floor girder questions

[Crappie Slayer]

I am fixing to pull the trigger on a build in the next couple of weeks on some lake property I recently purchased, waiting for the septic to be installed and then I will start with the post and beams.

My question is would 4x6's be large enough for floor girders if they were supported by 6x6 posts sank into the ground every 4 feet, or four feet apart, however you want to put it.

The cabin is going to be either 24x28 or 20x28, still doing my math and over all cost breakdown.

I think that the 6x6's every 4 feet would be over kill, but I over design stuff so I think it would be plenty sturdy.  Anyways as always all of your opinions area appreciated.  Thanks

Chris

[Squirl]

Here is a complete breakdown of minimum girder sizing as directed by the ICC.  It is a complete guide of how to use the chart.
http://countryplans.com/smf/index.php?topic=10511.0

The width of the building, snow load, number of floors, and spacing of the posts are the main factors in sizing.  Depending on these factors a 4x6 (2-2x6's in the chart) space 4 ft O/C might be far undersized for your application.

[MountainDon]

The first question that comes to my mind is, "Is there a permitting process in place on the property? What rules and regulations does the county/municipality enforce, which leads to the question what sort of foundation will be approved? If there is no governing body in place with requirements for foundation, etc. then you are on your own, or perhaps you need the services of an engineer and/or a geotech (soil specialist) to determine what foundations will work.

The requirements of the piers in a pier and beam foundation are such that the piers and their footings not only need to be able to support the weight of the structure, but they also must provide lateral resistance to movement. If the soil is even a little bit "squishy" some of the year the piers may settle unevenly and they will not have much resistance to lateral motion. By lateral movement I mean the wind and other natural forces may pull or push on the tops of the piers much like a levering force, and cause the piers to rotate (tilt) around some point in the ground. As the height above ground increases the depth below ground should be increased and serious rigid bracing added.

We have touched on piers in the IRC board, in the girder span topic. There are a lot of variables which is why piers are not covered prescriptively in any building code. So it's hard to give advice on piers without knowing what the soil is like. On the other hand the IRC does have prescriptive formulas for perimeter concrete footings and block foundations.

Girder size is hard to calculate when we don't know the dimensions of the structure, the height, the roof type, one floor or two or a loft, snow loads, etc. Design the structure as far as length and width, number of stories, roof, windows and doors desired, is there to be tile or stone flooring anywhere, a heavy wood stove, a king size water bed and all that stuff first. To my way of thinking all that must be known before the floor joists, girders and pier sizes and numbers can be determined. Twenty feet wide and wider may require a central beam or engineered floor trusses to provide a floor stiff enough to not bounce or have annoying harmonic vibrations as people move about.

Have you read through the girder span topic in the IRC board that Squirl referenced above?  It is a guide but it is not a complete course in structural engineering. It is a worthwhile read though, IMO, and gives insight into some of the complexities encountered when designing/engineering a good solid foundation.

I don't mean to sound discouraging; I do want everybody to be happy and safe with their completed project.


EDIT, when I hear lake property I think of high water tables and possible pier foundation issues.