Journal of Wind Engineering and Industrial Aerodynamics, 36 (1990) 689-698 689 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Wind Performance Limits of Roof Ballast Pavers

Jorge Pardo*

ABSTRACT

Given the importance of accurately determining the limit wind- speeds which loose-laid roof elements can withstand, this paper documents a survey of various wind test methodologies utilized by several researchers, and identifies the parameters of relevance to the subject.

The application of analytical tools such as utilized here to account for the effects of air turbulence upon the limit wind- speeds observed, makes possible the development of a standardized wind test format within which the "Micro-zone" concept is defined, and which permits the evaluation of full-scale roof system ele- ments within the relatively smooth-flow environment of aeronau- tical wind tunnels, where parameters such as roof fascia geometry, degree of element interdependence and underdeck air infiltration, among others, can be explored.

The study concludes by presenting maximum roof heights allow- able under different wind exposures for a particular type of roof ballast paver, in the context of the wind design requirements es- tablished by ANSI Standard A58.1-1982.

INTRODUCTION

The emergence of roofing single-plies as a major design alter- native, for the construction of low-slope roof systems, has re- sulted in a high degree of interest which focuses on the evalua- tion and quantification of their wind performance capabilities, particularly as applicable to the loose-laid roofing assemblies in which gravity ballasting is utilized in lieu of anchors or adhe- sives. The determination of the wind speed limits at which loose- laid roofing system elements initiate displacement has important economic and safety implications, since wind damage to building roofs and adjacent structures has been documented as being pre- ceded by such displacement (FM-TAB. 1-29, 1984).

* Director, Innovative Design Research Div. NCMA P. O. Box 781, Herndon, VA 22070 - U.S.A.

0167-6105/90/$03.50 © 1990--Elsevier Science Publishers B.V.

690

BACKGROUND

Aside from its economy in areas where abundant, stone ballast- ing has demonstrated a propensity for becoming an airborne missile when under extreme wind conditions, while under less severe con- ditions, it may be scoured by relatively low velocity winds. Stone ballast scouring may lead to deck overloading in areas of accumula- tion, or at best, necessitates periodic stone redistribution over bare spots of membrane.

For the above reasons, a number of roof paver systems have been developed for the ballasting of single-ply roofs, in which de- sirable features may be incorporated insofar as freeze-thaw resist- ance, drainage, interlocking, and color and texture variations.

Test Methods

Flat roofing assemblies have traditionally been tested for wind uplift through the application of static pressure differentials across full scale sections of the prototype specimen (FM-TAB 1-28, 1983).

The configuration and nature of loose-laid roofing systems pre- clude this type of static-load testing for their evaluation, as the displacement and uplift of loose-laid assemblies are the resultant of dynamic airflow conditions. (Kind et al, 1987).

Building macro-structures (entire buildings and building clus- ters) are currently tested using reduced-scale models in meteoro- logical wind tunnels, where the natural boundary layer character- istics of the atmospheric wind can be simulated in regards to variation of speed and turbulence with height. (Cermak, 1977). This method is essential for the study of macrostructure behaviour under high winds, and is hereinafter referred to as "Macro-zone" wind testing. (Figure i).

The wind testing of building roof Micro-structures on the other hand, such as the loose-laid subcomponents and elements scrutinized in this study, is more advantageous when performed on full scale specimens. This approach eliminates compromises in model charac- teristics, like joint interlock strength, membrane elasticity, and air permeability of the deck, to name a few. Such compromises are usually required to feasibly downscale microstructure elements for testing in atmospheric boundary layer (ABL) type tunnels. (Kind & Wardlaw, 1984).

In view of the preceding, the concept of "Micro-zone" wind test- ing has been advanced (Pardo, 1986), in light of Kind & Wardlaw's conclusion that the only wind characteristics ina~ediately relevant to the wind-induced displacement of roof micro-structures are those en- countered at roof-top level. (1984).

The microzone method described by the writer (1988) (Figure 2) permits testing of roof microstructures in aeronautical wind tunnels under relatively smooth-flow conditions, and independently of their location within a (simulated) boundary layer. (Matrix A).

Studies conducted by Bienkiewicz and Meroney (1985) have estab- lished a correlation between the static limits of loose laid roof pavers under heavy winds, and the longitudinal turbulence component of the wind at roof-top level. A semi-empirical procedure derived in part from that research, for converting limit wind speeds to dif- ferent turbulence conditions, has been proposed by the writer (1988) as a key element of application for the Microzone method.

691

WIND ) I

/ ~TURBULENCE GRADIENT

MICROZONE OF WIND SPEED GRADIENT ~,%~o~T°u~E /~--~ . . . . . . . . . . . . . . .

MACROZONE WIND TEST ABL ~ND TUNNEL

W, ND l-7 7 7 ~

=1/ / / I A / / / /

J ['- MICROZONE WIND TEST AERONAUTICAL WIND TUNNEL

Figure 1 Figure 2

EXPERIMENTAL CONDITIONS

The tests described in this study were conducted at the Glenn L. Martin Wind Tunnel facility of the Aerospace Engineering Depart- ment of the University of Maryland. The test facility is an aero- nautical, closed-loop wind tunnel with a 7.75 x 11.04 ft. (2.4 x 3.4 meter) test section at which point a maximum air speed of 230 mph (103 m/s) is attainable. Accurate velocity/pressures can be main- tained in the tunnel with the 2,000 hp synchronous motor driving a fixed-pitch blade fan.

The longitudinal turbulence level at the test table was augmented by an order of magnitude, through the increase of the fetch by 36.4 ft. (ll.10m), which also served to minimize blockage effects. Turbu- lence levels were measured at the leading edge of the test table utilizing hot film anemometers, and flow speed measurements were taken via Pitot-static tubes, at two levels near the point of inci- dence, with Gould type 590 Barocel differential pressure transducers comparing total pressure, as detected in the settling chamber, to the static pressure at the Pitot tube locations.

Static pressures from 148 taps on the test rig were fed to a minicomputer for digitization, via Scanivalves and pressure trans- ducers, interconnected by means of .054" I.D. tubing for relatively flat amplitude response in the I0 to 20 HZ range.

692

The ballast pavers tested included type "R", capable of varying degrees of interlock, of 8"x16" (20x40 cm) in plan, exerting an ave- rage ballast pressure of 12.7 ib/s.f. (608 N/m2). The larger non- interlocking pavers type "P", used as reference, were approximately 24"x24" (60x60cm) and exerted a ballasting pressure of 23.75 ib/s.f. (1136 N/m2).

Experimental Procedure

The wind studies referenced here indicate that the most sensi- tive areas of the roof, to wind-induced damage, are near the build- ing corners, particularly when the wind flow originates at 45 ° from the windward corner thereby bisecting it. (Kind & Wardlaw, 1976).

The tests described in this report explored the critical 45 ° wind direction identified, and made use of a rotating test table to vary prototype symmetry parameters.

Each experiment was conducted by gradually increasing the tunnel windspeed. Upon attainment of limit conditions, air velocity was maintained constant for a minimum of three (3) minutes in order to observe periodic motions of the system under scrutiny which might lead to fatigue failure.

All experiments were videotaped, while visual monitoring noted events of particular interest; tunnel airspeeds were recorded at limit cases, and at other relevant times as described in detail in the full study.

Limit Windspeeds

The critical windspeed identified for the purposes of this re- port as (VL), or limit windspeed, is defined here as the condition at which the measured mean tunnel windspeed is sufficient to effect up- lift at any one point, of any single roof element, by a maximum of 2" (5 cm), for a minimum period of three (3) minutes.

The experience gathered from the experiments described in a pre- vious Report (Pardo, 1986) confirms that, for the type and size of loose-laid elements under consideration, this amount of vertical displacement accurately foretells proximity of failure, which in turn is defined here as element dislodgement from the assembly.

DISCUSSION AND RESULTS

As elaborated by Kind et al, (1984) and Bienkiewicz & Meroney, (1985), the mechanics governing the uplift phenomena of loose-laid elements on low slope roofs appears to be understood in principle.

A number of building design parameters have been identified through this investigation, as critical determinants of the limit windspeeds which loose-laid roof assemblies can withstand:

• Underdeck pressurization • Fascia geometry • Parapet height • Loose-laid element weight and pressure equalization

capabilities. • Interlock capacity of roof pavers or boards • Perimeter restraint

Underdeck Pressurization

The effects of underdeck air pressure resulting from building envelope openings and/or infiltration, can be a major contributing factor to the overall uplift pressure exerted upon roof coverings by high winds. (FM-TAB 1-7, 1983). The correlation between different envelope infiltration areas and limit windspeed was investigated in this study by means of a set of variable vent ports located on two opposite sides of the test table. By operation of these ports, open- ings corresponding to specific percentages of the roof deck area could be vented to the incoming flow, or conversely closed totally. (Figure 3).

In this manner, the type of roof deck construction could be sim- ulated, as it is known that monolithic cementitious roof decks tend to isolate well the roof coverings from the pressures which may re- sult inside the building, while metal roof deck construction is rather permeable to internal building pressurization. The results of the underdeck pressure measurements, and their effect on (VL), are plotted in trivariate form in Graph i.

L ................. I . . . . . . ' . . . . .

NSTRUCTION 10(] 18

VERTICAL ENVELOPE -~" ~ ~ 1 2

INFILTRATION )ROOF DECK AREA)

VARIABLE INFILTRATION PORTS FOR TEST TABLE UNDERDECK PRESSURIZATION

B;24"x24"PAVERS L :24 "x24PAVERB O ;B x16 PAVERS-INTERLOCK ~8 ' x16PAVERS-NOINTERLOCK

EFFECT OF ENVELOPE INFILTRATION BALLAST PAVERS - WIND UPLIFT

693

Figure 3 Graph 1

694

Fascia Geometry

It has been noted that reduced scale models (1:10 to 1:15) util- ized for wind uplift research of microstructures grossly depict fascia/membrane interphase conditions, and as a rule, such highly abbreviated parapet configurations unrealistically protect membrane edges from air infiltration. Additionally, vortex formation and other air disturbances resulting from wind impact upon the roof eave are highly dependent upon the geometry of the fascia micro-structure, as evidenced by the results of this study; a fact usually dismissed in macrozone-type tests. (Figures 4 & 5).

WHERE CALLED FOR PARAPET ASSEMBLY

LOOSE-LAID ELEMENTS ,i'"""", m

S;NGLE-PLY MEMBRANE

RIGID INSULATION

GYPSUM FIRE BARRIER . . (WHERE SPECIFIEDI "

METAL ROOF DECK

MODEL ROOF CONSTRUCTION NO SCALE

............. ..... /:::::

PERIMETER C RESTRAINED

Contrary to expectations, this experimental fascia geometry generated higher negative pressures on the roof surface than conventional straight geometries.

Figure 4 Figure 5

The fascia frames tested were fitted with eave assemblies on three (3) sides only, while the fourth side was left flush with the surface of the loose-laid elements under test, and fitted with espec- ially shaped metal trim, designed to simulate the vertical restraint corresponding to the interlock provided by a continuing (virtual) series of elements, as would occur in a much larger roof surface.

Pressure Equalization

Previous research has shown that the pressure-equalization phe- nomenon which occurs across top and bottom surfaces of loose-laid roof elements may be related to joint configuration and joint length density per unit area. (Kind & Wardlaw, 1984).

In order to observe and quantify the pressure equalization ef- fects on loose-laid elements of different aspect ratios and with different joint densities per unit area, an elevator-type gauge deck assembly was utilized to take pressure measurements at precise elevations above and below the loose-laid elements;(Pardo, 1988), tap grid resolution was 2.06/sf. (22.16/m2). Characteristic results of the measurements are given in Graphs 2 & 3.

695

+45 WIND DIRECTION

"~ , C .... . . . . . .

FWS. ~.D ~ - -

+ 4 5 WIND DIRECTION

;L-1 I--Y I-1#

[ ' I I "I

L -

FLUSH END ..~r--

P R E S S U R E D I S T R I B U T I O N ' (Cp)* TOP SURFACE OF PAVERS

P E R I M E T E R : r ~ P E "A ~ FA SCI A

WINDSPEED : 80 MPH

Graph 2

P R E S S U R E D I S T R I B U T I O N (Cp)* 80~rOM SURFACE OF PAVERS

PERIMETER TYPE A " FASCIA WINDSPEED 80 MPH

Graph 3

Impact of Paver and Assembly Characteristics

Earlier Reports (Pardo, 1986, Rev.1987) describe a preliminary mathematical model for the prediction of paver uplift windspeeds, in which parameters such as paver weight, joint interlock capacity and joint density are integrated from empirical test data. (Graph 4).

1 25 140 I

,X•Fb, 02

2c

% - o~- - - ]V

/ /

/ /0

/ • / . o

/ 2/

,/ / +

Fb

MPH / LB/SF 10 20 30 40 50

BALLAST WEIGHT (Wu)

RESTRAINED vs. UNRESTRAINED ARRAYS BALLAST PAVER WIND TEST RESULTS - 6" PARAPET HEIGHT

8 ~ 8o

so

o : KIND - REF. I >~

: : PARDO- REF CJ 1.o0

E~ : PARDO = TS

+ : K I N D - REF 9 z~ \~ : BIENKIEWlCZ

: PARDO - REF "~

<~ : OPEN SYMBOl .~ • : SOLID SYMBO ~STRAINED J

7C

Graph 4

, SMOOTH / AS L FLOW FLOW -- ))

I ' "L i' i i: i i: i i: i'

5

UNRESTF

• , • RESTRAli% I '4"EOUAT'O~ (44~

I "~'. i I I

l o 15 20 'D15 C15 B15 A15

LONGITUDINAL TURBULENCE (lu>

EFFECT OF TURBULENCE ON VL FOR LOOSE-LAID ROOF ELEMENTS • A ~ o ~ t u~u~nc~ a~ 15 ft ht for Ex~ure Type g,ven

Impact of Exposure Graph 5

The application of wind test results to the exposure categories generalized by the Standard (ANSI A-58.1, 1982) must take into ac- count the effects of wind turbulence and velocity, and their varia- tion with height.

696

Bienkiewicz and Meroney (1985) found that a higher degree of roof top air turbulence results in a lowering of the mean windspeeds re- quired to effect uplift of roof elements. Based on those results, the writer has proposed a preliminary mathematical method to account for the effects of turbulence upon limit uplift windspeeds (Graph 5), so that tests need not be executed at more than one exposure, and therefore permitting the use of microstructure tests within the smooth flow environment of aeronautical tunnels. (Pardo, 1988).

Insofar as the effects of the height gradients characteristic of different exposure types, as defined by the Standard, the method de- veloped by Bienkiewicz and Meroney (1985) was utilized here to pro- ject maximum roof heights allowable for the roof paver system type "R", as reported in Table i.

Briefly, the method cited equates the mean limit windspeed ob- tained through tests, to the peak rooftop windspeed defined by the referenced Standard as:

VL2 = K z G Z Vm2 (i)

where, K Z = Velocity pressure exposure coefficient at height z, and, G z = Gust response factor at height z.

In addition to the safety margin inherent in the above approach, the mean limit windspeeds utilized to compute the maximum roof heights in Table I were conservatively adjusted (Graph 5) to the most severe turbulence conditions identified by the Standard for each Ex- posure Category (15 ft. ht.) (5m). (Matrix A).

TABLE 1

ALLOWABLE ROOF HEIGHTS* FOR TYPE "R" BALLAST PAVERS

6" Parapet, continuous engagement Computed per ANSI A58.1

ASSEMBLY TYPE BASIC WINDSPEED (Vm) EXPOSURE (mph) CATEGORY**

Roof Deck Interlock 70 80 90 100 110 120

Concrete 8" O.C. 380.6 225.9 138.8 86.4 59.1 40.2 16" O.C. 209.3 119 73.2 45.4 29.6 18.9 none 150.3 83.4 50.6 32.3 19.5 13.8

A Metal 8" O,C. 154 85.8 52.3 33.3 19.9 14.2

16" O°C. 99 54.9 32.7 19 12.9 none 78,9 44.9 26.1 16.1 10.2

..........................................................................................

Concrete 8" O,C. 294.2 132.7 65 33.9 18.4 9 16" O,C. 117.4 52 25.1 12.2 none 41.4 17.9

B Metal 8" O,C. 78.5 34 15.9

16" O,C. 38.8 16.2 none 16.4

..........................................................................................

Concrete 8" O.C. 350 111.3 38.6 14.4 16" O.C. 92.8 27.4 6.9

Metal 8" O.C. 50 14 16" O.C. 18.3 C

* Maximum in feet ** Definitions per ANSI A58.1 For V m Re = Wind Isotach

697

r N . C , M . A .

24" x 24" P~ers 26~. .....

24" x 24" pavers @ 26 PSF

24" x 24" Pavers 3.18 @ 26 PSF

I 16 l 24 " x 24" Pavers

?.S. 24" x 24 " Pavers ~-- @ 2~.75PSF

' ~ ' 24" x 24 Pavers ,c @ 23.75 PSF

24" x 24" Pavers

24" x 24" Pavers

24" x 24 " Pavers ~1 @ 15 PSF

9 24" x 24" Pavers

W I N D T E S T C O R R E L A T I O N M A T R I X A , o l ~OOI I - [A ;D lOOP I t l l l . T I i CO I~O~N, I . t i t W,NO YAW

T e l l M e t h o d

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~Z! . . . . . . . . . . . . . . . insuL . . . . . . . Full array over loose

1/1 ~.aid rnesabr and . . ~ . . . . . . . ~nsu~., ' . . . . . .

Full array on loose

_ "- ............ ln~ltr~ ckc:E.. _~-

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mer array w/taped

~ Qornex array on - - znsu l , boards

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.324 81

. )74 _ 00

.-5_ _ _ ~9

.5 73

. ~ . . . . . . . ~

. . 1 ~ . . . . ~

.242 61

.242

N . C . M . A . IDR D i v i s i o n

IA_

iA_

T.$ B" x 16" Pavers

T.S 8" x 16" Pavers ~ - @ 12.2 p~r

~4 8" x 16" Pavers

W I N D T E S T C O R R E L A T I O N M A T R I X A I FO~ Loose LAin ~OO' I ~ I~ I .T I I CO~O~IWVl - * l * w~No v~* I

E l e m e n t T e s t M e t h o d

= h a r a c t e r l s t l © !

8' x 16" Pavers @ 15 PSF

8" x 16" Pavers @ 1 3 . 9 P ~ ' . . . . . . .

8" x 16" Pavers

B" x 16" P ~ e r s 1 2 . 7 P ~

12" x 16%" Pm~exs 2 @ 12PS~

2 12" x 16%" Pavers @ 12 PSF

2 12" x 16%" Pavers @ 12PS~

2 12" x 16% N Pavers @ 12PS~

*mo~|ct: 87702

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M o d e l

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1 /1 .3 0 6" .K

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1/i P? _§'~..~.

I

Fu l l a r r ay sh /p l ap edg~ 1/10 17.5 O 6" U._; _og ._~_dg ,F - -~ . . . . . . .

Full a r ray shiplap e~ 1/15 9.1 O 6" U on ~oexm. deck

F u n a r r a y ~ l p l ~ edge~ 1115 17.5 0 ~ . . . . . U ) lus cccm. t~be

Full array ~iplap e419es III~ 9.1 8 6 ~ U ~lus conn. tabe.

Full array on membr. ÷ ¢rmul.

Full azray on mm~sr. ~nsuL

FUll array wlpartlal PL C~nn. on membr. + ins.

I F u l l a r ray unc~%n, cn I • ~mbr . + i nBu l .

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~ s u l . . . . . . . . . . . . . . . . .

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• 5 "~33 DS0 . ~ ,~ 62.7

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vm DSOO

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6 AI5

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Vm AI5

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vm C15 .331 }.03,7 113 02 .2

698

CONCLUSIONS

The results of this test program are consistent with the modest body of data found in the literature, and indicate that microzone test samples of reduced dimensions are capable of generating meaningful results, for the evaluation of flat roof micro-structure wind uplift performance.

The findings of this study appear to confirm that the failure of roof micro-structures by wind uplift is primarily dependent upon highly localized non-linearities in pressure distributions, and that such non-linear effects are a function of windspeed in the first place, and of longitudinal air turbulence and roof fas- cia geometry secondarily.

The effects of perimeter securement of ballast pavers to the roof eave are substantial, as is paver interlock, which results in significantly higher resistance to wind uplift, due to the corresponding increase in tributary area over which concentrated uplift loads are distributed.

ACKNOWLEDGEMENTS

The author is indebted to Drs. J. B. Barlow and A. Winkelman respectively of the G. L. Martin Windtunnel and the Aerospace En- gineering Dept. of the University of Maryland, for their assist- ance in setting up and interpreting the tests, and to A. Kassaee and W. Sekscienski of the G.L.M. Windtunnel who executed the tests and collected the data.

REFERENCES

American National Standards Institute Inc., American National Standard A 58.1, 1982.

gienkiewicz, B., Meroney, R. N., "Wind-Tttnnel Study of Westile Ballast Paver" -Fluid Mechanics and Wind Engineering Progr~, Fluid Dynamics and Diffusion Laboratory-Civll Engineering Dept. Colorado State University CSU Project 2-96460 Report CER85-86BB-RNM13, Nov. 1985.

Cermak, J. E., "Wind Tunnel Testing of Structures"-Journal of the Engineering Mechanics Division ASCE, Vol. 103 No. EM6, Proc. Paper 13445, December 1977, pp.l125-1140.

Factory Mutual Engineering Corp., "Wind Forces on Buildings and other Structures" -Loss Pre- vention Data, Technical Advisory Bulletin I-7, 1983.

Factory Mutual Engineering Corp., "Insulated Steel DecK" -Loss Prevention Data, Technical Advisory Bulletin 1-28, 1983.

Factory Mutual Engineering Corp., "Loose-laid Ballasted Roof Coverings" -Loss Prevention Data, Technical Advisory Bulletin 1-29, 1984.

Kind, R. J., Wardlaw, R. L., "Design of Rooftops against Gravel Blow-off" -National Research Council of Canada, Report NRS 15544, September 1976.

Kind, R. J., Wardlaw, R. L.,"Further Model Studies of the Wind Resistance of two loose-lald Roof-insulation Systems"-Natlonal Research Council of Canada, NAE Report LTR-LA-269,April 1984.

Kind, R. J., Imray, M. D., Pelosos, D., "An Engineering Study to Define Equipment for Full Scale Testing of Wind Resistance of Roofing Systems"-GasTops Ltd., Report GTL-52-I-TR.2, June 1987.

Pardo, J.,"The Effect of Variable Inter-connection on the Uplift Capacity of Roof Pavers" N.C.M.A. Report RCP-86702, July 1986, (Rev. 1987).

Pardo, J.,"Wind Test Methodology for Roof Elements and Components"-N.C.M.A. Report RCP-88702 May 1988.