Alternate Load Path Research Articles

Survival of Reinforced Concrete Flat Plate System against Column Loss

Column loss in a flat plate building, due to punching shear, explosion, impact load or any acci-dental event, can lead to what is termed progressive collapse. Progressive collapse is inherently a dynamic process, which makes it difficult to experimentally explore structures with real scale. Therefore, this paper aims to numerically investigate the behavior of flat plate systems due to column loss utilizing nonlinear finite element analysis with the aid of the computer software (ABAQUS). In this investigation, the nonlinear dynamic re-sponse of both an old flat plate building, designed according to the ACI 318-71, and a similar modern building, designed according the ACI 318-14, subjected to an instant removal of a column is examined. The obtained results clearly reveal that the old flat plate building without continuous slab bottom reinforcement at columns is highly vulnerable to progressive collapse and the efficiency of the continuous bottom reinforcement within the column strip, as recommended by the ACI 318-14, in preventing the disproportion or progressive collapse of a reinforced concrete flat plate building. Such reinforcement is able to produce alternate load path through the tensile membrane action, thus providing ductility and robustness in the system.

A Design Approach to Evaluate the Load-Carrying Capacity of Reinforced Concrete Slabs Considering Tensile Membrane Action

This paper proposes a novel simplified analytical approach, named the slab strip model, to assess the ultimate bearing capacity of reinforced concrete (RC) two-way slabs when large deflections are reached and tensile membrane action plays a key role. The motivation for the work derives from the fact that in recent years, investigations into the behaviour and modelling of laterally restrained and unrestrained RC slabs have been intensified owing to the reserve of bearing capacity provided by the tensile membrane action. The approach has been developed in the framework of the strip method and analyses the tensile membrane effect. Two failure criteria are accounted for: the maximum ultimate slab strip elongation and the maximum ultimate rotation of the structure at the supports. The rationality of the proposed approach is validated for laterally restrained slab strips and two-way laterally unrestrained simply supported slabs by comparing the analytical results with experimental data from the literature. The results show that despite its simplicity the method is accurate in estimating the load-carrying capacity of RC two-way slabs able to develop significant tensile membrane action. For its characteristics, the proposed method can be considered a useful initial design tool in a robustness context for conducting alternative load path analysis of RC structures characterised by two-way slabs.

Anti-collapse resistance mechanisms in corner-supported modular steel buildings

Due to different integration strategies and stability requirements, the robustness analysis of multi-story modular buildings, made of volumetric units, requires collapse capacity analysis of various scenarios, corresponding to the loss of supports and notional removal of a part of ground floor modules. In this paper the results of a parametric analysis on the progressive collapse response of typical corner supported modular steel buildings are presented. The focus is on the effects of inter-modular connections' design on the anti-collapse resistance of these buildings. To that end, the effects of inter-modular connections translational behaviour, rotational stiffness of inter-modular connections, their removal time, and the rigidity of modules on the buildings' response are studied. Various module loss scenarios are imposed to a six-story modular building and its dynamic progressive collapse responses are investigated through alternative load path method. The collapse modes and anti-collapse resistance capacity of the building are studied, and some design considerations are proposed accordingly. The results show the significant effect of the translational behaviour of the connections, especially in shear. The rotational stiffness is shown not to have considerable contribution to maintaining the overall stability, provided that the modules individually act as complete rigid moment frames. On the other hand, while increasing the removal time, to the level of vertical natural period of the building, can considerably reduce its dynamic collapse response, the modules' rigidity shows no significant effect on the gravity-induced collapse resistance of the modular buildings.

Robustness analysis of steel frames subjected to vehicle collisions

Among the accidental loading scenarios, the vehicle collision in the ground column of a building is surely one of the most common. When a building is subjected to such an action, the beam-to-column connections are responsible for the transfer of the actions through an alternative path, thus, playing an important role in the robustness of the building. This paper presents the results of a numerical study carried out on a steel frame subjected to a vehicle collision. Two different parameters are studied: i) the influence of the type of analysis, using alternative load path approaches (column loss scenario) or by explicitly considering the action deriving from the vehicle collision in a dynamic framework and ii) the influence of the beam-to-column connection typology, considering a connection with a reduced beam cross section and an innovative type of a slide-hinge joint. The type of analysis highlighted that the use of alternative-load path method can lead to a different structural response than that coming from the impact analyses, especially if high collision velocities are considered. Concerning the adoption of different connection typologies, the results showed that the main difference of using the slide-hinge joint rather than the connection with the reduced beam cross-section is in the delay of the yielding of beam and columns due to the slip mechanism of these connections.

Analysis of steel special moment frames including damaged column subjected to far and near-field ground motions

ABSTRACT In the present research, the performance of steel special moment frames having one damaged column designed based on seismic provisions has been evaluated during the progressive collapse under the ground motions. For this purpose, five- and ten-story two-dimensional steel frames with four bays have been modelled by utilising OpenSees software with and without considering a weak column. Moreover, nonlinear incremental dynamic analyses (IDA) have been carried out on the structures under far and near-field ground motions. In the case of the model with a weak column, the potential of progressive collapse has been assessed using the Alternative Load Path Method recommended in the General Service Administration (GSA) guideline. In this state, due to the defect in the implementation of column splice, the damaged column represents a situation in which it is weak in terms of bending performance and collapsed by the earthquake. Furthermore, the effect of floor number, weakness of column, and the type of applied earthquake have been discussed. According to the obtained results, a structure including one damaged column has a weaker performance in the near-field compared with in the far-field ground motions before a progressive collapse.

Interfacial Shear Bond Strength between Steel H-Piles and Polymer Concrete Jackets

Steel H-piles have been used widely in bridge construction throughout the U.S. because of their relatively large load-carrying capacity while occupying a small area. However, many H-piles suffer from corrosion, which may lead to abrupt collapse. A cost-effective repair technique, including encasing the corroded region of the steel pile into a concrete jacket, which acts as an alternative load path for the applied axial load, has been used by several state Departments of Transportation. Methyl methacrylate polymer concrete (MMA-PC) is a type of concrete that is commonly used as a repair material. However, there is limited research on the assessment of bond strength between MMA-PC and steel elements. This paper investigates experimentally the bond behavior of seven full-scale steel H-piles encased in concrete jackets. The jackets were cast using either MMA-PC or Portland cement concrete (CC). Different embedment lengths of 63.5 mm (2.5 in.), 127 mm (5 in.), and 190.5 mm (7.5 in.) were used for the MMA-PC and one embedment length of 254 mm (10 in.) was used for the CC jacket. Cylindrical and prismatic jacket configurations were used and tested using push-out. The experimental results revealed that using the MMA-PC jacket was more effective compared with the CC jacket in relation to the load-carrying capacity. For design purposes, a shear bond stress of 2.96 MPa [0.43 kips per square inch (ksi)] can be used for MMA-PC jackets having an embedment length of at least 127 mm (5 in.) whereas a value of 0.83 MPa (0.12 ksi) can be used for CC.

Dynamic performance of a real-scale reinforced concrete building test under a corner-column failure scenario

The topic of robustness and progressive collapse of structures has attracted significant attention within the field of structural engineering recently. This is reflected by the rise in the number of scientific papers published in recent years as well as efforts in reviewing and developing codes for design. Although important numerical and experimental studies have been carried out to date simulating the sudden removal of columns to reproduce the possible consequences of an extreme event, most of these studies focus on subassembly systems and internal columns. Edge and corner columns are most vulnerable to accidental events. This paper gives the results of a test carried out on a purpose-built full-scale reinforced concrete building with a specially designed corner steel column used for the sudden column removal. The test was highly instrumented, involving 38 strain gauges, 38 displacement transducers and 2 accelerometers to monitor the vertical and lateral response. The results were used to analyse the dynamic performance of the structure after the sudden column removal as well as the alternative load paths (ALPs) mobilised during the test (i.e. flexural and Vierendeel action). The test showed a clear dynamic amplification of the strains and displacements (with high peaks); dynamic amplification factors (DAFs) were obtained accordingly. The load initially carried by the removed column was redistributed through the entire building system (not just the neighbouring columns). Tests on full-scale buildings, including the one described here, can be used to compile a database to validate codes and future numerical studies.

Finite element analysis of alternative load paths in a platform-framed CLT building

Multi-storey cross-laminated timber (CLT) buildings are a comparatively recent construction type. Knowledge concerning the performance of CLT buildings regarding the prevention of disproportionate collapse after unforeseeable events (e.g. accidents or acts of terrorism) is not as refined as that for concrete and steel buildings. In particular, alternative load paths (ALPs) after the removal of a wall panel in platform-framed variants have not yet been studied in detail. The goal of this work was therefore to study ALPs in CLT buildings. An eight-storey bay of an existing building was evaluated by conducting a non-linear static pushdown analysis in a finite element analysis on three representative storeys. The analyses accounted for single fastener behaviour, timber crushing, friction, brittle failure and large deformations. The force–deformation behaviours elicited under the pushdown analyses were subsequently inserted in a simplified dynamic model to evaluate the transient response of the entire bay. Four ALPs were identified in this case – shear resistance in the floor panels, arching action of the walls, catenary action in the floor panels and hanging action from the roof. The dynamic analysis did not show a collapse, unless the inter-compartment stiffness was significantly reduced. The resistance mechanisms are described in this paper, which may provide information for improved building design.

Effect of Initial Local Failure Type on Steel Braced Frame Buildings against Progressive Collapse

In many studies, the alternative load path method (APM) has been used for progressive collapse analysis. In this method, one or several columns of the building are removed and the building response is investigated. This method ignores the initial local failure cause of building and this can affect the structural response. Investigation of ignoring the initial local failure cause of steel braced frames is the main purpose of this research. The variables include the type of progressive collapse investigation (ignoring the initial cause of failure or APM, considering blast loading and the heat caused by the fire as the initial causes of failure) and the location of the initial local failure in plan (outer and inner frame) and in floors (1st, 2nd and 3rd). 4-story braced steel buildings were simulated using ABAQUS software and the responses were compared using different methods. The most important results showed that the axial forces are very noticeable in the columns around the damaged site if the initial local failure caused by explosive loading; while these forces are ignored when APM is used. Therefore, due to this significant difference, if the design of a steel building is to be considered against progressive collapse, it is recommended to consider the initial local failure in order to make the appropriate design in accordance with it. Therefore, the initial loading type has a very significant effect on the structure response, and ignoring the initial local failure can lead to incorrect predictions of the structure response.

Arching followed by catenary response of steel shear connections in disproportionate collapse

Steel shear connections are mainly designed to sustain shear forces. There is limited research assessing the axial response of shear connections, which is important in the evaluation of robustness of steel structures. Structural collapse can be arrested following localized damage if an alternative load path with sufficient capacity is available. In this study, the formation of compressive arching action followed by tensile catenary action is investigated. It is shown that in a column loss scenario, connections may develop significant axial compressive force before catenary action begins; this phenomenon has often been neglected in assessments of connection robustness in the sense that axial force is reported as a tensile action only. The presence of arching action has been confirmed in two experimental programs, and one set is selected for further study using a numerical approach. A simplified analytical model is then presented and compared with the observed axial response of these connections. It is concluded that vertical eccentricity between the centres of rotation of the connections at the two ends of a beam is the principal factor causing the development of a compressive arching force. Another influential parameter that affects the formation of arching action is the stiffness of the surrounding structure.

Robust composite joints

“Disasters” such as seismic events, impact or fire loading to structural elements of crucial importance highlight the significance of structural design for robustness. Consequently, exceptional load scenarios are to be considered. The question whether the joints of structures designed according to the current version of Eurocodes have adequate load bearing and defor­mation capacity for such load cases arises. The “alternate load path method” plays a key role in structural design for robustness. By using this design approach, redistribution and distribution of the existing and additional loads, respectively, to the intact structural elements should be enabled after a local failure such as “loss of a column”, preventing so the progressive collapse of the structure. In order to pursue the aforementioned goal, sufficient bearing as well as rotational capacity of the joints are required. In particular, for exceptional events, change of the moment’s sign in the area of the joints is often to be observed. However, until today, attention has been paid mainly to the response of composite joints under negative moments. Herein, besides this load case, the corresponding one of positive moments is investigated as well as the development of a membrane action that supports the load redistribution but requires a tension force capacity of the joints. Experimental research for the load case “loss of a column” was undertaken on both single composite joint specimens and composite frames. Also, an integrated design method, the beamline method, is analysed and constructional recommendations for design of ductile compo­site joints were developed and are herein presented.

Steel-concrete frames under the column loss scenario: An experimental study

Accidental events, such as impact loading or explosions, are rare events with a very low probability of occurrence. However, their effects often lead to very high human losses and economic consequences. An adequate design against these events should reduce the risk for the life of the occupancy, minimize the damage extension and enable a quick rebuilding and reuse. A structure fulfilling these requirements is ‘robust’. Different strategies can be pursued for accidental events, and among them, methods based on the residual strength or the alternate load path are frequently adopted because applicable to a vast range of structures. Adequate design strategies based on them require an in-deep knowledge of load transfer mechanisms from the damaged to the undamaged part of the structure. As to the frames, the important role of joint ductility was pointed out in recent studies. Besides, the flooring systems substantially affect the spread of the damage, but the research on this subject is still very limited. The present study focuses on steel-concrete composite frames under the column loss scenario. It aims to better understand the influence of both frame continuity and floor systems in the development of 3D membrane action. Two geometrically different 3D steel-concrete composite full-scale substructures were extracted from reference buildings and tested simulating the column collapse scenario. This paper illustrates the preparatory studies, the main features of the specimens and the outcomes of the first test. The test provided an insight in the need for an enhanced design of joints and pointed out the key features of the response of the floor system.

Progressive collapse analysis and structural robustness of steel-framed modular buildings

This paper investigated the structural robustness of steel-framed modular buildings based on the alternative load path method using LS-DYNA. The structural responses of a reference structure were first simulated to understand typical building motion, load redistribution mechanisms and failure modes after removal of a base corner module. Based on the reference scenario, a comprehensive parametric study was then carried out to investigate the effects of structural configuration (number of modules of each floor), module posts (wall thickness and number of supporting posts) and other features (inclusion of floor slab, longitudinal wall bracing, connection stiffness and connection capacity) on the structural responses of steel modular buildings. It was found that the collapse resistance of steel modular buildings could be improved with an increase in the number of modules per floor, the number and capacity of supporting posts, higher rotational stiffness and capacity of inter-module connections, and the use of longitudinal wall bracing. Consideration of floor slabs provided a more realistic description of collapse mechanisms, due to the slab-related contact interaction and diaphragm action. Also, the structure was more vulnerable to the removal of modules (or posts) near the end corner of a building.

Robustness Assessment Based on Pseudo-Static Full Probabilistic Approach

Accordance with a new EN 1992 the robustness of the structural systems must be analyzed. When applying proposed new strategy, it has been verified that the structure has sufficient redundancy and possibility to mobilize so-called alternate load path (AP-method). Paper presents a modified complex method for robustness estimation in case of the RC-structural systems. Proposed pseudo-static method based on the energy-saving approach. Paper also presents a full probabilistic pseudo-static nonlinear method which was developed for structural robustness assessment, taking into account the statistical variability of the materials properties and, as a consequence, parameters of the plastic hinges and possible failure modes.

Progressive Collapse Analysis of Cable-Stayed Bridges

Several codes have proposed guidelines to prevent progressive collapse. Although most of these standards are in progress, few recommendations for progressive collapse analysis and design of cable bridges or even bridges can be found. In this paper, progressive collapse analysis of a cable-stayed bridge is investigated. In this regard, the effects of changes in Fy, E and cross-section area of cables to progressive collapse resistance are studied. The evaluation is performed by alternate load path method and the nonlinear time history tool in SAP2000V17 software. The results of the analysis show that as the cross section and the modulus of elasticity of the cables increase, displacement of bridge decreases and the bridge’s resistance increases against failures. Also, for the case where Fy of cables were increased, displacement of the bridge did not differ, and only the formation of the plastic hinges in the cables changed.

A non-iterative progressive collapse design method for RC structures based on virtual thermal pushdown analysis

Disproportionate progressive collapse is a structural failure mode with low probability and high consequences. Due to this nature, progressive collapse design is usually treated as a secondary design; namely, a design check after the design for primary loadings. Various design procedures have been recommended, notably the alternate load path method through nonlinear pushdown analyses. However, most of these design procedures are iterative by nature to satisfy the design acceptable criteria. To improve the design efficiency, this study presented an innovative design technique that allows to directly determine the proper amount of reinforcement in beams and slabs of a reinforced concrete (RC) structure in order to withstand the gravitational loads under column removal scenarios. Inspired from fire-induced progressive collapse research, the proposed method employs a virtual thermal pushdown analysis, in which the temperature increase affects only the strength of reinforced steel. With carefully developed strength-temperature relationships of the rebars, the virtual thermal analysis produces a displacement-temperature curve that represents the genuine nonlinear relationship between the amount of reinforcement and structural performance (often represented by vertical displacement). This curve is then used to directly determine the amount of reinforcement at the prescribed performance target. Two beam-column sub-assemblage examples and the Alfred P. Murrah Federal Building were analyzed and redesigned to demonstrate the effectiveness of the proposed method. All three examples showed that the proposal virtual thermal pushdown method can accurately determine the appropriate amount of reinforcements to meet the performance target.

Experiments on the collapse response of flat slab structures subjected to column loss

Flat slab structures are prone to progressive collapse due to their relatively heavy self-weight and susceptibility to punching shear failure. In this study, three one-third scaled slab–column sub-assemblages were statically and dynamically tested to examine the collapse behaviour of such structures when either an edge or a penultimate column is removed. In the static tests, the development of an alternative load path bridging over the removed column was investigated with two specimens that were loaded to failure by a load-control procedure using concrete weights. In the dynamic test, a specimen that had been loaded by concrete weight up to 80% of the yield-line capacity was subjected to a sudden column removal, which was simulated by a quick-release mechanism. Based on the experimental observations, the potential failure modes of flat slab structures, the development of tensile membrane action as an alternative load path, and the dynamic effects resulting from sudden column removal are discussed.

Recommendations for Reducing Progressive Collapse Potential in Flat Slab Structural Systems

Flat slab structural systems are amongst the oldest methods to transfer gravity loads and contribute to resistance of lateral forces. The popularity of the system is owed to the ease and speed of construction compared to other floor systems. Flat slabs also offer flexibility where light and heavy partitions may be placed freely anywhere on plan. Properly designed flat slabs possess considerable ability to transfer vertical loads through membrane action. However, in the event of loss of primarily load-carrying system, the potential for progressive collapse is relatively high. This is due to the inherent lack of alternate load path to transfer gravity load and mitigate progressive collapse. Compared to interior columns, Loss of corner and exterior columns produces higher local demands on the slab and increases the potential for global progressive collapse in the panels at the vicinity of lost corner columns. This paper discusses the dominant failure modes associated with progressive collapse in flat slab construction. The paper also explores options for geometric and structural design of flat slab system that will enhance resistance for progressive collapse with better life safety. The following measures for reducing progressive collapse potential are discussed: 1) use of enhanced local resistance approach for corner and penultimate columns, 2) use of edge beams along perimeters to increase stiffness and improve load transfer, 3) detailing requirements for flat slab system to decrease the potential for punching shear and where continuity and anchorage of bottom reinforcement through columns was noted as critical in improving collapse resistance; 4) selecting columns configurations with improved resistance to punching shear and buckling, 5) addressing the inherently lower collapse resistance in corner slab panels through judicial selection of shear wall locations incorporated with perimeter exterior beams to form a load path.

Design of Various Shear Connectors for Repair of Corroded Steel Girders with Ultra-High Performance Concrete

Corrosion accounts for approximately 20% of the structurally deficient bridges in the United States, causing a massive backlog of rehabilitation projects. The current repair procedure for corroded bridge girders is expensive, slow to implement, and necessitates complete closure of the bridge. Through an extensive study supported by the Connecticut Department of Transportation, a novel repair method has been developed to rapidly restore the strength of corroded bridge girder ends with minimal traffic interruption. First, shear connectors in the form of headed studs are welded on the uncorroded web plate above the bearing. Next, formwork is placed and ultra-high-performance concrete (UHPC) is cast down to the bottom flange. This creates an alternate load path around the section loss. Based on the experimental results, it may be concluded that the implementation of the repair reduces the strains on the web plate and strengthens the bridge girder, allowing it to surpass its original capacity. However, the success of the repair centers around the performance of the shear connectors. In addition to headed studs, threaded bars and UHPC dowels in perforated web were evaluated experimentally as alternative shear-transfer mechanisms. This paper presents the repair design objectives and considerations for a realistic girder end utilizing different shear connectors to demonstrate the flexibility and versatility of the repair. The details of the designs are illustrated to facilitate the transition of the research findings to practice.

Behavior and design for component and system of cold-formed steel roof trusses

The experimental investigation of the behavior of cold formed steel (CFS) roof trusses does not offer a prior knowledge about either which member will fail first or the capacity of the whole truss. This research aims to investigate the overall behavior of CFS roof trusses using finite element analysis and to predict the failure location in the truss assembly and its cause by calculating the demand to capacity (D/C) ratios using Direct Strength Method (DSM) for each component of the truss and compare it with finite element results. Twelve trusses were examined under the same gravity load and boundary conditions to study the effect of span, the truss web pattern, and the cross section type of truss members on the ultimate strength of the truss.Results revealed that for longer spans, Fink truss configuration is better than Howe truss. The member with the maximum D/C ratio will be the first member to fail in FEA, so engineer can use D/C components' ratio instead of FEA to predict the first member to fail, the cause of failure, and truss capacity. The results of D/C ratio values showed that each truss was designed with high efficient critical components and highly variable D/C ratios, which increase the probability of producing an alternative load path to the loads that are carried by the failed truss members. The reliability of truss system is much greater than the reliability of any component as the truss system's D/C ratio calculated from FEA is always less than the maximum component's D/C ratio from the design.