Calculating load on gate post for brace
When our son built our gates last year, replacing old funky stuff, he put in a driveway gate for us: each half 6 feet wide, 5″8″ tall at its highest (the gate top describes a catenary curve), 2″ welded steel perimeter frame with a mid-height cross member, sheathed one side in redwood 1″x6″. I’m not sure of the total weight (I know he put it in himself, but then he inherited somehow my railroad-mechanic uncle’s knowledge of moving big stuff). It pivots on pintle hinges on a 6×6 post.
One side ties to full-height fence, and is fine; the other post is free-standing. On the free-standing side, he sank a concrete-filled deep hole with a 3/4″ stud sticking up. We both predicted the post on that side would sag, but decided to see before investing in a brace from post to anchor. It did. I tightened the pintle on the top; it sagged again. Time to face up to a brace, which will be chain and turnbuckle.
I’m now trying to figure out how to calculate load so I can decide if 3/8″ chain (working load 2,650 pounds) and turnbuckle, which is all our hardware store carries, is enough, or if I have to go to town to get 1/2″ chain (working load 4,500 pounds, which should be almost enough to pick up my 1972 truck).
Replies
You haven't explained your solution well enough to answer your question. Determining if the chain will be strong enough depends on the geometry in which it's applied. It's kinda like Archimedes said, "give me a lever long enough and I can move the world". How tall is the point where you'll attach the chain? How far away will you anchor the chain?
For the sake of continuing the discussion:
Redwood weight 450 kg / cubic meter which is ~0.016 #/cubic inch.
a 6'x6' wood gate 1" thick weighs ~85lbs.
2x2 1/8thick steel tubing weight 3 #/ft and I'd guess that the mid height cross member is also 6' long that means about 30'feet (~6' per side) which results in 90# of steel.
85# + 90# = 175#, let's call that #200 to be sure about hardware, finishes, etc. etc. All of this evenly distributed so that the center of gravity is in the center of the gate.
If your chain is anchored where the gate hinges (5' off the ground say) and then extends that same distance along the ground horizontally, then the tension in the chain would be the hinge load divided by the cosine of 45 degrees (~0.707) My rusty free body diagramming and sum of moments skills has the hinge load at ~150lbs so the load in the chain would be #150/0.707 = 215#.
The 3/8" chain would be about 10 times stronger than the load applied. Therefore I conclude that the 3/8" will work even if my calcs and assumptions are off.
I'd say your calcs are good... as long as the gate is in the closed postion and the line of tension remains in line with the load... but as soon as you open the gate... you are right back where you started with a weak post...
dig up the damn post... put a post in that will hold the gate... the chain is a fix for a poor design at best
p
I totally agree with you about the chain idea being a poor design. I only addressed the question of whether the chain would carry the applied load. The applicability of the design is another issue all together.
I've never heard of any positioning of a "chain & turnbuckle" other than diagonal - from the top of the post hinge down to the far bottom corner.
To the OP - SS cable. Junkhound probably has some aircraft stuff to let you use.
Making a lot of assumptioins here, ie, your gate weighs about 500 lbs, is 7 ft wide, and the chain will anchor to the top of the post, about 6 ft high and pull on an angle of 45 degs, I would estimate a line tension of about 425 lbs in the chain, which would be fine capacity-wise. Make sure the turnbuckle is rated same grade as the chain, and make sure everything has a weather-proof finish (galvanized or better), or is well painted. One note, 1/2" chain may also look more to scale in that application, if it's visible?
I agree with Ponytl. I'd beef up the post and foundation.
A 3x3 0.10 wall steel tube, with a 4-ft bury depth, in a 2-ft concrete foundation, will hold for sure.
If you need/want the look of a wooden post, then clad it.
I have built dozens of double gates at least 10 ' wide or greater and have found by far the best technique is to put a wheeled trailer jack on both sides.
They make your gate adjustable and remove sag weight and easy to open and close.
ANDYSZ2
WHY DO I HAVE TO EXPLAIN TO FRIENDS AND FAMILY THAT BEING A SOLE PROPRIETOR IS A REAL JOB?
REMODELER/PUNCHOUT SPECIALIST
Update: I installed 3/8" chain and a turnbuckle today, and discovered something I should have realized before: the post was and is carrying most of the load just fine; the bracing was needed to keep it from tilting. I brought out about 75% of the problem when hand tightening the turnbuckle, finishing up with only a little more grunt with pliers.
I probably could have used vineyard wire.
The chain looks more appropriate, though - although LOML is not sure she likes it, but I've convinced her it's necessary, so please don't anybody tell her otherwise.