PERFORMANCE OF STANDING SEAM METAL ROOFS UNDER REALISTIC WIND LOADING

Transcription

1 PERFORMANCE OF STANDING SEAM METAL ROOFS UNDER REALISTIC WIND LOADING Filmon Habte, Maryam Asghari Mooneghi, Peter Irwin, Arindam Gan Chowdhury ABSTRACT: A full-scale experimental investigation was conducted to study the behavior of wind loads on the surfaces of two types of standing seam metal roofs (i.e. vertical-leg and trapezoidal) and evaluate their performance under high wind speeds. Pressure measurements on the roof surface and roof panel deflection measurements were conducted under realistic wind loading in Wall of Wind (WOW) facility at Florida International University (FIU). Roof profile and eave details were observed to have significant effect on the magnitude and distribution of the wind-induced pressures at the surface of the metal roofs. Comparison of experimental results with ASCE 7-10 provisions showed that the ASCE 7-10 might in some cases underestimate the wind loading on a mono-sloped trapezoidal-leg standing seam roof. Significantly higher deflections were experienced by the trapezoidal roof, and the metal panels were observed to become increasingly stiffer at high deflections. Roof failure occurred on the trapezoidal roof only, and the main mode of failure was clip rupture/breakage. The clip failures are assumed to be caused by excessive vibration at the free end. KEYWORDS: Standing seam metal roof; Full-scale testing; Aerodynamics; Deflection; Roof failure.

2 1 Performance of Standing Seam Metal Roofs under Realistic Wind Loading Filmon Habte 1, Maryam Asghari Mooneghi 2, Arindam Gan Chowdhury 1, and Peter Irwin 1 1 Department of Civil Eng., College of Engineering, Florida International University, Miami, FL, USA 2 Advanced Technology and Research, Arup, San Francisco, CA, USA [email protected], [email protected], [email protected], [email protected] ABSTRACT: A full-scale experimental investigation was conducted to study the behavior of wind loads on the surfaces of two types of standing seam metal roofs (i.e. vertical-leg and trapezoidal) and evaluate their performance under high wind speeds. Pressure measurements on the roof surface and roof panel deflection measurements were conducted under realistic wind loading in Wall of Wind (WOW) facility at Florida International University (FIU). Roof profile and eave details were observed to have significant effect on the magnitude and distribution of the wind-induced pressures at the surface of the metal roofs. Comparison of experimental results with ASCE 7-10 provisions showed that the ASCE 7-10 might in some cases underestimate the wind loading on a mono-sloped trapezoidal-leg standing seam roof. Significantly higher deflections were experienced by the trapezoidal roof, and the metal panels were observed to become increasingly stiffer at high deflections. Roof failure occurred on the trapezoidal roof only, and the main mode of failure was clip rupture/breakage. The clip failures are assumed to be caused by excessive vibration at the free end. KEY WORDS: Standing seam metal roof; Full-scale testing; Aerodynamics; Deflection; Roof failure. 1 INTRODUCTION Roofs have been observed to be the most hurricane vulnerable components of a building envelope. Roof damage during a storm event can result in myriad of problems ranging from water leakage to complete failure of the building structure. Current methodology for investigating the performance of a metal roof systems under wind loads involves: 1) determining the design loads using wind provisions of codes and standards; and 2) undertaking physical test on the roof system to test its adequacy in withstanding the design load, in which, loading is applied to the roof system in the form of a uniform static pressure and the deflections versus loading are recorded up to failure. This method of using uniform static wind loading may not be truly representative of real conditions, and might on one hand set higher minimum design requirements for the entire system than necessary or on the other hand it could underestimate effects of localized peak pressures on critical locations. Most physical testing protocols for evaluating the adequacy of metal roof panels in withstanding design wind loads use defined pressures to evaluate the uplift capacity of metal roof panels. They do not include temporal and spatial variations that are inherent in real wind loading; nor do such tests necessarily include realistic boundary conditions provided by the supports in actual applications. Moreover, the failure modes observed during static pressure tests might not be true representatives of field failures ([1];[2]) Identification of the roof component that fails first and correct simulation of actual roof failure modes are important as they determine the ultimate strength of the roof assembly. The method of using static uniform loading has been the center of continuous debates and appraisals by structural design engineers, wind engineering specialists, and scientific researchers. Several researches have been conducted to relate high local loads to their corresponding uniform static loads ([2]; [3]; [4]; [5]; [6]). Most of those previous researches used scaled models of metal panels which may introduce scaling errors due to Reynolds number mismatch and not being able to reproduce the effects of architectural features of the standing seams. Large- and fullscale experiments in properly simulated flows can circumvent these issues related to adverse scale effects, but such studies have been limited. Understanding the response of actual metal roof panels to wind loading, as well as identifying the different failure mechanisms, can facilitate the development of improved designs and design guidelines which can reduce roof system damages during wind storms. To this end, the objectives of this work are to understand the behavior of standing seam metal roof panels under realistic wind loading and to evaluate their performance under high wind speeds. Aerodynamic pressure and deflection measurement tests were carried out on mono-sloped full-scale vertical-leg and trapezoidal standing seam metal roof panels under simulated turbulent wind loading. Observation of roof failure mechanism and comparison of the pressure results with minimum design loads for building and other structures given in ASCE 7-10 [7] provisions for wind loading were also performed.

3 2 2 EXPERIMENTAL SETUP AND TESTING PROTOCOL In this study, two types of standing seam metal roofs (i.e. vertical-leg and trapezoidal) each with plan dimensions of 3.1 m by 4.6 m were used. The roofs were attached to a 1.52 m high base structure which was designed to support interchangeable monosloped roofs with slope of 4.6 deg. (Figure1). The vertical-leg standing seam metal roof (henceforth referred to as vertical-leg roof for brevity) had 0.5 m wide panels with 50.8 mm high ribs and floating clips were used to attach the panels to the purlins. The trapezoidal standing seam metal roof (henceforth referred to as trapezoidal roof for brevity) had 0.6 m wide panel with 76.2 mm high ribs, and sliding clips were used as attachments. As can be seen in Figure1, based on the manufacturers specifications, perimeter eave trims (attachments) were placed on all four sides of the vertical-leg roof while the trapezoidal roof had eave trims only along its two sloping edges. Moreover, in the vertical-leg roof the top levels of the eave attachments only extended to the level of the panel flat pan sections, but in the trapezoidal roof the eave attachments were raised to about 76.2 mm above the surface of the panel flat pan sections. The 12-fan Wall of Wind (WOW) open jet facility at Florida International University (FIU) was used for this experimental investigation. The WOW has the capability of generating up-to Category 5 hurricane wind speeds with desired wind flow characteristics. The WOW has 12 electric fans arranged in a two row by six-column pattern which produce a wind field 6.1 m wide and 4.3 m high, allowing aerodynamic testing of large-scale models or full-scale portions of buildings. Figure 2 shows WOW simulated mean wind speed and turbulence intensity profiles for open country condition (target power law coefficient of α = 1/6.5), used during the experiments. Figure1. Standing seam metal roof mock-ups used in testing: Vertical-leg (left) and Trapezoidal (right) Aerodynamic pressure measurement tests were conducted on both roofs for twelve wind directions at a low-eave height wind speed of 24.7 m/s. Each roof system was instrumented with external and internal pressure taps. The time-histories of windinduced external and internal pressures were collected using 6.35 mm ID polyurethane pressure tubes and a DSA 4000, ZOC 33 Scanivalve data acquisition system. One minute duration time history pressure data were collected for each test at a sampling frequency of 512 Hz. A transfer function designed for the tubing [8] was used to correct for tubing effects. Metal panel and seam line (panel rib) deflection measurements were also conducted to evaluate the structural performance of the roof specimens under high wind speeds. During deflection measurement experiments, the two sloping roof edges and the low-eave end were fastened using self-drilling steel screws, but the high eave end was left open and only one screw was used at each panel rib. This fixed free boundary condition was intended to represent a continuous roof property at the open end, and simulate a set-up condition followed in the American Society for Testing and Materials (ASTM) E1592 [9] to enable comparison of the results (which will be done in future work). The deflection measurement tests were conducted at several WOW mean wind speeds which ranged from 30.5 m/s to 65.7 m/s and at different wind directions. The deflections were recorded using RDP Electrosense LDC3000A linear variable differential transformers (LVDTs) and Celecso SP2 type string pots. Deflection measurement was conducted for an effective duration of 1 minute (i.e. excluding fan ramp up times), at sampling rate of 100 Hz. Reference (baseline) deflection measurements at zero wind speed were conducted before each deflection measurement test.

4 3 Figure 2: Simulated open terrain atmospheric boundary layer (ABL) in the WOW: Mean wind speed profile (left) and Turbulence intensity profile (right) 3 RESULTS 3.1 Aerodynamic Pressure Results The pressure results are presented in the form of mean and peak pressure coefficients in different roof regions. The uplift pressure in a roof region was computed by area-averaging time-histories of pressures from pressure taps within that region. The mean pressure coefficients C p mean were then obtained using Equation (1) C p mean Pmean (1) 1 2 Vmean 2 where P mean is the mean of 1 minute duration area-averaged pressure data obtained on the surface of the metal panels; V mean is the mean upstream wind speed at model low-eave height, and ρ is the density of air. The peak pressure coefficients were obtained using Equation (2) and were normalized to the 3-sec gust dynamic pressure: C p peak Ppeak 1 (2) V 2 3sec 2 where P peak is the estimated peak pressure obtained from 1 minute duration area-averaged pressure data obtained on the surface of the metal panels and V 3 sec is the 3-sec gust dynamic pressure at model low-eave height. The tests were performed in partial turbulence simulation, hence the turbulence intensity at roof height was lower than that of atmospheric boundary layer (ABL) which contains full spectrum of turbulence. In order to calculate the estimated peak pressures, a method called Partial Turbulence Simulation was used. In this method, the turbulence is divided into two distinct statistical processes, one at high frequencies which can be simulated correctly in WOW, and one at low frequencies which can be treated in a quasi-steady manner. The joint probability of load from the two processes is derived, with part coming from the WOW data and the remainder from the Gaussian behavior of the missing low frequency component. For the evaluation of these estimated values, the peak value with 95% probability of not being exceeded in one hour of full spectrum wind was selected [10, 11].

5 4 Figure 3. Roof layout and wind direction: trapezoidal standing seam roof Although the plan dimensions, eave height, roof slope and test setup in the vertical-leg and trapezoidal roofs were identical, much higher pressures with larger non-uniformity were observed in the trapezoidal roof specimen. For illustration purposes pressure results in the high-eave corner regions (shaded region in Figure 3) are shown. The pressures in the high eave corner regions were computed by averaging time-series of pressure readings from four pressure taps constituting a total area of 0.35 m 2 and 0.5 m 2 in the vertical-leg and trapezoidal roofs respectively. Figure 4 shows the mean and peak pressure coefficients versus wind direction for the high-eave corner roof region in both vertical-leg and trapezoidal roofs (definition of wind direction is shown in Figure 3). The corresponding external GC p value specified by ASCE 7-10 for this roof region is also shown in Figure 4. It should be noted that equivalent pressure coefficients GCp eq were calculated using the method explained in [12] before comparison with ASCE 7-10 provisions was performed. Higher pressures, exceeding the ASCE 7-10 provision, were observed in the trapezoidal roof. The significant difference in roof pressures between the two roof models is a clear indication that roof architectural features and eave attachment details highly affect the magnitude and distribution of wind pressures on roofs. This is similar in theory with findings of previous researches on roof-tiles ([13];[14]) which showed high dependence of wind pressure on the roof architectural features, and the fact that using bare decked roof models in aerodynamic testing might underestimate the actual roof pressures. Note that the effects of such finer geometrical details are generally better addressed using large or fullscale tests. Figure 4. Mean (top) and Peak (bottom) pressure coefficients for high-eave roof region It s important to mention that in contrast to the vertical-leg roof that had eave trims/attachments on all its 4 edges, the trapezoidal roof had eave trims/attachments installed at its two sloping edges only. Hence, the significantly lower uplift pressure at the surface of the vertical-leg roofs might be due to the effect of rounding or chamfering eaves which is known to alleviate

6 5 suction at the leading edge [15]. It can also be hypothesized that since the eave attachments and ribs in the trapezoidal roofs extend to about 76 mm above the surface of the roof, they might act as low-parapets which have been observed to significantly increase the roof corner suction pressures by strengthening vortices ([16]; [17]; [18]). Nevertheless, detailed flow measurement investigation is needed to precisely identify the actual causes of such significant pressure differences. According to the ASCE 7-10 wind loading provisions for components and cladding, the roof is divided into three zones namely Zone 3, Zone 3 and Zone 2 with external GC p of -2.60, and respectively. A detailed comparison of these GC p values with pressures measured in different roof regions with averaging-area of 0.93 m 2 showed that the ASCE 7-10 might produce un-conservative pressures in the case of the trapezoidal roof. For a further comparison of the experimentally measured external roof pressures with the ASCE 7-10 provisions, peak pressures of different roof regions within Zones 3 and 3 having different averaging areas were computed. Note that in this experimental model, Zones 3 and 3 represent 1.8 m wide strips near the low-eave and high-eave ends respectively. Figure 5 and Figure 6 show maximum Cp peak computed for different averagingarea within roof Zone 3 and 3 respectively, along with their ASCE 7-10 pressure coefficients. The Cp peak value for a specific averaging area (shown in Figure 5 and Figure 6) represents the highest peak pressure coefficient from all pressure taps combinations with similar sizes of averaging area. It can be seen that changing the averaging area highly affects the evaluated uplift pressures. In Zone 3, the ASCE 7-10 pressure coefficients were observed to be conservative for the vertical-leg roof for all considered averaging areas, the opposite was true in the case of the trapezoidal roof. Similarly, for Zone 3 the ASCE 7-10 pressure coefficients were conservative for all averaging areas used in the vertical-leg roof. But in the trapezoidal roof, the ASCE 7-10 pressure coefficients were shown to be un-conservative for averaging areas less than 0.93 m 2. Figure 5. Peak pressure coefficient versus averaging area, Zone 3 Figure 6. Peak pressure coefficient versus averaging area, Zone 3

7 6 3.2 Deflection Measurement Results Under real wind loading, metal roofs experience vibratory deflections that are either upward or downward dominated depending on the direction of the fluctuating wind-induced force at their surface. The wind-induced vibrations are unsteady as well as highly non-uniform, and increase with increasing wind pressure until permanent failure of the roof occurs. During an uplift deflection, the metal panels assume the shape of a cylindrical shell spanning between the ribs, with highest deflection usually occurring at the center of the flat-pan section. Relative to the flat pan sections, vibrations with lower amplitude but higher frequency were observed along the panel ribs and lower amplitudes were recorded at the ribs near the purlins (i.e. at structural supports). The deflections experienced by a roof region were highly dependent on wind direction, and were relatively higher at the edge panels. Net deflections were computed by deducting the baseline deflections (deflection measured before each test at zero wind speed) from the measured deflection during testing. In both standing seam roofs, the highest deflections were observed at the free end of the edge panels when they were situated at the upwind position and normal to the direction of approaching wind. The deflections experienced by the trapezoidal roof were significantly higher than the vertical leg-standing seam roof. These higher deflections might be justified by the higher (in magnitude) aerodynamic pressure on the trapezoidal roof. Moreover, the span between adjacent ribs in the trapezoidal seam roof was longer than that of the vertical-leg seam roof, making it less rigid hence more susceptible to high deflections. Mean and maximum deflections versus wind directions for the deflection measurement devices that recorded the highest mid-flat pan (mid-panel) and panel-rib deflections are shown in Figure 7 and Figure 8 for the vertical-leg and trapezoidal roofs respectively. The mid-panel deflections were recorded using linear variable differential transformers (LVDT) while panel-rib deflections were recorded using String Pots. The location of the LVDTs and SPs is shown in Figure 3. It should be noted that the deflections in Figure 7 and Figure 8 were recorded at a low-eave height mean wind speed of 57 m/s and positive values represent upward deflection. Figure 7. Mid-panel and panel-rib deflections in vertical-leg roof Figure 8. Mid-panel and panel-rib deflections in trapezoidal roof As previously indicated, Figure 7 and Figure 8 show that both roofs tended to experience the highest mid-panel and panel-rib deflections when their longer sides (the sloping sides) were situated approximately normal to the direction of the approaching wind, slightly different from the directions producing highest local uplift pressures, a fact probably attributable to details of pressure distribution over the roof surface combined with roof stiffness characteristics. The deflections were also observed to

8 7 increase as a panel s distance frame the edges reduces. Both mid-panel and panel-rib deflections of the trapezoidal roof were higher than those observed in the vertical-leg roofs. Increasing the wind speed increases the wind-induced pressures which in turn increases the vibratory deflections experienced by the roofs. The maximum deflections versus mean low-eave height wind speeds for vertical-leg roof (for critical wind directions of 85 o ) and trapezoidal roof (for critical wind directions of 275 o ) are shown in Figure 9. For both roofs results of the LVDTs and SPs which registered the highest deflections are shown here. In the vertical-leg roof the deflection increased at a relatively lower rate with increasing wind speed, but in the trapezoidal roof the deflection increased sharply up to wind speed of 47.8 m/s but at higher wind speeds it was almost constant. At low mid-panel deflections, the roof panel vibrations were observed to be mainly limited to the flat metal pan section only. But as wind speed increased, the vibrations started creeping into the panel-ribs and at a sufficiently high wind speed, the flat pan section lifted off entirely and the whole metal panel section started vibrating from the ribs. At this stage, the rate at which deflections increased with increasing wind speed reduced significantly and the dominant frequency of vibration also changed. In a sense, at very high deflections the metal panel assembly became increasingly stiffer until failure was encountered. For the range of wind speeds shown in Figure 9, metal panel stiffening was observed in the trapezoidal roof panels around mean wind speed of 40 m/s while in the vertical-leg roof the deflection were predominantly within the flat pan sections. Figure 9. Deflection versus wind speed in Vertical-leg and Trapezoidal roofs 3.3 Observations of Roof Failure The ability of a metal panel to undergo high deflections without any permanent deformation or failure to the roof structure might vary depending on several factors including metal panel property, spacing of clips, width of panel, etc. In the vertical-leg standing seam no roof failure was observed for the full range of wind speeds and directions tested. Failures in the trapezoidal roof occurred at wind speed of 65.7 m/s when the wind direction was approximately normal to the sloping edges. Failures occurred at the high eave sides of the upwind end panel. Figure 10. Panel ballooning (left) and clip rupture (right) failures Panel ballooning, seam disengagement and panel rib rupture were observed and the mode of failure was clip breakage/rupture (Figure 10). The dominant type of failure in ASTM E1592 tests is clip slippage from seam, but this protocol doesn t account for the non-uniform vibratory loading that might cause fatigue failures at joints of the roof, which is believed to have happened in this case. The interlocking property of standing seam roofs gives them strength, but makes them susceptible to progressive failure. Once clip-breakage occurred, patterns of progressive failure were observed, seam-clip separation started to initiate in the neighboring clips and ballooning continued to the next panel. It should be noted here that the free condition used at one end of

9 8 the roof to replicate the end condition used in ASTM testing was not a condition that would normally occur in real installations. The vibration behavior in this area may have been somewhat exaggerated as a result. Further testing would be desirable using a more realistic end condition Even though the highest roof uplift pressure was measured at the low eave corner region, failure occurred at the high eave corners because the high eave edge was not fastened. To check whether roof failure occurred because of the free boundary condition at high eave side, the free high eave ends of the two edge panels were fastened to the purlin (Figure 11) and the testing at the wind speed causing failure and critical wind direction was repeated. When the end panel s high-eave and low-eave ends were fastened, the shape of the uplift deflection changed and the uplift force was concentrated on the two middle clips of the upwind edge panel, and eventually clip-slippage occurred at the middle clip near the high eave end (Figure 11). Moreover, under this boundary condition the deflections of the metal panel adjacent to the upwind edge panel were observed to be higher than the case when the edge panel was free. This might be due to the fact that the edge panel, in its fixed- free boundary condition might have been acting as an aerodynamic shield to its neighboring panel. At high deflections the roof geometry changes can affect the mean pressure distribution and uplift forces experienced. Figure 11. Clip failures with free (upper) and fixed (lower) edge panel boundary conditions 4 CONCLUSION An experimental investigation was performed to study the behavior of wind loads on the surfaces of two types of standing seam metal roofs and evaluate their performance under high wind speeds. When compared to the vertical-leg roofs, significantly higher uplift pressures were recorded on the trapezoidal roof. This showed that roof architectural details and perimeter eave trims have significant effect on the magnitude and distribution of the wind-induced pressures at the surface of the metal roofs. The results point to the need for more experimental investigation aimed at identifying the effects of such significant differences in uplift pressures. This will assist in identifying and designing aerodynamically favorable roof profiles and eave details. Comparison with experimental results showed that the wind loading provisions in ASCE 7-10 (which is based on bare deck models), might in some cases underestimate the actual uplift on a mono-sloped trapezoidal-leg standing seam roof. Owing to the elastic property of the metal panels, high panel deflections may not necessarily cause permanent deformation or roof failures, as observed in the vertical-leg roof which did not fail. The trapezoidal roof experienced higher suction pressures and higher deflections which led to progressive connection and panel roof failure. The main mode of failure in the trapezoidal roof was clip breakage/rupture. This failure mode is very different from clip slippage, which is the dominant type of failure in testing protocols that use uniform pressure load, and is hypothesized to have occurred due to excessive vibrations at the free end. It should be noted that the vibration experienced at the failing high-eave ends were exaggerated due to the fixed-free boundary conditions used. ACKNOWLEDGMENTS The authors greatly appreciate the cooperation of Jaime Gascon of Miami-Dade County Regulatory and Economic Resources Department (RER); Ken Buchinger and Jason Allen of MBCI; Richard Starks and Trey Herren of BRS, Inc.; Terry Wolfe and Brandon Jasek of Force Engineering & Testing, Inc.; and Dale Nelson of Roof Hugger, Inc. This research was supported by the Florida Division of Emergency Management (DEM) and the National Science Foundation (NSF) (NSF Award no. CMMI ). The help offered by Walter Conklin, Roy Liu-Marquis and Alvaro Mejia is greatly acknowledged. The findings expressed in this paper are those of the authors only and do not necessarily represent the views of the sponsors.

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