IowaIowa’’s Highs High--Mast Lighting Towers: Mast Lighting Towers: A Proactive Approach to a ProblemA Proactive Approach to a Problem

Bruce BrakkeIowa Department of Transportation

Terry J. Wipf, Brent M. Phares, Byung-Ik

Chang

Iowa State University

Robert J. ConnorLehigh University

The ProblemThe ProblemThe Problem

Nationwide failures of HMLT.Inadequate design specifications based upon dissimilar structures.

Iowa DOT BackgroundIowa DOT BackgroundIowa DOT Background

233 High-Mast Towers.Statewideinspection in 2000.

140 ft tower failed near Sioux City in 2003 –

Fracture in base plate weld (37 mph NW wind).

Subsequent statewide inspection–

Other cracks found.

Presenter�Presentation Notes�Photos would be helpful�

Iowa DOT BackgroundIowa DOT BackgroundIowa DOT BackgroundInvestigation by Dexter andothers – speculated wind induced fatigueSeveral different retrofitsdeveloped and are currentlybeing implementedDetermined that further investigation needed to fully understand the problem (e.g., monitoring needed).

Presenter�Presentation Notes�Lehigh and Wiss, Janney�

Questions to be AnsweredQuestions to be AnsweredQuestions to be Answered

Design for vortex shedding–

Mode(s) of vibration.

–

Loading profile.–

Wind/pole interaction characteristics

»

Roughness length (z).»

Lift coefficient (CL

).

Questions to be AnsweredQuestions to be AnsweredQuestions to be Answered

Design for gusting–

Mode(s) of vibration.

–

Loading profile.–

Wind/pole interaction characteristics

»

Roughness length (z).»

Coefficient of drag (CD

).

Overall GoalOverall GoalOverall Goal

Develop a comprehensive long-term inspection and maintenance program.Add to the body of knowledge related to the design of slender high-mast structures.

Monitoring System -

GeneralMonitoring System Monitoring System --

GeneralGeneral

Two poles being monitored.Hardware–

Two dataloggers.

–

Long-range wireless.–

Satellite communications.

–

24x7 data collection »

With triggering –

specified wind speeds.»

Rainflow

stress cycle counting.»

1 minute averages calculated on “the fly”.

Monitoring System –

Pole 1Monitoring System Monitoring System ––

Pole 1Pole 1

14 strain gages.4 accelerometers.1 video camera.1 anemometer–

Wind speed.

–

Wind direction.

Monitoring System –

Pole 2Monitoring System Monitoring System ––

Pole 2Pole 2

6 strain gages.3 anemometers–

Wind speed.

–

Wind direction.

Video

InternetAnemometer

33 ft

Satellite

Transceiver

UPS

Data Storage Server

115.8’

14 Strain gages

4 Accelerometers

45.3’

148’

10 ft

Modem

Wireless Router

CR9000C

WAP (Bridge)

CR5000WAP

(Bridge)

B E C

5.8’

148’

6 Strain gages

Data ProcessingData ProcessingData Processing

Extract RMS and average wind speed, stress range, and acceleration (1 minute).Vibration information.Basic wind rose information (speed/direction)–

Daily.

–

Monthly.–

Seasonally.

–

Yearly.

Sample ResultsSample ResultsSample Results

Cumulative Wind Information Chart Since 2004-October1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16N NNE NE ENE E ESE SE SSE S SSW SW WSE W WNW NW NNW

219.75 242.25 264.75 287.25 309.75 332.25 354.75 17.25 39.75 62.25 84.75 107.25 129.75 152.25 174.75 197.25231 253.5 276 298.5 321 343.5 6 28.5 51 73.5 96 118.5 141 163.5 186 208.5

Min Max 242.25 264.75 287.25 309.75 332.25 354.75 17.25 39.75 62.25 84.75 107.25 129.75 152.25 174.75 197.25 219.75 Sum %0 5 625 423 508 664 742 451 938 693 590 541 374 586 741 927 736 737 10276 12.35 10 1495 1286 1286 975 1285 961 3765 2921 1958 1226 907 1777 2225 3212 2815 1680 29774 35.510 15 1043 845 578 734 996 488 3727 3148 1531 945 1072 938 890 1803 3683 1615 24036 28.715 20 737 537 256 457 1124 292 2214 1550 555 504 400 367 280 678 2413 1176 13540 16.120 25 218 188 120 71 271 101 976 600 172 89 198 106 11 104 1190 363 4778 5.725 30 40 2 8 14 35 10 121 316 10 17 42 4 2 25 470 25 1141 1.430 35 1 2 2 1 3 1 3 78 0 0 20 0 0 1 160 1 273 0.335 40 0 0 0 0 0 0 0 20 0 0 1 0 0 0 30 0 51 0.140 45 0 0 0 0 0 0 0 9 0 0 0 0 0 0 0 0 9 0.045 50 0 0 0 0 0 0 0 7 0 0 0 0 0 0 0 0 7 0.050 100 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.0

4159 3283 2758 2916 4456 2304 11744 9342 4816 3322 3014 3778 4149 6750 11497 5597 83885 100% 5.0 3.9 3.3 3.5 5.3 2.7 14.0 11.1 5.7 4.0 3.6 4.5 4.9 8.0 13.7 6.7 100 64%

Sum

Dir. No.

Speed

0

5

10

15

20N

NNE

NE

ENE

E

ESE

SE

SSE

S

SSW

SW

WSE

W

WNW

NW

NNW

% of Frequency

0

3000

6000

9000

12000

15000

N NNE NE ENE E ESE SE SSE S SSW SW WSE W WNW NW NNWWind direction

Win

d di

rect

ion

freq

uenc

y

50 ~ 100 mph

45 ~ 50 mph

40 ~ 45 mph

35 ~40 mph

30 ~ 35 mph

25 ~30 mph

20 ~ 25 mph

15 ~20 mph

10 ~15 mph

5 ~ 10 mph

0 ~ 5 mph

Sample ResultsSample ResultsSample Results

S+60 Wind

0.0

2.0

4.0

6.0

8.0

10.0

0 10 20 30 40 501 min. mean wind speed (mph)

Peak

stre

ss (k

si)

S10

S12

S13

S14

Sample ResultsSample ResultsSample Results

S10

S12

S13

S14

Sample ResultsSample ResultsSample Results

Mode 2: Frequency = 1.3 HzVortex Shedding

0

30

60

90

Time

Win

d di

rect

ion Wind direction

17.3 degree / 4.42 degree

0

10

20

30

Time

Win

d Sp

eed

Wind Speed

5.25mph / 0.47 mph

S1-S12

S2-S14

S4-S13S3-S10

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

1/17/0519:01

1/17/0519:01

1/17/0519:01

1/17/0519:01

1/17/0519:01

1/17/0519:01

1/17/0519:01

1/17/0519:02

1/17/0519:02

Time

Stre

ss (k

si)

S43.25 ksi

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

1/17/0519:01

1/17/0519:01

1/17/0519:01

1/17/0519:01

1/17/0519:01

1/17/0519:01

1/17/0519:01

1/17/0519:02

1/17/0519:02

Time

Stre

ss (k

si)

S10

Mode 2: Frequency = =1.3 Hz 3.22 ksi

Findings –

GustingFindings Findings ––

GustingGusting

Response primarily in first mode (f 1= 0.3Hz).Overall, largest stress ranges are caused by natural wind gusts.Max stress range is approximately 14 ksi.Relatively few cycles–

“Slow”

vibration.

–

High wind speeds are not very frequent.

Findings –

Vortex SheddingFindings Findings ––

Vortex SheddingVortex Shedding

Significant vortex shedding observed in the second mode (f2 = 1.3 Hz)Occurs during steady wind speeds of 4 - 11 mph (but, somewhat dependent upon wind direction).Stress range = 1.5 ~ 3.3 ksiMaximum stress range caused by vortex shedding is approximately 3.3 ksi.Relatively high number of cycles–

“Fast”

vibration.–

Low wind speeds more common.

Future WorkFuture WorkFuture Work

Wind tunnel testing–

Scale models to validate field CD

, CL

.–

Wind profile pressure information.

Analytical modeling–

Validated with field/wind tunnel data.

–

Extrapolate to nationwide pole geometries.Develop proposed specification modifications.

AcknowledgementAcknowledgementAcknowledgement

Iowa Highway Research Board.Midwest Transportation Center.

Thank YouThank You

Questions?

Iowa’s High-Mast Lighting Towers: A Proactive Approach to a ProblemThe ProblemIowa DOT BackgroundIowa DOT BackgroundQuestions to be AnsweredQuestions to be AnsweredOverall GoalMonitoring System - GeneralMonitoring System – Pole 1Monitoring System – Pole 2 Data ProcessingSample ResultsSample ResultsSample ResultsSample ResultsFindings – GustingFindings – Vortex SheddingFuture WorkAcknowledgementThank You