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Re: Loads on Floating Barge

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You are correct, there is a limit to the spring constant.  But, as depicted
by your example of the styrofoam block, it is limited to the case
technically known as "sunk".  Then the constant goes to zero, and the force
is constant.

When the first part of the barge goes under water, the water-plane area
starts to go down, and the average spring constant for the entire barge will
decrease linearly with the water-plane area.  So long as the floating
structure still has any water-plane area left there will still be a spring
constant, however small.

Locally, if an object is partially submerged, that portion of the structure
that is submerged will have a constant force upward, and a spring constant
of zero.  The portion that is still above water will have a linearly varying

We have used this feature by using completely submerged concrete marina
units (w/ styrofoam cores) to provide a constant force upwards to balance
the dead load and reduce the bending of an extra-long heavy-duty ramp to a
marina walkway.

A nice thing about floating structures is that exclusive of lateral loads
(waves, seismic) the loads are easy to accurately predict.


Kris P. Hamilton, PE
Geiger Engineers
114 W. Magnolia Street
Suite 505
Bellingham, WA  98225

Ph: 360 734 7194
Fx: 360 734 7399

> From: "Rich Lewis" <seaint04(--nospam--at)>
> To: <seaint(--nospam--at)>
> Subject: RE: Loads on Floating Barge
> I think there is a limit to the spring constant.  Once the section of the
> spring is submerged the stiffness is a constant and does not increase with
> depth.  I say this as a general statement, not specifically with barges.  I
> worked on structures supported by foam blocks.  Once a block was submerged
> it no longer increased in resistance with additional submersion.
> Rich
> -----Original Message-----
> From: Kris Hamilton [mailto:kph(--nospam--at)]
> Sent: Saturday, October 21, 2006 1:11 PM
> To: seaint(--nospam--at)
> Subject: Re: Loads on Floating Barge
> (I haven't posted in a couple of years; this is the first time in ages that
> I've felt like I have the time to read and respond to the threads.)
> If I understand the Loads on Floating Barge question correctly, Gary is
> asking for the global spring-constant for the analysis of the barge and
> crane.  In this case the spring constant is simply 62.4 #/sf/ft in fresh
> water, 64.2 in salt water.  Actual deflection of the barge, no matter how
> large (unless it is so soft that the ends lift out of the water-plane), will
> not change the spring constant.

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