Slender Braces Research Articles

Numerical study of the hysteretic behavior of energy dissipation braces based on Miura origami

Miura-ori pattern was adopted to form the tessellation design of slender braces to improve the energy dissipation capacity under the low-cycle repeated loading. The energy dissipation mechanisms of origami energy dissipation braces (OEDBs) and conventional square braces with equal bearing capacity were compared. Finite element models for hysteretic simulation were constructed and analyzed to figure out the influences of various geometrical design parameters on the hysteretic behavior of OEDBs. The results show that the OEDBs have an obvious improvement in the energy dissipation capacity compared to conventional square braces. The influence of parameters on energy dissipation capacity is quite different. The smaller folding angles, smaller aspect ratios, and larger width-to-thickness ratio play positive roles in improving energy dissipation performance rather than elastic stiffness. However, smaller slenderness improves both while the change of in-plane angle is sensitive to the hysteretic performance.

Linear Buckling Analysis Considering No-compression Property of Metal Grid Shells Stiffened by Tension Braces

In order to analyze the buckling behavior of lattice shells stiffened by cables or slender braces without pre-tension, it is necessary to consider the no-compression property of braces. This paper proposes an innovative method of linear buckling analysis that considers the no-compression property of braces. Moreover, in order to examine the proposed method's validity, its results are compared with the results from a nonlinear buckling analysis with geometrical nonlinearity and material nonlinearity to express the no-compression property of braces. The results show that the proposed method can well-predict the buckling behaviors of lattice shells stiffened by tension braces.

Simplified Lateral Torsional Buckling (LTB) Analysis of Glass Fins with Continuous Lateral Restraints at the Tensioned Edge

Within multiple design challenges, the lateral torsional buckling (LTB) analysis and stability check of structural glass members is a well-known issue for design. Typical examples can be found not only in glass slabs with slender bracing members but also in facades and walls, where glass fins are used to brace the vertical panels against input pressures. Design loads such as wind suction give place to possible LTB of fins with LR at the tensioned edge and thus require dedicated tools. In the present investigation, the LTB analysis of structural glass fins that are intended to act as bracers for facade panels and restrained via continuous, flexible joints acting as lateral restraints (LRs) is addressed. Geometrically simplified but refined numerical models developed in Abaqus are used to perform a wide parametric study and validate the proposed analytical formulations. Special care is spent for the prediction of the elastic critical buckling moment with LRs, given that it represents the first fundamental parameter for buckling design. However, the LR stiffness and resistance on the one side and the geometrical/mechanical features of the LR glass members on the other side are mutually affected in the final LTB prediction. In the case of laminated glass (LG) members composed of two or more glass panels, moreover, further design challenges arise from the bonding level of the constituent layers. A simplified but rational analytical procedure is thus presented in this paper to support the development of a conservative and standardized LTB stability check for glass fins with LR at the tensioned edge.

A new energy-absorbing system for seismic retrofitting of frame structures with slender braces

Past earthquakes experience regarding frame structures with slender braces, i.e. tension-only concentrically braced frames, has demonstrated poor seismic behavior of these systems. In these buildings, the bracing system does not have sufficient buckling strength, and the sudden buckling causes rapid drops in the structural stiffness and strength. In the present study, to improve the seismic performance of slender braces in frame structures, adding new, replaceable shear-flexural yielding hinges in the central yielding dampers is proposed. In the new system, by transforming the axial force of braces to shear force and flexural moment in the hinges, the seismic energy will be dissipated. In this regard, the influences of the proposed dampers on the seismic response, as well as response modification factor (R-factor), are evaluated along with the seismic reliability analysis for different performance levels. The results show that the proposed damper has an acceptable energy dissipation capacity and considerably increases ductility of the system. The response modification factor and corresponding acceleration for the frame structures equipped with the new damper are derived as 8.5 and 0.775 g, respectively. Also, it is observed that the proposed energy-absorbing system provides acceptable reliability, such that the bracing system remains in the immediate occupancy performance level in the case of design earthquake level.

Seismic Performance Evaluation of Steel Moment Frame Factory Building with Slender Braces

'Seismic Performance Evaluation Method for Existing Buildings (2013)' developed in accordance with the overseas guidelines ASCE 41 - 06 is the most widely used procedure among domestic seismic performance evaluation guidelines in Korea. However, unlike ASCE 41 - 06, it stipulates that the final performance should be derived as the gravity load distribution ratio of the lateral force resistance system in the guideline. Therefore, in the case of a dual steel structure system with slender braces, where the internal moment frame is mostly responsible for the gravity load, the evaluation of slender braces based on gravity load distribution ratio is difficult to be achieved. In this research, we propose an objective evaluation process for such system by evaluating seismic performance for large-scale factory facilities as an example.

11.62: A numerical study on the seismic performance of experimentally‐tested steel single‐storey concentrically braced frames

ABSTRACTThe paper presents an overview on nonlinear modeling and numerical evaluation of the seismic response of two steel single‐storey concentrically braced frames with X‐brace configuration previously tested by other researchers.The numerical simulations included series of nonlinear time history analyses using OpenSees computational framework. The brace members were modeled with fiber frame finite elements with distributed plasticity taking into account their initial imperfections. The material model accounted for the low‐cycle fatigue effects based on the approach proposed by several other research works on the topic.The maximum response displacements of the investigated specimens were also evaluated following the procedure given in Annex B of EN 1998–1 (Eurocode 8). However, the previously conducted shake table testing and the herein presented numerical results indicated that this procedure may significantly underestimate the displacements of single‐storey dissipative short‐period CBFs designed with high values of the behaviour factor.It was also found that the numerical estimation of the maximum and residual lateral response displacements of the analyzed CBFs based on nonlinear time history analysis is very sensitive to small variations of the material properties and assumed damping model (damping matrix definition). Therefore the experimentally‐obtained response displacements can be realistically predicted only if the parameter variations are taken into account in the numerical analyses.The authors believe that the use of high values for the behaviour factor in the design of low‐rise CBFs may result in excessive residual displacements and low‐cycle failure of the brace members. It is suggested the further revisions of Eurocode 8 impose lower reference values of the behaviour factor for the short‐period steel CBFs with slender braces in order to reduce their ductility demand and related damage in strong earthquakes.

Mid-length lateral deflection of cyclically-loaded braces

This study explores the lateral deflections of diagonal braces in concentrically-braced earthquake-resisting frames. The performance of this widely-used system is often compromised by the flexural buckling of slender braces in compression. In addition to reducing the compressive resistance, buckling may also cause these members to undergo sizeable lateral deflections which could damage surrounding structural components. Different approaches have been used in the past to predict the mid-length lateral deflections of cyclically loaded steel braces based on their theoretical deformed geometry or by using experimental data. Expressions have been proposed relating the mid-length lateral deflection to the axial displacement ductility of the member. Recent experiments were conducted on hollow and concrete-filled circular hollow section (CHS) braces of different lengths under cyclic loading. Very slender, concrete-filled tubular braces exhibited a highly ductile response, undergoing large axial displacements prior to failure. The presence of concrete infill did not influence the magnitude of lateral deflection in relation to the axial displacement, but did increase the number of cycles endured and the maximum axial displacement achieved. The corresponding lateral deflections exceeded the deflections observed in the majority of the previous experiments that were considered. Consequently, predictive expressions from previous research did not accurately predict the mid-height lateral deflections of these CHS members. Mid-length lateral deflections were found to be influenced by the member non-dimensional slenderness () and hence a new expression was proposed for the lateral deflection in terms of member slenderness and axial displacement ductility.

Compressive behavior of H-shaped brace strengthened with non-welded cold-formed element

As seismic design code was revised in Korea in 2009, the seismic performance of the structures which were not built in compliance with the code need to be reviewed. In middle and low-rise buildings, braces resist most of lateral force. Therefore, it is critical to decide how to reinforce the braces in middle and low-rise buildings to ensure structural safety when seismic load is applied repeatedly. This study suggests reinforcement details to restrain the flexural buckling of slender H-shaped braces and enable them to resist compressive force. In order to evaluate the behavior of the suggested reinforcing member, preliminary test and full-scale monotonic compressive loading test were carried out. The parameters in the test were slenderness ratio and reinforcement area ratio. It was observed from the test that the suggested reinforcing member restrained the flexural buckling of slender brace and induced stable behavior of the brace when compressive force was applied. According to the tests, the area of reinforcing member should be more than 1.7 times of that of core member and the length of the former should be at least 80% of that of the latter.

A hybrid friction-yielding damper to equip concentrically braced steel frames

This paper introduces the design of a new combination of friction/hysteretic damper installed in the middle of cross bracing for dissipating seismic energy. Due to its relative simplicity and easy application, with no need for any special material or technology, this high-performance system has gained more attention than any other energy dissipation devices. Choosing appropriate slip load and maximum sliding movement values in this system, in comparison with its counterpart i.e. circular ring yielding dissipator, this combined system may inherit the advantages of both friction and steel yielding damper. In weak-to-moderate ground motions, it dissipates energy by friction and in strong ground motions, it absorbs energy by yielding These circular damper systems can be used to rehabilitate existing buildings whose slender braces are only designed based on resistance of tension forces. Also because of failure localization in this simple-type system, a distorted and damaged damper can be promptly replaced with a new one after a strong earthquake.

Seismic performance of floor-by-floor assembled steel braced structures with stiffened connections

This paper presents an overview of a full-scale testing on a tension-only concentrically braced beam-through frame (TCBBF). The implementation of TCBBF facilitates the construction of low-rise industrialised residential steel houses by means of floor-by-floor assembling. This type of TCBBF system features cold-formed hollow structural section columns connected to H-section through beams by end plate with bearing-type high-strength bolts. A two-storey, four-span by one-span TCBBF subjected to vertical loads was cyclically loaded horizontally to examine the seismic behaviour. Stable behaviour was observed up to a storey drift angle of 1/10. The cyclic behaviour was characterised by a linear response, a slip range and a significant hardening response. Deteriorating pinched hysteretic behaviour was notable for cyclic loading primarily because of cyclic brace compression buckling and tension yielding. TCBBF incorporates very slender bracing members that are unable to bear much axial load when subjected to comp...

Comparison of European and Japanese seismic design of steel building structures

This paper compares EuroCode 8 (EC8) and the Japanese seismic design code (BCJ) for steel moment frames and braced frames. Soil classification, magnitude and shape of unreduced elastic response spectra, distribution of seismic shear along the height, member ductility requirements, and behavior factor are compared. It was found that the two codes are relatively similar except for the seismic force stipulated for the serviceability limit state. EC8 gives an approximately 2.5 times larger force for this limit state. Behavior factors that allow for system ductility are significantly different between the two codes; for moment frames BCJ is more conservative by about 50%. Strength demanded by EC8 and BCJ is evaluated for steel moment frames and chevron braced frames. Although the behavior factor is less conservative in EC8, the net strength required by EC8 is significantly greater than the corresponding BCJ strength for steel moment frames, and it occurs because of the significantly larger design force stipulated for serviceability in EC8. With regard to braced frames, BCJ leads to larger lateral story strength except for chevron braced frames with slender braces.

SEISMIC IMPACT LOADING IN INELASTIC TENSION-ONLY CONCENTRICALLY BRACED STEEL FRAMES: MYTH OR REALITY?

Tension-Only Concentrically Braced Frames (TOCBF) exhibit deteriorating pinched hysteretic behaviour during strong earthquakes. Slender braces transit between an elastic buckling state, a restraightening state, in which they carry almost no load, an elastic tensile loading state as they are suddenly taut and, finally, a tensile yielding state. It has long been suspected that the sudden increase in tensile forces in the braces of TOCBSF creates detrimental impact loading on the connections and other structural elements. No experimental evidence, however, has been provided so far to confirm, or to quantify, this impact phenomenon. This paper addresses this issue through shake table tests of half scale, two-storey, TOCBF models. By normalizing the hysteresis loops of braces obtained from shake table tests to the yield strength of steel obtained from quasi-static tests, the increase in tensile forces in the braces was obtained. Results of dynamic tensile tests on steel coupons under similar strain rates as observed during the shake table tests showed that this increase in tensile forces is not the result of impact, but is rather caused by a yield strength increase of the steel under high strain rate. A procedure is proposed to estimate and account for this increase in tensile forces in the braces at the design stage.