09 - Post Selection -

Post Selection

A fence post has two main jobs. It must support the fencing materials and any ice that can build up on it, and resist wind forces. In bending under the wind, it acts as a beam. Resisting the vertical forces, it acts as a column. The more bending stress from the wind, the less available capacity the post has to resist gravity forces and vice versa. The post needs to be checked for the combined forces in both full wind loading and the weight of the fencing, and for the lower wind speed for the ice loading case plus the weight of the fencing and ice, if applicable.

The charts below list the Bending Strength, F_a and Axial Strength, P_a of the posts at different heights. Also listed are the weight of the posts, D_p in lbs / ft.

Look for post types in the desired size range that have higher strengths than the f_w, fi, p_w & p_i values calculated in the previous section.

Entries with an * next to the Axial Strength, P_a values exceed the stability ratio for normal columns so they would only be used if the 5 × p_i value is less than the P_a value. (p_w or p_i / P_a ≤ 0.2)

To get the total axial load including the weight of the post, calculate the following values for each post being checked.

p_w‘ = p_w + (D_p × h)

p_i‘ = p_i + (D_p × h)

If the strength ratios add up to a value ≤ 1.0, the post is acceptable.

If f_w / F_a + p_w‘ / P_a ≤ 1.0, the post is OK for wind loading

If f_i / F_a + p_i‘ / P_a ≤ 1.0, the post is OK for wind & ice loading

Below is an example for a 10’ tall chain link fence in Pennsylvania. 105 mph wind, 40 mph gust speed for ice. (see Design Examples for full calcs)

Wind Loading

Wind force, f_w = 216 lbs Axial force, p_w = 100 lbs

Wind on ice Loading – lower wind speed, but larger wind area due to ice build up

Wind Force, f_i = 249 lbs

Axial Force including ice loading, p_i = 540 lbs 5 × p_i = 2,700 lbs

Ice loading controls. Desired post size is 2-7/8”. Look on the sample chart below for posts that have F_a & P_a values larger than the f_i and p_i values. If the post at that height has an * next to the P_a value, the 5 × p_i value should be used for selection. Also note the post weight per foot, D_p.

10’ tall posts being checked that may resist the worst case bending load (249 lbs) combined with axial load (540 lbs or *2,700 lbs)

Both the posts have an * next to the P_a value, so the P_a value has to be more than 5 × p_i which is 2,700 lbs for post selection only, not overall strength.

2-7/8” O.D. Schedule 40 / 30 ksi (Group 1A)

p_i‘ = p_i + (D_p × h) = 540 lbs + (5.8 lbs/ft × 10 ft) = 598 lbs

f_i / F_a + p_i’ / P_a = 249 / 408 + 598 / 3,418 = 0.79

Post is stressed to 79% of allowable capacity – OK

2-7/8” O.D. 40 Wt. / 50 ksi (Group 1C)

p_i‘ = p_i + (D_p × h) = 540 lbs + (4.6 lbs/ft × 10 ft) = 586 lbs

f_i / F_a + p_i’ / P_a = 249 / 547 + 586 / 2,818 = 0.66

Post is stressed to 66% of allowable capacity – OK

Post Strength Tables

ASTM F1043 Group IV / IV-L Circular Tubes – Residential / Light Industrial / Commercial

ASTM F1043 Group IA & IC Circular Tubes – Heavy Industrial

ASTM F2589 Square Steel Tubes (coming soon)