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A New facility to simulate simultaneous wind and thermal effects on
roofing systems
Baskaran, B. A.; Liu, K. K. Y.; Lei, W.; Delgado, A. H.
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https://nrc-publications.canada.ca/eng/view/object/?id=887e298d-0be8-4483-800f-b281b1b533a1 https://publications-cnrc.canada.ca/fra/voir/objet/?id=887e298d-0be8-4483-800f-b281b1b533a1A New facility to simulate simultaneous wind and
thermal effects on roofing systems
Baskaran, A.; Liu, K.; Lei, W.; Delgado, A.
NRCC-45672
A version of this document is published in / Une version de ce document se trouve dans: Journal of Testing and Evaluation, v. 31, no. 4, July 2003, pp. 1-10
A New Facility to Simulate Simultaneous Wind and Thermal Effects on
Roofing Systems
A. Baskaran1, K. Liu, W. Lei and A. Delgado Institute For Research in Construction
National Research Council Canada Ottawa, Ontario, K1A OR6
613-990-3616 bas.Baskaran@nrc.ca
ABSTRACT
Roofing systems are exposed to wind pressures and thermal stresses. Through a North American roofing consortium (Special Interest Group for Dynamic Evaluation of Roofing Systems – SIGDERS) the National Research Council Canada fabricated a facility to evaluate roof assemblies under simultaneous wind and thermal effects. The overall objective of this paper is to present the commissioning process of the facility based on experimental investigations that were carried out on a single-ply roofing system with a PVC membrane. For this study, roofing system responses were measured under simulated simultaneous wind and heat/cold conditions. A systematic attempt also was made to quantify the system response and to characterize the membrane properties. This was performed on a cold-conditioned system as well as by subjecting the membrane samples to the same cold-conditioning program in a laboratory freezer. Membrane samples were characterized by mechanical and chemical methods. Comparison of laboratory-conditioned membranes and wind-tested systems revealed the effects of cold conditioning and wind loading. Neither the wind uplift nor the mechanical properties (tensile breaking strength, elongation at break, tear strength, and seam strength) nor the chemical properties (glass transition temperature and weight loss) were affected
by the selected cold conditioning. Based on these findings, directives for further studies are presented for this on going project.
Keywords
wind uplift, roofing membrane, dynamic mechanical analysis, mechanical testing, thermal conditioning, chemical characterization, PVC membrane, thermogravimetric analysis
1
INTRODUCTION
In a conventional roofing system, the roofing membrane is installed at the top surface where it experiences environmental forces such as wind and temperature. Most conventional systems have five main components with associated functions (from bottom up):
1. Deck - Structural support
2. Barrier – Air, moisture, and fire control 3. Insulation - Thermal control
4. Membrane - Waterproofing
5. Fasteners/Adhesives – Integrating the above components to act as a system
The service life of a roof assembly is based not only on the characteristics of the individual components but also on the performance of the whole assembly. Existing research knowledge and standards are applicable mainly for component evaluations. In construction specifications of roof assemblies, engineers and architects mainly refer to standards that call for certain minimum requirements for the membrane’s physical, chemical, and mechanical properties. In roofing, system evaluation is not a common routine as it is in the wall/cladding manufacture sector. The reason may be two-fold: first, in roofing, the emphasis is on waterproofing and the membrane is the primary component of that function. Second, there exists neither an acceptable protocol nor an experimental setup for the evaluation of roofing systems. This was noted in the conclusions of the state-of-the-art report prepared by Baskaran and Dutt [1]. In roofing, system evaluations are performed only for static wind uplift rating and in certain scenarios for fire rating. Through a North American roofing consortium (Special Interest Group for Dynamic Evaluation of Roofing Systems – SIGDERS), the National Research Council Canada (NRC) built a facility to evaluate roofing systems under simultaneous wind and thermal effects. The facility can provide a platform for system performance evaluations and as such it can help facilitate the development of test protocols to rate roofing systems. This paper describes the facility and presents response data from a pilot study on a mechanically attached single-ply roofing system with a poly(vinyl chloride) (PVC) membrane. Behavior of the roofing system, with and without thermal conditioning, was also investigated under dynamic wind uplift conditions. With the idea of developing an evaluation relationship between small-scale and system testing, membrane samples were also subjected to the same conditioning program using laboratory
freezers. Mechanical properties (tensile breaking strength, elongation at break, tear strength, and seam strength) were determined. Dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA) were also used to investigate chemical changes in the membranes. By comparing mechanical and chemical properties of wind tested and laboratory-conditioned samples, the effects of thermal conditioning and wind loading on the membrane were determined.
DEVELOPMENT OF A FACILITY
In 1997, the North American roofing consortium SIGDERS established a Dynamic Roofing Facility (DRF) at the NRC/IRC that can evaluate the dynamic effects of wind on roofing systems, [2]. More than 200 systems have been investigated at the DRF for wind performance at room temperature using
new roofing components. Systems, which varied in membrane types, layout, and configurations, were subjected to static and dynamic load cycles. Based on this extensive study, it can be said that SIGDERS brought the benefit of dynamic testing to the roofing community [3].
Recently, SIGDERS enhanced the DRF so that it can simulate simultaneously wind and thermal effects on roof assemblies. For this enhancement, six weather stations were selected to represent the weather variation across North America. Their weather data were analyzed for input into the facility [4]. Table 1 presents the analyzed values for the six stations. Assuming independence between air temperature and wind speed, it is possible to approximate a 10–year return-period event (which has an annual probability of 0.1) as the combination of air temperature reached 1% of the time and wind speed achieved at least 10 times per year. Taking Ottawa as an example, the 10-year return-period event is the combination of air temperature reaching 1% of the time (-24°C and +30°C or –11°F and +86°F) and wind speed achieving at least 10 times per year (12 m/s or 27 mph). Based on the analysis and the choice of stations, the air temperature ranges from +43°C to – 33°C (109°F to –27°F) across much of North America. Since the membrane surface temperature can be quite different from the air temperature due to many factors such as membrane color (black versus white) and snow coverage, weather data analysis therefore provides a design direction for the DRF enhancement rather than a set of design conditions.
In 1998, a new second chamber, DRF-WT – Dynamic Roofing Facility for Wind and Thermal, was
constructed. DRF-WT is identical to the DRF in overall size. . Similar to the DRF, DRF-WT Itconsists of a
bottom frame of adjustable height upon which roof specimens are installed, and a movable top chamber (Figure 1). The bottom frame and top chamber are 6100 mm long (l = 240 in.), 2200 mm wide (w = 86 in.) and 800 mm (h = 32 in.) in height. The top chamber is equipped with six windows for viewing roof specimens, and a gust simulator, which consists of a flap valve connected to a stepping motor through a timing belt arrangement. Pressure suction over the roof assembly is produced by a 75 kW (100 HP) fan with a flow rate of 2500 L/s (5300 cfm). A computer-based feedback system controls the operation of the DRF-WT. The computer regulates the fan speed in order to maintain the required pressure level in the top chamber. Operation of the flap valve simulates the gusts. Closing the flap valve allows pressure to build in the chamber while opening the valve bleeds the pressure.
There are several design differences between DRF and DRF-WT and some of these are highlighted
below:
• Top chamber of the DRF-WT was well insulated to ensure minimal thermal losses through the chamber assembly and to maintain a near-constant temperature distribution inside the chamber.
• Wind flow path in the DRF-WT was generated by circulating air in a closed loop (Figure 2) rather than an open loop to the laboratory.
• The fan that produces wind suction was placed in an enclosure that was insulated both acoustically and thermally.
Once assembled, the top chamber can be configured to simulate either heating or cooling using airflow patterns shown in Figure 2. Using a circulating fan and a series of ducting hoses with dampers, air is preferentially conditioned before entering the chamber. As shown in Figure 2, the air is conditioned either by a heating unit or cooling (using a 5 ton cooling unit and air compressor. Cooling and then
heating rather than cooling alone precisely controls the inlet air temperature. The experimental set-up allows for a closed-circuit flow of air through the test chamber, which is continuously monitored using an analog thermal controller with feedback signals, and a computer-based data acquisition system connected to thermocouples installed inside the chamber. Testing can be configured into the
experimental set-up using the 75 kW (100 HP) blower and gust simulator flap either alone or combined with temperature conditioning. During heat aging or cold conditioning of a roof assembly, a circulating fan is used for better air circulation. The existing capacity of the DRF-WT is summarized in Table 2.
The existing capacity of the DRF-WT is summarised in Table 2.
SYSTEM INVESTIGATION UNDER
ÄP PRESSURE AND TEMPERATUREand T
Selected System
To commission the DRF-WT facility, a roofing system with a PVC membrane was selected. The system consisted of three roofing components whose properties are summarized below:
Deck: The steel deck was 0.76 mm thick (22 gage) with a profile height of 38 mm (1.5 in.) and a flute width of 150 mm (5.9 in.). The steel deck was fastened at every flute to the wood frame of the DRF-WT with size-10 round-head screws.
Insulation: The polyisocyanurate (ISO) insulation boards were 102 mm thick by 1.25 m by 3.0
2.4 m (4 in thick thick by 4 ft by 8 ft). They were attached to the deck with six fasteners per
board. The fasteners were 127 mm (5 in.) long with a plastic discplate 76 mm (3 in.) in diameter.
Membrane: The membrane was 1.2 mm (0.047 in.) thick and 1.83 m (6 ft) wide. It is composed of a polyester reinforcing fabric sandwiched between two PVC sheets.
The membrane was installed as follows. First, one sheet of the PVC membrane was unrolled over
the insulation and was mechanically fastened to the steel deck. Then, another sheet was placed beside the first one such that it overlapped the row of fasteners. Finally, the two membranes were
hot air welded using hot air at the overlaps to achieve continuity. Figure 3 shows the Ddetails of
the tested system are now described (Figure 3). The bottom sheet was fastened every 305 mm
(12 in.) along the seam. Fasteners were 127 mm (5 in.) long with a plate disc 51 mm (2 in.) in
diameter. Each seam had an overlap of 127 mm (5 in.), with the fastener placed 51 mm (2 in.) from the edge of the bottom membrane, and 76 mm (3 in.) from the edge of the overlapping membrane. The membranes were welded with a hot air welder at the overlap to form a seam of 38 mm (1.5 in.) in width.
Dynamic Wind Test Protocol
The SIGDERS load cycle was developed based on extensive wind tunnel studies of full-scale roof systems measuring 3 m X 3 m (10 ft × 10 ft). These studies were carried out in the NRC's 9 m X 9 m (30 ft × 30 ft) wind tunnel. Details of the method by which the load cycle was developed can be found in Baskaran and Chen [5]. A summary of the developed load cycle follows:
As shown in Figure 4, the SIGDERS dynamic protocol has five rating levels (A to E). To evaluate a roof assembly for a specific wind resistance (e.g. Level D), all the gusts from Level A upward must be applied.
Each level consists of eight load sequences with different pressure ranges (Figure 4). The eight load sequences can be divided into two groups. Group 1 represents wind-induced suction over a roof assembly. It consists of four sequences, where the pressure level alternates between zero and a fixed pressure. Group 2 represents the effects of exterior wind fluctuations combined with a constant interior pressure on a building [6]. Internal pressure variations are explicitly codified in recent North American wind standards [7, 8]. The SIGDERS test protocol accounts for such variations.
Test pressures (Y-axis) can be calculated from the design pressure, in accordance with local building codes or wind standards. The pressures for each load sequence are calculated from specified ratios of the test pressure. For the present investigation, all the tests started from Level A using a test pressure of 60 psf (2.87 kPa).
To evaluate the ultimate strength of a roofing system, testing should start at Level A and continue to the next higher level. To obtain a level rating, all specified numbers of gusts in each lower level and the specified level must be completed.
To study the roofing system response, design parameters (wind pressure, fastener force, and membrane deflection) were monitored. Instrumentation locations [pressure transducers (P1, P2, P3, and P4), load cells (L1, L2, and L3) and deflection sensors (D1, D2, D3, and D4)] are shown in Figure 3. Signals from all of these instruments were monitored and collected by a Hewlett-Packard mainframe computer. The responses recorded by pressure sensor P1, load cell L3, and deflection sensor D1 will be discussed in this paper. These locations respectively represent the membrane surface pressure, maximum fastener load, and deflection.
Figure 5 shows the DRF-WT operation in heating mode. On the horizontal axis, time taken for each wind gust is shown. In accordance with the SIGDERS protocol, each gust has a period of 8 s with a loading time of 2 s and an unloading time of 2 s. Two such gusts are displayed in Figure 5 with the target and measured pressures. The pressures shown on the left Y-axis were suction pressures induced by the fan on the membrane top. They correspond to loading sequence 2 of level A of the SIGDERS cycle (refer to Figure 4). Measured and target temperatures are also shown in the figure. Similar to pressure, thermocouples were installed on the membrane top surface to record temperature. It is clear that the roof assembly was subjected to a pressure difference of 1436 Pa (30 psf) and at the same time a membrane surface temperature of 65°C (149°F). The figure also shows that measured temperatures fit the target temperature better than measured pressures do target pressures. Each simulated wind gust had two pressure targets: a maximum (1436 Pa or 30 psf) and a minimum, the base pressure. Measured base pressures were higher than the target due to the closed-circuit operation of the system (Figure 2).
Figure 6 presents cooling mode data using the same format as Figure 5. As can be seen, the below-freezing target temperature (–7°C or 19°F) was readily achieved during the wind gusting. The data represent gusts from loading sequence 1 of level A in the SIGDERS cycle (refer Figure 4). Figures 5 and 6 clearly demonstrate the satisfactory simultaneous wind and thermal operation of the DRF-WT.
Figure 7 displays measured membrane deflection (D1) and fastener force (L3) when the system was subjected to a cold wind (on the membrane surface –7°C or 19°F) and an induced suction across the roof assembly of 718 Pa (15 psf). Responses measured with the same system at the same pressure, but at room temperature are also shown for comparison. Note the maximum deflection at room temperature was about 160 mm (6.3 in.) compared to 120 mm (4.7 in.) at – 7°C or 19°F - a difference of about 40 mm or 1.8 in. However, only a small difference was observed in the measured fastener force. This may be due to the fact that the system was subjected to a pressure difference of only 718 Pa (15 psf). It is rather difficult to attribute the above large difference in deflection to the simulated cold wind gusts because of the many complexities. To obtain a better understanding of the process, the effect of cold conditioning on system response was investigated first.
Two identical roof systems having components as described in the previous section were investigated. The first system was subjected to the SIGDERS protocol immediately after it was installed in the DRF. This was designated as the Reference system. The second system was cold conditioned at – 15°C (5°F) for 7 days after it was installed in the DRF-WT. The system was allowed to return to room temperature overnight. It was then tested using the SIGDERS protocol. This is the Cold-Conditioned system. In our study, the target temperature (–15°C or 5°F) was reached in 15 min and steady state pressure cyclic condition was achieved in under half a day. Thermocouples were instrumented on and under the PVC membrane to monitor the surface temperature at various locations on the roof system and the temperature difference on the roof system across the width of the table was found to be less than 1°C (2°F).
Measured responses of the two systems tested in the DRF and DRF-WT are presented in Table 3. Both systems were subjected to the SIGDERS dynamic load cycle with 60 psf as the test pressure at Level A. As shown in Figure 4, Level A has eight load sequences, which can be divided into two groups. Group 1 consists of four sequences, in which the pressure level alternates between zero and prescribed fixed pressures:
• Sequence 1 applies 400 gusts at 718 Pa (15 psf) pressure.
• Sequence 2 applies 700 gusts at 1436 Pa (30 psf) pressure.
• Sequence 3 applies 200 gusts at 2155 Pa (45 psf) pressure.
• Sequence 4 applies 50 gusts at 2873 Pa (60 psf) pressure. Similar to Group 1, Group 2 consists of four sequences:
• Sequence 5 applies 400 gusts with pressure alternates from 718 Pa to 1436 Pa (15 psf to 30 psf).
• Sequence 6 applies 400 gusts with pressure alternates from 718 Pa to 2155 Pa (15 psf to 45 psf).
• Sequence 7 applies 25 gusts with pressure alternates from 718 Pa to 2873 Pa (15 psf to 60 psf).
• Sequence 8 applies 25 gusts with pressure alternates from 1436 Pa to 2873 Pa (30 psf to 60 psf).
In accordance with the SIGDERS test method, to get a wind uplift rating of 2873 Pa (60 psf), the system should successfully pass all eight sequences. Neither the DRF system nor the DRF-WT system
obtained a wind uplift rating. This suggests that the simulated cold conditioning did not affect the wind uplift performance of the PVC system. Measured fastener forces also support this observation. Nevertheless, there is a difference of about 8% on the measured membrane deflections. The membrane may have become stiffer (although mechanical tests did not show an increase in modulus but a slight increase in strength only) due to cold conditioning and thus billowed about 8% less than the new membrane. Similar findings were also observed under simultaneous wind and cold testing (Figure 7).
In Table 3, to validate the findings, the present data were compared with previously published data of [2]. Note that the Ref [2] data were obtained from room temperature testing. Components such as the membrane, insulation fastener and plate, and membrane fastener and plate had similar physical and mechanical properties as the present study. However, they were not from the same batch of the manufacturing process. Every effort was also made to have system layouts and instrument locations identical to those shown in Figure 3. The same roofing contractor installed all the specimens. However, the test pressure selected was 3352 Pa (70 psf) by [2] instead of 2873 Pa (60 psf). As shown in Table 3, the Ref [2] system passed SIGDERS load sequences 1, 2, and 3 with maximum applied pressures of 838 Pa, 1676 Pa, and 2514 Pa (17.5, 35, and 52.5 psf), respectively. The system failed during the application of 3352 Pa (70 psf), during loading sequence 4. In the present study, the system passed 2155 Pa (45 psf) and failed at 2873 Pa (60 psf). Measured fastener force (774 N vs 776 N) and membrane deflection (201 mm vs 200 mm) were also similar. Moreover, the failure modes of the present test were the same as that of the original test [2] as explained below.
Examination of the failed roof specimens revealed that the PVC roof system failed from membrane tear at the fasteners and delamination of the membrane at the seams. The applied pressure stretched the membrane so much that it started to tear at the fasteners under the discs and delaminate along the seams. The tear propagated with each load cycle until the membrane finally broke free from the fasteners and the system failed (Figure 8). As the delamination propagated along the seam, it shifted from the welded interface into the membrane itself. At failure, delamination had occurred between the reinforcement and the polymer sheet within the membrane (Figure 8). The cold-conditioned system also
exhibited the same failure mode as the reference system, i.e. membrane tear at the fasteners and delamination of the membrane at the seams.
MEMBRANE CHARACTERIZATION
To investigate the effects of thermal conditioning and wind loading on the PVC membrane, samples of the membrane were thermally conditioned in laboratory freezers using the same cold-conditioning program (i.e. –15°C or 5°F for 7 days) as the tests in DRF-WT. Samples of the PVC membrane from the original roll (not conditioned, not wind tested) were kept as control. Unlike the membranes conditioned and tested on the DRF-WT, the laboratory samples were free from stresses incurred during roof installation, retained contraction due to cold conditioning, and mechanical fatigue from SIGDERS wind testing.
Mechanical Characterization of the Membrane
Cold-Conditioned Samples: Four sets of PVC membrane samples (Table 4) were characterized by mechanical and chemical methods. The DRF–SYS samples were conditioned and wind tested in the DRF-WT. The LAB–MEM samples were conditioned in a laboratory freezer and were not wind tested. The effects of thermal conditioning can be examined by comparing the Reference and the Cold-Conditioned samples within the same group (DRF–SYS or LAB–MEM). In addition, the effects of wind loading can be investigated by comparing either the Reference or the Cold-Conditioned samples between different groups (DRF–SYS or LAB–MEM). The DRF–SYS samples were selected from the center and the seam sections, which corresponded to areas of the highest and the lowest membrane deflections.
Breaking Strength and Elongation at Break: The breaking strength of the membrane was determined by the cut strip test method in accordance with ASTM D 751 [9]. The test specimen was 152 mm by 25 mm (6 in. by 1 in.), with a gage section of 76 mm (3 in.). The specimens were cut carefully to ensure that each contained the same number of fiber tows across its width. The specimens were pulled in tension at a constant rate using an Instron universal-testing machine.
The breaking strength and the elongation at break of a minimum of five specimens per sample were determined for all four sample sets described in Table 4.
Effects of Cold Conditioning: Mechanical tests showed that cold conditioning affected neither the breaking strength nor the elongation at break of the PVC samples.
Effects of Wind Loading: The mechanical properties of the DRF–SYS samples have been normalized to those of the corresponding LAB–MEM samples to illustrate the effects of wind loading. Figure 9 shows the effects of wind loading on the breaking strength of the Reference and the Cold-Conditioned systems. The breaking strength of the central portion of the membrane, which had been subjected to SIGDERS dynamic wind cycles, was about 10% lower than either the seam or the laboratory membranes. This decrease was independent of cold conditioning. Figure 10 shows the effects of wind loading on the elongation at break of the Reference and the Cold-Conditioned systems. The elongation at break of the central portion of the membrane, which had been subjected to SIGDERS dynamic wind cycles, was again the lowest, about 15% less than the other two sets of membranes. Again, the decrease was independent of cold conditioning. Note that the breaking strength and the elongation at break of the membrane taken from the seam area were comparable to the laboratory membranes. Since the central portion of the membrane experienced higher deflection than the seam area, it is possible that repeated straining of the membrane by the SIGDERS dynamic load cycles introduced mechanical fatigue on the membrane.
Tear Strength: The tear strength of the PVC membrane was determined by the tongue tear test method in accordance with ASTM D 751 [9]. The test specimen was 203 mm by 203 mm (8 in. by 8 in.). A slit was cut from the center of one edge to a length of 75 mm (3 in.). The specimen was pulled apart in tear mode (mode III fracture) using an Instron universal-testing machine. The tear strength is defined as the average of the five highest peaks registered in the first 25 mm (1 in.) on the force-displacement curve.
The tear strength ranged from 300 to 450 N (67 lbf to 101 lbf) in the machine direction (along the rolling direction during membrane manufacture) and 150 to 250 N (34 lbf to 56 lbf) in
the cross direction (perpendicular to the rolling direction). Figure 11 shows the tear strength (in the machine direction) of the Reference and the Cold-Conditioned systems. The tear strengths of the DRF–SYS samples have been normalized to those of the corresponding LAB–MEM samples to illustrate the effects of cold conditioning and wind loading. Cold conditioning and wind loading did not appear to affect tear strength.
Seam Strength: The seam strength of the PVC membrane was also determined in accordance with ASTM D 751 [9]. The test specimen was 203 mm (8 in.) by 51 mm (2 in.). It was cut across the weld line, which was about 51 mm (2 in.) wide. The seam was pulled apart at a constant rate using an Instron universal-testing machine. The seam strength of an individual specimen was defined as the peak on the force-displacement curve. The average of three specimens was reported as the seam strength of the sample.
Seam specimens were cut from locations S1, S2, S3, and S4 of the PVC roofing system (Figure 3). Seams at location S2 and S3 were severely stressed during the SIGDERS wind test but those at locations S1 and S4 were not. Examination of the failed system confirmed that delamination of the seam had occurred at locations S2 and S3 whereas the seams at locations S1 and S4 remained intact. As a result, seam samples obtained from locations S1 and S4 were used as the reference.
Figure 12 shows the seam strength of the Cold-Conditioned and Reference systems. The seam strength of the samples obtained from locations S2 and S3 have been normalized to those obtained from locations S1 and S4 to illustrate the effects of wind loading. Cold conditioning did not appear to affect significantly the seam strength of the PVC system.
Chemical Characterization of the Membrane
Dynamic Mechanical Analysis (DMA): Dynamic mechanical analysis measures the changes in modulus and or damping capacity of a polymer subjected to forced oscillations or mechanical vibrations. The viscoelastic behavior of the polymer is determined over a range of temperatures. DMA can also measure an important property of amorphous polymer called the glass transition
temperature (Tg). The glass transition temperature is an important parameter in the
characterization of polymeric roofing membrane [10]. At Tg, a polymer goes from a rubbery state
to a glassy state and recovers without any phase change. Above Tg, segments of polymer
molecules are rapidly moving and the polymer becomes soft and flexible. Below Tg, the
segmental movement of polymer molecules has completely stopped; the polymer is stiff and brittle. Tg is governed by the chemical structure of the polymer molecules. However, cross-linking
between polymer chains and the addition of small molecules with low Tg, such as plasticizers, can
change the Tg of a polymer [11, 12]. It is therefore possible to observe chemical changes in a
polymer by measuring its Tg.
A Rheometric Scientific Solid Analyzer RSA II equipped with an environmental controller was used to measure the glass transition temperature of the specimens. Rectangular strips were cut from each specimen and placed on a dual cantilever fixture. The calibration of the instrument was checked weekly and recalibrated when required using the manufacturer’s specifications and procedures. The glass transition temperature was measured according to ASTM D 6382-99 and D 5418-99. Table 5 shows the measured glass transition temperature of the PVC membrane samples. Thermal conditioning and wind testing of the membranes did not significantly affect the Tg of the PVC membrane.
Thermogravimetric Analysis (TGA): Thermogravimetric analysis measures the change in mass of a material as a function of time at a fixed temperature or over a temperature range using a predetermined heating rate. This technique is useful in monitoring thermal stability and loss of components (e.g., oils, plasticizers) of a material upon heating [11, 12]. Small amounts of PVC (in the mg range) were heated from 25 °C to 1000 °C at 20°C/min in an inert atmosphere (150 mL/min Ultra High Purity nitrogen). The mass loss over the temperature range of the analysis was found to be the same for all PVC samples. Thermal conditioning and wind loading of the membranes did not affect the thermal stability of the PVC membrane.
FUTURE STUDIES WITH DRF-WT
The data show that the cold-conditioning program did not significantly affect the mechanical and chemical properties of the PVC membrane. This is probably due to the moderately low temperature
(–15°C or 5°F) and the fairly short exposure time (7 days). Also, it may be due to the physical, chemical, and mechanical properties of the PVC membrane. Other membranes may perform differently and researches carried out on modified bituminous membranes were reported by Baskaran et al. [13]. Since many parts of Canada experience much lower temperatures in the winter, colder conditioning temperatures and longer exposure times would be more realistic and should be the subject of future study. It is also not uncommon for a light-colored roof membrane to reach over 60°C on a hot summer day in North America [14]. Therefore, heat conditioning should also be considered as important as cold conditioning.
The DRF-WT offers many opportunities for system evaluation that were not possible before. To date, wind uplift resistance is rated for new roofing systems only. Although the roofing community has been evaluating membrane durability for a long time, no data or test method exists to evaluate how the wind uplift resistance of a roofing system changes over time. Small specimens are heat aged in laboratory ovens (e.g. ASTM method D 5869) and their initial and final properties (such as tensile strength, elongation and seam strength) are measured and compared. A membrane is considered acceptable for use if the aged material retains its original properties within specified variations. The DRF-WT has the capability to heat condition a system and to determine its wind uplift resistance afterwards. This creates a new potential to evaluate the wind uplift resistance of conditioned roofing systems.
Perhaps the most important feature of the DRF-WT is the capability to test a roofing system simultaneously under both wind and thermal conditions. A polymeric membrane becomes more flexible at elevated temperatures and stiffer at low temperatures. This change in mechanical properties might not be critical to the performance of the membrane itself but it may change the load transfer path (pneumatic or structural) and thus affect the system response to wind gusting. A plastic fastener plate might creep under prolonged heat, affecting its load transfer capacity. The effectiveness of an adhesive might increase due to further curing or decrease due to softening at elevated temperatures. These changes can affect the seam strength and consequently the overall response of the roofing system to wind gusting. High temperatures also can remove residual stress induced during installation while cold temperatures can produce stress due to restrained contraction. These thermal stresses may not be detrimental by themselves but might become critical when the roofing system is subjected to wind
loading. These are few effects that temperature has on the system and that cannot be predicted from measurements conducted on individual components. With careful experimental design, the DRF-WT can be used to examine these factors on a full system basis.
SUMMARY AND CONCLUSIONS
• Through a consortium effort, a new test facility to evaluate roofing systems for simultaneous wind and thermal effects was established and commissioned.
• A single-ply roofing system with a PVC membrane was subjected to simultaneous thermal and SIGDERS loading sequences. Design parameters of the roof assembly (pressure, fastener forces, and membrane deflections) were measured and compared to control specimens.
• Comparison of the design parameters, although limited to the tested PVC roof assembly and the simulated cold-conditioning program (–15°C or 5°F for 7 days), indicated that neither the wind uplift rating nor the membrane’s mechanical and chemical properties were affected by the dual testing program.
• SIGDERS dynamic testing influenced the membrane mechanical properties. The breaking strength and elongation at break of the most deflected section of the membrane decreased by 10% and 15%, respectively, whereas the seam strength of the cold-conditioned system increased by 8%.
• To validate the above findings, investigations are in progress using various single-ply systems, including a reinforced EPDM (ethylene propylene diene monomer), TPO (thermoplastic polyolefin) and modified bitumen with appropriate cold/heat conditioning programs.
ACKNOWLEDGEMENTS
The presented research is being carried out for a consortium - Special Interest Group for Dynamic Evaluation of Roofing Systems (SIGDERS). SIGDERS was formed from a group of partners who were interested in roofing design. These partners included: Manufacturers - Atlas Roofing Corporation, Canadian General Tower Ltd., GAF Materials Corporation, GenFlex Roofing Systems, Firestone Building Products Co., IKO Industries Ltd., Johns Manville, Sarnafil, Soprema Canada and Stevens Roofing Systems. Building Owners - Canada Post Corporation, Department of National Defence, Public Works and Government Services Canada. Industry Associations -Canadian Roofing Contractors' Association, Canadian Sheet Steel Building Institute, Industrial Risk Insurers, National Roofing Contractors’ Association and Roof Consultants Institute. Research Agencies-Institute for Research in Construction, Institute for Aerospace Research and Canadian Construction Material Centre.
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Roofing Systems”, Proceedings of the fourth international symposium on roofing technology, NRCA/NIST, Washington, DC, USA, 168-179.
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System for Wind Performance," Journal of Architectural Engineering, 5, (1), pp. 16-24.
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America, Report BLWT-SS35-1997, University of Western Ontario, London, Ont. Canada.
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Attached Single Ply Roofs," Journal of Wind Engineering and Industrial Aerodynamics, Vol. 77-78, pp. 83-96.
6. Zarghamee, M.S. 1990. "Wind Effects on Single-Ply Roofing Systems", Journal of Structural
Engineering, Vol. 116, pp. 177-87.
7. ASCE. 2002. “Minimum Design Loads for Buildings and Other Structures”, ASCE Standard 7 –
98, American Society of Civil Engineers, Reston, Va., p. 13.
8. NBCC. 1995. National Building Code of Canada, National Research Council of Canada, Ottawa,
Canada, pp. 145.
9. ASTM 1996 D751-89 “Standard Test Methods for Coated Fabrics”, Annual Book of ASTM
Standards, vol. 09.02.
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Materials: Comparison of DMA, TMA and DSC Techniques”, Assignment of the Glass Transition,
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269-276.
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Captions
Figure 1 Facility to Simulate Simultaneous Wind and Thermal Effects on Roofs Figure 2 Wind Flow Paths in DRF-WT for Cooling Mode and Heating Mode Figure 3 System Layout, Seam Details and Instrumentation Locations Figure 4 SIGDERS Dynamic Wind Load Cycle
Figure 5 Operation of the DRF-WT in Heating Mode Figure 6 Operation of the DRF-WT in Cooling Mode
Figure 7 Simultaneous Wind and Thermal Effects on System Response Figure 8 Failure Mode of the Cold-Conditioned PVC system
Figure 9 Effects of Wind Gusting on the Breaking Strength of the PVC Membrane Figure 10 Effects of Wind Gusting on the Elongation at Break of the PVC Membrane Figure 11 Normalized Tear Strength of PVC Membrane
Figure 12 Normalized Seam Strength of PVC Membrane
Table 1 Wind Speed and Temperatures Obtained from Weather Data (10-year Return Period).
Table 2 Capacity of the DRF-WT under Simultaneous Wind and Thermal Operation Table 3 Measured System Response of the Cold Conditioned PVC system
Table 4 Sample Identification for Membrane Characterization Table 5 Glass Transition Temperatures of PVC Membrane Samples
Figure 1: Facility to Simulate Simultaneous Wind and Thermal Effects on Roofs Top Chamber Enclosed Fan Heating Unit DAS Cooling Unit Bottom Frame
Figure 2: Wind Flow Path in DRF-WT for Cooling Mode and Heating Mode Wind Controller Thermal Monitor Reference Pressure 2m X 6m Roof AC Inverter 100 HP Blower (350 psf) Air Circulator 5-ton Cooling Unit (-35°C) 30 kW Heating Unit (350°C) Gust Simulator
Figure 3 System Layout, Seam Details and Instrumentation Locations P P P P P 6 ’-Sh e e t 1 6’-S h eet 2 6 ’-Sh ee t 3 Fr = 1 .7 m ( 6 7 in .) D e fl ecti o n sen so r L o ad ce ll P ress u re tr an sd u cer D3 S1 S2 S3 S4 C 305 m m (12 i n .) o c D4 P2 P4 P3 P1 D 1 L1 D2 L2 L3 PVC Membrane Steel Deck
Fastener and Plate ISO Insulation
51mm (2in.)
102mm (4in.) ISO Insulation
Membrane fastener with 51mm (2in.) plate
127mm (5in.) overlap 38mm (1.5in.) heat welded seam PVC membrane
Insulation fastener with 76mm (3in.) plate Steel Deck
Figure 5 Operation of the DRF-WT in Heating Mode 20 25 30 35 40 45 50 55 60 65 70 Te m p e rat u re, °C 65 °C (149 °F) 0 5 10 15 20 25 30 35 40 45 Pr es su re , p s f 0 2 4 6 8 10 12 14 16
Time, sec
Target Temperature Measured Temperature Measured Pressure Target Pressure (30psf) Base PressureFigure 6 Operation of the DRF-WT in Cooling Mode -10 -5 0 5 10 15 20 Te m p e ratu re, °C - 7 °C (19 °F) 0 5 10 15 20 25 30 35 40 45 Pre ssu re , ps f 0 2 4 6 8 10 12 14
Time, sec
Measured Temperature Measured Pressure Target Temperature Target Pressure (15psf) Base PressureFigure 7 Simultaneous Wind and Thermal Effects on System Response 0 50 100 150 200 250
Time, sec
3 4 5 6 7 De fl ect ion, i n 20 °C(68 °F) -7 °C(19 °F) 60 80 100 120 140 160 180 Deflection, mm∆
P = 15 psf 0 50 100 150 200 250Time, sec
0 20 40 60 80 100 Loa d, lb f 0 100 200 300 400 Lo ad , N 20 °C (68 °F) - 7 °C (19 °F)∆
P = 15 psfFigure 8: Failure Mode of the Cold-Conditioned PVC system.
305 mm
Figure 9: Effects of Wind Gusting on the Breaking Strength of the PVC Membrane
Cold Conditioned
0
0.2
0.4
0.6
0.8
1.2
Reference
N
or
m
a
li
ze
d
B
re
a
k
ing
S
tre
n
g
th
SEAM
LAB-MEM
DRF-SYS
CENTER
1.0
Figure 10: Effects of Wind Gusting on the Elongation at Break of the PVC Membrane
Cold Conditioned
0
0.2
0.4
0.6
0.8
1.2
Reference
No
rm
al
iz
ed
B
re
aki
n
g
S
tr
e
n
g
th
SEAM
LAB-MEM
DRF-SYS
CENTER
1.0
Figure 11: Normalized Tear Strength of PVC Membrane LAB-MEM Cold Conditioned Reference No rm al iz ed T e ar S tr e ng th
Figure 12: Normalized Seam Strength of PVC Membrane SEAM #4 1.0 Cold Conditioned Reference No rm aliz ed S e a m St re n g th
Table 1 Wind Speed and Temperatures Obtained from Weather Data (10-year Return Period).
Station Air Temperature
°C (°F) - Low
Air Temperature °C (°F) - High
Hourly Mean Wind Speed m/s (mph) Ottawa, Ontario, Canada -24 (-11) +30 (+86) 12 (27) St.John’s, Newfoundland, Canada -15 (+5) +24 (+75) 18 (40) Edmonton, Alberta, Canada -33 (-27) +27 (+81) 10 (23) Seattle, Washington, USA -5 (+23) +29 (+84) 10 (22) Phoenix, Arizona, USA +1 (+34) +43 (+109) 10 (23)
Table 2 Capacity of the DRF-WT under Simultaneous Wind and Thermal Operation.
Hot Cold
Conditioning for Hot and Cold 150°C (302°F) -35°C (-31°F) Simultaneous Wind and
Temperature 9576 Pa @ 65°C (200 psf @ 149°F) 718 Pa @ -7°C (15 psf @ 19°F) Relative Humidity 40-120°C (104-248°F) @ 5-95% 10-30°C (50-86°F) @ 40-70%
Table 3 Measured System Response of the Cold-Conditioned PVC System.
Pressure, Pa (psf) Force, N (lbf) Deflection, mm (in.) Test
Completed Failed @ 45 psf @ 60 psf @ 45 psf @ 60 psf
Reference System 2155 (45) 2873 (60) 796 (179) 1063 (239) 200 (7.9) 234 (9.2) Cold-Conditioned
System 2155 (45) 2873 (60) 796 (179) 992 (223) 190 (7.5) 218 (8.6) Data from Reference [2] 2538 (53) 3352 (70) 774 (174) 1023 (230) 201 (7.9) 260 (10.2)
Table 4 Sample Identification for Membrane Characterisation
System/Membrane Cold Conditioning Wind Testing
Reference System N Y DRF–SYS Cold-Conditioned System Y Y Reference Membrane N N LAB–MEM Cold-Conditioned Membrane Y N
Table 5 Glass Transition Temperature of PVC Membrane Samples
System/Membrane Cold
Conditioning Wind Testing Tg
Reference System N Y -42 ± 1 °C (-44 ± 2°F) DRF–SYS Cold-Conditioned System Y Y -41 ± 1 °C (-42 ± 2 °F) Reference Membrane N N -40 ± 1 °C (-40 ± 2 °F) LAB–MEM Cold-Conditioned Membrane Y N -40 ± 1 °C (-40 ± 2 °F)
