Tension Chords Research Articles

Performance of Novel U-Connector in CFS Truss-to-Column Bolted Connection under Axial Force

This paper presents an experimental and numerical investigation of the tensile and compressive behaviour of a novel U-connector in the cold-formed steel (CFS) truss-to-column connection. Tensile tests were performed on 12 specimens representing the tension chords of the trusses in the connection. The results were used to validate a finite element model. The validated model was then subjected to both compressive and tensile loads, which revealed low stiffness in both the compressive and tensile components of the proposed connection. An optimisation of the geometry by using one long nut instead of two nuts was carried out to improve the behaviour and stiffness of the connection. The optimised results were compared with both experimental and numerical data, and conclusions were drawn regarding the effectiveness of the components in the proposed connection. The use of long-nut optimisation in the tension and compression components of the proposed connection shows a significant increase in load-bearing capacity, which makes it very promising for future applications in CFS truss-to-column connections. However, further validation through experimental testing is required to confirm the effectiveness and reliability of the connection in full-scale structures.

Time‐dependent elongation and cracking behavior of fiber reinforced concrete tension chords

AbstractThe long‐term (LT) tension stiffening effect in fiber reinforced concrete (FRC) tension chords is investigated in this paper. Six tension tie specimens with varying fiber types (either no fibers, polypropylene, or steel fibers) and reinforcement ratios were subjected to a sustained tensile in‐service load for 560 days. The LT specimens were fitted with gauges to monitor their elongation over the test period. In addition, 12 specimens were subjected to instantaneous loading to capture the entire load‐deformation response of the tension ties, up to and beyond the yielding of the reinforcement. A complete suite of material characterization tests were conducted in conjunction with the tension tie testing. The results from this investigation indicate that the inclusion of the fiber types and dosages adopted in this program improve the short‐term performance with respect to stiffness, tension stiffening, and cracking. Steel fibers improved the LT behavior, while the polypropylene fibers used in this study led to mixed results. Models previously developed by the authors have been shown to describe the behavior with good accuracy.

A general numerical model for predicting the flexural behavior of hybrid FRP-steel reinforced concrete beams

Hybrid fiber reinforced polymer (FRP)-steel reinforced concrete (RC) beams exhibit different flexural failure modes with different reinforcement ratios and the contributions from tensile region and compressive region of the beams to flexural behavior vary accordingly. Therefore, a general model to predict the global nonlinear behavior of hybrid FRP-steel RC beams featured by common flexural failure modes is critical to analysis and design. In this paper, flexural behavior of a hybrid FRP-steel RC beam is considered as the systematic combination of mechanical behavior of FRP and steel RC tension chords and concrete strut. The Tension Chord Model (TCM) for steel and its extended application for FRP, properly considering the bond performance between FRP/steel reinforcement and the surrounding concrete, are employed to represent the tension stiffening effect in tensile region; the Linear Softening Model (LSM), incorporating the compressive fracture energy and considering the confinement effect, is applied to predict the softening behavior of compressive concrete. Based on the primary principles of equilibrium and compatibility, the aforementioned two models are assembled to numerically evaluate the advanced moment–curvature relationship and flexural behavior. Due to the proper combination and application of mechanical models, the global evaluation process is significantly simplified and avoids the lengthy iterative calculations performed in conventional numerical methods. Finally, the proposed calculational model is validated by comparisons between the predictions and experimental results of hybrid FRP-steel RC beams with common flexural failure modes available in the literature.

The Steel Roof of the New Lille Stadium: Instability Phenomenon in Cover-Plates of Tension Plates

The steel roof of the Pierre Mauroy Stadium in Lille (France) is supported by two mega-trusses, each 200 m long and 16 m high. The main design challenge was to propose a technique to assemble the elements of the mega-trusses with minimum welding on site to save time, with a limited number of bolts for aesthetic reasons. The members of the compression chords were assembled using full contact and those of the tension chords were joined with cover-plates and a single pin instead of multiple bolts. During erection of the mega-trusses, an unknown instability phenomenon appeared on the cover-plates used only to transmit tension forces. The lifting operations were stopped immediately to maintain safety. After verifications, it was concluded that the final design based on Eurocodes was correct for both the service conditions and construction stages. Large parametric studies with nonlinear analyses were conducted to understand the origin of this behavior. Ignoring this phenomenon would be impossible even if the tension forces were smaller for the service life. Thus, new design requirements were defined to consider the phenomenon and reinforcement was added to some cover-plates to secure the structure. This article presents the observed phenomenon, the main numerical simulations to understand the origin of the instability, and the design requirements to verify the safety of this type of joint.

Openings Effect on the Performance of Reinforced Concrete Beams Loaded in Bending and Shear

Transverse openings are often provided to reinforced concrete beams to accommodate utility ducts and pipes. This technique is usually adopted to avoid the creation of dead space in structures caused by extended dropped ceilings and leads to significant cost saving. On the other hand, the provision of openings through a beam creates a reduction in its strength and affects serviceability. In this study, ten reinforced concrete beams were cast using C30 concrete. Material characterization and engineering properties tests were carried out to ensure compliance with the requirements provided by the codes of practice. The effect of vertical positioning and size of openings was investigated through subjecting the beams to a four-point bending test after 28 days of curing. Maximum load capacity, first cracking load, and deflections at mid-span were recorded and crack pattern and failure mode were evaluated. Test data showed that openings of depth greater than 0.4d significantly affect the beams’ strength and lead to earlier cracking, while the failure mode remains essentially the same, a diagonal tension crack through the opening except for opening of 0.5d size where the failure occurred by a sudden formation of two independent shear cracks above and below the opening. When holes were located above the centroid of the section, the beams exhibited a lesser deflection characterized by the absence of plastic deformation. Furthermore, a significant reduction in strength was recorded compared to cases where the positioning of openings was in tension chords. This was validated using equations from the ACI code of reinforced concrete design.

ПРОЧНОСТНЫЕ ХАРАКТЕРИСТИКИ ЭЛЕМЕНТОВ ФЕРМЫ ИЗ ЭКОЛОГИЧЕСКИХ КОНСТРУКЦИОННЫХ ПИЛОМАТЕРИАЛОВ

Introduction: Softwood lumber is widely used to manufacture load-bearing structures. However, the quality of round wood used to manufacture such lumber has been deteriorating lately. Round wood decreases in diameter and often has heart rot. The article looks into the possibility of manufacturing elements of load-bearing structures using beams made of round wood of small diameter that have not been previously used to manufacture structural materials for construction purposes. It is suggested to make beams of such round wood preserving the trunk structure to the maximum (heartwood beams). Due to the preservation of the annual growth ring pattern, such beams have better strength properties as compared to traditional structural lumber. Purpose of the study: The study is aimed to determine the strength properties of engineering structures’ elements made of heartwood beams sawn from round wood of small diameter. Methods: The authors tested an experimental truss made of heartwood beams by means of incremental loading until destruction. Results: The strength properties of the truss elements made of heartwood beams sawn from round wood of small diameter were determined. There is a good fit between the calculated values of stress in the truss elements and the experimental data. The structure was damaged in the panel points connecting the compression strut with the elements of the tension and compression chords. The tension elements and their joints remained undisturbed. The experimental structure has a safety factor of 2. Compared to the design load, such a value shows that the experimental truss has the required bearing capacity and is robust. The findings confirm that the strength properties of heartwood beams match the requirements for elements of load-bearing structures. The strength properties of heartwood beams make it possible to use them to manufacture load-bearing structures.

Humerus osteology, myology, and finite element structure analysis of Cheloniidae.

Adaptation of osteology and myology lead to the formation of hydrofoil foreflippers in Cheloniidae (all recent sea turtles except Dermochelys coriacea) which are used mainly for underwater flight. Recent research shows the biomechanical advantages of a complex system of agonistic and antagonistic tension chords that reduce bending stress in bones. Finite element structure analysis (FESA) of a cheloniid humerus is used to provide a better understanding of morphology and microanatomy and to link these with the main flipper function, underwater flight. Dissection of a Caretta caretta gave insights into lines of action, that is, the course that a muscle takes between its origin and insertion, of foreflipper musculature. Lines of action were determined by spanning physical threads on a skeleton of Chelonia mydas. The right humerus of this skeleton was micro-CT scanned. Based on the scans, a finite element (FE) model was built and muscle force vectors were entered. Muscle forces were iteratively approximated until a uniform compressive stress distribution was attained. Two load cases, downstroke and upstroke, were computed. We found that muscle wrappings (m. coracobrachialis magnus and brevis, several extensors, humeral head of m. triceps) are crucial in addition to axial loading to obtain homogenous compressive loading in all bone cross-sections. Detailed knowledge on muscle disposition leads to compressive stress distribution in the FE model which corresponds with the bone microstructure. The FE analysis of the cheloniid humerus shows that bone may be loaded mainly by compression if the bending moments are minimized.

Bending tests for the structural safety assessment of space truss members

This article compares two nondestructive static methods used for the axial load assessment in prismatic beam-columns of space trusses. Examples include the struts and ties or the tension chords and diagonal braces of steel pipe racks or roof trusses. The first method requires knowledge of the beam-column’s flexural rigidity under investigation, whereas the second requires knowledge of the corresponding Euler buckling load. In both procedures, short-term flexural displacements must be measured at the given cross sections along the beam-column under examination and subjected to an additional transverse load. The proposed methods were verified by numerical and laboratory tests on beams of a small-scale space truss prototype made from aluminum alloy and rigid connections. In general, if the higher second-order effects are induced during testing and the corresponding total displacements are accurately measured, it would be easy to obtain tensile and compressive force estimations.

03.16: Multiplanar K‐joints on cold‐formed open sections: An experimental study with high strength steels

ABSTRACTThe design, laboratory investigation and main results of an experimental programme on the joints of an innovative 3D truss made of high strength steel (HSS) is presented. An entire truss of triangular cross‐section was fabricated (two compression chords and a single tension chord) to provide realistic loading conditions. Both compression and tension chords were fabricated by cold forming hot‐rolled HSS plates by means of press braking. The chords were as follows: — an open angle‐type profile with 45° contained angle as tension chord (U‐chord), — a semi‐closed octagonal profile made of 3 sectors bolted along their length every 1.2 diameters as compression chords (P‐chord). The focus of this study is on the tension chords joints where a pair of tension and a pair of compression diagonals converge. Joints were tested to ultimate load by introducing shear to one bay of the truss at a time. Two design approaches for the joints were tested resulting in four individual tests.Ultimate load levels, stiffness and failure modes varied between the considered joint configurations. Stiffening increased the ultimate load by as much as 170% of that of the unstiffened version achieving higher utilisation of the tension chord, thus making more efficient use of HSS. The stiffness loss at loads close to failure varied between 60% – 80%. Two distinctive failure modes were observed, one caused by cracking of the welds connecting the diagonal's gusset plates to the tension chord while the second by cross‐section tensile fracture of the circular hollow section (CHS) diagonal.

Shear Strength of Arch-Shaped Members without Transverse Reinforcement

Arch-shaped members are used in many reinforced concrete structures such as tunnels, bridges, silos, or offshore constructions. They are generally designed to carry significant axial forces, and, in most cases, are not provided with transverse reinforcement. Depending on the applied actions, however, bending moments and shear forces may also develop. This can potentially lead to spalling failures of the concrete cover or to shear failures where the strength may be reduced in comparison with straight members. Such reduction is due to the curvature of the element and to the corresponding deviation forces originated by the compression and tension chords. In spite of the significance of the topic, little research has been performed. In this paper, the authors summarize the results of a comprehensive investigation comprising an experimental part on nine arch-shaped specimens with thickness equal to 400 mm (15.7 in.). On the basis of these results, the basic failure mechanisms and shear-transfer actions are identified. By using the principles of the critical shear crack theory for assessment of the shear strength, a design approach for such members is presented that accounts for the various mechanical and geometrical parameters. The approach provides consistent and accurate predictions when compared with the tests presented in the paper.

Out-of-plane buckling and bracing requirement in double-angle trusses

Truss members built-up with double angles back-to-back have monosymmetric cross-section and twisting always accompanies flexion upon the onset of buckling about the axis of symmetry. Approximate formulae for calculating the buckling capacity are presented in this paper for routine design purpose. For a member susceptible only to flexural buckling, its optimal cross-section should consist of slender plate elements so as to get larger radius of gyration. But, occurrence of twisting changes the situation owing to the weakness of thin plates in resisting torsion. Criteria for limiting the leg slenderness are discussed herein. Truss web members in compression are usually considered as hinged at both ends for out-of-plane buckling. In case one (or both) end of member is not supported laterally by bracing member, its adjoining members have to provide an elastic support of adequate stiffness in order not to underdesign the member. The stiffness provided by either compression or tension chords in different cases is analyzed, and the effect of initial crookedness of compression chord is taken into account. Formulae are presented to compute the required stiffness of chord member and to determine the effective length factor for inadequately constrained compressive diagonals.

Tension Stiffening and Crack Formation in Reinforced Concrete Members with Fiber-Reinforced Polymer Sheets

Models to estimate crack spacings and tension stiffening effects in reinforced concrete (RC) members with externally bonded fiber-reinforced polymer (FRP) sheets are presented. Due to the lack of experimental evidence on the tension stiffening effect of FRP sheeting, a theoretical approach based on the concept of tension chords is introduced. Crack formation and the tension stiffening of tension chords with FRP sheets are subjected to parametric analyses using bond stress-slip relations. The analytical results are then reduced into simple model equations. In addition to the FRP sheet, the bond characteristics between steel bars and concrete is also modeled according to a concept of average bond. These models are incorporated into the distributed stress field model, enabling reasonable estimations of the crack widths and the tension stiffening effects in RC members with the FRP sheets.