Global Structural Response Research Articles

In-plane response of masonry infill walls: Comprehensive experimentally-based equivalent strut model for deterministic and probabilistic analysis

It is widely recognised that the presence of infills modifies the global structural response of frame buildings subjected to seismic loads. Despite the substantial research carried out in the last decades, the assessment of the seismic performance of infilled frames remains an open issue due to the complexity of the interaction between the infill and the frame and to the large number of parameters involved. In this study, a data set of 162 experimental tests available in the literature is initially employed to assess the reliability of different models based on the equivalent strut approach. The comparison between predicted and experimental values highlights large uncertainties, especially for the stiffness prediction. A novel model is thus developed, that duly takes into account these uncertainties. The constitutive model is piecewise-linear, where characteristic force–displacement points are clearly defined and the related error terms are estimated. Beside the masonry mechanical characteristics, the model considers different features, such as the presence of vertical or horizontal hollows in the units. An example of application of the proposed model is finally presented. Specifically, the cyclic response of experimental tests not included in the original data set is reproduced by making use of the suggested strut model.

Hydroelastic analysis of global and local ship response using 1D–3D hybrid structural model

ABSTRACTFor relatively ‘soft’ floating bodies (very large ships, floating airports, etc.) using the classical rigid body seakeeping analysis and modelling the hydro-structure interface by the transfer of the pressure is not sufficient. The most appropriate method for rational design is the hydroelastic coupling by solving the interaction between the surface waves and the rigid-elastic body. In this paper, a hybrid structural model based on one-dimensional (1D) beam elements and three-dimensional (3D) finite element elements is developed and used for the analysis of wave induced ship vibrations. Using a hybrid model, both global and local structural response can be determined. The coupling between structural and hydrodynamic model is performed using the modal superposition method. The complete hydroelastic response (motions, accelerations, stresses, …) of a floating unit is performed by using 1D–3D hybrid structural model with 3D hydrodynamic model and compared with full 3D structural model coupled with 3D hydrodynamic model.

Effect of connection modeling on the seismic response of steel braced non-moment resisting frames

Non-moment beam-to-column connections, which are usually referred to as simple or shear connections, are typically designed to carry only gravity loads in the form of vertical shears. Although in the analysis of structures these connections are usually assumed to be pinned, they may provide a small amount of rotational stiffness due to the typical connection details. This paper investigates the effects of this small rotational restraint of simple beam-to-column connections on the behavior and seismic response of steel braced non-moment resisting frames. Two types of commonly used simple connections with bolted angles, i.e., the Double Web angle Connection (DWC) and Unstiffened Seat angle Connection (USC) are considered for this purpose. In addition to the pinned condition - as a simplified representation of these connections - more accurate semi-rigid models are established and then applied to some frame models subjected to nonlinear pushover and nonlinear time history analyses. Although the use of bracing elements generally reduces the sensitivity of the global structural response to the behavior of connections, the obtained results indicate considerable effects on the local responses. Namely, our results show that consideration of the real behavior of connections is essential in designing the column elements where the pin-connection assumption significantly underestimates design of outer columns of upper stories.

Methodological approach for reducing computational costs of vehicle frontal crashworthiness analysis by using simplified structural modelling

ABSTRACTA challenge in the design and optimisation of the vehicle front structures is the high computational costs required for crash analysis. A methodological approach to simplifying finite element vehicle models and crash barriers is presented in this paper. The methodology uses global deformation characteristics of structures which are obtained from the global crash model. For the simplification of the vehicle crash model, structural regions which sustain only elastic deformations during the frontal crash are replaced by kinematic numerical representations which describe both stiffness and load paths at the interface of the substituted structures. Verification studies of the simplified vehicle model show a very good agreement of the global and local structural response during the frontal crash. Further simplifications were applied to the offset deformable barrier by replacing its detailed crushing behaviour by kinematic descriptions. Through the combined use of both simplified numerical representations, the computational cost of a Euro new car assessment program (NCAP) offset crash analysis can be reduced by around 92%. With the obtained time reduction, structural optimisation studies of the remaining structure can be conducted efficiently for the identification of weight reduction potentials.

Performance of highway bridges subjected to blast loads

Since the collapse of the WTC towers in September 2001, concern about the protection of buildings and infrastructures against blast loads has increased significantly. Comprehensive experimental and numerical studies of blast loading effects on buildings have been carried out in recent years, whereas for bridge engineers, blast-resistant design is still a new area which requires separate and systematic investigation. The objective of this paper is to simulate the performance of three modern types of reinforced concrete bridges under various blast loads, including a slab-on-girder bridge, a box-girder bridge and a long-span cable-stayed bridge. To solve the computational constraints in performing numerical analysis on a personal computer, a Multi-Euler domain method based on the fully-coupled Lagrange and Euler models is adopted and further developed for long–span bridge application. This study investigates various detonation scenarios in terms of the explosive weight and location, and their interactions with bridge structures. Both the localized damage mechanism and the global structural response of three bridges are examined. By studying the blast-resistance of each bridge under different explosion threats, the most critical scenarios are identified respectively. Studies of bridge protection against potential attacks by using Carbon Fibre Reinforced Polymer (CFRP) strengthening are also discussed. Numerical results in this study provide bridge owners and engineers with thorough and important information on the structural performance of highway bridges under blast loads, helping them in choosing effective protection strategies for possible potential explosion events.

Effect of vertical structural irregularity on seismic design of tall buildings

SummaryMany tall buildings are practically irregular as an entirely regular high‐rise building rarely exists. This study is thus devoted to assessing the approach and coefficients used in the seismic design of real‐life tall buildings with different vertical irregularity features. Five 50‐story buildings are selected and designed using finite element models and international building codes to represent the most common vertical irregularities of reinforced concrete tall buildings in regions of medium seismicity. Detailed fiber‐based simulation models are developed to assess the seismic response of the five benchmark buildings under the effect of 40 earthquake records representing far‐field and near‐source seismic scenarios. The results obtained from a large number of inelastic pushover and incremental dynamic analyses provide insights into the local and global seismic response of the reference structures and confirm the inferior local response of tall buildings with severe vertical irregularities. Due to the significant impacts of the severe irregularity types on the seismic response of tall buildings, the conservative code approach and coefficients are recommended for design. It is also concluded that although the design coefficients of buildings with moderate irregularities are adequately conservative, they can be revised to arrive at more consistent safety margins and cost‐effective designs.

Assessment of load carrying capacity enhancement of an open spandrel masonry arch bridge by dynamic load testing

ABSTRACTHealth monitoring of masonry railway bridges is vital due to their long life in service, an increasing demand for higher axle loads, and the recent trends in sustainability that requires the preservation of aged masonry bridges. Safety assessment of a 70-year-old masonry arch bridge is presented. The bridge is situated in a S-shaped curved section of the track and has long spans of 36 m. Added the deterioration of masonry due to old age of the bridge and complex behavior of masonry arch bridges, dynamic load tests are employed and global structural responses to allowable loading schemes of the bridge are determined. Dynamic amplification factors are determined and compared to those proposed by standards and it is concluded that experimental values are higher than those proposed by standards. Model calibration is carried out using deflection signatures. Experimental and analytical natural frequencies are compared for model verification. Having the model calibrated and verified, safety of bridge in serviceability and ultimate limit states due to application of operational loading schemes and higher axle loads are assessed. It is concluded that the bridge is safe regarding operational loading patterns, and could withstand an increased axle load of 25 tons.

On the capability of the Thick Level Set (TLS) damage model to fit experimental data of size and shape effects

Size and shape effects are important issues in predicting the global response of concrete structures. Small-scale tests performed in laboratory to determine the material properties are not enough to simulate large-scale structures. Many models are used to extrapolate small scale results to large scale simulations, but only few are able to recover size and shape effects. Recently a model of graded damage (TLS) has been proposed and comparison with cohesive zone models shows that this new model contains a new degree of freedom, the length of transition between totally damaged material and undamaged zone (ie the process zone size). In this paper, the capability of the model (TLS) to represent size and shape effects for two recently published experimental campaigns is studied.

Column splice fracture effects on the seismic performance of steel moment frames

The influence of welded column splice fracture on the seismic response of steel moment frames is examined. The study is motivated by pre-Northridge moment frames with welded column splices with crack-like flaws that are highly vulnerable to fracture. Costly retrofit strategies to repair these splices are usually intentioned to preclude splice fracture, without an explicit examination of its effects on global response. This study simulates post-fracture response of splices through a new material model, which is informed by fracture-mechanics based estimates of splice strength, and reproduces phenomena such as gapping and re-seating that occurs in the splices after fracture. Nonlinear response history simulations (incorporating this model) are used to examine the response of 4- and 20-story moment frames. The simulations, using 100 ground motions, and reflecting key aspects of nonlinear response are conducted within a Performance Based Earthquake Engineering (PBEE) framework, to examine global and local structural response in a probabilistic sense. The simulations indicate that neither the collapse potential nor building deformations are significantly affected by splice fracture when compared to benchmark simulations without fracture. This is attributed to a combination of phenomena; these include the mobilization of building rocking due to splice fracture, and the tendency of fractures to cascade upwards through individual columns rather than across a story. The results suggest that splice fracture may not necessarily trigger structural collapse, and retrofit strategies that consider global, rather than local response may be more cost effective. Limitations of the study are outlined.

Anomalous influence of grain size on the global structure, ferroelectric and piezoelectric response of Na0.5Bi0.5TiO3

Na0.5Bi0.5TiO3 (NBT) based lead-free piezoelectrics have received significant attention in the last few years for their anomalous high-field electrostrain response. The positional disorder on the nano/meso scale in these systems have a profound effect on their perceived global structure and dielectric properties. Here we have carried out a systematic analysis of the grain size dependent structural, ferroelectric, piezoelectric and high temperature electrical conduction behaviour of NBT. We show that the global monoclinic lattice distortion collapses even while the grains are unusually large (∼2.5 μm). We also found that the polarization switching ability and piezoelectric response of bulk NBT ceramics reduce drastically when grains are submicron in size. We use a powder poling technique to prove that the significant reduction of piezoelectricity in the submicron grain ceramic is not due to suppression of ferroelectricity within the grains itself, but rather due to inability of the grains to transform collectively to a long range rhombohedral ferroelectric state, either because of the clamping effect and/or enhanced incoherence of the grain boundaries. We also noted a qualitative change in the electrical conduction behaviour as a function of grain size and comment on a possible correlation between the grain size, global structure, piezoelectric response and oxygen ion conductivity of this lead-free compound.

Passive Vibration Control of Existing Structures by Gravity-Loaded Cables

Structures with high concentration of mass at or close to the top such as highway bridge piers are vulnerable in earthquakes or accidents. In this paper, a simple and convenient retrofit strategy is proposed for minimizing vibrations and damages, extending service life and preventing collapse of existing structures. The proposed system comprises of tension-only cables secured to the sides of the structure through gravity anchor blocks that are free to move in vertical shafts. The system is installed in such a way that the cables do not induce unnecessary stress on the main structure when there is no lateral motion or vibration. The effectiveness of controlling global structural responses is investigated for tension-only bilinear-elastic behaviour of cables. Results of a realistic case study for a reinforced concrete bridge pier show that response reduction is remarkably well under seismic excitation.

A model updating method with strain measurement from impact test for the safety of steel frame structures

Abstract Conventional model updating methods for structures mainly use modal parameters, which are global structural responses such as natural frequency, mode shape, and modal damping obtained through vibration measurements. A model updated using those modal parameters can accurately estimate global structural responses and is used to evaluate the state of a structure. However, when using modal parameters only in model updating, a difficulty arises regarding safety assessment and response prediction of local structural members. To ensure safety of structural members, this study proposes a model updating method using the strains measured from the impact tests in structural members of steel frame in addition to modal parameters. In the model updating technique proposed in this study, error functions are set as the differences between measured and model’s modal parameters and the maximum strain subject to impact force, and the functions are minimized using a multi-objective optimization algorithm. The strain responses predicted in the updated model are used in the safety assessment of structural members in steel frames. The proposed method was experimentally verified through impact hammer loading tests on a planar steel moment frame. Furthermore, the stresses obtained from the model updated with modal parameters only and the model updated by the proposed method were compared.

Local Mechanical Behavior of Steel Exposed to Nonlinear Harmonic Oscillation

The local mechanical behavior of fatigued steel specimens was probed using nanoindentation. High-carbon steel cantilevers were exposed to nonlinear harmonic oscillation. The indentation modulus on the beam surface and plastic work during indentation decreased as a function of cycles, which was attributed to grain fragmentation and reorientation as well as the continuous reduction in inherent energy dissipation capacity of the material. X-ray diffraction, electron backscatter diffraction, and atomic force microscopy were used to characterize this microstructural evolution during early stages of the beam fatigue life, which altered 1) the local mechanical properties and 2) the global structural dynamic response. The results provide insight into fatigue damage precursors and provides a framework for connecting materials evolution with nonlinear structural dynamics.

Response attenuation in a large-scale structure subjected to blast excitation utilizing a system of essentially nonlinear vibration absorbers

Limiting peak stresses and strains in a structure subjected to high-energy, short-duration transient loadings, such as blasts, is a challenging problem, largely due to the well-known insensitivity of the first few cycles of the structural response to damping. Linear isolation, while a potential solution, requires a very low fundamental natural frequency to be effective, resulting in large nearly-rigid body displacement of the structure, while linear vibration absorbers have little or no effect on the early-time response where relative motions, and thus stresses and strains, are at their highest levels. The problem has become increasingly important in recent years with the expectation of blast-resistance as a design requirement in new construction. In this paper, the problem is examined experimentally and computationally in the context of offset-blast loading applied to a custom-built nine story steel frame structure. A fully-passive response mitigation system consisting of six lightweight, essentially nonlinear vibration absorbers (termed nonlinear energy sinks – NESs) is optimized and deployed on the upper two floors of this structure. Two NESs have vibro-impact nonlinearities and the other four possess smooth but essentially nonlinear stiffnesses. Results of the computational and experimental study demonstrate the efficacy of the proposed passive nonlinear mitigation system to rapidly and efficiently attenuate the global structural response, even at early time (i.e., starting at the first response cycle), thus minimizing the peak demand on the structure. This is achieved by nonlinear redistribution of the blast energy within the modal space through low-to-high energy scattering due to the action of the NESs. The experimental results validate the theoretical predictions.

Multi-scale model updating of a transmission tower structure using Kriging meta-method

A multi-scale model is often constructed using different finite elements and consists of a global scale model for the structural system and a few local scale models for critical structural components so that the multi-scale simulation can concurrently exhibit both global performance and local behavior of the structure. To ensure the multi-scale model can best represent the real structure, multi-scale model updating technique shall be developed accordingly. This paper thus presents a multi-scale model updating method for a transmission tower structure using the Kriging meta-model that actually is a surrogate for the multi-scale model. Firstly, the multi-scale model of a transmission tower is established by using beam elements to simulate global structure and solid elements to simulate local joints with bolt connections. Secondly, the multi-objective optimization problem that involves multiple objective functions is established to update key parameters of the multi-scale model so that the errors between the measured and predicted structural dynamic characteristics and multi-scale responses can be minimized. To improve the computational efficiency and accuracy of optimization, the Kriging meta-method is used to find the updated key parameters of the tower after a comparison with other meta-methods is made. Finally, the proposed method is applied to a physical transmission tower model, which has been tested in a laboratory, to demonstrate the feasibility and accuracy of the proposed model-updating method. The updated results show that the proposed updating method can improve the accuracy of the multi-scale model of the tower in both global and local structural responses.

Composite joints under M-N at elevated temperatures

The Eurocodes recognise robustness as a way to ensure the structural integrity of a building frame subjected to an unforeseen event and therefore to avoid a so-called “progressive failure” mode in extreme loading situations. However, few practical guidelines exist nowadays which would allow a designer to design a structure accordingly. Within the European RFCS ROBUSTFIRE project, the behaviour of steel and composite car parks subjected to localised fire leading to a column loss was investigated. Under such a scenario, the beam-to-column joints play a key role in the global structural response. Indeed, these joints, initially loaded in bending, may be subjected to elevated temperatures and to combined axial load “N”, bending moment “M” and shear forces “V”. In this paper, a methodology to predict the mechanical response of bolted composite beam-to-column joints at elevated temperatures under M-N is presented and validated through comparison with detailed numerical results and experimental tests. This methodology is based on an analytical method able to predict M-N resistance interaction curves for steel and composite joints and which is in full agreement with the component model recommended by the Eurocode.

Damage Probability Matrices for Three-Nave Masonry Churches in Abruzzi After the 2009 L’Aquila Earthquake

ABSTRACTThis article provides the main outcomes obtained downstream from a survey concerning the damage to 64 three-nave masonry churches in Abruzzi resulting from the 2009 L’Aquila earthquake. First, they are classified according to the typological, structural, and architectural features. Next, the main failure mechanisms observed in the immediate aftermath of the 2009 earthquake are identified and analyzed, taking into account both the global structural response and the local mechanisms involving specific parts of the churches. Damage related to the local mechanisms is classified by means of scores based on both the severity and extension of the revealed cracks. The single scores are combined to define an index measuring the global damage of each church. Such indices are therefore associated with six damage levels, the frequencies of which are organized into damage probability matrices. These matrices can be used in order to quantitatively evaluate the damage scenarios that the studied population of buildings revealed after the seismic event, as well as to predict the damage level that could be expected on similar churches for future earthquakes.

Stochastic dynamic response analysis of a floating vertical-axis wind turbine with a semi-submersible floater

Floating vertical-axis wind turbines (FVAWTs) provide the potential for utilizing offshore wind resources in moderate and deep water because of their economical installation and maintenance. Therefore, it is important to assess the performance of the FVAWT concept. This paper presents a stochastic dynamic response analysis of a 5 MW FVAWT based on fully coupled nonlinear time domain simulations. The studied FVAWT, which is composed of a Darrieus rotor and a semi-submersible floater, is subjected to various wind and wave conditions. The global motion, structural response and mooring line tension of the FVAWT are calculated using time domain simulations and studied based on statistical analysis and frequency-domain analysis. The response of the FVAWT is compared under steady and turbulent wind conditions to investigate the effects of turbulent wind. The advantage of the FVAWT in reducing the 2P effect on the response is demonstrated by comparing the floating wind turbine with the equivalent land-based wind turbine. Additionally, by comparing the behaviour of FVAWTs with flexible and rigid rotors, the effect of rotor flexibility is evaluated. Furthermore, the FVAWT is also investigated in the parked condition. The global motions and structural responses as a function of the azimuthal angle are studied. Finally, the dynamic response of the FVAWT in selected misaligned wind and wave conditions is analysed to determine the effects of wind-wave misalignment on the dynamic response. Copyright © 2016 John Wiley & Sons, Ltd.

A non-linear procedure for the numerical analysis of crack development in beams failing in shear

In this work, a consistent formulation for the representation of concrete behavior before and after cracking has been implemented into a non-linear model for the analysis of reinforced concrete structures, named 2D-PARC. Several researches have indeed pointed out that the adoption of an effective modeling for concrete, combined with an accurate failure criterion, is crucial for the correct prediction of the structural behavior, not only in terms of failure load, but also with reference to a realistic representation of crack initiation and development. This last aspect is particularly relevant at serviceability conditions in order to verify the fulfillment of structural requirements provided by Design Codes, which limit the maximum crack width due to appearance and durability issues. In more details, a constitutive model originally proposed by Ottosen and based on non-linear elasticity has been here incorporated into 2D-PARC in order to improve the numerical efficiency of the adopted algorithm, providing at the same time an accurate prediction of the structural response. The effectiveness of this procedure has been verified against significant experimental results available in the technical literature and relative to reinforced concrete beams without stirrups failing in shear, which represent a problem of great theoretical and practical importance in the field of structural engineering. Numerical results have been compared to experimental evidences not only in terms of global structural response (i.e. applied load vs. midspan deflection), but also in terms of crack pattern evolution and maximum crack widths.

IRREGULAR STEEL BUILDING STRUCTURES SUBJECTED TO BLAST LOADING

In seismic design, structural irregularity has been found to have a significant influence on structural response. The impact of structural irregularity on the global response of steel frame structures subjected to blast loading has not been examined. In the paper, six seismically designed steel framed structures are considered: moment resisting frames (MRF), concentrically braced frames (CBF) and eccentrically braced frames (EBF) each with geometric irregularity in the plan and with a geometric irregularity in the elevation. The blast loads are assumed to be unconfined, free air burst detonated 15 ft from one of the center columns. The structures are modeled and analyzed using the Applied Element Method, which allows the structure to be examined during and through structural failure. A plastic hinge analysis is performed as well as a comparative analysis observing roof deflection and acceleration to determine the effect of geometric irregularity under extreme blast loading conditions. Two different blast locations are examined. Conclusions of this research are a concentrically braced frame provides somewhat of a higher level of resistance to blast loading for irregular structures and geometric irregularity has an impact on the response of a structure subjected to blast loading.