Cold-formed Angle Research Articles

Mechanical properties and constitutive model of cold-formed Q1100 ultra-high strength steel sections at low and elevated temperatures

The Q1100 ultra-high strength steel (UHSS) has broad application prospects in bridges and super high-rise buildings due to its ultra-high strength properties. However, no research has been conducted on the mechanical properties of Q1100 UHSS at low and elevated temperatures. Therefore, an experiment was conducted to examine the low- and elevated-temperature mechanical properties of Q1100 UHSS at the flat and corner regions of cold-formed angle sections. The temperature considered in this study ranged from −80 ℃ to 500 ℃. The failure modes, stress-strain curves, and key mechanical parameters of the flat and corner specimens of Q1100 UHSS at different temperatures were obtained. The experimental results showed that both the flat and corner specimens underwent ductile fracture with obvious necking at different temperatures. The stress-strain curves of all specimens had no yield plateau, showing obvious nonlinear characteristics. The strength properties of Q1100 UHSS at low temperatures were increased by less than 10 %. However, the strength properties of Q1100 UHSS at elevated temperatures were significantly reduced, where the reduction exceeded 60 % at temperatures reaching 500 °C. Notably, the ultimate strength of Q1100 UHSS at 200 °C was about 5 % higher than that at ambient temperature. The cold-forming process significantly reduced the ductile properties of Q1100 UHSS, but improved its strength properties. At low and elevated temperatures, the most significant cold-formed strengthening effects of yield strength, ultimate strength, and elastic modulus occurred at −40 ℃ and 300 ℃, respectively. Moreover, the predictive equations of key mechanical parameters and a two-stage constitutive model were proposed for the Q1100 UHSS flat and corner specimens. The verification results indicated that the proposed predictive equations and constitutive model could accurately predict the mechanical parameters and stress-strain curves of the Q1100 UHSS flat and corner specimens at different temperatures. The experimental results and constitutive model provided a basis for the numerical and theoretical studies on the resistant performance of the cold-formed UHSS structures servicing in low and elevated temperatures, which could be used as the material model.

Experimental and numerical analysis of joints and thin-walled steel beams fabricated through resistance spot welding and hot-dip galvanizing

The study conducted a numerical and experimental analysis of lap joints and thin-walled beams made of DC01 steel. The beams consisted of webs formed by two flat bars, which were connected to flanges using cold-formed angles through advanced resistance spot welding (RSW) technology. The beams varied in geometry and mass. Load capacity evaluation was performed on the beams using a test stand consisting a testing machine and additional bending tooling. Prior to testing, some of the beams underwent hot-dip galvanization to create a bimetallic structure. The application of hot-dip galvanization resulted in the filling of free spaces between the welded sheets of the beams, leading to increased load-bearing capacity and stiffness in the galvanized beams compared to the non-galvanized ones. The galvanized beams tested during the experimental study exhibited a bending load capacity that was 59–83 % higher than that of the non-galvanized beams. The RSW lap joints were subjected to a uniaxial tensile test using digital image correlation and tracking (DIC). Three variants of joints were analyzed, differing in the number and arrangement of welds. The results obtained from the experimental tests were then compared with those from numerical analyses conducted in the ADINA software, utilizing the Finite Element Method (FEM). The initiation of cracks in the lap joints occured at the location of maximum plastic deformation, determined experimentally and confirmed by numerical simulations. The strength of the joint was influenced by the number and arrangement of welds in the joint. The use of 3D solid models to analyze RSW lap joints and resistance welded beams allowed for more accurate results in the weld area. The paper also presented the possibilities of using the designed beams.

Reinforcement concrete beams with external steel elements

The reinforcement of structural elements is widely practiced worldwide. It can be applied due to deterioration of the concrete, either due to age, change of use, extensions, or updating of the design code. However, it is sometimes used with inappropriate materials or without technical supervision or structural design. The main purpose of this paper is to investigate the method of strengthening based on the external addition of steel elements and to determine its influence on the flexural load-carrying capacity of reinforced concrete (RC) beams. For the same area of external steel, two types of beams are defined: the first one with cold-formed angle sections at the bottom corners, and the second one with a steel plate in the center of the tension face of the beam. The behavior of a total of 12 beams under monotonic and cyclic loading is experimentally studied. The connection was made with post-installed expanding bolts, without adhesives. The configuration that presents adequate results in terms of strength, flexibility, and energy, this obtained with the addition of a steel elements. On the other hand, the use of angled sections, contrary to expectations, reduces the load-carrying capacity and ductility of the section.

Experimental study on horizontal low-cycle loading of high-strength concrete medium high shear wall with cold-formed steel braces

This study presents the findings from experimental tests conducted on four medium-high shear walls with a shear span ratio of 1.8. The focus of the study is to investigate the effect of different types of cold-formed steel diagonal braces on the seismic performance of the shear walls. The specimens were subjected to horizontal low-cycle loading to observe their failure behavior, including cracking load, ultimate displacement, yield load, and ultimate load. Additionally, a comparative analysis of the stress distribution in the cold-formed steel braces, braces with different steel ratios but of the same type, and braces with the same steel ratio but of different types, was conducted. The research findings indicated that the inclusion of an appropriate amount of cold-formed steel braces with an appropriate steel ratio significantly improved the ultimate displacement and shear stiffness of the structures. The cold-formed steel braces effectively restrained crack development and prevent structural collapse after failure. However, it was observed that the lattice-type braces failed to confine the internal concrete to form a compressed concrete strut capable of resisting both shear and bending moment simultaneously. The addition of cold-formed steel braces was found to be advantageous in improving the shear stiffness of the specimens. In particular, the cold-formed angle steel braces in SRHCW-2 were more effective in enhancing the shear stiffness compared to the lattice-type cold-formed steel braces in SRHCW-3. Moreover, an increase in the steel brace ratio on top of the lattice-type cold-formed steel braces in SRHCW-3, as observed in SRHCW-4, contributed to a certain improvement in the structural shear stiffness.

Investigation on single-bolted cold-formed steel angles with geometric imperfections under compression

The behaviour of cold-formed steel angle members with bolted connections under compression is usually complicated, whereas not significant number of experimental tests is found in the literature. This study offers a better insight on the issue, by comparing experimental tests with a thorough numerical study, both carried out at the Institute of Steel Structures of NTUA. Significant attention is paid for the elaborate measurement of initial geometric imperfections along the member's length. An in-house built deformation plotter was designed and upgraded for this reason. The compression failure loads of the experimental tests are illustrated both in graphical and tabular form and compared with similar tests of the literature. The results denote a prediction of the EN 1993-3-1 towards the safe side, for the case of the examined bolted cold-formed angles. A simple formula is proposed for the accurate prediction of angle columns' buckling deformation, wherein the rigidity of the connection is also included. Finally, interesting conclusions are derived for the numerical simulation of cold-forming effects, as well as the shape and amplitude of the initial imperfect geometry, on the basis of the critical buckling load. Further research is in progress, aiming to capture the stochastic buckling response for the case of bolted cold-formed steel angles.

Efficacy of roof-to-beam mechanical connections on the diaphragm behaviour of precast decks with spaced roof elements

The diaphragm action of slab/roof decks ensures the collaboration of different parts of the lateral load resisting system of a building. In reinforced concrete precast buildings for industrial/commercial halls, not seldom the deck is not provided with a cast-in-situ topping: this results into diaphragm action to rely upon the floor mechanical connections alone. In this study, the efficacy of three different typical floor-beam connections, namely hot-rolled angle brackets, cold-formed angle brackets and dowel bars, on the diaphragm action of the deck has been investigated. To this purpose, simplified macroscopic “behaviour models” have been proposed, based on the results of monotonic and cyclic tests carried out within the framework of the research project Safecast (FP7-SME-2007-2; GA 218417/2009). A numerical model of a dry-assembled precast structure with mechanical floor-to-beam connections has been checked against the results of cyclic and pseudo-dynamic tests carried out on a full-scale prototype within the framework of the Precast Structures EC8 project (GA G6RD-CT-2002-70002). Non-linear dynamic analyses have then been performed to investigate the diaphragm action effectiveness of the three different technological solutions to connect slab and beams considering a seismic action orthogonal to the roof elements. Different stiffness distributions of the lateral load resisting system have been considered, investigating the possible bracing effect induced by an integrated connection system of the external cladding panels. A simplified “design-wise” analytical interpretation of the phenomenon is also formulated and checked against the numerical results.

Experimental and Numerical Study on Single-Bolted Cold-Formed Angles under Tension and Compression

Angle sections are commonly designed to bear only axial force, usually neglecting the additional bending moments resulting from the eccentric connection and the shift of the effective centroid. The current work deals with the capacity of single-bolted equal angle sections made from cold-formed steel. The experimental investigation presented herein includes tension and compression tests which subsequently are compared with the corresponding code provisions. Numerical analyses are also presented based on a detailed finite element simulation. Finally, a reliability analysis is implemented in order to demonstrate the reliability of the design rules for cold-formed steel angle columns. Results indicate a small discrepancy on the strength prediction in general by EN 1993-1-3, as well as by the AISI for slender columns and a more conservative one by EN 1993-1-1. A comparison of the above results is clearly illustrated herein in graphical forms.

Study on Behaviour of Bolted Cold-Formed Steel Angle Tension Members

Background: The main objective is to investigate the behaviour of cold-formed steel single and double angles subjected to tension. Methods: Sixteen single plain and lipped angle specimens and thirty two numbers of double angle specimens connected to the same side and opposite side of the gusset plate were tested in a Universal Testing Machine using ordinary black bolts of 10mm diameter. The experimental loads are compared with ultimate load calculated using BS 5950(Part V)-1998, and AS/NZS 4600:2005. Results: Most of the single and double angles failed by net section fracture failure and the outstanding leg are subjected to local bend due to shear lag effect. Application: Angles are used in a variety of structures such as trusses, transmission towers etc. as tension members. Angles may be used as single angles or double angles and the connection may be bolted or welded.

Base connections of single cold-formed steel portal frames

Abstract An investigation of the behaviour of base connections, fabricated from cold-formed channels and hot-rolled angle cleats, is presented in this paper. This investigation is part of an extensive research to develop portal frames made out of single cold-formed steel channels. The use of bolted angle cleats allows for a simple connection to be developed, which can result in significant cost savings within the steel construction industry. Stiffer angle cleats are used to prevent premature failing of the base connections. The base connections are subjected to four loading configurations and these are dependent on the eccentricity of the load. In base connections where cold-formed angle cleats were used, failure of the bases was largely caused by premature deformation of the angle cleats. Base connections with hot-rolled angle cleats were so stiff that failure occurred in the column channels instead of the angle cleats. A significant amount of bearing distortion was observed in the heavily loaded flange. A simple formula is suggested to design these bases.

Residual Stress Research of Cold-Formed Thin-Wall Steel Members Using the Finite Element Method

This paper analyzes the results of experience and rules of distribution of the cold – formed residual stress. Distribution models of residual stress in cold-formed thin-wall steel that are close to the real condition and initial stress files of ANSYS which can describe the magnitude and distribution of the residual stress are established. This method can be applicable to simulate different distribution of the residual stress. Through importing the files into the stability analysis of ANSYS, ultimate load in the effect of residual stress can be gained. The stability coefficient of cold-formed thin-wall crimping angle steel under influence of the initial bending of 1/750 is studied, comparing the result curve with the one in GB50018-2002, it is proved that the curve is reasonable and the stability coefficient of the code should be suitable to improve.

Study on the buckling behaviour of cold-formed angles in transmission towers

Considering the structural characteristics of transmission towers, four sections of cold-formed angles with different slenderness ratios and constrained types were selected for the experimental and numerical study. Experiments and finite element analysis for the ultimate loads of the compression cold-formed angles were carried out. The finite element model well predicts the buckling behaviour of the cold-formed members. For the axial compression members, the load-strain curves as well as the ultimate loads were analyzed, and the experimental ultimate loads were compared with those of calculated by the applicable standards. It shows that the applicable standards aren’t adaptive to the strength design of the compression cold-formed members in transmission towers. Through the analysis of the experimental and FEA results, the fitting curve of the stability coefficients for the cold-formed members in transmission towers was determined. The ultimate loads calculated by the fitting curve are well agreed with the experimental values, especially for the members with relatively low slenderness ratios. Some modification suggestions were proposed for the calculation of the ultimate load for the axial compression cold-formed angle. Experimental analysis on the cold-formed members of the other five constrained types was completed, and the modification formulas of the slenderness ratios for members of different constrained types were obtained.

Buckling Behavior of Steel Bridge I-Girders Braced by Permanent Metal Deck Forms

Permanent metal deck forms (PMDFs) are often used in the bridge industry to support wet concrete and other loads during construction. Although metal formwork in the building industry is routinely relied on for stability bracing, the forms are not permitted for bracing in the bridge industry, despite the large in-plane stiffness. The forms in bridge applications are typically supported on cold-formed angles, which allow the contractor to adjust the form elevation to account for changes in flange thickness and differential camber between adjacent girders. Although the support angles are beneficial toward the constructability of the bridge, they lead to eccentric connections that substantially reduce the in-plane stiffness of the PMDF systems, which is one of the reasons the forms are not relied on for bracing in bridge applications. This paper documents the results of an investigation focused on improving the bracing potential of bridge deck forms. Modifications to the connection details were developed to improve the stiffness and strength of the forming system. Research included buckling tests on a 15-m (50-ft) long, twin-girder system with PMDFs for bracing. In addition, twin-girder tests were also used to validate computer models of the bracing systems that were used for parametric finite-element analytical studies. The buckling test results demonstrated that modified connection details make PMDF systems a viable bracing alternative in steel bridges, which can significantly reduce the number of cross-frames or diaphragms required for stability bracing of steel bridge I-girders during construction.

Some Advances in the Application of Weathering and Cold-Formed Steel in Transmission Tower

Application of weathering and cold-formed steel in transmission lines can reduce steel consumption and environmental pollution. Some advances in the studies on the weathering and cold-formed steel in transmission tower are introduced. Firstly, corrosion-resistant tests of weathering steel samples under different simulating technical atmospheres were carried out separately for 240 hours. It shows that the corrosion degree of joint samples is higher than that of single chip samples, and the corrosion-resistant performance of weathering steel is superior to common carbon steel. The corrosion-resistance of weathering steel meets with the requirement of transmission tower. Secondly, experiments and finite element analysis for cold-formed angles and a 220kV prototype tower were completed, and the stability coeffi-cient fitting curves as well as the modification formulas of slenderness ratio for cold-formed members were determined. According to the structural characteristics of transmission towers, four sections of cold-formed angles with different sections and slenderness ratios were selected in this study. The finite element model well predicted the buckling behav-iour of the cold-formed members. Ultimate loads calculating by the fitting curve were well agreed to the experimental values, especially for the members with small slenderness ratios. Weight of the cold-formed steel tower can be reduced by more than 5 percent after considering the strength enhancement. Cost of the weathering and cold-formed steel transmission tower is nearly equivalent to that of hot-rolled steel tower with hot galvanizing.

Plastic and Lateral-Torsional Buckling Behavior of Single Cold-Formed Channels Connected Back-to-Back

This paper describes the results of a series of experiments on semi-rigid and inelastic joints, and lateral-torsional buckling of the eaves region of portal frames comprising cold-formed single channels bolted back to back at the eaves and apex joints. The tests demonstrate the considerable ductility that is achieved in the back to back bolted connections, which should be sufficient in many cases to accommodate plastic analysis of the portal frames. Variables in the tests include the number of bolts in the connection, the lengths between lateral restraints, the width of the channel flanges, and the strength of the channels. Adjacent column and rafter channels are oriented successively in opposite transverse directions to take advantage of the counterbalancing moments and forces in the back to back connections. This system is important in enhancing the lateral buckling strength of the channels. An innovative method is proposed for connecting each purlin to the web of the column or rafter through a cold-formed angle. Experiments have shown this system to be capable of restraining the frames from failing due to lateral-torsional buckling. This eliminates the idea of having fly bracings, which is normally done in practice to restrain torsional instability.

Compression Tests on Cold-Formed Angles Loaded Parallel with a Leg

This paper describes a series of compression tests on cold-formed equal angles with slender cross section. The angles were tested between pinned ends and loaded axially with eccentric load, which caused bending parallel with a leg. The test data are compared with the design rules of the Australian and American specifications for cold-formed and hot-rolled steel structures, as well as an ASCE standard. The rules of the specifications for cold-formed steel structures are shown to be very conservative. The cause of the conservatism is explained and improved design rules are proposed.

Angle and Z Beams in Bending: Tensor Matrix Method for Spreadsheets

This paper presents the generalized classical flexure formula for unsymmetrical bending in a matrix format. The new formula is presented as a product of three matrixes: a matrix of all the moments of inertias of the beam or the inertia tensor; the coordinates; and the applied bending moment matrices. Presented as such, the solution to a problem generally regarded as tedious is greatly simplified and is suited for computations using spreadsheets. The calculations for effective bending moments, effective loads, deflections, and principal moments of inertia are made easy. A uniform computational format for all types of unsymmetrical bending problems is provided. The application of the procedure is described for the analysis of hot-rolled or cold-formed angle and Z unsymmetrical steel sections. This paper should be of interest to civil, structural, mechanical, and aeronautical engineers.

Axial Compression Tests of Cold-Formed Angles

This paper presents fixed-ended and pin-ended compression tests on cold-formed, in-line galvanized equal angles. Three nominal section sizes were tested ranging from nonslender to slender sections. Detailed measurements of material properties, residual stresses, and geometrical imperfections were conducted. The pin-ended specimens were tested with a nominal eccentricity of \iL/1,000 applied about the minor axis to cause compression at the tips of the legs. The test data are compared with the Australian and American specifications for cold-formed and hot-rolled steel structures. The section capacities obtained from the stub column tests are found to be between 15 and 40% higher than those calculated according to the specifications. It is concluded that there is no need to include the additional eccentricity specified in the Australian and American cold-formed steel structures standards accounting for the shift of the effective centroid, and that the eccentricity of \iL/1,000, specified for all section classes, should be applied to slender sections only. These conclusions lead to increases in the design strengths on the order of 25%.

Effect of temperature and galvanization on the compressive strength of cold-formed angles

An experimental program was conducted at The University of Manitoba to investigate the effect of subfreezing temperatures and galvanization on the compressive strength of cold-formed steel angles. The study involved the testing of 20 cold-formed angles, 55 × 55 × 4 mm, with a slenderness ratio of approximately 70. Equal numbers of galvanized and ungalvanized angles were tested at various temperatures ranging from −45 to 25 °C. The material properties were obtained through 48 standard tension coupon tests conducted at the same temperature range. The results showed that the capacity of the angles measured at temperatures below −40 °C was approximately 8% higher than the capacity at room temperature. Similar results were obtained during testing of the standard tension coupons where the yield and tensile strengths of the steel used were approximately 10% higher at temperatures below −40 °C than at room temperature. On the average, the ultimate capacity of galvanized angles was approximately 9% higher than that of ungalvanized angles. Although the yield strength of the corner coupons was between 13% and 27% higher than that of flat coupons tested at room temperature, the compressive strength of the full-size angles was influenced mainly by the material properties of the flat regions. A comparison of the measured and predicted capacities using the current Canadian standard and American specification indicated that the Canadian standard overestimated the ultimate capacity of the angles by as much as 37%, while the American specification underestimated the capacity by as much as 45%. Key words: angles, cold-formed, galvanized, temperature, compression, ultimate.

Biaxial bending of cold-formed angle members

Theoretical load–deflection relationships for cold-formed angles under biaxial bending using the Galerkin method are presented. The computational difficulties encountered in the exact solution of differential equations of equilibrium involving 12 unknown constants in 12 simultaneous equations are pointed out. A computer program for pinned-end boundary conditions was developed to estimate the deflection components of the shear centre, to calculate the total stress at various points in the cross section, and to predict the ultimate strength of the cold-formed angle sections connected by one leg. Failure is assumed to have occurred when the total stress at any point on the cross section reaches the value of yield stress, compressive or tensile, or when there is a change of sign for at least one of the deflection components. A table giving the ultimate compressive strength of two commonly used cold-formed angles for various gauge distances is included. The theoretical load–deflection curves are compared with experimental results and typical curves for three test specimens are also presented.

Cross-sectional properties of cold-formed angles

Cross-sectional properties of both equal and unequal leg cold-formed angle sections are presented. Besides cross-sectional area, location of centroid, moments of inertia, and torsional constant, the properties listed include the location of shear centre and the magnitude of warping constant. These two latter properties are required for determining failure loads of angles subjected to torsional–flexural buckling. Also listed are two important parameters, β1, and β2, that are required for the calculation of theoretical buckling loads of eccentrically loaded columns. Key words: buckling, cold-formed angles, columns, cross-sectional properties, shear centre, stability, torsional–flexural buckling, warping constant.