Fence Design Variables

B = Length of straight line fence run in feet

h = Overall height of the fence in feet

g = Gap (if any) between the bottom of the fence and the ground in feet

s = h – g = Height of the fencing in feet

L = Post spacing in feet

B/s = Aspect Ratio

s/h = Clearance Ratio

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Velocity Pressure Exposure Coefficient, K_z

Wind forces increase based on the height of the fence.  The Velocity Pressure Exposure Coefficient, K_z is used to adjust the wind force based on the fence height and Exposure Category of the installation site.

For open fencing, (ε ≤ 0.7) taller than 15′, use ½h to determine the K_z value.  If ice loading raises the iced solidity ratio, ε’ greater than 0.7 for open fencing over 15′, use K_zw for Wind loading based on ½h and K_zi for Wind on Ice loading based on the full height, h.  (see below for ε & ε’ definitions)

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Wind and Ice variables

The wind force is mainly driven by the surface area and aerodynamics of the fencing material.  The dead load of the fencing material affects the load bearing capacity of the post and the footing.  Ice buildup can increase the wind area as well as add additional weight.  The following variables are used to determine the forces the posts and footings must support.

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Solidity Ratios, ε & ε’

Solidity Ratio, ε = The net area of the fencing divided by the gross area

Iced Solidity Ratio, ε’ = The net area of the iced fencing divided by the gross area

Gross area, A_g = Fencing height, s times the post spacing, L

Net area, A_n = The actual solid or iced area of the fencing material between the post centers.

For open fencing with wind screen, use just the wind screen area, neglecting the fencing in the un-iced condition.

For wind screen with flaps, still count it as solid as the flaps only let air through in one direction.

For ε and/or ε’ ≤ 0.7, the fence is considered an open fence / lattice framework / single plane open frame.

For ε and/or ε’ > 0.7, the fence is considered a solid fence / solid wall.

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Inverted Fence Opening Reduction Factor, R₁  (See Solid Wall Tables, Note 2)

This is a modifier for mostly solid fencing to adjust the wind force based on the open areas of the fencing.  The Note 2 reduction factor is inverted for this guide to allow the solid and open fencing force calculations to use the same formulas.  The value may be different in the normal condition, R_1w vs the iced condition, R_1i, depending on their respective Solidity Ratios, ε & ε’.

For an open fence (e.g. chain link) with a Solidity Ratio, ε and/or ε’ ≤ 0.7, R₁ = 1.0.

For a solid fence with a Solidity Ratio, ε and/or ε’ = 1.0, R₁ = 1.0.

For a mostly solid fence with a Solidity Ratio, ε and/or ε’ > 0.7, R₁ values are shown on the table below. 

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Force Height Adjustment Factor, F_h (see Solid Wall Tables, Note 3)

The post strength tables are based on the wind force being applied at the center of the overall fence height, ½ h. 

The Solid Wall Table, Note 3 requires the force to be applied 5% of the total height above the center of the post for solid / mostly solid fences with no gap at the bottom (s/h = 1.0). 

For fences with gaps at the bottom, as the gap gets larger, the center of the fencing area, s rises above the center of the overall fence height, which raises the effective force application height.

The Force Height Adjustment Factor, F_h increases the calculated wind force to give equivalent results compared to raising the force application height.

F_hw would only be different from F_hi if ice would be expected to close up the gap at the bottom of the fence (if any).

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Dead Load of fencing material, D_m

This is the weight per square foot of the fencing material.  Take the weight of all the materials between the posts and divide it by the gross area.

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Dead Load of Ice, D_i

This is the weight per square foot of the ice that can build up on the fencing.  The Chain Link and Ornamental / Wrought Iron Fencing tables list estimated ice loading.  For other fencing types, this would be calculated per ASCE 7 §10.  See Design Examples.

The tables below show the ε, ε’, C_fw, C_fi, D_m and D_i values for common fence types.

Chain Link Fencing

Mesh Sizes

2-3/8”         2-1/4”        2”         1-3/4”        1-1/2”

1-1/4”        1”        5/8”      1/2”        3/8”

Note:  Chain link with privacy slats or wind screen, ε and ε‘ should be set to 1.0, and the C_fw & C_fi values must be taken from the Case A & Case C Solid Wall Tables.

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ASTM F2408 / F2589 / F2814 / F2957 – Ornamental / “Wrought Iron” Fencing

ASTM F2453 – Wire Mesh Fencing w/ openings ≤ than 6 in²

ASTM F2919 – Wire Mesh Fencing w/ openings > than 6 in² – coming soon

ASTM F1267 / F2780 – Expanded Metal Fencing

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Next – C_f Values