Computer analysis and design of concrete shell roofs

This paper is a preliminary version of Chapter 3 of a State-of-the-Art Report by the IASS Working Group 5: Concrete Shell Roofs. The intention of this chapter is to set forth for those who intend to design concrete shell roofs information and advice about the selection, verification and utilization of commercial computer tools for analysis and design tasks.The computer analysis and design steps for a concrete shell roof are described. Advice follows on the aspects to be considered in the application of commercial finite element (FE)computer programs to concrete shell analysis, starting with recommendations on how novices can gain confidence and competence in the use of software. To establish vocabulary and provide background references, brief surveys are presented of, first,element types and formulations for shells and, second, challenges presented by advanced analyses of shells. The final section of the chapter indicates what capabilities to seek in selecting commercial FE software for the analysis and design of concrete shell roofs. Brief concluding remarks summarize advice regarding judicious use of computer analysis in design practice.

Modelling of chloride penetration into non-saturated concrete: case study application for real marine offshore structures

The aim of this paper is to explain the chloride concentration profiles obtained experimentally from control samples of an offshore platform after 25 years of service life. The platform is located 12 km off the coast of the Brazilian province Rio Grande do Norte, in the north-east of Brazil. The samples were extracted at different orientations and heights above mean sea level. A simple model based on Fick’s second law is considered and compared with a finite element model which takes into account transport of chloride ions by diffusion and convection. Results show that convective flows significantly affect the studied chloride penetrations. The convection velocity is obtained by fitting the finite element solution to the experimental data and seems to be directly proportional to the height above mean sea level and also seems to depend on the orientation of the face of the platform. This work shows that considering solely diffusion as transport mechanism does not allow a good prediction of the chloride profiles. Accounting for capillary suction due to moisture gradients permits a better interpretation of the material’s behaviour

Effects of hydrogen assisted stress corrosion on damage tolerance of a high-strength duplex stainless steel wire for prestressing concrete

The study brings new insights on the hydrogen assisted stress corrosion on damage tolerance of a high-strength duplex stainless steel wire which concerns its potential use as active reinforcement for concrete prestressing. The adopted procedure was to experimentally state the effect of hydrogen on the damage tolerance of cylindrical smooth and precracked wire specimens exposed to stress corrosion cracking using the aggressive medium of the standard test developed by FIP (International Prestressing Federation). Stress corrosion testing, mechanical fracture tests and scanning electron microscopy analysis allowed the damage assessment, and explain the synergy between mechanical loading and environment action on the failure sequence of the wire. In presence of previous damage, hydrogen affects the wire behavior in a qualitative sense, consistently to the fracture anisotropy attributable to cold drawing, but it does not produce quantitative changes since the steel fully preserves its damage tolerance.

Design of Reinforcement in Concrete Shells: a unified Approach

The problem of design/verification of reinforcement in concrete shells is reviewed. Methods of analysis are classified, and the elastic-plastic approach is described in detail in the general case of shells subjected to both bending and membrane action. The procedure is then reduced to membrane shells (applicable also to concrete walls) and to pure bending, as in the case of plates. The procedure, which is based on previous research,generally requires the use of a desk-top computer.

Debonding detection of FRP strengthened concrete beams by using impedance measurements and an ensemble PSO adaptive spectral model

An impedance-based midspan debonding identification method for RC beams strengthened with FRP strips is presented in this paper using piezoelectric ceramic (PZT) sensor?actuators. To reach this purpose, firstly, a two-dimensional electromechanical impedance model is proposed to predict the electrical admittance of the PZT transducer bonded to the FRP strips of an RC beam. Considering the impedance is measured in high frequencies, a spectral element model of the bonded-PZT?FRP strengthened beam is developed. This model, in conjunction with experimental measurements of PZT transducers, is used to present an updating methodology to quantitatively detect interfacial debonding of these kinds of structures. To improve the performance and accuracy of the detection algorithm in a challenging problem such as ours, the structural health monitoring approach is solved with an ensemble process based on particle of swarm. An adaptive mesh scheme has also been developed to increase the reliability in locating the area in which debonding initiates. Predictions carried out with experimental results have showed the effectiveness and potential of the proposed method to detect prematurely at its earliest stages a critical failure mode such as that due to midspan debonding of the FRP strip.

Shake-table tests of a reinforced concrete frame designed following modern codes: seismic performance and damage evaluation

This paper presents shake-table tests conducted on a two-fifths-scale reinforced concrete frame representing a conventional construction design under current building code provisions in the Mediterranean area. The structure was subjected to a sequence of dynamic tests including free vibrations and four seismic simulations in which a historical ground motion record was scaled to levels of increasing intensity until collapse. Each seismic simulation was associated with a different level of seismic hazard, representing very frequent, frequent, rare and very rare earthquakes. The structure remained basically undamaged and within the inter-story drift limits of the "immediate occupancy" performance level for the very frequent and frequent earthquakes. For the rare earthquake, the specimen sustained significant damage with chord rotations of up to 28% of its ultimate capacity and approached the upper bound limit of inter-story drift associated with "life safety". The specimen collapsed at the beginning of the "very rare" seismic simulation. Besides summarizing the experimental program, this paper evaluates the damage quantitatively at the global and local levels in terms of chord rotation and other damage indexes, together with the energy dissipation demands for each level of seismic hazard. Further, the ratios of column-to-beam moment capacity recommended by Eurocode 8 and ACI-318 to guarantee the formation of a strong column-weak beam mechanism are examined.

Seismic performance and damage evaluation of a reinforced concrete frame with hysteretic dampers through shake-table tests

Passive energy dissipation devices are increasingly implemented in frame structures to improve their performance under seismic loading. Most guidelines for designing this type of system retain the requirements applicable to frames without dampers, and this hinders taking full advantage of the benefits of implementing dampers. Further, assessing the extent of damage suffered by the frame and by the dampers for different levels of seismic hazard is of paramount importance in the framework of performance-based design. This paper presents an experimental investigation whose objectives are to provide empirical data on the response of reinforced concrete (RC) frames equipped with hysteretic dampers (dynamic response and damage) and to evaluate the need for the frame to form a strong column-weak beam mechanism and dissipate large amounts of plastic strain energy. To this end, shake-table tests were conducted on a 2/5-scale RC frame with hysteretic dampers. The frame was designed only for gravitational loads. The dampers provided lateral strength and stiffness, respectively, three and 12 times greater than those of the frame. The test structure was subjected to a sequence of seismic simulations that represented different levels of seismic hazard. The RC frame showed a performance level of "immediate occupancy", with maximum rotation demands below 20% of the ultimate capacity. The dampers dissipated most of the energy input by the earthquake. It is shown that combining hysteretic dampers with flexible reinforced concrete frames leads to structures with improved seismic performance and that requirements of conventional RC frames (without dampers) can be relieved.

Inelastic torsional seismic response of nominally symmetric reinforced concrete frame structures: shaking table tests

This paper discusses the torsional response of a scaled reinforced concrete frame structure subjected to several uniaxial shaking table tests. The tested structure is nominally symmetric in the direction of shaking and exhibits torsion attributable to non-uniform yielding of structural components and uncertainties in the building process. Asymmetric behavior is analyzed in terms of displacement, strain in reinforcing bars, energy dissipated at plastic hinges, and damage at section and frame levels. The results show that for low levels of seismic hazard, for which the structure is expected to perform basically within the elastic range, the accidental eccentricity is not a concern for the health of the structure, but it significantly increases the lateral displacement demand in the frames (about 30%) and this might cause significant damage to non-structural components. For high levels of seismic hazard the effects of accidental torsion become less important. These results underline the need to consider accidental eccentricity in evaluating the performance of a structure for very frequent or frequent earthquakes, and suggest that consideration of torsion may be neglected for performance levels associated with rare or very rare earthquakes.

Modified Gutenberg-Richter coefficient for damage evaluation in reinforced concrete structures subjected to seismic simulations on a shaking table

This paper presents analysis and discussion of the b- and ib-values calculated from the acoustic emission (AE) signals recorded during dynamic shake-table tests conducted on a reinforced concrete (RC) frame subjected to several uniaxial seismic simulations of increasing intensity until collapse. The intensity of shaking was controlled by the peak acceleration applied to the shake-table in each seismic simulation, and it ranged from 0.08 to 0.47 times the acceleration of gravity. The numerous spurious signals not related to concrete damage that inevitably contaminate AE measurements obtained from complex dynamic shake-table tests were properly filtered with an RMS filter and the use of guard sensors. Comparing the b- and ib-values calculated through the tests with the actual level of macro-cracking and damage observed during testing, it was concluded that the limit value of 0.05 proposed in previous research to determine the onset of macro-cracks should be revised in the case of earthquake-type dynamic loading. Finally, the b- and ibvalues were compared with the damage endured by the RC frame evaluated both visually and quantitatively in terms of the inter-story drift index.

Polyolefin fiber-reinforced concrete enhanced with steel-hooked fibers in low proportions

Over the past few years, polyolefin fiber reinforced self-compacting concrete has shown high performance in both fresh and hardened state. Its fracture behavior for small deformations could be enhanced with a small amount of steel-hooked fibers, obtaining a hybrid fiber-reinforced concrete well suited for structural use. Four types of conventional fiber-reinforced concrete with steel and polyolefin fibers were produced on the basis of the same self-compacting concrete also manufactured as reference. These concrete mixtures were manufactured separately with the same fiber contents being subsequently used for two more hybrid mixtures. Fracture properties, in addition to fresh and mechanical properties, were assessed. The research showed both synergies (with the two types of fibers working together in the fracture processes) and an improvement of the orientation and distribution of the fibers on the fracture surface

A methodology to calibrate structural finite element models for reinforced concrete structures subject to blast loads

Civil buildings are not specifically designed to support blast loads, but it is important to take into account these potential scenarios because of their catastrophic effects, on persons and structures. A practical way to consider explosions on reinforced concrete structures is necessary. With this objective we propose a methodology to evaluate blast loads on large concrete buildings, using LS-DYNA code for calculation, with Lagrangian finite elements and explicit time integration. The methodology has three steps. First, individual structural elements of the building like columns and slabs are studied, using continuum 3D elements models subjected to blast loads. In these models reinforced concrete is represented with high precision, using advanced material models such as CSCM_CONCRETE model, and segregated rebars constrained within the continuum mesh. Regrettably this approach cannot be used for large structures because of its excessive computational cost. Second, models based on structural elements are developed, using shells and beam elements. In these models concrete is represented using CONCRETE_EC2 model and segregated rebars with offset formulation, being calibrated with continuum elements models from step one to obtain the same structural response: displacement, velocity, acceleration, damage and erosion. Third, models basedon structural elements are used to develop large models of complete buildings. They are used to study the global response of buildings subjected to blast loads and progressive collapse. This article carries out different techniques needed to calibrate properly the models based on structural elements, using shells and beam elements, in order to provide results of sufficient accuracy that can be used with moderate computational cost.

Accommodating cabins as a new way of building houses

The current economic crisis has meant, particularly in Spain, the almost cessation of new buildings construction. This deep crisis will mean in future an irreversible change in the Spanish construction model, based to date almost exclusively on the brick. The project “Accommodating cabins as a new way of building houses” is part of a larger research within the line “Modular Architecture” developed by the Research Group “Design and Industrial Production”, belonging to the Technical University of Madrid, which aims to respond to the need for decent housing at an affordable price, by offering through Internet the plans, resources and other technical details required to build a house oneself. The proposed houses are built from the combination of industrially made modules (accommodation cabins, which are prefabricated modules usually used as provisional constructions in conventional building works), prefabricated subsystems and other catalogue components available on the market, all they set together by dry joints.

Polyolefin fibre-reinforced concrete : from material behaviour to numerical and design considerations

La presente Tesis proporciona una gran cantidad de información con respecto al uso de un nuevo y avanzado material polimérico (con base de poliolefina) especialmente adecuada para ser usada en forma de fibras como adición en el hormigón. Se han empleado fibras de aproximadamente 1 mm de diámetro, longitudes entre 48 y 60 mm y una superficie corrugada. Las prometedoras propiedades de este material (baja densidad, bajo coste, buen comportamiento resistente y gran estabilidad química) justifican el interés en desarrollar el esfuerzo de investigación requerido para demostrar las ventajas de su uso en aplicaciones prácticas. La mayor parte de la investigación se ha realizado usando hormigón autocompactante como matriz, ya que este material es óptimo para el relleno de los encofrados del hormigón, aunque también se ha empleado hormigón normal vibrado con el fin de comparar algunas propiedades. Además, el importante desarrollo del hormigón reforzado con fibras en los últimos años, tanto en investigación como en aplicaciones prácticas, también es muestra del gran interés que los resultados y consideraciones de diseño que esta Tesis pueden tener. El material compuesto resultante, Hormigón Reforzado con Fibras de Poliolefina (HRFP o PFRC por sus siglas inglesas) ha sido exhaustivamente ensayado y estudiado en muchos aspectos. Los resultados permiten establecer cómo conseguidos los objetivos buscados: -Se han cuantificado las propiedades mecánicas del PFRC con el fin de demostrar su buen comportamiento en la fase fisurada de elementos estructurales sometidos a tensiones de tracción. -Contrastar los resultados obtenidos con las bases propuestas en la normativa existente y evaluar las posibilidades para el uso del PFRC con fin estructural para sustituir el armado tradicional con barras de acero corrugado para determinadas aplicaciones. -Se han desarrollado herramientas de cálculo con el fin de evaluar la capacidad del PFRC para sustituir al hormigón armado con las barras habituales de acero. -En base a la gran cantidad de ensayos experimentales y a alguna aplicación real en la construcción, se han podido establecer recomendaciones y consejos de diseño para que elementos de este material puedan ser proyectados y construidos con total fiabilidad. Se presentan, además, resultados prometedores en una nueva línea de trabajo en el campo del hormigón reforzado con fibras combinando dos tipologías de fibras. Se combinaron fibras de poliolefina con fibras de acero como refuerzo del mismo hormigón autocompactante detectándose sinergias que podrían ser la base del uso futuro de esta tecnología de hormigón. This thesis provides a significant amount of information on the use of a new advanced polymer (polyolefin-based) especially suitable in the form of fibres to be added to concrete. At the time of writing, there is a noteworthy lack of research and knowledge about use as a randomly distributed element to reinforce concrete. Fibres with an approximate 1 mm diameter, length of 48-60 mm, an embossed surface and improved mechanical properties are employed. The promising properties of the polyolefin material (low density, inexpensive, and with good strength behaviour and high chemical stability) justify the research effort involved and demonstrate the advantages for practical purposes. While most of the research has used self-compacting concrete, given that this type of matrix material is optimum in filling the concrete formwork, for comparison purposes standard vibration compacted mixes have also been used. In addition, the interest in fibre-reinforced concrete technology, in both research and application, support the significant interest in the results and considerations provided by the thesis. The resulting composite material, polyolefin fibre reinforced concrete (PFRC) has been extensively tested and studied. The results have allowed the following objectives to be met: -Assessment of the mechanical properties of PFRC in order to demonstrate the good performance in the post-cracking strength for structural elements subjected to tensile stresses. -- Assessment of the results in contrast with the existing structural codes, regulations and test methods. The evaluation of the potential of PFRC to meet the requirements and replace traditional steel-bar reinforcement applications. -Development of numerical tools designed to evaluate the capability of PFRC to substitute, either partially or totally, standard steel reinforcing bars either alone or in conjunction with steel fibres. -Provision, based on the large amount of experimental work and real applications, of a series of guidelines and recommendations for the practical and reliable design and use of PFRC. Furthermore, the thesis also reports promising results about an innovative line in the field of fibre-reinforced concrete: the design of a fibre cocktail to reinforce the concrete by using two types of fibres simultaneously. Polyolefin fibres were combined with steel fibres in self-compacting concrete, identifying synergies that could serve as the base in the future use of fibre-reinforced concrete technology.

Lime rendering of the houses of historical core of Maracaibo, Venezuela

An advance of the study undertaken of the plastering of the few dwellings that still exist in the historical area of Maracaibo is presented, with a base of restoration mortar of the plastering of sandstone known as “piedra de ojo”. The mortars are made of lime with a ferruginous stone aggregate that gives them a pinkish color, with additions of ceramics, bamboo, sand, wood and straw, and kneaded with salty water from Lake Maracaibo

Analysis of Combined Bending and Punching Tests of Reinforced Concrete Slabs within IMPACT III Project

The paper reports on a collaborative effort between the Swiss Federal Nuclear Safety Inspectorate (ENSI) and their consultants Principia and Stangenberg. As part of the IMPACT III project, reduced scale impact tests of reinforced concrete structures were carried out. The simulation of test X3 is presented here and the numerical results are compared with those obtained in the test, carried out in August 2013. The general object is to improve the safety of nuclear facilities and, more specifically, to demonstrate the capabilities of current simulation techniques to reproduce the behaviour of a reinforced concrete structure impacted by a soft missile. The missile is a steel tube with a mass of 50 kg and travelling at 140 m/s. The target is a 250 mm thick, 2,1 m by 2,1 m reinforced concrete wall, held in a stiff supporting frame. The reinforcement includes both longitudinal and transverse rebars. Calculations were carried out before and after the test with Abaqus (Principia) and SOFiSTiK (Stangenberg). In the Abaqus simulation the concrete is modelled using solid elements and a damaged plasticity formulation, the rebars with embedded beam elements, and the missile with shell elements. In SOFiSTiK the target is modelled with non-linear, layered shell elements for the reinforcement on both sides; non-linear shear deformations of shell/plate elements are approximately included. The results generally indicate a good agreement between calculations and measurements.