Prestressed reinforced concrete beam 1

[sbkenn]

Hi all.
I am considering getting a prestressed reinforced concrete beam fabricated to stiffen the keel of a 100ft wooden boat. The proposal is a 16" x 8" or maybe 12 x 6 box section with 4 reinforcing rods, then filled with concrete. The concrete & rods would provide most of the stiffness, with the box protecting it from impact, and for attaching it to the existing keel. It would be required to resist bending loads in both directions. It it bends a little after releasing the traction, I would be content to pack the concave side to meet the keel.
Questions:
1) Is it a reasonable idea ?
2) Should I use fibreglass rods rather than steel for corrosion resistance.
3) Should I have the inside of the box coated with epoxy first.
4) Should the box be stretched as well as the rods, more/less than the rods, or not at all ?

Please advise.

TIA
Shane

 

[centondollar]

1) No. To resist any significant amount of bending while staying elastic (the purpose of prestressing) and possessing a large stiffness, the tendons must be curved and placed closer to one face of the beam in every cross-section: stress = P/A + M/W < concrete_tensile_strength.

If the load is buoyancy minus weight, as is usual in ships, the longitudinal moment distribution is approximately parabolical, and when the boat rides a wave, it will continuously switch between sagging and hogging moments, cracking the beam due to the large tension generated by bending stresses in the "wrong" face (i.e., the face where tendons do not create compression) of the beam. Tendons will also not be able to resist any significant torsion or bending around the vertical axis (from "wobbling" of the ship). In short, the tendons will not serve their intended purpose and the concrete blocks will cause a massive weight with a large eccentricity, reducing the stability and maximum live load tonnage of the ship.

There is a reason why ships are built with steel, a homogeneous material with highly predictable behavior.

2) See 1).

3) see 1).

4) see 1).

 

[sbkenn]

Thanks for that.
Weight really isn't an issue. The vessel (ex-Minesweeper) was designed to be 116tons dry, 164 fully loaded. She is currently about 100tons. The British Navy removed about 15 tons of equipment when converted for training. I removed about another 4, then added about 5 in internal ballast in boxed concrete poured in situ. Several boxes of about 1/2 ton each, resting on the heavy frames, 16" pitch. Fore n aft trim is spot on. She would almost certainly benefit from extra weight, esp' low down, the lower the better. Weight further from the roll axis will slow the roll frequency too.
As for the beam itself, I can't grasp why a curve is necessary. By my inexperienced thinking, if the concrete is in uniform compression, as long as a forced bend doesn't fully relieve the compression and move into tension, it isn't going to crack.

 

[centondollar]

The "forced bend" will relieve axial compression almost immediately. Read some worked examples on design of a prestressed beam in bending or a basic textbook on the subject, and you'll notice that the curvature effect (counteracting normal stresses by inducing an eccentric normal force in the section) is very important.

You're better off stiffening the ship with stifened steel plates and web frames. Less useless weight, more guaranteed bang for the buck. There's a reason for why you don't see prestressed concrete in ships. In this case, google is not hiding search results - there simply aren't any.

 

[sbkenn]