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FULL SCALE EARTHQUAKE DESTRUCTION TEST

Melbourne University

 

Authorised by DCI Pty Ltd, the University of Melbourne did the CORCONâ  Concrete Slab System Seismicity Test during 2002 AND 2003. This testing has demonstrated the CORCONâ   system to be the most earthquake proof and the strongest such system in the  world. (Figure below: Andy with the test group)

 

 

1.0 Test Specimen

 

The test specimen selected was interior beam-column joint with 300 mm deep Corcon beam and 250x 250 mm column. The height of the column is 1900 mm and the beam length is 4800 mm. The width of the slab is 1200 mm, which represents the standard flange width for Corcon beam-slab system.

 

The ribbed beam was detailed according to current Australian design practice with no special provision for seismicity. The entire slab and the column were concreted in the lab where the test rig was set-up. The specimen hung freely at the center of the test rig from the upper column end while bottom column was fixed to the actuator arm, which was used to impose horizontal displacements. The beam-ends were connected to the test rig with pin connections, which can provide limited horizontal movements.

 

 

2.0 Instrumentation for data collection

 

Three types of instrumentations, i.e. Strain gauges, displacement transducers and load cells, were used to monitor the behavior of the specimen during the test. All the data from the instruments were collected through a computer based data logging system.

 

 

 

3.0 Observations

 

There were no visible cracks in the specimen before the test and after applying the initial gravity loading. All new cracks and extensions of old cracks were numbered according to the cycle number in which they were first seen.

 

The first flexural crack in the top surface of the beam was observed at a nominal specimen drift ratio of 1.0 %. The width of the crack at this stage was very small and varied from around 0.25 mm to 0.1 mm along the length of the crack. As the specimen drift was increased, flexural cracks propagated away from the beam column connection. At the most extreme level of displacement (Cycle 8 - drift ratio of 3.0%) the crack that appeared first was widened in the order of 0.8 mm. During the last cycle with nominal drift ratio of 4 %, a large crack which is in the order of 4-5 mm appeared on the flange slab and propagated in to the beam depth, on the corresponding location where the beam top reinforcements were curtailed. It was also noted that slight concrete crushing & spalling at the column face where the ribbed beam connects the column.

 

The specimen appears to perform fairly well under lateral deformations up to high drift levels. The damage that was observed did not appear to be catastrophic; rather, cracking which occurred was associated with a ductile flexural mechanism. There were no cracks appeared due to secondary effects.

 

The recorded hysteretic response is shown in figure 1.0. It is observed that in the positive load cycles the maximum actuator load is achieved in cycle 8 with 3 % drift ratio. However in negative load cycles it was achieved in cycle 6 with 2.0% drift ratio. As it can be seen from the "fatness" of the hysteretic response, the system as a whole seems to have little energy absorption capacity. At large displacements, the system remains fairly elastic though at stiffness lower than the original stiffness. This degradation in the stiffness is associated with flexural cracking and loss of anchorage of beam reinforcements.

Figure 1.0

4.0 Comparison of Corcon slab system performance with other similar slab system

The main objective of this section is to evaluate and compare the performance of the Corcon ribbed slab system with similar slab system. Ideally Corcon slab system should be compared with a conventional ribbed slab system with 600 mm rib spacing. As such test data is not available readily, the comparison was done using previous test results of three interior beam-to-column sub assemblages. (Ahmad et al. 1987)

4.1 Test Specimens used in previous study

Each sub assembly consisted of main beams, transverse beam, top and bottom columns, and a floor slab. The length of the beams and the height of the columns represented one half of the span and the storey height, respectively, which is exactly similar to Corcon test specimen. The typical member cross sections have been used as follows:

Main Beam – 419x279 mm, Transverse Beam- 381x279 mm, column – 362x362 mm and slab –100 mm thick & 1003 mm wide.

The typical column height of 2248 mm and beam length of 2496 mm has been used in all specimens. The three different specimens had been tested with different joint shear stress and the amount of joint transverse reinforcement. The design of frame members was based on the ACI 318-77 Building code.

4.2 Results of Previous Test

Testing of beam column sub assemblies had been done similar to Corcon test set up except the hydraulic actuator had been connected to upper end of column instead of connecting to lower end. The test had been performed for seven cycles starting from 1% to 4% storey drift.

The change in strength and stiffness of the three specimens during the loading cycles could be observed from the lateral load versus lateral displacement (hysteresis loops) as reported by Ahmad et al. 1987.

The results of test specimen can be analysed in terms of (1) crack width and cracking pattern, (2) strength degradation, (3) loss of stiffness, (4) energy dissipation, and (5) the slippage of beam column bars through the joint. For the comparison purpose of two structural formes, out of above, loss of stiffness and energy dissipation aspects could be used more effectively to evaluate performance of the system under earthquakes. It should be noted that most of above aspects measures the behaviour of sub assembly as a whole, the marginal design of the joints and the column to beam flexural strength will make characteristically different behaviour. The energy dissipation of each specimen distinctly varies with specimens having different joint shear stress level and Joint reinforcement. (Ahmad et al. 1985) Considering the all aspects, loss of stiffness character was selected to evaluate the two systems.

4.3 Comparison of Loss of stiffness of Corcon and ‘T’ Beam system

The hysteresis loops of both systems were used to determine the stiffness degradation. The average peak-to-peak stiffness degradation of the specimens is illustrated in Figure 2. For each specimen the stiffness is shown as a percentage of the initial stiffness. As reported by Ahmad et al. 1985, specimens with different levels of joint shear stress and joint confinement reinforcements have very little effect on the behaviour of the stiffness degradation. It noted that the loss of average peak-to-peak stiffness at the end of the seventh cycle was approximately the same magnitude for all three specimens in spite of the different level of confinement and the joint shear stress. However Corcon beam showed slightly lower stiffness at its last cycle (9th cycle), which may be due to the local failure of reinforcement anchorage near the top reinforcement curtailment point.

4.4 Conclusions

The following conclusions can be drawn from the results of Corcon specimen and three specimens used for comparison purposes.

·         Reference to the figure 2, it is clear that CORCONâ  specimen distinctly displayed a superior behaviour over the other specimens.

·         It should be noted that Corcon test specimen used was not provided with any form of joint confining reinforcement or transverse beam. Whereas all other specimens were provided confining reinforcements and transverse beam, both will improve the joint behaviour and thus the behaviour of subassembly as a whole.

Figure 2

4.5 Proposal and Further study of Corcon slab system

4.5.1 Feasibility of use of Deformed Wire Fabric as Shear Reinforcement in Corcon Ribbed Beams.

As a case study if we consider 8.4 m x 8.4 m Corcon slab panel, the number of ribbed beam in one panel will be 7 Nos. Considering nominal shear link requirement of R6-200 mm c/c, the number of links required per one rib beam would be 43 Nos. Therefore the total number of links in a 8.4x8.4 m panel (70.56 m2) would be 301 Nos, which means that 4.26 shear links per square metre.

Just consider a floor slab having 8.4 x 8.4 m panel 5 bay ribbed slab, the amount of shear links would be 5x5x301 = 7525 Nos. I believed that lot of on site work as far as labour involvement is considered, Specially in Australian context where the labour cost is very high. Therefore the use of bent-up Deformed Welded Wire Fabric (WWF) should be financially feasible.

The WWF as a shear links has been studied previously. As reported by Mansur 1987, Deformed WWF as web reinforcement in beams provides a significant improvement in the control of diagonal cracking than an equivalent amount of conventional mild steel stirrups or smooth WWF. Since it is difficult and uneconomical to provide standard hooks or bends in the bent-up WWF cages, two cross wires welded at a spacing of 50 mm at the open end of bent-up WWF cages as used in previous study provided satisfactory anchorage of the stirrups.

 References:

1.      Ahmad j., Durrani and James K. Wight " Behaviour of Interior Beam –to column Connections under Earthquake Type loading," ACI Structural Journal, 82(3), May-June 1985, pp. 343-349.

  1. Ahmad j., Durrani and James K. Wight " Earthquake Resistance of Reinforced Concrete Interior Connections Including a Floor Slab," ACI Structural Journal, 84-S42 (5), September-October 1987, pp. 400-406.
  2. Mansur, M. A., Lee, S. L., and Lee, C. K., " Deformed Wire Fabric as Shear Reinforcement in Concrete Beams" ACI Structural Journal, 84-S41, September-October 1987, pp. 392-399.  

 


SEISMIC PERFORMANCE OF CONCRETE BEAM-SLAB-COLUMN SYSTEMS CONSTRUCTED WITH A RE-USEABLE SHEET METAL FORMWORK SYSTEM

 

by

 

Upul Perera

 

Submitted in total fulfillment of the requirements

of the degree of

Master of Engineering Science by Research

ABSTRACT

This report describes an investigation of seismic performance of a ribbed slab system constructed with an innovative re-usable sheet metal formwork system. Experimental results from quasi-static cyclic lateral load tests on half-scale reinforced concrete interior beam-slab-column subassemblages are presented. The test specimen was detailed according to the Australian code (AS 3600) without any special provision for seismicity. This specimen was tested up to a drift ratio of 4.0 %. Some reinforcement detailing problems were identified from the first test. The damaged specimen was then rectified using Carbon Fibre Reinforced Polymer (CFRPs), considering detailing deficiencies identified in the first test. The repaired test specimen was tested under a lateral cyclic load as per the original test arrangement up to a drift level of 4%. The performance of the repaired specimen showed significant improvement with respect to the level of damage and strength degradation. The results of the rectified specimen indicate that the use of CFRPs may offer a viable retrofit/repair strategy in the case of damaged structures, where this damage may be significant.

Two finite element analysis models were created and results of the first test were used to calibrate the FE model. The second FE model was used to obtain detail information about stress and strain behaviour of various components of the beam-column subassemblage and to check the overall performance before carrying out expensive lab tests. It was concluded that finite element modelling predictions were reliable and could be used to obtain more information compared to conventional type laboratory tests.

Time-history analyses show that the revised detailing is suitable to withstand very large earthquakes without significant structural damage.

 

Acknowledgements

 

I would like to express my gratitude to my supervisor Dr. Priyan Mendis for initiating this project and for providing continuous support and encouragement.

I thank other academics, Dr Nelson Lam and Dr.John Wilson for teaching me a lot about earthquakes and also Dr. Nick Haritos for teaching me about structural theory. I thank Dr John Stehle for helping me in finite element modeling issues.

 

The financial support provided by Andy Stodulka of Decoin Pty. Ltd and Australian Research Council are greatly appreciated.

I would like to sincerely thank Andrew Sarkady of MBT (Aust) Pty. Ltd. for materials support for this test program. I would like to acknowledge Richard O’Connorat staff of Structural Systems Pty.Ltd. for carrying out the rectification work.

I would like to thank Grant Rivett and Graeme Bannister, the laboratory technicians for their help, thoughtfulness and dedication in the undertaking of the experimental work.

I thank my wife Thushari, and two sons Matheesha and Kaveesha for their patience and support without which this project would not have been possible.

Chapter 1

 

INTRODUCTION

 

A.      

1.1                Background

When designing for earthquake induced loading, most conventional, popular gravity dominated structural systems possess a major inherent deficiency because of undesirable member proportions. Many structures designed and constructed in Australia belong to this category. The purpose of this study is to investigate the seismic performance of a beam-slab-column system constructed with a re-usable sheet metal formwork system, which is becoming popular in Australia and overseas. This innovative formwork system, Corcon, has been developed and patented throughout the world, by the industry partner, Andy Stodulka of Decoin Pty Ltd.

‘Corcon’ derives its name from the combination of CORrugation and CONcrete. This reusable lightweight sheet metal form system optimises the traditional rib slab construction by using corrugated arch metal sheet spanning over series of sheet metal beam moulds to form the suspended concrete slab. The corrugated arched metal sheet enables the rib beam spacing to be increased to 1200 mm from the conventional 600 mm.

There have been no investigations reported on the seismic behaviour of concrete beam-arch slab systems, both locally and internationally. Decoin Pty Ltd., the industry partner will work with the University of Melbourne to find an appropriate and economical solution for this important problem.

1.2                 Purpose

The purpose of the research presented in this thesis is to investigate the seismic performance of Corcon slab system for various levels of seismicity, with the aim that design recommendations are to be formulated.

The main goal is to assess current Australian design practice and to provide design guidelines for these beam-slab-column systems constructed with the Corcon form work system and to find a detailing strategy which will ensure a sufficient level of ductility for various levels of seismic demands.

1.3                Means to achieve outcomes

The seismic performance of Corcon slab system is to be assessed through experimentally and analytically.

A theoretical model of four-storey framed structure equivalent to those in a typical frame structure constructed with Corcon system is designed and detailed according to the existing rules given in the Australian Concrete Structures Code, AS 3600. The Program RUAUMOKO is used to predict the inelastic dynamic responses of the frame structure, and to determine the expected maximum drift levels for different levels of seismicity.

The experimental work, consists of two tests and is conducted taking an isolated half-scale Corcon interior beam-column subassembly to understand the real Corcon slab performs under cyclic lateral load. The second test was conducted after repairing the damaged first specimen to test the effectiveness of the modified detailing.

The finite element modeling of the sub assemblage is performed using Program ANSYS. The experimental results are used to calibrate the finite element model. The second finite element model is prepared and used to test the performance with improved reinforcement detailing to overcome deficiencies identified in the experiment. 

1.4                Aims

·         Seismic performance of existing Corcon system designed for gravity loads.

·         --------- performance of a similar system retrofitted with CFRP.

1.5                Arrangement of the thesis

This thesis is presented in the following manner:

 

Chapter 2 presents a range of earthquake engineering topics and structural modelling aspects; a review of literature related to experimental testing, current design practice, theoretical strength evaluation and modeling techniques such as finite element analysis.

Chapter 3 deals with construction and testing of interior Corcon rib beam-column subassemblages tested in the Francis Laboratory at The University of Melbourne.

Chapter 4 presents the results from the half scale interior Corcon rib beam-column subassemblage.

Chapter 5 presents the analytical component of this investigation, such as finite element analysis and time history analysis.

Chapter 6 gives the overall conclusions and future work.

Chapter 2

 

LITERATURE REVIEW

 

B.      

1.6                Introduction

This chapter presents an overview of previous work on related topics that provide the necessary background for the purpose of this research. The literature review concentrates on a range of earthquake engineering topics and structural modelling aspects. For the understanding of seismic capacity, a review of literature is required in experimental testing, current design practice, theoretical strength evaluation and modelling techniques such as finite element modelling. The literature review begins with a coverage of general earthquake engineering topics, which serves to set the context of the research.

At present, there is no information available on seismic performance of arched rib slab systems. However, research on similar types of systems have been conducted and the available literature on those projects reviewed in following sections.

1.7                Earthquake design techniques

The objective of design codes is to have structures that will behave elastically under earthquakes that can be expected to occur more than once in the life of the building.  It is also expected that the structure would survive major earthquakes without collapse that might occur during the life of the building. To avoid collapse during a large earthquake, members must be ductile enough to absorb and dissipate energy by post-elastic deformations.  Nevertheless, during a large earthquake the deflection of the structure should not be such as to endanger life or cause a loss of structural integrity. Ideally, the damage should be repairable.  The repair may require the replacement of crushed concrete and/or the injection of epoxy resin into cracks in the concrete caused by yielding of reinforcement.  In some cases, the order of ductility involved during a severe earthquake may be associated with large permanent deformations and in those cases, the resulting damage could be beyond repair.

Even in the most seismically active areas of the world, the occurrence of a design earthquake is a rare event.  In areas of the world recognised as being prone to major earthquakes, the design engineer is faced with the dilemma of being required to design for an event, which has a small chance of occurring during the design life time of the building. If the designer adopts conservative performance criteria for the design of the building, the client will be faced with extra costs, which may be out of proportion to the risks involved.  On the other hand, to ignore the possibility of a major earthquake could be construed as negligence in these circumstances. To overcome this problem, buildings designed to these prescriptive provisions would (1) not collapse under very rare earthquakes; (2) provide life safety for rare earthquakes; (3) suffer only limited repairable damage in moderate shaking; and (4) be undamaged in more frequent, minor earthquakes.

The design seismic forces acting on a structure as a result of ground shaking are usually determined by one of the following methods:

 

·         Static analysis, using equivalent seismic forces obtained from response spectra for horizontal earthquake motions.

·         Dynamic analysis, either modal response spectrum analysis or time history analysis with numerical integration using earthquake records.

 

C.     Static analysis

Although earthquake forces are of dynamic nature, for majority of buildings, equivalent static analysis procedures can be used. These have been developed on the basis of considerable amount of research conducted on the structural behaviour of structures subjected to base movements.  These methods generally determine the shear acting due to an earthquake as equivalent static base shear.  It depends on the weight of the structure, the dynamic characteristics of the building as expressed in the form of natural period or natural frequency, the seismic risk zone, the type of structure, the geology of the site and importance of the building.

The natural frequency, which is the reciprocal of natural period, can be calculated using the following formulae (Smith et al., 1991) as given in Table B‑1.

Table B1: Formulae to calculate the fundamental natural frequency of a building

(Smith & Coull, 1991).

Formula

Notation

Type of lateral load resisting system

No = D1/2/0.091H

D = base dimension in the direction

      of motion in meters. 

H = height of the building in meters

Reinforced concrete shear wall buildings and braced steel frames

No = 10/N

N = number of storeys

Moment resisting frame

No = 1/CTH3/4

CT= 0.035 for steel structures, 0.025

        for concrete structures,

H = height of the building in feet

Moment resisting frame is the sole lateral load resisting system.

No = 46/H

H = height of the building in meters

For any type of building

 The static equivalent earthquake load mainly depends on the accuracy of natural period calculation. The Australian code