Venice Biennale 2013 Sandbag Installation Details of sandbag wall load testing and construction proposals Project number: 000100 March 2013 Sheraton House Cambridge CB3 0AX Tel. 01223 370 126 www.smithandwallwork.com
Contents Introduction Project Team Project Evolution Summary of Structural Design Recommendations Description of Sandbag Wall Testing Results from Sandbag Wall Testing Discussion of Results Structural Design Recommendations Appendices Appendix A Engineering sketches Appendix B Photos from Test Panel Build Process Appendix C Photos from Test Panel Load Testing Appendix D Results from Test Panel Load Testing Report Authors Simon Smith Smith and Wallwork Michael Ramage University of Cambridge Report Revision History Author Description Date Ref Simon Smith First issue 15/03/2013 0 SaW_Venice Biennale_Sandbag Wall Load Testing_March 2013 Page 2
Introduction This report has been compiled to provide information on the results of sandbag wall load testing and construction proposals for a sandbag installation conceived by Roz Barr Architects for the Venice Biennale 2013. The structural stability of the sandbag walls is discussed and options are proposed to achieve a structurally safe and robust installation. The report will cover the evolution of the design, load testing carried out at Cambridge University and construction proposals. Project Team The project is led by Roz Barr Architects. The structural design of the sandbag walls is a joint venture between the architecture department at Cambridge University and engineering consultant, Smith and Wallwork. The construction of the sandbag test panel was carried out by the Architecture Masters Degree students at Cambridge University. The construction of the final sandbag installation will be carried out by an Italian contractor in collaboration with Roz Barr Architects. Architect and Lead Designer: Roz Barr and Emma Tubbs, Roz Barr Architects Structural Engineer: Simon Smith, Smith and Wallwork Structural Engineer: Michael Ramage and Emily So, University of Cambridge Structural Testing and Construction: Architecture Masters Degree students at Cambridge University Project Evolution The project was conceived by Roz Barr Architects who had previously designed and constructed a successful sandbag installation in central London as part of the 2012 London Festival of Architecture. The plans for the Venice Biennale 2013 sandbag installation differ from the London project; it uses a sandbag floor and 2.5m tall sandbag walls to create a courtyard space approximately 51m x 13m. Some 17,000 sandbags will be required to complete the installation. From an early stage the importance of the stability of the sandbag walls was recognised and the design was influenced by this consideration; hence the saw-tooth profile of the main walls on plan. From the experience gained at the 2012 London Festival of Architecture it was known that a test panel build would prove useful in terms of providing data on buildability and structural stability. SaW_Venice Biennale_Sandbag Wall Load Testing_March 2013 Page 3
Summary of Structural Design Recommendations The sandbag wall load testing carried out at Cambridge University has provided extensive structural performance data. This data allows a structural design to be completed for the proposed Venice Biennale installation. It is proposed that the sandbag walls are designed to resist a horizontal loading of 3.0kN/m in areas of potential crowd loading (ie entry and exit points) and 1.5kN/m in all other areas. For the purposes of design this load would be applied at 1.1m above ground level. The minimum factor of safety against collapse should be 2.0. Using these design loads and the data from the load test completed it will be necessary to provide reinforcement to certain areas of the sandbag wall; adjacent the openings and in the flat walls at each end of the installation. The reinforcement proposed is a form of vertical pre-stressing similar to that trialled during the load tests. A metal bar is concealed in the depth of the wall or pier and is fixed to a baseplate and top plate. A pre-stress of 0.85kN is introduced and maintained using a head spring or regular pre-stress visits. Details of this pre-stressing can be seen in appendix A and further information associated with the reinforcement can be found in this report in the section titled Structural Design Recommendations. Description of Sandbag Wall Testing A sandbag wall test panel was built at Cambridge University Architecture Department during March 2013. The test panel layout was chosen to incorporate the saw-tooth profile of the main walls and comprised three wall panels and four piers that formed a test sample that was 2.0m tall and 5.1m x 1.8m on plan constructed from approximately 400 sandbags. On completion of the test panel construction it was evident that the massive nature of the wall and the saw-tooth profile provided a structure that was extremely robust within the central portion. Initial horizontal push tests detected almost no movement in these central portions and it was decided to concentrate all testing at the wall ends, in particular to test the piers that formed the wall ends. A series of load tests were devised including monitoring and recording of structural behaviour. The tests carried out on the panel included static, dynamic and push-over scenarios. Monitoring comprised recording loads applied, horizontal deflection and recovery and accelerometers were place on top of the wall to record data during the dynamic load tests. SaW_Venice Biennale_Sandbag Wall Load Testing_March 2013 Page 4
Static load test location unreinforced pier Dynamic load test location unreinforced Static load test location reinforced pier Accelerometer locations Static load tests: Involved testing an unreinforced wall end/pier. Horizontal loads were applied incrementally, increasing in magnitude to 120kg at 135cm above ground level. Between load increments the wall was allowed to recover to enable recording of any plastic deflection. Deflections were recorded. No failures occurred. Dynamic load tests: Involved a 50kg pendulum test to an end wall panel. The test was designed to follow BS EN 12600:2002 as close as possible. Pendulum heights of 65cm and 115cm above ground level were investigated. Pendulum back swings varied from 40cm to 200cm from the face of the wall. Deflections and accelerations were recorded. No failures occurred. Abuse load tests: 3no. students scaled the wall simultaneously and stood on the wall. Push-over tests: Both wall ends/piers were pushed over, one unreinforced and one pre-stressed (approximately 85kg) with strapping. Loads were applied 135cm above ground level and recorded at failure. Each hessian sandbag was filled with typically 24kg of building sand and when placed in the wall formed a brick of typically 600x300x90mm. Photos from the test panel build process can be seen in appendix B. SaW_Venice Biennale_Sandbag Wall Load Testing_March 2013 Page 5
Results from Sandbag Wall Testing Data from the load testing can be found in appendix D and photos of the load testing can be seen in appendix C. During both the static and dynamic testing the sandbag wall remained stable albeit with some permanent deflection recorded during both tests. Failures were only encountered in the push-over tests. Static load test on unreinforced pier: An elastic deflection of 38mm (measured at 175cm above ground) under a 1.2kN load (applied at 135cm above ground) was recorded. On release of the load recovery was 30mm giving a permanent deflection of 8mm. As can be seen from the plot of the deflections against load below the relationship begins linear moving to exponential at the 1kN load stage. 40 Static Load Test - Unreinforced Pier 35 30 25 20 15 10 5 Elastic deflection (175cm) Plastic deflection (175cm) Elastic deflection (121cm) Plastic deflection (121cm) Trend line (175cm) Trend Line (121cm) 0 0 20 40 60 80 100 120 140 Push-over test to unreinforced pier: The pier failed at 2kN load applied 135cm above ground. As can be seen from the photos in appendix C the pier failed approximately 80cm above ground accompanied by a partial collapse of the adjoining wall panel. Push-over test to reinforced pier: The pier failed at 3kN load applied 135cm above ground. As can be seen from the photos in appendix C the pier failed approximately 60cm above ground accompanied by a partial collapse of the adjoining wall panel (albeit not as much as with the unreinforced pier). The pre-stressing strap failed in tension at the point of push-over. The failure occurred at a point of damage to the strap. SaW_Venice Biennale_Sandbag Wall Load Testing_March 2013 Page 6
Dynamic load test to unreinforced wall panel: In both pendulum tests the deflections measured, both elastic and plastic, were negligible (less than 5mm). The accelerations experienced by the wall ranged from zero up to 0.7g and can be seen in the graphs below. The sampling rate was 0.1sec. 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 65cm High Pendulum Wall Accelerations End of wall accelerations Central wall accelerations 1 501 1001 1501 2001 2501 3001 The duration of the test was 5 minutes and 30 seconds and the lag shown between each location represents approximately 8 seconds. 0.7 115cm High Pendulum Wall Accelerations 0.6 0.5 0.4 0.3 0.2 End of wall accelerations Central wall accelerations 0.1 0 1 501 1001 1501 2001 2501 3001 The duration of the test was 5 minutes and 30 seconds and the lag shown between each location represents approximately 8 seconds. SaW_Venice Biennale_Sandbag Wall Load Testing_March 2013 Page 7
Other: During the tests it was possible to observe a potential weakness in the wall. At the pier locations the bond pattern introduced a clear vertical joint that ran the full height of the wall for half its depth. Under horizontal loading this joint was seen to open up. The robustness of the wall could be increased if the wall was reinforced here, either through a revised bond pattern or barbed wire reinforcement. Image highlights points at which joint opening-up was observed under load testing Discussion of Test Results The load testing provided considerable data from which it is possible to gain a fuller understanding of the way in which the sandbag wall responds to various load conditions. The discussions here are limited to the wall ends or piers as the central sections of the sandbag wall were observed to be extremely robust and not thought to require any further discussion. Unreinforced pier (static load and push-over test results): From both the static load test and push-over test it is clear that the pier does not exhibit full rigid body behaviour (at higher loads) and that its strength is influenced by the adjoining wall panel. This is evident from non-linear deflection behaviour, the failure at 800mm above ground and the wedge shape failure of the adjacent wall panel (see photos in appendix C). Furthermore the theoretical push-over moment of a 0.6x0.6x2m tall sandbag pier with a mass of 1100kg (ie 46no. 24kg sandbags) acting as a rigid body is 3.3kNm. This represents a push-over load of 2.5kN which is 25% higher than the 2kN experienced in the test. SaW_Venice Biennale_Sandbag Wall Load Testing_March 2013 Page 8
Deflection (mm) Using the static load deflection curves it is possible to predict the failure of the unreinforced pier as can be seen in the graph below. Assuming a tipping point of 300mm horizontal deflection at pier mig-height (ie when the centre of mass of the pier rotates outside of the pier footprint) the predicted force to achieve push-over is 195kg applied at 135cm above ground. This can be compared to the 200kg load observed in the test. The extrapolation of the load deflection lines was undertaken in MS Excel using 3 rd order polynomial functions. 500 450 400 350 300 Prediction of failure load of unreinforced pier 250 200 150 deflection at 175cm deflection at 121cm 100 50 0 0 50 100 150 200 250 Load (kg) The failure of the unreinforced pier at 2kN is not acceptable when considering the regulation loads and the deflections experienced at this magnitude of load. For crowd loading regulations will require up to 3kN/m applied at 1.1m above ground (equivalent to stadia balustrade loading) which on a 0.6m wide pier equates to 1.8kN. Crowd loading has been used due to the potential for increased pedestrian traffic at points of entry and exit. According to the tests carried out this would give a factor of safety of 1.35 for the unreinforced pier which is not acceptable and a factor of safety of at least 2.0 will be required by the authorities. By extending the wall height to 2.5m (as per the project proposals) the push-over load is increased to 2.5kN and factor of safety to 1.7 although this is a slight simplification of what would happen in reality. Reinforced pier (push-over test results): From push-over test it is clear that the reinforced pier is tending to rigid body behaviour and that its strength is only slightly influenced by the adjoining wall panel. This is evident from the failure being SaW_Venice Biennale_Sandbag Wall Load Testing_March 2013 Page 9
lower down at 600mm above ground and the failure of the adjacent wall panel being limited to only a few sandbags (see photos in appendix C). The failure of the pre-stress strap during the push-over test indicates that the pre-stress load increased during the deformation of the pier. The final pre-stress design solution should take this in to consideration. The reinforced pier failed above the theoretical push-over load of 2.5kN. With a failure load of 3kN the factor of safety against push-over is 2.0 which is adequate. By extending the wall height to 2.5m (as per the project proposals) in the push-over load is increased to 3.75kN and factor of safety to 2.5 although this is a slight simplification of what would happen in reality. Dynamic load test to unreinforced wall panel: The wall performed extremely robustly during the pendulum tests. Both elastic and plastic deflections were negligible and dissipation of the impact energy in the wall was excellent, as can be seen by back analysis of the estimated force exerted during the maximum pendulum swing. These forces are estimated to be 10kN (115cm height) and 15kN (65cm height) and when viewed against the deflections experienced and compared to the static load tests results the sandbag wall energy adsorption properties are significant. The dynamic load test also reinforced our empirical observations that the central portion of the sawtooth profile wall was highly stable as an unreinforced sandbag structure. Again the difference in magnitude of the accelerations measured at the two points in the test panel (typically a factor 5 to 10 difference) provides measured evidence of this. Structural Design Recommendations The following recommendations are made in order to achieve a structurally stable sandbag wall installation to the saw-tooth wall profile: 1. The saw-tooth end walls should be designed to crowd horizontal loading of 3kN/m at 1.1m above floor level. 2. The saw-tooth wall profile requires pier reinforcement at opening points, ie at wall ends. This will provide adequate stability against push-over for a wall height of 2m or 2.5m. 3. The reinforcement could be formed in a number of ways. If pre-stressing is adopted then a minimum pre-stress force of 0.85kN is required. 4. The pre-stress force may dissipate over time and therefore a detail should be developed that maintains a constant force (ie spring head) or a detail that is adjusted on a regular basis. 5. The baseplate to the reinforcement would benefit from extending it under the sandbag floor and/or adjacent wall sandbags. 6. The joint opening up observed during load tests should be eliminated through revising the bond pattern or introducing local barbed wired bed joint reinforcement. SaW_Venice Biennale_Sandbag Wall Load Testing_March 2013 Page 10
The following recommendations are made in order to achieve a structurally stable sandbag wall installation to the flat end walls: 7. The flat end walls should be designed to horizontal loading of 1.5kN/m at 1.1m above floor level. 8. The flat end walls require reinforcement at 1.2m centres throughout their length. This will provide adequate stability against push-over for a wall height of 2m or 2.5m. 9. The reinforcement could be formed in a number of ways. If pre-stressing is adopted then a minimum pre-stress force of 0.85kN is required. 10. The pre-stress force may dissipate over time and therefore a detail should be developed that maintains a constant force (ie spring head) or a detail that is adjusted on a regular basis. 11. The baseplate to the reinforcement would benefit from extending it under the sandbag floor. Details of the proposed sandbag wall reinforcement can be seen in appendix A. Roz Barr Architects will specify the construction details to ensure that this detail is applied during the construction process. The insertion of the pre-stress detail within each end of wall pier section will offer greater stability of the structure and the 2.5m wall. The stability of all sandbag walls built will also rely on a well-constructed installation. The tolerances achieved during construction should be closely monitored so that vertically plumb walls are achieved. Closely controlling the sandbag filling process so that each bag is filled with the same amount of sand is essential. The test panel build process has indicated that for the bags used, the ideal fill weight of sand is around 24kg to 25kg. SaW_Venice Biennale_Sandbag Wall Load Testing_March 2013 Page 11
Appendices Appendix A Engineering sketches Appendix B Photos from Test Panel Build Process Appendix C Photos from Test Panel Load Testing Appendix D Results from Test Panel Load Testing SaW_Venice Biennale_Sandbag Wall Load Testing_March 2013 Page 12
Appendix A Engineering Sketches Sketch showing sandbag wall reinforcement proposals SaW_Venice Biennale_Sandbag Wall Load Testing_March 2013 Page 13
Appendix A Engineering Sketches Sketch showing early thoughts SaW_Venice Biennale_Sandbag Wall Load Testing_March 2013 Page 14
Appendix B Photos from test panel build process SaW_Venice Biennale_Sandbag Wall Load Testing_March 2013 Page 15
Appendix C Photos from test panel load testing SaW_Venice Biennale_Sandbag Wall Load Testing_March 2013 Page 16
Appendix C Photos from test panel load testing SaW_Venice Biennale_Sandbag Wall Load Testing_March 2013 Page 17
Appendix C Photos from test panel load testing SaW_Venice Biennale_Sandbag Wall Load Testing_March 2013 Page 18
Appendix D Results from test panel load testing SaW_Venice Biennale_Sandbag Wall Load Testing_March 2013 Page 19
Appendix D Results from test panel load testing SaW_Venice Biennale_Sandbag Wall Load Testing_March 2013 Page 20