Venice Biennale 2013 Sandbag Installation - engineers€¦ · Sheraton House Cambridge CB3 0AX Tel. 01223 370 126 Venice Biennale 2013 Sandbag Installation Details of sandbag wall

Sheraton House Cambridge CB3 0AX Tel. 01223 370 126 www.smithandwallwork.com

Venice Biennale 2013

Sandbag Installation

Details of sandbag wall load testing and construction proposals

Project number: 000100

March 2013

SaW_Venice Biennale_Sandbag Wall Load Testing_March 2013 Page 2

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


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.


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.


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.

Static load test location

unreinforced pier

Dynamic load test

location unreinforced

pier Static load test location

reinforced pier

Accelerometer locations


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.

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.

0

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0 20 40 60 80 100 120 140

Static Load Test - Unreinforced Pier

Elastic deflection (175cm)

Plastic deflection (175cm)

Elastic deflection (121cm)

Plastic deflection (121cm)

Trend line (175cm)

Trend Line (121cm)


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.

The duration of the test was 5 minutes and 30 seconds and the lag shown between each location

represents approximately 8 seconds.

The duration of the test was 5 minutes and 30 seconds and the lag shown between each location

represents approximately 8 seconds.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

1 501 1001 1501 2001 2501 3001

65cm High Pendulum Wall Accelerations

End of wall accelerations

Central wall accelerations

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

1 501 1001 1501 2001 2501 3001

115cm High Pendulum Wall Accelerations

End of wall accelerations

Central wall accelerations


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.


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 3rd order polynomial

functions.

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

0

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250

300

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400

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500

0 50 100 150 200 250

De

fle

ctio

n (

mm

)

Load (kg)

Prediction of failure load of unreinforced pier

deflection at 175cm

deflection at 121cm


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 saw-

tooth 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.


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.


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


Appendix A – Engineering Sketches

Sketch showing sandbag wall reinforcement proposals


Appendix A – Engineering Sketches

Sketch showing early thoughts


Appendix B – Photos from test panel build process


Appendix C – Photos from test panel load testing






Appendix D – Results from test panel load testing


Appendix D – Results from test panel load testing

Documents

Venice Biennale 2013 Sandbag Installation - engineers€¦ · Sheraton House Cambridge CB3 0AX Tel. 01223 370 126 Venice Biennale 2013 Sandbag Installation Details of sandbag wall