Structures Congress 2018: Blast, Impact Loading, and Response; and Research and Education

A new platform, based on distributed computing concepts, is proposed with the potential to model the physical, social, and economic interactions that occur during disasters. The platform uses lightweight communications and marshalling (LCM) libraries for message passing and data marshalling thereby enabling various types of simulators to interact together. Each simulator is discipline specific, represents one part of a disaster scenario, and is viewed as a black box that subscribes to data from other simulators and/or publishes its results for other simulators to use. The new LCM framework is versatile and scalable and, as such, enables the developed system to grow through addition of new simulators provided by other users. The new concepts are demonstrated through an introductory case study that is focused on the simulation of wind-induced progressive damage due to both the structural response of the building as well as the external wind pressures.

Composite transmission towers are hybrid systems that have been introduced to practice. Being composed of the most two used materials, structural steel and concrete, composite transmission towers present many advantages such as: economy, rigidity, fire resistant, durability, high stiffness due to the use of concrete; high strength, high ductility, easy to assemble, and fast to erect due to the use of steel. Recently, composite columns in transmission towers have been an alternative to steel transmission towers worldwide, especially in USA and Europe, yet little research has been performed in this field. Being composed of structural steel and non-homogenous concrete makes the bevahior of these structures challenging to understand. Therefore, this study presents the analysis and mechanical behavior of composite transmission towers. Three-dimensional non-linear finite element models were built using ABAQUS to compare and validate the behavior of the towers and materials against previous experimental work. The models include detailed analysis and investigation of the concrete and steel behavior up to failure using concrete damage plasticity (CDP) and bilinear elasto-plastic model respectively. Further extensive study was performed using different load cases to predict the behavior of bonds and load transfer between concrete and steel in composite transmission towers. Good comparison between the FE results and the experimental work has been achieved which lead to studying and investigating multiple forms of composite transmission towers.

This paper will present a feasibility study to show applicability of high-fidelity physics-based (HFPB) simulations to screen performance of breakaway devices for highway lighting poles. Collision of two types of vehicles with standard lighting pole configuration is simulated using LS-DYNA, an explicit, large-displacement finite element program that is the de-facto industry standard for crashworthiness evaluation. The paper discusses the model used for the evaluation and presents results of the analyses. Lessons-learned and application of HFPB simulations in other fields are discussed in the conclusions and outlook section.

This paper presents a new beam-column element model based on stress-resultant plasticity and damage mechanics for the collapse simulations and damage assessment of steel columns subjected to a variable axial force. The element formulation adopts a resultant-plasticity model with linear kinematic and isotropic hardening and utilizes a yield envelope to account for the axial-flexure interaction. The element model adopts a recent 1D damage hysteretic model to describe the strength and stiffness deterioration in the column response. The proposed element is able to capture the main characteristics of steel columns with softening response, including the strength and stiffness deterioration under monotonic and cyclic loading, the more severe strength deterioration under high axial compression, and the non-symmetrical flexural response under a variable axial force. The excellent accuracy is confirmed by the agreement with experimental results from a collection of steel column specimens under monotonic and cyclic loading. A case study of a multi-story steel special moment-resisting frame under a strong ground motion further showcases the capabilities of the proposed model. The dynamic response of the archetype structure suggests that if element damage is not accounted for in the structural model, both global collapse behavior and local element response are inaccurately described.

Recent advancement in ultrasonic technology has led to the ability to directly determine stress (strain) in steel members through measurement of the acoustic birefringence. Birefringence is differences in the velocity of orthogonally polarized shear waves. The measured birefringence is proportional to the stress in a steel plate, and thus the magnitude and orientation of the applied stress can be deduced from the ultrasonic velocity measurements. Direct measurement of stress via ultrasonic waves provides a potential solution to the existing structural evaluation limitations. Because the total value of stress can be determined, the existing dead and any additional live load will be measured. The value of the total stress can be compared to the known stress-strain relationship of the steel to determine how close to failure that component may be, or whether the measured amount of stress in accordance with that predicted through analysis. Additionally, the ultrasonic measurement of stress is nondestructive, portable, and fast allowing it to be used quickly and easily. This research looks specifically at the application of this technology to W-flange steel sections commonly used in buildings under axial loads. The research sought to determine the variation in constants used to convert the ultrasonic readings to stress measurements. The values of B_o and m₁ for two samples from different mills differed by 8% and 16% respectively. The comparison between the predicted and actual stresses of three tests demonstrated that ultrasonic stress measurement (USM) using the procedure described could accurately predict the stress in the member to 13.8 MPa (2000 psi).

Current vibration based-structural health monitoring strategies have relied on vibration measurement methods using affixed accelerometers as a standard practice, but a fundamental limitation of this approach is that these sensors only provide point measurements. This low spatial resolution ultimately limits the effectiveness of structural monitoring and damage localization methods that rely on changes in global response to identify local damage features. To overcome these limitations, this paper focuses on the use of a high-speed imaging approach for measuring full-field vibration data for damage identification and localization in engineering structures. The framework is based on the design of a novel monitoring system that leverages a high-speed imaging system for capturing vibration response in a non-contact manner. A digital image processing algorithm is used to reconstruct full-field structural vibration response, which, in turn, enable modal properties of structures to be identified via a novel modal identification scheme. Processing the full-field modal data, the current state of the art in damage identification is extended to identify damages without in-place sensors. A series of experimental tests were performed on small-scale specimens to explore the potential of the proposed method. The preliminary study demonstrated the capability of the proposed approach to localize damage from non-contact and full-filed measurement.

This paper explores a cyber-physical systems (CPS) approach to optimize the design of rigid, low-rise structures subjected to wind loading. The approach combines the accuracy of physical wind tunnel testing with the ability to efficiently explore a solution space using numerical optimization algorithms. The approach is fully automated, with experiments executed in a boundary layer wind tunnel (BLWT), sensor feedback monitored by a computer, and actuators used to generate physical changes to a mechatronic structural model. The approach was demonstrated for a low-rise structure with a parapet wall of variable height. A non-stochastic optimization algorithm was implemented to search along the domain of parapet heights to minimize both positive and negative pressures on the roof a of a 1:18 length scale low-rise building model. Experiments were conducted at the University of Florida Experimental Facility (UFEF) of the National Science Foundation’s (NSF) Natural Hazard Engineering Research Infrastructure (NHERI) program.

Earthquake-induced loss of functionality and downtime in individual buildings can cause major economic and societal impacts, ranging from displacement of individual families and businesses to disruption of emergency medical services and government functions. The ability to predict recovery times for buildings after an earthquake is a critical component in understanding and mitigating the seismic risks facing a community. The seismic performance assessment methodology proposed by FEMA P-58 created a framework that translated structural performance into metrics that would enable stakeholders to make risk-informed decisions. Upon that framework, the REDi downtime assessment methodology was created to convert repair times from a FEMA P-58 assessment into building downtime through realistic labor allocation, consideration of delays to initiation of repairs, and calculation of intermediate goals along the path to full recovery. Since its initial development in 2013, the REDi downtime assessment methodology has been implemented in several projects. From these project and research applications, further improvements have been made to the previously published methodology. These improvements, key takeaways, and remaining challenges are discussed.

Coupling between flexure and shear responses affects the global behavior of reinforced concrete shear walls subjected to cyclic earthquake loading. A recent version of the multiple vertical line element model (MVLE) is shown to capture shear-flexure interaction for concrete structural walls, which often serve as the essential (and/or sole) lateral load-carrying element in buildings. Assessment of the predictive potential of such numerical tools in the context of global response and particularly with regard to capturing the shear component of response is needed. Moreover, the prediction of damage to the shear wall, which can be associated with the evolving crack pattern, may help in the design of nonstructural components, such as anchoring systems. In this regard, the numerical simulations presented in this paper demonstrate the reasonable predictive capability of the MVLE models, notably without an ad hoc tuning of the models’ material parameters. The models’ predictive response is assessed with regards to global force-drift response, global crack pattern, and residual crack widths.

In different high seismic regions around the world, post-earthquake reconnaissance has shown that nonductile concrete frame structures are much more susceptible to collapse than modern code-conforming frames. Therefore, for this type of structures, it is necessary to accurately model materials and members to capture the flexure, shear, and flexure-shear failure modes in members and the potential collapse of the structure. In this paper, alternative retrofit methods are evaluated for these older frame buildings using a probability-based framework, based on nonlinear dynamic cloud analysis, in order to assess the structural performance and safety at each chosen performance level. As a case study, the longitudinal frame of an existing building is modeled, including the effect of flexural-shear-axial load interaction and the longitudinal bar slip deformation component in order to be able to capture column shear and axial failures. The critical demand to capacity ratio, corresponding to the component or mechanism that leads the structure closest to the onset of limit state (e.g., near collapse), is adopted as the structural response parameter. This structural response parameter, that is equal to unity at the onset of limit state, can encompass both ductile and brittle failure mechanisms. It can also register a possible shift in the governing failure mechanism with increasing intensity. Finally, the estimates of expected life cycle cost are compared for the retrofit methods considered in this research.

Particular emphasis of this study is placed on modeling the panel-to-panel connection. As such, there is a need to evaluate the connection behavior which influenced the connection design shift of non-prestressed deck-to-deck panels using aramid fiber reinforced polymer (AFRP) bars. Due to the non-ductile behavior of the AFRP bars, the vertical displacement at each connection (seam) due to various loading cases is analyzed as a non-linear system. A numerical model of a bridge deck panel-to-panel seam is developed, using COMSOL5.2 a software, and validated by previous published experimental data of similar strip beam tests partitioned from an 18 ft by 16 ft bridge deck with embedded reinforced and non-prestressed longitudinal AFRP bars used in joint. Harmonic forced vibration analysis and strength degradation of each connection are examined for two different cases, with and without a shear plate, to improve the ductility at the member level. The results show that the overall structural performance can be enhanced using shear-plate connections in panel-to-panel connections to increase ductility.

The ability to simulate the nonlinear dynamic response of RC buildings up to significant levels of component damage is of fundamental importance to evaluate their expected performance for different levels of seismic hazard. In the United States there is an increasing reliance on the provisions in the ASCE-41 standard for developing models to simulate the nonlinear response of structures during strong earthquakes. This paper presents fragility relationships from incremental dynamic analyses performed with ASCE 41 modeling parameters on a non-ductile reinforced concrete building located in Van Nuys, California, that was instrumented during several earthquakes. Analyses were performed for the ground motion sets in FEMA P695 and ground motions from earthquakes recorded at the building site. Results from incremental dynamic analyses are used to evaluate modeling parameters and acceptance criteria in ASCE 41 as well as the calculated intensity measure at collapse.

This research focuses on new shear transfer model for reinforced concrete membrane elements. The formulation is based on the recent work on aggregate interlock carried out at University of Toronto. The model is further adapted for displacement-based smeared-crack analysis based on a fixed-crack approach. The proposed model fully couples axial–shear responses of the crack, providing accurate descriptions of the crack dilatancy process due to aggregate overriding under cyclic shear. An important aspect is the definition of the aggregate contact surface, which is used as a criteria to determine when the axial-shear coupled model has to be applied. The formulation is implemented into a four-node isoparametric finite element and validated against uniaxially reinforced panels under cyclic and reversed cyclic tension-shear loading.

Movable bridge structures are challenging for owners and engineers to maintain and keep in good working order due to the typical complexity of the structural system, incorporation of machinery, and the many movement mechanisms included in such a structure. One property of movable bridges that is of particular concern to those who maintain them is the balance between the span and the counterweights. This is especially true in older bridges where many years of construction and rehabilitation projects may have added undocumented weights to the structure, removed portions of the original structure, and/or altered the mass of the counterweights. Maintaining proper balance between the span and its counterweights is crucial to the service life of the motor and other machinery that drives the spans open and closed. There are few documented cases where the state of balance of movable bridges is assessed in real time to provide immediate feedback to the operators and owners of the structure. One might argue that the balance need only be assessed after a construction project where it is known ahead of time that weight will be added or removed from the span. However, the distribution of balance is not expected to be constant over time as temperature, live load effects, and the operation of the span could alter this distribution. Traditional testing methods assess the overall state of balance typically in a discrete fashion and require a significant amount of post-processing to provide interpretation. This presentation will discuss an application where a structural health monitoring (SHM) system was designed and deployed on an 85-year-old, 540-foot vertical lift span to capture the continuous system balance at all four corners of the truss when the span was seated and provide real time assessment of the state of balance of the bridge, and its distribution, to key personnel. A framework will also be presented for determining the state of balance for movable structures in general, as well as a discussion on lessons learned and critical challenges faced.

This paper provides a novel method to quantitatively assess the resilience of communities at various scales. The proposed method is based on the PEOPLES framework and it takes an indicator-based approach as an engine for its algorithm. PEOPLES is a framework for identifying the different resilience aspects of a community and for providing new ways through which the decision makers can take actions. The framework comprises seven dimensions, each of which is the collection of more specific components and indicators. Each indicator is accompanied with a measure allowing the analytical computation of the indicator’s performance. The measures are presented in the form of continuous functions whose parameters can be analytically obtained. The output of the methodology is a performance function for each indicator and a resilience index for the whole community. A case study illustrating the application of the methodology is also provided in the paper.

Although it is well acknowledged that the severity of earthquake ground motions (GMs) can be categorized considering different features of GMs, non-spectral aspects of GMs are not explicitly considered in seismic design requirements. This study examines the impact of neglecting non-spectral parameters of GMs on the intensity- and risk-based structural responses. The structural responses of three steel moment resisting buildings in terms of maximum inter-story drift ratio and maximum floor acceleration are compared when subjected to different ensembles of GMs, which are selected based on the generalized conditional intensity measure. It is found that neglecting non-spectral features of GMs could induce a bias up to 50% in the structural responses depending on the considered building and damage metric. In addition, it was observed that ignoring non-spectral features of GMs in the GM selection process can lead to an overestimation in the estimated probability of collapse. In overall, the findings confirm that non-spectral GM features are important parameters to be incorporated in GM selection process for both intensity- and risk-based seismic assessment.

This paper presents an investigation of possible disproportionate collapse for a nine-storey flat-plate timber building, designed for gravity and lateral loads. The alternate load-path analysis method is used to understand the structural response under various removal speeds. The loss of the corner and penultimate ground floor columns are the two cases selected to investigate the contribution of the cross-laminated timber (CLT) panels and their connections, towards disproportionate collapse prevention. The results show that the proposed building is safe for both cases, if the structural elements are removed at a speed slower than 1 sec. Disproportionate collapse is observed for sudden element loss, as quicker removal speed require higher moments resistance, especially at the longitudinal and transverse CLT floor-to-floor connections. The investigation also emphasises the need for strong and stiff column-to-column structural detailing as the magnitude of the vertical downward forces, at the location of the removed columns, increases for quicker removal.

In this paper, progressive collapse performance of an existing multi-story building has been investigated through field experiments and computational simulations. The masonry load-bearing wall structure, Nosker House was located on the Ohio State University campus in Columbus, Ohio. During the experiments, four exterior first-story load-bearing walls were consecutively removed from the building, which was demolished immediately after the experiment. Due to lack of full-scale building test data, the data produced in this research has been a valuable addition to the state of knowledge on progressive collapse assessment of masonry buildings. The main goal of field experiments was to simulate the structural dynamic and static response of buildings that may experience collapse after sudden loss of walls. Another objective was to investigate how the internal forces are redistributed within the building after each wall is removed. In this study, three-dimensional model of the building was developed and analyzed to simulate the building performance and collapse potential. Computational models and simulations were examined and compared to the experimental data from the field tests. This study showed robustness of existing buildings and contribution of different structural components to collapse resistance.

The performance of cementitious materials against high-speed hard projectile is a complex problem as compressive shock waves followed by reflected tensile waves cause brittle tensile failure and spalling. The conventional finite element models result in inaccurate response prediction due to their incapability of handling very large deformation of the elements. This issue could be addressed by introducing element erosion or smooth particle hydrodynamics (SPH) methods. In this study, the structural performance of steel-concrete-steel (SCS) sandwich and reinforced concrete (RC) panels under high-speed missile impact are assessed using a commercial finite element analysis package. These panels are frequently used in nuclear structures and they are expected to maintain a sufficient level of safety under extreme loading scenarios such as airplane crash. The two configurations for panels are compared in terms of penetration depth and rear deformation and validated with the experiments and empirical equations. Local and global failure mechanisms including bending, punching, and penetration are studied. Strain rate effect for the concrete and steel parts is considered via a dynamic increase factor and rate-dependent constitutive models. Several criteria for element failure for the element erosion approach are tried in order to accurately capture the penetration depth. Furthermore, the accuracy of the SPH method in predicting the penetration depth is investigated and compared with the element erosion method. The advantages and modeling issues for each method are discussed.

Building envelope is the most critical line of defense to protect structural components and occupants against external explosion threats. Much research has been done, and design methods have been developed and verified, for exterior building wall systems under blast loads. Limited research exists for roof truss systems under blast loading, and the resistance of roof trusses to failure are not well-defined. In addition, the blast load on a roof system varies with space and time, which makes it more complex compared to that experienced by building walls. Current design methods calculate the response based on equivalent blast roof loading. In this paper, the dynamic response of roof system under blast loading will be predicted numerically using the actual blast load on the roof and compared to the dynamic response calculated based on the equivalent blast roof loading. The dynamic predictions will be verified using full-scale field experiments using live explosions. In addition, the static resistance function of open-web steel joist roof trusses will be developed using full-scale experiments and numerical simulations. The verified numerical models will be used to analyze the dynamic structural response of open-web steel joist roof truss system under blast loading.

Now a days, concrete filled steel tubes are being used in building construction owing to the faster rate of construction, reduced labor requirements, and less material costs. Accidental or intentional vehicle collision with building columns can result in significant damage to these building columns and potentially lead to partial or full collapse of structure. Also the steel tube exposed to atmosphere will be subjected to corrosion. This will further increase the probability of failure. In this study, multi-hazard analysis in the event of corrosion and vehicular impact of concrete filled steel tubes is carried out. A detailed finite element (FE) analysis is conducted in this current study using the LS-DYNA package. FE models for truck is used from National Crash Analysis Center and the National Transportation Research Center. The concrete filled steel tube models are first validated with the experimental results by comparing the impact force time history and maximum deflection. To account for corrosion, the material loss in steel tube is first considered uniform throughout the height of column. Secondly, stochastic fields for steel tube thickness are generated to account for non-uniform corrosion along the height of column. The impact analysis is performed for different truck impact velocities. The vulnerability of the concrete field steel tube columns under the expected impact loads is assessed by calculating impact force and its lateral load capacity at the impact location. A damage index is proposed to under the vulnerability of CFST columns under vehicle impact and corrosion. The analysis results for FE models with and without corrosion are compared. It is believed that the results from this study will help in improving the design of such structural members.

A partial damage caused by an abnormal load could trigger progressive collapse of high rise buildings which may lead to fatal consequences. To meliorate structural robustness, one types of joint with improved connection details–modified bolted connection and their corresponding design methods are proposed on the basis of the conventional RHS column joint with through diaphragm. The experimental results indicate that the modified bolted connection can enhance the vertical deformation capacity of the beam-column assemblages before the force transmission failure at the bottom of connection region, which is conductive to the development of catenary action by axial force in the beams. For that reason, the beam-column assemblages can provide greater vertical carrying capacity and the improved connection details can be used in designing and transforming structures against disproportionate collapse.

In flat plate structures, the slab-column joints are prone to punching shear failure, which would trigger progressive collapse of the entire structural systems. The joints subjected upward punching shear (UPS) and downward punching shear (DPS) actions exhibit different post-punching behavior, which will affect the progressive collapse of flat plate structures. A static collapse test was conducted on two slab-column joint specimens (UPS-1 and DPS-1). The structural resistance and deformation were analyzed to explore the collapse-resistant mechanism of the joints after punching shear. The test results illustrate that the post-punching bearing and deformation capacities are governed by the through column longitudinal reinforcement (including both integrity reinforcement (IR) and flexural reinforcement (FR)) in the slab. The longitudinal bars ruptured in reverse orders under UPS and DPS failure scenarios. In DPS-1, the peak resistance under the suspension mechanism was reached when the first FR bar ruptured while the IR bars remained intact. The peak resistance of UPS-1, on the other hand, was achieved at the onset of the IR bars being ruptured and the FR bars being detached from the concrete.

The performance of multi-bay flat slab substructures subjected to the loss of an interior column scenario is investigated experimentally. The influence of multi-column missing on the performance of flat slab substructures against progressive collapse is quantified. The test results including the dynamic load redistribution response, crack pattern, and displacement distribution response. The test results indicated that compressive membrane action could increase the dynamic load resistance of flat slab substructures efficiently. However, when both interior and exterior columns are lost, much larger dynamic response is achieved due to less compressive membrane action is able to develop.

This paper investigates the collapse resistance of three-dimensional steel moment-resisting frame structures. Two static push-down analysis are conducted experimentally using 1/3 scale one-story steel frame structures. A bare steel frame structure without slab system is firstly tested to study its behavior subjected to loss of the interior column, following by a second experimental test on an identical frame with reinforced-concrete slab system under the same column loss scenario. The overall behavior and the failure modes of the two structures are recorded and compared. The contribution of the RC slab to the collapse resistance of the system is also explored. The test results show that both substructures exhibit remarkable robustness to prevent the occurrence of progressive collapse under column loss scenarios. The presence of the RC slab system can increase the initial stiffness and load carrying capacity of the structural system substantially.

After several progressive collapse incidents, many researchers try to find the way to mitigate progressive collapse of building structures. In case of the failure of column, beam, and beam-column connection, the falling impact of failure members and debris from upper floor is an important reason to result in progressive collapse. In this paper, steel frames with five types of beam-column connections, i.e. fin plate connection, welded unreinforced flange-bolted web connection, reduced beam section connection, reverse channel connection with flush end plate, and reverse channel connection with extended end plate, were tested under impact loads. The impact tests were carried out by using high-performance drop weight machine. All impact loads were applied at the beam end. The main objective of the experiments is to study the impact resistance of steel frames with different beam-column connections subjected to falling floor impact. Failure mode, input kinetic energy, and residual displacement of each specimen were obtained and compared. The test results show that fin plate connection has the worst impact resistance. Both WUF-B connection and RBS connection have better impact resistance than fin plate connection. Different failure modes appear on reverse channel connection with flush end plate and reverse channel connection with extended end plate. Reverse channel connection with extended end plate dissipates more energy than it with flush end plate.

Extensive studies had been carried out to understand the load resisting mechanisms and load re-distribution behavior of reinforced concrete (RC) frames to resist disproportionate collapse. It is found that RC frames with irregular layout or longer design span have high vulnerability for collapse. Thus, it is necessary to look for measures to improve the robustness of the frames against disproportionate collapse. Post-tensioning strand (PTS) is one of the strengthening approaches. Thus, a series of experimental and analytical studies are carried out to quantify the efficiency of PTS for upgrading the load resisting capacity of RC frames. The test results including load-displacement curves, deformation shape, and local strain are captured. In addition, the load resisting capacity of PTS in different load carrying stages is evaluated. The change of the failure modes due to externally installed PTS is also determined.

In reinforced concrete (RC) structures, the sudden failure of a column is likely to cause severe progressive collapse due to the relatively weak tie force from the surrounding elements. Therefore, some effective strengthening measures can be used to improve the progressive collapse behavior of the structures. Fiber reinforced polymer (FRP) has been widely used in retrofitting the RC structures because of its advantages such as high strength, light weight, corrosion resistance, etc. Thereby, in this paper the progressive collapse behaviors of the glass-FRP (GFRP) strengthened RC substructures subject to a column-removal scenario were studied. A quasi-static experimental method was adopted to test the progressive collapse behavior of the substructures including control specimen and strengthened specimens. The influences of the strengthening schemes on the behavior of the tested specimens were studied by comparing all the test results. The experimental results demonstrate that the strengthening technology can enhance the progressive collapse resistance of the RC structures.

Falling debris imparts a dynamic loading on the floor below. A simplified estimation of this loading depends on the amount of kinetic energy transmitted from the upper floor during impact. The kinetic energy of the impacting floor that is transferred to the impacted floor is first theoretically determined for the two extreme impact possibilities, namely fully rigid and fully plastic impact. Estimation of the falling debris loading is carried out for a reinforced concrete flat-plate system and compared to an earlier analysis using a steel framed composite building. Theoretically, a plastic impact results in an amount of energy imparted to a lower floor of 33% and 37.5 % of the initial kinetic energy of the falling floor for the reinforced concrete flat plate building and composite steel-concrete framed building, respectively. For a rigid impact this value is between 44%–97% for reinforced concrete flat plate and 41%–98% for steel framed building. The relative impact loads and ability of the impacted floors to arrest the progressive collapse is also analyzed. For both the concrete flat plate buildings and steel frame, the impacted floor is highly unlikely to withstand an impact from the floor above. The analysis of the flat-plate system includes a detailed look at how the dynamic loading can change the strength and ductility of the slab-column connection.

In reinforced concrete (RC) frames, horizontal progressive collapse (H-PC) may be triggered due to insufficient lateral stiffness of the structural systems. Current research studies have been mainly focused on vertical progressive collapse (V-PC) rather than H-PC. To analyze the effect of lateral stiffness and capacity on the behavior of progressive collapse (PC) of RC frames, a static collapse test of two asymmetric one-story beam-column substructure specimens with flange slabs was conducted. The two specimens S1 and S4 were subjected to an interior column removal scenario, in which S1 has one edge column and S4 has an edge and a penultimate column. The test results show that under the compressive arch action (CAA), S1 developed a smaller peak resistance and a larger vertical displacement as compared to S4 due to insufficient horizontal restraint of the columns. In addition, the contribution of the CAA within the beams and slabs in S1 was significantly reduced. Under the catenary action (CA), the collapse resistance of S4 developed rapidly and eventually exceeded the peak resistance of CAA; while the resistance of S1 remained almost constant, caused by the continuously weaken horizontal constraint provided by the edge column. Additionally, the final failure mode for both specimens was found to be in form of the compression-bending failure of the edge and penultimate columns.

Despite advances in design methods and building code development, collapse still occurs in structures considered to be “safe” and code-compliant. These occurrences have led the structural engineering community to seek “robust design” solutions to mitigate disproportionate collapse from an initial local failure event. While compounding uncertainties from life-cycle demands and material behavior negate the ability to eliminate the possibility of structural failure, an acceptance of structural damage allows one to mitigate such an event from propagating. The acceptance of structural damage requires a means to limit loss to an acceptable level. Prediction of structural elements’ performance under collapse debris impact is a critical step in estimating the extent of expected loss due to local failure. The uncertainties associated with structural collapse debris demand that analysis methods be flexible and not cumbersome. It is beneficial to employ methods that can be expeditiously used, while accounting for complexities associated with highly nonlinear and dynamic material response. An overview of such analysis methods is provided with accompanying validation against experimental test data. The benefits, drawbacks, and limitations of each method are discussed and examples are provided to demonstrate how each can be implemented in a design setting.

Reinforced concrete (RC) beams against lateral impact loading usually undergo a local response phase and an overall response phase. In view that the RC beams are prone to suffer shear failure in the first stage, the aim of this work is to investigate the transient response and the mechanism underlying the RC beams subjected to impact loading in the local response stage. A refined FE model has been established by using LS-DYNA. Parametric studies are conducted to investigate the effect of the impact velocity, the impact mass and the span-depth ratio on the transient response of RC beams. Numerical results show that there is a big difference between the impact phenomena of RC beams against impact loading and static loading, such as the negative bending moment and the reverse reaction which may have great effect on the internal force distribution. A physical model and an empirical formula are proposed to establish the resistance mechanism and the internal force distribution of RC beams under impact loading, which are in good agreement with the experimental and simulation results.

The research focuses on the dynamic behavior of bridge components under various loading scenarios and effectiveness of possible protective measures. In order to accomplish this, a 3D numerical bridge model is being built by using ANSYS/LS-DYNA considering one simply-supported girder bridge as the prototype. The influence of essential parameters including section shape of components, equivalent amount of TNT, explosive locations are studied to comprehensively explore the dynamic behavior of bridge components subjected to blast. Effective anti-explosion measures including increasing reinforcement ratio, adding FRP layer and foamed-aluminum sacrificial layer have been put forward and comparative analysis has been done. Results show that damage condition of bridges under blast mainly appears in the form of local damage. Response of bridges with various section types basically coincides with each other for girder but of obvious difference for pier. As the amount of TNT increases, damage area and displacement response for girder and pier both increase. Foamed-aluminum sacrificial layer and FRP layer could be adopted to reduce the damage condition for girder and pier. Results in the paper show the sensitivity of different bridge components to blast load and relative effectiveness of possible protective measures.

The stability of steel columns that have suffered localized web damage is studied through analytical and computational means. Local web damage can result from either intentional or unplanned events. Recent retrofitting methods to repair column splice welds in pre-Northridge buildings have involved cutting a large portion of the web out. Web damage in steel members exposed to the elements can also occur due to corrosion where the material is weakened to the point of no longer being able to carry the load. While several papers address the issue of holes being intentionally placed in the webs of steel beams, this work does not extend to steel columns where reductions in load carrying capacity could result in catastrophic failure of a structure. This work can aide practicing engineers in assessing the safety of damaged structures.

In the case of a confined explosion, the resulting pressure signals are very complex. Ability to predict the pressure-time history with high fidelity is mandatory for any attempt to properly predict the dynamic structural response of the envelope panels and assess the damage caused by the confined explosion. The present research program comprises experimental, analytical, and numerical work. The experimental part included full scale tests of TNT confined explosions, carried out in a single-room structure. This study yielded high quality pressure signals that were used as a data-base for comparisons with our computational results. The analytical part focused at developing an extensive theoretical model for the prediction of the blast load, mainly characterized by the peak gas pressure and its decay with time. The developed model for the peak gas pressure prediction is based on the ideal gas equation of state. Moreover, a simplified method for calculating the gas pressure decay, based on Bernoulli theory, was developed for a partially confined explosion. The derived formulas are found to yield gas pressure predictions that agree well with our experimental results and with experimental results available in the open literature. The experimental results were compared with numerical calculations performed using AUTODYN hydro-dynamic code, aiming at simulation of the experimental test cases. It clarified the behaviour of the blast waves, the gas pressure, and the effect of the detonation products afterburning on the acting pressures. According to the analytical work, that had been done within the framework of this multistage study, the afterburning energy release, as a result of exothermic reaction of the detonation products with the oxygen in the air, helped to improve the numerical simulations. The objective of the present paper is to overview the accumulated contributions to the subject matter and present the overall understanding that has been gained as a result.

In this paper, the impact behaviour of sandwich panels constructed of flax fibre-reinforced polymer (FFRP) facings and closed cell polyisocyanurate foam cores is studied. A total of 27 sandwich beam specimens (1200 mm long and 150 mm wide) made of 75 mm thick foam have been fabricated. As a part of this study, three of these specimens have been tested under impact load at several energy levels. The main test parameter is the facing thickness (one, two, or three layers of FFRP). A bidirectional flax fabric (400 g/m²) was used for the face sheets and a foam with a density of 64 kg/m³ was used for the core. A bio-based epoxy resin, with an approximate bio-content of 30%, was used to make the FRP facings. Each specimen was tested multiple times increasing the kinetic energy until failure. The kinetic energy was increased by first increasing the drop height and then, if necessary, adding additional weight to the impactor and resetting the height to keep the increase in energy constant. The acceleration of the impactor, the drop weight velocity, and the top and bottom facing strains at mid-span will be measured. It is expected that ultimate energy absorption and strength of the specimens will increase as both the core density and facing thickness increase. The aim of this paper is to provide data to the field of study, provide insight into the structural behavior of these types of structures, and show their viability for use in infrastructure. With further research, there is the potential that these types of structures could be included in structural building codes. This research is part of a larger on-going study and more results will be available at the time of the conference.

Adobe construction is becoming popular in the American Southwest, but has always been used for construction in many parts of the world. When U.S. personnel move into an area, they often will temporarily occupy housing that is made out of adobe; therefore, it is necessary to understand the characteristics of such materials and building systems under blast. Therefore, the objective of this work is to characterize the mechanical properties of adobe to be able to model its response as a material type in structural elements. This research focused on evaluating the static resistance function of adobe walls, which was used for blast response calculations and modeling. An experimental program was conducted to evaluate the material and structural response of adobe materials and walls, as well as develop and evaluate possible blast retrofit designs. In this paper, material testing for shear, compression, and rupture properties and walls and wallettes under uniform pressure were evaluated. In addition, measured mechanical properties were used in a 3D finite element model and compared to the experimental results of the full-scale adobe wall systems. Blast response predictions and numerical models were verified using shock tube dynamic test results, and are presented in this paper.

Ultra-high performance fiber reinforced concrete (UHPFRC) has high compressive strength of about 150–200 MPa, but does not have sufficient ductility requires for optimum performance in protective structures, which are usually subjected to large inelastic deformation unless steel fibers are introduced to the mix. However, it has been shown that incorporating steel fibers complicates the mixing process and disrupts UHPFRC production rate. Nano-modified polymer concrete (NMPC) has a superior ductility, about 3% failure strain as against 0.3% failure strain of normal concrete, appreciable tensile strength, and an outstanding bond strength. This study focuses on the performance of NMPC walls under blast loading using a nonlinear finite element program, LS-DYNA, which has been used in different blast loading simulations, by choosing sufficient time steps, material constitutive models, and numerical integration technique. Hybrid composite concrete panels produced by interlocking elements made of UHPFRC and NMPC are studied with respect the charge weights, standoff distances, composite action of UHPFRC and NMPC configurations, and stress distribution across the wall.

Results from a series of blast tests performed in October 2016 on three two-story, single-bay cross-laminated timber (CLT) structures demonstrated the ability of CLT construction to resist airblast loads in a predictable fashion. These tests were performed on structures without superimposed load to limit inertial resistance. Subsequently, a follow-on series of tests was performed to investigate the response of axially-loaded CLT construction. Panels damaged during the preceding test were removed and replaced. Axial load was applied using precast concrete blocks to simulate the loaded condition of a five-story building at the first-floor front panel of the structures. These test structures were exposed to two shots: the first was designed to keep the structures within their respective elastic ranges while the second was designed to push the structures beyond their elastic limits. Reflected pressure and peak deflections were recorded at the front panels of the test structures to document the two-way panel load distribution behavior under a dynamic load event and the clearing of the shock wave. Prior to conducting the blast tests, a small number of tests were performed on a load tree test apparatus to aid in test planning by investigating the post-peak response of individual CLT panels of various lengths to quasi-static out-of-plane and axial loads applied simultaneously. This paper provides an overview of the results obtained from both the quasi-static and blast tests of axially-loaded CLT. Additionally, the paper compares CLT structure, component, and connection response across the suite of data. Conclusions are offered to assist engineers in the design of load bearing CLT construction exposed to airblast loads.

Because automated construction is a developing construction practice, no current building codes control structural design of printed structures; therefore, many unknowns exist concerning how printed structures respond to different load cases. Here, an analysis of the dynamic response to blast loading of concrete wall sections produced using automated construction techniques is presented and compared to traditional construction techniques such as cast-in-place concrete and concrete masonry unit construction. Wall sections were subjected to simple one-way action, as well as applied vertical stresses to simulate roof loads. These laboratory experiments of simulated wall sections allow for a more complete understanding of when automated construction is an acceptable method of construction for structures potentially subjected to blast loads. In addition, the findings herein indicate areas where refinement of both materials and construction methods can improve upon the response to dynamic loads. Also included is a discussion of the dynamic response of printed concrete beam sections reinforced with reinforcing bars and/or fiber mesh. Furthermore, descriptions of material development and construction methodology are given.

Open web steel joists (OWSJs) are widely used for carrying gravity and environmental loads on roofs and floors. However, when they are subjected to blast loads, it can be a challenge to determinate their ultimate blast capacity, due to limited availability of data on their blast performance. Current blast design criteria for OWSJs appear to be conservative by permitting rather small deformation, considering certain OWSJs appeared to have experienced significant amount of deflection without collapsing after some explosion accidents. Adding to the seemingly conservative criteria, the commonly used simplified analyses techniques tend to over-predict the deformation of OWSJs under blast loads. A blast testing and numerical study was carried out to investigate the blast response and failure modes of certain OWSJs. The results showed that these joists performed better than predicted by simplified analyses using the current response criteria, and the joists did not fail in the more brittle shear mode.

Current design criteria and specifications increasingly require specified charge weight and site-specific standoff distance combinations in blast analysis and design of buildings, as opposed to prescribed uniform loadings. Even so, typical air blast calculation methodologies usually do not account for site-specific phenomena that may affect the air blast wave characteristics for high explosives. For example, environmental variables, clearing effects, or alternate methods of calculation may allow for more efficient and cost-effective blast-resistant designs. A brief review is provided of air blast characteristics for high explosives and factors which can influence blast loads applied to the structure. The authors discuss how variables and alternate methods of determining design blast loads may potentially impact design and provide analytical comparisons of calculated air blast loads and arena explosive test data. Real-world examples from explosive testing are provided for comparison and context and relevant key recommendations are presented for design professionals and researchers.

Designing structures to withstand blast loads can be complex and very costly. The complexity and cost can often be amplified when retrofitting existing buildings. As such, designing blast-resistant buildings often requires a series of compromises and innovative/creative design solutions to balance budget constraints, construction schedules, and architectural aesthetics while meeting the project specific blast criteria. This paper will describe a variety of unique and practical solutions used to retrofit an existing façade system. The design solutions presented complement the existing building design, simplify construction leading to a shortened construction schedule, and minimize the effect on the architectural design. Lastly, the paper will present a novel way to utilize fiber reinforced polymers that could be used in both blast and non-blast applications.

Electrical enclosures are a fundamental part of any process facility. Often referred to as power control rooms (PCR), power distribution centers (PDC), or simply substations, these enclosures host the equipment that distributes, controls, and monitors the electrical power used for process equipment in the plant. Depending on the application, these enclosures typically contain low voltage or medium voltage switchgear and motor control centers. Often a battery room is included to provide DC power for switchgear control or for uninterrupted power systems (UPS) to ensure power supply in cases of standby or emergency. Although these enclosures are not continuously occupied by personnel during typical plant operations, industry design practices indicate that they be protected against potential explosions due to their critical function in the plant during an emergency. ASCE Design of Blast-Resistant Buildings for Petrochemical Facilities (2010) states that critical equipment must be protected to enable safe shutdown and must be designed for low response. Similarly, Process Industry Practice (PIP) Blast Resistant Building Design Criteria (2014) indicates that main electrical stations are classified as “critical/essential” and are to be designed for low response. These design practice documents, however, are limited to specifying ductility and end rotation for structural components of the enclosure, which may not be directly related to the functionality of equipment after the blast event. The paper discusses a real case of electrical enclosure that was subject to vapor cloud explosion loading during the Texas City accident in 2005. The paper illustrates the blast response of typical construction details of electrical enclosures such as cable wall penetrations, elevated floor, air conditioning duct penetrations, rear-access equipment doors, and interior non-structural items such as lights and cable trays. Considerations are also made on the correlation between dynamic excitation and equipment functionality by comparison with existing seismic experience data on electrical equipment published by the Department of Energy (DOE).

This forensic investigation centered around the failure of a vertical weld seam in the first ring of the Pine Hill potable water ground storage tank. This failure resulted in a leak emptying most of the water in the tank. The investigative team was required to research the cause of failure, identify any related structural issues and propose an immediate repair procedure. The investigation revealed that the failure was preventable and outlined specific, documented observations from preceding years which provided clear warning signs. The investigation and resulting repair consisted of the following general steps: step one: identifying the problem, step two: diagnosing the cause of the problem, step three: locating the source of the problem, step four: emergency repairs, and step five: long term repair.

On the evening of July 27, 2012, three raw coal conveyor belt structures simultaneously collapsed, shutting down the processing facilities at the largest underground coal mining complex in North America. Two miners working next to the conveyors heard the structures failing above and fled the area. Two of the conveyors were new and had only been in service for a little over two weeks. The third belt was old and had recently been taken out of service when the new ones became operational. One of the new belts had been operating at a reduced capacity since it was first put into service, but its capacity had been increased on the day of the failure. Telltale signs that the structure had deficiencies had been discovered weeks prior when the belts were commissioned, but had not been comprehensively investigated. Following the failure, the Mine Safety and Health Administration (MSHA) investigated the collapse and found that a bolted connection failed at the end of a cantilever beam that supported a skewed beam. The skewed beam was supporting the trusses for one of the new belts at the top of a shared bent. The bolt failure then led to load redistribution, resulting in twisting of the cap beam and buckling of the outer bent column. Instability at the bent caused both new belts to partially collapse, with one of them falling onto the old one below. This paper discusses the background of the collapse, a description of the conveyors and their supports, the nature of the loading, a summary of the events preceding the failure, post-collapse observations, structural analyses, failure modes considered, a design deficiency identified, and conclusions.

A very large metal building with approximate plan dimensions of 900 ft × 400 ft collapsed following a severe weather event. Our client retained Simpson, Gumpertz & Heger (SGH) to investigate the cause of the collapse. One of the investigation objectives was to estimate the collapse load of the as-built structure. The authors performed a detailed finite element (FE) analysis to predict the collapse load of a typical three-span 400-ft long steel frame of the as-built structure. We used the general purpose FE analysis software, ABAQUS, to perform the analysis. We considered initial imperfections in the FE model in order to capture local buckling and lateral torsional buckling. We also performed sensitivity studies considering different material models, initial imperfection values, lateral spring stiffness values, and boundary conditions, among others. We present a summary of the FE analysis results in this paper, as well as the important lessons learned in the design of ultra-large metal buildings.

On November 4, 2008, an approximately 160-foot tall scaffold collapsed as it was being installed inside of a boiler, causing one fatality and several injuries. The boiler measured approximately 46 by 52 feet in plan, and 178 feet in height. At the time of the incident, the installation of the scaffold was substantially complete. The authors investigated the collapse at the request of the attorneys representing the supplier of the scaffold components. To investigate the collapse, the authors performed field observations; reviewed the scaffold drawings, reports, calculations, relevant codes, and standards; and performed linear and nonlinear structural analysis and calculations. This paper presents the method of the investigation, and provides the key findings of the investigation including the likely cause of the collapse and lessons learned.

Engineered wooden I-joists are efficient and effective structural members that are typically used in residential and commercial floor and roof diaphragms. The American Wood Council has established that wooden I-joists should be used in dry-service conditions. However, the effects of submerging wooden I-joists in water for extended periods of times is not well studied and if joists have been exposed to long periods of moisture, as is typical in flood events, they are often replaced. The replacement of the I-joists often leads to considerable expenses and the replacement of the entire structure. A small-scale study was initiated to begin to investigate the effects of submerging wooden I-joists in water for extended periods. Twelve I-joists were tested: two each were tested dry, and submerged for one day, two days, three days, five days, and seven days. The strength and load-deflection behavior of the I-joists were determined during the testing. It was determined, that regardless of the amount of time submerged, the ultimate strength decreased by approximately thirty-five percent and the bending stiffness decreased by approximately thirty percent.

Countless efforts have been placed in studying if, when and how to employ building information modeling (BIM) into various academic programs and courses. Common topics at conferences and in journal publications around BIM academia seem to focus on construction, architecture, and multidisciplinary applications. An area which is leading BIM usage that has not been explored heavily in academia is structural engineering and BIM. This paper will look at traditional curriculums for structural engineering and propose where BIM could be deployed along with different mechanisms to deploy it. Based on the nature of BIM and core content in structural engineering, there are several key aspects of the technology and align such that it can provide significant enhancement to current programs. They are: analysis and design integration, visualization of solutions and problems, multi-disciplinary engagement beyond structures, and documenting design intent. These areas align strategically with what industry is looking for in future engineers.

As computational power perpetually increases, high-fidelity structural analysis is increasingly used by structural engineering professionals. Just as an undergraduate course on strength of materials complements a course on matrix structural analysis, a typical graduate-level course on continuum mechanics complements a graduate course on high-fidelity finite element analysis (FEA). While high-fidelity FEA is a sophisticated method of structural analysis with a steep learning curve, the underlying continuum mechanics theory is elegant and conceptually simpler than many of the concepts that are already included in the typical undergraduate structural engineering curriculum. Carefully selected continuum mechanics topics, namely, three-dimensional strain, stress, and linear elasticity, have been recently included in undergraduate courses on "Structural Mechanics" and "Advanced Structural Analysis." It is demonstrated that some subtle simplifications are necessary to tailor the pedagogy for an undergraduate audience, and significant "hands-on" time should be devoted to the post-processing of high-fidelity FEA simulations that are relevant to structural engineers. However, with some careful lesson-planning, the key principles of continuum mechanics and high-fidelity FEA can certainly be taught to an undergraduate level, and will provide the undergraduate student with a broader perspective of structural engineering.

Traditional structural engineering pedagogy has consisted of students preparing for class by reading a textbook, followed by a professor giving a lecture, followed by students doing individual homework. Students received feedback in terms of a grade from the professor, and, ideally, the student filed the graded work and possibly reviewed it again before an exam. This process repeats itself as the professor moved from one topic to the next with little consideration given to anyone or anything more being part of the learning process besides the professor, the textbook, and the student. Most faculty would agree that undergraduate students often skip the reading prior to class. Furthermore, studies have shown that almost half of all students do not pay attention to material presented during a lecture. Thus, it is critical for the structural engineering profession, a profession for which small mistakes can lead to death or millions of dollars in repairs, to modify its pedagogy. This past semester, the civil engineering students in structural analysis participated in a cooperative learning technique aimed at improving student learning. Instead of turning in individual homework to the instructor for a grade, students paired up with a peer and checked each other’s work prior to submitting their completed homework. The expectation was that when disagreements were discovered between students, they would discuss with each other where the error or misunderstanding existed and subsequently correct the error. This not only required students to explain the work they completed, but it provided additional contact time with the material. From a faculty perspective, this provided the essential cooperative learning characteristic of positive interdependence because individual student learning increased as the pair improved in their design review. As a student incentive to complete a thorough review, the quality of review counted for at least 10% of each assignment. In addition, each pair of students turned in the assignment together for one grade with work chosen for the instructor to grade separated by work for the instructor to check for completion. In the event one student did not complete the assignment in time to conduct a review with their partner, the individual student’s grade was negatively impacted, providing individual accountability. This cooperative learning was used on seven of the nine homework assignments in the course. At the end of the course, student feedback was collected with five Likert scale questions and two open-ended questions. This paper will make the case that this pedagogy benefits the structural engineering profession by: getting young engineers in the practice of what engineer’s already do (check each other’s work), increasing student understanding of structural analysis, and improving the ability of future structural engineers to communicate their work clearly and effectively.