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Roofing, Siding and Insulation, 78, January 1, pp. 28, 30, 32-34, 2001-01-01
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Wind fatigue effects on the seam strength of TPO roofs
Liu, K. K. Y.; Baskaran, B. A.
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Discussion Material and NOT for Distribution
Page 1 of 6
WIND FATIGUE EFFECTS ON THE SEAM STRENGTH OF TPO ROOFS WITH
DOUBLE SIDE WELD TECHNOLOGY
K. Liu and A. Baskaran National Research Council Canada,
Ottawa, Ontario, K1A OR6 613-990-3616 (phone) Bas.baskaran@nrc.ca
ABSTRACT
The objective of this study was to investigate the effects of wind fatigue on the seam strength of mechanically attached TPO systems. Conventional one side weld system and the new double side weld system was installed on the Dynamic Roofing Facility and subjected to the SIGDERS dynamic wind test protocol. The seam strengths before and after the wind test were determined. Seam samples failed cohesively in the polymer and had strength from 3.0 to 8.3 kN/m (17 to 47 lbf/in.). Wind fatigue caused a reduction of about 30% on the seam strength. Wind uplift rating of double side weld systems is better than that of the one side weld system.
EXPERIMENTAL APPROACH
Wind Fatigue Testing on Systems
Roof assemblies with TPO sheets 1910 mm (74 7/8 inch) wide and 1.1mm (45 mil) thick were installed1 on the Dynamic Roofing Facility (DRF) of the National Research Council of Canada. Systems were installed over metal deck with ISO board insulation. Two weld configurations were obtained using a hot air robotic welder (Figure 1): One Side Weld (OSW) using fastener plates and Double Side Weld (DSW) using batten strips (Figure 2). A summary of the welding parameters is shown in Table 1. All the systems were then subjected to the SIGDERS dynamic wind test protocol.2
Seam Strength Testing on Membranes
At the termination of the SIGDERS protocol, the seams that remained intact were removed and tested for their strengths. Samples that had not been subjected to the wind test were also tested to provide control data for wind fatigue comparisons. Samples were tested using ASTM D7513 with modifications to specimen dimensions and crosshead speed. A minimum of three specimens, 25 mm wide by 300 mm long (1 inch wide by 12 inch long) was cut across the lap seam of the membrane sample using a knife. The specimen was placed in the pneumatic grips approximately 50 mm (2 inch) from the edges of the overlap area and pulled at a constant crosshead speed of 50 mm/min (2 inch /min). In cases where only limited material was left under the overlap, the grip was placed closer than 25mm (1 inch) from the welded area to achieve proper gripping. Specimens cut from two seam locations (Figure 3) on the DRF were tested and compared. All samples were tested using an Instron 4502 Automated Materials Testing System and the test condition was kept at a temperature of 23 ± 2°C (73 ± 4°F) and a relative humidity of 50 ± 5%. The load and displacement were recorded by a data acquisition system on a computer. The highest peak achieved was noted as the seam strength of each individual specimen (Figure 4).
1
The authors acknowledge GenFlex Roofing Systems for their support. Authors also appreciate David Scott of
GenFlex Roofing Systems, William Lei of NRC for the system installations and Jayne Irwin of NRC for the mechanical testing.
2 Baskaran, A and F. Nabhan, (2000), “Standard Test Method for the Dynamic Wind Uplift Resistance of
Mechanically Attached Membrane Roofing Systems”, Internal Report IRC-IR 699, National Research Council, Canada.
3
EXPERIMENTAL DATA AND DISCUSSION
Effect of Wind Fatigue on Seam Strength
Table1 shows that the seam strengths of the samples ranged from 3.0 to 8.3 kN/m (17 to 47 lbf/in). All the seams (seam of OSW and inner and outer seams of the DSW) had similar strength. To identify the wind fatigue reduction, the median seam strengths were considered. Normalized percentages, {(Wind Tested / Control)*100}, were plotted in Figure 5.
One Side Weld: After the SIGDERS test, the seam strength was reduced by about 28%. Specimens cut from two seam locations (#1 and #2) had similar strengths; indicating uniform welding quality was achieved during installation.
Double Side Weld: After the SIGDERS test, the strength of the seam was reduced by 26-33%. Specimens cut from two seam locations (#1 and #2) had similar strengths; indicating uniform welding quality was achieved during installation.
From the above one can notice that the seams strength reductions were similar for both OSW and DSW systems. Nevertheless, what is important to observe is: the OSW system passed only 60psf (withstood 2200 gusts). The DSW system passed 90psf (withstood 4100 gusts). During the dynamic wind test, the seams in the OSW were subjected to dynamic stretching and peeling stresses (Figure 6a), and their strengths were reduced mainly due to dynamic peeling stress. Figure 6b shows the loading geometry and stress development at the inner and outer seams of the DSW during the SIGDERS test. The inner seam was simultaneously subjected to dynamic stretching stress and peeling stress by the top sheet: whereas the bottom sheets, introduced stretching stresses in the opposite direction. The combination of these created dynamic shear
stress at the bonded area. The outer seam was subjected to dynamic stretching stress by the
bottom sheet only; the top sheet did not exert any stress on the bonded area. This created dynamic tensile stress at the bonded area.
Double Side Weld Vs One Side Weld
The DSW offered two major advantages over the OSW due to its seam geometry (Figure 6). It reduced the load bore by each seamed area and the tendency of membrane tear at the fastener. In the DSW, the wind load is transferred by two seamed areas at each seam location instead of one as in the case of the OSW. Since twice the seamed areas are available to share the load, the load bore by each seamed area is reduced. Therefore, the DSW is expected to withstand higher wind load than the OSW. SIGDERS test confirmed that the DSW system sustained significantly higher wind load than the OSW system 4. Also, because the load bore by each seamed area is reduced, the double side weld configuration is expected to be more forgiving to poor seam quality (low seam strength) than the one side weld configuration. The DSW provided a “balanced” loading geometry around the batten (Figure 6b). In the OSW, the bottom sheet tended to pull itself from the fastener due to the unbalanced wind load exerted on the bottom sheet (Figure 6a). In fact, the SIGDERS test showed that the OSW systems failed by membrane tear at the fastener. In the DSW systems, on the other hand, the bottom sheet was pulled almost equally from both sides of the batten in opposite directions. This balanced load at the fastener/batten reduced the possibility of membrane tearing. In fact, the SIGDERS test showed that the DSW system failed by fastener pullout.
Failure Mode
All the tested seams showed cohesive failure mode, i.e., the ply interface at the weld remained intact (Figure 7). After the polymer sheet had broken, the broken sheet peeled across the width
4
Baskaran, A and G. Xu, (2000), “Wind Uplift Rating of TPO Systems – New Data Confirms the Theory of Double
Sided Weld Technology” Accepted for the Interface Journal, Roof Consultant Institute, 7424 Chapel Hill Road, Raleigh, NC 276-7, USA.
Discussion Material and NOT for Distribution
Page 3 of 6
of the seam along the reinforcement-polymer interface until final failure. This process was indicated by the irregular peaks on the force-displacement curve as shown in Figure 4. This failure mode indicates that proper fusion of the sheets was achieved, thus producing good bonding.
CONCLUSIONS
The tested TPO samples had seam strength from 3.0 to 8.3 kN/m (17 to 47 lbf/in.). Seam samples failed cohesively in the polymer.
The seam strength of the TPO samples was significantly reduced (26 – 33%) by wind fatigue regardless of the weld configuration. However, the double side weld configuration (90psf) had higher wind uplift rating than the one side weld configuration (60psf).
RECOMMENDATION
During the service life, the seam strength of TPO membrane could be reduced by wind fatigue. However, the current TPO ASTM draft5 specifies the factory seam strength only, which does not take into the account of the service conditions of the roofing system. It is recommended that ASTM should consider adding maximum allowable level of seam strength reduction due to wind fatigue and other environmental stresses in their specification.
Table 1 Summary of welding parameters and seam strength of TPO membrane samples
Welding Parameters* Seam Strength
kN/m (lb/in.) Sample Seam Type Speed m/min (ft/min) Weight kg (lb) Seam Location
Low Median High
One side 2.4 (7.9) 2.7 (6) 5.00 (28.6) 5.74 (32.8) 6.09 (34.8) Outer 5.77 (32.9) 6.00 (34.3) 8.27 (47.2) Control Double side 2.0 (6.5) 4.5 (10) over both seams Inner 4.49 (25.6) 5.64 (32.2) 6.31 (36.0) #1 4.03 (23.0) 4.08 (23.3) 4.27 (24.4) One side 2.4 (7.9) 2.7 (6) #2 4.20 (24.0) 4.20 (24.0) 4.24 (24.2) #1 Outer 3.32 (19.0) 4.26 (24.3) 4.44 (25.4) #1 Inner 3.00 (17.1) 3.79 (21.6) 4.03 (23.0) #2 Outer 3.96 (22.6) 4.10 (23.4) 4.34 (24.8) After Wind Fatigue Double side 2.0 (6.5) 4.5 (10) over both seams #2 Inner 4.13 (23.6) 4.17 (23.8) 4.36 (24.9) * Welding temperature 510°C (950°F) 5
Figure 1: Hot air distribution system in the robotic welder for the double side weld configuration. (a) One Side Weld
(b) Double Side Weld
Figure 2: Weld configurations of TPO system (a) one side weld with fastener plates and (b) double side weld with batten strips.
INSULATION 48mm (1 7/8”) 60mm (2 3/8”) SEAM FASTENER PLATE TOP SHEET BOTTOM SHEET DECK 51mm (2”) 35mm (1 3/8”)
OUTER SEAM INNER SEAM
25mm (1”) BATTEN STRIP INSULATION BOTTOM SHEET 35mm (1 3/8”) TOP SHEET DECK
Discussion Material and NOT for Distribution
Page 5 of 6
Figure 3: Specimens were obtain from two locations: SEAM #1 and SEAM #2.
Figure 4: Load-displacement curves from the double side-welded configuration
2 007m m ( 79 ” ) 1791mm (70 1/2”) SIGDERS TABLE 5994mm (236”) SEA M SEA M # 1 SEAM #2 SEA M
(a) One Side Weld (b) Double Side Weld Figure 5: Effect of wind fatigue on the seam strength
(a) One Side Weld (b) Double Side Weld
Figure 6: Stress distribution on the seams due to wind fatigue.
(a) One Side Weld (b) Double Side Weld
Figure 7: Failure mode of the tested seams
0 20 40 60 80 100 120 SEAM #1 SEAM #2 SEAM LOCATION N O R M A L IZ E D S E A M S T R E N G T H ( % ) SEAM #1 SEAM #2 SEAM LOCATION N O RM AL IZ E D S EA M S TR EN G TH ( % ) INSULATION SEAM PEELING FORCE STRETCHING
FORCE STRETCHINGFORCE
BO TTO M SHEE T TOP S HEET DECK
OUTER SEAM INNER SEAM
INSULATION PEELING FORCE STRETCHING FORCE STRETCHING FORCE STRETCHING FORCE BOTTOM SHE ET TOP SH EET DECK
