308 research outputs found
Efficient strategy for modelling punching failure of flat slabs
This thesis develops a novel modelling strategy depicted JSPM (joint-shell punching model) for simulating punching failure of reinforced concrete (RC) slabs in which non-linear joint elements are combined with nonlinear 2-D shell elements. Punching failure of the nonlinear joint is governed by the failure criterion of the critical shear crack the-ory (CSCT) of Muttoni (2008). A notable feature of the JSPM is that joint punching resistance is continually updated during the analysis in terms of the slab sector rotation calculated at the previous load step. This feature enables the JSPM to accurately simu-late the slab-column connection behaviour from the initial load stage, occurrence of punching (peak), followed by a transition to post-punching stage without the need of post-processing. This modelling strategy has been implemented in the nonlinear struc-tural analysis program ADAPTIC (Izzuddin, 1991).
Throughout the thesis, the proposed JSPM has been extended to simulate various practical scenarios, including punching of: slabs supported on elongated column, slabs subjected to eccentric loading (both monotonic and reversed-cyclic), and slabs with shear reinforcement. In total, 90 internal slab-column connections from test database were simulated to verify the proposed JSPM. In addition, NLFEA based on 3-D solid elements were simulated in ATENA (Cervenka et al., 2018) to provide an objective comparison (benchmark). The JSPM is shown to produce accurate predictions of the measured slab-column connection behaviour while requiring significantly less computa-tion time than the NLFEA with solid elements.
The analysis and comparison of the numerical and test data were used to inform design procedures: including: a) shear-field method to design slabs supported on elongated column or wall; b) simplified analytical method to determined drift-induced punching for slabs subjected to reversed-cyclic loading.Open Acces
Numerical and analytical investigation of internal slab-column connections subject to cyclic loading
Properly designed flat slab to column connections can perform satisfactorily under seismic loading. Satisfactory performance is dependent on slab column connections being able to withstand the imposed drift while continuing to resist the imposed gravity loads. Particularly at risk are pre 1970’s flat slab to column connections without integrity reinforcement passing through the column. Current design provisions for punching shear under seismic loading are largely empirical and based on laboratory tests of thin slabs not representative of practice. This paper uses nonlinear finite element analysis (NLFEA) with ATENA and the Critical Shear Crack Theory (CSCT) to investigate the behaviour of internal slab-column connections without shear reinforcement subject to seismic loading. NLFEA is used to investigate cyclic degradation which reduces connection stiffness, unbalanced moment capacity, and ductility. As observed experimentally, cyclic degradation in the NLFEA is shown to be associated with accumulation of plastic strain in the flexural reinforcement bars which hinders concrete crack closure. Although the NLFEA produces reasonable strength and ductility predictions, it is unable to replicate the pinching effect. It is also too complex and time consuming to serve as a practical design tool. Therefore, a simple analytical design method is proposed which is based on the CSCT. The strength and limiting drift predictions of the proposed method are shown to mainly depend on slab depth (size effect) and flexural reinforcement ratio which is not reflected in available empirically-based models which appear to overestimate the drift capacity of slab-column connections with dimensions representative of practice
A model for the prediction of the punching resistance of steel fibre reinforced concrete slabs centrically loaded
With the aim of contributing for the development of design guidelines capable of predicting with high
accuracy the punching resistance of steel fibre reinforced concrete (SFRC) flat slabs, a proposal is presented
in the present paper and its predictive performance is assessed by using a database that collects the
experimental results from 154 punching tests. The theoretical fundaments of this proposal are based on the
critical shear crack theory proposed by Muttoni and his co-authors. The proposal is capable of predicting the
load versus rotation of the slab, and attends to the punching failure criterion of the slab. The proposal takes
into account the recommendations of the most recent CEB-FIP Model Code for modelling the post-cracking
behaviour of SFRC. By simulating the tests composing the collected database, the good predictive
performance of the developed proposal is demonstrated.Fundação para a Ciência e a Tecnologia (FCT
Deformation capacity evaluation for flat slab seismic design
The authors acknowledge the dedicated and careful work carried out by the Associate Editor and Reviewers whose constructive criticism contributed to a very significant improvement of the quality of the paper. Authors wish to dedicate this work to the memory of their co-author Prof. Ion Radu Pascu, UTCB Bucharest, who passed away on June 10, 2021.
Publisher Copyright:
© 2021, The Author(s).In flat-slab frames, which are typically designed as secondary seismic structures, the shear failure of the slab around the column (punching failure) is typically the governing failure mode which limits the deformation capacity and can potentially lead to a progressive collapse of the structure. Existing rules to predict the capacity of flat slab frames to resist imposed lateral displacements without losing the capability to bear gravity loads have been derived empirically from the results of cyclic tests on thin members. These rules account explicitly only for the ratio between acting gravity loads and resistance against concentric punching shear (so-called Gravity Shear Ratio). Recent rational models to estimate the deformation capacity of flat slabs show that other parameters can play a major role and predict a significant size effect (reduced deformation for thick slabs). In this paper, a closed-form expression to predict the deformation capacity of internal slab-column connections as a function of the main parameters is derived from the same model that has been used to develop the punching shear formulae for the second generation of Eurocode 2 for concrete structures. This expression is compared to an existing database of isolated internal slab-column connections showing fine accuracy and allowing to resolve the shortcomings of existing rules. In addition, the results of a testing programme on a full-scale flat-slab frame with two stories and 12 columns are described. The differences between measured interstorey drifts and local slab rotations influencing their capacity to resist shear forces are presented and discussed. With respect to the observed deformation capacities, similar values are obtained as in the isolated specimens and the predictions are confirmed for the internal columns, but significant differences are observed between internal, edge and corner slab-column connections. The effects of punching shear reinforcement and of integrity reinforcement (required according to Eurocode 2 to prevent progressive collapse after punching) are also discussed.publishersversionpublishe
Numerical modelling and parametric assessment of hybrid flat slabs with steel shear heads
This investigation examines the performance of hybrid reinforced concrete flat slabs, incorporating fully-integrated shear-heads at connections to steel columns, through a series of numerical evaluations and parametric studies. Validations of the adopted nonlinear finite element procedures, which employ concrete damage plasticity constitutive models, are carried out against experimental results on hybrid members. Complementary verifications on conventional reinforced concrete flat slabs are also undertaken to ensure the reliability of the selected ranges for key modelling parameters. Comparison of the numerical simulations against the test results shows close correlations in terms of ultimate strength, deformations and stress levels in the constituent elements of hybrid members. This is followed by a series of parametric assessments on key structural parameters for hybrid flat slabs with steel shear heads. The results of these investigations enable the identification of three modes of failure as a function of the interaction between the shear-head and surrounding concrete. The findings permit the development of improved analytical models for predicting the response as well as the ultimate strength of such members. In addition, recommendations are given for the determination of shear-head dependent parameters, which are required for practical design purposes, with a particular focus on the embedment length and section size of the shear-head elements. The suggested expressions for assessing the shear-head characteristics offer a more reliable design approach in comparison with existing methods and are suitable for effective practical application and implementation in codified procedures
An Assessment to Benchmark the Seismic Performance of a Code-Conforming Reinforced-Concrete Moment-Frame Building
This report describes a state-of-the-art performance-based earthquake engineering methodology
that is used to assess the seismic performance of a four-story reinforced concrete (RC) office
building that is generally representative of low-rise office buildings constructed in highly seismic
regions of California. This “benchmark” building is considered to be located at a site in the Los
Angeles basin, and it was designed with a ductile RC special moment-resisting frame as its
seismic lateral system that was designed according to modern building codes and standards. The
building’s performance is quantified in terms of structural behavior up to collapse, structural and
nonstructural damage and associated repair costs, and the risk of fatalities and their associated
economic costs. To account for different building configurations that may be designed in
practice to meet requirements of building size and use, eight structural design alternatives are
used in the performance assessments.
Our performance assessments account for important sources of uncertainty in the ground
motion hazard, the structural response, structural and nonstructural damage, repair costs, and
life-safety risk. The ground motion hazard characterization employs a site-specific probabilistic
seismic hazard analysis and the evaluation of controlling seismic sources (through
disaggregation) at seven ground motion levels (encompassing return periods ranging from 7 to
2475 years). Innovative procedures for ground motion selection and scaling are used to develop
acceleration time history suites corresponding to each of the seven ground motion levels.
Structural modeling utilizes both “fiber” models and “plastic hinge” models. Structural
modeling uncertainties are investigated through comparison of these two modeling approaches,
and through variations in structural component modeling parameters (stiffness, deformation
capacity, degradation, etc.). Structural and nonstructural damage (fragility) models are based on
a combination of test data, observations from post-earthquake reconnaissance, and expert
opinion. Structural damage and repair costs are modeled for the RC beams, columns, and slabcolumn connections. Damage and associated repair costs are considered for some nonstructural
building components, including wallboard partitions, interior paint, exterior glazing, ceilings,
sprinkler systems, and elevators. The risk of casualties and the associated economic costs are
evaluated based on the risk of structural collapse, combined with recent models on earthquake
fatalities in collapsed buildings and accepted economic modeling guidelines for the value of
human life in loss and cost-benefit studies.
The principal results of this work pertain to the building collapse risk, damage and repair
cost, and life-safety risk. These are discussed successively as follows.
When accounting for uncertainties in structural modeling and record-to-record variability
(i.e., conditional on a specified ground shaking intensity), the structural collapse probabilities of
the various designs range from 2% to 7% for earthquake ground motions that have a 2%
probability of exceedance in 50 years (2475 years return period). When integrated with the
ground motion hazard for the southern California site, the collapse probabilities result in mean
annual frequencies of collapse in the range of [0.4 to 1.4]x10
-4
for the various benchmark
building designs. In the development of these results, we made the following observations that
are expected to be broadly applicable:
(1) The ground motions selected for performance simulations must consider spectral
shape (e.g., through use of the epsilon parameter) and should appropriately account for
correlations between motions in both horizontal directions;
(2) Lower-bound component models, which are commonly used in performance-based
assessment procedures such as FEMA 356, can significantly bias collapse analysis results; it is
more appropriate to use median component behavior, including all aspects of the component
model (strength, stiffness, deformation capacity, cyclic deterioration, etc.);
(3) Structural modeling uncertainties related to component deformation capacity and
post-peak degrading stiffness can impact the variability of calculated collapse probabilities and
mean annual rates to a similar degree as record-to-record variability of ground motions.
Therefore, including the effects of such structural modeling uncertainties significantly increases
the mean annual collapse rates. We found this increase to be roughly four to eight times relative
to rates evaluated for the median structural model;
(4) Nonlinear response analyses revealed at least six distinct collapse mechanisms, the
most common of which was a story mechanism in the third story (differing from the multi-story
mechanism predicted by nonlinear static pushover analysis);
(5) Soil-foundation-structure interaction effects did not significantly affect the structural
response, which was expected given the relatively flexible superstructure and stiff soils.
The potential for financial loss is considerable. Overall, the calculated expected annual
losses (EAL) are in the range of 97,000 for the various code-conforming benchmark
building designs, or roughly 1% of the replacement cost of the building (3.5M, the fatality rate translates to an EAL due to
fatalities of 5,600 for the code-conforming designs, and 66,000, the monetary value associated with life loss is small,
suggesting that the governing factor in this respect will be the maximum permissible life-safety
risk deemed by the public (or its representative government) to be appropriate for buildings.
Although the focus of this report is on one specific building, it can be used as a reference
for other types of structures. This report is organized in such a way that the individual core
chapters (4, 5, and 6) can be read independently. Chapter 1 provides background on the
performance-based earthquake engineering (PBEE) approach. Chapter 2 presents the
implementation of the PBEE methodology of the PEER framework, as applied to the benchmark
building. Chapter 3 sets the stage for the choices of location and basic structural design. The subsequent core chapters focus on the hazard analysis (Chapter 4), the structural analysis
(Chapter 5), and the damage and loss analyses (Chapter 6). Although the report is self-contained,
readers interested in additional details can find them in the appendices
Recommended from our members
Punching shear of concrete flat slabs reinforced with fibre reinforced polymer bars
Fibre reinforcement polymers (FRP) are non-corrodible materials used instead of
conventional steel and have been approved to be an effective way to overcome
corrosion problems. FRP, in most cases, can have a higher tensile strength, but
a lower tensile modulus of elasticity compared to that of conventional steel bars.
This study aimed to examine flat slab specimens reinforced with glass fibre
reinforced polymer (GFRP) and steel bar materials for punching shear behaviour.
Six full-scale two-way slab specimens were constructed and tested under
concentric load up to failure. One of the main objectives is to study the effect of
reinforcement spacing with the same reinforcement ratio on the punching shear
strength. In addition, two other parameters were considered, namely, slab depth,
and compressive strength of concrete.
The punching shear provisions of two code of practises CSA S806 (Canadian
Standards 2012) and JSCE (JSCE et al. 1997) reasonably predicted the load
capacity of GFRP reinforced concrete flat slab, whereas, ACI 440 (ACI
Committee 440 2015) showed very conservative load capacity prediction.
On the other hand, a dynamic explicit solver in nonlinear finite element (FE)
modelling is used to analyse a connection of column to concrete flat slabs
reinforced with GFRP bars in terms of ultimate punching load. All FE modelling was performed in 3D with the appropriate adoption of element size and mesh.
The numerical and experimental results were compared in order to evaluate the
developed FE, aiming to predict the behaviour of punching shear in the concrete
flat slab. In addition, a parametric study was created to explore the behaviour of
GFRP reinforced concrete flat slab with three parameters, namely, concrete
strength, shear load perimeter to effective depth ratio, and, flexural reinforcement
ratio. It was concluded that the developed models could accurately capture the
behaviour of GFRP reinforced concrete flat slabs subjected to a concentrated
load.
Artificial Neural Networks (ANN) is used in this research to predict punching
shear strength, and the results were shown to match more closely with the
experimental results. A parametric study was performed to investigate the effects
of five parameters on punching shear capacity of GFRP reinforced concrete flat
slab. The parametric investigation revealed that the effective depth has the most
substantial impact on the load carrying capacity of the punching shear followed
by reinforcement ratio, column perimeter, the compressive strength of the
concrete, and, the elastic modulus of the reinforcement
Assessment of the effectiveness of steel fibre reinforcement for the punching resistance of flat slabs by experimental research and design approach
The present paper deals with the experimental assessment of the effectiveness of steel fibre reinforcement in terms
of punching resistance of centrically loaded flat slabs, and to the development of an analytical model capable of
predicting the punching behaviour of this type of structures. For this purpose, eight slabs of 2550 x 2550 x 150 mm3
dimensions were tested up to failure, by investigating the influence of the content of steel fibres (0, 60, 75 and 90
kg/m3) and concrete strength class (50 and 70 MPa). Two reference slabs without fibre reinforcement, one for each
concrete strength class, and one slab for each fibre content and each strength class compose the experimental
program. All slabs were flexurally reinforced with a grid of ribbed steel bars in a percentage to assure punching
failure mode for the reference slabs. Hooked ends steel fibres provided the unique shear reinforcement. The results
have revealed that steel fibres are very effective in converting brittle punching failure into ductile flexural failure, by
increasing both the ultimate load and deflection, as long as adequate fibre reinforcement is assured. An analytical
model was developed based on the most recent concepts proposed by the fib Mode Code 2010 for predicting the
punching resistance of flat slabs and for the characterization of the behaviour of fibre reinforced concrete. The most
refined version of this model was capable of predicting the punching resistance of the tested slabs with excellent
accuracy and coefficient of variation of about 5%.The study presented in this paper is a part of the research project titled "SlabSys-HFRC - Flat slabs for multi-storey buildings using hybrid reinforced self-compacting concrete: an innovative structural system", with reference number of PTDC/ECM/120394/2010. The second author acknowledges the support provided by the Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior (CAPES) and Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq) grant, and the grant provided by the project SlabSys. The authors would acknowledge the collaboration of Casais Company on the preparation of the moulds and flexural reinforcement, CiviTest on the design of the SFRSCC for the slabs, and Maccaferri, Secil (Unibetao), and Sika companies for the supplying of steel fibres, concrete and superplasticizers, respectively
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