1、An ACI Technical Publication SYMPOSIUM VOLUME SP-309 Structural Integrity and Resilience Editors: Mehrdad Sasani and Sarah OrtonStructural Integrity and Resilience SP-309 Editors: Mehrdad Sasani and Sarah Orton Discussion is welcomed for all materials published in this issue and will appear ten mont
2、hs from this journals date if the discussion is received within four months of the papers print publication. Discussion of material received after specified dates will be considered individually for publication or private response. ACI Standards published in ACI Journals for public comment have disc
3、ussion due dates printed with the Standard. The Institute is not responsible for the statements or opinions expressed in its publications. Institute publications are not able to, nor intended to, supplant individual training, responsibility, or judgment of the user, or the supplier, of the informati
4、on presented. The papers in this volume have been reviewed under Institute publication procedures by individuals expert in the subject areas of the papers. Copyright 2016 AMERICAN CONCRETE INSTITUTE 38800 Country Club Dr. Farmington Hills, Michigan 48331 All rights reserved, including rights of repr
5、oduction and use in any form or by any means, including the making of copies by any photo process, or by any electronic or mechanical device, printed or written or oral, or recording for sound or visual reproduction or for use in any knowledge or retrieval system or device, unless permission in writ
6、ing is obtained from the copyright proprietors. Printed in the United States of America Editorial production: Aimee Kahaian ISBN-13: 978-1-944487-04-0 First printing, June 2016Preface The need for structural integrity has been recognized ever since the 1968 failure of the Ronan Point Apartment build
7、ing. Improvements to the ACI code in 1989 required additional reinforcement for structural integrity, however those requirements were based on generally good building practices with little research or analysis to support them. However, since the disproportionate failure of the Murrah Federal buildin
8、g in Oklahoma City, these requirements have received renewed interest and new research conducted. More recently, and primarily due to the aftermath of natural and manmade disasters, the need for designing buildings that are resilient against various hazards has been recognized. While most of the lat
9、est research does not directly analyze the efficacy of the structural integrity requirements, it does consider the overall collapse resistance and robustness of reinforced concrete buildings. Research using field experiments conducted in the last decade indicates that reinforced concrete structures
10、are generally robust against local damage like single column removal. Although structural integrity requirements have been included in ACI 318 since 1989, there still exists areas of improvement. For example, recent laboratory experiments show that flat plate structures may still be vulnerable due t
11、o the high likelihood of progressive punching shear failures. Furthermore, for structures designed and built without structural integrity provisions, new research highlights ways to improve their robustness and collapse resistance. Finally, improved analysis models and predictions on the likelihood
12、of collapse lead to better assessment of the risks of collapse. ACI Committee 377 sponsored two sessions during the Fall 2014 ACI convention in Washington, DC to highlight the importance of structural integrity and resilience of reinforced concrete and precast/prestressed structures subjected to ext
13、reme loading conditions. The sessions sought papers on topics including improving the structural integrity of structures, minimum level of required integrity, integrity of precast/prestressed structures, performance-based structural integrity and resilience, infrastructure resilience, issues and new
14、 developments in modeling, and assessment of existing structures. Both experimental and analytical investigations were presented. The sessions presented 10 papers covering the design of reinforced concrete buildings against progressive collapse, evaluation of NYC code provisions, analysis and experi
15、mental testing of post-tensioned and precast/prestressed structures, methods to improve collapse resistance, and probabilistic analysis of collapse. This special publication includes eight papers that were presented during the sessions. The papers are alphabetically ordered based on the last names o
16、f the first authors. Editors Sasani, M., and Orton, S. TABLE OF CONTENTS SP-3091 A Simple Method of Enhancing the Robustness of R/C Frame Structures 1. Authors: Yihai Bao, H.S. Lew, Joseph A. Main and Fahim Sadek SP-3092 New York City Building Code Flat Plate Integrity Provisions .2. Authors: Ramon
17、Gilsanz, Karl Rubenacker, and Jennifer Lan SP-3093 Response of a Post-Tensioned Floor Following a Column Loss 3. Authors: Leila Keyvani and Mehrdad Sasani SP-3094 Uncertainties in Predicting Structural Disproportionate Collapse 4. Authors: Shalva Marjanishvili and Serdar Astarlioglu SP-3095 Post-Pun
18、ching Capacity of Flat-Plate Floor Systems 5. Authors: Sarah Orton, Ying Tian, Zhonghua Peng, and Jinrong Liu SP-3096 Recent Progress in Understanding of Load Resisting Mechanisms 6. for Mitigating Progressive Collapse Authors: Kai Qian, Bing Li, and Ying Tian SP-3097 Assessment of Urban Building Co
19、mplexes Subjected to Natural and 7. Man-Made Hazards Authors: Malte von Ramin, Alexander Stolz, Oliver Millon, and Tassilo Rinder SP-3098 Risk Assessment of Reinforced Concrete Buildings Against .8. Progressive Collapse Authors: Bing Xue and Jia-Liang LeSP-30901 1.1 A SIMPLE METHOD OF ENHANCING THE
20、ROBUSTNESS OF R/C FRAME STRUCTURES By: Yihai Bao, H.S. Lew, Fahim Sadek, and Joseph A. Main Synopsis: A simple debonding technique was proposed to reduce strain localization in reinforcing bars in the region of wide flexural cracks in reinforced concrete (R/C) beams, in order to enhance the resistan
21、ce of R/C buildings to disproportionate collapse. Debonding was achieved by heat-shrinking polyolefin tube over the reinforcing bar. Results from testing of a No. 8 reinforcing bar showed that with an 8 in (203 mm) debonding length on both sides of a in (6.35 mm) wide gap, simulating a wide flexural
22、 crack, the elongation of the reinforcing bar prior to fracture was about 38 % more than for the case without the debonding technique. This observation demonstrated that the debonding method could effectively reduce strain localization, thereby delaying the fracture of reinforcing bars. To analyze t
23、he effects of debonding, detailed finite-element models of the test specimens were developed, which adequately captured the experimental results. R/C frame structures were analyzed by applying the debonding model under a column removal scenario. The results indicated that the debonding method could
24、enhance the development of catenary action in the beams of R/C frame structures. Keywords: disproportionate collapse, reinforced concrete, debonding, catenary action, nonlinear finite element analysis Bao et al. 1.2 Biography: Yihai Bao is an IPA researcher at the Engineering Laboratory (EL), the Na
25、tional Institute of Standards and Technology (NIST) and a project scientist at the Department of Civil and Environmental Engineering, the University of California - Davis. He received his BS and MS degrees from Tongji University of China and PhD from the University of California - Davis, Davis, CA.
26、His research interests include nonlinear modeling of structural behavior under extreme loading conditions, large-scale experimental methods and advanced scientific computational methods. ACI Honorary Member H. S. Lew is a Senior Research Engineer at the Engineering Laboratory (EL), the National Inst
27、itute of Standards and Technology (NIST). He received his PhD from the University of Texas at Austin, Austin, TX. He has served on the ACI Board of Direction, and a member of standard and technical committees. His research interests include performance of structural members and systems, construction
28、 safety, failure investigations, and earthquake engineering. Fahim Sadek is Leader of the Structures Group at the Engineering Laboratory (EL), the National Institute of Standards and Technology (NIST). He received his PhD from the Southern Methodist University, and is a licensed professional enginee
29、r. His research interests include vulnerability of structures subjected to multi- hazard conditions, response of structures to dynamic and impulse loading and structural health monitoring. Joseph A. Main is a Research Structural Engineer at the Engineering Laboratory (EL), the National Institute of
30、Standards and Technology (NIST). He received his PhD from Johns Hopkins University. His research interests relate to the computational analysis of structural performance under extreme loads, wind, fire, and disproportionate collapse. INTRODUCTION Recent experimental studies 1-4on reinforced concrete
31、 (R/C) frame structures under column removal scenarios have identified the failure mechanisms that occurred when they underwent large deflections. In the tests of two full-scale frame assemblies 1 , it was noted that the beam-end rotations under monotonic loads were approximately 7 to 8 times as lar
32、ge as those based on seismic tests under cyclic loads. However, the rotational capacity can be further increased if more ductility is achieved in the reinforcing bars at critical locations, such as at beam ends near column faces. The ductility of reinforcing bars is adversely affected by strain loca
33、lization, which may occur at a wide flexural crack at a critical beam section, such as the one shown in Fig. 1. The exposed bar segment at the crack developed larger plastic strain than the embedded portions adjacent to the crack, due to the bond of surrounding concrete to the reinforcing bar. Prema
34、ture fracture of reinforcing bars limits the further development of catenary forces in the beams. The existence of strain localization was observed in the numerical analysis of two reinforced concrete frame assemblies, where the numerical models have been validated against experimental results 5 . F
35、ig. 2 shows the calculated plastic strain distribution of the beam bottom reinforcing bar near the unsupported center column just before the fracture. A steep strain increase in the bottom reinforcing bar was observed at the beam section near the unsupported column, where a wide flexural crack occur
36、red during the test. To relieve the strain localization, a debonding concept is applied and illustrated in Fig 3. By debonding the reinforcing bar from the surrounding concrete at the potential crack zone, a more uniform strain distribution can be achieved, reducing the strain localization. To demon
37、strate the effectiveness of this concept, a detailed finite element model was created for one of the above-mentioned frame assemblies. The numerical model considered the debonding of the beam- bottom reinforcing bars near both sides of the unsupported center column with the debonding length equal to
38、 the beam depth. The plastic strain distribution of the bottom reinforcing bar is plotted in Fig. 2. A relatively uniform distribution was observed as a result of detaching the reinforcing bar from the surrounding concrete. The reduction of strain localization delayed the bar fracture, which resulte
39、d in a 14 % increase in the ultimate vertical load capacity and a 10 % increase in the corresponding vertical deflection of the center column as presented subsequently in Table 1 and Fig. 14(a). Larger increases in capacity and deflection at failure were achieved with a larger length of debonding. A
40、 simple approach is proposed herein to accomplish debonding between reinforcing bars and concrete. The effectiveness of this method is demonstrated through an experimental study. The experimental data are also compared with computational models, which are later used to investigate the influence of d
41、ebonding length and evaluate the behavior of frame structures under seismic loads. A Simple Method of Enhancing the Robustness of R/C Frame Structures 1.3 Fig. 1Reinforcing bar fracture at a wide flexural crack. 0.0 101.6 203.2 304.8 406.4 508.0 0.00 0.05 0.10 0.15 0.20 0.25 0 4 8 12 16 20 Distance
42、from the center column face (mm) Effective plastic strain Distance from the center column face (in) No debonding Debonded Length = 1D (one beam depth)Fig. 2Computed strain distribution in beam bottom reinforcing bar. Strain Distribution F F F F Concrete Crack Debonded Zone Fig. 3Debonding concept. B
43、ao et al. 1.4 RESEARCH SIGNIFICANCE Few research studies on enhancing the disproportionate collapse resistance of reinforced concrete frame structures have been conducted to date. Based on the previous experimental studies on the failure mechanisms of reinforced concrete frame structures under colum
44、n loss scenarios, this paper presents an innovative yet simple method which has the potential to significantly enhance the structural performance under column loss scenarios. The feasibility and effectiveness of the proposed method are demonstrated experimentally and numerically. These studies provi
45、de a solid basis for further study on this method and its practical application in reinforced concrete structures. EXPERIMENTAL INVESTIGATION A series of tensile tests of an embedded reinforcing bar in concrete with a simulated wide crack was carried out to demonstrate (1) the effectiveness of the t
46、echnique used in this study to debond the reinforcing bar from the surrounding concrete and (2) the efficacy of the debonding concept to delay the fracture of the embedded bar at a simulated crack location. Two different debonding lengths were considered, as well as a specimen with no debonding, in
47、order to investigate the influence of debonding length on the efficacy of the debonding technique. A key objective of the embedded bar tensile tests with different debonding lengths was to provide experimental data for comparison with computational models. As is discussed subsequently, the computati
48、onal models were used to further investigate the influence of debonding length on the performance of an R/C frame. Seismic loading was considered as well as column loss, because it is important that the debonding technique should not negatively impact the overall performance of an R/C frame. Design
49、and fabrication of specimens A total of three specimens were tested in this study. Each specimen consisted of two 48 in (1219 mm) long 10 in (254 mm) diameter concrete cylinders with one No. 8 reinforcing bar embedded in the center of both cylinders (Fig. 4). A in (6.4 mm) gap was preset between the two cylinders, simulating a wide crack opening. The only parameter varied between specimens was the debonding length on each side of the gap. The No. 8 bar was fully embedded (no debonding) for the first specimen (SN). The second (SD-2) and third specimens (SD-8) were designed w
