Binishell Domes

Carl Hubben, BAE/MAE, Penn State 2010

Introduction

Binishell domes are thin-shelled reinforced concrete structures which became popular after the first was built in the mid 1960’s by the Italian architect Dante Bini. Since then, roughly 1600 Binishells have been built in 23 countries across the world (Levy, 39). The domes were well accepted due to their quick construction time, low cost, high strength and reduced carbon footprint when compared to conventional construction.
Binishell Example
Binishell Example
They range in height from roughly 36 to 115 feet and have been built to span up to 300 feet. Due to the versatile nature of Binishells, they have been used in the construction of schools, housing, sports arenas, shopping malls, storage buildings and silos (Binisystems). The domes are constructed by inflating a membrane which lifts reinforcement steel and wet concrete into the desired shape by varying the air pressure within the membrane. The unique construction needs to be followed accurately and if it isn’t the domes quickly lose their structural integrity. Due to faulty construction practices, there have been issues with the domes’ strength and in a few cases, Binishells have failed to the point of collapse.

Construction

Binishell construction is a successful and innovative application of pneumatic forming. Traditionally, pneumatic forming is done by fastening a membrane to the edges of a mold and then then formed with the assistance of heat and compressed air. This technique was used to mold small items such as sinks, bathtubs, and aircraft windows. Dante Bini simply applied the theory on a much larger scale.

1.) Placing the springs and reinforcement
1.) Placing the springs and reinforcement

Construction of Binishell domes begins with placing a concrete anchorage beam circular in plan with a molded recess. A slab on grade is also poured at this time to act as the floor of the dome. The anchorage device of the inflatable neoprene-coated nylon membrane is placed within the recession in the beam. Steel springs of various strengths are then stretched across the membrane at which time reinforcing bars are inserted inside the springs.

2.) Pouring the concrete
2.) Pouring the concrete

A thin layer of concrete is then poured over the membrane, covering the springs and reinforcement. The concrete is then covered with light PVC sheeting which is also secured to the anchorage. Once the sheeting is in place, the spool and roller vibrators are placed at the center of the membrane.

3.) Inflating the membrane
3.) Inflating the membrane

Now, the membrane is inflated through the ducts under the membrane. Although the pressure in the membrane is very low, the entire mass of the wet concrete, springs and reinforcement rise. As the pneumatic form fills with air, the different tensions in the springs control the lifting speed and shape of the dome. The springs also ensure the location of the reinforcement as the dome is lifting.This process takes roughly 1-3 hours.

4.) Using the roller virbators
4.) Using the roller virbators

When the dome has reached the desired height and shape, the rolling vibrators are pulled over the exterior of the surface to re-consolidate the concrete. When this is completed, the height of the dome is held constant by maintaining the pressure in the dome for 1-3 days depending on the size of the structure. During this time the PVC sheeting can be removed. Once the dome has reached sufficient strength, the form is deflated, leaving the 3 day old Binishell to support itself.

The construction process has been further developed and patented by Binisystems. Certain techniques have been implemented to have a more accurate structure in an effort to ensure the strength of the completed domes.

Failures and Causes

The unconventional and tedious construction of Binishells has caused some domes to fail and in a few, collapse. Two failures are covered in detail in this report along with one which were not well documented. The Fairvale High School and the Pittwater High School failure generated a large amount of interest because Dante Bini had been an architectural consultant during the design and construction of both of these domes. These collapses resulted in the investigation of Binishells with similar geometry. Some of these investigations were responsible for the deconstruction of standing Binishells.

In the poorly documented cases, the dome was located in Australia and was going to be used as a school. The first collapse occurred just two days after the inflation of the balloon and nearly immediately after the balloon was removed.

Structural strength of Binishells depends on four factors during construction. These are: air pressure of the balloon, location of reinforcing steel, concrete weight and distribution, and air temperature during while the concrete is curing. Improper responses to complications of these factors experienced during construction are the causes of the Binishell collapses. Luckily, there was no loss of life in any of the collapses.

Fairvale High School

Fairfield West, Sydney, Australia

Youtube clip of it's Construction and photos of the collapse: Credit: Ron Cheers

http://www.youtube.com/watch?v=q5ELKiTekwc


Introduction

On January 4th, 1975 the center of the Fairvale High School Binishell collapsed, leaving the perimeter approximately 3-4 meters (10-13 feet) high standing and undamaged. No one was injured in the collapse because damage to the dome was noticed the day before, allowing time for the building to be evacuated. The Fairvale Binishell was a 36 m (118 ft) radius dome, built as part of the 1973 Department of Public Works program. To help manage the program, which was responsible for building 15 Binishells in roughly 4 years, Dante Bini was hired as an architectural consultant. As a result of the Fairvale collapse, an investigation was conducted to determine the strength of the other 36 m domes constructed under the program.

Collapse

On January 2nd, 1975 there were unusually high temperatures in Fairfield, reaching a maximum of 41 C (106 F). Around 4:30 in the evening, an intense rainstorm moved through the area, causing a drop in temperature of 9 C (48 F) accompanied by strong, gusting winds. When the storm was analyzed after the collapse, it was estimated that it caused a temperature gradient of roughly 25 C (77 F) through the shell.

8:30 the next morning, significant cracking was noticed on the crown of the dome in an area of a circle with a 6 m (20 ft) radius. Additionally, there was spalling on the underside of the dome, which was beginning to sag at the center in an uneven way. These warning signs were taken very seriously and the building was evacuated. The very next day, January 4th, the center of the dome collapsed. When the dome fell, it left a 3-4 m high perimeter wall which was completely undamaged and was described as very gentle as it came to rest on tables and blockwork, which showed little evidence of a heavy impact.

Causes

The collapse of the dome was a surprise to everyone involved. Although the school was still under construction, the dome standing since October 15th, 1975 and was constructed using the same techniques as the other Binishells in the area. In response to the Fairvale collapse, an investigation of the physical properties of the Fairvale dome and the other program domes was made. The investigation covered dome shape, thickness, concrete strength, rod reinforcement, and temperature gradient. Despite a few minor areas where Fairvale differed from the other Binishells, the investigation concluded that the Fairvale shell collapse was caused by the presence of severe bending moments induced by a large thermal gradient.

The severe weather on January 2nd set off a chain of events responsible for collapsing the Fairvale dome. Based on a study performed by the N.S.W. Department of Public Works, a temperature gradient of 12 C can overstress a 36 m dome like the Fairvale Binishell. Fairvale experienced a gradient of 25 C, well over the necessary temperature difference. The bending moments caused by the large thermal gradient resulted in deflections in the domes shape. This caused a further increase in the dead and live load moments due to the departure of the Binishells elliptical cross section.

Buckling was the failure state reached by the Fairvale Binishell collapse. When large bending moments and deflection occur in a member, the compressive stress can be increased to a level greater than what can be supported. When concrete is overstressed, it can begin to creep, a deformation of the original shape which disrupts the load path of the structure. In the gradual failure of Fairvale, concrete which was creeping was relied on to support the structure. In a dome, the strength of the structure is directly related to the shape and any departure from the intended geometry is extremely dangerous. The uneven sagging of the dome the day before the collapse was a dead giveaway that the concrete was beginning to creep and that failure was imminent. The concrete membrane resisted the collapse of the dome until the critical state of creep was reached, at which point the dome fell. The buckling failure in this case was gradual but it has the potential to occur instantly which can be much more dangerous due to the lack of warning signs.

Lessons Learned

The Fairvale Binishell was still under construction at the time of the collapse. Although the dome had been standing under its own strength for some months, additions and modifications of the shell were still being made during the January storm. Since construction was still taking place, no insulation or weather proofing had been installed on the exterior of the dome.

The insulation is vital to the dome because it prevents possible thermal gradients to occur through the shell of the dome. Prior to the Fairvale collapse, installing the insulation was one of the final steps of the construction process. As a result of the Fairvale collapse, construction procedures were modified to suggest insulating the dome as soon as possible to prevent any effects that inclimate weather could have.


http://www.youtube.com/watch?v=q5ELKiTekwc


Pittwater High School

Northern Beaches, Sydney, Australia

Introduction

As part of the North South Wales Department of Public Works program responsible for constructing the Fairvale dome, a 36 m Binishell was built and used as the Pittwater High School. The dome at Pittwater was constructed a few months before Fairvale and in response to the collapse, the Pittwater dome was tested and determined to have appropriate strength. However, on August 4th, 1986, roughly 10 years after construction was complete, the Pittwater High School Binishell collapsed, only minutes after the area had been occupied by students. Thankfully only person was reported as injured. The collapse at Pittwater resulted in every Binishell across Australia being temporarily closed and inspected. Following these inspections, Binishells across the country either had supports added or were taken down.

Collapse

Pittwater.jpg
Inside Pittwater High School after failure
Immediately following the collapse of the Pittwater Binishell the NSW Public Works Department commissioned three engineering firms to determine the cause of the collapse. In one of the reports it was stated that the failure was “sudden and without warning”. However, the dome had been showing signs of an eminent failure for some time before. In the same report, comments were made on the flattening of the crown of the dome and how it had been getting more and more noticeable during the months and weeks leading up to the collapse. On August 4th, 1986, the Pittwater dome fell in a sudden collapse only minutes after only over 100 students had been in the space.

Cause

Although some of the Pittwater collapse reports have different opinions on some of the engineering concepts responsible for the collapse, they all reach a common ground on what the cause of the failure was. Problems for the Pittwater Binishell began on the first night of construction and were never resolved during the 10 years the dome was standing.

During the first night shift of the construction, one of the blowers used to keep the dome static was accidentally shut off. This caused the partially cured concrete to develop internal buckling. This would cause the concrete to never reach its intended strength and large cracks were formed on the crown of the dome. The engineer on this project realized the problem that was developing and took remedial action. He decided to add a concrete stiffening cap over the entire crown as well as the cracks that had formed.

addedconc.JPG
Picture taken from: Why Buildings Fall Down: How Structures Fail

Despite realizing the problem and acting quickly, the engineer did not consider the bond between the partially cured original concrete and the wet concrete cap. No bonding agent was used and the repair work performed added very little strength to the compromised dome while effectively adding a large dead load. The additional dead load was too great for the pneumatic form to support and the membrane sagged, causing the dome to have a more flattened shape than intended. When the dome was finished, it had the larger than normal radius of curvature, giving it the flattest crown of any of the 36 m domes. Additional air pressure in the form would have supported the additional weight but it would have caused damage to the overall structure and the equipment. The dome was tested upon completion and it was determined to have adequate strength leaving everyone to believe the repair had served its purpose.

In addition to the problems related to the construction difficulties other factors contributing to the collapse were identified in the investigative reports. There was a loss of stiffness of the dome cause by cracking initiated by slight reinforcement corrosion. Additionally, cracking due to moments induced by a thermal gradient were identified. This discovery led to the suggestion that the thermal insulation had been moist prior to collapse, deeming it ineffective. Finally, tubular voids were found enclosed by the springs causing additional cracking. The voids suggest improper vibration techniques during the concrete pouring.

For 10 years, the crown of the Pittwater dome continued to flatten as the concrete began to creep. The strength of the dome weakened more and more as the shell continued to deform further from its intended shape. Finally on August 4th, 1986 years of creep caused the concrete to be overstressed and the dome inverted followed by an instantaneous and dramatic collapse.

Lessons Learned

The problems responsible for the Pittwater collapse began during construction when the pneumatic form was partially deflated. This mistake led to the Department of Public Works to employ a new monitoring device that which was not part of the traditional Binishell equipment. The device was used during the construction of the Binishells constructed after Pittwater and no similar mishaps occurred.

Warning Signs

Several warning signs presented themselves throughout the duration of the Pittwater Binishell but they were either ignored or handled incorrectly. After the collapse of the Fairvale collapse, the Pittwater High School was subjected to a load test. Deflections of the Pittwater Binishell were three times larger than any of the other identical Binishells. This is a clear indication that the dome was not performing as expected but this was ignored and the report determined the dome had adequate strength.

The final warning signs that the Pittwater Binishell gave was the accelerated flattening of the crown leading up to the collapse. When it was observed that the dome was changing shape an engineer should have been notified. Shell structures depend on their shape for strength not the quantity of material in which they are constructed with. If a dome is rapidly changing its shape it is only a matter of time before the structure is over stressed and failure occurs.

Alternative Theories

There are some sources which claim the Pittwater High School collapse to be a result of lightning striking the structure in which the reinforcement steel was not properly grounded (Scott 35). There is little information available to support this theory.

Case A

In 1975, a 120 foot dome suffered a partial collapse of a Binishell just 2 days after construction began. There were uncommonly warm temperatures during the inflation of the pneumatic form. The warm temperature caused the air within the balloon to expand, reaching the desired pressure sooner than expected. T
Overinflated.JPG
Picture taken from: Why Buildings Fall Down: How Structures Fail
his would not have been an issue if the air temperature did not drop 50 F the next day due to a thunderstorm. The air pressure within the balloon went down with the temperature change and could no longer hold the desired shape of the dome. This resulted in the crown of the dome flattening out over a 40 foot diameter circle. The contractor for this project was inexperienced in Binishell construction and made errors when trying to correct the pressure loss. The contractor immediately began to re-pressurize the form but overcompensated for the loss of pressure and ended up over-inflating the balloon. Multiple iterations of lowering and raising the pressure attempting to reach the original shape resulted in crack forming around the top of the dome. When the form was deflated and removed, the top of the dome inverted and then sheared off the rest of the shell and fell to the ground. Although the middle of the dome collapsed, the exterior of the shell remained standing. (Levy 39)


Note: The majority of the content of this article was taken from the Engineers Australia article "Binishell Collapse - the Inventor's View" and the book published by the Department of Public Works New South Wales "Construction of Binishell Reinforced Concrete Domes New South Wales Australia".

Bibliography:


Bini, Dante. "Binishell Collapse - the Inventor's View." Engineers Australia 63 (3 May 1991): 5-8.
This is a letter written to Engineers Australia giving his point of view on the cause for the collapse of the Pittwater High School Binishell.

Grad, Paul. "Binishell Collapse." Engineers Australia 63 (8 March 1991): 27-28.
This article covers the information in the three engineering reports written on the causes for the Pittwater High School Binishell collapse.

BINISYSTEMS. Web. 22 Sept. 2010. http://www.binisystems.com/.
This website contains information relevant to the research and development of automation in the construction of Binishell systems.

Department of Public Works N.S.W. (1978). “Construction of Binishell Reinforced Concrete Domes New South Wales Australia.
This book describes the techniques used in the early stages of the Binishell dome construction

FUTURE / The Architect of Now / Dante Bini addresses our immediate and timely need --> http://articles.sfgate.com/2005-02-20/living/17361725_1_balloon-dome-concrete/2.

Hamed, Ehab, Bradford, Mark, Gilbert R. Ian (September 2009). “Time-Dependent and thermal behavior of spherical shallow concrete domes.”
Engineering Structures (1919-1929).
This journal enrty provides outcomes and insight to enhance the effective design and safe use of shallow concrete domes.

Latrobe City Council (2005). Latrobe City Heritage Study 2005: Places of Potential State Significance (1-4).
This article describes the historical importance of the Binishell and how it has been implemented over the past 40+ years.

Levy, Matthys; Salvadori, Mario (1994). Why Buildings Fall Down: How Structures Fail (38-41)
This book on famous building failures covers the history of Binishell domes and gives a description of a dome failure in Australia due to added weight from an attempted crack fix during construction.

Meyer, Christian, Sheer, Michael H (October 2005). “Do Concrete Shells Deserve Another Look?” Concrete International.
This journal entry discusses the modern era of thin concrete shell construction and mentions the prominence of Binishells in 1960-1986.

Scott, John S (1992). Dictionary of Civil Engineering, 4th edition (35).
This book provides a short description of the construction method with a possible explanation of the cause of failure of the Pittwater School (Australia).

Wolfe’s Method of Graphical Analysis. Web. 9 Nov. 2010. http://web.mit.edu/masonry/wwlau/wolfeMethodology.htm
This website discusses the structural theory concrete domes and how concrete domes distribute applied forces.