1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Container stack height on a ship: Mechanical strength

  1. Mar 29, 2015 #1

    rollingstein

    User Avatar
    Gold Member

    In the photo of a container ship cargo seems stacked 12 high. I do know that there's lateral supports etc. that are not visible in the figure, yet the vertical load, isn't that transferred downwards to the bottom container?

    Or is there some way to support the stack at an intermediate point so that the bottom container in a stack does not take the compressive load of the 11 units above it. At 30 tons a container, with 4 corner posts / twist locks that'd be 80 tons per bottom container's corner post.

    I do know there's ship mounted structural columns / cells / bulkheads above deck on these vessels but I thought they provided lateral stability. If you wanted to transfer vertical loads I cannot think how to do that without obstructing access to the cranes that lower these containers into the stack.

    So how do they manage the vertical loads? Or are these 12 high stacks composed of empties high in the stack?

    XXW3UiF.jpg
     
  2. jcsd
  3. Mar 29, 2015 #2

    SteamKing

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Containers are designed to take mostly vertical loads.

    If a container ship makes deliveries to multiple ports on a single voyage, the containers must be loaded so that the ones scheduled to be discharged first can be reached first. These vessels normally operate on a tight schedule, so you don't want to be unloading containers to reach one which is to be discharged, and then have to re-load the others before sailing to the next port.

    30 tons is the maximum load for certain sizes of containers, but that does not necessarily mean that all containers are loaded to the maximum. This is somewhat of a concern in shipping circles currently, because the weight of container contents are often not declared properly (many are declared at lower than their true weight) and container weights are not verified before loading.

    Certain hazardous cargoes are not supposed to be shipped in regular containers, but these regulations are sometimes ignored as well, leading to explosions and fires while the ships are at sea.

    Container cranes use special loading frames to attach to the corners of a container in order to load or discharge it from a ship, as shown below:

    The vertical loads are transmitted thru the vertical posts at each corner of the container, and the stacks are designed to be held in place by special twist locks.

    Given the apparent draft of the ship in the photo, I would venture that most, if not all, of the containers aboard are loaded. Now, that doesn't mean that each container is loaded with its maximum weight capacity, because there are some cargoes which are light but bulky.

    The 411 on shipping, or intermodal, containers can be found here:

    http://en.wikipedia.org/wiki/Intermodal_container

    The strength of containers is governed by various standards organizations, like ISO and DIN, and the containers themselves are often made with high-strength steel to allow for greater stack heights:

    http://www.containerhandbuch.de/chb_e/stra/index.html?/chb_e/stra/stra_03_01_00.html

    Often, if an outboard stack of containers above deck suffers a collapsed container at the bottom, this can result:

    1184318230469.jpg

    In the early days of containers, stack heights were limited to 5 or 6 containers; now, stacks of 10-12 are apparently permissible. However, there are thousands of containers lost at sea every year due to failure of the securing mechanisms, the container structure, or both.
     
    Last edited by a moderator: Apr 28, 2017
  4. Mar 29, 2015 #3

    Doug Huffman

    User Avatar
    Gold Member

    There is no profit in transporting empties, they accumulate around port facilities, even to being repurposed as housing and housing material.
     
  5. Mar 29, 2015 #4

    rollingstein

    User Avatar
    Gold Member

    I don't think that is true. My impression is empties are transported back too. Just maybe not as often perhaps.

    e.g. See this press release from Maersk circa 2014.

    While the container vessel Svendborg Maersk was sailing from Rotterdam to Colombo 520 containers were lost at sea during a storm in the Bay of Biscay.

    Of those 520 containers 440 were reported as empties. Pretty sure such a high proportion of empties isn't typical but that's one data point. I'd be curious if someone knows industry averages.

    http://www.maerskline.com/en-nz/countries/int/news/news-articles/2014/02/svendborg-maersk-incident
     
  6. Mar 29, 2015 #5

    rollingstein

    User Avatar
    Gold Member

    Thanks @SteamKing for lots of useful information.

    Though my core question remains. Is there a standard, specifying how high containers can be stacked, in general, on a ship? Even assuming lashings, twist-locks are in place for lateral restraint.

    Or must the max allowable stack height have to be calculated case to case based on the exact weights of the individual containers. e.g. The upper layers in (say) a 9 high stack must be empties / light weight containers?

    And if so, what's the max allowable loading on the corner post of an individual container. Will all the standardization at the heart of containerization I suppose this is spelt out somewhere? My intuition says 80 tons per corner post in compression seems a tad too much. So I'm curious what mechanisms these ships use to work around this restriction.
     
    Last edited: Mar 29, 2015
  7. Mar 29, 2015 #6

    rollingstein

    User Avatar
    Gold Member

    Wouldn't it be relatively easy for them to incorporate a load cell in the massive, multi million dollar dockside gantry cranes that they use to stack these containers? Or into the stack-loader or forklift that arranges them in the dock's container yard?

    Is this already done? To get true weight estimates I mean.
     
  8. Mar 29, 2015 #7

    SteamKing

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Yes. The ISO and the DIN are two standards organizations which have regulations governing the construction of shipping containers. Depending on the design of the container and the materials used, the maximum allowable stack height will vary. An individual container will be marked to indicate the size of the stack which is permitted.

    If you check out the Container Handbook which I cited previously, you'll see some of the placards affixed to typical containers spelling out how high they can be stacked:

    http://www.containerhandbuch.de/chb_e/stra/index.html?/chb_e/stra/stra_03_01_00.html

    Scroll down at the link to see photographs of these placards.

    The largest ships now being built are capable of loading 20,000 TEUs for a single voyage, so doing such calculations would be costly and time-consuming. The container structure is designed to handle fully loaded units in the stack, however many high that the container is certified to carry. At sea, the containers themselves are subject to dynamic loads in addition to the static loads from the weight of the container and its contents. These dynamic loads will vary due to the position of the container on the ship, the speed of the ship and the condition of the sea in which it is traveling, the size of the ship, its stability characteristics, etc.

    Like I said previously, standards organizations like DIN and ISO spell out how containers are designed and tested. The major ship classification societies, like DNV-GL and ABS, also govern the testing of containers and securing apparatus.

    All of these organizations famously eschew the use of intuition in their evaluation of container strength and securing mechanisms and prefer to use science and data obtained from testing actual containers.

    Ships which do not follow the rules of the classification societies or the guidelines of the standards organizations risk being monetarily liable for the loss or damage to container cargo which may result, especially if insurance underwriters find evidence of malfeasance on the part of the ship's owners or crew. With ships now being capable of carrying 8,000 to 20,000 containers on a single voyage, the potential value of the cargo can run into the hundreds of millions of dollars. The value of the cargo on a typical voyage can exceed the value of the ship itself several times over.
     
  9. Mar 29, 2015 #8

    SteamKing

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    In light of recent accidents involving container vessels at sea, it has been proposed that verification of the weights of containers should be included in maritime regulations.

    Regulations have been adopted recently by the IMO requiring the verification of container weights before loading on a vessel, but these regulations will not take effect until 2016:

    http://www.worldshipping.org/industry-issues/safety/cargo-weight
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Container stack height on a ship: Mechanical strength
  1. Yield Strength (Replies: 6)

Loading...