Rigid Diaphragm Research Articles

SEISMIC VULNERABILITY ASSESSMENT OF A 17th CENTURY ADOBE CHURCH IN THE PERUVIAN ANDES

ABSTRACT A large part of Peruvian heritage corresponds to Colonial Andean churches, which are economically and culturally essential for the development of several villages. Particularly, adobe churches are very vulnerable to earthquakes due to the brittle material behavior, the weak connections, and the absence of a rigid diaphragm. This article aims at performing the seismic vulnerability assessment of an adobe heritage church: “Virgen de la Asuncion” of Sacsamarca located in the province of Ayacucho in the Andean region of Peru; using a rapid screening method and a more complex numerical analysis. First, the damage survey was carried out through visual inspection and infrared thermography, while the material characterization was performed by applying flat-jack tests. Later, the seismic vulnerability assessment was carried out in two stages. The first was based on simplified criteria for masonry structures, and the other on a pushover analysis of global performance. From this evaluation, it was possible to conclude that the church is indeed vulnerable to Peruvian seismic demands.

Full-scale Experimental Pushover Testing of an Existing URM Building

Due to the large number of existing unreinforced masonry (URM) buildings worldwide, and the significant seismic risk that this building stock represents, it is important to quantify the seismic behaviour of such buildings in order to provide assessment guidance to practicing structural engineers. Current assessment guidelines have received little benchmarking against the results of large scale lateral testing of typical URM buildings. In response, field testing was performed on a decommissioned vintage URM building to investigate the in-situ behaviour of URM piers and compare actual behaviour to that predicted by existing assessment frameworks. The tested building was a prototypical, three-storey, 1930s fired clay brick URM building located in Auckland, New Zealand. A pushover test was performed by applying a load to the roof level of the building with a 22 tonne excavator, resulting in the formation of a pier rocking mechanism consistent with the predicted modes determined using current assessment guidelines for URM piers. Finite element modelling was able to replicate the results of the experimental testing, but a reduction of the masonry elastic modulus when acting in tension was required in order to match experimental force-displacement response. The calibrated finite element model was subjected to a pushover analysis of the entire building and was used to benchmark the ability of common seismic assessment techniques and simplified macro-element models to predict failure mechanism and building capacity. It was found that macro-element models tended to under-predict the capacity of the building unless an artificial rigid diaphragm constraint was introduced, and that current assessment techniques and 2D macro-element modelling of individual facades provided a dependable lower bound on the capacity of URM frame-type buildings.

Proposals for seismic retrofitting of timber roofs to enhance their in-plane stiffness and diaphragm action at historical masonry buildings in Cairo

Abstract The in-plane stiffness of flat roofs plays crucial role in the structural stability and safety of historical load bearing masonry buildings under lateral hazardous loads. For decades, many restorers of historical buildings used to provide the original timber roofs with rigid diaphragms action using modern engineering systems; such as building reinforced concrete (R.C.) thin slabs. The present research studies analytically the installation of simple and robust timber bracing system above timber boards and under flooring layers of timber flat roofs; to improve the seismic behavior and stability of their historical load bearing masonry buildings in Cairo. The study will evaluate among various alternatives of bracing systems using numerical modelling techniques of computer software that applies Finite Element (F.E.) method. The research applies the numerical investigation to one of archaeological buildings in Cairo from Ottoman era, which is the “Sabil” of “Khusraw Pasha” (1535 A.D./942 A.H.). The timber x-bracing system provides remarkable results relative to rigid diaphragm and steel bracing for seismic retrofitting of historical timber roofs.

Design, fabrication and testing of reduced graphene oxide strain gauge based pressure sensor with increased sensitivity

Pressure sensors with high sensitivity, repeatable output, wide sensing range, suitable for mass production and which can be manufactured cost effectively are highly desirable for pressure sensing applications. Over the years Graphene and rGO (reduced graphene oxide) based strain gauges have typically been prepared on substrates such as silicon, polydimethylsiloxane, polyethylene terephthalate, kapton, and rubber. They have mainly been utilized in the wearable and temperate condition applications. In the proposed work we would like to present a stainless steel based pressure sensor using rigid centre diaphragm and a fixed guided beam along with rGO-filter paper based strain gauges (reduced graphene oxide on a cellulose filter paper). The resulting strain on the fixed guided beam due to the applied pressure is sensed by the rGO-filter paper based strain gauges and the output is measured as change in resistance. The analytical, numerical simulation and experimental studies of this sensor are discussed and the results obtained are in good agreement to each other. The performance of the sensor is evaluated experimentally and is compared against a standard strain gauge for a pressure range of 0 to 20 bar. The developed sensor exhibits a pressure sensitivity of 2.29 Ω/bar, with an enhanced gauge factor of 112 and the effect of temperature is nullified. The high sensitivity and a wide sensing range enable it for a broad variety of applications.

Seismic rehabilitation of cultural heritage masonry buildings with unbonded fiber reinforced elastomeric isolators (U-FREIs) – A case of study

In order to assess the structural behavior and to evaluate the seismic vulnerability of masonry structures of relevant historical and artistic significance, which is a widespread building type in Italy and in the world, an historical masonry church is analyzed under earthquake loading. Linear and non-linear analyses are performed on the finite element models of the structure. From these analyses it is pointed out that the structure does not behave elastically in its existing condition even when subjected to the frequent design earthquake (81% probability of being exceeded over 50 years). Two traditional rehabilitation methods are studied: the placement of a rigid diaphragm which connects the top of the masonry walls only enclosing the church entrance area and the placement of a rigid diaphragm which connects the tops of all masonry walls. None of the traditional method is sufficient for the structure to survive basic design earthquake (10% probability of being exceeded over 50 years). Hence an advanced seismic retrofit solution using innovative carbon fiber reinforced elastomeric isolators is proposed. The proposed intervention consists in the installation of six Unbonded Fiber-Reinforced Elastomeric Isolators (U-FREI) and six Flat Surface Sliders (FSS) as passive protective devices besides the placement of a rigid diaphragm which connects the tops of all masonry walls. The process of installation of the devices is illustrated. The use of the proposed solution leads to a remarkable enhancement of the seismic response capacities of the structure; indeed a general elastic response under the Basic Design Earthquake (BDE) is attained.

Design, fabrication and testing of fiber bragg grating based fixed guided beam pressure sensor

A stainless steel pressure sensor using rigid center diaphragm and a fixed guided beam with FBG sensor is proposed in this paper. Two bare FBGs are bonded on to the tensile and compressive regions of the fixed guided beam. The deflection of the diaphragm due to the applied pressure induces strain on to the fixed guided beam which in-turn shifts Bragg wavelength of the FBG. The sensor is fabricated and tested for its performance in the range of 0–20 bar. The test results are in close agreement with the results obtained from analytical and experimental models. The sensitivity, nonlinearity and resolution of the sensor are found to be 0.103 nm/bar, 0.0008% full scale deflection (FSD) and 0.05 bar. The use of rigid center diaphragm improves the linearity of the sensor for a wider pressure range, and the fixed guided beam provides equal magnitudes of tensile and compressive stresses on the surface of the beam. The differential output in wavelength shifts of the two FBGs nullifies the temperature effect and hence offers temperature compensation.

The Influence of Overhead Cranes in the Seismic Performance of Industrial Buildings

The paper investigates the influence of overhead cranes with a hanging mass under earthquake type loading, considering the Emilia 2012 seismic sequence. The structural layout of precast concrete industrial buildings typical of the Italian territory is considered. The equations of motion describing the behaviour of the hoist load are derived and a sensitivity analysis is carried out on simplified three degrees of freedom systems by solving the governing differential equations. The influence of various parameters on the roof displacement and on the horizontal load transferred by the hanging mass is addressed. The considered parameters are the relative damping of the hanging mass, the length of the hoist ropes, the earthquake record, the hysteretic type of the plastic hinges at the column base, and the behaviour factor of the structural system. The results show that for a horizontal component of the considered seismic sequence the structural displacements are amplified in the case of a behaviour factor greater than 2.5. A simplified modelling strategy considering small displacements is also investigated. Such model is suitable for response spectrum analyses. Finally, a three-dimensional case study is analysed by means of non-linear time history analyses. The results show the influence of the overhead crane on the local performance of some structural and non-structural elements, such as columns and cladding panels, especially when the assumption of rigid roof diaphragm does not apply.

Effects of podium interference on shear force distributions in tower walls supporting tall buildings

High rise constructions featuring a podium surrounding tower walls are often favoured for the versatile functionality of the building. It is shown in this paper that the podium can impose significant differential restraint on coupled tower walls. Incompatible tower wall displacements under lateral loads were found to be the main contributor to the generation of in-plane strutting forces in floors above and below the podium-tower interface level. Shear force localisations in the interior tower wall immediately above the interface was found to be the direct consequence of these actions. Key parameters contributing to this detrimental shear force localisation in a tower wall were analysed by way of parameter studies on representative models of the building and sub-assemblages. It is revealed that the in-plane rigid diaphragm assumption commonly adopted in practice can significantly suppress compatibility forces generated within the building floor leading to unconservative design of the tower walls. Elaborate nonlinear model has been examined to showcase the consequences of understating the shear demands on these walls.

Role of diaphragm flexibility modelling in seismic analysis of existing masonry structures

Numerical modelling considering inelastic response often becomes essential for seismic analysis and assessment of existing masonry structures. Post-earthquake surveys and past experimental studies have demonstrated that flexible diaphragms significantly alter the seismic behaviour of masonry structures. The absence of a rigid diaphragm alters the seismic performance of a structure due to local mechanisms or out-of-plane actions that could compromise the global in-plane capacity. Consistent inferences from different numerical modelling approaches for masonry structures with flexible diaphragms have however been elusive in previous research. Macro-element (or equivalent frame) modelling and non-linear finite element modelling approaches have important differences, particularly in the way interactions between out-of-plane actions and in-plane shear response can be modelled, and in the way they can represent flexible diaphragms. The current paper examines the role of diaphragm flexibility modelling in the global seismic performance of existing masonry structures through a numerical study. Differences observed in the results of equivalent frame modelling versus non-linear finite element modelling, for a set of representative structural models with rigid and flexible diaphragms, under both static and dynamic analyses, are examined. The structural models examined range from single-storied to multi-storied (G+2) and plan symmetric to asymmetric configurations. The approach to modelling diaphragm flexibility in equivalent frame models is also discussed. Significant differences between approaches imply repercussions on seismic assessment and retrofit design for existing masonry structures.

Effect of longitudinal joint on the shear-key of hollow core slab which function as an rigid diaphragm

Various types of reinforced concrete floors can be applied to building construction. The floor of the building can be made of cast in place or precast concrete. The floors should function as a rigid diaphragm, in order to distribute the lateral forces on the building frame. While the floor is made of precast concrete, especially those without overtopping, should be arranged in such a way to function as a diaphragm. In order for the precast slab can function as a diaphragm, the components must be connected to the building frame and between the component itself . Some of them are drag strut, boundary element (chord), transverse joint, collector and longitudinal joint. This paper will discuss the influence of the longitudinal joint shear key with some types. The test specimen is modeled as two HCS are mounted parallel to each other , then the both of HCS are combined with perimeter beams to obtain the same model with the actual conditions and simplify to execute. All models of the test specimen are made to represent the three types of shear key. The test specimen is loaded the shear force and observed the structure behavior and crack patterns.

Seismic Vulnerability Reduction of Masonry Churches: A case study

This paper presents some advanced FE analyses conducted on a masonry church damaged by the recent Emilia earthquake, occurred on 20th-29th May 2012. The Nativity of the Virgin Mary parish, located in the province of Ferrara (Italy), was subjected to a post-earthquake strengthening intervention, which consisted of introducing steel profiles, rigid diaphragms and repointing technique for the damaged masonry material. The earthquake highlighted a series of pre-existing structural vulnerabilities. A probable mechanism involving the detachment of the façade was also observed due to the presence of vertical cracks. Three FE models were created in order to investigate the seismic behavior of the church in the original, retrofitted and proposed FRP-retrofitted configurations. The mechanical properties of the material were adapted from National Technical Code recommendations for existing masonry constructions by introducing a macro-scaling homogenization technique. The performed numerical analyses highlighted the seismic vulnerability of the church in the original and retrofitted configurations. A strengthening intervention performed by means of FRP strips was numerically investigated. The results obtained by using the three different FE models were compared and the use of FRPs was found to be a quite reasonable strengthening intervention, ensuring a considerable seismic upgrading.

Skyscraper Design and Behaviour of Steel Structures and Connections Complexities from Constructability Perspectives

The use of high grade steel and concrete in high-rise buildings has increased significantly in the past 20 years mainly owing to improvement in all the materials, technologies and associated methods related to prepare, supply and pour the concrete. Cementations materials, admixtures, aggregates, pumping techniques, transportations and elevation methods…etc. all these enchased possibilities are illustrated by taking 150 story high rise structural model; analyzed and designed by using the software ETABS-2013, to withstand the gravity loads and also the lateral loads considering Wind 100 mph, Exposure-Seismic Zone-I, soil profile type SD, Occupancy category 1.0 and Ductility factor, R=5.5. The type of ultra-high strength concrete cylindrical strength has been considered as 107 MPa @ 28 days to bear the high load and straining action at lower portion of the core wall, Steel sections and plates are confirming to ASTM-A992-Gr:70 Ksi are considered for Built-up column sections and floor beams. In addition Shear Studs conforming to ASTM-A106-Gr:1020 with composite metal deck have also been considered to be have as rigid diaphragm to act as monolithic unit against the heavy lateral loads. This paper clearly would show that the Design and constructability considerations, serviceability requirements and international codes compliances such as ACI-318, ASCE-7, IBC-2011, UBC-1997, further it would prove that the combination of R.C. concrete and steel composite sections could be the best solution for such tall skyscrapers to meet the client interest and expectations.

Evaluation of an alternative seismic design approach for rigid wall flexible wood roof diaphragm buildings through probabilistic loss estimation and disaggregation

Rigid Wall Flexible roof Diaphragm (RWFD) buildings, commonly referred to as “big-box” buildings are the most prevalent type of construction for low-rise industrial and warehouse facilities in the United States (US). These buildings usually incorporate rigid-in plane concrete tilt-up walls and flexible wood roof diaphragms, which is a commonly seen construction technique in the Western United States. Due to their vulnerability in high seismic areas (e.g. California) observed in past earthquakes, an alternative design methodology was introduced in the FEMA P1026 document to account for the response of the flexible roof diaphragm. The FEMA P1026 design approach has been validated through numerical collapse assessment studies. In this study, the Performance-Based Earthquake Engineering framework, introduced by the Pacific Earthquake Engineering Research (PEER) center, is combined with Monte Carlo Simulation to evaluate, in a probabilistic sense, the earthquake-induced economic losses for these structures. The results are presented in terms of expected losses for two hazard intensities: Maximum Considered Earthquake (MCE) and Design Earthquake (DE), while loss disaggregation plots for collapse and no-collapse losses are also presented. The results demonstrate the ability of the FEMA P1026 design approach to reduce earthquake losses compared to current code-conforming RWFD buildings. Additionally, the results can provide damage and loss information for modeling of these types of buildings within a resilient community and other spatially focused analyses.

Review of Different Pushover Analysis Methods Applied to Masonry Buildings and Comparison with Nonlinear Dynamic Analysis

This article presents the comparison among different nonlinear seismic analysis methods applied to masonry buildings, i.e., pushover analyses with invariant lateral force distributions, adaptive pushover analysis and nonlinear dynamic analysis. The study focuses on the influence of lateral force distribution on the results of the pushover analysis. Two simple benchmark case studies are considered for the purpose of the research, i.e., a four-wall masonry building prototype without floor rigid diaphragms and a two-wall system with a cross-vault. The comparative study offers a useful review of pushover analysis methods for masonry structures and shows advantages and possible limitations of each approach.

Seismic control of irregular multistory buildings using active tendons considering soil–structure interaction effect

Much research has been conducted in recent decades on structural control to improve the performance of different towers and high-rise buildings against severe earthquakes and strong winds. Most studies on building vibration control have been considered just two-dimensionally using shear frame models. In reality, most of the buildings might have irregular plans and thus experience torsion when subjected to earthquakes. Such torsion would further increase the structural response. On the other hand, some buildings are located on soft soil that would trigger the soil-structure interaction (SSI) effects required to be considered for design purposes. The main dynamic behavior parameters like natural frequencies, damping ratios and mode shapes would depend on construction site conditions and thus the SSI effects must be taken into account for buildings on soft soil. In this paper, a mathematical model is developed for calculating the seismic response of an irregular multi-story building equipped with active tendons. The SSI effect is then introduced by changing structure mass, stiffness and damping matrices. The model is employed to obtain the seismic response of 10-story buildings using active tendon with LQR algorithm. The building is modeled as a structure composed of members connected by rigid floor diaphragms with three degrees of freedom at each story; i.e. lateral displacements in two perpendicular directions and a rotation with respect to a vertical axis. Results showed that active tendons have low effects on the reduction of structural response when the building has been located on soft soils.

Timber–Reinforced Concrete Core Hybrid System: Shake Table Experimental Test

AbstractThe law to promote the use of wood in the construction industry was announced by the Japanese government in 2010. As a result of this initiative, timber and timber-based hybrid buildings are increasingly used in public buildings. As the buildings are located in high seismic zone areas, however, validation of the current design guideline was of paramount importance. This paper reports the design and a shake table test of timber frame–reinforced concrete (RC) core hybrid systems. Three specimens (S1, S2, and S3) are tested: S1 and S2 are both one-story buildings with a nailed plywood floor (flexible diaphragm) and a rigid RC floor (rigid diaphragm), respectively. S3 is a two-story building with a nailed plywood floor. Each specimen is tested with increasing magnitude of earthquake loading. The shake table test results showed that the adopted design guideline is sufficient. At higher seismic loads, however, the prevalent plan irregularity induced higher torsional demand damage at the RC core and timb...

A method for preliminary seismic design and assessment of low-rise structures protected with buckling-restrained braces

This paper proposes a method for preliminary Performance-Based Seismic Design (PBSD) of low-rise structures protected with Buckling-Restrained Braces (BRBs). It is assumed that a frame structure protected with BRBs, termed as a dual structure, is rationally represented by a dual single-degree-of-freedom (SDOF) oscillator whose parts yield at different displacement levels. The formulation of the method is presented for SDOF structures. This simplification is validated using a case study example. A comparison of the responses between conventional and dual structures shows that, when designing dual structures, the common practice of using conventional design spectra may lead to biased designs. One of the main advantages of the method is that, during its application, information useful for preliminary and quick assessment of structures is generated, facilitating the application of the PBSD philosophy. A case study example is conducted to show its applicability and its potential for preliminary assessment of structures. Regarding its limitations, the method is valid for low-rise regular buildings with rigid in-plane diaphragms, and whose dynamic response is dominated by their fundamental mode of vibration.

Nonlinear Dynamic Analysis of Masonry Buildings and Definition of Seismic Damage States

A large part of the building stock in seismic-prone areas worldwide are masonry structures that have been designed without seismic design considerations. Proper seismic assessment of such structures is quite a challenge, particularly so if their response well into the inelastic range, up to local or global failure, has to be predicted, as typically required in fragility analysis. A critical issue in this respect is the absence of rigid diaphragm action (due to the presence of relatively flexible floors), which renders particularly cumbersome the application of popular and convenient nonlinear analysis methods like the static pushover analysis. These issues are addressed in this paper that focusses on a masonry building representative of Southern European practice, which is analysed in both its pristine condition and after applying retrofitting schemes typical of those implemented in pre-earthquake strengthening programmes. Nonlinear behaviour is evaluated using dynamic response-history analysis, which is found to be more effective and even easier to apply in this type of building wherein critical modes are of a local nature, due to the absence of diaphragm action. Fragility curves are then derived for both the initial and the strengthened building, exploring alternative definitions of seismic damage states, including some proposals originating from recent international research programmes.

ASSESSMENT OF SENSITIVITY OFPUSHOVER ANALYSIS

In this present study, an attempt has been made to determine the uncertainty occurring in pushover analysis results for a Reinforced Concrete (RC) G+2 storied frame modeled as a bare frame and frame with rigid diaphragm slab by considering userdefined hinges using non-linear static pushover analysis using SAP2000. Since the output of the analysis is very much sensitive and susceptible to design parameters, to interpret the sensitivity analysis by varying the strength of steel, concrete and hinge lengthon the performance of structures and compare the variation in the performance which is randomly generated and incorporated into the analysis. The nonlinear behavior of the elements wasmodeled using plastic hinges based on the momentcurvature relationship as described in Federal Emergency Management Agency (FEMA)-356 and Applied Technology Council (ATC)-40. A seismic response like base shear, roof displacement corresponding to performance point was evaluated using nonlinear static analysis. The experimental results were compared with analytical results.

Evaluation of Ambient Vibration Test Method for Historic Wooden Buildings Based on the Rigid Diaphragm Assumption

To retrofit historic wooden buildings to improve their seismic and wind resistance, the precise structural assessment to such loads is essential. However, little research has been done in Korea on the ambient vibration test (AVT) of historic wooden buildings for the collection of dynamic characteristics and the corresponding structural assessment. The reason for this is that, for wooden buildings, unlike reinforced concrete (RC) and steel buildings, a systematic in situ AVT method has not been established. In this paper, an in situ AVT method is presented for historic wooden buildings based on the assumption that rigid diaphragm action plays a role in the structural responses of wooden buildings. This assumption is reasonable when the roof and rafters restrain the rotation of columns, as do slabs in RC and steel buildings. The proposed method was employed in an evaluation of the dynamic characteristics of a small traditional wooden building to assess its applicability. The method was also applied to in situ AVT of eight historic wooden buildings that are designated as national treasures in Korea.