The result picture of Bird Nest, the white part is the the membrane structure
Zhang Chao, Hi-tech Olympics Net
Speaking of the membrane structure of Olympic venues, we would first think of the light blue "water cube". And another symbol Olympic venue, the Bird Nest also has the membrane structure. Different from the water cube which is all wrapped up by the membrane, the membrane structure of Bird Nest lies in the roof. So what are the characteristics of this membrane structure? How is the membrane material installed? With the introduction of the expert, the veil of the membrane structure of bird nest was uncovered ˇˇ
Speaking of the membrane structure of Olympic venues, we would first think of the light blue "water cube". And another symbol Olympic venue, the Bird Nest also has the membrane structure. Different from the water cube which is all wrapped up by the membrane, the membrane structure of Bird Nest lies in the roof. So what are the characteristics of this membrane structure? How is the membrane material installed? With the introduction of the expert, the veil of the membrane structure of bird nest was uncovered ˇˇ
Two thin layers of membrane cover the steel bone
The project contracted party is the combo of Beijing N & L Fabric Technology Co., Ltd. and a Germany company. Board chairman Liao Yang of N & L Company gave introduction to the overall situation of Bird Nest membrane structure.
The membrane structure is divided into two layers. The outermost layer is roof enclosure membrane, and the second layer is acoustics suspended membrane.
The roof enclosure membrane is made up of 1024 diaphragms with the whole area of about 42000 ©O. The biggest diaphragm is about 250 ©O and the minimum is 5 ©O. The material used is ETFE membrane, which is also used in the National Aquatics Center. The difference is that the Water Cube is puff air ETFE membrane, and the Bird Nest is monolayer stretching kind.
The project contracted party is the combo of Beijing N & L Fabric Technology Co., Ltd. and a Germany company. Board chairman Liao Yang of N & L Company gave introduction to the overall situation of Bird Nest membrane structure.
The membrane structure is divided into two layers. The outermost layer is roof enclosure membrane, and the second layer is acoustics suspended membrane.
The roof enclosure membrane is made up of 1024 diaphragms with the whole area of about 42000 ©O. The biggest diaphragm is about 250 ©O and the minimum is 5 ©O. The material used is ETFE membrane, which is also used in the National Aquatics Center. The difference is that the Water Cube is puff air ETFE membrane, and the Bird Nest is monolayer stretching kind.
The acoustics suspended membrane is divided into underneath acoustics suspended ceiling, stand acoustics and inner acoustics. It is made up of 1044 diaphragms in the totally area of about 53000 square meters. The material is monolayer PTFE.
Why install two layers of membrane like this? Professor Zhang Xi'an of China Academy of Building Research who has ever participated in the membrane structure project demonstration of Bird Nest explains the ETFE membrane is superstrong to keep out the ultraviolet. Its durability is very good. So it is placed on the outermost layer to resist the erosion of wind and rain and irradiation of ultraviolet.
But this layer is transparent that the sunshine can penetrate easily. So the second layer of opaque PTFE membrane plays a role in sheltering. And PTFE membrane has sound absorption, which can reduce the noise and impact on surrounding environment. This opaque membrane cover the steelframe and the audiences can concentrate on competition for building better match atmosphere.
It is reported the membrane does not cover the whole structure of the "bird nest", but mainly focus on the roofing extending to nearby "shoulder". Liao Yang says it was such in the original design, which can save expenses as well as make favorable ventilation.
The reason to use membrane for the roofing instead of other materials like aluminum plate or glass sheet is that the membrane can greatly reduce the burden of Bird Nest, said Zhao Xi'an. Since the span of the bird nest is large and the steel needed is very big. The total reduced weight of the Stadium by membrane is great.
So how was the membrane installed?
Install ˇ°windowˇ± on Bird Nest
Liao Yang says install the membrane on the Bird Nest is just like make window. Make a metal frame by aluminium alloy and stainless steel, and then put the membrane into it. Then link it up with the main steel structure by a set of jigs system.
Liao Yang says install the membrane on the Bird Nest is just like make window. Make a metal frame by aluminium alloy and stainless steel, and then put the membrane into it. Then link it up with the main steel structure by a set of jigs system.
While installing, the staff member used the vertical transportation tool such as crane or pulley block to roll the completed membrane material, carry to the pointed position, and afterwards exert pretension to the membrane. Exerting pretension is the most important procedure in the membrane structure installation. The principle is just like the drum leather. The tighter it stretches, the stronger it is. So the workers will use pull the membrane along with sides and fix when it is especially tight. Thus finish the installation.
In order to meet the demand of draining, the workers need to weld a circle of drainage gutter around the unit of membrane. The peripheral ETFE membrane is connected directly with the drainage gutter. Owing to the ETFE is transparent and can't bear large intensity, the workers add the auxiliary support like girder to increase its horizontal steel degree and the ability to resist external force.
Optimization innovation during design
The membrane structure of Bird Nest has gone on constant optimization and innovation during the design that made the design more reasonable and adaptable. Liao Yang introduces in the original design, several small steel arches are found in each unit of the ETFE membrane. And the single ETFE membrane was propped up as a convexity. The Association studies and proves this design to not in conformity with the real situation of the Bird Nest.
The membrane structure of Bird Nest has gone on constant optimization and innovation during the design that made the design more reasonable and adaptable. Liao Yang introduces in the original design, several small steel arches are found in each unit of the ETFE membrane. And the single ETFE membrane was propped up as a convexity. The Association studies and proves this design to not in conformity with the real situation of the Bird Nest.
Because the increasing slope of the membrane would be disadvantage to solve the problems of ponding and snow if one side is high while the other is low. Also the roof of the Bird Nest has originally slope of 9 degrees at the least.
Finally the Association persuaded the owners and general contractor to change the design to plane. And utilize its natural slope for better draining and defending snow. This is a big improvement that it would save a lot of expenses, more economized and effective.
After optimizing, the steel amount reduced, but also brought the new difficulty to the membrane construction. Owing to the decrease of steel amount, the thickness of the girder steel has been reduced, so did the strength bearing ability.
The workers have found out a ingenious solution. The additional girders are stuck in the units. In case that there is outside force, the additional girder will give a pull or push to the main beams. Through adjustment, they made a extension on the original basis. Thus the additional girder can move to release the pressure which would formally give to the main bean, playing a similar function of buffer.
At the moment, the membrane structure of the Bird Nest has finished designing and ready to make. It will begin installation in February or March next year.