Equipment rack stress analysis

[daak]

Hi! I'm wondering if anyone has had any experience in performing analysis to determine the airworthiness of equipment racks. The equipment rack in question consists of a frame made of 1" steel tubing with the necessary bracing made of the same and is covered with thin aluminum sheathing. It is rigidly attached to a pallet in the cargo compartment of a C-130. If the rack consists of 3 stacked shelves, what are some possible approaches to take to determine the maximum mass that can be placed on each shelf in order that the contents will not become projectiles when subjected to loadings of 16g (fwd)?
Without becoming to detailed here, I'm just interested in knowing what approaches could be taken. And if you have any books or websites I should visit, let me know! Thanks!

 

[rb1957]

9g fwd, 3g side ... FAR25 crash
military operators may want more
up/down load factor according to airplane data

it becomes a very complicated process if you don't know what or how they're attaching to the shelves.

it depends on how detail they want to get. you can sensibly placard your shelves of Xlbs, assume the load to be in the middle of the shelf, 1/2 the height to the shelf above. you could assume the load is sheared into the rack at the shelf face (and neglect the over-turning moment due to the weight being above the shelf, as being small). Presumably someone mounting stuff on the shelf will check the attmt to the shelf, and verify that they don't exceed the CG limits.

 

[daak]

Thanks for your reply. The contents within the rack will not be secured, but loose inside each compartment. I'm interested in the resulting stress in the frame due to the energy of the contents as they make contact with the frame. From there, I need to find out if the frame will fail and contents be ejected. Determining the max allowable weight for each compartment in the rack may be a difficult, iterative process. Thanks for your suggestions on how to set the problem up. Please let me know if you know any websites covering this similar topic, or books.

 

[SAITAETGrad]

Send me a personal message, we can do that design work for you.

 

[rb1957]

ok, the rack has loose equipment on it, the the compartments are closed off with solid doors or maybe mesh.

two issues ...
1) strength of the rack ... apply a load at the mid-height of each compartment, adjust to you heart's content (eg something simple like 100 lbs/compartment, or moe complex 120 lbs/lower compartment, 100 lbs/middle compartment ...)

2) strength of the comparmtne closing, door or mesh ... usually the attmt to the rack is critical

 

[HANDCALC]

One design approach to installing equipment racks onto commercial aircraft, is to restrict the overall weight and CG of the rack to less than that of a seated passenger. Make sure the equipment rack's forward and aft attachment points on the floor are spaced further apart than the seats, and the inertia loads generated in a crash condition will not exceed the floor allowables for the seat-track system.

Simple hand stress calculations can clear the equipment rack itself, and the floor load allowables are "cleared by similarity" to that of a seated passenger.

In your case, check if their are structural limitations for the C-130 pallets.

 

[40818]

I have experience in tubular equipment racking on C-130s.
First ensure it works statically @16g and the other critical cases, by hand preferably and dont immediatley worry about the dynamic effects.

The second part your looking at seems to be impact analysis really. Ballpark figures would be to calculate the maximum energy and also a dynamic magnification factor to apply and start stressing, otherwise for accurate results go for implicit dynamic impact FE, which would probably be more effort than its worth. The C-130 isn't an elegent and weight optimised aircraft, make it quick and conservative.

 

[Sparweb]

Our approach is usually to find the loads and then test the rack.
We find the loads and their distribution through the shelving, sometimes add a bit so that the user can add equipment later.
Then the test apparatus has to hold the rack in various orientations and either pull (chains) or pile weight on it. Distribution of the test load must approximate the distribution of equipment load.

If the structure is too complex, we have at times tested only representative parts of the structure or certain joints - feeding the strength of these parts into the loads analysis to show that the frame stays straight by analysis.

The rack's weight represents a mass on a lever, which in the forward "g" situation (eg. crash) puts a tremendous force couple on the floor, or pallet in your case. Load allowables for the pallet are necessary, or the pallet must become part of the structural test.

As you can see by the number of bidders for your contract, it's the kind of thing many DER's and DAR's like us are equipped to do.

Usually our customers do us the courtesy of welding these frames up with drain-pipe and coat-hanger before ever asking us to demonstrate they're strong enough...


Steven Fahey, CET