Hog & Sag effects on Rack placed on Ship Deck

[Sjqlund]

Dear Engineers

I'm working on a rack used for sea transport of some long objects. The objects are stacked in the rack. Please see attached figures.

I'm worried about the influence of hog & sag effects on the rack. Due to this effect, i think i need a longitudinal degree of freedom between the rack and the stacked objects.
I would like to calculate how much longitudinal displacement can be expected due to this hog and sag effect.

I think it would be sufficient for the calculation to know a measure of vertical ship displacement (mm) per longitunal (m) of deck.

This value, i expect is dependant on the ship. But i would like an estimate value, just to see what 'ballpark' I'm in - e.g. is it 10 mm or 100 mm displacement at the top of the rack?
Suggestions on where i can get an approximate value from?

 

[Sjqlund]

There seems to be a problem with the Figures. Please see them on the below links:

Rack on deck: Link
Hog and sag: Link

 

[dhengr]

Sjqlund:
You haven’t defined your problem at all well. What is the product and what makes you think this will be a problem on the ship, as opposed to any other means of handling or transport? What are hogging and sagging anyway, (upward and downward deflection?), and why can’t your product tolerate this? I don’t know what the ship deck deflection is, but I can’t imagine it should be a problem, you have to design your rack to protect the product, of course. Put the rack on only four bearing points (hold down points, feet) and design it to span btwn. them and support your load. You say... “10 mm or 100 mm displacement at the top of the rack?”, and either one of these would seem like a lot of movement. I can imagine that the rack should be designed for lateral loads due to the ship rocking side to side. You have to show a much better, more detailed, sketch of your rack (skid?) and product and how they are attached together and to the deck, with dimensions, member sizes, weights, loads, etc., if you want a meaningful discussion of your problem here on E-Tips. We can’t see it from here. How do you lift the rack on and off the ship, is your product on the rack then?

 

[PersonalProfile]

I propose you consider consulting with your shipping agent etc, though there are some generalised guides available, including:

https://infostore.saiglobal.com/store/Details.aspx?productID=1672559

I believe the above is the reference that I am thinking off, though I cannot recall with certainty.

Although I am obviously not aware of your situation in detail, in a "similar" application where the shipped item could not tolerate the imposed actions, we: fully constrained one end of the item, and left the other end to "float" [not literally] using a "slide" arrangement.

Regards,
Lyle

 

[racookpe1978]

dhengr (Structural)

Hogging and Sagging are "ship fabrication and design issues" as the entire ship's hull either bends up (supported in the middle by the wave with both ends drooping down) or is supported at both ends by a wave (with the middle of the long hull "sagging" down).

I agree with you: It depends on the product. If the product is supported at two points for shipping (truck, crane, warehouse, or on the parking lot) at two points, then it will be supported at those same two points on a ship whose basic structure is moving up and down. (And rotating about its center point.)

So, the result is: "So what?" The two points move, and the cargo is still supported.

But! If the cargo (the product) is supported by a long-row-of-perfectly-equal-points - then it (the cargo) has to be planned for support during a sea voyage (and a crane lift!) for movement of each of those long-row-of-support-points independently of the rest.

 

[Sjqlund]

dhengr:
I'm aware that the problem as described here, is not very well posed. But i'm afraid i can't provide any more information on an open forum like this.
What I'm looking for is really some knowledge about how "Hog & Sag" is generally taken into account. Perhaps some users have knowledge of a set of guidelines for "long deck-side mounted structures" or similar.

lylebrown00:
Thank you for your input. I'm afraid that your link does not work. Can you provide the name of the reference you're thinking of?
With regards to letting one end "float" - this is pretty much my plan. However - i'm constrained in the way that i'm not designing a new product - i'm merely modifying an existing product. Therefore it would be a great help if i could use a guideline to estimate how much "floating" is needed.

racookpe1978:
The cargo is only supported at two points. However when the two points move relative to the cargo, the friction is causing wear on the cargo which is not optimal. Therefore at least one of the two support points need adequate degree of freedom. But how much is adequate is really the question.

 

[Tmoose]

"the friction is causing wear on the cargo which is not optimal."

I'd expect the length, width, and any lay of the wear marks might be useful clues how much swing and twist the hammock needs.

 

[MikeHalloran]

You might consider adding or improving the dunnage around the product, restricting the friction damage to the dunnage, not the product.



Mike Halloran
Pembroke Pines, FL, USA

 

[concretemasonry]

Have you suggested the use of containers to your client?

They are easier to handle from a logistics and shipping standpoint. They are available in open top and closed versions. Open racks probably more costly and difficult to schedule.

If you are with a shipping company, there should be someone the help you with limitations once you have established the length and loads.

I worked with a company in northern MI that shipped to customers in every continent, but always avoided open loads, but some had to be shipped open because of height restrictions and despite the loads (40,000 pounds) per container since weight was not a problem. Our electronics and control panels were never a problem. In one case we added about 500 brooms to one container for a group to clean/restore old churches in Belarus because they we not that heavy and were able to be loaded before and after the deck mounted or open containers.

If the product is steel pipes, there are some long established standards compatible with most ships. the ship manufacturers (mainly European) should be able to give you help since they are aware of most type of ship configurations and practices. Highly specific products/cargo can be handled if the limitations and restrictions are identified.

Dick

A very interesting problem, but more information on the physical loads would be needed before going into specifica.

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[PersonalProfile]

The link [that apparently works for me] was supposed to be "NOBLE 0030:2013 Guidelines For Marine Transportations".

Try Googling the same.

Regards,
Lyle