Sheet metal rib spacing

[pinhead4]

I'm trying to calculate how far apart to weld ribs for a sheet metal deck with a specified pressure.

I have 1/4" plate and I'm assuming a deck pressure of 160PSI, looking for some resources/calculations to figure out how far I can space apart the rectangle tubing supports underneath with an acceptable level of bending in the sheet metal.

**I realize 1/4" plate isn't sheet metal but I figured the principles would be the same and it describes my situation better

 

[racookpe1978]

What is the "acceptable" level of bending of the final plate assembly?

How is the plate supported at ALL edges and across the center?

What is the load and how is it distributed?

What are live loads and dead loads?

See, this isn't so much a "rib" problem under a theoretical plate in the middle of nowhere, but a "how is the entire plate supported around the tank (?) or deck circumference"?

 

[pinhead4]

Yes the plate is supported around the rectangle circumference. X amount of cross struts will be welded inside the rectangle in parallel to one another (visually it would look like a ladder lying flat). The load as I mentioned is assumed to be a static 160PSI evenly distributed across the plate (includes safety factor for live and dead loads).

 

[MikeHalloran]

Check your units.

A design load of 160 pounds per square foot requires a pretty sturdy floor.

A design load of 160 pounds per square inch requires a much more extreme design.




Mike Halloran
Pembroke Pines, FL, USA

 

[racookpe1978]

Right. So how big is the floor?

 

[pinhead4]

18' x 8', it's a car deck. Through some research I've came up with 160PSI. Does anybody have any leads on where I can read more into this? I've been searching through my mechanics of materials textbook as well as the net and havn't really been able to find anything on plate calculations other than K-factors and how much the plate will stretch when bent.

I should also mention I am an EIT so my skills are not fully developed.

 

[MikeHalloran]

What I'm guessing that your research found is the local pressure under the contact patches of a ~10 ton vehicle.

... which would ordinarily be modeled as four concentrated loads. Specifically, a car deck would probably be designed with two beams under the left and right wheels, sized to carry the car weight at any arbitrary axial position,
plus a lightweight deck to support a few itinerants or a driver, and maybe to catch whatever fell out of their pockets.


If you size an 18' x 8' deck for a uniformly distributed load of 160 psi, the supports for the deck need to be able to support an aggregate load of ~7.5 million pounds.
That's basically a machine tool's table, which is probably overkill for carrying a vehicle.





Mike Halloran
Pembroke Pines, FL, USA

 

[pinhead4]

Thanks Mike, I have a better idea of how to go about this now. I still find it strange I couldn't find any information on sheet metal stress for a given pressure/load though. If I say a deck pressure of 40psf then how could I find the optimum spacing of the beams for the 14" plate?

And as for the supports, is it valid to argue that I am not saying the deck will support 40psf distributed across the entire surface at once (~5000lbs) but that any 1 square ft could support 40 lbs with an acceptable level of deflection? This was my thinking when I said 160 PSI, I realized that if the entire deck was subject to that pressure at once the supporting structure would be insane. Of course in this case the supports will need to support more than 5000lbs anyways but I'm just clarifying this thought.

 

[MikeHalloran]

It's not at all uncommon for structural load cases to have multiple requirements superimposed, e.g., aircraft floors able to carry xx lb/ft^2 representing the passenger load, and also able to withstand the pressure exerted by a stiletto heel.

Your last message suggests that you understand the concept of overlapping requirements, but the way you have restated the numbers suggests you were in a hurry, or need to work on your self-checking and problem statement skills. I am now more confused than before about the exact nature of the problem you are trying to solve.

FYI, 'sheet' ordinarily refers to product thinner than 1/4", or 6mm, depending on your age, whereas 1/4" is unequivocally 'plate'. That's just industry custom.

Mike Halloran
Pembroke Pines, FL, USA

 

[dhengr]

Pinhead4:
What kind of a car is this, a VW Beetle or a Go Cart? Give us some meaningful design info. instead of some arbitrary/imaginary loads and terms, some well proportioned sketches with some dimensions, etc., so we have a real idea of what you are trying to design. What is it, how is it used, what does it actually do? There are cars and then there are cars, and they aren’t all the same. As MikeH suggests, the term sheet metal and your loads just don’t add up, nor does 8'x18' deck and sheet metal add up, they just don’t make sense, and you won’t get any meaningful answers. For the life of me, I don’t understand why an engineer can’t adequately describe his own design problem, so other engineers have a vague idea what he’s talking about. Remember, we can’t see it from here. Is it a railcar, what kind, what loadings?

 

[Ron]

Agree with others...you're loading is not correct.

For vehicles, you will have point loads at each tire. Further, since you are driving the vehicles onto the deck, you will have a rolling load. This affects the structural supports more than the decking. For the decking, the worst case is the point load between supports (but you need to check shear at other locations as well). This will give you the maximum deflection and the maximum moment for stress analysis. For decking, a 12" wide strip is usually taken. That's conservative, but it works.

As for the framing supports (beams), the deflection limits of the deck must be determined and from there you can determine the spacing of the supports.

You can determine basic plate stresses using techniques in Roark or do an FEA analysis and check the von Mises stress levels.

 

[racookpe1978]

With only 8 ft wide - and "walls" on both sides that absolutely prevent the car from going anywhere else sideways! - your problem (as pointed out above) is now TWO things: A stable high-capacity pair or rails that can resist the car's ROLLING weight on 4 point locations (each the 12x12 width of the tire tread) and a separate thin 1/4 ribbed plate that will carry ONLY a person's walking weight (with point loads of the impact of high heels or the like) PLUS extra loads like packages carried or carts or baggage of whatever also as pointed out above.

You've got a structural problem of like a pair of bridges or beams. Then a lightweight plate problem between container edge and beam, beam-to-beam across the middle, then beam to container on the other side.