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I'm building a 6' x 6.5' gate that will be mounted to a 2 3/8" schedule 40 pipe with sleeve hinges. The frame needs to be steel and the pickets will be Ceder. As per a prior question on weight tolerances I need to keep the entire gate at or under 105lbs. Given the weight of ceder pickets I have 35lbs available for the frame.

If the frame is a 5' x 6.5' rectangle with a diagonal cross brace then I need 31.2 lineal feet of steel. According to this chart that would allow for 16awg 1.5" square tubing.

would 16awg steel be heavy enough for this application? If not, are there other design choices I could adjust to make this work?

Solomon Bothwell
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  • How are you connecting the steel? Do you intend to weld it? Is bolting preferred? (16 gauge--awg is American wire gauge--is thin for butt joint welding unless you're a pretty skilled welder. MIG or TIG would typically be the process, with emphasis on TIG.) – popham Oct 29 '23 at 17:24
  • Aluminum or Magnesium would offer more strength-to-weight for a metal frame, but you've restricted to steel, so your options are likewise restricted. And the cedar will weigh more when it rains. – Ecnerwal Oct 29 '23 at 17:24
  • @popham I've got a MIG welder and was planning to to do butt joints – Solomon Bothwell Oct 29 '23 at 17:26
  • @Ecnerwal I'm limited to steel. As far as the ceder and rain, I live in a very dry climate so unless its enough water weight to bring the whole gate down i'm not worried about temporary sagging from water. – Solomon Bothwell Oct 29 '23 at 17:33
  • Scroll down to the rectangular tube calculator here, perhaps. https://www.roguefab.com/tube-calculator/ but it still depends what loads you assume to apply. I get acceptable results for 500 lbs evenly distributed - whether that's an accurate reflection of the loads that will be applied to the gate is unknown. Might be overkill, might be less than your wind loading... – Ecnerwal Oct 29 '23 at 17:35
  • That 5.2# per picket sounds heavy, so I double checked the Lowes website. Their pickets are green, so that 5.2# is for wet cedar. – popham Oct 29 '23 at 17:49
  • Using A500 rather than A36 will improve the loading somewhat without increasing the weight. 4130 (chrome-moly) would be a bit more than twice as strong as A36 for basically the same weight. Your linked chart is actually weights for round mechanical tube and pipe, NOT square section. Anyway, larger cross-section with thinner wall to keep the weight down is stronger for the same weight, as you can see in the calculator linked earlier. – Ecnerwal Oct 29 '23 at 18:14
  • This is a local supplier? You're sure you can get this 16 gauge steel tube? Strength to strength, I have a local supplier whose aluminum sections are price competitive with steel. Due to its lower density, you get substantially thicker walls. That combined with the better corrosion resistance, I like it better if that's locally available to you. (Zinc is very similar galvanically to aluminum, where you can use hot dip galvanized fasteners between steel and aluminum.) 1-1/2" x 1/8" is 0.80#/ft and $4.88/ft versus $3.75/ft for your steel section. – popham Oct 29 '23 at 20:50
  • Use angle instead of tube and save some weight? – Huesmann Oct 30 '23 at 13:06
  • @Huesmann, angle has terrible flexural properties for this. And it wants to bend along the line connecting the heel to the point splitting the toes, so any sag would look terrible. And with that diagonal tension tie, the bottom rail takes compression. Single angles hate compression. I love the idea for taking paint in a corrosive environment, but closed sections have nice torsional properties that he probably wants for this. – popham Oct 30 '23 at 15:49

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I think this can be much simpler than the steel frame. I think that two aluminum rails and some steel close to the post will work. My knee jerk reservation is that the gate will warp out of plane. You push on the top and the bottom sits there like a lump and then follows after a lag time. You close the gate and it naturally deforms out of plane to look dilapidated. For exposition purposes, imagine assembling this gate from the post out to the free edge of the gate.

First you've got your two hinges. Move one hinge, the other just sits there. Alone, the hinges provide no warping resistance to the free edge. Welding a vertical steel bar between the hinges, you synchronize their movement. When one hinge moves, so too must the other. The torsional stiffness of this vertical steel bar determines just how synchronized the hinges are. Zero stiffness? No synchronicity. Infinite stiffness? Perfect synchronicity. This bar is so close to the post that it contributes very little to the post's toppling load. This location, then, is the ideal location for heavy components.

Now moving on to the gate's rails, where I propose constructing these rails out of aluminum. The problem is transferring the stiffness of your synchronized hinges out to the gate's free edge. In the limit, perfectly flexible connections between the aluminum and steel are new unsynchronized hinges. I propose a steel connection plate welded to the vertical bar for each of the rails. If the steel overlaps, say 8" of the rail, then bolts will maintain a nice tight connection. The thickness and height of these steel plates will determine how much force applied at the gate's free edge would permanently bend them. I can provide sizes for these steel plates later that match the bending strength of the rails. Any strength beyond that is wasteful.

It's tempting to put a vertical member out at the gate's free edge, but way out at the edge is the least efficient place for that. The biggest problem to my mind is the curving of green fence pickets as they bake in the sun. My instinct is to compute the bending moment it takes to straighten a slightly curved board and add this as a stiffness constraint on the rail torsion. Maybe choose a slightly larger rail if a problem looks possible. It probably makes more sense, though, to just do some quality control on the boards. You don't have to wait for the boards to dry totally, but allow enough drying time so that the board outer fibers dry and can restrain movement as the rest of the board dries. Just a week or two makes a profound difference.

You should figure out what material you have access to:

  • 1-1/2"x1/8" steel almost certainly works and gives you a budget of 50# for the stuff by the post as long as the net eccentricity is 4".

  • 1-1/2"x1/8" aluminum probably works and gives you a budget of 240# for the stuff by the post as long as the net eccentricity is 4". This is for structural grade aluminum like 6061-T6, not "architectural" aluminum (which is weaker and more expensive, although manufactured to tighter dimensional tolerances kinda like cold rolled steel).

popham
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  • I'm using sleeve hinges which you weld to the horizontal member closest to the post. So I've got the exact hinge synchronization you described.

    1.5" x 1/8" steel horizontal members would be about 22lbs of weight and 6061 aluminum would be 10.4lbs.

    Why does aluminum give so much more budget for weight close to the post?

     – Solomon Bothwell Oct 30 '23 at 02:16
  • @Solomon Bothwell, like I said, you need to link the two of them with something with lots of torsional stiffness (closed hollow sections like tube steel would be best). Depending on the thickness of the sleeves, the weld distortion could hurt how smoothly your gate swings. I hope they provided instructions on weld size, length, placement, etc. Be sure to think it through, where increased resistance implies that framing should be heavier. – popham Oct 30 '23 at 02:26
  • The post's toppling load is a moment, not a force (ft-pound units, not pound units). That moment is statically equivalent to 105# of downward force applied at the gate's center. 210.# located at 39"/2 is statically equivalent to 105# located at 39". (105#-70#-31#)(39"/4") versus (105#-70#-10.4#)(39"/4")--I computed 0.285(1.5^2-1.25^2)6.5212 for the steel. – popham Oct 30 '23 at 02:58
  • @Solomon Bothwell, when you figure out what material you can use and want to use, then say so. I don't want to take the time checking a design that can't be built. – popham Oct 30 '23 at 19:33
  • sorry for the slow reply. I needed a vertical member on the free end of the gate for my latching mechanism. I think I found a solution to brace the post. Perpendicular to the gate is a concrete driveway and chainlink fence. The concrete spills over onto my side of the fence by about 6 inches. I think its enough that I anchor a post to it and then use that to brace the gate post. This second post would be 12 awg 1.75" steel square tube. – Solomon Bothwell Nov 06 '23 at 18:28
  • @Solomon Bothwell, I anticipated a piece of 2x2 cedar at the free end between rails. No bracing for the top rail means that it wants to sag under gravity. That sag, resisted by the pickets, could induce warping in the pickets as their moisture levels track seasonal humidity. – popham Nov 06 '23 at 18:40
  • @Solomon Bothwell, since the gate probably spends 99% of its time closed, this is the most likely configuration where somebody climbs on it. A judiciously chosen and properly installed latch mechanism could hold the weight in the closed position if there's only one gate. If two gates, then you could design a rod that engages into the ground with a collar installed on the rod to carry the weight in the closed position. If you keep the gates open sometimes and the ground level cooperates, you could install a rod receiver in the open position that also holds up the weight. – popham Nov 06 '23 at 18:52
  • @Solomon Bothwell, tube is relatively weak in bending compared to tension and compression, so the optimal configuration for your kicker would be diagonally from the ground to the post. Just remember that tension is the more likely scenario, so the weight of its foundation and continuity with its foundation are critical. "Nelson shear studs" are typically used for continuity like that--you just need a blob at the end so that the steel can't slide. The weight of the concrete blob times the distance from center of gate post to center of blob is the moment resistance provided by your new blob. – popham Nov 06 '23 at 19:03
  • For context this is part of a double gate where the other side is braced to the side of a building.

    I like your idea of supporting with a rod in the closed position. I was planning to add something like that anyways as part of my latching system.

     – Solomon Bothwell Nov 06 '23 at 19:22