2005 Whidbey Island DGPS Narrative.pdf

U.S. Department of Homeland Security United States Coast Guard

Civil Engineering Unit Oakland

Oakland, California

October 2005

Tower Inspection and Maintenance for DGPS Towers

Whidbey Island – Oak Harbor, Washington

Purpose: Support a Differential Global Positioning System Antenna

Contract No. HSCG83-05-D3-WF062 Record Nos. 24, 25, 27

TEC Project No. 05049.00

Inspected by: Adam Radke, P.E.

Peter Lundquist, P.E.

Doug Westcott

Deon Yirka

Inspection Date: August 4, 2005

USCG Civil Engineering Unit Oakland DGPS Towers, Record Nos. 24, 25, 27 Contract No. HSCG83-05-D3-WF062 Whidbey Island – Oak Harbor, Washington

Tower Engineering Corporation i 1601 5th Avenue, Suite 1600

(206) 926-0508 Fax (206) 622-8130 Seattle, WA 98101

TABLE OF CONTENTS

NARRATIVE REPORT

Introduction Structural Subsystem Guy Subsystem Verticality Antenna Subsystem Obstruction Lighting Subsystem Ladder, Safety Rail, and Platforms Maintenance Accomplished Recommended Maintenance References Maintenance Cost Estimates

APPENDIX A: PHOTOGRAPHS

Evaluation Photographs ....................................................................... A1-A17

APPENDIX B: TOWER & SITE INFO

Location Maps...................................................................................... B1-B2 Site Map............................................................................................... B3 120’ Guyed Tower Elevation and Plan ................................................. B4 North 30’ Self Support Tower Elevation and Plan ................................ B5 South 20’ Self Support Tower Elevation and Plan................................ B6 Tension Measurements........................................................................ B7 Deflection Measurements..................................................................... B8-B9 Graphical Representation of Twist and Deflection ...............................B10-B11 Definition Sketch For Deflection Calculations....................................... B12

APPENDIX C: FIELD DATA FORMS

Field Logs............................................................................................. C1 Tower Inspection Summaries............................................................... C2-C3

APPENDIX D: SUPPLEMENTAL INFO

Maintenance Information...................................................................... D1-D2

Tower Engineering Corporation 1 1601 5th Avenue, Suite 1600

(206) 926-0508; Fax (206) 622-8130 Seattle, WA 98101

NARRATIVE REPORT

INTRODUCTION

Tower Engineering Corporation (TEC) was retained by the U.S. Coast Guard Civil Engineering Unit Oakland to conduct a field visit and inspection of the 120-foot guyed, 30- and 20-foot self-supporting Differential GPS (DGPS) towers located near Oak Harbor, Wash. The inspection consisted of field observations of the overall condition of the towers and antenna systems. Our tower subcontractor, Seacomm Erectors, was also on site to perform minor maintenance, including retensioning and plumbing of the guyed tower.

The towers are located at the Whidbey Island Naval Air Station, south of the Gallery Golf Course, as seen in the tower location maps in Appendix B (B-1 and B-2). The towers were inspected on Aug. 4, 2005, by Adam Radke, P.E., and Peter Lundquist, P.E., of TEC. Doug Westcott and Deon Yirka of Seacomm Erectors were also present to perform maintenance and assist with the inspections. The weather was sunny and clear with calm winds of 0 to 5 mph. The temperature varied from 70 to 75 degrees.

The guyed tower is a 120-foot three-sided tower that acts as an antenna for the DGPS system (Photo No. 1). The tower was manufactured by Rohn and appears to be a galvanized Rohn 45GSR model. The face width of the towers is 1 foot, 5 inches, measured at the leg centerlines. A safety rail that provides access to the top of the tower is located on face A of the tower (Photo Nos. 3, 7).

Both the 30- and 20-foot self-supporting towers are three-sided and appear to be galvanized Rohn 45G sections. The north 30-foot tower supports two DGPS cone antennas, a GPS unit, and weather-measurement equipment (Photo Nos. 18-25). The south 20-foot tower supports two DGPS cone antennas and a GPS unit (Photo Nos. 26-31).

Refer to Appendix B (B-4 to B-6) for the typical tower orientation and elevation sketches.

STRUCTURAL SUBSYSTEM

The towers were each built on concrete foundations, which were found to be in satisfactory condition (Photo Nos. 2, 19, 27). The base isolator for the 120-foot guyed tower was inspected and found to in satisfactory condition (Photo Nos. 2, 5). No build up from arcing was found on the ball gap at the base of the tower (Photo No. 5). In order to determine if the gap is set at the correct distance the USCG procedure for determining proper ball gap distance should be followed. A copy of the pages of the 2002 USCG Tower Manual2 that describe this procedure can be found in Appendix D (D-1, D-2).

On the 120-foot tower at the leg splice above the 2nd guy level there is a slightly bent horizontal member on the south face. All other tower members and connections were found to be in good condition structurally, with no major bent or damaged members. No action is recommended for the towers’ structural components.

Tower Engineering Corporation 2 1601 5th Avenue, Suite 1600

GUY SUBSYSTEM

The 120-foot tower is guyed at three levels in three directions by 1/4- and 5/16-inch diameter guy wires looped through shackles at the 45-, 78-, and 122-foot levels (Photo Nos. 6-9, 12). The guy wires and insulators were found to be in satisfactory condition; however, rusted areas were found on the bolts and nuts in the shackles both above and below the four-tiered insulator assemblies (Photo Nos. 8, 10, 11). These insulator assemblies are located just under the guy pull-offs at the 45- and 78-foot guy levels and at the mid-guy wire length on the top guy wire. The areas of rust on the bolts and nuts that were accessible from the tower were cleaned and coated with a cold galvanizing compound (Photo No. 9). However, the areas of rust that were located a distance out on the guy wires were not cleaned or painted (Photo No. 10, 11). The shackles both above and below the four-tiered insulators should eventually be replaced.

The guy anchors were inspected and found to generally be in satisfactory condition. Areas of tall grass were cleared from around all the guy anchors, it is suggested that the guy anchors be kept clear (Photo No. 14, 16, 17). The attachment of the grounding wires to the buried grounding anchor was repaired at guy anchor B (Photo Nos. 14, 15).

Guy and antenna-curtain tensions were measured with a 5,000-pound-capacity dynamometer, guy grips, and a hand winch as part of this project and are tabulated in Appendix B (B-7) (Photo Nos. 16, 17). Of 9 initial tensions measured, all 9 were beyond the tolerance of 5 percent of design values allowed in the USCG FY05 Tower Inspection and Maintenance contract. The initial tensions in the bottom and middle guy wires were found to be 81 percent and 70 percent greater than the design tensions, respectively. The tensions in the bottom wires were lessened and brought to within 11 percent of the design tension. The guy-wire tensions were adjusted in conjunction with the plumbing of the tower with the final adjusted tensions ranging from 5 percent less to 11 percent greater than the design tensions.

VERTICALITY

The tower was observed with a digital theodolite from three directions before and after guy-tension adjustments in order to determine twist and deflection. The results of the initial tower alignment measurements showed deflections of 0.25, 0.45, and 1.13 inches at the 1st, 2nd, and 3rd guy levels, respectively. Segmental deflections of 0.25, 0.51, and 0.69 inches were measured for the 45-, 33-, and 44-foot segments below the 1st, 2nd, and 3rd guy levels, respectively. Maximum twist was measured to be 2.70 degrees. Compared to the USCG specified limitations1 of 0.31, 0.54, and

0.85 inches at the 1st, 2nd, and 3rd guy levels, only the top level was found to be outside the acceptable range. Additionally, segmental deflections measured between consecutive guy levels were found to be above the limits1 of 0.23 and 0.31 inches at the sections below the 2nd and 3rd guy levels, respectively.

After the tensions were adjusted the verticality of the tower was measured again and recorded, with the maximum deflection at the top of the tower being reduced to 0.85 inches and within the USCG limitations. However the maximum deflections were found to be 0.61 and 0.60 inches at the 1st and 2nd guy levels, respectively, which exceed the USCG limitations. Adjusted segmental deflections of 0.61, 0.16, and 0.37 inches were measured below the 1st, 2nd, and 3rd guy levels, respectively.

The 0.61-inch segmental deflection exceeds the 0.32-inch limitation, and the 0.37-inch deflection below the 3rd guy is slightly more than the 0.31-inch limitation set by the USCG. Maximum twist was measured to be 2.86 degrees.

Tower Engineering Corporation 3 1601 5th Avenue, Suite 1600

Refer to Appendix B (B-8 to B-12) for verticality and deflection results and deflected shapes.

ANTENNA SUBSYSTEM

The 120-foot guyed tower serves as the transmitting antenna for the DGPS system with the top-level guys serving as top loading elements (TLE) for the antenna. The top-level guys are bonded to the tower through clamps and are the insulated at approximately their midpoints from the guy wires below (Photo No. 13). As previously mentioned, rusted shackle nuts and bolts were found above and below the insulator assemblies. Rust was also found on the guy pull-offs at the 45-foot and 122-foot elevations. These areas of rust were cleaned with a wire brush and coated with cold galvanizing compound.

The tuning unit that is located near the base of the tower and feeds the antenna signal was inspected and found to be in satisfactory condition (Photo Nos. 2, 4).

The 30- and 20-foot towers support auxiliary equipment needed for the operation of the DGPS system (Photo Nos. 18, 26). On both self-supporting towers the top horizontal angle supporting the two DGPS cone antennas is severely rusted and corroded. Both angles were cleaned and painted with cold galvanizing compound, but corrosion is to the point where replacement is warranted (Photo Nos. 22, 23, 30, 31). The 30-foot tower also supports a variety of weather monitoring equipment, which is in good condition (Photo No. 20).

OBSTRUCTION LIGHTING SUBSYSTEM

The towers are not marked with obstruction lighting. Lighting is unnecessary on towers below 200 feet in height.

LADDER, SAFETY RAIL, AND PLATFORMS

There is no ladder attached to any of the towers, instead the tower members are used for climbing.

There is a safety rail attached to face A of the 120-foot guyed tower and it is generally in good condition (Photo Nos. 3, 7). There are no safety rails attached to the two self-supporting towers.

MAINTENANCE ACCOMPLISHED

Cleaning with a wire brush and touch-up painting with a cold galvanizing compound was performed on rusted areas found on the tower members and connections as well as the guy wire anchors (Photo Nos. 8, 9, 22-25, 28-31).

Areas of tall grass were cleared from around all the guy anchors (Photo Nos. 14, 16, 17).

Attachment of grounding wires to the buried grounding anchor at guy anchor B was repaired (Photo Nos. 14, 15).

Guy wire tensions that were found to be out of tolerance were adjusted to the best judgment of the engineers present. The towers were plumbed to within the appropriate tolerances, except for two instances where the segmental displacement was above the deflection limitation.

Tower Engineering Corporation 4 1601 5th Avenue, Suite 1600

RECOMMENDED MAINTENANCE

At the time of inspection there were no deficiencies or other conditions adversely effecting the structural integrity or functionality of the tower. However, the recommendations tabulated below should be considered in order to avoid more serious and costly future repairs or unsafe conditions.

It should be noted that nearly all these maintenance items will require off-air time for the DGPS system.

The following tabulation of recommended maintenance includes all items mentioned previously in this report, in order of priority.

Continuous Maintenance:

• Keep grass trimmed and clear at all guy anchors.

Within One Year:

• Adjust ball gaps per USCG manual, see Appendix D, D-2 and D-3. (Requires off-air time)

• At both self-supporting towers, replace the top horizontal angles support the DGPS Cone antennas.

Within Five Years or Next Major Repair Project:

• Replace (18) galvanized shackle bolts above and below the four-tiered insulators on all guy wires. Note: This service requires off-air time.

REFERENCES

1. "8.G.1.e Allowable Deflections," Tower Manual, COMDTINST M11000.4A, Department of

Transportation, U.S. Coast Guard, Washington, D.C. January 2002, p. 8-9.

2. "9.F Lightning Protection," Tower Manual, COMDTINST M11000.4A, Department of

Transportation, U.S. Coast Guard, Washington, D.C. January 2002, p. 9-7 – 9-8.