Container Loading

[Engineer2345]

We've got a tilted shipping container here on a frame that we're using to house material. Now we're switching material to one of a higher density and I've been asked to find out how much weight the system can withstand. My problem is figuring out how to model this problem and what failure modes to look for.

I began by treating the frame as a truss problem and using the method of joints. Summing Fx & Fy for each joint basically said that the load is supported by the four vertical suppors made of 4x4 square tubing and the rest of the tubing is there for bracing and/or variations in fabrication but doesn't support the load in a perfect system. There's no open end to measure thickness, so I'm going to assume 3/16" (worst case for common materials). I'm not exactly sure how to model the loading though...I know that as the container (and weight) becomes more angled the load is shifted towards the lower end from a practical standpoint, but don't know how to show the numbers. I should note that the container is only supported at the four corners where the posts are.

I'm also not sure what failure methods I need to analyze. I thought about buckling of the supports, but don't know if that's the most likely method of failure, or if some sort of shear force on the weld points would be.

This can be a simplified model and that's what I'm looking for. I know the container will hold 40,000 lbs as that's how we currently use it, but I need to know if loading the denser material will reduce factor of safety of the system to a concerning level. If it does I'll hire a structual engineer.

 

[XR250]

Are you modeling the container or the tilted supports or both?

 

[Engineer2345]

I want to model both, but as two systems.

I believe the frame is the most concerning system so I wanted to model it first. I'll just represent the container with a load on each column (the part I'm having a problem with), and go from there.

Then if the frame is in fact strong enough, I'll see if the container will hold the weight. If it does buckle, then supports can always be added in neccesary areas.

Any help would be appreciated. Thanks.

 

[XR250]

The frame is the easy thing to analyze. Shipping containers are not. Looks like the product will be loading up the back and side walls of the container. I doubt you will be able find any tabulated deign values for the out-of-plane capacity of the shipping container walls.
Sounds like you need to hire a structural engineer.

 

[TLHS]

You can start by looking for a certification marking somewhere on the shipping container. There are different standards, and depending on the agency and type of container you may be able to get at least some guidance.

Really though, it may come down to measuring the corrugations, drilling a hole so you can get the thicknesses and then making some reasonable guesses at the framing members on the corners so that someone can check the whole thing as, effectively, a hopper.

 

[BAretired]

Volume of container = 96*101*236.8 = 2,296,000 in³ = 1329 ft³.
Weight = 40000#
Density of material = 30pcf.

When flat, c.g. container is 108" left of right end and 50.5" above bottom.
When tilted 24^o, c.g. container moves to 78.2" from right end.
Reaction at left end = 40,000*78.2/216 = 14,500# (7,250#/col). Columns are loafing if they are HSS 4x4x3/16 and braced as shown.

If the upper beam is carrying half of the container load, it may be critical in bending unless the sidewall of the container is acting as a deep beam. Welds are not likely to be critical, but that depends on the detail used (not shown on sketch).

The container floor would have to be capable of spanning horizontally from beam to beam, loaded with the denser material. No information has been provided about the construction of the container.

Notwithstanding the above, you should retain the services of a structural engineer to provide a detailed review of the design to accommodate the denser material proposed.


BA

 

[racookpe1978]

Isn't the probable failure point the "end walls' of the titled container being pushed out of their circumferential welds across the lowest edge of the tiled end? (Assuming the unsupported floor is not pushed out from the weight of the entire cargo mass.)

If so, the "un-tilt" the container for the limiting condition on the floor: From the standard container tables for that original unit, there should be a "mass weight (or weight of the max volume of liquid" that it is rated to carry. That max weight for a uniformly distributed load on the floor of a flat container will be the same for a tilted container with a load not completely full.

Now, you come back to that tilted container with all of weight of the maximum load run up against the lower wall, right? (You're not opening the end wall to get the load out, so there must be some sort of hopper or discharge chute on the bottom? Regardless, the original hopper or chute seems to work, and you have indicated no problems with the new material density.)

At this moderate tilt angle, the weight on the lowest wall will be weight of the flat container completely full multiplied by a the cosine function of the tilt angle. (Check yourself: If the wall is vertical, the force sideways is 0.0 at the top, increasing as the depth gets bigger according to density and height. The "worst case" load condition would be a liquid as dense as the new material. If the load were a solid (steel or solid concrete for example) the is no "sideways force" on a vertical wall. If the wall were liquid or very dusty like cement, then the sideways force on the wall depends on density and depth.) As the tilt angle goes towards 90, then the force on the lowest point on the lowest part of the wall (that bottom weld) is going to again become proportional to density and depth.

When you've done both (bottom floor stress and lowest wall stress), you will which is the limiting condition.

 

[BAretired]

What is the nature of the stored material? Is it a solid granular material like sand?

BA

 

[Engineer2345]

Thanks for the replies.

The material we currently use is polyethylene resin, and the container holds about 40,000 lbs.

The material we were going to switch to is a white colorant additive (again in a granular form) and would be ~95,500lbs in the same container.

I'll make sure and check the container label for any kind of loading or at least contact information, thanks for the tip.

You're right about the discharge, the doors are kept in their shut position, and there's a big plastic ball valve cut in one of the doors at the bottom that lets material flow into a take-off box where it's drawn inside the plant.

Having said that and looking at your comments, I think I should be calculating bending in the doors as well, because you're right that's a lot of weight on those doors.

From looking at the reaction provided, you're working off the center of gravity, but please explain how you arrived at the force given.

Thanks.

 

[BAretired]

I believe I made a mistake in my earlier post. I now find that the c.g. of load ends up 87" left of the right support, not 78" as originally stated. Assuming the supports are 216" apart, the left hand reaction should be 40,000*87/216 = 16,100# (8,050# per column). The right hand reaction would be about 23,900# (11,950# per column).

If the total weight is W, the left reaction is 0.403W and the right reaction is 0.597W, so if the weight is increased from 40,000 to 95,500 the reactions are 38,500# and 57,000# left and right respectively (column reactions of 19,300# and 28,500#).

For an increase of this magnitude, I would definitely recommend having the design of the frame and container checked by a structural engineer.

BA

 

[dhengr]

Engineer2345:
I would take a long hard look at the doors and their latches and hardware, on the lower end of your container, as they are loaded by these high lateral loads. The side walls will be subjected to higher lateral loads at the lower end too. They may not have been designed for these high lateral loads. The floor framing would likely be o.k. as long as you haven’t exceeded the container’s max. design load. It would pay for you to find out who built your container, and to what spec. it was designed and built. Then get copies of that container code/spec. and maybe talk with the original mauf’er. Knowing the loads and design criteria for your container should be very helpful in directing your investigation. What materials were used in its building, what are the various section dimensions and thicknesses, what are their conditions, given the fact that it has been retired from container service? I’ll bet you might have been better off using the solid end of the container as the lower end.