strength of mgp10 pine

hi all

I am building a multihull boat and want to know if my crossbeams are strong enough. I am on a bit of a budget. The boat consists of a large dory and a smaller ama. The ama has a volume of 220L, thus I assume max load is 220kg. The ama is 5ft from the main hull.

I intend to build the crossbeams out of 90x 35mm mgp radiata pine, cut in two, so that I have 2 strips of 90 x 17mm each. These will be separated by spacers of 6 inches deep. Now I can calculate the load on these two strips of timber, after applying safety factor they need to be able to stand 1500kg each. The top one in compression, the bottom one is tension. This is the extreme load, and I have applied a safety factor as well.

What I want to know is how strong in mpg 10 radiata pine in compression and tension. Googling this has brought up thousands of websites and it is hard to sort the wheat from the chaff as they say. Does anyone know approximate values for these strenghts?

The non-mathematical method is of course to build my crossbeams and jump up and down on them at their midpoint whilst they are supported at their ends. I will do this anyway, but it would be nice to know how strong pine is. I have read values from 8MPa, all the way up to 40MPa... I am just wondering if the maths will get me anywhere or whether I have to revert to physical testing? Yes I know radiata pine is not ideal, however it will be coated in epoxy resin and should last a few years.

regards
n peter evans

In South Aus we have a Timber Ass of South Aus.they supposerdly can give you and answer you need not always what you want Ph 1902 282 000 Fax8297 2772 You may have the same in Vic.
Tony

When you say 8-40 mpa are you talking UTS?

Yes I am talking ultimate tensile strength,

I have also put this thread in the boatbuilding group and I know it is bad to put same thread in 2 places. Think I will delete the thread in this room soon, perhaps any replies can be put in boatbuilding room?

My material science skills are based on year12 engineering science.

I feel one good way of knowing if it will be strong enough is to see what other people have done and do similar. Many similar designs have solid laminated beams, not box beams or trusses

I can reinforce the wood beams by laminating some strips of ply on top and bottom of cross beams. I can find ways of adding more weight (more people standing on them) to increase load for physical testing

What I am looking for and am as yet unable to find (maybe I am not knowledgeable enough to know what to ask for) is how strong a piece of pine is in comprssion and tension. At present I can use judgement and commom sense and physical testing to ensure strenght, but some maths would be nice as a backup.

Hoop pine is an option, but only supplier I know is in seaford, miles away. Would cost more

youngs modulus is somehting differnet, the strain in relation to load. But for strenght of crossbeams it would be best to look at UTS rather than youngs modulus.

as a reminder,, if any replies can be put in boatbuilding room, so that I can delete this duplicated thread quite soon

n peter evans

What about making the beams of a torsion box construction from ply board.

sorry, but if you cut any graded timber lengthwise, it loses any pretense of maintaining that grading. This is due to the potential for weak points (ie, knots) to constitute a far greater proportion of the wood's thickness, and therefore is a far greater weakness in the overall member. In reality, you can get round the physical problem by very careful selection of the individual pieces, but is there any sort of inspection you have to submit to in the interests of declaring the craft seaworthy? This is why the good old woods were used in the old days...Huon pine etc.

Bill

According to AS 1720.1-1997 - Timber Structures it gives the following:

Characteristic properties for 35 and 45mm thick MGP
MGP10:
Bending (f'b) 16MPa for 35mm, 19MPa for 45mm thick
Tension parallel to grain (f't) 8 MPa
Shear in beams (f's) 5 MPa
Compression parallel to grain (f'c) 24MPa

Hope that helps.