Designing a boathouse

[idecharlotte]

So I'm designing a boathouse that has 8" diameter wood piles. The piles will be driven in at a 6 ton capacity. Assuming a fixed base, the piles will be very tall to support an upper story deck with a boat slip below. I'm concerned about the sway and want to check for the need for bracing. I can calculate the wind load easily but what load should I use for the impact from a boat on the side of one of the piles. I don't want a light bump to cause the entire structure to shake but I don't know what impact force to assume.

http://www.idecharlotte.com

 

[bridgebuster]

AASHTO Guide Spec for Vessel Collision provides a formula for equaivalent static force

Ps = 220DWT^2(V/27), where

Ps = equivalent static force in kips
DWT = deadweight tonnage of the vessel
V= impact speed in ft/s

Wouldn't you be better off with a fender system to absorb the impact energy rather than putting the force into your piles?

 

[idecharlotte]

I found this after further looking:

P = 8.15 (DWT)^0.5*V

Either way, if I assume a 1 ton boat hitting at 1 ft./sec I get at least 8.15 kips of force. Seems pretty high to me.

http://www.idecharlotte.com

 

[dhengr]

And, 8150lbs. assumes you have a pretty damn strong boat too, or else don’t forget that damage, over and above a spilled drink on the deck above. Remember, energy absorbed or caused on impact has to do with velocity, mass, deflections on impact, and a big assumption about the time interval of the impact. The two equations above make some effort to take this into account. The energy is absorbed by both masses involved in the impact. Look in most Civil or Heavy Construction Handbooks, the Navy has specs. and good refs. on pier designs and loadings, fenders and the like. You may want some protection at the two outer corners of your boat house, against errant boats much larger than your own.

The piles are only partially fixed some distance below the mud line, and you’ve given no dimesnions, then canti. up to the water elev. which may vary; there may be some framing support or bracing at approx. this elev., at least in one wall line direction; then there may be some support or bracing under the upper deck depending upon how you frame it, in two directions. Then you train your skipper to kiss the dock, and live with some amount of movement up on the roof deck. Sorry, no answers, just some food for thought.

 

[SAIL3]

Looks way high to me too.
Blodgetts "Solutions to Design of Weldments" (a great little book)
has impact examples.
Kinetic energy=Potential energy
Kinetic energy=1/2mv^2
Potntial energy=1/2F(delta L), k=F/(delta L)
=F^2/2k
1/2mv^2=F^2/2k, solve for F, k=stiffeness of pile, fixed at base
and cantilevered.

 

[a2mfk]

Someone throw out an anchor on this one! What kind of boats are we talking about here? Anything below 25 feet if coasting towards a dock can usually be stopped by a man using his foot or worse case a decent push or shove. Wind will control lateral loading by a large margin. Now if he come crushing in there at 20 knots, the last thing I'd be worried about is a full lateral failure of your piles...

Plus that stuff dhengr said too