savageseadog
Well-Known Member
Can anyone suggest what the maximum load on a 2m long 90 x 90 x 3.6mm section in pure compression would be please?
Strength of Hollow Square Box Section Steel For Boat Cradle
- Thread starter savageseadog
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rogerthebodger
Well-Known Member
This online calculator may help you
It will give you various tube sizes including square tune for a particular load so you may need to go through several collections to ge max load for your size of tube.
It does beams as well and compression struts.
http://www.atc-mechanical.com/calculators/tube-size-using-structural-properties/
DownWest
Well-Known Member
Good link Roger, thanks. I was wondering where I had put my tables... other than thnking that the size quoted would be pretty tough for most boats, considering that Acro struts are commonly used and they are way below the size quoted.
I have a cradle stashed for a Swan 43, and none of the struts are as big as the OP's size. Designed by an Aerospace engineer...
I have a cradle stashed for a Swan 43, and none of the struts are as big as the OP's size. Designed by an Aerospace engineer...
savageseadog
Well-Known Member
Thanks for the link. The load will be about 2 Tonnes per strut estimated, just trying to verify if it's big enough.
JumbleDuck
Well-Known Member
You need to consider two main possibilities: buckling of the member or failure of the material.
The Euler buckling load Pe pi2 E I / L2.
For a 90 x 90 x 3.6 standard square section [1] the second moment of area I = 76 cm4 = 7.6 x 105 mm4
The Young's modulus E for most steel is about 210 kN / mm2
Putting them all together and working in kN and mm gives
Pe = pi2 x 210 x 7.6 x 105 / 20002 = 394 kN ~ 40 tonnes.
However, the cross sectional area is 1230 mm2 and the yield strength of mild steel is typically 250 MPa = 250 N / mm2, so compressive failure of the material would occur at a load of
Pc = 1230 x 250 = 307 kN ~ 30 tonnes.
The lower load is the limiting case, so 30 tonnes is the theoretical maximum; I'd probably treat half that (15 tonnes) as a safe maximum, but I'm a mathematical engineer and someone with practical civil engineering experience should probably be able to recommend a proper factor of safety.
[1] Google "standard square section properties, find something like http://www.roymech.co.uk/Useful_Tables/Sections/SHS_hf.html, note that the I value there is the polar second moment and halve it to get the second moment around either the x or y axis.
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JumbleDuck
Well-Known Member
Easily.
NormanS
Well-Known Member
I'm struggling to visualise a cradle design which would have any really significant compression load on members 2m long. Particularly loads of 2 tonnes.
James_Calvert
Well-Known Member
Perhaps one where the boat has a lifting keel which needs to be dropped for maintenance?
And allowing for the situation where a gale force wind had the effect of transferring all the boat's weight onto the leeward props?
rogerthebodger
Well-Known Member
I agree. If I had knows that was all the load I would have not even bothered to to the analysis but of cause that where experience in structure design comes in.
This again goes back to the difference between an Engineer ( with a capital E) and an Technician and engineer (with a small e)
savageseadog
Well-Known Member
The yacht is suspended in poppets also known as splashes. On one hand the theoretical load is one thing but one has to consider distribution of weight, load distribution when boat is placed into cradle and pushed about, wind, vibration and weakening due to rust and flaws in the steel.
NormanS
Well-Known Member
Ah, not your average cradle then. From others' calculations, your dimensions sound plenty adequate.
