Unified Facilities Criteria Research Articles

Resilience of code compliant reinforced concrete buildings to progressive collapse: A numerical analysis investigation

This study investigates the resilience of dual-system reinforced concrete (RC) structures impacted by progressive collapse where the local failure of a primary structural component leads to the sequential collapse of adjoining elements. The study is conducted on buildings designed in the United Arab Emirates in the context of extreme events. Nonlinear dynamic analysis of twenty-seven different building models is performed to evaluate the impact of critical variables, building height, and column spacing on progressive collapse potential under the Unified Facilities Criteria (UFC) regulations. The research methodology aims to study impacts associated with sudden removal of critical columns (corner, edge, and internal) that replicate severe accident scenarios in buildings. The structural responses were assessed based on the ASCE 41 performance design criteria. The formation of plastic hinges was analyzed to determine the ductility and resilience of the structures. The study finds that buildings with removed corner columns exhibit more significant vertical displacements than those with edge or internal column removals. The results also indicate that columns adjacent to the removed ones generally remained elastic and within their structural capacities, showing structural resilience to localized damage. However, the flexural strength of RC flat slabs near removed or adjacent columns demonstrates insufficient flexural capacities. This study underscores the imperative for enhanced design strategies and emphasizes the critical importance of bolstering structural resilience against extreme events. Moreover, it intends to draw the attention of policymakers locally to the need to consider progressive collapse when designing reinforced concrete structures.

Automatic evaluation of progressive collapse performance of braced frames

Braced frames are among the most effective systems for resisting progressive collapse; however, determining the optimal bracing system remains challenging. Evaluating different bracing systems in this study was simplified using VBA programming tools and a flowchart that complied with the Unified Facilities Criteria (UFC)regulations (chapter 5). Applying control processes and automated VBA programming tools was beneficial for examining plastic hinges and guaranteeing adherence to the Unified Facilities Criteria (UFC)Design Strategies for Preventing Progressive Collapse in Building Structures.The study examined several types of braced frames, such as CBF and EBF, and found that the inverted V-braced systems (CBF and EBF) were the most efficient. They reduced the amount of vertical movement and dispersed the stresses on the remaining linked parts. Although there was a substantial vertical displacement, all beams and columns showed a decrease in overall force. The absence of a bracing system in the frame led to a violation of the regulatory criteria outlined in the control flowchart. The remaining structural systems exhibited exceptional performance.

Progressive Collapse of RCC Structure: A Review

Progressive collapse is a sudden, severe failure of a structure, spreading from an initial point of damage to other parts. It occurs when a critical component, like a column, is compromised, often triggered by accidents, explosions, or natural disasters. The concern about the progressive collapse increased during the late sixties after the Ronan Building incidences in the UK. Mitigation efforts against the progressive collapse focus on minimizing the risk and potential catastrophic consequences. The critical issue with progressive collapse is the significant difference between the initial and final damage, which can destroy the whole building. Various strategies prevent the initial damage from spreading and causing more destruction. These include strengthening certain load-bearing elements or designing the building plan more effectively. The General Services Administration (GSA) and the Unified Facilities Criteria (UFC) provide crucial guidelines for preventing progressive collapse. One standard method they recommend is the Alternative Path Method (APM). This method helps evaluate how a building would respond to potential progressive collapse by ensuring that the rest of the structure can still stand safely if one part fails. An attempt has been made in the current research to assimilate the research carried out in the recent past in the field of progressive collapse and its mitigation and put forth recommendations.

Progressive Collapse Evaluation of a Reinforced Concrete High-rise Building Designed According to Turkish Earthquake Code

In this study, a numerical progressive collapse response evaluation was performed on a 40-story high-rise building designed according to the Turkish Earthquake Code of 2018 (TEC-2018). The alternate path method specified in the General Services Administration of 2016 (GSA2016) and the Unified Facilities Criteria (UFC 4-023-03) was used for the evaluation. A total of 18 scenarios were investigated for column and shear wall removals. In the cases where hinges were observed, the extent of damage was evaluated using the performance criteria given in the Turkish Seismic Code and the American Society of Civil Engineers (ASCE) 41-17. In this study, the most damage was observed when the corner column was removed at the bottom floor, while the least damage was observed on the shear walls close to the center and core of the building. For all the scenarios evaluated, no collapse was observed. The study deduced that the high-rise building designed according to TEC-2018 showed sufficient resistance to progressive collapse.

Nghiên cứu tải trọng nổ theo tiêu chuẩn UFC (Mỹ)

This paper describes the determination of the blast loading based on the Unified Facilities Criteria (UFC) and provides a numerical example of a fictive structure under this blast load. The objective of the paper is to introduce the readers to be familiar with the blast loadings calculated based on the UFC. The blast load was, then, determined using the time-pressure history analysis as well as the structure was numericallly modeled using ABAQUS computer software. The results obtained show that the conventional computer software can be used to simulate explosion effects and possibly give an initial assessment of the structure damages.

Effect of thermal modification on the ballistic performance of coastal western hemlock Cross-laminated Timber (CLT)

To test and understand the potential for integration of thermally modified coastal western hemlock CLT systems in military applications, 109 ballistic experiments were conducted at the United States Army Engineer Research Development Center's Fragmentation Simulation Facility. For a range of CLT thicknesses, the depth of penetration and residual velocities were recorded for different striking velocities utilizing a 0.50 caliber steel sphere projectile. This information was used to compare and add to the existing ballistics data for CLT, investigate the effects of thermal modification on high strain rate loading of wood, assess physics-based empirical design equations used by the Unified Facilities Criteria, and evaluate energy absorbing behavior at a range of thicknesses and striking velocities.The experimental results showed that thermal modification does not significantly alter the full and partial penetration performance of coastal western hemlock CLT. However, the mechanism of fracture along the path of the projectile differed, which indicates that the model parameters used in the UFC design equation (density, hardness, and projectile properties) may not fully encompass the ballistic failure mechanisms of CLT.

Blast Resistance Capacities of Structural Panels Subjected to Shock-Tube Testing with ANFO Explosive.

This study presents a series of shock-tube tests conducted on structural panels using ammonium nitrate fuel oil (ANFO) as the explosive. The characteristics of the blast waves propagating through the shock tube were analyzed by measuring the pressure generated at specific locations inside the shock tube. The extent of differences in blast pressure generated in a confined space, such as the shock tube, was compared to that predicted by the proposed method in the Unified Facilities Criteria 3-340-02 report. The target specimens of this study were plain reinforced concrete (RC), high-performance fiber-reinforced cementitious composites (HPFRCCs), and composite panels. Polyurea-coated RC panels and steel plate grid structure-attached RC panels were used as composite panels to evaluate the effectiveness of the coating and structural damping methods on the enhancement of structural blast resistance. The tests were conducted with different ANFO charges, and the crack patterns and lengths on the rear surface of each panel were measured. Based on the measured results, discussions regarding the blast resistance capacities of each panel type are provided.

Computation of Surface Wave Parameters and Peak Particle Velocity

In recent times, the rise in terrorist activities has underscored the critical need for designing structures with blast resistance capabilities. However, many Indian codes currently in place do not adequately address the issue of blast loads. This knowledge gap poses a significant challenge to engineers who must ensure the safety and security of structures against such threats. In light of this, the objective of this study is to provide an educational resource to bridge this gap by exploring various methods for calculating blast loads on structures. This paper delves into the subject matter, discussing different methodologies and approaches for estimating blast loads on structures, with a specific focus on the Unified Facilities Criteria (UFC) guidelines. To illustrate the practical application, a numerical example is presented, demonstrating the computation of blast loads on structures using the UFC guidelines. Notably, the results reveal that the surface blast load exhibits an exponential variation along the front wall of the structure. Furthermore, the study introduces the concept of peak particle velocity (PPV) as a parameter for identifying the threshold vibration level that would cause no structural damage. The methodology suggested by the United States Bureau of Mines (USBM) is adopted to calculate the PPV and the corresponding results are presented. By providing insights into blast load computation and the determination of safe vibration levels, this study contributes to enhancing the understanding and preparedness of engineers when it comes to designing structures that can withstand and mitigate the destructive effects of blasts.

Machine Learning-based Optimum Reinforced Concrete Design for Progressive Collapse

This paper investigated progressive collapse analysis of three-dimensional (3D) reinforced concrete (RC) frames that are optimized for carrying structural loads by introducing a unique simultaneous multi-column removal load path using Machine Learning. The investigation includes formulating an integrated computational framework that incorporates a self-training machine learning algorithm. This algorithm is used to train the largest machine learning models of 3D RC frames containing more than 600 optimized structures to predict the posterior based on the trained priors. The efficiency of the computational framework was shown by conducting a comprehensive study on the optimization and behavior of structures considering seismic loading, alternative load path due to progressive collapse, and second order (P–delta) effects. The results show that the proposed framework ensures that system solutions meet both structural integrity and constructability requirements of the ACI and the Unified Facilities Criteria.

Effects of Entrance Shape and Blast Pocket on Internal Overpressure Mitigation for Protective Tunnels Exposed to External Detonation on the Ground

This study presents a numerical analysis to reduce the overpressure inside protective tunnels for external detonations. A three-dimensional computational fluid dynamics model of a tunnel subjected to detonation for a hemispherical charge with a charge weight of 555 kg and a standoff distance of 7.6 m was established, based on a mesh sensitivity study to obtain an optimal element size, stability analysis of overpressure, and validation study to evaluate the accuracy of the numerical results based on Unified Facilities Criteria (UFC) 3-340-02. A parametric analysis was performed using the validated numerical model to investigate the effects of the entrance shape and blast pockets on the reduction in the maximum overpressure. The maximum overpressures were effectively reduced as the slope angle of the tunnel entrance decreased and the length of the blast pocket divided by the tunnel width decreased. An optimized shape of the tunnel was proposed based on the numerical results, where the peak overpressures were reduced by a maximum of 64.5%. This study aims to protect facilities, personnel, and equipment and further reduce construction costs by lowering the overpressure rating of blast valves in protective tunnels.

Effects of Tsunami Forces on RC Framed Structure Subjected to Bracing and Shear Wall

Abstract— The Indian Ocean Tsunami on 26 December 2004 resulted into massive destruction to coastal communities with more than 3,00,000 people losing their lives, and causes severe damage to buildings, bridges and other infrastructure. Such catastrophic event made the coastal community realize the need for the preparedness against initial ground shaking and subsequent effects followed by Tsunami. The current study focuses on progressive collapse analysis of 10 storey reinforced concrete frame with bracing and shear wall subjected to different Tsunami forces. The Tsunami forces were calculated according to the guidelines of FEMA P-646, ASCE 7-16. The December 26, 2004 Tsunami parameters on the coast of Tamilnadu were considered for the evaluation of effect of Tsunami forces on structure. To simulate similar conditions structure was designed according to Zone-III and was subjected to Tsunami forces for the 12m, 9m and 6m runup. This study consists of two parts: 1) The calculation of Tsunami forces and application of forces on structure and 2) Progressive collapse analysis of structure by removing the vertical load bearing critical members due to Tsunami forces. The progressive collapse analysis was carried out with the help of commercially available software named ETABs 16. The critical members were removed. The analysis was carried out by referring General Services Administration (GSA-2013, Revised 2016) and Unified Facility Criteria - Department of Defense (UFC - DoD-2009, Revised 2016) guidelines. From the analysis results it was found out that structure becomes critical when Bracing and Shear wall were oriented normal to the flow of tsunami. Upon performing the progressive collapse analysis of structure which was found to be critical due to Tsunami forces reveals that the joint at which the vertical member was removed did not exceed the acceptance criteria of collapse prevention. Hence structure was able to redistribute the unbalanced gravity load and thus prevents the progressive collapse of structure. Keywords— RC Frame, Shear wall and Bracing, Tsunami Forces, Nonlinear Static Analysis, Progressive Collapse

Improved resistance functions for RC elements accounting for compressive and tensile membrane actions

Membrane actions commonly present in reinforced concrete elements as a result of restrained boundary conditions and geometry of deformations, which could substantially improve the ultimate flexural load-resistance as compared to that using yield line theory. Nevertheless, most current design manuals do not consider membrane effect because of a short of proper analysis method. This paper proposed an improved resistance model for RC (reinforced concrete) elements which considers both compressive and tensile membrane actions. Firstly, the derivation of the proposed membrane model was presented in detail. It was then validated with available testing data, in which good agreement was found on the load-deflection relationship of RC element between the estimation using the proposed model and testing data. Combining with the equivalent SDOF (single-degree-of-freedom) analysis method, the dynamic responses of structural elements subjected to blast loads could be more accurately predicted as compared to the common elastic-perfectly-plastic resistance assumption in design guides. The proposed method was further verified with existing field blast testing results. Parametric studies were then carried out to examine the influences of critical design parameters for membrane behaviors including reinforcement ratio, span-to-depth ratio, and restraint stiffness. Last but not the least, based on the proposed analytical method a series of diagrams for modifying the design loading capacity estimated by UFC (Unified Facilities Criteria) design guides without considering the membrane effects were derived for more accurate and easy predictions of loading capacities in engineering applications.

Comparison of seismic and gravity progressive collapse in dual systems with special steel moment-resisting frames and braces

Progressive collapse studies generally assess the performance of the structure under gravity and blast loads, while earthquakes may also lead to the progressive collapse of a damaged structure. In this study, the progressive collapse response of concentrically braced dual systems with steel moment-resisting frames was assessed under seismic loads through pushover analysis using triangular and uniform lateral load patterns. Two different bracing types (X and inverted V braces) were considered, and their performances were compared under different lateral load patterns using the nonlinear static alternate path method recommended in the Unified Facilities Criteria (UFC) guideline. Eventually, the seismic progressive collapse resistance of models was compared to their progressive collapse response under gravity loads. These studies showed that models under the seismic progressive collapse loads satisfied UFC acceptance criteria and limited rehabilitation objective. The structures had better performance under seismic progressive collapse than models under gravity loads because of more resistance, ductility, suitable load redistribution, and more structural elements that participated in load redistribution. Furthermore, despite studies on progressive collapse under gravity loads, the dual system with X braces showed better progressive collapse performance (more resistance, residual reserve strength ratio and ductility) under seismic loads than the model with inverted V braces.

Progressive collapse assessment of precast prestressed reinforced concrete beams using applied element method

Due to the repetitive progressive collapse events, it became necessity to form theories of designs against those cases of loadings. Prestressed reinforced concrete elements become widely used in construction field due to various properties that contribute to enhancing the overall structural stiffness, increasing the loading capacity, and elevating the crack resistance threshold compared to the non-prestressed elements. However, few researches in prestressing members and its capacity towards progressive collapse is covered. According to the Unified Facilities Criteria (UFC) and the General Services Administrations (GSA) guidelines, the pre-stressed precast structure’s assessment under progressive collapse are taken into consideration using approximate solutions for analysis. The UFC stated the types of the structure under progressive collapse assessment regardless of the structural system used. This is a great challenge needs a lot of experimental and numerical testing and validation. In this research, a numerical analysis is carried out for a typical five-story framed prestressed precast reinforced concrete structure subjected to column loss (corner column, edge column and internal column near to the structure’s edge), and designed according to Precast/Prestressed Concrete Institute (PCI) and (ACI 318−14). Non-linear dynamic analysis for the structure was carried out using Extreme Loading for Structures (ELS) software depending on the AEM. The investigation was done on two levels. Total collapse took place in case 1. As a result, the necessity of extending the research to include case (2) is essential for having an overview assessment of this type of structures. Case 2 showed high capability to resist progressive collapse against all column removal scenarios. The results are indicated in terms of; prestressed beam behavior, prestressing cable contribution, and flexure and axial loads’ changes with respect to time. Beam and column rotations are calculated and compared to the UFC limitations to assess its safety towards progressive collapse.

Assessment of SMC Frames under Different Column Removal Scenarios

Throughout the past decades, failure of structures threatening the lives of humans had been popular whether through structure failure due to human error such as Hyatt Regency walkway collapse, 1981, terrorist attacks on the American embassy attack in Nairobi, Kenya 1998 and the famous 9/11 attacks in 2001 and many more. As a result of these incidents, The Unified Facilities Criteria (UFC) was developed concerning the progressive collapse issues by analyzing different types of structures under column loss and studying the overall structural behavior. However, the (UFC) didn’t scope on the local behavior of the structural components and its connection under column loss. In this research, the main objective is to study the local behavior of the special moment frame connection (SMC) under column loss. A detailed study is conducted on a 3D model fully designed by adopting the strong-column weak-beam approach following the ACI318-14 regulations. Two frames are selected from the designed structure, interior and exterior frames, to apply the column loss scenario in different locations and different floor levels. The Applied Element Method is adopted in the study. Non-linear time-dependent dynamic analysis is implemented to apply the different column removal scenarios. Twelve case studies are modeled in detail using the Extreme Loading for Structures (ELS) software at which all elements are modeled and analyzed in a 3D model technique. After analyzing the different case studies, structure behavior is observed. Some cases encountered total collapse, other cases encountered partial /local collapse and finally, some survived the column loss scenario. Many parameters are involved and studied in the research. Failure pattern is observed for collapsed cases, the cause of failure is monitored and studied. Special moment connection behavior is studied concerning the shear connection capacity. The location of the column removal with the type of frame selected played an important role in changing the structural behavior from one case to another. As a result, it is not applicable to assume that due to the special moment connection ductility, the structure will be able to resist the column loss in all cases.

Effect of Steel Bracing in Progressive Collapse of Multi Storey RC Buildings

Rapid increase in manmade and natural disasters can lead to structural instability which triggers the partial or complete collapse of the building. These manmade disasters include gas cylinder explosion or terrorist attack or instant removal of the primary structural element which leads to the progressive collapse of the building. To avoid the progressive collapse of building due to man-made disasters, incorporate the mechanism which resists the man-made disaster while the design for existing or newly constructing building. In this paper, an attempt is made to understand the effect of with and without providing steel bracing at various positions column removal in multi-storey building one at a time as per General Service Administration and Unified facilities criteria codes. The propagation of progressive collapse with and without the effect of steel bracing is studied in this paper. A seven-storey RC building is modelled and analysed for various column removal scenarios in SAP 2000 software. The Loads are applied according to General Service administration and Unified facilities criteria guidelines, column removal positions namely centre, edge and corner column removal at ground floor are also considered from guidelines. The three scenarios are then analysed individually as a bare frame, bare frame with steel bracing in the wall and bare frame with steel bracing in the slab. Push down analysis is carried out on models and results of mode shapes, time periods, force Vs displacement curves, plastic hinge deformation are discussed. From results, it is observed that wall bracings have better performance in all three and can help in arresting the progressive collapse from spreading throughout the building.

Progressive collapse assessment of post-tensioned reinforced concrete flat slab structures using AEM

Progressive collapse is defined as the failure of a primary vertical element of a structure, which may result in the failure of adjoining elements, consequently, leads to a partial or total collapse of the structure. Unified Facilities Criteria (UFC) and General Services Administration (GSA) guidelines used to assess the behavior of structures subjected to progressive collapse as a result of the loss of primary vertical support. For the current case study, numerical analysis is carried out for a typical ten-story reinforced concrete post-tensioned flat slab structure subjected to primary vertical element loss (corner column, edge column, internal column, edge shear wall & internal shear wall). The structure is designed according to (ACI 318-14) and its progressive collapse is assessed according to UFC guidelines. Non-linear dynamic analysis for the structure is carried out using Extreme Loading for Structures (ELS) software. The Applied Element Method (AEM) is used to create a 3D model to assess the structure’s progressive collapse behavior as a result of primary support removal. Evolution of axial forces in columns, principal stress contour for slabs and tendon’s prestressing losses are the parameters used to investigate the post-tensioned flat slab progressive collapse as a result of support removal.

Progressive Collapse Assessment: A review of the current energy-based Alternate Load Path (ALP) method

The Alternate Load Path (ALP) is a useful method that has generated a considerable recent research interest for the assessment of progressive collapse. The outcome of the ALP analysis can be assessed either using the force-based approach or the energy-based approach. The Unified Facilities Criteria (UFC- 4- 023-03) of progressive collapse guideline - have outlined that the force-based approach can either be analysed using static or dynamic analysis. The force-based approach using static analysis is preferable as it does not require a high level of skill and experience to operate the software plus no effort is required in scrutinising the validity of the analysis results output. However, utilising the static approach will eliminate the inertial effect in capturing the actual dynamic response of the collapsed structure. In recent years, the development of the energy-based progressive collapse assessment is attracting widespread interest from researchers in the field; as the approach can produce a similar structural response with the force-based dynamic analysis by only using static analysis. Most of the current energy-based progressive collapse assessments are developed following the requirements which are given in the progressive collapse guidelines provided by the Unified Facilities Criteria. However, little attention is given to the development of the energy-based approach using the Eurocode standards as a base guideline. This article highlights the merits of utilising the energy-based approach against the force-based approach for a collapsed structure and explains the collapse mechanism of a steel frame in the perspective of the energy concept. The state of the art of energy-based progressive collapse assessment for a structural steel frame is reviewed. The comprehensive review will include insights on the development of the energy-based method, assumptions, limitations, acceptance criterion and its applicability with the European standards. Finally, potential research gaps are discussed herein.

Damage Prediction for an AP1000 Nuclear Island Subjected to a Contact Explosion

The reinforced concrete (RC) shield building is an important defence layer of AP1000 nuclear islands, as it protects the steel containment vessel of the nuclear power plant. Therefore, the safety and integrity of the RC shield building must be ensured throughout the plant’s operational life under all conditions—especially the application of blast loading. This paper presents the prediction formulae for the volume and shape of damage to the RC shield buildings of AP1000 nuclear islands resulting from contact explosion. The coupled Euler–Lagrange (CEL) algorithm and the Riedel–Hiermaier–Thoma (RHT) concrete damage constitutive model were used to numerically simulate the fluid–structure interaction (FSI) between the explosives, the air and the AP1000 nuclear island. The limitations of empirical spall prediction and Unified Facilities Criteria (UFC) 3-340-02 curves for shell structures are discussed. A detailed analysis reveals the relation between the volume and shape of the damage caused to the AP1000 nuclear island and the mass of explosives and concrete strength. It is also found that the lateral stiffness and tensile stiffness are the main factors that affect the damage volume and damage shape resulting from contact explosion, respectively.

Evaluation of the existing triple point path models with new experimental data: proposal of an original empirical formulation

With the increasing use of improvised explosive devices (IEDs), the need for better mitigation, either for building integrity or for personal security, increases in importance. Before focusing on the interaction of the shock wave with a target and the potential associated damage, knowledge must be acquired regarding the nature of the blast threat, i.e., the pressure–time history. This requirement motivates gaining further insight into the triple point (TP) path, in order to know precisely which regime the target will encounter (simple reflection or Mach reflection). Within this context, the purpose of this study is to evaluate three existing TP path empirical models, which in turn are used in other empirical models for the determination of the pressure profile. These three TP models are the empirical function of Kinney, the Unified Facilities Criteria (UFC) curves, and the model of the Natural Resources Defense Council (NRDC). As discrepancies are observed between these models, new experimental data were obtained to test their reliability and a new promising formulation is proposed for scaled heights of burst ranging from 24.6–172.9 $$~\hbox {cm}/\hbox {kg}^{1/3}$$ .