Progressive Collapse of Reinforced Concrete Structures Devising Realistic

Progressive Collapse of Reinforced Concrete Structures Devising Realistic Scaled Modeling for Catenary Action RET Participant: Mark Joseph Casto, Amesbury Public Schools Faculty Mentor: Mehrdad Sasani (Department of Civil and Environmental Engineering) Research Mentor: Serkan Sagiroglu (Department of Civil and Environmental Engineering) ABSTRACT The extent of initial damage to the World Trade Center Towers and the Pentagon during the September 11, 2001 terrorist attacks was beyond what was perhaps practical to be considered for progressive collapse resistant design. The extreme assaults and the tragic outcomes have initiated wide spread interest and research in progressive collapse of structures under more moderate initial damage scenarios. As part of an on-going research on progressive collapse of structures at Northeastern University, experimental and analytical studies are being conducted on potential collapse of reinforced concrete structures due to loss of columns. Following loss of columns, the beams bridging over these columns will need to dynamically redistribute the gravity loads to other parts of the structures. In order to reliably model the behavior of these critical beams and its effect on the response of the buildings, small scaled models of RC buildings are being tested. (Sasani, 2009) INTRODUCTION METHODS RESULTS Much of the 6 week lab experience was devoted to choosing, testing, and evaluating materials that could be used to model a real-world scenario in regards to; 1) the typical loading of a simple beam through gravitational loading, and 2) to provide a lab set-up to demonstrate catenary action. The goal of the research was to: ● select materials that are minimal in cost ● select materials that would provide as close to a scale model as scientifically possible To accomplish this there were several tasks that needed to be completed. Below is a list of the variables that needed to be tested and researched in order for the product to model a realistic model. means of supporting beams (fixed and unfixed) Figure B. Progressive Collapse Resistance Competition (pcrc 2007) A recent competition, hosted at Northeastern University, was designed to “provide undergraduate and graduate students, as well as professionals (engineers, architects and other professionals) an opportunity to learn about progressive collapse resistance of reinforced concrete structures” (pcrc 2007). In the competition a 2 -D model was constructed and contestants had to model the outcome after a column was removed. Research in this area still continues at Northeastern University in collaboration with the university’s new center known as ALERT funded by the Department of Homeland Security. The Summer 2009 RET Experience in this lab was designed to introduce these concepts to students in a K-12 setting, specifically in the areas of math, science and engineering technology. For a more comprehensive look at research conducted at Northeastern University in regards to the progressive collapse of structures visit the Progressive Collapse Reinforcement Challenge Website @ Design of the test setup allowed to provide fixed end support conditions at the end of the beam tested. Visible cracks formed at 91. 8 lbs load and 0. 105” vertical displacements at: properties of concrete properties of rebar beam dimensions catenary action lab set-up WHAT IS CATENARY ACTION? One significant requirement of Northeastern University’s Research Experience for Teachers Program is that the teachers develop a lesson plan that can be incorporated into their pre-existing curriculum. The lesson plan must utilize aspects of the lab experience and should be a product that can be used by other teachers on a national level. The lesson(s) should effectively incorporate best practices, specifically inquiry based learning strategies. This lab experience can provide enrichment in the following curriculum areas: Physics (9 -12) Properties of Matter Strength of Materials (Composites) Stress vs. Strain Relationships Forces of Motion Engineering Technology (9 -12) Reinforcement Engineering Properties of an Aggregate Material Building Technology Strength of Building Materials Physical Science (5 -8) Forces of Motion Compression and Expansion Simple Bridge Building (a) Left support A complete lesson entitled, Reinforcement Engineering of a Simple Beam, A concrete example was developed as a result of this experience. The lesson can be download from the Northeastern RET website. (b) Center The bending of the beam element is result of the deformational strain caused by the flexural stresses due to the external load. As the load is increased, the beam sustains additional strain and deflection, leading to development of flexural cracks along the span of the beam. Continuous increases in the level of the load lead to the failure of the structural element when the external load reaches the capacity of the element. Such a load level is termed the limit of failure in flexure. (Nawy, 2000) Figure A. Column Removal of a Frame Structure (pcrc 2007) Details of the Test Setup ● select materials that could be easily obtained by a classroom teacher means of loading the beam Reinforced concrete structures are built by combining concrete and steel. Concrete is relatively strong in compression and weak in tension. Steel reinforcement (rebars) is placed to carry the tensile forces that develop in the structural elements. Therefore the tensile zones in the structural elements needs to be identified in the design phase and the placement of the reinforcement should be done accordingly. Due to several unfortunate human-induced collapses especially in the recent past, consideration of the performance of the structures following a loss of one of more structural elements, (i. e. columns) has received greater attention. Removal of one of more columns from a structure drastically changes the load patterns and could lead to structural collapse. Sequence of the Test CLASSROOM CONNECTIONS Providing proper reinforcement, catenary action as a resisting mechanism could be formed and may allow the beam to carry vertical loads at large displacements even if the limit of failure in flexure has been reached in critical sections and flexural reinforcement at those sections are fractured following removal of a supporting element. http: //www. ret. neu. edu (c) Right support ** see Back in the Classroom** Force-Displacement graph of the specimen tested 3 – Dimensional Drawing of a 3 Column / 2 Beam Structure Under Gravity Loading - Before Removal of Column #2 - *** The lesson was developed to allow students to have a ‘hands-on’ experience in which actual materials (concrete and rebar) are used to explore the concept of composite materials and their strength benefits. The lesson also highlights the fact that in today’s post 9/11 world engineers not only have to prepare for natural hazards, they now have to prepare for human-induced hazards (acts of terrorism) when they design buildings. Collapse scenarios connected to this lesson include the following well-known examples: Alfred P. Murrah Federal Building World Trade Center Column 1 = Area of Tension Beam B = Area of Compression = REBAR position Figure C. Simple Supported beam (RET ’ 09) http: //www. pcrc 2007. neu. edu Research Experience for Teachers at Northeastern University Claire Duggan, Program Director - Sept 11 th Tower Collapses Column 3 Column 2 Beam A - Oklahoma City Bombing At 191. 8 lbs load is applied the beam was still able to carry the load. After applying additional 10 lbs of load beam collapsed. All the reinforcement at critical sections are fractured and in turn beam cut at its supports as well as at the center. Fractured reinforcement at the critical section shown below: (a) Left support (b) Center (c) Right support ACKNOWLEDGEMENTS Mehrdad Sasani, (Professor) Northeastern University Serkan Sagiroglu, (Research Assistant) Northeastern University Michael Mac. Neil, (Lab Technician) Northeastern University REFERENCES 1. http: //www. pcrc 2007. neu. edu/ 2. http: //www 1. coe. neu. edu/~sasani/ 3. http: //cedb. asce. org/cgi/WWWdisplay. cgi? 0802113 4. Nawy Edward G. 2000. Reinforced concrete : a fundamental approach. Prentice Hall. Saddle River, NJ. This work was supported by the National Science Foundation Grant #0227577, 9986821, 0425826