Experimental assessment of the in-plane lateral drift capacity of precast concrete building cores

Transcription

1 Experimental assessment of the in-plane lateral drift capacity of precast concrete building cores Scott J. Menegon Structural Engineer and PhD Candidate Centre for Sustainable Infrastructure Swinburne University of Technology, Melbourne John L. Wilson Professor and Executive Dean Faculty of Science, Engineering and Technology Swinburne University of Technology, Melbourne Nelson T. K. Lam Associate Professor and Reader Department of Infrastructure Engineering University of Melbourne, Melbourne Emad F. Gad Professor and Dean Department of Civil and Construction Engineering Swinburne University of Technology, Melbourne

2 Acknowledgements 1. The Dr William Piper Brown AM scholarship. 2. Australian Research Council (ARC) Discovery Project: Collapse Assessment of Reinforced Concrete Buildings in Regions of Lower Seismicity.

3 Presentation Overview 1. Overview of experimental testing being performed into RC walls and building cores 2. Preliminary results of large scale testing 3. Addressing the conference theme: the roles of structural engineers

4 4 large scale tests in total: 1x cast in-situ rectangular wall specimen 1x cast in-situ building core specimen 2x jointed precast building core specimens Plus 2x jointed precast specimens 1x monolithic cast in-situ specimen 1x monolithic cast in-situ specimen

5 The MAST System at Swinburne 4,000 kn vertical capacity with ±250 mm stroke 1,000 kn horizontal capacity with ±250 mm stroke (x-direction) 1,000 kn horizontal capacity with ±250 mm stroke (y-direction) 3,500 knm torque capacity with ±8.1 degrees rotation 4,500 knm moment capacity with ±6.3 degrees rotation (x-axis) 4,500 knm moment capacity with ±6.3 degrees rotation (y-axis) The MAST system is being used to test the behaviour of a 4 storey wall using a 1 storey test specimen

6 Not quite full scale 60 to 70 per cent scale. Still very large specimens; approximately 3.5 metres tall, weighing about 10 tonnes.

7 Cast in-situ building core specimen front view Cast in-situ building core specimen back view

8 Preliminary results R f ~2. 6 Key take home point: 1. Generally an inelastic reduction factor of 2.6, as specified in AS for limited ductile RC walls, was achieved. 2. The lap splice / dowel connection results in a region of overstrength forcing either a single-crack or two-crack plastic hinge R f ~2. 6 Splice creates region of overstrength with only hairline cracking developing Major crack forms at weak points above and below splice with concentrated plasticity Traditional plastic hinge model Two-crack plastic hinge model seen during testing

9 Bit different to the classic Priestley curvature diagram as shown Preliminary results Two-crack plastic hinge model seen during testing

10 Preliminary results Key take home point: 3. Premature vertical cracks in the bottom of the panels can lead to compression failures occurring at much lower displacement levels as the panels have very little inherent robustness (we generally don t confine walls in Australia). Care should be taken when detailing precast panel connections such that it is ensured good bearing surfaces can be achieved on site. Panel-to-panel connection (grout tubes not shown) Poor bearing surfaces can result in stress concentrations on the underside of the panel and lead to premature vertical cracks (same applies for panel-tofoundation connections)

11 THEME: THE ROLES OF STRUCTURAL ENGINEERS 1. Understand seismic design criteria 2. Understand how to appropriately design and detail RC structures for seismic actions While earthquakes are uncommon in Australia, they still occur and present a serious risk and could be surmised as low-probability high-consequence events. Earthquake design cannot be ignored nor considered to be just another load case the same as wind loading

12 What is the seismic design criteria for Australia? Key take home question: are our codes and standards really safe guarding our population against building collapses under very rare long return period earthquakes? Idealistic (Sullivan, Priestley and Calvi, 2012) Australia Sullivan, Priestley and Calvi (2012) recommends for regions of lower seismicity to only check seismic compliance for no collapse under a long return period earthquake and ignore the other objectives

13 Questionable detailing practices: with respect to seismic loading Commonly seen detailing practices of RC precast panels and jointed cores that significantly limit the drift capacity of a structure under seismic loading: 1. Walls with one central layer of reinforcement 2. Walls detailed with low ductility reinforcement 3. Walls detailed with low reinforcement ratios These three things are often seen in precast panels that are referred to as load bearing only elements. An earthquake does not know what a load bearing only element is. If it is attached to the floor diaphragm, it will take load Earthquakes are very good at identifying the least strong, least ductile elements in a building. These three detailing practices have one thing in common: they limit the tensile capacity of an RC wall and hence its ability to develop ductility. The underlying assumption in AS is that you re trading strength for ductility (i.e. the μ = 2 reduction factor).

14 Questionable detailing practices: with respect to seismic loading Commonly seen detailing practices of RC precast panels and jointed cores that significantly limit the drift capacity of a structure under seismic loading: 1. Walls with one central layer of reinforcement 2. Walls detailed with low ductility reinforcement 3. Walls detailed with low reinforcement ratios AS 3600 Cl limits the use of centrally reinforced walls in any scenario where tension can be developed in the wall.

15 Questionable detailing practices: with respect to seismic loading Commonly seen detailing practices of RC precast panels and jointed cores that significantly limit the drift capacity of a structure under seismic loading: 1. Walls with one central layer of reinforcement 2. Walls detailed with low ductility reinforcement 3. Walls detailed with low reinforcement ratios Low ductility reinforcement has low ductility. The ratio of ultimate stress to yield stress is also very low (in comparison to D500N rebar), which significantly reduces its ability to distribute plastic strain and hence effects the walls ability to develop ductility.

16 Questionable detailing practices: with respect to seismic loading Commonly seen detailing practices of RC precast panels and jointed cores that significantly limit the drift capacity of a structure under seismic loading: 1. Walls with one central layer of reinforcement 2. Walls detailed with low ductility reinforcement 3. Walls detailed with low reinforcement ratios Key take home question to ask yourself: if you haven t thought about how ductility can be developed in your structure and you re detailing RC walls or panels using these practices then maybe you should not be using a force reduction factor when performing seismic analysis Low reinforcement ratios reduce a walls ability to develop distributed cracking, which can result in a single crack being developed in the wall with very concentrated plasticity. Rule of thumb to achieve distributed cracking: N40 concrete: p v.min = 0.8 per cent N50 concrete: p v.min = 0.9 per cent S65 concrete: p v.min = 1.0 per cent

17 ~ 2.3 reduction in drift 500 Axial load The simplest method to increase the drift capacity of an element is to reduce its axial load, or more specifically, axial load ratio: 8-N20 N LIGS f c =40MPa 500 n = N A g f C

18 Conclusion 1. Recently completed a large series of RC walls tests, including 4 large scale wall specimens. 2. The lap splice / dowel connection at the base of the wall creates a two-crack plastic hinge, further work is being performed to better understand and predict this behaviour. 3. Good detailing of panel-to-foundation and panel-to-panel connections are recommended to ensure stress concentrations don t occur leading to premature compression failures. 4. Serious care and attention must be taken when detailing RC walls and panels with: Central layers of reinforcement Low ductility reinforcement Low vertical reinforcement percentages (try to stay above 0.8 to 1.0 per cent depending on concrete grade) Thank you. Questions? Recent testing has shown J-hooks or s don t work