Mechanical properties and structural performance of ETFE (ethylene-tetrafluoroethylene) cushion structures at low temperatures

Engineering Structures 136 (2017) 420–429

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Engineering Structures journal homepage: www.elsevier.com/locate/engstruct

Mechanical properties and structural performance of ETFE (ethylenetetrafluoroethylene) cushion structures at low temperatures Bing Zhao a,b, Leiting Dong c, Wujun Chen a,⇑, Jianhui Hu a, Zhenyu Qiu a, Jinyu Zhou a a

Space Structures Research Center, Shanghai Jiao Tong University, Shanghai 200240, China School of Aeronautics and Astronautics, Shanghai Jiao Tong University, Shanghai 200240, China c Center for Aerospace Research & Education, University of California, Irvine, USA b

a r t i c l e

i n f o

Article history: Received 27 August 2015 Revised 10 January 2017 Accepted 12 January 2017 Available online 31 January 2017 Keywords: ETFE cushion structure Wind load simulation Environment simulation Low temperature Ultimate limit state Failure mode

a b s t r a c t ETFE (ethylene-tetrafluoroethylene) cushion structures have been widely used as building roofs and façades in recent years. However, mechanical behaviors and structural performance of ETFE cushions in cold regions have not been well-understood, which limits its applications in such regions. This paper is thus aimed to study the behaviors of ETFE cushion structures subjected to wind loads at low temperatures. To this end, a novel method of simulating static wind loads was proposed, by pumping out air from the load simulation chamber (LSC) and blowing air into the LSC, generating wind suction and pressure loads, respectively. These simulated wind loads were directly acted on the inner layers of the ETFE cushions in the normal direction. In this case, a corresponding experimental system, integrating ETFE cushion model, load simulation device, environment simulation subsystem, automatic pressure control subsystem and measurement subsystem, was designed and manufactured. Destructive tests of three ETFE cushion models were performed under progressive wind suction loads at 0 °C, 25 °C and 50 °C, respectively. Temperature, internal pressure, deformation, ultimate bearing capacity and failure mode were obtained and analyzed. Experimental results demonstrate that low temperature environment leads to smaller deformation, larger ultimate bearing capacity, larger failure stress and higher frangibility for the ETFE cushion structures. Such insights gained from experimental observations are useful for the analysis and optimal design of ETFE cushion structures in cold regions. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction ETFE (ethylene-tetrafluoroethylene) has received considerable attentions in recent years due to its excellent chemical, thermal and electrical properties [1,2]. This is because the ETFE is a copolymer synthetized with ethylene and tetrafluoroethylene (TFE) [3], which has advantageous properties of both polytetrafluoroethylene (PTFE) and polyethylene (PE) [4,5]. Therefore, ETFE foil can be used in single or multi-layer configurations for building structures due to its transparence, lightweight and durability [1,2,6,7]. In 1982, the first ETFE membrane structure was designed and constructed for the Arnhem Burger Botanical Garden in Netherlands. Subsequent applications of ETFE membranes were firstly in mild-climate Western European countries, and then gradually spread all over the world [2]. ETFE cushion enclosures became widely known primarily through the eight domes of the ⇑ Corresponding author. E-mail addresses: [email protected], [email protected] (B. Zhao), [email protected] (L. Dong), [email protected] (W. Chen), [email protected] (J. Hu). http://dx.doi.org/10.1016/j.engstruct.2017.01.031 0141-0296/Ó 2017 Elsevier Ltd. All rights reserved.

Eden Project in 2001 [8], the dome of Tropical Islands in 2004 [9], and the building envelope of Allianz Arena in 2005 [10]. In 2008 Beijing Olympics, the ETFE membrane structure was introduced into China. It was utilized for the National Stadium ‘‘Birds Nest” and the National Aquatics Center ‘‘Water Cube”, the largest ETFE building envelopes in the world so far [11]. However, the ETFE membrane structures have not been widely applied in the very cold regions, such as those in Northern Europe, Russia, Northeastern China and Northern America where the minimum temperature may reach as low as 50 °C. This is because the mechanical behaviors and structural performance of ETFE cushions in these regions have not been well-understood, while only a few tests have been performed on ETFE foils at low temperatures [12]. When being used as structural components, the ETFE foil is mostly in a cushion form which consists of two or more layers of ETFE foils, an enclosing framework and the pressurized air between layers [1,2]. Such ETFE cushion structures have a wide range of applications [13], such as being used as building roofs or facades in greenhouses [8], stadiums [10], gymnasiums [14], exhibition halls [15], etc. More novelly, the ETFE cushion has been

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applied in the field of building integrated photovoltaic [16–19]. Unlike traditional rigid structures, this kind of inflatable membrane structure requires internal pressurization to maintain structural integrity [20]. When being utilized as building roofs or façades, the ETFE cushion is a very significant structural element, to transfer the external loads from the roof or facade to steel frames [21–23]. Most of the external loads acting on ETFE cushions are wind load, rain load and snow load. In order to resist these external loads, the internal pressure of ETFE cushion is adjusted automatically by the corresponding control devices. Meanwhile, ETFE cushions are directly affected by the environmental temperature due to their exposing to the external air [24]. Therefore, it is very important to investigate the behaviors of ETFE cushion considering these aspects. Attentions have been paid on the analysis and tests of ETFE cushion structures. Form finding [25,26], load analysis [26,27] and cutting analysis [2,13] are common types of analysis for the design of ETFE cushion structures. When taking the wind load into account, the aerodynamic response (wind load to pressure) [28] and the mechanical response (pressure to structural response) are generally studied. For the structural response, the vibration corresponding to the wind load should be mitigated [29,30]. As for the pressure, the fluctuating wind load is commonly replaced by the equivalent static load in civil engineering applications [31–33]. In order to verify the numerical analysis, experiments were conducted by means of paving sandbags on the upper layers of ETFE cushion structures to simulate the uniform loads [34,35]. Compared to the actual wind load in the normal direction, these methods could cause significantly different structural responses and failure modes, due to externally applied in-plane forces [36]. However, simulating static wind loads in the normal direction is very difficult, rarely found from the existing reports. Moreover, the above-mentioned studies were mainly at room temperature (around 25 °C). However, mechanical behaviors and structural performance of ETFE cushions under extreme environments, such as at extremely high/low temperatures, can be significantly different from those at room temperature due to the temperature sensibility of ETFE foils [4,12,37,38]. For example, the temperature of ETFE foils used in the PV-ETFE cushion structure in Shanghai can reach to 70 °C in summer [39,40]. And to verify the applicability of ETFE cushions in the Middle East, a series of comprehensive tests were performed on ETFE cushions experiencing high temperature variations from 40 °C to 80 °C [41]. More importantly, the tests on the full-scale structures are more difficult [42] compared with the tests of constructive materials [43,44]. Therefore, very few studies can be found on the mechanical behaviors and structural performance of ETFE cushions at low temperatures, resulting in limiting its applications and promotions in the cold regions. This paper presents a novel method for simulating static wind loads to evaluate the responses of ETFE cushions at low temperatures. Wind loads were simulated by pumping out air from the load simulation chamber (LSC) and blowing air into the LSC, generating suction and pressure loads, respectively. These simulated wind

loads were directly acted on the inner layers of ETFE cushions in the normal direction. An experimental system, integrating the square ETFE cushion model, a load simulation device, an environment simulation subsystem, an automatic pressure control subsystem and the measurement subsystem, was designed and manufactured. Destructive tests of three ETFE cushion models were performed under progressive wind suction loads at 0 °C, 25 °C and 50 °C, respectively. The temperature, internal pressure, deformation, ultimate bearing capacity and failure mode were recorded and analyzed to study the mechanical behaviors and structural performance of ETFE cushions at low temperatures. Detailed experimental set-up and results are presented in the following sections. 2. Experimental system To investigate the mechanical behaviors and structural performance of ETFE cushions subjected to the varied wind loads at different low temperatures, the experimental system consisted of a square ETFE cushion model, a load simulation device, an environment simulation subsystem, an automatic pressure control subsystem and the measurement subsystem, as illustrated in Fig. 1. The detailed descriptions of the experimental system are presented as follows. 2.1. ETFE cushion model Three identical ETFE cushion models were designed and manufactured for three different environment temperatures. The planar dimension of ETFE cushion was limited by the sizes of the environment simulation subsystem and the load simulation device, which was finally set as 1.5 m  1.5 m. The ETFE foil was 250 lm in thickness. The rise of the ETFE cushion was determined by capacity evaluation and was finally set as 188.0 mm. The form of the ETFE cushion was found by EASY Version 9.2 [45] and the initial stress was calculated from the expected geometry and the internal pressure (3.5 kPa) [46,47]. To have a smooth surface, four symmetrical ETFE patterns were welded through the heat bars at about 275 °C, with the welding seam width of 10 mm. Two layers were welded together at the four edges to form an enclosed cushion. Finally, two inlets with the diameter of 6 mm were set up for installing the automatic pressure control subsystem. The planar dimension and cutting pattern of the ETFE cushion model are shown in Fig. 2. 2.2. Load simulation device The load simulation device, including a load simulation chamber (LSC), a steel cover and the ETFE cushion, was developed to simulate wind loads. As shown in Fig. 3, the size of the LSC was 1863 mm  1703 mm  1600 mm in length, width and height, respectively. The chamber was made of steel frame of rectangular hollow section (RHS) cladding with thin steel plates, as well as stiffener plates with different bays. The top and bottom of the LSC were a steel cover and the ETFE cushion, respectively. The steel

Experimental system of ETFE cushion

ETFE cushion model

load simulation device

automatic pressure control subsystem

environment simulation subsystem

Fig. 1. The frame of the experimental system.

measurement subsystem

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(a) Planar dimension

(b) Cutting pattern (1/2) Fig. 2. Planar dimension and cutting pattern of the ETFE cushion.

(a) Design scheme

(b) Snapshot Fig. 3. Load simulation device.

cover and the ETFE cushion were connected to the top and bottom of the LSC with bolts and rubber mats to reduce air leakage. The load-carrying capacity of the LSC was designed to reach to 50 kPa

at least in order to satisfy the requirements of the destructive tests. To simulate the building façade, the ETFE cushion was vertically place=d as shown in Fig. 3.

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After closing the LSC, the automatic pressure control subsystem was installed on the LSC and ETFE cushion. The vacuum pumps were used to pump out or blow the air. And the pressure sensor was utilized to monitor the internal pressure of the LSC, as shown in Fig. 4. Three groups of pressures, including the internal pressure of ETFE cushion PETFE, the internal pressure of the LSC PLSC, and the atmospheric pressure PATMOS, were defined. To be clear, PETFE and PLSC were defined as the differences between the ETFE/LSC internal pressure and the atmospheric pressure PATMOS. When the air in the LSC was pumped out, negative pressure (PLSC) was generated. As a result, wind suction load acting on the inner layer in the normal direction was obtained. Correspondingly, wind pressure load can be simulated by blowing air into the LSC. When wind pressure load was simulated, decrease of the tension force and change of the shape would cause structural instability of the ETFE cushion. Therefore, simulated wind suction load was used to investigate the ultimate limit state of the ETFE cushion. 2.3. Automatic pressure control subsystem As mentioned above, the ETFE cushion was inflated to a certain pressure to maintain its shape, and wind loads were simulated by

controlling the internal pressure of the LSC. Therefore, the automatic pressure control subsystem was composed of an ETFE cushion part and a load simulation part, as illustrated in Fig. 5. In the subsystem, some components were shared for both parts, such as the computer, the multifunction USB module, the power relay board and the three stabilized voltage supplies, while other components were different to satisfy the functionality requirement of each individual part. The equipment specifications are listed in Table 1. The control software, including the main interface, data processing and basic logic control, was coded using C++. To control and measure the ETFE/LSC internal pressures, a series of actions were automatically performed in the developed control subsystem. The pressure signals measured via the pressure sensor were collected by the multifunction USB module and transmitted to the control software. Then, the software module processed the pressure signals and sent commands to the power relay board through the USB module. Finally, the performer controlled the internal pressures of the objects by executing comments sent through the USB module. In summary, these actions constituted a complete control loop of the automatic pressure control subsystem, as shown in Fig. 6.

Fig. 4. Principle diagram of load simulation.

(b) Snapshot

(a) Schematic diagram Fig. 5. Automatic pressure control subsystem.

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Table 1 Equipment specifications of the automatic control subsystem. Equipment

Type

Voltage (V)

Frequency/accuracy/response time

Multifunction USB module Power relay board Pressure sensor (pressure control) Pressure sensor (load simulation) High-speed pneumatic on/off valves Solenoid valves Air compressor machine Vacuum pumps

USB-4711A-AE PCLD-885 KQ-SPB KQ-SPB MHE2-MS1H-3/2G-QS-4 ZQDF-1 OTS-1100 XZ-1.3 XZ-4.2

– 5, 12 24 24 24 24 220 220 220

150 kS/s 6 ms 15 Pa 60 Pa 7 ± 10% ms 5 Hz 50 Hz 50 Hz 50 Hz

Range Input

Output

0–10 V ±5, ±10 V – – 0–15 kPa 0–10 V 30 to +30 kPa 0–10 V –0.09 to 0.8 MPa, 100 L/min 0–0.8 MPa – 0–0.7 MPa – 88 to 40 kPa – 88 to 60 kPa

Air supply (air compressor machine or vacuum pumps)

Computer (control software)

Performer (highControl speed pneumatic on/ off valves or solenoid valves) Transmit signal

Power relay board

Transmit signal and command Multifunction USB module

Collect signal

Control pressure

Pressure sensor (ETFE cushion or LSC )

Object (ETFE cushion or LSC)

Measure pressure

Fig. 6. Control loop of the pressure control.

Fig. 7. Environment simulation subsystem.

Fig. 8. The direct displacement measurement device.

The ETFE cushion part of the pressure control subsystem required higher control accuracy and faster response. This was because the internal pressure of ETFE cushion was easily affected

by the wind suction or pressure load. Considering the small volume of the ETFE cushion model, the pressure control part was composed of a laptop computer, an air compressor machine, a multi-

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(a) Measuring principle

(b) Measured points on inner layer

Fig. 9. Displacement measurement of ETFE cushion.

Begin Automatic control subsystem works: measures the pressure of ETFE cushion Pm, ETFE and load simulation chamber (LSC) Pm, LSC The ETFE cushion is inflated to 3.5 kPa No

3.47 kPa
No

The ETFE cushion is inflated to 3.5 kPa; the LSC is pumped to PLSC

No

3.47 kPa
The experimental system enters into stable state ETFE cushion destroys

System stops

End

No

No Time≥ 10 min The subsystem updates the set pressure of chamber by reducing an increment: PLSC=PLSC - 1 kPa

Fig. 10. Work flow of pressure control.

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30

20 15 10 5 0 -5

0

100

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20

40

60 80 Time (min)

100

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-4 -6

Load simulation chamber

-8

(a) 0°C

Time (min) 4

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-2

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-4 -6 Load simulation chamber

-8 -10 -14

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(b) -25 °C Environment simualtion chamber Outer layer Load simulation chamber Inner layer

20 10 0

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ETFE cushion

2

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Internal pressure (kPa)

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ETFE cushion

2

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Internal pressure (kPa)

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0

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-4 -6 -8 -10

-20

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Load simulation chamber

(c) -50°C

-40 Fig. 12. Internal pressures of the experimental system.

-50 -60

(c) -50°C Fig. 11. Temperatures of the experimental system.

function USB module, a power relay board, a one-way valve, a pneumatic FRL (Filter-Regulator-Lubricator), two high-speed pneumatic on/off valves, a pressure sensor (0–15 kPa) and three stabilized voltage supplies, as shown in Fig. 5. As for load simulation part of the pressure control subsystem, it required faster speed and greater flow due to the large volume and poor airtightness of the LSC compared to the ETFE cushion. Three vacuum pumps were employed to simulate wind suction and pressure loads by pumping and blowing, respectively. Two magnetic valves were used to control the airflow according to the output commands of the coded control software. As shown in Fig. 5, the load simulation part consisted of a computer, a multifunction

USB module, a power relay board, two one-way valves, a pressure sensor ( 30 to +30 kPa), three stabilized voltage supplies, three vacuum pumps and two solenoid valves. 2.4. Environment simulation subsystem A stable environmental temperature was particularly required to test the structural performance of ETFE cushions in cold regions. Hence, an environment simulation subsystem with a 2.0 m  1.8 m  2.2 m space chamber was employed to simulate the low temperature environment. As shown in Fig. 7, the environment simulation chamber was closed by two insulated doors with transparent windows. The temperature range of the environment simulation subsystem was from 65 °C to 100 °C with ±2 °C accuracy. The temperature in the chamber was reduced through circu-

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3. Experimental procedures As described in the previous section, an ETFE cushion experimental system was designed and integrated. The double-layer ETFE cushion model was connected to the LSC by using aluminum profiles, rubber mats and bolts. The automatic pressure control subsystem was installed onto the ETFE cushion at the two inlets on the outer layer. When the ETFE cushion was fixed on the chamber, the measurement subsystem was installed to monitor the experimental process. After that, the steel cover was connected to the LSC by bolts and then the automatic pressure control subsystem was installed onto the LSC. Finally, the experimental system was moved horizontally into the environment simulation chamber. In order to reveal the structural performance of ETFE cushion under various loads at different temperatures, the following experimental procedures were carried out. Firstly, the temperature was reduced to 0 °C, 25 °C and 50 °C, respectively, for the three ETFE cushion models. Each ETFE cushion was inflated to 3.5 kPa. Then, the internal pressure of the LSC (PLSC) was reduced from 1 kPa with an increment of 1 kPa in every 10 min. While changing PLSC, the airflow through the ETFE cushion was stopped without inflating and deflating. After PLSC reached to its target value, the internal pressure of the ETFE cushion PETFE was reset to 3.5 kPa and kept constant. After the pressure was stabilized by the pressure control subsystem after each increment, displacements, temperatures, and pressures were measured and recorded. After the measurements were completed, the automatic pressure control subsystem updated PLSC by reducing an increment of 1 kPa. The procedures described above were repeated, until the ETFE cushion was destroyed. This corresponding work flow is illustrated in Fig. 10.

Temperatures of the three ETFE cushions were reduced from room temperature to the target values of 0 °C, 25 °C and 50 °C, respectively. Fig. 11 demonstrates the temporal variations of temperatures at the four measurement points of the experimental system. It is found that the four curves can be divided into two groups: the outer layer and the air in the environment simulation

180

D1 D3 D5 D7 D9

150

Displacement (mm)

A direct method, using rulers, needles, threads and cameras, was employed to measure the displacements on membrane surfaces of the ETFE cushions in the environment simulation chamber, as shown in Fig. 8. This direct displacement measurement device included ten scale rulers, ten threads, ten needles, four cameras and two computers. The needles connected to measurement points by threads were hanged near the scale rulers. The needles would move with the deformation of the inner layer, and their displacements were shown in the computers captured by four cameras, as shown in Fig. 9(a). Meanwhile, considering that the ETFE cushion model was a centrosymmetric structure, ten measured points were located on the middle diagonal seams of the inner layer, as shown in Fig. 9(b). In detailed, D1 was on the midpoint of ETFE cushion, D10 was near the corner, and the other measured points (D2-D9) were arranged between these two points in sequence. Four resistance thermometers were employed to measure the temperatures of the air in the environment simulation chamber, the outer layer of the ETFE cushion, the air in the LSC, and the inner layer of the ETFE cushion. The measurement points were located at the corner area of the ETFE cushion to avoid affecting the experimental process. Moreover, the internal pressures of the ETFE cushion and the LSC were measured by the automatic pressure control subsystem. Another two cameras were used to monitor the inner layer of the ETFE cushion model during the whole loading process.

4.1. Temperature

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D2 D4 D6 D8 D10

90 60 30 0

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80

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Time (min)

(a) 0°C 180

D1 B3 D5 D7 D9

150

Displacement (mm)

2.5. Measurement subsystem

4. Results and discussions

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D2 D4 D6 D8 D10

90 60 30 0

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(b) -25°C 180

D1 D3 D5 D7 D9

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Displacement (mm)

lating cold air from the inlet to the outlet by using two high power refrigeration compressors. A temperature sensor was installed near the air inlet to monitor the temperature of the cold air. And a control process for constant temperature would be performed when the temperature of the air inlet reached to the target value.

120

D2 D4 D6 D8 D10

90 60 30 0

0

20

40

60 80 Time (min)

(c) -50°C Fig. 13. Deformations of ETFE cushions.

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(a) 0°C

(b) -25°C

(c) -50°C

Fig. 14. Failure modes of the ETFE cushions.

chamber are considered as one group, while the inner layer and the air in the LSC are regarded as the other group. In the cooling process, very small difference (around 2–3 °C) can be found between two curves in the same group. However, the temperature difference between these two different groups is significant, which are about 10 °C. It means that the cooling rates of ETFE foils are fast while directly contacting with the cold air. However, temperatures of the two cushion layers are significantly different, due to the low thermal conductivity of ETFE foils and the pressurized air between two layers. Therefore, the ETFE cushion can provide the considerable heat insulation. 4.2. Internal pressure Fig. 12 shows internal pressures of the experimental system during the whole process of loading tests at 0 °C, 25 °C and 50 °C, respectively. Overall, variation of internal pressures demonstrates the precision of the automatic pressure control subsystem. A decrease in the ETFE cushion internal pressure (PETFE) is found at the beginning of each load increment ( 1 kPa decrease of PLSC), agreeing with the Ideal Gas Law proposed by Clapeyron, as inflating and deflating of the ETFE cushion was stopped when reducing PLSC. At the ends of load tests, internal pressures of the experimental system returned to 0 kPa, which means that the ETFE cushion models were destroyed. 4.3. Deformation Fig. 13 shows deformations of ten measured points on the middle diagonal seams of the inner layers, for the three ETFE cushion models at 0 °C, 25 °C and 50 °C, respectively. Overall, trends of displacement time-history curves at three different temperatures are similar, showing increasing deformations over time. Maximum displacements (D1) of the three ETFE cushion models, which are on the midpoints of the middle diagonal seams, are clearly different, i.e. 165 mm, 153 mm and 79 mm at 0 °C, 25 °C and 50 °C, respectively. Meanwhile, minimum displacements (D10) are 71 mm, 44 mm and 15 mm at 0 °C, 25 °C and 50 °C, respectively. The creep behavior can be seen from continual deformations of the ETFE cushion when constant pressure is maintained. It is found that creep deformations of the ETFE cushion in the first three 1 kPa load increments are very small at 50 °C. This is because mechanical properties of ETFE foils have been obviously strengthened at 50 °C compared to the other two ETFE models at 0 °C and 25 °C. 4.4. Ultimate limit state The ultimate bearing capacity and failure mode determine structural safety of the ETFE cushion. As shown in Fig. 12, it is found that the ultimate wind suction load bearing capacities of ETFE cushions are 10 kPa, 12 kPa and 12 kPa at 0 °C, 25 °C

and 50 °C, respectively. The ultimate bearing capacity of the ETFE cushion at 25 °C is slightly higher than that at 0 °C, although slightly smaller deformation is found at 25 °C as shown in Fig. 13. This means that the maximum stress of ETFE cushion at 0 °C was slightly larger when the ETFE cushion was destroyed, according to the theory on determining the stress distribution of inflatable membrane structure [2,46–48]. The ultimate bearing capacities of ETFE cushions at 25 °C and 50 °C are similar, but the deformation at 25 °C is much greater than that at 50 °C. It can thus be deduced that the maximum stress of ETFE cushion at 50 °C is significantly larger than that at 25 °C. Failure modes of the three ETFE cushion models are similar, showing slits on the middle diagonal seams of the ETFE cushions, as shown in Fig. 14. This is due to the stress concentration at the connection between the single foil and the seam. However, lengths of slit and speeds of spreading are found to be much larger with the decrease of the environment temperature, showing brittle failure at lower temperature. 5. Conclusions This paper presents a novel method for simulating static wind loads for ETFE cushions at low temperatures. The wind loads were obtained by pumping out air from the LSC and blowing air into the LSC, generating wind suction and pressure loads, respectively. These simulated wind loads were directly acted on the inner layers of ETFE cushions in the normal direction. An experimental system, integrating the square ETFE cushion model, a load simulation device, an environment simulation subsystem, an automatic pressure control subsystem and the measurement subsystem, was developed for the structural tests. Destructive tests of three ETFE cushion models were performed with progressive wind suction loads at 0 °C, 25 °C and 50 °C, respectively. The temperature, internal pressure, deformation, ultimate bearing capacity and failure mode were recorded and analyzed. According to the experimental results, some conclusions and remarks can be given: 1. Cooling rate of ETFE foils is high while directly contacting the cold air. However, temperatures of the two cushion layers are significantly different, due to the low thermal conductivity of ETFE foils and the pressurized air between two layers. Therefore, the ETFE cushion can provide considerable heat insulation. This finding would be beneficial to its application in the cold regions. 2. Mechanical behaviors of the ETFE cushion are sensitive to temperature. At low temperature, deformations of ETFE cushion are relatively small, and the ETFE foil demonstrates significant delay phenomenon with the increase of viscosity and elastic modulus. This means that ETFE foils becomes stiffer at lower temperature. This phenomenon can reduce the deformation of ETFE cushion while resisting the loads.

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3. The ultimate bearing capacity of the ETFE cushion is not significantly improved with the decrease of the temperature, but the maximum stress is significantly increased due to the smaller deformation under the same internal pressure. These improvements are advantageous to bear the external loads for ETFE cushion. 4. Failure modes of all the three ETFE cushion models show slits at the middle diagonal seams of the inner layers. It means that the mechanical behavior of the ETFE foil seam plays a key role in the structural performance of the ETFE cushion at low temperatures. To reduce this influence, it is recommended that the welding seam should not be located at the places with large deformation, such as the middle part of ETFE cushion. 5. The length of the slit and the speed of spreading are found to be larger with the decrease of the environment temperature, indicating brittle failure at low temperatures. For the structural safety, the intense brittle failure should be avoided.

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