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    Zircon's Avatar
    Zircon Posts: 1, Reputation: 1
    New Member

    Jun 9, 2006, 02:57 PM
    Wind Loads
    Does anyone out there know how to calculate the wind load on a wooden fence? I have used 12 foot 4 x 4 pressure treated posts set in 4 feet of concrete spaced 8 feet apart. The concrete was poured in a 12 inch in diameter concrete tube. I used a strong concrete mix in the ratio 3:2:2. I want to use the full length of the posts that project above the concrete (8 feet) to construct a solid board (1/2 inch spacing between boards) privacy fence. However, before I do decide to use the full 8 feet rather than cutting the posts to some shorter length, I would like to know what the wind load will be on such a fence. There must be some kind of formula out there somewhere to tell me how to do this. I can calculate the weight of the concrete no problem and the weight of the actual fence (dead load) is relatively easy to obtain as well. I'm not concerned that the concrete will crack or uproot(?). What I am concerned about is whether the 4 x4 's can break. I was planning on using three 2 x4 rails for each section attached to the posts with fence clips (similiar to 2 x4 joist hangers). Thanks in advance for any help you might be able to provide.
    GuruVT's Avatar
    GuruVT Posts: 1, Reputation: 1
    New Member

    Jul 22, 2006, 04:09 AM
    Ok, first of all, I didn't find this problem easy and I'd like to get a second opinion before I claim to be correct in my calculations. I expect a civil engineer receives more training in this area than I have, but I think I figured it out. I found an equation online for wind pressure on a vertical wall and I rearranged it for your application. I came up with an equation for maximum stress (psi) in each post. The derivation is too complex to explain here, but here is the end result:

    (I developed an equation for moment, integrated it over the surface, and divided by the section modulus to get stress)

    Stress = 0.000234*(Height^3)*(Wind Speed^2)
    This is assuming 8-foot post spacing, height in feet, and wind speed in mph

    Stress = 0.000234*(8ft^2)*(75mph^2) = 675 psi

    A chart indicates that the maximum stress for structural timber is approximately 5000 psi. Therefore, you shouldn't have any problems based on that calculation. If that equation is correct, the fence should be able to withstand winds in excess of 200 mph. (Which seems pretty unlikely to me, but I can't find my mistake)

    Someone please check me on my calculation or offer a different solution.

    This calculation is based on information from these websites:
    Blackfellow's Avatar
    Blackfellow Posts: 1, Reputation: 1
    New Member

    Sep 5, 2009, 11:09 AM
    My fence was 4X4 posts in 12 in dia holes, in concrete (Quickcrete), on 7 ft centers, 2X12 rotboards at the base and nailed to the posts, 2X4 runners placed horizontally along rotboard (nailed to it), runner in the middle of the posts, 15 ft 2X4 top plates capped by overlaping 15 ft 2X6 along the tops of the posts to eleminate joint movement, cedar pickets all along the fence perpendicular to the runners, nailed to the runners and the top plate... Hurricane Ike, with winds far less than 200 MPH sheared off the posts at the top of the concrete base. I suggest at least 4X6 posts with the long axis perpendicular to the fence line, on 5 ft centers.
    dfence's Avatar
    dfence Posts: 1, Reputation: 1
    New Member

    Jan 19, 2010, 11:45 AM
    I'm not sure I'd rely on any of those answers, and would recommend getting a civil engineer involved if you live where there are high winds. I've got an engineering degree in aeronautical, not civil, and there are a lot of factors in play here. What you're trying to find is the total torque at the point of entry just above the footings on each post under a given wind load and see if it will snap off the 4x4s, right? The best answer is to research what others have used and ask if its stood up to winds in your area. If you want to do this mathematically, the angle of the wind from perpendicular in both the vertical and horizontal axis, the gaps in your boards, span of fence sections, space under the fence, and even the finish of the boards and trim will all play a role in establishing the pressure on the fence surface which is translated to this moment at the footing. Even when all these are known, there is the matter of soil saturation and composition. That is, will the footing stay upright if you build the fence strong enough to withstand record winds. Don't forget that if you have gates their span may create greater moment on the posts.

    Regarding the structural timber maximum stress of 5000 psi quoted above: Isn't that if you use the 4x4 to pull or lift something? Stress and compression numbers for any material are not the same as the torque it can stand before breaking if you try to bend it. I don't have numbers for this stuff, but what you need to know is how much force is required to snap a 4x4 if you had it supported at each end and pushed down in the middle.
    michiganhunter's Avatar
    michiganhunter Posts: 1, Reputation: 1
    New Member

    Feb 28, 2013, 05:17 PM
    I am working on a quote right now for a potential customer that had the same problem. After a strong wind storm (70mph gusts) this customer had 4 eight foot sections snap off 12 inches above ground. The 4x4s look like snapped like you would see in a dry twig snapped. I am researching now to see if 4x4 galvanized steel posts will work and use shadowbox style panels made with treated 1x6x6 with a 6 inch space from ground level to bottom of the boards. This should give privacy yet allow air flow, I would think. However it will be more labor intensive because of having to use carridge bolts to attached the 2x4 cross bracing.

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