Bracing for uprights

[FL Guy]

Based on the picture below, would I be better off going with a longer or shorter bracing? Note the shorter bracing will give me more of the 45 deg angle to the horizontal and vertical surfaces. Going with a longer bracing does not. Ive only got 20"x20" of weldable surfaces to play with.
These legs are for a gantry crane im going to be putting up in the garage

 

[joe_padavano]

The answer is, "it depends". The longer braces better stabilize the uprights, but then the braces themselves become the weak link due to their greater length. The short braces don't add a lot of stability to the uprights. I'm hoping these drawings aren't to scale, because the width of the base member is too short for overall stability. You might want to start by mimicing the Harbor Freight gantry crane. I have one of those and it's pretty stable, even off-road.

 

[manwithtools]

The base of 20" x 20" is way too small unless it's being bolted to a concrete footing of proper design. IMHO you are in way over your head with this project. Either find someone to design / build this properly or buy a commercial one. It could end up being a very costly if not deadly outcome the way you are headed.

 

[FL Guy]

The base of 20" x 20" is way too small unless it's being bolted to a concrete footing of proper design. IMHO you are in way over your head with this project. Either find someone to design / build this properly or buy a commercial one. It could end up being a very costly if not deadly outcome the way you are headed.

There is a steel plate reinforced with rebar below the concrete surface. Each plate has (4) 5/8” bolts sticking through the concrete for the top plate to bolt too. The uprights/ bracing will be welded to the top plate

 

[readhead]

Is this a rotating gantry? Is it supported at the top? If it isn’t supported at the top then 20”x20” will barely support itself. More info would be helpful.

 

[kbs2244]

gantry cranes need a deep fondation or a support at the end of the boom to fight the leverage forces

 

[PCustoms]

If I were an engineer I would bet that some basic calculations would show that a 20x20 base on a 7x7 tube 11' tall is inadequate.

Where did you get 7" square tube, how thin is the wall?

 

[Snip]

In my own shop with me being the primary worker I would be ok with the OP's set up, welding to the plate ( as described in one of his earlier posts) secured to the floor, using the longer braces. I say this because the gantry should be used lifting straight up, not so much side loading. Now if you were to try to pull the load away from center, all bets are off. I would also suggest some gussets at the top to prevent side racking under load. Reminder this is only one old mans opinion. I have worked under a lot skechier.

 

[FL Guy]

Is this a rotating gantry? Is it supported at the top? If it isn’t supported at the top then 20”x20” will barely support itself. More info would be helpful.

Not rotating. Two uprights with an H beam sitting on top of it. Each up right is welded to a plate that is then bolted to the floor where another plate/ rebar was placed before concrete was poured. The bolts were welded to the plate buried in the concrete. I’ve got about 2” of thread sticking upwards.

 

[FL Guy]

gantry cranes need a deep fondation or a support at the end of the boom to fight the leverage forces

This is not a jib crane. See reply above

 

[FL Guy]

If I were an engineer I would bet that some basic calculations would show that a 20x20 base on a 7x7 tube 11' tall is inadequate.

Where did you get 7" square tube, how thin is the wall?

The walls are 1/4” thick.

 

[FL Guy]

In my own shop with me being the primary worker I would be ok with the OP's set up, welding to the plate ( as described in one of his earlier posts) secured to the floor, using the longer braces. I say this because the gantry should be used lifting straight up, not so much side loading. Now if you were to try to pull the load away from center, all bets are off. I would also suggest some gussets at the top to prevent side racking under load. Reminder this is only one old mans opinion. I have worked under a lot skechier.

Thanks. I already have plans for the bracing under the beam connected to the upright like you mentioned.
This will be in my shop, and I will be the only one in there working with it.
I’m not lifting thousands of pounds. Maybe 3-400..

 

[jack stand]

Stationary, 2 uprights, steel in the slab and 8 bolts...
Keeping within a couple of degrees of plumb in hoisting, I think you have it covered and if you're a good enough welder and practice common sense using it lower bracing shouldn't be too much concern how you brace it.

 

[firebirdparts]

Sounds fine, really; use the longer braces. Ignore the people that don't know what they're talking about.

 

[PCustoms]

Sounds fine, really; use the longer braces. Ignore the people that don't know what they're talking about.

@FL Guy what welder are you using?

 

[readhead]

The extra info tells me you are okay lifting straight up.

 

[FL Guy]

@FL Guy what welder are you using?

HF Omni Pro 220.

 

[FordTruckWench]

Each up right is welded to a plate that is then bolted to the floor where another plate/ rebar was placed before concrete was poured. The bolts were welded to the plate buried in the concrete. I’ve got about 2” of thread sticking upwards.

Go to someplace with traffic signals on a wide road. Take a look at how the poles are attached at the ground. You'll notice there is no bracing. A plate welded to the bottom of the pole works. Of course, that plate is really thick. You're probably going to use a much thinner plate. I'd suggest you weld on solid gussets, perhaps with a 45 degree top edge. The purpose of these is not to brace your 7x7 tube, but instead to create a robust connection from the plate to tube.

While you're looking at the traffic signal, also check how it is attached to the bolts. The plate on the pole won't be sitting on the ground. Instead it will be held elevated via nuts on the bolts.

 

[FordTruckWench]

Did some math:

Assume your 7" x 7" x 1/4" wall x 11' long tube is mounted horizontally with one end firmly in a wall. If you hang a 4,000 pound load from the unsupported end, that end of the tube will deflect (droop) 2" and the tube will experience a max of 36,000 psi of stress.* (Garden variety A36 steel has a strength of 36,000 psi.)

In other words, when mounted vertically, with nary any horizontal load, you will never bend the tubes. You need no bracing except that which is needed to keep the bottom plate from bending.

* My math if someone wants to check:
I = 7^3*7/12 - 6.5^3*6.5/12
S = I / 3.5
deflection = (132^3*4000)/(3*29000000*I)
stress = (132*4000)/S

 

[NUTTSGT]

Did some math:

Assume your 7" x 7" x 1/4" wall x 11' long tube is mounted horizontally with one end firmly in a wall. If you hang a 4,000 pound load from the unsupported end, that end of the tube will deflect (droop) 2" and the tube will experience a max of 36,000 psi of stress.* (Garden variety A36 steel has a strength of 36,000 psi.)

In other words, when mounted vertically, with nary any horizontal load, you will never bend the tubes. You need no bracing except that which is needed to keep the bottom plate from bending.

* My math if someone wants to check:
I = 7^3*7/12 - 6.5^3*6.5/12
S = I / 3.5
deflection = (132^3*4000)/(3*29000000*I)
stress = (132*4000)/S

He's using a H beam as the horizontal member. I'm not sure why you would even calculate numbers for him using the 7" sq tubing as such.

 

[jack stand]

^^^ hey, I'm sure that we've all answered after "speed reading" with a wacky response. I have.

 

[PCustoms]

He's using a H beam as the horizontal member. I'm not sure why you would even calculate numbers for him using the 7" sq tubing as such.

^^^ hey, I'm sure that we've all answered after "speed reading" with a wacky response. I have.

I don't think that was a wacky response, I think he made some (erroneous) assumptions/generalizations and did a quick static loading calculation. In pure column loading beams/tubes can take literally tons of weight, but once that has and torque or bending load the calculation gets more involved and the capacity drops.


@FL Guy good luck. Under absolutely perfect conditions you might be ok, but if anything is off (i.e. load not centered) this could get sketchy in a hurry and maim or kill you.

 

[dfiler2]

I would use the long brace and make it out of some plate steel so it is connected on the bottom and all the way along the vertical post.

 

[FordTruckWench]

He's using a H beam as the horizontal member. I'm not sure why you would even calculate numbers for him using the 7" sq tubing as such.

Yes I know that FL Guy is using the 7x7's as a vertical members. My calculation was to show that the 7x7's don't need any bracing even under absurd overloading conditions.

Here's an alternate scenario with the same numbers: The 11 foot tall 7x7 is firmly anchored into the floor. A 4000 pound load is applied horizontally at the top of the 7x7. This will stress the 7x7 to its limit.

In actual use, a 4000 pound load will act nearly parallel to the tube, with only a tiny horizontal component.

 

[FordTruckWench]

OK, lets calculate buckling of the 7x7 vertical members: The critical point is reached with a (vertical) load of 67,000 pounds.

buckling = 0.25 * Pi * 29000000 * I / 132^2

 

[tehach]

Failure mode is likely either the column to base plate connection, base plate deformation, or the anchor bolts themselves.