Experimental study on progressive collapse of reinforced concrete frames under a corner column removal scenario

Progressive collapse mechanisms of post-tensioned reinforced concrete frames considering effect of infill walls

Progressive collapse is defined as the spread of an initial local failure of a critical load bearing structural element to the remaining structure, ultimately leading to the collapse of the entire building or of a large portion of it. After the partial collapse of the Ronan Point apartment tower in 1968, when a localised gas explosion ultimately led to the collapse of the whole corner of the building, progressive collapse firstly drew public attention due to its catastrophic consequences on human casualties and economic losses. Other recent collapse events, such as those of the Murrah Federal Building in 1995, Sampoong department store in 1995 and World Trade Centre towers in 2001, have further highlighted the importance of this topic and made engineering communities aware of the necessity to prevent the occurrence of progressive collapse in building structures. In the past decades, considerable research has been devoted to investigate progressive collapse of building structures. These research typically followed an alternate load path approach. This approach allows an initial failure to occur, while the remaining structure needs to seek alternate load paths to dissipate damage and prevent progressive collapse. In practice, the progressive collapse resistance of building structures is generally evaluated by notionally removing one or more load bearing structural elements, representing the initial failure. Despite all research efforts made, they have predominately focused on frame systems. Relevant investigations on reinforced concrete (RC) flat plate structures, a commonly used structural system, are still scarce. The flat plate system contains slabs, of uniform thickness, directly supported by columns without using beams, drop panels and column capitals. This provides structures with increased storey height, architectural convenience and significant savings in construction costs. Nevertheless, the slab-column connection is prone to punching shear failure, which may propagate to the vicinity of connections, potentially leading to progressive collapse. The present study aims at filling some of the current knowledge gap with comprehensive experimental, analytical and numerical investigations on RC flat plate structures. The experimental program consists of four quasi-static large-deformation tests performed on three 1/3-scale, 2×2-bay RC flat plate substructures subjected to critical column removal scenarios. The test specimens included one specimen tested under two consecutive corner column removal scenarios, one specimen under an edge column removal and one specimen under the scenario of concurrently removing both an edge and an interior column. For the loading scheme, a multi-point loading system was specially designed to simulate an increasing uniformly distributed load (UDL). The loading points of this system were equally positioned on the bays adjacent to the removed column(s) to apply the load until failure of the slab. Throughout each test, the structural behaviour of the slab was monitored by extensive apparatuses mounted to the slab both internally and externally. Failure and post-failure behaviours, failure modes, and collapse resisting mechanisms were observed and analysed. Additionally, a complement of analytical studies to the experimental tests, based on the observed yield lines, was carried out to estimate the flexural capacity of the test specimens. To numerically investigate the progressive collapse behaviour of RC flat plate structures, a set of 3D nonlinear finite element modelling approaches, using LS-DYNA software, was established. A total of five numerical models were explicitly created based on the four experimental tests in present research and one from the literature. The proposed modelling approach was basically composed of appropriate definitions of element types, bond-slip relationships, loadings and boundary conditions, and more importantly development of the material models for concrete and reinforcements and calibration of failure criteria for element erosion. A quasi-static loading scheme in displacement control in the numerical models was adopted, the same as that in the conducted tests, through explicitly modelling the multi-point loading system in order to replicate the true test conditions. The numerical models were respectively validated by comparing the load displacement responses, failure modes, crack patterns, and displacement and strain results against the experimental ones, which confirmed the appropriateness and reliability of the proposed models. The validated modelling techniques could be applied to further perform parametric studies of the progressive collapse resistance of RC flat plate substructures and prototype structures. In summary, the significant and original contribution of this research is to (1) obtain a solid understanding of the collapse behaviour of RC flat plate structures under critical column removal scenarios, (2) examine the analytical solution using the yield line theory to predict the flexural capacity of RC flat plate structures, (3) establish a set of numerical modelling techniques which can reasonably replicate the experimental process and provide valuable references of numerically analysing progressive collapse of RC flat plate structures, and (4) give design insights into Australian and international Standards in mitigating progressive collapse of RC flat plate structures.

Seismic behavior of hinged steel frames with masonry infill walls

Effect of loading methods on progressive collapse behavior of RC beam-slab substructures under corner column removal scenario

In open ground storey buildings, along the building height, sudden change of stiffness takes place. Therefore, the storey is more flexible than adjacent storey. As a result, beams and column in these storey get heavily stressed. The behaviour of building gets changed due to infill walls under lateral loads. However, in industry, while analysing the framed building, the stiffness of infill wall is ignored. To compensate for stiffness, discontinuity IS1893:2002 allows analysis of open ground storey building with a multiplication factor of 2.5 without considering infill walls. According to design office, for low-rise building, the multiplication factor of 2.5 is not real. This gives a call for review and assessment of code recommendation on multiplication factor for low-rise buildings with open ground storey. The present study deals with the modelling of the reinforced concrete structure with infill walls in the software. The results shall be studied for the fully infill wall, partially infill walls and without infill wall.KeywordsInfill wallStorey stiffnessLateral displacementTime periodStorey shearStorey drift

The progressive collapse analysis of a reinforced concrete (RC) building occurs when an extreme event arises (vehicle collision, gas explosion, bomb blasts, etc.), which often cause damage to the RC structural frame were a sudden loss of primary structural element takes place or due to the failure of one or more critical structural element leading the structure to partial or entire collapse. The fear of progressive collapse is growing, due to the partial collapse of the 22-story Ronan point apartment in 1968, leading to a gas burst and attacks in Oklahoma City, the Alfred P. Murrah Federal Building in 1995, and the attacks on the world trade center in Lower Manhattan, New York in the year 2001. The trend is now to attain strong buildings which can be operational after such extreme events, especially when the building is occupied by a large number of persons. This paper reviews the progressive collapse study on RC frame structure based on various numerical, analytical, and experimental methods which are reported and discussed in the literature. In comparison with bare RC frame, the advantageous impact of the masonry infill wall on the RC structural frame is summed up against the progressive collapse.

Designing infilled reinforced concrete frames with the ‘strong frame-weak infill’ principle

The influence of masonry infill walls on the progressive collapse performance of reinforced concrete (RC) frame structures was investigated in this paper, using a nonlinear dynamic analysis approach. Based on ANSYS/LS-DYNA finite element software, two finite element models of RC frame structures with and without masonry infilled walls were established. Then, the collapse modes of the two RC frame structure models were analyzed for different scaled distance blast loads, different locations of column damage, and different span numbers. The results show that with the increase of explosive amount, the collapse degree of the structure is more serious in the same time. Under the condition of destroying the outermost central column, the degree of progressive collapse of the RC frame model with infilled walls in the same time is lower than that of the RC frame model without infilled walls. The RC frame model with infilled walls is more resistant to collapse when the outermost side columns are damaged. With the increase of span number, the structure is more likely to be damaged and collapsed.

Effect of infill walls on the seismic performance of a severely damaged substandard RC building during the February 6, 2023, Kahramanmaras earthquake sequence

Experimental study on separating reinforced concrete infill walls from steel moment frames

The effectiveness and the sensitivity of a Wireless impedance/Admittance Monitoring System (WiAMS) for the prompt damage diagnosis of two single-storey single-span Reinforced Concrete (RC) frames under cyclic loading is experimentally investigated. The geometrical and the reinforcement characteristics of the RC structural members of the frames represent typical old RC frame structure without consideration of seismic design criteria. The columns of the frames are vulnerable to shear failure under lateral load due to their low height-to-depth ratio and insufficient transverse reinforcement. The proposed Structural Health Monitoring (SHM) system comprises of specially manufactured autonomous portable devices that acquire the in-situ voltage frequency responses of a network of twenty piezoelectric transducers mounted to the RC frames. Measurements of external and internal small-sized piezoelectric patches are utilized for damage localization and assessment at various and increased damage levels as the magnitude of the imposed lateral cycle deformations increases. A bare RC frame and a strengthened one using a pair of steel crossed tension-ties (X-bracing) have been tested in order to check the sensitivity of the developed WiAMS in different structural conditions since crack propagation, damage locations and failure mode of the examined frames vary. Indeed, the imposed loading caused brittle shear failure to the column of the bare frame and the formation of plastic hinges at the beam ends of the X-braced frame. Test results highlighted the ability of the proposed SHM to identify incipient damages due to concrete cracking and steel yielding since promising early indication of the forthcoming critical failures before any visible sign has been obtained.

A diverse range of masonry infill walls is frequently used for low and medium rise reinforced concrete residential and office buildings. Exterior and interior partitioning walls are usually regarded as non-structural elements, even though masonry infill panels alter the seismic behaviour of buildings. Under earthquake loadings, the influence of infill panels may significantly increase stresses developed in some structural elements of the building. Residential and office buildings that have ground floor commercial or parking spaces develop a particularly dangerous pattern, where soft storey mechanisms might lead to premature failures. This paper examines the structural behaviour of masonry infilled RC frames under in-plane loading. A numerical modelling approach is adopted. Infill masonry walls are modelled as equivalent single diagonal struts. This model depicts the general behaviour of a building adequately enough, even if it cannot accurately reproduce local effects resulting from the interaction between frames and infill panels. Nonlinear push-over analyses are carried out for a range of alternative configurations of an outer wall of a building. Bare frame and masonry infilled frame behaviour are compared and the influence of masonry types is assessed. Subsequently, the influence that the ground floor infill walls absence exerts on the failure pattern is analysed. Conclusions reflect the influence that masonry infills have on the behaviour of RC frames for in-plane lateral loads.

Concrete block infill walls, due to their high strength and stiffness, can significantly influence the seismic behavior of the frames. The infill walls can increase the overall lateral frame strength and can also induce local failures in the surrounding frames. In general, if local failure can be avoided, the presence of the infill walls can provide additional reserved strength that can be beneficial to the frames. For this reason, a new RC frame with infill walls could be designed as a bare frame without considering the increase in strength, but measures must be taken to prevent local failures due to the infill wall-frame interaction. However, this infill wall-frame interaction is very complex, and clear design strategies to prevent local failures are not normally given in design codes. For this reason, two full-scale intermediate RC moment-resisting frame specimens with infill concrete block walls were tested under a horizontal cyclic load to study the infill wall-frame interaction. Finite element analysis was carried out to evaluate the response particularly on the infill-frame interactions. Based on the results, a design strategy to eliminate local failure in the surrounding frame is suggested. The proposed method is based on local plastic mechanism analysis of the column considering the variation in the column shear demand and column shear capacity at different states of the response as the frame deforms. The shear demand depends on the magnitude and direction of the bearing force from the infill wall. The column is considered as a captive or slender column depending on the restraint from the infill wall, thereby affecting the shear capacity of the column. The proposed approach can be readily applied without radically changing common design practices and design codes.

In the composite walls stiffness system of reinforced concrete walls with composite boundary members, boundary members resist most of the moment due to seismic loading. In contrast, the reinforced concrete (RC) wall provides shear resistance. The SRCW system comprises partially restrained steel frames with reinforced concrete infill walls. The steel columns and beams act as boundary members to resist gravity loads and most of the overturning moment due to seismic loading, while the reinforced concrete (RC) infill wall acts as shear-resisting web. The RC infills increase the lateral stiffness dramatically compared to a bare steel frame, thus avoiding excessive drift and reducing the seismic demands on the steel frames. In that way, why not use just steel shear frames instead of partially restrained steel frames with reinforced concrete infill walls? This paper presents a numerical study of the behavior of a composite structural system consisting of steel shear frames with reinforced concrete infill walls (SFRCW). The composite interaction is achieved using welded bolts as shear connectors along the steel frame–infill interfaces. Welded bolts were used as shear connectors because they are frequently used in Colombia due to their ease of installation. The SFRCW system may be particularly appropriate for low-to-moderate-height structures. In addition, the steel shear frame will support gravity loads at the construction stage, allowing progress in height. The system can also be used to strengthen existing steel buildings. The relatively light steel frame constructed using shear connections maximizes the system's economy.This study compared the behavior of the SFRCW with columns acting in their weak and strong axes and with different numbers of shear connectors. It also compared the behavior of the bare steel moment frame, bare steel shear frame, and reinforced concrete wall. The numerical models show interesting results.

Recent earthquakes have demonstrated a significant contribution of the masonry infill walls in the structural response of the reinforced concrete (RC) buildings. Field observations after the 25th April’s earthquake in Nepal reinforce the conclusion regarding the influence of the infill walls increasing significantly the structural stiffness, which has a direct impact on the natural frequencies of the structure and of structural elements. Firstly, the manuscript focuses on the performance of the infilled RC structures. Secondly, a bare frame existing building in Nepal was studied, modelled and calibrated with results from data collected on ambient vibration tests. The seismic vulnerability of the building and a parametric study was conducted to study the influence of each material properties in the structural response. The influence of the infill masonry walls presence was also evaluated.