Diaphragm Action Research Articles

UNI-axial dynamic tests on a stone masonry subassembly before and after interventions

The present study deals with the seismic response of three-leaf stone masonry subassemblies before and after the application of intervention techniques. In detail, shaking table tests are performed on a scaled substructure in the Laboratory for Earthquake Engineering, at the National Technical University of Athens (NTUA). The substructure consists of two parallel walls. One wall is of rectangular section, whereas the other is provided with a portion of transverse wall at its mid-length. A flexible timber floor rests on the top of the walls. Sine-sweep and dynamic tests are performed perpendicular to the two walls, before and after interventions. The aim of this work is to experimentally study the response of three leaf masonry walls against in-plane and out-of-plane seismic actions, to investigate the effect of the transverse wall to the out-of-plane behavior of walls (and vice versa) and to assess the effect of a selected set of interventions on the behavior of the substructure. The results of this study confirm the vulnerability of three leaf masonry structures, evidenced by the premature detachment of masonry leaves. They prove the significant role of the transverse wall, in reducing the width of the separation cracks observed in the walls subjected to out-of-plane actions followed by an increase of its in-plane damage, as well as the positive effect (in terms of load and deformation capacity) of grouting the walls and enhancing the diaphragm action of the floor. Additionally, the experimental results of this study can serve as a validation tool for the calibration of numerical models and as input for further analytical research. In this paper, the calibration of a smeared crack model that is based on macro-modeling approach is presented, along with a comparison between experimental and numerical results for the specimen before interventions, based on the performed calibration.

Structure cladding interaction in sandwich panel roofs

A few years ago, several failures in the sandwich panel cladding systems of large warehouses or distribution centres (“Big Sheds”) in the UK highlighted shortcomings in the current design philosophy, which ignores the global effect of cladding on the performance of the structure. The current European Standard for including the effect of cladding (i.e., diaphragm action) in the structural analysis is inappropriate for modern cladding systems, especially in association with ultra large portal frame structures.In this paper, a new state-of-the-art for sandwich panels acting as a diaphragm to resist in-plane forces in portal frame structures is defined. The modern terminology for such interactive behaviour is now given the more general title of “Structure Cladding Interaction”. Potential cladding failure modes are highlighted and where possible quantified. The “component method” is advocated as the best way to include the global effects of sandwich panels in a 3D finite element model of the complete structure. Guidance on generating such models, obtaining the critical component properties and validation is provided, together with three detailed validation examples. The method is both scalable and general; it may be used to model the effects of cladding in any size of structure or of any structural form. It is strongly recommended that the influence of the cladding should be included in the design of any large portal frame structure. In such buildings the serviceability limit state is likely to be defined by cladding failures which may occur below the working loads. Detailed outcomes obtained using sophisticated 3D models will be presented in subsequent papers.

Shake table tests of a full-scale two-story plate-type modular composite building with semi-rigid corner connections

To study the dynamic characteristics and seismic response of a novel plate-type modular steel–concrete composite building with different semi-rigid corner connections under different seismic levels, a full-scale two-story two-room test model with L-shaped and T-shaped II corner connections was designed and built according to the Chinese typical area with an eight-degree seismic precautionary intensity and a design basic acceleration of ground motion of 0.2 g. The test model was subjected to white-noise tests and horizontal biaxial seismic tests by applying two natural seismic waves (the EL Centro wave and the Chi-Chi Taiwan wave) and one artificial wave on a shake table. The test program and the results were presented in detail in this paper. The experimental phenomena were analyzed, and the dynamic characteristics (fundamental frequency and damping ratio) and seismic performance (acceleration dynamic amplification factor, global lateral displacement, building drift, shear force distribution, and strain response at key positions) were investigated and discussed. The test results showed that the full-scale two-story plate-type modular composite building has good seismic performance and little damage under a severe earthquake due to the strong diaphragm action of the exterior glass fiber-reinforced concrete wall panels.

Combining Architectural Conservation and Seismic Strengthening in the Wood-Based Retrofitting of a Monumental Timber Roof: The Case Study of St. Andrew’s Church in Ceto, Brescia, Italy

ABSTRACT This work presents the application of timber-based retrofitting techniques to a case-study stone masonry church featuring a wooden roof from 18th century. From the static point of view, the original roof structure presented a number of undersized structural elements, and its members were poorly or not connected among each other and to the masonry, making the church vulnerable to seismic loads as well. Thus, the roof was retrofitted with wood-based techniques, including an overlay of plywood panels, against seismic actions. These affordable, rapid, easily realizable interventions enabled both the conservation and seismic retrofitting of the roof, providing an adequate load-carrying capacity for static loads, and an effective diaphragm action against seismic loads. The conducted numerical analyses showed that the realized interventions greatly improve the seismic behaviour of the building. Besides, when the additional energy dissipation provided by the plywood panels overlay is taken into account in the numerical model, the church would even potentially be able to fully withstand the expected seismic action of the site.

Seismic Performance Assessment of Low-Rise Unreinforced and Confined Brick Masonry School Buildings Using the Applied Element Method

Masonry buildings are generally vulnerable to seismic action, as evidenced extensively in past earthquakes. In order to improve their seismic performance, several modifications have been introduced, such as reinforcing or confining the masonry. This paper presents a seismic analysis and fragility assessment procedure for non-engineered masonry building typologies, employing the applied element method (AEM). Compared to buildings with stiff diaphragms, the conventional pushover-based procedure is challenging for the seismic assessment of masonry buildings with flexible diaphragms, due to the lack of a global box-like behaviour. This study first presents a novel and validated method for nonlinear pushover analysis, independent of the type of diaphragm action on the building, by applying incremental ground acceleration and by considering suitable engineering demand parameters for the assessment of lateral capacity. Based on the failure mechanisms, a seismic performance assessment and fragility evaluation approach is then proposed, for reliable accounting of both the in-plane and out-of-plane failure modes. Finally, the proposed methodology is applied to a number of unreinforced and confined masonry school buildings with different seismic detailing levels, as often found in the Himalayan belt and beyond.

Modeling Knee-Brace Fastener Slippage in Post-Frame Buildings with Diaphragm-Action Design

Highlights Effects of knee braces, with and without fastener slippage, on a post-frame structure with diaphragm-action design. Modeling techniques of knee braces with fastener slippage. Effects of fastener slippage on a primary frame with knee braces. Knee brace may have an undesired (negative) effect on posts and roof trusses. Abstract. Knee braces should not be an add on after the post-frame building is designed because they do affect the behavior of the structure. It is recommended that the building designer first check the need for knee bracing when diaphragm action is included in the design. Knee braces may benefit buildings with certain geometrical configurations and loading conditions where diaphragm action alone is not enough. Modeling knee bracing within a post-frame building is complex. The complexity extends to the roof-truss design because the truss design must include the knee brace reaction forces. The building designer is responsible for reviewing truss drawings to verify that knee brace loads are applied correctly, and that correct governing load combinations are applied. In this study, a structural analog that includes knee braces, with and without slippage, was developed to model the effects of knee braces on designs of post-frame buildings with diaphragm action. In the five buildings analyzed in this study, use of knee braces produced inconsistent results. Knee braces may increase or decrease horizontal eave deflection, load demand on the diaphragm and endwalls, load demand on the foundation, and stress unity in the posts. In all buildings with knee braces, posts were subjected to additional bending stresses under gravity loads. This behavior was more pronounced in buildings with long truss spans. Knee braces should not be specified in the design documents unless their effects on the building are considered by structural analysis. Keywords: Diaphragm action, Knee braces, Modeling connection slippage, Post-frame buildings.

Diaphragm effectiveness of multistory precast parking structures with floor‐to‐floor mechanical connectors

AbstractMultistory parking structures are a promising exploration avenue to address the acute need for parking spaces, and their floors are often joined with mechanical connectors to form a floor system. The properties of floor‐to‐floor connectors directly affect the seismic behavior of parking structures. Thus, the effectiveness of diaphragm action with several different typical floor‐to‐floor connectors was investigated and the natural frequencies and seismic performances of the parking structures were obtained. The results show that the mechanical characteristics of connectors in precast elements have a crucial influence on the diaphragm effectiveness of large‐span concrete structures with precast double‐tees under strong seismic action.

Seismic Behaviour of Reinforced Concrete Structure with Flexible and Rigid Discontinuous Diaphragm

In general, the structure of a building may be thought to be composed of many vertical systems finite by horizontal diaphragms. Diaphragms play a very important role within the distribution of force to the vertical components. The common assumption is formed whereas analysis is that diaphragm can act as rigid diaphragm. In rigid diaphragm action in plane deformation is not allowable. This kind of assumption is good for few of buildings; however, a number of buildings might exhibit in plane flexibility in their floor system. To compare the influence of rigid and flexible diaphragm, an analysis was carried out in the finite element software Midas gen. Rectangular, L shape, U shape, T shape were employed, as well as storey variation G+5, G+10, G+15. Each model examined both with and without a shear wall in place. IS 1893:2002 response spectrum analysis was used to analyse response quantities like storey drift, storey displacement, fundamental natural period, and column axial force.

In-plane capacity of existing post-WWII beam-and-clay block floor systems

A growing attention has been paid to the deep renovation of RC buildings, particularly focusing on their structural vulnerability and on the development of retrofit strategies; however, the issue of the in-plane diaphragm action and the capacity of existing floors has rarely been addressed. Although floor capacity does not seem critical for the seismic capacity of existing structures, commonly affected by greater vulnerabilities, it may become critical when an additional lateral force resisting system is introduced. This paper investigates the in-plane capacity of beam and hollow-clay-block floor system, typical of the European post-WWII RC buildings. Considering the diaphragm action as associated with an in-plane tied-arch mechanism developing within the floor thickness, the main failure mechanisms are discussed, and some simplified equations are provided to preliminary estimate the maximum capacity of floors. Experimental and numerical analyses are than carried out to validate the simplified analytical model. The relevant influence of possible staircase openings on the in-plane load paths and on diaphragm flexibility and capacity are also considered. Finally, the influence of the floor capacity on the seismic vulnerability assessment and in the conceptual design of a seismic retrofit intervention is discussed. This preliminary study shows that only some of the beam-and-block floor systems have a reliable in-plane capacity, while other typologies cannot serve as floor diaphragms. When the diaphragm action can be relied upon, the diaphragms often exhibit a fairly stiff behaviour up to a brittle failure, which is commonly associated with the ultimate capacity at the tied-arch supports.

Shake table testing of braced and Friction-Added suspended ceilings and associated numerical study

Suspended ceilings have long been shown as one of the most fragile non-structural components during past earthquakes. Under high seismic application, suspended ceilings are often required to be restrained using ceiling braces. However, because of the complicated behavior of suspended ceilings under seismic loading, the efficiency of braced ceiling systems is still unclear, and effective performance-enhancing methods for suspended ceiling systems have not yet been established. In this study, shake table tests were first conducted for unbraced (or non-seismic) and braced suspended ceiling specimens. Despite the seismic bracing according to ASTM E580, substantial damage, comparable to the non-seismic ceiling specimen, was observed in the braced ceiling specimen because of the lack of rigid diaphragm action of the tested ceiling grid. In order to enhance the seismic performance of a non-seismic suspended ceilings cost-effectively, a novel rotational friction mechanism was added, and its effectiveness was experimentally evaluated. The proposed friction damper exhibited a stable hysteretic behavior under various input motions and effectively improved the seismic performance; only minor damage was observed at the ceiling perimeters. The friction damping parameters, such as the maximum static friction force, were experimentally calibrated, and an analytical model of the friction-added ceiling system was developed. A numerical case study was conducted to evaluate the applicability of the proposed friction-added ceiling system based on an extensive time history analysis of steel moment-frame buildings.

Shear resistance of sandwich panel connection at elevated temperature

PurposeSandwich construction has developed and has become an integral part of lightweight construction. In the recent projects, it has been shown that by using sandwich panels as stabilizing members, a considerable amount of savings of steel can be achieved for structural members at ambient temperature. These stabilizing effects may also help to achieve similar savings in case of fire.Design/methodology/approachThe response of a sandwich single panel as well as the behaviour of the whole structure at ambient temperature and in case of fire is influenced by joints between the sandwich panels and the sub-structure. The fastenings used to fix the sandwich panels to a sub-structure may be loaded by shear forces caused by self-weight, live loads or diaphragm action. Therefore, an experimental investigation was conducted to investigate the shear behaviour of sandwich panel joints in fire.FindingsThis paper summarized briefly the experimental results, numerical simulations and analytical models on the shear behaviour of sandwich panel joints at ambient and elevated temperatures.Research limitations/implicationsThe work is limited to studied types of screws and sandwich panels which are generally used in current sandwich construction.Practical implicationsThese stabilizing effects in sandwich construction help to achieve savings in case of fire.Social implicationsSandwich construction has developed and has become an integral part of lightweight construction. In the recent projects, it has been shown that by using sandwich panels as stabilizing members, a considerable amount of savings of steel can be achieved for structural members at ambient temperature. These stabilizing effects help to achieve similar savings in case of fire.Originality/valueThis paper summarized briefly the experimental results, numerical simulations and analytical models on the shear behaviour of sandwich panel joints at ambient and elevated temperatures, which were not published yet.

Stressed skin theory and structure cladding interaction: Safety concerns with Big Sheds

Stressed skin theory was born in the 1960’s and intensive research and development took place in the UK and Europe up to the mid 1980’s. During this time, much of this research was directed towards utilising stressed skin action to reduce the amount of structural steelwork in the portal frames of relatively stocky, low rise “shed” type structures. By the end of this period, the theory appeared to be quite stable and almost all further development ceased. Subsequent Standards and Codes of Practice therefore reflect the state-of-the-art at or around the mid-1980s. Since then, there has been a massive evolution in shed type structures (e.g. large warehouses and distribution centres also known as ‘Big Sheds’) as they have increased greatly in size, the frames have become taller and more slender and alternative cladding systems have proliferated. Some recent roof failures have been attributed to ‘parasitic’ (unwanted) diaphragm action in such ‘Big Sheds’ and this has drawn attention to the inadequacy of current stressed skin theory. This paper summarises an on-going response to this situation within which interest broadens to include the response of the unclad frames and the interface between the frames and the cladding, as well as the cladding system. Within this context, the theory is better discussed under the more general heading of ‘structure cladding interaction’.

Response Behaviour of High‐Rise Modular Building under Wind and Seismic Loads

AbstractSingapore has adopted modular construction method known as Prefabricated Prefinished Volumetric Construction (PPVC), where this method requires modules to be prefabricated in a factory environment and subsequently transported to site for installation. A 40‐storey steel‐concrete composite PPVC building consisting of a modular structural frame and a central reinforced concrete (RC) core wall subjected to wind and seismic loading combinations are studied on the global response behavior building drift behavior. The case study is used to investigate the structural response behavior throughout the working life of the building under the service limit state. The adjacent modules are connected horizontally using a tie steel plate idealized by a pin‐pin connection while upper and lower modules are connected vertically using a vertical steel thread rod secured with locking nut idealized by a pin‐ended connection. The modules are braced to a central RC core wall. Continuous semi‐rigid floor diaphragm action is also considered in each floor. The study reveals the importance of inter‐module connection and the floor diaphragm action for the structural interaction between modules and central RC core wall.

Seismic Performance of Precast Slab to Beam Connection: An Overview

Precast construction is common across the world, but in India the level of acceptance is less, although it offers several benefits compared to the cast-in-place construction. Adopting a suitable connection for pre-cast elements is very important in providing the over-all robustness to the structure. Among all precast connections, the precast slab to beam connection is considered a vital one as the horizontal load is trans-ferred to the vertical load resisting structural elements by the diaphragm action. This article mainly focuses on the seismic performance of the precast slab to beam connections with an experimental evaluation of the basic concepts of design and detailing of the con-nections. This overview will pave way to refine work on the slab to beam connections in future research.

Numerical analysis of a clad portal frame structure tested to destruction

A full-scale steel portal frame structure, explicitly designed to utilize stressed skin action, was tested to destruction in the early 1970′s. At this time, it was not possible to perform any detailed analysis of such structures; however, using modern computational techniques, detailed Finite Element Models (FEMs) have now been developed. These FEMs show an excellent agreement with the reported test results, including following the nonlinear behaviour to complete collapse. The validated model was then extrapolated into a multi-bay portal structure of more realistic length, and the effect of cladding on the strength and flexibility of the frames was established under both vertical and horizonal loading. Under a distributed vertical load, the cladding initially had little influence although, once the internal frames reached a complete collapse mechanism, the cladding prevented structural collapse until the cladding also failed. However, under horizontal load, the coupling between frames was significant, particularly in the multi-bay case, leading to high forces in the cladding and the structure/cladding interface. The tested frame was stocky and low rise, typical of its time. Today, with taller, more flexible modern frames, stiffer and potentially less robust cladding systems will resist an even more significant proportion of the horizontal load; a warning to the construction industry that parasitic diaphragm action should not be ignored.

Research on Initial Behavior of Screwed Lap Connections of Thin Plates – Experimental Tests and FEM Analysis

The paper continues the past research on the screwed thin-plate single-lap connections undertaken by the author. This re-search was focused on stressed skin diaphragm action, where it is essential to know values of connections behavior. Until now, initial stiffness is used for common engineering estimations. The main aim of this paper is to compare the experimental results of the shear tests conducted on the connections with the theoretical predictions of their initial behavior obtained from the FEM models. The experiments followed the Eurocode rules for design aided by testing and the European Convention for Constructional Steelwork (ECCS) recommendations. FEM parametric models are introduced as a consideration to use, taking into account screw threads and EPDM washers. In the technical literature, various techniques are described for the modelling of threaded connections. A modification to a known solution is applied in this analysis where a spiral thread is substituted by simple horizontal rings; this simplification allows use of symmetry in a static system, which is particularly desirable for large models such as those prepared in this paper. In contrast to the mentioned known solutions, the thread mod-el presented here has a triangular cross-section related to the geometry used in real screws. The numerical results correspond reasonably well to the test’s early stage measurements from the lab experiments and can be useful for initial stiffness examination (e.g. where no data is provided for a particular connector by a producer) or as a base to further research on the model.

Rubble Stone Masonry Buildings with Cement Mortar: Base Shear Seismic Demand Comparison for Selected Countries Worldwide

Full base shear seismic demand analyses with calculated examples for heavy stone masonry buildings are not present in the literature. To address this shortcoming, analyses and calculations are performed on nominally reinforced rubble stone masonry house and school designs, as typically built in Nepal. The seismic codes are literally applied for countries where the technique is still allowed (Nepal, India, China, Tajikistan, Iran, Croatia), or should be reintroduced based on current practices (Pakistan, Afghanistan, Turkey). First, this paper compares the base shear formulas and the inertia forces distributions of these codes, as well as material densities, seismic weights, seismic zoning, natural periods of vibration, response spectra, importance factors and seismic load combinations. Large differences between approaches and coefficients are observed. Then, by following Equivalent Lateral Force-principles for Ultimate Limit State verifications (10%PE50y), the base shear and story shears are calculated for a design peak ground acceleration of 0.20 g, as well as the effects of critical load combinations on the forces and moments acting on the lateral-resisting elements. It is concluded that Pakistan has the most tolerant code, Nepal represents an average value, whereas India and China are most conservative toward the case study buildings. Overall, it is observed that heavy-masonry-light-floor systems with negligible diaphragm action behave different under seismic motion than most other building typologies. Given the observations in this paper, the applicability of conventional ELF, S-ELF and S-Modal methods for heavy masonry buildings is questionable. The codes however do not introduce modified approaches that address these differences. Possible implications of the exclusion of plinth masonry and large portions of seismic weight need further assessment and validation, for which different (possibly more sophisticated) concepts must be considered, such as the equivalent frame method or distributed mass system. Since Nepal allows stone masonry in areas with higher seismic hazard levels >0.40 g (opposed to India <0.12 and China <0.15 g), their code is taken as the reference and starting point for follow-up research, which aims to verify the seismic demand by performing seismic capacity checks of the masonry piers and spandrels. The paper ends with an appeal for global collaboration under the research project SMARTnet.

An engineering approach for evaluating the dynamic response of acceleration‐sensitive secondary systems in flexible structures

AbstractSecondary systems are nonstructural components that serve important functions within a structure, but are not considered as an integral part of the main structural system. There are many categories of secondary systems addressing architectural aspects, mechanical, electrical, communication, water supply and plumbing functions, as well as being part of the structure's content. The main issue regarding the installation of secondary systems in structures placed in seismically prone regions is preservation of their integrity and functionality during earthquake motions, since failure can have dramatic consequences in terms of property losses and safety concerns. This study introduces a new methodology for approximating the seismic response of acceleration‐sensitive secondary systems with a mass lower than 1% of the total mass of the primary structure, and whose seismic excitation derives exclusively from the motion of that structure, that is, interaction phenomena are ignored. The methodology applies to secondary systems installed on regular and flexible primary structures exhibiting floor diaphragm action and quantifies the seismic demand from information based solely on the deflected shape of the structure at the instant it reaches peak seismic response. The methodology can be applied in alternative ways, depending on the required accuracy of the results and on acceptable computational cost, yielding good quality results when compared to those derived from detailed time‐history analyses. Thus, the present methodology signifies a departure from the current state‐of‐design of acceleration‐sensitive secondary building components, which does not fully account for the vibrations of the primary structure. Finally, the accuracy of the methodology is gauged through a number of application examples involving different combinations of primary‐secondary systems.

Lateral Load Performance of Panelized Wood I-Joist Floor Systems

Abstract Panelized light wood frame construction is becoming more popular due to the faster construction time and shortage of onsite skilled labor. To use light wood frame panels effectively in panelized floor systems, panel-to-panel joints must be fastened adequately to allow load transfer between panels. They must also possess in-plane shear strength and stiffness comparable to stick-built, staggered-sheathed assemblies. This study was designed to develop efficient and effective panel-to-panel joints for connecting adjacent floor panels built with wood I-joists and evaluate the efficiency of the joints in achieving diaphragm action. At first, a number of these panel-to-panel joints were tested in the laboratory using a small-scale diaphragm test setup to determine their efficiency in transferring in-plane forces between panels. Test results showed that a small decrease in in-plane stiffness was expected for the most effective joints, but their strengths were significantly higher than at the same location in a conventional site-built floor diaphragm. The presence of blockings and use of two-row nailing were found to considerably improve stiffness and strength. These features can be used to mitigate the potential reduction in mechanical performance of panelized floor construction, in comparison with the site-built wood I-joist floor.

Seismic performance of precast shear wall-slab connection under cyclic loading: experimental test vs. numerical analysis

The structural behaviour of precast shear wall-diaphragm connection was compared with the monolithic connection under seismic loading. The monolithic connection was made by using U-bars connecting shear wall and slab, and the precast connection was made by using dowel bars in two steps. Firstly, U-shaped dowel bars from the precast shear wall lower panel and precast slab were connected by the longitudinal reinforcement, and screed concreting was done above the precast slab. Secondly, the shear wall upper panel was connected using the dowel bar protruding from the shear wall lower panel. The gap between the dowel bars and the duct was filled with non-shrink grout. The specimens were subjected to reverse cyclic loading at the ends of the slab. This study also aimed to develop a 3-D numerical model using ABAQUS software. The non-linear properties of concrete were defined by using the concrete damaged plasticity (CDP) model to analyse the response of the structure. The precast dowel connection between the shear wall and slab showed superior performance concerning ductility, strength, stiffness and energy dissipation. The developed finite element model exactly predicted the behaviour of connections as similar to that of experimental testing in the laboratory. The average difference between the results from finite element analysis and experimental testing was less than 20%. The results point to the conclusion that the shear resistance is provided by the dowel bars and the stiffness of the precast specimen is due to the diaphragm action of the precast slab. The damage parameter and the interaction between structural members play a crucial role in the modelling of precast connections.