Are these actually barges as your title says? If so structural is the least of your worries. Ships and barges carrying liquids must be carefully designed for stability. Swash plates may need to be installed to prevent liquids from moving too much. Visualize carrying a roaster pan full of soapy water versus carring a full coffee cup. How hard is it to keep the pan from spilling as opposed to the coffee?
This same principle applies to the stability of waterborne vessels and it can be calculated. If you don't know how to calculate the metacentric height of a vessel and how to take deductions from the metacentric height for free surface effect you are asking for a disaster. Barges look deceptively stable but when they flip they flip with lightning speed. Contact a Naval Architect or a Civil Engineer with experience in this area. Naval Architects are not architects they are actually a type of Mechanical Engineer. They have knowledge which is critical to your project. Beware though as in any field of engineering there is the occaisional idiot. Check references.
You need to be able to calculate KG (distance from the keel to the center of gravity of the combined vessel and cargo) KM (keel to metacenter) GM (center of gravity to metacenter) plus the free surface effect of the liquid which is also a metacentric height which is deducted from the vessels metacenter but it is not a straight process of subtraction.
Stucturally the vessel needs to be strong enough so that it can be supported on the crest of a wave with both ends in the air and also with each end supported on a wave with the center up in the air. But that part is easy.
