ABSTRACT

The stress loading herein uses the "method of joints" type of analysis and is presented in load as pounds of force in bending moment. All joints are considered simple single point cantilevers and use conventional load polarities in terms of tension and compression and are expressed as foot pounds. Only first order load distribution is used to ensure an adequate over design and also to ensure that the supporting members are not stressed beyond their modulus of elasticity (maximum allowable load).

ASSUMPTIONS AND FACTORS

Below is a list of assumptions and factors used in this analysis.

1) The antenna manufacturers’ load ratings are accurate and true.

2) Failures are wind related and antennae failure will occur first.

3) Use no more than 0.6 load distribution factor per support level.

4) The design consideration is 80 mph wind loading.

5) The area is subject to ice build up during the winter. Use 1.2 as factor for ¼" ice loading.

6) Use EIA wind loading of 20 pounds / square ft. of rated antenna area for 80 mph.

7) Top load on the roof mounted tripod guy support is equal to the top load of an earth mounted guy support.

CRITICAL LOAD MEMBER DEFINITIONS

Load members are identified on the drawing as having an [n] designation, whereas, "n" is a number identifier. Below is a description and area value of each critical load member.

1 - Area = 0.3 square ft. Comet GP-15 tri-band vertical omnidirectional antenna.

2 - Area = 0.7 square ft. Cushcraft AS-270S dual band vertical yagi antenna.

3 - Area = 5.7 square ft. Mosley TA-33M tri-band horizontal yagi antenna.

4 - Area = 0.5 square ft. 1 ¼" hot galvanized steel mast plus antenna feed lines.

5 - Area = 0.2 square ft. Unsupported mast below lowest antenna plus unsupported tower.

CRITICAL LOAD POINT DEFINITIONS

Critical load points were determined as point of failures of the antenna support structure and do not preclude the failure of the antennas. Load points are identified on the drawing as having an {x} designation, whereas, "x" is an alphabetic letter identifier. Below is a description each critical load point.

A - The point where the mast engages the supporting tower.

B - The point where the uppermost guy wire is connected to the supporting tower.

C - The point where the lowermost guy wire is connected to the supporting tower.

D - The point where the supporting tower is attached to the dwelling.

E - The point where the supporting tower connects to the tower foundation.

F - The point where a single plane guy wire set is attached to an earth mounted guy support.

G - The point where an earth mounted guy support engages the earth.

H - The points where the roof mounted tripod are attached to the roof.

LOADING

1) Determine static load for each load member at design wind load of 80 mph.

SL{n} = square ft of {n} * 20

SL{1} = 0.3 * 20 = 6.0

SL{2} = 0.7 * 20 = 14.0

SL{3} = 5.7 * 20 = 114.0

SL{4} = 0.5 * 20 = 10.0

SL{5} = 0.2 * 20 = 4.0

2) Determine bending moment at point {A} for each load member.

SM{n} = (SL{n} * length in feet from A) + SL{n}.

SM{1} = 6.0 * 9.0 = 54.0 + 6.0 = 60.0

SM{2} = 14.0 * 5.0 = 70.0 + 14.0 = 84.0

SM{3} = 114.0 * 0.6 = 57.0 + 114.0 = 171.0

SM{4} = 10.0 * 0.0 = 0.0 + 10.0 = 10.0

3) Determine initial bending moment at point {A}.

IM{A} = SM{1} + SM{2} + …

IM{A} = 60.0 + 84.0 + 171.0 + 10.0 = 325.0

4) Determine final bending moment at point {A} using ice loading factor.

FM{A} = IM{A} * 1.2

FM{A} = 325.0 * 1.2 = 390.0

5) Determine isolated static load for point {B} using load member [5] static load and ice loading factor.

ISL{B} = SL[5] * 1.2

ISL{B} = 4.0 * 1.2 = 4.8

6) Determine final bending moment for point {B}.

FM{B} = FM{A} + ISL{B}

FM{B} = 390.0 + 4.8 = 394.8

7) Determine individual maximum guy wire load at point {B} using 0.6 load distribution factor.

GL{B} = FM{B} * 0.6

GL{B} = 394.8 * 0.6 = 236.88

8) Determine individual maximum guy wire load at point {C} using 0.6 load distribution factor.

GL{C} = GL{B} * 0.6

GL{C} = 236.88 * 0.6 = 142.13

9) Determine maximum side load at point {D} using 0.6 load distribution factor.

SL{D} = GL{C} * 0.6

SL{D} = 142.13 * 0.6 = 85.88

10) Determine maximum side load at point {E} using 0.6 load distribution factor.

SL{E} = SL{D} * 0.6

SL{E} = 85.88 * 0.6 = 51.53

11) Determine maximum bending moment at point {F}.

FM{F} = FM{B}

FM{F} = 394.8

12) Determine maximum bending moment at point {G}.

FM{G} = (FM{F} * length in feet from G) + FM{F}

FM{F} = 394.8 * 6.0 = 2368.8 + 394.8 = 2763.6

13) Determine maximum bending moment at points {H}.

FM{H} = FM{F} / 3

FM{H} = 394.8 / 3 = 131.6 (2 in tension, 1 in compression)