Ductile Link Research Articles

Flexural behaviour of ECC slabs reinforced with high-strength stainless steel wire rope

Engineered cementitious composites (ECCs) have ultrahigh ductility and multiple-cracking properties. High-strength stainless steel wire rope (HSSSWR) has high tensile strength and good corrosion resistance. Taking advantage of these two materials, ECCs reinforced with HSSSWR (HSSSWR-ECCs) promise to be attractive when used in flexible and ductile link slabs in bridge deck systems, in permanent formworks of concrete members and for strengthening existing members. In this work, bending tests were performed on HSSSWR-ECC slabs with different HSSSWR reinforcement ratios and ECC formulations. The results showed that the HSSSWR-ECC slabs exhibited excellent crack-width control and deformation capacities under bending moments. An increase in the HSSSWR reinforcement ratio enhanced the flexural capacity of the HSSSWR-ECC slabs, but reduced their ductility. Adding a thickener to the ECC enhanced the crack-width control ability and ductility of the HSSSWR-ECC slabs by improving the dispersion of polyvinyl alcohol fibres in the ECC, but reduced the flexural capacity by reducing the strength of the ECC. Formulas for predicting the flexural capacity of HSSSWR-ECC slabs were developed based on related mechanics theories. The accuracy of the proposed formulas was verified by comparing the test results and predicted results using a finite-element model for HSSSWR-ECC slabs.

Moment-resisting self-drilling dowel connections between steel link beams and CLT for coupled walls

Coupled cross-laminated timber (CLT) walls can be created by connecting steel link beams between adjacent CLT wall panels and are an efficient alternative to conventional cantilevered CLT shear walls. Effective coupling in the coupled wall system requires the beam to wall connections to have adequate strength to ensure a ductile link beam response and adequate stiffness to yield the link beams at relatively low inter-storey drifts. This study evaluates beam to wall connections with a group of self-drilling dowels (SDDs) installed through an 8mm steel knife plate to connect 200UB18 steel link beams to 5-ply CLT walls. A simplified method for determining the connection strength is presented and used to capacity design the test specimens. The capacity design method was validated for the tested specimens because the damage was concentrated in the steel link beams and the connection’s peak strength was not exceeded. This study indicates that the SDD connections may be a feasible beam to wall connection solution in coupled CLT wall systems because the capacity protected connections had adequate strength and relatively high stiffness.

Design concepts and seismic behavior of ductile linked rocking steel frames

This paper proposes a novel ductile linked rocking steel frame (DLRF) that relying on gravity load and rocking column base components rather than post-tensioning elements to provide the self-centering capacity. In the DLRF, rocking steel braced frames remain elastic by being permitted to uplift and rock at the rocking column bases, coupled with ductile links that act as structural fuses to dissipate energy through accommodating vertical movement between the two-rocking steel braced frames. Rocking column bases can dissipate energy through fuses that can be repaired easily by being bolted to the column. The design equations to govern the corresponding resistance mechanism of the DLRF for each limit state are presented and applied to a four-story prototype structure. Nonlinear dynamic analyses were performed to examine the behavior of the DLRF. The seismic performance of the DLRF is contrasted with a corresponding fixed-base structure, in which the column base is designed as a rigid connection. The comparison indicated that the DLRF exhibited uniformed interstory drift distribution as well as negligible residual drift subjected to DBE and MCE intensities, and thus the soft-story failure can be effectively avoided. Dynamic analyses also confirmed that the proposed design equations can predict the mechanical behavior of the DLRF with expected drift capacity and flag-shaped hysteretic response. Therefore, the proposed DLRF is a viable alternative to a seismic load-resisting system which could avoid severe and expensive damage to the structure, enhancing resilience capacity.

Damage-control steel frames equipped with SMA connections and ductile links subjected to near-field earthquake motions: A spectral energy factor model

To achieve a delicate balance between self-centring behaviour and energy dissipation, it was proposed to develop the damage-control steel frame equipped with shape memory alloy (SMA) connections and ductile links showing the partially self-centring behaviour. This paper examined the inelastic seismic demand of the novel structure, and the emphasis was given to the spectral energy factor of the system subjected to near-field earthquake motions. Based on single-degree-of-freedom (SDF) systems representing the novel structure and a near-field earthquake database, nonlinear spectral analyses were performed considering a wide range of structural period and hysteretic parameters, and more than 25 million energy factors were obtained. The probabilistic characteristics of the energy factors were examined in detail. The analysis database confirmed that the energy factors were sensitive to structural period and hysteretic parameters. In addition, a right-skewed distribution feature of the energy factors was observed. Thus, a spectral energy factor model for damage-control steel frames equipped with SMA connections and ductile links was developed using the lognormal distribution model. Nonlinear regression analyses were conducted to develop a series of prediction equations. The good agreement between the histograms of the energy factor and predictions by the regression equations confirmed the adequacy of the proposed model. The proposed model was eventually applied to evaluating the damage-control behaviour of a prototype structure under near-field earthquake motions, and the sufficiency of the model for assessing the structural damage-control behaviour from a statistic perspective was confirmed.

Development of Resilient Seismic Solutions for Timber Structures in New Zealand Using Innovative Connections

There is increasing public pressure for damage avoidance structural systems using new engineered wood products, especially cross-laminated timber (CLT), in countries with high levels of seismic activity, such as New Zealand, to minimise the destruction after severe earthquakes with no post-event maintenance. This study presents damage avoidance lateral load-resisting systems (LLRSs) using rocking timber panels and the innovative resilient slip friction joint (RSFJ) technology. RSFJs can be used as hold-downs or as ductile links between adjacent walls or between the walls and boundary columns. These joints are capable of providing a self-centring behaviour (the main deficiency of conventional friction joints) in addition to a high rate of energy dissipation, all in one compact package. In addition, a novel type of shear-transferring device is introduced to be used as the shear key for rocking structural members. The proposed concepts are described and verified by large-scale experimental tests and numerical simulations. The results confirm that the introduced systems can be considered as the new generation of resilient LLRSs for timber construction.

Modular tied eccentrically braced frames for improved seismic response of tall buildings

Two alternative braced frame configurations are examined to improve the vertical distribution of seismic storey drifts and inelastic demands in multi-storey steel eccentrically braced frames (EBFs): the tied braced frame (TBF) system and a modular tied-braced frame (M-TBF) system. In TBFs, the concentration of inelastic deformation is prevented by connecting all ductile links over the frame height with vertical tie members. In the M-TBF configuration, the ties are interrupted at one or more locations along the frame height to reduce member forces while achieving the same objective as the TBF. The performance of each bracing system is investigated through nonlinear response history analysis for 8-, 16-, and 24-storey prototype building structures. For the 16-storey building, two M-TBF configurations are examined, two 8-storey modules and four 4-storey modules, to investigate the influence of this parameter. The analysis results show that the TBFs exhibited smaller and more uniform storey drifts and residual drifts at the expense of high axial force demands in the tie members. Soft-storey response could also be mitigated with M-TBFs, but member forces diminished significantly when adopting a modular tied braced frame configuration, which led to a reduction in the required steel tonnage with limited increases in peak and residual drift demands. Performance assessment using incremental dynamic analysis showed that the 16-storey EBF could not pass the FEMA P695 collapse safety criteria whereas all tied braced frames could exhibit satisfactory performance.

Investigation of the behaviour of flexible and ductile ECC link slab reinforced with FRP

In this paper, a flexible and ductile link slab used to eliminate the expansion joints in the bridge deck system was proposed and investigated through a comprehensive study, including material design and structural experimental investigation. This novel link slab utilises highly ductile Engineered Cementitious Composites (ECC) and non-corrosive Fibre Reinforced Polymer (FRP) reinforcement. At the first stage of this study, an experimental investigation was carried out to evaluate the influence of compositions on the mechanical behaviour of ECC materials, particularly emphasis on flexural behaviour. The test and analytical results established the feasibility of attaining a type of ECC composite material with high deflection capacity, multi-cracking behaviour and small crack width (0.05 mm), which is suitable for the application of link slab in bridge deck system. Thereafter, the proposed ECC material combined with FRP reinforcement was adopted to conduct link slab specimens in full scale bridge decks. All the test specimens were tested under monotonic repeated loading. The performance was described based on the load-deflection/moment-rotation response, strain development, cracking and energy absorption. The influence of FRP reinforcing materials on the link slab structural performance was presented and discussed. The significant enhancement of deflection capacity and crack-width control in ECC link slab reinforced with FRP bars suggests that the use of ductile ECC and non-corrosive FRP reinforcement can be effective in extending the service life of joint-free bridge deck system.

11.32: Linked columns with friction dampers as a technique for seismic retrofit of steel moment resisting frames

ABSTRACTThe existing steel moment‐resisting frames (MRFs) designed to seismic codes of older generation typically have limited local ductility capacity and may not perform to the desired “strong‐column‐weak‐beam” concept due to the lack of capacity design provisions. All these deficiencies require modifications for reducing their seismic vulnerability.The paper proposes a seismic retrofit technique for steel moment frames based on the linked column concept. The linked columns are dual piers connected throughout their height with short horizontal replaceable ductile link elements. Developed in the last decade for new seismic‐resistant steel structures, the linked column frame structural system combines two sub‐systems – a primary lateral force resisting system composed of linked columns and secondary moment resisting frames mainly involved in resisting gravity loads.During a seismic event, the relative deformations of the closely spaced piers activate the links which are designed to yield in shear or bending for seismic energy dissipation, but they also help in controlling interstorey drifts and limiting the forces transferred to the surrounding structural members.The authors present a new innovative design of the linked columns. Instead of short steel beams, rotational friction dampers (RFDs) are used as seismic links in order to avoid the damage expected in conventional link elements. The RFDs are supplied by a Danish company and have been used for seismic protection in many projects for the last decade. They have proven energy‐dissipation capacity and can be delivered in different shapes and slip capacities at relatively low cost.In this study three‐, six‐ and nine‐storey four‐bay steel MRFs have been analyzed using the capacity spectrum method. The added linked columns are arranged symmetrically on both sides of the first interior columns of the MRFs.The results obtained for three different arrangements of the seismic links are presented and summarized. They indicate that the seismic retrofit using linked columns with rotational friction dampers is a promising technique for seismic upgrade of existing steel moment frames.

Seismic resistant rocking coupled walls with innovative Resilient Slip Friction (RSF) joints

Multi-story hybrid timber-steel structures are becoming progressively desirable owing to their aesthetic and environmental benefits and also to the relatively higher strength to weight ratio of timber. Moreover, there is an increasing public pressure to have low damage structural systems to minimize the destruction after severe earthquakes. A recent trend in the timber building industry is the use of cross laminated timber (CLT) wall systems. CLT is a relatively novel engineered wood based product well suited for multi-story structures. Latest research findings have shown that CLT buildings constructed with traditional steel connectors can experience high damage mainly because of stiffness degradation in the fasteners.It has been proven that friction joints can provide a perfectly elastoplastic behaviour and a stable hysteretic response. Up until now, the main disadvantage of the friction joints has been the undesirable residual displacements after an earthquake. This study presents a hybrid damage avoidant steel-timber wall system using the innovative Resilient Slip Friction (RSF) joint. The proposed system includes coupled timber walls and boundary steel column as the main lateral load resisting members. RSF joints are used as ductile links between the adjacent walls or between the walls and the steel boundary columns. The efficiency of the system has been investigated by experimental joint component tests on the RSF joint followed by reversed cyclic numeral analyses and dynamic non-linear time-history simulations on the wall system. The results confirmed that the proposed system has the potential to be recognised as an efficient lateral load resisting system.

Study of an Optimum Design Method for Links

Under severe earthquakes, eccentrically braced frames might experience large inelastic deformations, and the inelastic action is restricted primarily to the ductile links. In order to study the reasonable design method for links, seventy analyses of links are conducted to investigate the effect of different flange width-thickness ratio and length, fifty links are also designed to study the influence of stiffeners spacing, stiffeners thickness and placing on side(s), and thirty-six links are designed to consider the effect of axial loads, which are all based on the material properties of Q235 steel. The accuracy of finite element models is verified using the experimental data during cyclic loading. Numerical analysis results show that the flange width-thickness ratio of short and long links can be relaxed to 10√235/fy , and stiffeners can only be placed on one side. However, the flange width-thickness ratio of intermediate links is limited to 8√235/fy , and stiffeners should be placed on both sides due to the unstable behavior. Stiffener thickness has no significant influence on the performance of links with varying length. Unlike short links, intermediate and long links are susceptible to the axial forces. Then an optimum design method is proposed by analyzing the main influencing factors, so links can have good ductility and stiffness at high load levels.

DUCTILE STEEL CONNECTIONS FOR SEISMIC RESISTANT PRECAST BUILDINGS

Welded connections are extensively used by the precast concrete industry to ease construction. When the seismic resistance and ductility of a building incorporating precast concrete elements needs to be assured, the connections should be carefully designed and constructed to avoid undesirable structural performance. This paper discusses aspects related to an analytical and experimental programme conducted to investigate the response of a connection detail for coupling precast concrete walls in low to medium rise buildings. The connection consists of a rectangular steel plate with a concentric circular perforation. In this way, a weak but ductile link is introduced in the system. Using the principles of capacity design, the walls can be designed as if coupled and with energy dissipation taking place in the connection. Simplified expressions are presented to evaluate the shear stiffness and the yield and ultimate strength of the perforated steel plates, based on theory and the experimental data collected during the test.

Cyclic Testing of Ductile End Diaphragms for Slab-on-Girder Steel Bridges

Ductile end diaphragms have been proposed as a seismic retrofit strategy to protect the substructures of existing steel slab-on-girder bridges from damage during earthquakes. This paper presents the results from an experimental program to investigate the adequacy of some proposed details. Cyclic tests on full-size girder specimens having the proposed ductile diaphragms demonstrate that these can possess adequate initial elastic stiffness, strength, and capacity to dissipate hysteretic energy in the intended manner. The specimens developed a rotational capacity of 0.2 rad when energy dissipation devices of the TADAS (Triangular-plate Added Damping And Stiffness) type were used, and link distortion angles of 0.08 to 0.11 rad when eccentrically braced frame and shear panel systems were in place, corresponding to average ductilities of 8 to 10 before failure. Better performance of the eccentrically braced frame system would have been possible had lateral support been provided at the ductile link. However, ductile end diaphragms having bolted connections suffered significant slippage, leading to pinched hysteretic curves. Tests show that welding significantly improves the seismic behavior of these ductile systems. Specimens with nominal channel diaphragms and those without any diaphragm dissipated significantly less hysteretic energy, and suffered bolt rupture, buckling of the web stiffeners, and fracture of the stiffeners welds at large drifts.

Sierra Nevada uplift: A ductile link to mantle upwelling under the Basin and Range province

Recent geophysical studies in the southern Sierra Nevada found no significant Airy-type crustal root; the mountain range seems largely supported by the buoyant asthenosphere beneath an abnormally thin mantle lithosphere. We suggest that the late Cenozoic uplift of the Sierra Nevada may have resulted from mantle upwelling under the Basin and Range province, which tends to push ductile material within the surrounding lithosphere, causing it to flow away and downward. Numerical modeling indicates that such ductile flow could lead to pronounced lithospheric thinning under the High Sierra and lithospheric thickening under the western Sierra Nevada, comparable to the observed structure.

Ductile Connectors for a Precast Concrete Frame

46 Vice President Dywidag Systems International , USA, Inc. Long Beach , Cal ifornia A precast concrete ductile frame system is presented that takes advantage of the inherent discrete nature of precast concrete by providing ductile links in the connections. These ductile connectors contain a rod that yields at a welldefined strength, effectively limiting the load that can be transferred to less ductile components of the frame. The ductile connectors also transfer all vertical shear forces, eliminating the need for corbels. High strength [150 ksi (1 034 MPa)] reinforcing steel is used in the beams to reduce congestion. A building designed for Uniform Building Code Seismic Zone 2A is presented; however, the system is versatile and can be used in all seismic zones.

Seismic analysis of links of various lengths in eccentrically braced frames

Eccentrically braced steel frames have excellent elastic stiffness under moderate lateral loads and good ductility when subjected to severe seismic loading conditions. Under extreme loading, the inelastic behaviour is designed to be confined to a ductile link element. The behaviour of links of various lengths in eccentrically braced frames is evaluated using a finite element model. The link is subjected to extreme cyclic loading simulating a severe earthquake. The effect of the link length on its performance, capacity for energy dissipation, plastic mechanisms, and mode of failure are investigated. Measures for improving the performance of long links are examined. It was found that the most efficient link is the short shear link. However, the short link deformation is characterized by large angles of deformation, which may cause substantial damage to the nonstructural elements. On the other hand, long links were found to have smaller angles of deformation than short links, but with reduced ductility levels. An effective approach to improve the performance of long links is by increasing the flange thickness of the link section with the appropriate adjustment to the stiffener design. This technique can be applied to links of length up to 1.4 times the critical shear link length. Key words: steel, eccentric, braced, frame, link, seismic, ductility, design.

Effect of axial forces on the performance of links in eccentrically braced frames

The damage to an eccentrically braced frame due to a major earthquake is designed to be confined to a highly ductile link. The link acts as a safety valve for the structure. The post-elastic behaviour of the link and its performance up to failure has been evaluated using a finite element model, with the link subjected to a cyclic loading. The results obtained from the finite element model were verified against test data. The model was used to evaluate the effect of axial forces on the performance of the link. High axial normal forces may be developed in the link as a result of the particular configuration of the eccentrically braced frame. It was found that it is possible to develop a simple yet effective finite element model for the link. Comparison between the analytical model predictions and the test results indicated good agreement. When the link is subjected to axial forces, deterioration in both the load-carrying capacity and ductility were observed. The adverse effects of axial forces on the link performance were found to be most pronounced for link lengths close to the value of the critical shear link length limit.

On Design of Eccentrically Braced Frames

Seismic-resistant Eccentrically Braced Frames (EBFs) are becoming a widely used lateral resisting system for steel buildings, with even wider application anticipated as design requirements are put into building code format. This paper addresses a number of EBF design issues, which in the opinion of the authors are inadequately considered either in current practice or in the emerging code provisions. The overall design philosophy for EBFs is reviewed, with specific reference to the concept of “Capacity Design”. Application of capacity design principles assures that yielding will be restricted primarily to the ductile link elements, an important goal of EBF design. Further, through careful choice of frame geometry and link length at the preliminary stages, many potential design difficulties can be avoided. The paper also presents some important observations from experimental work currently underway on EBFs with long, flexural yielding links.

Formation of plastic enclaves at running brittle cracks

Onset of enclave formation and their growth at the tip of a steady state crack which propagates under plane strain conditions in a mild steel plate is investigated. It is recognized that the crack tip boundary conditions are determined by the stresses transmitted in the weakened ductile links within a discrete crack zone which forms as a result of selective microcrack propagation in the elastic material ahead of the main crack. A general condition for the beginning of nonuniform viscoplastic yielding is proposed for an arbitrary path in the stress-time space and used for investigation of initial yielding and for an approximate description of enclave growth, while the loading path is derived from a dynamic elasticity solution of the crack problem. The plastic enclaves form initially at some distance from the crack line and may adjoin the crack zone only in later stages of their development. It is suggested that enclave formation and their eventual interference with the local separation process are respectively responsible for energy transition and crack arrest in brittle fracture.