Standart ISO for FERROCement hull

Hi,
I'm about to start the construction of a ferrocement boat (21 meters) in Italy.

I got stuck cause the no one can tell me what are the specifications for this kind of material.

The Standard ISO 12215 do not provide any info on it.

Please help!!!!

Best regards,
Sergio

 

Ferro-cement resources

I know of at least four resources for you. Chances are that if ISO knew about these, they would base their specifications on what these say:

Book:
"Ferro-Cement: Design, Techniques, and Application" by Bruce Bingham, Cornell Maritime Press, 1974. Long out of print, but maybe you can find used copies. Bruce is a well-know boat designer and excellent marine artist. His book is probably the single most comprehensive treatise on the subject.

Technical Papers, Society of Naval Architects and Marine Engineers (SNAME), Marine Technology quarterly journal:

"The 1973 University of Michigan Ferro-Cement Canoe Project" by Tracy T. Coveyou (student at UofM), April 1974, Vol. 11, No. 2.

"Material Development, Design, Construction, and Evaluation of a Ferro-Cement Planing Boat" by A.L. Dinsenbacher and F.E. Brauer (of the US Naval Ship Research and Development Center), July 1974, Vol. 11, No. 3.

"Design and Construction of the Concrete Canoe California II", by R. Cengiz Ertekin and Bradford A. Porter (faculty at University of California at Berkeley and the University of Hawaii, respectively), July 1986, Vol. 23, No. 3.

The Dinsenbacher-Brauer paper contains probably the most scientific engineering data that would help you the most.

All these papers are available from SNAME in New Jersey: www.sname.org. Others may be able to chime in with additional resources.

I hope that helps.

Eric

 

Hartley designs have a very informative book as well:

http://www.hartley-boats.com/books.html

 

ISO 12215-5 (Hull construction scantlings) can be applied to any materials. The main issue will be determining the design stress of the material.

One option would be to consider that the cement has no tensile strength, and use only the steel elements as contributing to the strength of the elements.

Any areas specific to ferro-cement, such as corrosion protection of steel elements, would need to be covered by some other ISO rules, I guess.

 

Assuming that the cement has no tensile strength is a gross underestimation. Of course it is going to have some tensile strength, and in fact, one table in the Dinsenbacher/Braueer paper shows tensile strengths of 2,000 psi at initiation of cracking, and 4,200 psi ultimate tensile strength. Flexural and compression properties are also shown.

Eric

 

My answer was a bit too abbreviated to be clear.

By 'cement', I meant the mixture of portland cement and sand, excluding the steel rods and mesh embedded in it. In civil engineering, it is generally assumed that this has no tensile strength under extension. This makes sense, as all cement contains very small cracks, which can propogate quickly and reduce the strength under extenstion in a non-linear and unpredictable manner. In a ferro-cement plate subject to a load, the beam will bend. For calculation purposes, it is assumed that the tensile strength stretched side is entirely derived from the reinforcing steel.

In static structures, the compressive strength of cement/concrete is exploited by reducing the amount of rebar in compressed parts (the top side of a supported floor, or in a supporting column). However ISO 12215-5 does not allow this - 'other materials' is limited to isotropic materials (equally strong in all directions). That's why I suggested assuming the cement part has no tensile strength.

If the cement contains glass fibre strands, or something equivalent, it may have some tensile strength under extension, but I am assuming this is not the case.

In practice, this would mean assessing the amount of steel in the ferro-cement, (cross-sectional area, not volume). Say this is 10%, then the ferro-cement has a design stress (sigma_d) of 212 x 10% = 21.2 N/mm2 (using ISO values for mild steel).

Let's assume that the ISO calculation for the pressure on the bottom, P is 10.(T + L/17). [This applies to RCD Cat A motor craft, assuming dynamic loads are lower than this minimum requirement.] For a 2m draft and L=17m, P= 30kN/mm2.

Assume that the short dimension of any panel of the hull is no more than 600mm. Then the minimum hull thickness for bending strength is:

t1 = 600.squareRoot( (P.k2) / (1000 sigma_d ) )

Suppose k2=1/2, then t1 = 600.sqr(30/2)/21200) = 16mm. You should also check the minimum hull thickness for bending stiffness, t2 = 600.cubeRoot( (P.k3) / (1000.k1.Ef) ), but will always much lower than t1 for this material. 16mm seems thin to me, even if there's a lot of steel in it - maybe there should be an extra safety coefficient in there. You also need to allow for the thickness of cement to cover the steel adequately - this is not included in the 16mm.

You can use this formula for decks and bulkheads, and can proceed in an analogous manner for calculation of stringer dimensions.

I think applying ISO rules to ferro-cement is quite technical and unusual, so it may be worth consulting a specialist, rather than relying on an unqualified and anonymous forum member. If you want to build a RCD A, B or C certified boat, you will need to convince the certifier in advance that the calculations meet the standard.