Diagonal Braces Research Articles

Progressive collapse mitigation approaches for steel-concrete composite buildings

This paper investigates the robustness of steel-concrete composite frames under sudden column loss. Various lateral load resisting systems, including contributions from bracing systems, composite slab and joints are taken into account in analysing buildings against progressive collapse. Robustness of ten-storey steel buildings with (1) diagonal braces at corners, (2) centre core wall, (3) rigid moment joints are analysed for corner, perimeter and internal column loss. The analyses conclude that simple braced frames are more susceptible to the progressive collapse compared with the moment resisting frames which has higher redundancy to redistribute the loads arising from sudden column loss. Various mitigation approaches are proposed to improve the progressive collapse resistance of the simple braced frames. End-plate beam-to-column connection is proposed instead of fin-plate connection due to greater rotational stiffness and higher moment resistance. Vierendeel truss is proposed at the top floor level in a multi-storey building to redistribute the load due to sudden column loss. In addition, modified fin-plate connection is proposed in which a plate is welded to the bottom beam’s flange to increase joint’s rotational restraints helping to reduce beam’s vertical displacement in the event of sudden removal of column.

가설울타리 골조의 횡력에 대한 구조적 거동

Temporary fencing is a mandatory installation in construction sites because of environmental regulations and pedestrian safety. Contractors are responsible for the structural integrity of frames. The fence frame composed of a ledger and post is very weak in the out-of-plane direction, which is normally reinforced by diagonal bracing with a sub post. In this study, the location of lateral bracing for the main post was investigated analytically. Three types of frame were suggested and their performance was compared in terms of the maximum moment and deflection. A construction site may be located at city centers or crowed areas, and a contingent accident should be considered. The EOD type was found to be most efficient among the three frames suggested.

Study on the Strength of Different Door-Type Structure of Attached Type Lifting Scaffold

Attached type lifting scaffold is an important engineering machine which is used for building construction. In this paper, the strength of door-type structure of attached type lifting scaffold is studied. The diagonal brace is removed in the new structure and replaced with two short braces. The two type structure elastic FE models are founded and ANSYS software is used to compute the stress and deformation. The results are compared. The comparison study shows the new structure’ strength and stiffness are both better than the old one. Therefore, the new structure of door-type frame is much better than the old structure not only in cost-saving but also in easy through.

A Capacity Design Procedure for Columns of Steel Structures with Diagonals Braces

According to modern seismic codes, in concentrically braced frames the seismic input energy should be dissipated by means of the hysteretic behaviour of braces while all the other members (i.e. beams and columns) have to remain elastic. Accordingly, the design internal forces of braces are determined in these codes by elastic analysis of the structure subjected to seismic forces obtained by the design spectrum. The internal forces of the non-dissipative members, instead, are calculated by means of specified rules for the application of capacity design principles. According to some recent numerical analyses, the yielding or buckling of columns may take place before braces achieve their axial deformation capacity. This paper investigates the reasons of this unsatisfactory behaviour and proposes technological suggestions and a design procedure to improve the seismic performance of columns of building structures with diagonal braces.

Seismic Upgrading of Steel Moment-Resisting Frames by Means of Friction Devices

In recent decades, several passive energy dissipation systems have been conceived in order to minimize the damage in structural and non-structural components of either new or existing buildings. In this study, the use of friction damped tension-compression diagonal braces for seismic upgrading of a steel moment resisting frames is investigated. To this aim, nonlinear time history analyses have been carried out on a set of representative frames with and without friction damped braces. In the nonlinear time history analyses, two sets of natural accelerograms compatible with seismic hazard levels of 10% and 2% probability of exceedance in 50 years have been considered. Under these records, the structural response has been comparatively investigated in terms of the maximum inter-storey drift ratio, maximum storey acceleration, residual drift ratio and displacement demand for the friction device. The results clearly highlighted that the application of friction damped braces allows reducing the damages to the main structural elements, thus significantly improving the seismic behaviour of the frame.

Seismic retrofitting of timber framed walls

After the 1755 earthquake that destroyed Lisbon, an innovative anti-seismic structural system was developed consisting of a timber skeleton, that included timber framed masonry walls. After more than 250 years these structures need rehabilitation to face the present demands. The research presented in this paper aimed at experimentally characterizing the cyclic behaviour of timber framed walls reinforced with three different methods, namely: (i) elastic-plastic dampers on diagonal braces, (ii) reinforcement of timber connections with steel plates, (iii) application of a reinforced rendering. The elastic-plastic damper showed an unsymmetrical behaviour and some difficulties to implement in practice. The strengthening with reinforced render led to an initial stiffness increase but showed a limited deformation capacity. The walls with reinforcing steel plates at the timber connections showed the best behaviour in terms of strength, stiffness and energy dissipation.

Failure mechanism of steel arch trusses: Shaking table testing and FEM analysis

To study the failure mechanism of steel arch truss structures that are subjected to severe earthquakes, a series of shaking table tests were conducted, and the results are discussed in this paper. Furthermore, the effect of bracing forms on the structure failure mode and the cumulative damage effects on the member internal forces were evaluated using the Finite Element Method (FEM). The results from the numerical model are compared with the experimental measurements in terms of displacement, acceleration, steel strain and amplification factor of the structural model. The comparison shows that the numerical results are significantly consistent with the experimental measurements. Under severe earthquakes, structural cumulative damage and plastic deformation increased, whereas the structure stiffness declined, which leads to structural failure. The failure load of the structure was 0.8–1.0g, and in-plane anti-symmetric deformation was found in the main trusses after loading. The maximum strain in the test model is larger than those in the cumulative damage model and the ideal elasto-plastic model by 7–15% and 8–26%, respectively. Increasing the number of rigid tied bars slightly enhances the stiffness of the main truss, whereas using cross diagonal bracings enhances the stiffness of the main truss by 2.9 times.

Seismic Design of MRF-EBF Dual Systems with Vertical Links: EC8 vs Plastic Design

Despite the fact that Eccentrically Braced Frames with Vertical Links (VL-EBFs), also referred to as inverted Y-scheme, are codified in Eurocode 8, the issues related to their seismic response and design have not been widely investigated, so that design criteria commonly applied for Eccentrically Braced Frames with Horizontal Links (HL-EBFs) are commonly applied. However, the Theory of Plastic Mechanism Control (TPMC) has been recently extended to the case of VL-EBFs. The aims of this article, on one hand, are to provide a further validation of the recently proposed design procedure, based on TPMC, and, on the other hand, are to compare the seismic performance of dual systems composed by a moment-resisting part and VL-EBF part designed by means of TPMC with those occurring when Eurocode 8 design criteria are applied. The validation of the proposed design procedure is carried out by means of Incremental Dynamic Analyses (IDA). The main purpose of such analyses is the check of the fulfilment of the design goal of TPMC, i.e., the development of a pattern of yielding consistent with the collapse mechanism of global type. Such mechanism is universally recognized as the one leading to the highest energy dissipation capacity. In case of MRF-EBF dual systems, it is characterized by the yielding of all the links and all the beams at their ends. Conversely, all the columns and the diagonal braces remain in elastic range. Obviously, exception is made for the base sections of first story columns. In particular, two case studies are analyzed which are characterized by a different number of stories. Each building structure is designed according to both TPMC and Eurocode 8 provisions. The seismic response obtained is investigated by both push-over and IDA analyses. The attention is focused on the pattern of yielding obtained, the maximum interstory drift demand, the link plastic rotation demand and sharing of the seismic base shear between the moment-resisting part and the bracing part of the structural system. The results obtained point out improvement of the seismic response, compared to Eurocode 8 provisions, achieved by means of TPMC.

Dynamic Parameters of Pultruded GFRP Structures for Seismic Protection of Historical Building Heritage

The seismic events in the last decades evidenced the vulnerability of the architectural heritage with respect to these phenomena. Therefore, timely provisional and permanent interventions are often required in post-seismic scenarios.In response to the call for more efficacious solutions, a great deal of interest has been given to glass fiber-reinforced polymers (GFRPs) composite. GFRP composites offer the advantage of high strength, low self-weight and durability.This research proposes some first evaluations on the dynamic behaviour of pultruded FRPs (Fiber Reinforced Polymers) strut and tie spatial structure for seismic protection of historical building heritage. The dynamic identification has been carried out using the ambient vibrations test to measure the mode of vibration, frequencies, displacements and damping ratios of the structures. The Operational Modal Analysis (OMA) has been carried out to identify the modal characteristics through poly-reference Least Square Complex Frequency-domain (pLSFC) estimator.Basing on the experimental results the dissipative capacity has been evaluated through the calculation of the behaviour factorq.The experimental dynamic parameters were used to calibrate a numerical finite element model employed under the hypothesis of kinematic equivalence to evaluate theqfactor considering the elastic-brittle behaviour of FRP material. The dissipative capacity of structure was assigned to the global geometric configuration - frame with concentric diagonal bracings - through the strength hierarchy criteria locating the dissipative zones in the tensile diagonals. The analysis was carried out considering some typological variations to investigate the variability of the behaviour factorq.

Metal–strip bracing versus diagonal timber bracing in timber trussed tiled roofs

Diagonal bracing is a common method used in South Africa for the bracing of timber trussed roofs with spans of less than 9m. In the typical diagonally braced roof system, the brace is placed on the underside of the compression chord, which is as far as it can be placed away from the battens that brace the trusses not directly connected to the diagonal brace. Some of the stiffness of the bracing is lost because the low torsional rigidity of the top chord as well as the batten to top chord nailing is all part of the structural system. In this paper the authors investigate bracing of the entire roof system and contend that it is better to connect the bracing member closer to the battens as this improves the stiffness and bracing ability of the system. The authors study both the diagonal brace as well as the alternative bracing method that uses criss-cross metal strapping, sometimes called speed brace. The speed brace is nailed to the top of the compression chord and is therefore much closer to the battens.Two different truss spans were studied to ascertain the buckling length of the top chord for both the diagonal bracing as well as speed bracing. By moving the bracing system closer to the battens the authors show that the speed brace gives a stiffer system which thereby reduces the buckling length of the top compression chord.

Pseudodynamic test and numerical simulation of a large direct air‐cooling structure

SummaryIn this study, pseudodynamic test was performed to evaluate the seismic performance of air‐cooling structures built in the high‐intensity earthquake fortification zones. The testing prototype was designed with the scale factor of 1/8, and the structural responses subjected to five peak ground accelerations (PGAs) were investigated followed by a cyclic test. Experimental results, including the lateral displacements, strains and crack patterns, were recorded and presented. The experimental load–displacement hysteretic loops subjected to the cyclic displacement history indicate that the direct air‐cooling structure exhibits a good seismic performance with a displacement ductility factor greater than 4. It is particularly noted that the structural integrity is well enhanced by the inter‐column connections, which are composed by using the steel truss and diagonal braces, and these steel members mainly remain elastic throughout the testing process. The natural period and the equivalent viscous damping ratio of the first‐order mode are also identified on the basis of the test results, which are utilized to calibrate the finite element model. In addition, the predicted lateral displacements based on a damaged plasticity model of concrete agree well with the experimental results. Both the lateral bearing capacity and ductility of the model demonstrate that the direct air‐cooling structure meets the seismic design requirement with respect to the high‐intensity earthquakes. Copyright © 2014 John Wiley & Sons, Ltd.

Towards lateral performance of CBF with unwanted eccentric connection: A finite element modeling approach

This paper presents a parametric investigation into lateral performance analysis of Concentrically Braced Frames (CBFs) having in-plane eccentricities at the connection of their braces. Nonlinear behavior analyses of a single bay, two-story steel frames with single diagonal concentric braces are taken as numerical examples. Static pushover analyses is carried out to show the impact of eccentricity values (5, 10, and 15 cm), location of the meeting point of brace and frame member axes, and the angle of brace with respect to beam axis. Finite element modeling of the whole members is conducted. Fine meshes are used in regions where inelastic deformations are expected. Numerical results show that in-plane eccentricities can be acceptable within a safe region (max. 10 cm for this specific case). However, no safe region exists for in-plane eccentricity in columns since the strength loss may be significant. Stiffeners are recommended in column web to reduce the strength loss. Their impact is less in strength loss but high in plastic strain developed in the critical region. Eccentricities have positive or negative effects on seismic behavior depending on the geometric properties of the frame, type of eccentricity and bracing configuration.

Vibration Control of Relative Tool-Spindle Displacement for Computer Numerically Controlled Lathe With Pipe Frame Structure

A new computer numerically controlled (CNC) lathe with a pipe frame bed has been developed. This structure is expected to have enough space between the truss bars to solve the space problem and have enough rigidity for machine tools. Therefore, a CNC lathe whose frame consists of pipes, joints, and diagonal braces has been developed with enough rigidity and space utility for chip evacuation. From the viewpoint of machine tool usage, real-time vibration control theory is applied to control the relative displacement between the tool post and the spindle to suppress specific relative vibration modes.

Damping systems that are effective over a wide range of displacement amplitudes using metallic yielding component and viscoelastic damper in series

SUMMARYA damper system that absorbs energy over a wide range of displacement amplitudes during building vibration was proposed. This system uses a serial connection of a metallic yielding component and viscoelastic damper with a displacement limit mechanism. Three types of the system were developed and tested: a diagonal bracing type, inverted V bracing type, and wall type. The test results showed that all these systems have damping ratios higher than 8% at small vibration amplitudes on the order of 0.1 mm. For a large vibration, a displacement limit mechanism with two pins limited the displacement of the viscoelastic damper as designed. Analytical simulations established that the system reduced the acceleration and the story drift to 60–70% and 80%, respectively, during a small earthquake compared with a conventional metallic yielding damper system. Furthermore, it showed an equivalent control performance during a severe earthquake. The damper system requires that a clearance be maintained for the displacement limit mechanism. However, this may be lost through construction error, residual displacement after an earthquake, and temperature effects. The changes in the clearance due to these effects were discussed. Copyright © 2014 John Wiley & Sons, Ltd.

Dissipative capacity on FRP spatial pultruded structure

The dissipation capacity of a generic structure depends on its many characteristics – like material, shape of the cross-sections, structural scheme, regularity and typology of structures – and it is accomplished by means of the behavior factor q through a very strategic reduction of seismic action. This is the reason why this research shows an overview on the evaluation of the q factor, preparatory to an analysis related to a spatial frame structure of built-up members made of FRPs (Fiber Reinforced Polymers) pultruded profiles, FRP gusset plates and steel bolts. The factor q proposed starts directly from experimental data, in detail the outcomes due to the dynamic identification on site carried out. Subsequently, a finite element model under the hypothesis of kinematic equivalence for the measurement of q was calibrated. The adopted structure investigated is a spatial frame with concentric diagonal bracings for which its dissipation capacity by means of the strength hierarchy criteria was assigned. To exalt the overall behavior of the all-GFRP structure a case of rigid connection is assumed with a comparison with semi-rigid conditions. The analysis was carried out also on the variability of the q factor, considering some typological variants of the basic structure.

Comparative Response Assessment of Different Frames with Diagonal Bracings Under Lateral Loading

In this study, nonlinear static and dynamic analyses were performed to compare the structural response of different types of moment-resisting frame buildings equipped with concentric diagonal braces under lateral loading. For this purpose, the existing moment-resisting frames designed according to two different stiffness levels were used as case studies. They were referred to as (a) flexible moment-resisting frames (MRF) and (b) rigid MRFs. The buildings having 4, 8, 12, and 16 storeys consist of the same plan and three bays on both directions. Then, concentric steel braces were inserted in the middle bay of each frame. For the braced frame structures, diagonal type configuration was utilized. Capacity curve, inter-storey drift index, global damage index, variation of storey displacements with height, roof displacement time history, and plastification in the frame members were evaluated for all frame systems. The results exhibited a substantial improvement in the earthquake performance of the frames with the incorporation of concentric diagonal braces, depending on the storey height and especially stiffness level of the frame.

Steel concentrically braced frames using tubular structural sections as bracing members: Design, full-scale testing and numerical simulation

This paper presents the results of experimental and analytical studies carried out on two full-scale, one-bay, two-story steel concentrically braced frames. Square hollow and round hollow structural sections were used for the bracing components. The specimens were designed and detailed according to the 2005 AISC Seismic Provisions, and tested cyclically under displacement control but with a fixed lateral load distribution over height. Numerical computational models including the brace components, gusset plate details and frame members were implemented in OpenSees. Numerical simulations were then performed to investigate the cyclic behavior of brace components, brace failure mechanisms and overall system response. Satisfactory agreement was obtained in comparisons of experimental and numerical results. Premature failures observed suggest that beamto-gusset plate connections could be pinned to accommodate large rotational demands at this location without the need to form plastic hinges. Test results also showed that for the braced frames having the same configuration, designed for similar base shear capacities, and subjected to the same roof level displacement history, the braced frame specimen using round tubular sections as diagonal braces was able to sustain larger story drifts without brace fracture than the specimen employing square tubular sections. Fracture of the column base in the second specimen, although inconclusive from a single test, suggests more study is needed of design requirements for column to base plate connections where large variations of axial, bending and shear load are expected.

Cyclic response of hollow and concrete-filled circular hollow section braces

The behaviour of seismic-resistant buildings relies heavily upon the inclusion of energy dissipating devices. For concentrically-braced frames, this function is accomplished by diagonal bracing members whose performance depends upon both cross-sectional properties and global slenderness. Traditionally preferred rectangular hollow sections are susceptible to local buckling, particularly in cold-formed tubes, owing to the residual stresses from manufacture. This paper explores the response of hollow and concrete-filled circular tubes under cyclic axial loading. The uniformity of the circular cross-section provides superior structural efficiency over rectangular sections and can be further optimised by the inclusion of concrete infill. A series of experiments was conducted on filled and hollow specimens to assess the merit of the composite section. Comparisons were drawn between hot-finished and cold-formed sections to establish the influence of fabrication on member performance. Two specimen lengths were utilised to assess the influence of non-dimensional slenderness. Parameters such as ductility, energy dissipation, tensile strength and compressive resistance are presented and compared with design codes and empirically derived predictions.

An experimental investigation on the ultimate strength of epoxy-repaired braced partial infilled RC frames

Due to earthquake, buildings are damaged partially or completely. Particularly, structures with soft storey are mostly affected. In general, such damaged structures are repaired and reused. In this regard, an experimental investigation was planned and conducted on models of single bay, single storey of partial concrete infilled reinforced concrete (RC) frames up to collapse with corner, central, and diagonal steel bracings. Such collapsed frames were repaired with epoxy resin and retested. The initiative was to identify the behavior, extent of restored ultimate strength, and deflection of epoxy-retrofitted frames in comparison with the braced RC frames. The performance of such frames has been considered only for lateral loads. In comparison with bare RC frames, epoxy-repaired partial infilled frames have significant increase in lateral load capacity. Central bracing is more effective than corner and diagonal bracing. For the same load, epoxy-repaired frames have comparable deflection than similar braced frames.

Experimental and analytical investigations on seismic behavior of ductile steel knee braced frames

Knee Braced Frame (KBF) is a special form of ductile eccentrically braced frame having a diagonal brace connected to a knee element, as a hysteretic damper, instead of beam-column joint. This paper first presents an experimental investigation on cyclic performance of two knee braced single span one-story frame specimens. The general test arrangement, specimen details, and most relevant results (failure modes and hysteretic curves) are explained. Some indexes to assess the seismic performance of KBFs, including ductility; response reduction factor and energy dissipation capabilities are also subsequently discussed. Experimental results indicate that the maximum equivalent damping ratios achieved by test frames are 21.8 and 23% for the specimens, prior to failure. Finally, a simplified analytical model is derived to predict the bilinear behavior of the KBFs. Acceptable conformity between analytical and experimental results proves the accuracy of the proposed model.