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Re: Center of Gravity & Sling Loadings

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Jeremy,

        The problem you have is that there are FOUR lift points rather than
three, which makes the thing statically indeterminate.  Statically indeterminate
structures, of course, can not be solved without taking internal deformations
within the system into account or by otherwise eliminating the redundancies.

        Your first method seems to be correct but you will need some means such
as use of spreader bars to equalize some forces in order to eliminate the
redundancies in order to get an accurate calculation.  As a practical approach
you could also do it by taking your best shot then using a higher than normal
impact factor (such as 1.5, or even 2.0) to size your individual sling members.

        Hope these thoughts are helpful.

Regards,

H. Daryl Richardson

Jeremy Kuhn wrote:

> I have a trolley which I need to lift which has 4 lifting lugs on it...I
> have determined the C.G. of the load and now I am in the process of
> determining the different vertical load which each lug sees due to its
> proximity to the C.G. and then I will determine the tension in the slings by
> trigonometry.
>
> Here is the question...I have come up with 2 different methods of
> determining the vertical loads on each lug - both seem to be logical...but
> results are very different ! Is there a correct way to determine vertical
> loads at 4 different locations which are at unequal distances from the C.G.
> ? I looked through all the Riggers Handbooks in the office and came up
> empty.
>
> The first method I used (working in an XZ plane) was to divide the problem
> into 2...two lugs had a distance of 30% of the length to the C.G. in the X
> direction - so I assumed those two lugs saw 70% of the load - then one of
> the lugs which saw 70% of the load had a distance of 40% of the length to
> the C.G. in the Z direction so it got 60% of the 70% (42% of entire
> load)...and so on for each of the other lugs..
>
> The second method which I used was to take a straight line distance from
> each lug to the C.G. and then determine the vertical load seen by assuming
> that each vertical load would be inversely proportional to the distance from
> the C.G. - little tough to explain the exact process.
>
> Anyhow...any insight into an approved method would be appreciated ! Thanks !
>
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