- Email: [email protected]
Local wind loading on the roof of a low-rise building
Journal of Wind Engineering and Industrial Aerodynamics, 45 (1992) 11 24
11
Elsevier
Local wind loading on the roof of a low-rise building Bogusz Bienkiewicz and Yawei S u n Colorado State University, Department of Civil Engineering, Engineering Research Center, Fort Collins, CO 80523, USA (Received May 16, 1991; revised version received March 18, 1992)
Summary Fundamental studies of wind loading on the roof of a 1 : 25 geometrical scale model of the Texas Tech University test building were performed in a boundary-layer wind tunnel. The investigation was focused on the pressure distribution in the windward corner region of the roof. A range of wind directions and roof parapet heights was considered. The surface flow patterns were visualized and compared with the roof pressure. The largest negative mean and peak pressure coefficients were found for cornering winds, wind direction of 235 . The pressure coefficients obtained using the dynamic pressure at the roof height were, respectively, 3.5 and 11. Very low roof parapets increased the overall maximum roof suction, while high parapets decreased it. For the wind direction of 225, the highest negative pressure occurred in the secondary vortex region and the highest pressure gradient occurred in the reattachment region. It was found that the fluctuating roof pressure was sensitive to the intensity and the integral scale of turbulence in the oncoming flow. Comparison of the wind tunnel data with field data showed that the mean and the positive peak pressures were in good agreement. The negative peak pressures measured in the wind tunnel underestimated the field data.
Notation
Cp H L
R
S U
pressure coefficient height of building model height of roof parapet l e n g t h of b u i l d i n g model reattachment secondary separation mean wind speed at building height
Correspondence to: B. Bienkiewicz, Colorado State University, Department of Civil Engineering, Engineering Research Center, Fort Collins, CO 80523, USA.
0167-6105/92/S05.00
1992 Elsevier Science Publishers B.V. All rights reserved.
B. Bienkiewicz. }1. Sun~Local wind loading
12
W X Y
width of building model coordinate along the long edge of building root' coordinate along the short edge of building roof wind direction
1. Introduction Many modern industrial buildings have flat roofs of rectangular shape covered by loose-laid roofing systems consisting of light-weight pavers and gravel laid on the top of an impermeable membrane. In recent years, the dynamic wind effects for such roofs on low-rise buildings have been investigated and progress in understanding of the performance of the loose-laid roofing systems has been achieved [1- 7]. Due to the complexity of the phenomenon, several unanswered questions remain. They include the critical wind direction for roof loading, the effects of roof parapets, and the influence of the characteristics of the oncoming flow. In order to provide more information on wind loading on low-rise buildings, a cooperative research program involving the Colorado State University (CSU) and the Texas Tech University (TTU), USA, was established in 1989. A full-scale low-rise building was set up and instrumented at TTU. Meanwhile, wind-tunnel studies were initiated at the Fluid Dynamics and Diffusion Laboratory, CSU. One of the studies was focused on roof wind loading and the performance of loose laid roofing systems. The purpose of this paper is to present representative results for wind loading on the roof of a 1 : 25 geometrical scale model of the TTU test building. Discussed are the effects of wind direction, roof parapets, and the characteristics of the oncoming flow. The presented results include the roof pressure and the visualization of the flow near the surface of the roof. A comparison between the wind-tunnel and full-scale data is also presented.
2. Experimental configuration The experiments were performed in a boundary-layer wind tunnel (the Meteorological Wind Tunnel at CSU) with a test section 29m long and 2.1 m x 1.8 m in cross-section. The lower part of the atmospheric boundary layer (ABL) the atmospheric surface layer (ASL) - originally developed for a 1 : 100 geometrical scale model [8] was utilized. In the present study the flow was interpreted as an ASL at a 1 : 25 scale. The mean velocity and turbulence intensity profiles of the simulated and prototype ASL are shown in Fig. 1, where the velocity profile is normalized using the velocity at the roof height. Two simulations of the ASL, denoted as flows R1 and R2, are included. They represent two experimental approximations of the prototype flow conditions. For the model scale of the present study, the R2 flow had a better agreement
13
B. Bienkiewicz, Y. Sun~Local wind loading 30[ R1 Flow
R1 Flow •
-
£
-
/
R2 Flow Field Data (n=0.14) Field Data (n=0.17)
R2 Flow u - Field Data •
/ •
/
N 2O o /
/ o,
/ ~dJ--
Model
ols-
i
1
U(z)/U(H) (a)
0 m
1.5
I
[ .
10 20 Urms(z)/U(z)
I.a 30
(b)
Fig. 1. Properties of ASL: (a) mean velocity profile, (b) turbulence intensity profile. with the field d a t a t h a n the R1 flow. The t u r b u l e n c e (longitudinal) i n t e g r a l scale of the R2 flow at r o o f level was 33 m, w h i c h was in good a g r e e m e n t w i t h the field data. All of the d a t a p r e s e n t e d in this p a p e r were t a k e n in the R2 flow, except w h e n m e n t i o n e d otherwise. A detailed d e s c r i p t i o n of the modelled ASL c a n be found in Ref. [8]. F i g u r e 2 shows s c h e m a t i c a l l y the 1:25 scale building model of the T T U building. The r o o f slope was r e p l i c a t e d in the model. One h u n d r e d and e i g h t y
Fig. 2. Model of TTU building (1 : 25 geometrical scale).
14
B. Bienkiewicz, Y. Sun~Local wind loading
\
270 °
I
180 °
0.4
0.8
:1•"!:-:-::::-:•:!
• 193 .
"Jr >-
X/H
i
.
.
•
.
"
.
•
.
.
.
"
.
•
.
•
.
.
"
.
.
"
•
.
.
'] ] ]
.'!'i i'i i'i i'i i'i'l
0,4 •
•
•
•
.
•
•
•
.
•
t
B
t
J
4
.
•
"
•
.
•
.
I
0.8
Fig. 3. Arrangement of pressure taps (B tap on full-scale building).
four closely spaced pressure taps were provided in one corner region of the roof. The tap locations are indicated in Fig. 3. The pressure taps were arranged so th at the taps existing on the full-scale building were represented in the model• Additional taps were added in between the locations of the full-scale taps and also very close to the roof edge. Roof parapets of various heights were employed in the experiments. The parapet relative height (Hp/H) ranged from 0 to 0.24. The prototype parapet thickness was 3.125 inches• A four-channel scanivalve in conjunction with four pressure transducers and tygon tubing (28 cm in length and 1.59 mm in internal diameter) were employed in the pressure measurements. Effort was made to improve the frequency response of the scanivalve tubing system• Figure 4 shows the frequency response of the original and optimized systems• The optimized system was used in the measurements• As shown, this system had a transfer function close to unity (with deviation less t han 5%) for the frequency range up to 200 Hz. The system incorporated a restrictor, made of 8 cm long PVC tube with an internal diameter of approximately 0.4 mm, which was installed between the scanivalve and each of the four pressure transducers. The pressure signal was low-pass filtered with the frequency cut-off of 200 Hz. The sampling frequency was 400 Hz and the sampling time was about 41 s, which corresponded to the fullscale record length of about 17 min. The record length of the field data was 15 min.
3. R e s u l t s a n d d i s c u s s i o n 3.1. Pressure measurement The worst wind direction (wind direction resulting in the highest roof peak suction) was expected to be between 180° and 270 °. Therefore, the roof pressure
B. Bienkiewicz, Y. Sun~Local wind loading 15 9
1.59 mm
/"
1.5
\
/" /
0 "m
28cm
"N,k,k
/./
o u_
S'-. /I
/
.
0.5
.
.
i I
\,
\.
\. ~.
-
.
.
.
.
.
.
.
.
.
.
.
.
.
-
Tran~lucer
\
/
.
mm
9.Scm
Scanivalve ~
o/
I
~
15
-
-
-
-
-
-
-
-
"~,..,-~. :'~,- ............. -"
"[~-5% +_..5~
,]
"-N~ ~ 2g
0
scanlvalw
0
50
1O0
Tran~lucer
150 Frequency (Hz)
200
250
300
Fig. 4. Frequency response of pressure measurement system.
X/H 0.4
0.8
•~.I 0.4 >-
0.8
(a)
(b)
Fig. 5. Pressure distribution at ~.= 205 : (a) mean, (b) negative peak. d a t a w e r e t a k e n f o r w i n d d i r e c t i o n s (~) in t h i s r a n g e , w i t h a n i n c r e m e n t Representative distributions of the measured mean and negative peak s u r e s a t ~ = 2 0 5 :, 225 °, a n d 2 4 5 a r e s h o w n i n F i g s . 5 t h r o u g h 7. T h e pressure contours are based on the worst single peak measured at a
of 5. prespeak given
B. Bienkiewicz, Y. Sun~Local wind loading
16
XIH
0.4
0.8
"
>..
0.4
0.8
(a)
(b)
Fig. 6. P r e s s u r e d i s t r i b u t i o n a t ~ = 2 2 5 : (a) m e a n , (b) n e g a t i v e p e a k .
XlH 0.4
0.4
0.8 !
~
\''
0.8
(a) Fig.
(b)
7. Pressure distribution at ~= 245 : (a) mean. (b) negative peak.
p r e s s u r e tap. In e a c h case the c o n t o u r s of the m e a n a n d p e a k p r e s s u r e s are similar. F o r ~ e q u a l to 225 '~ (Fig. 6), the p r e s s u r e d i s t r i b u t i o n s are n e a r l y s y m m e t r i c a b o u t the b i s e c t o r of the w i n d w a r d r o o f corner. Two h i g h s u c t i o n r e g i o n s a p p e a r n e a r the r o o f edges, (X/H, Y/H)= (0.15, 0.02) a n d (0.02, 0.l 5). T h e o v e r a l l m a x i m u m m e a n a n d p e a k suction, and t h e m a x i m u m of p r e s s u r e s t a n d a r d d e v i a t i o n o c c u r at t a p No. 191 and are e q u a l to - 2 . 8 , - 8 , and 1.3, r e s p e c t i v e l y . B o t h the m e a n a n d p e a k s u c t i o n in the p r e s e n t s t u d y c o m p a r e r e a s o n a b l y well w i t h the d a t a r e p o r t e d in Ref. [3]. As the wind d i r e c t i o n was c h a n g e d (Figs. 5 t h r o u g h 7), the c e n t e r line of the h i g h s u c t i o n r e g i o n r o t a t e d in the s a m e sense as the wind. In o r d e r to
B. Bienhiewicz, Y. Sun/Local wind loading
17
I
-2
-4
Q. O
-6'
\
)-"
-8
\
/ \\
-10
-12 180
-~O---
Mean
_ _,c
Negative Peak
1/
'-~- -
,
,
I 210
,
,
[
240
i
i
i
270
Wind Direction
Fig. 8. Overall maximum mean and peak suction.
d e t e r m i n e the w o r s t wind direction, the overall m a x i m u m m e a n and p e a k s u c t i o n at e a c h wind d i r e c t i o n are plotted in Fig. 8. The d a t a show t h a t the w o r s t wind d i r e c t i o n is 235, w h e r e the m a x i m u m m e a n and p e a k suction are - 3 . 5 and 9.7, respectively. B o t h of the values o c c u r at tap No. 191. The c u r v e s in Fig. 8 are not s y m m e t r i c a b o u t ~.= 2 2 5 possibly due to the lack of s y m m e t r y in the l o c a t i o n of the p r e s s u r e taps. It should be m e n t i o n e d t h a t o t h e r a u t h o r s [9,10] r e p o r t e d similar a s y m m e t r y of the r o o f p r e s s u r e distributions. W h e n the p r e s s u r e m e a s u r e m e n t s were r e p e a t e d for a = 2 3 5 , the overall maximum peak suction reached 11.3 at tap No. 193. The effects of p a r a p e t s on the r o o f p r e s s u r e d i s t r i b u t i o n h a v e been investigated by v a r i o u s r e s e a r c h e r s , [2,3,5 7]. T h e s e effects were also addressed in the p r e s e n t study. F i g u r e s 9 and 10 show the p r e s s u r e d i s t r i b u t i o n s at the wind d i r e c t i o n ~ = 2 2 5 with low (Hp/H=O.04) and high (Hp/H=O.08) p a r a p e t s , respectively. W h e n these d a t a are c o m p a r e d with the result for the r o o f w i t h o u t a p a r a p e t , Fig. 6, it is evident t h a t the p r e s e n c e of p a r a p e t i n c r e a s e s the width of the high s u c t i o n edge region, w h i c h is in a g r e e m e n t with o t h e r r e p o r t e d results [2,3,5,6]. In addition, a lower p a r a p e t i n c r e a s e s the p r e s s u r e g r a d i e n t in the c o r n e r region, w h e r e a s a h i g h e r p a r a p e t d e c r e a s e s it. The effects of the p a r a p e t on the overall m a x i m u m m e a n and p e a k suction are i l l u s t r a t e d in Fig. 11, w h e r e the results of the p r e s e n t s t u d y are c o m p a r e d with the d a t a of Ref. [5]. It can be seen t h a t the p r e s e n c e of low p a r a p e t s (Hp/H< 0.04) results in an i n c r e a s e in t h e overall m a x i m u m m e a n and p e a k suction. H i g h p a r a p e t s (Hp/H> 0.08) r e d u c e b o t h the values. Therefore, o n l y p a r a p e t s w h i c h are high e n o u g h are beneficial.
B. Bienkiewicz, Y. Sun/Local wind loading
18
X/H
0.4
>..
0.8
,
.----~
0.4
0.8 (a)
(b)
Fig. 9. Pressure distribution at u = 225: with low parapet (Ho/H= 0.04): (a) mean, (b) negative peak. X/H
0.4
0.8
~0 0.4
>-
\
\ 0.8
(a)
I
(b)
Fig. 10. Pressure distribution at ~=225 with high parapet (H,/H=O,08): (a) mean, (b) negative peak.
3.2. Flow effects on roof pressure To i n v e s t i g a t e the effects of the c h a r a c t e r i s t i c s of the o n c o m i n g flow, the m e a n a n d p e a k p r e s s u r e s in t h e R1 a n d R2 flows a r e s h o w n in Figs. 12 and 13 for t w o r e p r e s e n t a t i v e l o c a t i o n s on t h e roof. T h e r o o f level t u r b u l e n c e intensity of t h e t w o flows was, r e s p e c t i v e l y , 15% a n d 20%. T h e i n t e g r a l l e n g t h scale of t h e R2 flow was 74% h i g h e r t h a n for the R1 flow. T h e d a t a in Figs. 12 a n d 13 s h o w t h a t the h i g h e r level of t u r b u l e n c e in the R2 flow r e s u l t s in h i g h e r
B. Bienkiewicz, Y. Sun~Local wind loading
1
19
•
-4
/
/
/
/ /
ZY
-6 J J
/
-8'
/
/
Mean
)~
-E'--
/ \
/ ,
/
/ \
Negative Peak
•
/
Ref. [5]
-10
I
-12 0.1
0.05
i
0.15
I 0.2
0.25
Relative Parapet Height, Hp/H Fig. 11. Overall maximum mean and peak suction.
-4
,
°
--.
Cp -6
£~
4',
Z:. - -
2.
,
-----
'
....
M e a n (R2) +Peak Mean
,
-10
•
Tap No. 11 12
, 180 °
(R2)
P e a k (R2) (R1)
÷ P e a k (R1) P e a k (R1) M e a n (Field' + P e a k (Field) P e a k (Field)
I 240 o
210 °
270 o
Wind Direction Fig. 12. Flow effects and comparison with field data (tap No. 11). pressure fluctuations, with the m a x i m u m increase in the fluctuations being 100%. The effects on the mean pressure are not significant. This sensitivity is more p r o n o u n c e d for the pressure taps close to the roof corner (e.g., tap No. 1]) than for the taps away from the roof edge (e.g., tap No. 85).
B. Bienkiewicz Y. Sun/Local wind loading
20
21
Cp '~,L
/
:i: *~ -4
'::::
,
~,
•
::::'
Mean {R2)
-6
-8
L 180
°
~
,
~
- P e a k {R2) M e a n (R1) + Peak (R1)
• (~
Mean (Field)
Tap No.85
- P e a k (R1)
;
I 210
• •
°
I 240
"
270
Wind Direction Fig. 13. Flow effects and comparison with field data (tap No. 85).
3.3. Comparison with field data Figures 12 and 13 also include the field data. By comparing the wind tunnel data with the field data, it is found that both the mean and positive peak pressures in the R2 flow are in a good agreement with the field data. However, for most wind directions, the magnitude of the negative peak pressure in the R2 flow is lower than the field data. For some wind directions, the wind tunnel data underestimate the negative peak pressure by a factor of two. This discrepancy has not been resolved. It may be attributed to possible non-stationarity effects in the field data and/or Reynolds number or flow modeling effects on the wind tunnel data. 3.4. Surface flow pattern The two conical vortices separating from the windward edges of the roof are responsible for the high suction in the roof corner region. In order to investigate the surface flow pattern in this region, flow visualization experiments were conducted. Figure 14 shows representative flow visualization results illustrating the effects of the roof parapet. The surface traces of the conical vortices and the traces of the zone of interference with the secondary vortices near the roof edge can be identified. The two vortices become wider and they begin to interact with each other near the roof corner as the parapet height is increased. These patterns are in good agreement with the pressure distributions in Figs. 6, 9, and 10. Figure 15 shows the reattachment lines (R) of the primary vortices and the secondary separation lines (S) inferred from Fig. 14a.
Fig. 14. Surface flow patterns (~=225'=): (a)
Hp/H=O,(b) Hp/H=O.04,(c) H,/H=O.08.
22
B. Bienkiewicz, Y. Sun~Local wind loading X/H
0.8
0.4
CS)
"7"
0.4
\
\ \
0.8
Fig. 15. Sketch of reattachment and secondary separation lines (~= 225~, H,/H= 0). X/H
0.4
0.8
"--- (s)
"T- 0.4 "
'
~
~
'
"-- (R)
0.8 Fig. 16. Relation between mean pressure distribution and surface flow pattern. A comparison of the results in Figs. 15 and the mean pressure distribution in Fig. 6 is presented in Fig. 16. As shown, the highest negative roof pressure occurs in the secondary vortex region while the highest pressure gradient is located in the reattachment region, between the reattachment and secondary separation lines.
B. Bienkiewicz, Y. Sun/Local wind loading
23
4. Conclusions The r e s u l t s of the p r e s e n t s t u d y can be s u m m a r i z e d as follows: (1) M a x i m u m s u c t i o n on the r o o f of the tested low-rise building a l w a y s occurred at a l o c a t i o n v e r y close to the w i n d w a r d edges. The w o r s t wind direction was 2 3 5 , w h e r e the m a g n i t u d e of the overall m a x i m u m m e a n and p e a k suction coefficients r e a c h e d - 3 . 5 and 11, respectively. (2) The effects of p a r a p e t s on the r o o f p r e s s u r e depended on the r e l a t i v e p a r a p e t height, defined as the r a t i o of p a r a p e t h e i g h t to building height: v e r y low p a r a p e t s i n c r e a s e d the overall m a x i m u m suction, w h e r e a s high p a r a p e t s d e c r e a s e d it. (3) The l o n g i t u d i n a l t u r b u l e n c e i n t e n s i t y and the i n t e g r a l scale of o n c o m i n g flow affected the r o o f p r e s s u r e distribution, especially in the region close to the r o o f edge. An i n c r e a s e in the t u r b u l e n c e i n t e n s i t y from 15% to 20% and the 74% i n c r e a s e in the i n t e g r a l l e n g t h scale, b o t h e v a l u a t e d at the r o o f level, r e s u l t e d in a significant i n c r e a s e in the f l u c t u a t i n g pressure, w i t h no s u b s t a n t i a l c h a n g e in the m e a n pressure. (4) W i n d - t u n n e l d a t a showed good a g r e e m e n t with field d a t a for the m e a n and positive p e a k pressures. The n e g a t i v e p e a k p r e s s u r e m e a s u r e d in the wind tunnel u n d e r e s t i m a t e d the field data. (5) S u r f a c e flow v i s u a l i z a t i o n r e v e a l e d the effects of r o o f p a r a p e t on the p a t t e r n s a s s o c i a t e d with the m a i n and the s e c o n d a r y v o r t i c e s of s e p a r a t i n g and r e a t t a c h i n g flow in the r o o f c o r n e r region. The h i g h e s t n e g a t i v e r o o f p r e s s u r e o c c u r r e d in the s e c o n d a r y v o r t e x region while the h i g h e s t pressure g r a d i e n t was located in the r e a t t a c h m e n t region.
Acknowledgements S u p p o r t of the s t u d y was provided by the N a t i o n a l Science F o u n d a t i o n C o o p e r a t i v e A g r e e m e n t BCS-8821542 (Colorado S t a t e U n i v e r s i t y / T e x a s T e c h U n i v e r s i t y C o o p e r a t i v e P r o g r a m in Wind Engineering). Field p r e s s u r e d a t a were provided by the r e s e a r c h g r o u p at T e x a s T e c h U n i v e r s i t y .
References [11 C. Kramer and H.J. Gerhardt, Wind pressures on roof of very low and very large industrial buildings, Proc. 8th Colloquium on Ind. Aerodyn., Aachen, 1989, Part 2, pp. 79 94. [2J G. Lythe and D. Surry, Wind loading of flat roofs with and without parapets, J. Wind Eng. Ind. Aerodyn., 11 (1983) 75 94. [3] R.J. Kind, Worst suction near edges of flat rooftops on low-rise buildings, J. Wind Eng. Ind. Aerodyn., 25 (1986) 31 47. [4] R.L. Wardlaw and R.J. Kind, Wind speeds for gravel scour and paver lifting on roofs, Proc. Specialty Conf., Hurricane Alicia: One Year Later, ASCE, 1984, pp. 245 260.
24
B. Bienkiewicz, Y, Sun/Local wind loading
[5] R.J. Kind, Worst suctions near edges of flat rooftops with parapets, J. Wind Eng. Ind. Aerodyn., 31 (1988) 251 264. [6] T. Stathopoulos, Wind pressures on low buildings with parapets, J. Struct. Div., ASCE, Vol. 108, No. ST12, 1982, pp. 2723 2736. [7] T. Stathopoulos and A. Baskaran, Wind pressures on fiat roof's with parapets, J. Struct. Eng., ASCE, Vol. 113, No. 11, 1987, pp. 2166 2180. [8] J.E. Cermak and L.C. Cochran, Physical modelling of the atmospheric surface layer, Preprints 8th Int. Conf. on Wind Eng., London, Ontario, Canada, July, 1991. [9] T. Stathopoulos, Wind pressures on flat roof edges and corners, Preprints 7th Int. Conf. on Wind Eng., Aachen, 1987, Vol. 3, pp. 39 48. [10] D. Surry, Recent and current research into wind loading of low buildings at the University of Western Ontario, Proc. 6th US National Conf. on Wind Eng., Houston, Texas, 1989, Vol. 2, pp. A9-40 A9-50.
11
Elsevier
Local wind loading on the roof of a low-rise building Bogusz Bienkiewicz and Yawei S u n Colorado State University, Department of Civil Engineering, Engineering Research Center, Fort Collins, CO 80523, USA (Received May 16, 1991; revised version received March 18, 1992)
Summary Fundamental studies of wind loading on the roof of a 1 : 25 geometrical scale model of the Texas Tech University test building were performed in a boundary-layer wind tunnel. The investigation was focused on the pressure distribution in the windward corner region of the roof. A range of wind directions and roof parapet heights was considered. The surface flow patterns were visualized and compared with the roof pressure. The largest negative mean and peak pressure coefficients were found for cornering winds, wind direction of 235 . The pressure coefficients obtained using the dynamic pressure at the roof height were, respectively, 3.5 and 11. Very low roof parapets increased the overall maximum roof suction, while high parapets decreased it. For the wind direction of 225, the highest negative pressure occurred in the secondary vortex region and the highest pressure gradient occurred in the reattachment region. It was found that the fluctuating roof pressure was sensitive to the intensity and the integral scale of turbulence in the oncoming flow. Comparison of the wind tunnel data with field data showed that the mean and the positive peak pressures were in good agreement. The negative peak pressures measured in the wind tunnel underestimated the field data.
Notation
Cp H L
R
S U
pressure coefficient height of building model height of roof parapet l e n g t h of b u i l d i n g model reattachment secondary separation mean wind speed at building height
Correspondence to: B. Bienkiewicz, Colorado State University, Department of Civil Engineering, Engineering Research Center, Fort Collins, CO 80523, USA.
0167-6105/92/S05.00
1992 Elsevier Science Publishers B.V. All rights reserved.
B. Bienkiewicz. }1. Sun~Local wind loading
12
W X Y
width of building model coordinate along the long edge of building root' coordinate along the short edge of building roof wind direction
1. Introduction Many modern industrial buildings have flat roofs of rectangular shape covered by loose-laid roofing systems consisting of light-weight pavers and gravel laid on the top of an impermeable membrane. In recent years, the dynamic wind effects for such roofs on low-rise buildings have been investigated and progress in understanding of the performance of the loose-laid roofing systems has been achieved [1- 7]. Due to the complexity of the phenomenon, several unanswered questions remain. They include the critical wind direction for roof loading, the effects of roof parapets, and the influence of the characteristics of the oncoming flow. In order to provide more information on wind loading on low-rise buildings, a cooperative research program involving the Colorado State University (CSU) and the Texas Tech University (TTU), USA, was established in 1989. A full-scale low-rise building was set up and instrumented at TTU. Meanwhile, wind-tunnel studies were initiated at the Fluid Dynamics and Diffusion Laboratory, CSU. One of the studies was focused on roof wind loading and the performance of loose laid roofing systems. The purpose of this paper is to present representative results for wind loading on the roof of a 1 : 25 geometrical scale model of the TTU test building. Discussed are the effects of wind direction, roof parapets, and the characteristics of the oncoming flow. The presented results include the roof pressure and the visualization of the flow near the surface of the roof. A comparison between the wind-tunnel and full-scale data is also presented.
2. Experimental configuration The experiments were performed in a boundary-layer wind tunnel (the Meteorological Wind Tunnel at CSU) with a test section 29m long and 2.1 m x 1.8 m in cross-section. The lower part of the atmospheric boundary layer (ABL) the atmospheric surface layer (ASL) - originally developed for a 1 : 100 geometrical scale model [8] was utilized. In the present study the flow was interpreted as an ASL at a 1 : 25 scale. The mean velocity and turbulence intensity profiles of the simulated and prototype ASL are shown in Fig. 1, where the velocity profile is normalized using the velocity at the roof height. Two simulations of the ASL, denoted as flows R1 and R2, are included. They represent two experimental approximations of the prototype flow conditions. For the model scale of the present study, the R2 flow had a better agreement
13
B. Bienkiewicz, Y. Sun~Local wind loading 30[ R1 Flow
R1 Flow •
-
£
-
/
R2 Flow Field Data (n=0.14) Field Data (n=0.17)
R2 Flow u - Field Data •
/ •
/
N 2O o /
/ o,
/ ~dJ--
Model
ols-
i
1
U(z)/U(H) (a)
0 m
1.5
I
[ .
10 20 Urms(z)/U(z)
I.a 30
(b)
Fig. 1. Properties of ASL: (a) mean velocity profile, (b) turbulence intensity profile. with the field d a t a t h a n the R1 flow. The t u r b u l e n c e (longitudinal) i n t e g r a l scale of the R2 flow at r o o f level was 33 m, w h i c h was in good a g r e e m e n t w i t h the field data. All of the d a t a p r e s e n t e d in this p a p e r were t a k e n in the R2 flow, except w h e n m e n t i o n e d otherwise. A detailed d e s c r i p t i o n of the modelled ASL c a n be found in Ref. [8]. F i g u r e 2 shows s c h e m a t i c a l l y the 1:25 scale building model of the T T U building. The r o o f slope was r e p l i c a t e d in the model. One h u n d r e d and e i g h t y
Fig. 2. Model of TTU building (1 : 25 geometrical scale).
14
B. Bienkiewicz, Y. Sun~Local wind loading
\
270 °
I
180 °
0.4
0.8
:1•"!:-:-::::-:•:!
• 193 .
"Jr >-
X/H
i
.
.
•
.
"
.
•
.
.
.
"
.
•
.
•
.
.
"
.
.
"
•
.
.
'] ] ]
.'!'i i'i i'i i'i i'i'l
0,4 •
•
•
•
.
•
•
•
.
•
t
B
t
J
4
.
•
"
•
.
•
.
I
0.8
Fig. 3. Arrangement of pressure taps (B tap on full-scale building).
four closely spaced pressure taps were provided in one corner region of the roof. The tap locations are indicated in Fig. 3. The pressure taps were arranged so th at the taps existing on the full-scale building were represented in the model• Additional taps were added in between the locations of the full-scale taps and also very close to the roof edge. Roof parapets of various heights were employed in the experiments. The parapet relative height (Hp/H) ranged from 0 to 0.24. The prototype parapet thickness was 3.125 inches• A four-channel scanivalve in conjunction with four pressure transducers and tygon tubing (28 cm in length and 1.59 mm in internal diameter) were employed in the pressure measurements. Effort was made to improve the frequency response of the scanivalve tubing system• Figure 4 shows the frequency response of the original and optimized systems• The optimized system was used in the measurements• As shown, this system had a transfer function close to unity (with deviation less t han 5%) for the frequency range up to 200 Hz. The system incorporated a restrictor, made of 8 cm long PVC tube with an internal diameter of approximately 0.4 mm, which was installed between the scanivalve and each of the four pressure transducers. The pressure signal was low-pass filtered with the frequency cut-off of 200 Hz. The sampling frequency was 400 Hz and the sampling time was about 41 s, which corresponded to the fullscale record length of about 17 min. The record length of the field data was 15 min.
3. R e s u l t s a n d d i s c u s s i o n 3.1. Pressure measurement The worst wind direction (wind direction resulting in the highest roof peak suction) was expected to be between 180° and 270 °. Therefore, the roof pressure
B. Bienkiewicz, Y. Sun~Local wind loading 15 9
1.59 mm
/"
1.5
\
/" /
0 "m
28cm
"N,k,k
/./
o u_
S'-. /I
/
.
0.5
.
.
i I
\,
\.
\. ~.
-
.
.
.
.
.
.
.
.
.
.
.
.
.
-
Tran~lucer
\
/
.
mm
9.Scm
Scanivalve ~
o/
I
~
15
-
-
-
-
-
-
-
-
"~,..,-~. :'~,- ............. -"
"[~-5% +_..5~
,]
"-N~ ~ 2g
0
scanlvalw
0
50
1O0
Tran~lucer
150 Frequency (Hz)
200
250
300
Fig. 4. Frequency response of pressure measurement system.
X/H 0.4
0.8
•~.I 0.4 >-
0.8
(a)
(b)
Fig. 5. Pressure distribution at ~.= 205 : (a) mean, (b) negative peak. d a t a w e r e t a k e n f o r w i n d d i r e c t i o n s (~) in t h i s r a n g e , w i t h a n i n c r e m e n t Representative distributions of the measured mean and negative peak s u r e s a t ~ = 2 0 5 :, 225 °, a n d 2 4 5 a r e s h o w n i n F i g s . 5 t h r o u g h 7. T h e pressure contours are based on the worst single peak measured at a
of 5. prespeak given
B. Bienkiewicz, Y. Sun~Local wind loading
16
XIH
0.4
0.8
"
>..
0.4
0.8
(a)
(b)
Fig. 6. P r e s s u r e d i s t r i b u t i o n a t ~ = 2 2 5 : (a) m e a n , (b) n e g a t i v e p e a k .
XlH 0.4
0.4
0.8 !
~
\''
0.8
(a) Fig.
(b)
7. Pressure distribution at ~= 245 : (a) mean. (b) negative peak.
p r e s s u r e tap. In e a c h case the c o n t o u r s of the m e a n a n d p e a k p r e s s u r e s are similar. F o r ~ e q u a l to 225 '~ (Fig. 6), the p r e s s u r e d i s t r i b u t i o n s are n e a r l y s y m m e t r i c a b o u t the b i s e c t o r of the w i n d w a r d r o o f corner. Two h i g h s u c t i o n r e g i o n s a p p e a r n e a r the r o o f edges, (X/H, Y/H)= (0.15, 0.02) a n d (0.02, 0.l 5). T h e o v e r a l l m a x i m u m m e a n a n d p e a k suction, and t h e m a x i m u m of p r e s s u r e s t a n d a r d d e v i a t i o n o c c u r at t a p No. 191 and are e q u a l to - 2 . 8 , - 8 , and 1.3, r e s p e c t i v e l y . B o t h the m e a n a n d p e a k s u c t i o n in the p r e s e n t s t u d y c o m p a r e r e a s o n a b l y well w i t h the d a t a r e p o r t e d in Ref. [3]. As the wind d i r e c t i o n was c h a n g e d (Figs. 5 t h r o u g h 7), the c e n t e r line of the h i g h s u c t i o n r e g i o n r o t a t e d in the s a m e sense as the wind. In o r d e r to
B. Bienhiewicz, Y. Sun/Local wind loading
17
I
-2
-4
Q. O
-6'
\
)-"
-8
\
/ \\
-10
-12 180
-~O---
Mean
_ _,c
Negative Peak
1/
'-~- -
,
,
I 210
,
,
[
240
i
i
i
270
Wind Direction
Fig. 8. Overall maximum mean and peak suction.
d e t e r m i n e the w o r s t wind direction, the overall m a x i m u m m e a n and p e a k s u c t i o n at e a c h wind d i r e c t i o n are plotted in Fig. 8. The d a t a show t h a t the w o r s t wind d i r e c t i o n is 235, w h e r e the m a x i m u m m e a n and p e a k suction are - 3 . 5 and 9.7, respectively. B o t h of the values o c c u r at tap No. 191. The c u r v e s in Fig. 8 are not s y m m e t r i c a b o u t ~.= 2 2 5 possibly due to the lack of s y m m e t r y in the l o c a t i o n of the p r e s s u r e taps. It should be m e n t i o n e d t h a t o t h e r a u t h o r s [9,10] r e p o r t e d similar a s y m m e t r y of the r o o f p r e s s u r e distributions. W h e n the p r e s s u r e m e a s u r e m e n t s were r e p e a t e d for a = 2 3 5 , the overall maximum peak suction reached 11.3 at tap No. 193. The effects of p a r a p e t s on the r o o f p r e s s u r e d i s t r i b u t i o n h a v e been investigated by v a r i o u s r e s e a r c h e r s , [2,3,5 7]. T h e s e effects were also addressed in the p r e s e n t study. F i g u r e s 9 and 10 show the p r e s s u r e d i s t r i b u t i o n s at the wind d i r e c t i o n ~ = 2 2 5 with low (Hp/H=O.04) and high (Hp/H=O.08) p a r a p e t s , respectively. W h e n these d a t a are c o m p a r e d with the result for the r o o f w i t h o u t a p a r a p e t , Fig. 6, it is evident t h a t the p r e s e n c e of p a r a p e t i n c r e a s e s the width of the high s u c t i o n edge region, w h i c h is in a g r e e m e n t with o t h e r r e p o r t e d results [2,3,5,6]. In addition, a lower p a r a p e t i n c r e a s e s the p r e s s u r e g r a d i e n t in the c o r n e r region, w h e r e a s a h i g h e r p a r a p e t d e c r e a s e s it. The effects of the p a r a p e t on the overall m a x i m u m m e a n and p e a k suction are i l l u s t r a t e d in Fig. 11, w h e r e the results of the p r e s e n t s t u d y are c o m p a r e d with the d a t a of Ref. [5]. It can be seen t h a t the p r e s e n c e of low p a r a p e t s (Hp/H< 0.04) results in an i n c r e a s e in t h e overall m a x i m u m m e a n and p e a k suction. H i g h p a r a p e t s (Hp/H> 0.08) r e d u c e b o t h the values. Therefore, o n l y p a r a p e t s w h i c h are high e n o u g h are beneficial.
B. Bienkiewicz, Y. Sun/Local wind loading
18
X/H
0.4
>..
0.8
,
.----~
0.4
0.8 (a)
(b)
Fig. 9. Pressure distribution at u = 225: with low parapet (Ho/H= 0.04): (a) mean, (b) negative peak. X/H
0.4
0.8
~0 0.4
>-
\
\ 0.8
(a)
I
(b)
Fig. 10. Pressure distribution at ~=225 with high parapet (H,/H=O,08): (a) mean, (b) negative peak.
3.2. Flow effects on roof pressure To i n v e s t i g a t e the effects of the c h a r a c t e r i s t i c s of the o n c o m i n g flow, the m e a n a n d p e a k p r e s s u r e s in t h e R1 a n d R2 flows a r e s h o w n in Figs. 12 and 13 for t w o r e p r e s e n t a t i v e l o c a t i o n s on t h e roof. T h e r o o f level t u r b u l e n c e intensity of t h e t w o flows was, r e s p e c t i v e l y , 15% a n d 20%. T h e i n t e g r a l l e n g t h scale of t h e R2 flow was 74% h i g h e r t h a n for the R1 flow. T h e d a t a in Figs. 12 a n d 13 s h o w t h a t the h i g h e r level of t u r b u l e n c e in the R2 flow r e s u l t s in h i g h e r
B. Bienkiewicz, Y. Sun~Local wind loading
1
19
•
-4
/
/
/
/ /
ZY
-6 J J
/
-8'
/
/
Mean
)~
-E'--
/ \
/ ,
/
/ \
Negative Peak
•
/
Ref. [5]
-10
I
-12 0.1
0.05
i
0.15
I 0.2
0.25
Relative Parapet Height, Hp/H Fig. 11. Overall maximum mean and peak suction.
-4
,
°
--.
Cp -6
£~
4',
Z:. - -
2.
,
-----
'
....
M e a n (R2) +Peak Mean
,
-10
•
Tap No. 11 12
, 180 °
(R2)
P e a k (R2) (R1)
÷ P e a k (R1) P e a k (R1) M e a n (Field' + P e a k (Field) P e a k (Field)
I 240 o
210 °
270 o
Wind Direction Fig. 12. Flow effects and comparison with field data (tap No. 11). pressure fluctuations, with the m a x i m u m increase in the fluctuations being 100%. The effects on the mean pressure are not significant. This sensitivity is more p r o n o u n c e d for the pressure taps close to the roof corner (e.g., tap No. 1]) than for the taps away from the roof edge (e.g., tap No. 85).
B. Bienkiewicz Y. Sun/Local wind loading
20
21
Cp '~,L
/
:i: *~ -4
'::::
,
~,
•
::::'
Mean {R2)
-6
-8
L 180
°
~
,
~
- P e a k {R2) M e a n (R1) + Peak (R1)
• (~
Mean (Field)
Tap No.85
- P e a k (R1)
;
I 210
• •
°
I 240
"
270
Wind Direction Fig. 13. Flow effects and comparison with field data (tap No. 85).
3.3. Comparison with field data Figures 12 and 13 also include the field data. By comparing the wind tunnel data with the field data, it is found that both the mean and positive peak pressures in the R2 flow are in a good agreement with the field data. However, for most wind directions, the magnitude of the negative peak pressure in the R2 flow is lower than the field data. For some wind directions, the wind tunnel data underestimate the negative peak pressure by a factor of two. This discrepancy has not been resolved. It may be attributed to possible non-stationarity effects in the field data and/or Reynolds number or flow modeling effects on the wind tunnel data. 3.4. Surface flow pattern The two conical vortices separating from the windward edges of the roof are responsible for the high suction in the roof corner region. In order to investigate the surface flow pattern in this region, flow visualization experiments were conducted. Figure 14 shows representative flow visualization results illustrating the effects of the roof parapet. The surface traces of the conical vortices and the traces of the zone of interference with the secondary vortices near the roof edge can be identified. The two vortices become wider and they begin to interact with each other near the roof corner as the parapet height is increased. These patterns are in good agreement with the pressure distributions in Figs. 6, 9, and 10. Figure 15 shows the reattachment lines (R) of the primary vortices and the secondary separation lines (S) inferred from Fig. 14a.
Fig. 14. Surface flow patterns (~=225'=): (a)
Hp/H=O,(b) Hp/H=O.04,(c) H,/H=O.08.
22
B. Bienkiewicz, Y. Sun~Local wind loading X/H
0.8
0.4
CS)
"7"
0.4
\
\ \
0.8
Fig. 15. Sketch of reattachment and secondary separation lines (~= 225~, H,/H= 0). X/H
0.4
0.8
"--- (s)
"T- 0.4 "
'
~
~
'
"-- (R)
0.8 Fig. 16. Relation between mean pressure distribution and surface flow pattern. A comparison of the results in Figs. 15 and the mean pressure distribution in Fig. 6 is presented in Fig. 16. As shown, the highest negative roof pressure occurs in the secondary vortex region while the highest pressure gradient is located in the reattachment region, between the reattachment and secondary separation lines.
B. Bienkiewicz, Y. Sun/Local wind loading
23
4. Conclusions The r e s u l t s of the p r e s e n t s t u d y can be s u m m a r i z e d as follows: (1) M a x i m u m s u c t i o n on the r o o f of the tested low-rise building a l w a y s occurred at a l o c a t i o n v e r y close to the w i n d w a r d edges. The w o r s t wind direction was 2 3 5 , w h e r e the m a g n i t u d e of the overall m a x i m u m m e a n and p e a k suction coefficients r e a c h e d - 3 . 5 and 11, respectively. (2) The effects of p a r a p e t s on the r o o f p r e s s u r e depended on the r e l a t i v e p a r a p e t height, defined as the r a t i o of p a r a p e t h e i g h t to building height: v e r y low p a r a p e t s i n c r e a s e d the overall m a x i m u m suction, w h e r e a s high p a r a p e t s d e c r e a s e d it. (3) The l o n g i t u d i n a l t u r b u l e n c e i n t e n s i t y and the i n t e g r a l scale of o n c o m i n g flow affected the r o o f p r e s s u r e distribution, especially in the region close to the r o o f edge. An i n c r e a s e in the t u r b u l e n c e i n t e n s i t y from 15% to 20% and the 74% i n c r e a s e in the i n t e g r a l l e n g t h scale, b o t h e v a l u a t e d at the r o o f level, r e s u l t e d in a significant i n c r e a s e in the f l u c t u a t i n g pressure, w i t h no s u b s t a n t i a l c h a n g e in the m e a n pressure. (4) W i n d - t u n n e l d a t a showed good a g r e e m e n t with field d a t a for the m e a n and positive p e a k pressures. The n e g a t i v e p e a k p r e s s u r e m e a s u r e d in the wind tunnel u n d e r e s t i m a t e d the field data. (5) S u r f a c e flow v i s u a l i z a t i o n r e v e a l e d the effects of r o o f p a r a p e t on the p a t t e r n s a s s o c i a t e d with the m a i n and the s e c o n d a r y v o r t i c e s of s e p a r a t i n g and r e a t t a c h i n g flow in the r o o f c o r n e r region. The h i g h e s t n e g a t i v e r o o f p r e s s u r e o c c u r r e d in the s e c o n d a r y v o r t e x region while the h i g h e s t pressure g r a d i e n t was located in the r e a t t a c h m e n t region.
Acknowledgements S u p p o r t of the s t u d y was provided by the N a t i o n a l Science F o u n d a t i o n C o o p e r a t i v e A g r e e m e n t BCS-8821542 (Colorado S t a t e U n i v e r s i t y / T e x a s T e c h U n i v e r s i t y C o o p e r a t i v e P r o g r a m in Wind Engineering). Field p r e s s u r e d a t a were provided by the r e s e a r c h g r o u p at T e x a s T e c h U n i v e r s i t y .
References [11 C. Kramer and H.J. Gerhardt, Wind pressures on roof of very low and very large industrial buildings, Proc. 8th Colloquium on Ind. Aerodyn., Aachen, 1989, Part 2, pp. 79 94. [2J G. Lythe and D. Surry, Wind loading of flat roofs with and without parapets, J. Wind Eng. Ind. Aerodyn., 11 (1983) 75 94. [3] R.J. Kind, Worst suction near edges of flat rooftops on low-rise buildings, J. Wind Eng. Ind. Aerodyn., 25 (1986) 31 47. [4] R.L. Wardlaw and R.J. Kind, Wind speeds for gravel scour and paver lifting on roofs, Proc. Specialty Conf., Hurricane Alicia: One Year Later, ASCE, 1984, pp. 245 260.
24
B. Bienkiewicz, Y, Sun/Local wind loading
[5] R.J. Kind, Worst suctions near edges of flat rooftops with parapets, J. Wind Eng. Ind. Aerodyn., 31 (1988) 251 264. [6] T. Stathopoulos, Wind pressures on low buildings with parapets, J. Struct. Div., ASCE, Vol. 108, No. ST12, 1982, pp. 2723 2736. [7] T. Stathopoulos and A. Baskaran, Wind pressures on fiat roof's with parapets, J. Struct. Eng., ASCE, Vol. 113, No. 11, 1987, pp. 2166 2180. [8] J.E. Cermak and L.C. Cochran, Physical modelling of the atmospheric surface layer, Preprints 8th Int. Conf. on Wind Eng., London, Ontario, Canada, July, 1991. [9] T. Stathopoulos, Wind pressures on flat roof edges and corners, Preprints 7th Int. Conf. on Wind Eng., Aachen, 1987, Vol. 3, pp. 39 48. [10] D. Surry, Recent and current research into wind loading of low buildings at the University of Western Ontario, Proc. 6th US National Conf. on Wind Eng., Houston, Texas, 1989, Vol. 2, pp. A9-40 A9-50.