Foundation requirements for 30' x 30' by ~2T bridge crane?

JasonPAtkins said:

Hi all!

My non-profit shop construction project in West Africa is on hold as I'm back in the U.S. to be with my wife's family because her mom is sick. I'm trying to finish up the drawings for the building so that when we're able to go back, we can hit the ground running.

While I was recently over there, I was able to scavenge some steel beams for the building which have opened up some new possibilities. I don't have enough to replace all of the building's reinforced concrete columns with steel, but I do have enough to replace a bunch of the central ones, enough to make it worth the effort to design to allow for a future bridge crane, I think.

Here's a render of the shop (13mb): https://drive.google.com/open?id=1yaF2UKLx1Dw-3kqoWzFsZvmcr69jmoPu

Basically, the large central area will be the sawing, forming, and fab area. The long room furthest away is the machining room. The red columns are the ones that I can now replace with steel. (The 9th one, that doesn't have an opposing twin, is just to get rid of that jib crane with one attached to the beam, just strong enough to hold a small mig welder).

Our normal method of construction would have each of those columns sitting on top of a 2' x 2' footing that's sunk about 2' deep. What I'm wondering is if I need to beef that up if those newly-steel columns were to support a 2 ton bridge crane. I have a very helpful installation doc from one of the crane companies which clearly lists the foundation requirements for each size of jib crane they sell, but I haven't been able to find such a document related to bridge cranes. The span is about 30' and the total travel of the bridge would also be about 30' (supported every ~10') in this scenario.

So, while I'm not asking for an engineering stamp from anyone here - I know you've all been around a lot of cranes. Do you know if any special foundation requirements are normal for a relatively-light-as-far-as-a-bridge-crane situation like this? Also, does anyone have any experience for what beam size would be used for the two stationary transverse beams, and for the single movable one?

As a side note, I'm also considering finding a few more shorter beams to replace ones on the outside wall of the machining room which would allow a second, shorter-spanned and lighter duty (maybe 1T) bridge in the machining room for moving heavy parts around. The problem is that the outer walls are only 10' and I'm afraid a bridge in there will be really low by the time it's all mounted.

Both of these cranes also have the unfortunate disadvantage that I don't have enough columns to extend them far enough to be able to park against a wall, so they'll be in the way, hanging significantly lower than the point of the roof trusses, in the middle of the room. But, they can be moved when needed.

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steve-l said:

I can appreciate your desire to save money, we all do, but you are dealing with life risk here. Under no circumstances should you go with engineering advice from this forum. It isn't just legal litigation, which I suspect is a non-issue in Africa, unlike in the first world, but a mistake here can kill people. Get a professional to do the specs, don't be stupid.

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hanermo said:

Lots of good advice in the thread, imho.

For the OP, a reality check is also useful.
Go to your local steel merchants, with similar 30 foot spans.
Look at their beams.

The typical code is 10x overbuilt for roof gantry cranes, that are "critical" in that they will kill people if they fail.
Like amusement parks, and anything lifting 3.5 m + overhead of people.

From what I see, in my local industrial steel merchants, they have a gantry of maybe 25 m width, rated for 5 tons.
The top beam is maybe 40-50 cm tall, at most.
9 m or 30 feet is about 12x less stress, vs 25 m, it´s length x pwr 3 for a free unsupported beam.
(Yes, the gantry is somewhat supported at both ends but it´s not theoretically a rigid mount).

2T as in 2000 kg is little.
It´s the weight of a modern CUV or small SUV.

But smaller beams, that will technically be way more strong than necessary, will BEND a huge amount over a 9 m free span, under a 2 metric ton load.
This means any smaller beams will extert huge tension on the tracks, and try really hard to jump out of the tracks, when they bend.
And the tension can be way more than the mass of the load, by several multiples.
E.g. It´s how some blocks in sailboats work.
Or a centerpoint static winch.

If the overhead tracks are not anchored to the sides of a building, they can easily bend a lot.
So a relatively low 2 MT weight can bend the tracks, and stress the vertical support pillars in the horizontal direction - and they are quite flexible.
Unles the gantry is rigid, which again means taller gantry beams.

E.
Watch lifting a 6 m std flat beam, at the steel merchants.
It bends about 10-20 cm, 7-14" or so, from its own weight.
Over 9 m length, it will bend about double.
Length pwr 3.

Perhaps find a similar crane somewhere, and just copy the dimensions they use.
Give or take, it will be stronger than 10x the rated load, and the strength is more than the failure load.

I also suggest looking at construction cranes in building sites.
They use space struts, and are mostly air, and are ridiculously light for the huge leverage and often big loads they swing.
The space strut steel lattice should be available in builders merchants, and is cheap.

You maybe could use it to stabilise the rails horizontally and probably carry the crane into the ends of the building, getting it out of the way.
It´s quite cheap, and rigid in in all planes, unlike anything else cheap, afaik.
The builders merchants struts are similar to grade 8.8 bolts vs 12.8 bolts from a top industrial supplier.
They are not tested, tempered, certified, for bridge use.
Half the strength in 8.8 vs 12.8, with maybe 30% less reliability.
Just get a bigger one.


But if You go up in section size, which is quite cheap, the end result is very good.
Section size increases strength and rigidity enormously, and You don´t have the constraint of looks, or space.
A bigger section is not much more expensive.

Anecdote.
I once designed and built an amusement park ride, with a similar load, from wood, 4.5 m beam clearance 4 m high, with overhead gantry and 1000 kg "max load" rated to 450 kg, (4 people, plus 250 kg plastic ball of 3.5 m diamter, aka 12 feet D).
The actual carrying capacity was somewhere above 2000-3000 kg, and impossible to measure.
About 16 m long with the steep ramp at the end.
Needed to be ultralight, so I built it from plywood, with a a reverse stressed-skin structure, like a wood boat inside out.

Like a house deck but with ribs and diagonals lke a boat below, crossed, but decked with a *double* layer of plywood screwed to both longitudinal and transverse ribs.
Screwed with == 3000 screws, 65 x 6 mm, every 120 mm or so, 30 cm centers.
So the (double) skin was the structural load-carrying member, not the beams underneath.
And any failure would have required shearing the screws, simultaneously, in great qty, all at once.
And shearing 2 layers of the plywood, all at once, across tens to hundreds of screws.

It was very much inspected by a very good industrial engineer from wales, nice guy, who officially inspected and signed lots of UK amusement park rides.
And it was insured like an overhead crane.
We had no issues passing the inspection.
I talked to the very nice guy in advance, who advised us, and said that as long as you use industrial components and build it 10x the rated load, there will be no issues.

We guesstimated that the deck would have held 8000 kg, maybe more, steadily distributed.
On a 2 person passenger load, aka customers, plus 2 staff and 250 kg ball, there was no issue at all.

My anecdote is a bit to show that really safe and really good structures can be done with not much money, and can be perfectly safe and pass industrial inspections.

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steve-l said:

Dale,
This is NOT one of those hard choices you mentioned. This is NOT an emergency situation. The few hours spent with a structural engineer would not be expensive and might even save money in the end. On the other hand, not going to one does risk lives necessarily.

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