Alternate Load Path Research Articles

Connection behaviour and the robustness of steel-framed structures in fire

The full-scale fire tests at Cardington in the 1990s, and the collapse of at least one of the WTC buildings in 2001, illustrated that connections are potentially the most vulnerable parts of a structure in fire. Fracture of connections causes structural discontinuities and reduces the robustness provided by alternative load paths. An understanding of connection performance is essential to the assessment of structural robustness, and so to structural design against progressive collapse. The forces and deformations to which connectionscan be subjected during a fire differ significantly from those assumed in general design. The internal forces i generally start with moment and shear at ambient temperature, then superposing compression in the initial stages of a fire, which finally changes to catenary tension at high temperatures. If a connection does not have sufficient resistance or ductility to accommodate simultaneous large rotations and normal forces, then connections may fracture, leading to extensive damage or progressive collapse of the structure. Practical assessment of the robustness of steel connections in fire will inevitably rely largely on numerical modelling, but this is unlikely to include general-purpose finite element modelling, because of the complexity of such models. The most promising alternative is the component method, a practical approach which can be included within global three-dimensional frame analysis. The connection is represented by an assembly of individual components with known mechanical properties. Component characterization must include high-deflection elevated-temperature behaviour, and represent it up to fracture.In reality a connection may either be able to regain its stability after the initial fracture of one (or a few) components, or the first failure may trigger a cascade of failures of other components, leading to complete detachment of the supported member. Numerical modelling must be capable of predicting the sequence of failures of components, rather than considering the first loss of stability as signifying building failure. It is necessary to use a dynamic analysis, so that loss of stability and re-stabilization can be tracked, includingthe movements of disengaging members and the loadsharing mechanisms which maintain integrity and stability within the remaining structure, until total collapse occurs.

Progressive Collapse Analysis of Latticed Telecommunication Towers under Wind Loads

As the antenna-supporting structures, latticed telecommunication steel towers are considered as critical members of telecommunication infrastructures. It is necessary to perform progressive collapse analysis of lattice telecommunication towers under wind loads. The present study conducts a nonlinear dynamic analysis on 50 m high typical standard latticed telecommunication tripole tower and angle tower by alternative load path method. The finite element models for two towers subjected to design wind loads are developed by ABAQUS. The analysis results show that, for 50 m high standard tripole tower, the member failure in the first three tower sections from tower top would not trigger the collapse of the tower. From the fourth tower section to tower bottom, the member failure at certain wind direction may cause a collapse. For 50 m high standard angle tower, the single member failure in any tower section would not cause the collapse of the tower. A dynamic sensitivity index is proposed to identify the most unfavorable wind direction for tripole tower and angle tower. A progressive collapse fragile curve based on collapse probability of telecommunication tower under wind loads is proposed to assess the anticollapse performance of the towers.

Concept proposal and feasibility verification of the Annular Crossed Cable-Truss Structure

A new concept for improving the robustness of pre-stressed spatial structures was introduced. The purpose of this concept is to avoid existing key elements by creating an alternative load path for every structural member. With this new concept, the Annular Crossed Cable-Truss Structure was proposed. How to defining the target geometry, force-finding, and designing connections for the Annular Crossed Cable-Truss Structure were illustrated by means of designing an experimental set-up. Feasibility of the structure, including self-weight, structural stiffness, and structural adaptability in asymmetric load arrangement, was discussed based on experimental results. The results show that the Annular Crossed Cable-Truss Structure can have a light self-weight, reasonable structural stiffness, and acceptable adaptability in asymmetric load arrangement. It is concluded that the Annular Crossed Cable-Truss Structure is a feasible pre-stressed spatial structure.

A simplified model of catenary action in reinforced concrete frames under axially restrained conditions

From previous experimental studies on missing column scenarios, catenary action could provide beneficial effects on structures during load redistribution after a target column is suddenly removed under an alternative load path procedure to assess structural resistance to progressive collapse. However, full-scale structural tests are costly and time consuming. Equally, finite-element models require in-depth expertise and large computing resources. Practising engineers are not amenable to this approach for preliminary sizing of members or for a quick assessment of structural resilience. To address this need, this paper provides a simplified semi-analytical model for predicting structural response under a column loss scenario. The model is constructed based on finite-element simulations and is verified by test results. The simplified model idealises the structural response of reinforced concrete beam–column sub-assemblages under column loss scenarios as a piecewise multi-linear curve. It shows good agreement with published test results and validated finite-element simulations.

Progressive Collapse Analysis of Reinforced Concrete Structures: A Simplified Procedure

In this paper, a simplified analysis procedure to calculate the column removed point displacement at progressive collapse analysis of reinforced concrete structures is proposed. The energy absorption capacity under the column missing event is used for formulations. The approximate method is simple to utilize, user friendly, yet accurate. For progressive collapse analysis of structures, linear static analysis, nonlinear static analysis, linear dynamic analysis and nonlinear dynamic analysis can be performed. In this paper, the nonlinear static analysis from alternate load path method is used and the reason of initial local collapse has not been considered. In fact, an energy-based method by using load-displacement curve of RC frame and considering the effect of floor slab for the progressive collapse analysis is considered. The accuracy of the proposed method is demonstrated by comparing the results to three experimental and analytical results. Finally, the effects of the spans length, sections dimensions, material properties and the beams reinforcements of column removed spans on substructure behavior is studied, as well.

Progressive Collapse Analysis of Reinforced Concrete Structures: A Simplified Procedure

In this paper, a simplified analysis procedure to calculate the column removed point displacement at progressive collapse analysis of reinforced concrete structures is proposed. The energy absorption capacity under the column missing event is used for formulations. The approximate method is simple to utilize, user friendly, yet accurate. For progressive collapse analysis of structures, linear static analysis, nonlinear static analysis, linear dynamic analysis and nonlinear dynamic analysis can be performed. In this paper, the nonlinear static analysis from alternate load path method is used and the reason of initial local collapse has not been considered. In fact, an energy-based method by using load-displacement curve of RC frame and considering the effect of floor slab for the progressive collapse analysis is considered. The accuracy of the proposed method is demonstrated by comparing the results to three experimental and analytical results. Finally, the effects of the spans length, sections dimensions, material properties and the beams reinforcements of column removed spans on substructure behavior is studied, as well.

Robustness analysis of steel structures with various lateral load resisting systems under the seismic progressive collapse

Abstract Robustness is called to non-sensitivity of a structure to local damage. In the literature of Civil Engineering, the sensitivity of structures to the local damage which can cause a progressive collapse is called Robustness Index. After 9/11, progressive collapse drew the attention of researchers and since that time, increasing the Robustness Index in a structure has been considered as one of the factors in strengthening the structure against progressive collapse. The aim of this study is assessing the Robustness Indices of steel structures having various lateral load resisting systems. So, 8-story, 4-story and 15-story structures with Moment Frames, Concentrically Braced Frames and Eccentrically Braced Frames were modeled and assessed under Robustness analysis. For a progressive collapse analysis, the Alternative Load Path method was used and after emerging local damage, the potential of progressive collapse was assessed using the Robustness Index. First, Robustness analysis was carried out on the basis of Robustness Index assessment using Stiffness and Base Shear methods. Subsequently, it continued by introducing a simple method based on Energy. The results show that despite their simplicity, the methods which are based on Stiffness and the Base Shear, have lots of disadvantages. But, the Energy method has better capabilities for understanding the behavior of structure under seismic loads in progressive collapse scenario. Furthermore, structures which have bracing had a better reaction in progressive collapse scenario.

Resilience of tall steel moment resisting frame buildings with multi-hazard post-event fire

Infrastructure resilience is the ability of an infrastructure asset to limit the effect and duration of damaging extreme events. The four main components of the concept of resilience are robustness, resourcefulness, recovery and redundancy most of which are very difficult to be quantified. In addition to this difficulty, a careful study of extreme event cases can demonstrate that it is very common for an extreme event scenario to include cascading multi-hazard events such as blast, floods, earthquake, and fire. This paper studies the resilience of a multi-story steel frame with multi-hazard considerations which include a post-event fire scenario. The initiating extreme event is simulated through the threat-independent alternate load path method of analysis and a post-event fire is considered following the extreme event. The work in the paper combines previous work by the authors on stability-induced collapse of damaged steel structures and closed-form solutions for temperature predictions of wide-flange components during a fire scenario. For the purposes of the fire scenarios, new fire time-temperature curves are developed based on experimental data from the well-known Cardington fire tests. The results show that even when a structure can withstand an extreme event scenario, a post-event fire consideration is highly critical to evaluate the remaining time of survival of the structure before collapse. It is shown that the sequential method of multi-hazard analysis can lead to very short available time periods before the post-event fire leads to the complete collapse.

Span length effect on alternate load path capacity of welded unreinforced flange-bolted web connections

It is intuitive that for a building of set dimensions, a shorter span length will increase the ultimate load carrying capacity, subjected to an arbitrary column loss. However, the capability of beam-to-column connections to develop and maintain the catenary action of the beams in this situation is not considered directly in the design stage. Therefore, to determine the optimum span length, which provides the required load carrying capacity and cost-effectiveness, sensitivity analyses are needed. In order to save the computational efforts, a component-based model for improved Welded Unreinforced Flange-Bolted web (WUF-B) connections is developed in this study. The capability of this model for predicting the failure and ultimate load carrying capacities was validated. Using this model, analyses results of three case study structures of set dimensions showed that by decreasing the span length by 25%, and 40% the ultimate load carrying capacity increased by 33%, and 72%, respectively. However, this increase was not as much as what was expected by using concentrated plasticity model recommended by UFC. Therefore, the concentrated plasticity model does not always conservatively predict the ultimate load carrying capacity of the studied connection. Moreover, although the distributed plasticity model could predict ultimate load capacity of the connections reliably in the longest span it overestimated the load carrying capacity for the shortest span by 13%. This explains the significance of component-based modelling approach in order to simulate the structural behaviour subjected to a column loss.

Experimental study on dynamic responses of reinforced concrete frames under sudden column removal applying concentrated loading

This paper focuses on structural behaviour of storeys above the targeted column that is suddenly lost due to accidental/terrorist events. The pertinent question is whether these storeys will remain stable, or the ensuing dynamic motion will cause them to fail during the free-fall stage. It is assumed that the two-dimensional frame structure above the targeted column is in pristine state. Previous quasi-static studies on reinforced concrete beam-column structures under missing column scenario have highlighted the potential of catenary action on providing alternate load paths to prevent catastrophic collapse. However, corresponding dynamic tests, either did not share the same loading configuration with the static tests, or did not have sufficient headroom and the specimen hit the ground before catenary action could be fully mobilised. On this paper, a series of dynamic tests was carried out for two-dimensional reinforced concrete beam-column frames simulating the sudden removal of a supporting column via a quick-release device. The specimens were loaded and hung by a mechanism which could be released to effect free vibration. Development of catenary action, which has not yet been confirmed in any previous dynamic tests, was well captured. The study also showed the influence of inertial and strain rate effects on structural response. Most importantly, the dynamic tests applied the same method of concentrated loading at the middle joint as the previous quasi-static tests conducted on similar specimens. As a result, comparisons could be made on damage patterns and failure modes between the dynamic tests and the static tests. Compared to static environment, the dynamic tests only took a few seconds and their behaviours are closer to the actual behaviour of the storeys that undergo free-fall acceleration.

Deconstruction Monitoring of a Steel Truss Bridge

A research study was performed on a steel through-truss bridge more than 80 years of age that was being taken out of service. The main objective of the project was to learn as much as possible about the bridge and to further the state of knowledge about the behavior of steel truss bridges. The study began with a review of the existing condition and load testing of the structure. The main experiment was to monitor the truss members during deconstruction to identify the intrinsic forces present when the bridge was still in service. Those measurements were then compared with simulation results from a finite element model and the original design drawings. This study produced two main conclusions about steel through-truss behavior. First, it was found that the fracture-critical lower chord was redundant because of an adequate alternate load path provided by the floor system. This redundancy was identified through load testing of the structure as well as from numerical simulation results. The floor system was found to take up to 34% of the tensile horizontal forces from vehicular loading. The second main finding was the uncertainty over intrinsic forces within the steel truss members. Comparison of measured intrinsic forces and the finite element model indicated differences of roughly 9% to 34%. These differences were found to be greater than those from the load test (average of 12% difference). In addition, a comparison of the measured intrinsic forces with the original design plans illustrated substantial differences. The findings indicate that significant uncertainty about the intrinsic force distribution within constructed bridges can be present.

Progressive Collapse of Exterior Reinforced Concrete Beam–Column Sub-assemblages: Considering the Effects of a Transverse Frame

Many experimental studies have evaluated the in-plane behavior of reinforced concrete frames in order to understand mechanisms that resist progressive collapse. The effects of transverse beams, frames and slabs often are neglected due to their probable complexities. In the present study, an experimental and numerical assessment is performed to investigate the effects of transverse beams on the collapse behavior of reinforced concrete frames. Tests were undertaken on a 3/10-scale reinforced concrete sub-assemblage, consisting of a double-span beam and two end columns within the frame plane connected to a transverse frame at the middle joint. The specimen was placed under a monotonic vertical load to simulate the progressive collapse of the frame. Alternative load paths, mechanism of formation and development of cracks and major resistance mechanisms were compared with a two-dimensional scaled specimen without a transverse beam. The results demonstrate a general enhancement in resistance mechanisms with a considerable emphasis on the flexural capacity of the transverse beam. Additionally, the role of the transverse beam in restraining the rotation of the middle joint was evident, which in turn leads to more ductile behavior. A macro-model was also developed to further investigate progressive collapse in three dimensions. Along with the validated numerical model, a parametric study was undertaken to investigate the effects of the removed column location and beam section details on the progressive collapse behavior.

Failures of US bridge rocker bearings

Bridge bearings made of iron or steel, first used in early railroad bridges about 150 years ago, are relatively small structural components that support the loads of bridge superstructures. Fixed bearings directly transfer forces from a bridges superstructure to its substructure. Movable bearings, such as sliding bearing plates, rollers and rockers, accommodate the thermal expansion of the superstructure without transferring it to the substructure. Rocker bearings tilt to accommodate thermal movement, theoretically without imposing stresses on the substructure. This paper concerns rocker bearings which are non-redundant components and have two states: stable and toppled over. When these bearings topple over, no alternate load paths exist, which results in a failed superstructure. As such, rocker bearings are critical bridge components. An overview is given of the history of use and failures of rocker bearings from nineteenth-century railroad bridges to US highway bridges with tall and slender rocker bearings that date from the late 1950s.

After-fracture redundancy analysis of an aged truss bridge in Japan

In the next 20 years, more than half of the bridges in Japan would be in service for more than 50 years. These include truss bridges those are generally considered to be non-redundant and fracture critical. With ageing, these bridges are facing an increasing risk of bridge collapse under extreme conditions. Therefore, it is essential to evaluate alternate load paths on existing bridge structures to avoid bridge collapse or replacement. In this paper, after-fracture redundancy of truss bridges is investigated through a case study for a five-span continuous truss bridge. Field tests were performed, and the displacements on key sections were measured in the test. According to the experimental observations and test results, a numerical model capable of simulating the present truss bridge was built and was used to investigate the after-fracture redundancy of the target bridge. Fracture of truss members in typical locations was considered in the numerical analyses, and the ‘R’ values of both intact and damaged trusses were compared. On the basis of the numerical results, a critical member in this bridge was also determined.

Membrane behavior in RC slabs subjected to simulated reinforcement corrosion

Deterioration due to corrosion has been a growing concern in the last decades since it compromises the serviceability of structures and results in a decrease of the structural safety. Apart from deterioration, also the awareness of the importance of structural robustness has increased due to several failures with progressive collapse. The activation of membrane action in reinforced concrete slabs can significantly increase the structural robustness by providing an alternate load path. A validated numerical model for the tensile membrane behavior of reinforced concrete slabs is used to investigate the influence of (simulated) corrosion effects on this membrane behavior. A two-step analysis is adopted: first a cross-section analysis is performed, followed by an analysis of the structural member. It was observed that even for small corrosion levels, there was a significant influence on the ultimate bearing capacity of the slab. Finally, also the influence of different corrosion locations was investigated.

An approximate method of dynamic amplification factor for alternate load path in redundancy and progressive collapse linear static analysis for steel truss bridges

Linear static analysis with an alternate load path using dynamic amplification factor (DAF) is often used for redundancy and progressive collapse analysis of steel truss bridges to avoid using the more time-consuming dynamic analysis. This study presents an empirical equation to calculate the DAF for this type of analysis against the initial sudden member fracture. Currently, this analysis employs an approximate model with a single degree of freedom to calculate the DAF. With a 5% damping ratio, the constant DAF of 1.854 is used for all types of steel truss bridges. However, this approach is inaccurate because the DAF varies between bridges and with the location of the fractured members as well. Considering some of the approaches developed for building structures but adapting them to steel truss bridges, this paper proposes an empirical equation that allows for the computation of the DAF from the maximum norm stress σis/σiy in static linear elastic analysis of the damaged model with a member removal. A total of 30 illustrative cases for two typical steel truss bridges are investigated to obtain the data points for the empirical equation. The proposed empirical equation is the enveloped line offset from the best fit line for the data points in illustrative cases.

Progressive collapse analysis of power transmission towers

Lattice transmission towers are critical components of power infrastructures. Collapse of a transmission tower may cause major economic and social impacts. The present study aims to evaluate the progressive collapse vulnerability of a lattice tower in a 400kV power transmission line. The load increase factors (IF) after an instantaneous element removal are determined for application in static analyses and for design purpose. The capacity-to-demand ratios (C/D) are proposed to identify critical members after different removal scenarios. This proposed parameter is examined by comparison with Overload Factor (OF) from pushdown analysis. In addition, the pushdown analysis is used to determine the remaining capacity of the structure after removing an element. It is found that tower may resist progressive collapse because of the possible alternative load paths. It is also observed that the studied structure has less susceptibility to progressive collapse in scenarios of element removal at higher elevations.

Evaluation of performance of non‐invasive upgrade strategy for beam–column sub‐assemblages of poorly designed structures under seismic type loading

SummaryBeam–column sub‐assemblages are the one of the most vulnerable structural elements to the seismic loading and may lead to devastating consequences. In order to improve the performance of the poorly/under‐designed building structures to the critical loading scenarios, introduction of steel bracing at the RC beam–column joint is found to be one of the modern and implementable techniques. In the present work, a diagonal metallic single haunch/bracing system is introduced at the beam–column joints to provide an alternate load path and to protect the joint zone from extensive damage because of brittle shear failure. In this paper, an investigation is reported on the evaluation of tae influence of different parameters, such as angle of inclination, location of bracing and axial stiffness of the single steel bracing on improving the performance through altering the force transfer mechanism. Numerical investigations on the performance of the beam–column sub‐assemblages have been carried out under cyclic loading using non‐linear finite element analysis. Experimentally validated numerical models (both GLD and upgraded specimen) have been further used for evaluating the performance of various upgrade schemes. Cyclic behaviour of reinforcement, concrete modelling based on fracture energy, bond‐slip relations between concrete and steel reinforcement have been incorporated. The study also includes the numerical investigation of crack and failure patterns, ultimate load carrying capacity, load displacement hysteresis, energy dissipation and ductility. The findings of the present study would be helpful to the engineers to develop suitable, feasible and efficient upgrade schemes for poorly designed structures under seismic loading. Copyright © 2016 John Wiley & Sons, Ltd.

A new partial-distributed damage method for progressive collapse analysis of steel frames

The alternate load path method has so far dominated the field of progressive collapse of structures; in order to assess the resilience of structural systems, the concept of the removal of a key element is utilized as a means of damage introduction to the system. Recent studies have indicated that the complete column loss notion is unrealistic and unable to describe a real extreme loading event, e.g. a blast, that will introduce damage to more than one elements in its vicinity. This paper presents a new partial distributed damage method (PDDM) for steel moment frames, by utilizing powerful finite element computational tools that are able to capture loss of stability phenomena. Through the application of a damage index δj and the investigation of damage propagation, it is shown that the introduction of partial damage in the system can significantly modify the collapse mechanisms and overall affect the response of the structure.

Experimental study on the progressive collapse performance of RC frames with infill walls

The interaction between the infill walls and the reinforced concrete (RC) frame members in the progressive collapse process was examined experimentally in this study. Two 1/3 scaled, four-bay, two-story RC frame specimens, one of which was featured without infill walls while the other with infill walls, were tested. The frame specimens were designed in such a way that the center column of the first story was missing, in order to simulate the failure of the structural component due to abnormal loads or design flaws. The frame specimens were quasi-statically pushed downward at the top of center column under displacement control to investigate the progressive collapse mechanism of the RC frames, with a focus on the effects of infill walls. Specifically, the physical quantities and phenomena of great interest in this study include the collapse resistance force and mechanism, strain variation and crack development in structural components, and local and global failure modes of the frames. The test results showed that the infill walls can provide alternative load paths for transferring the loads originally only supported by the beams, and thus, improve the collapse resistance capacity of the RC frame. The infill walls, however, may reduce the ductility of the RC frame and may change the failure mode of the frame. It is concluded that the infill walls may affect (i.e., either improve or impair) the performance of RC frames against progressive collapse in different aspects.