HSS) bolted connections subject to double shear. A total of 22 HSS and 11 mild steel (MS) bolted connection specimens were tested to net section fracture. HSS grades of Q690 and Q960, and MS grade of Q345 were studied. Although the HSS material has relatively lower ductility and a lower ratio of tensile strength to yield strength (fu/fy) than those of the MS material, in general, the HSS connection specimens were able to reach the connection efficiency (i.e. the ratio of the ultimate load of the connection specimens to the calculated net section resistance) of above 1.0. Subsequently, the structural behaviour of the connections was studied by finite element (FE) analysis. The effects of material ductility and fu/fy ratio on the stress development across the net section of the specimens were examined. It was found that HSS materials possess sufficient ductility to allow an efficient stress redistribution across the net section. Besides, the beneficial influence of the ‘reinforcement’ or the biaxial stress effect due to the presence of holes in the connection increases the ultimate capacity of the perforated main plate, and hence the HSS specimens were able to reach the net section resistance. However, the overall deformation capability of the HSS specimens was significantly lower than that of the MS specimens. A reliability analysis was carried out to re-examine the partial factor used in the current design equation for predicting the net section resistance in Eurocode 3.