Structural Support

[sajoco]

I have a 24’ X 20’ one story structure (camp) that is supported by 9 concrete piers. The floor joists are 2x6s spanning the 20' width of the structure. The original concrete piers were not set below the frost line and need to be replaced. To do this, we plan to remove the 8 perimeter piers and leave the middle pier because of the difficulty of removing and installing a new one. For the middle section we plan to use a girder that will span the 24 foot width of the structure without using a pier in the middle. Would this be an acceptable way of doing this and if so what type of girder for the middle would you recommend to support this structure?

[Don_P]

You need a girder, a 24'er will be a large one though. I'd try hard to get to that midpost. Between you me and the bedpost, how much moisture is under the middle of the building to frost and heave?

Assuming the roof is bearing on the outside walls only. A mid girder will support the 10 feet down the middle of the 20 foot way, the length of the 24 foot building. Thats 240 square feet x 40 pounds per square foot load = 9600 pounds on a 24 foot span.
If you can get a mid support under there you'll have a 4800 lb load on a 12 foot span.

One way to look at it is by the maximum bending moment the girder is under. The 12' girder supporting 4800 lbs has a bending moment of 7200 lbs. The 24 foot girder supporting 9600 lbs has a bending moment of 28,800 lbs... 4 times the shorter span. The span doubled and the load doubled, 2x2=4.

Another thing to think about is allowable deflection. This is usually limited to 1/360th of the span. 288"/360 = .8" or about 13/16" bounce in the long girder is allowable, half that in the short span. You can design the long girder stiffer than code, and should on that long a span.

Just roughly figuring you are looking at about 3 ply's of 18" deep LVL's at the 24' span, even at that the girder will be unacceptably soft feeling. There are several configurations to talk through with the supplier if you go that route.
With the midpost it looks like 3- 2x12's are sufficient.
You should have double 2x12 rims on the perimeter, or better, to span the 8' pier to pier with floor and roof load.

Your mileage may vary by inspector.

[sajoco]

Don,

Thank you for your response. I’ll need to get a better look at that middle pier but it looks like there has been some shifting. The piers used to build this foundation are what looks like 55 gallon steel drums filled with concrete and embedded in the ground. I don’t see any way to remove this middle barrel. Any suggestions?

[Don_P]

Ignore that pier and put in another beside it? Put in 2 piers at third points? How much working room do you have under there? What is the flooring above?
We've had to dig a pit just to put the jacks in to lift some of the houses around here, built dang near on the ground, and then settled. I've dug my way in before, that sure can get old fast. Parts of some jobs... just suck.

I was working under one house trying to place a jack under a sagging kitchen to get more support and relevel it. It was tight enough that I had to choose face up, arms overhead, and drag myself by the joists. I was well under when something came into my peripheral vision moving fast. Being certain in my compromised condition that this was a life and death battle, I beat myself nearly unconcious, the kitchen floor was probably bouncing, but I did managed to dispatch that little mouse.

[sajoco]

This is a lake front property I am looking to purchase that has been abandoned for a while and is pretty much infested with mice. So the thought of getting underneath the structure isn’t something I look forward to.

There is only about 12 to 18 inches of space under the structure. As for the flooring above, if I’m understanding the reason for the question (remove the flooring and dig between the joist?) this might be a good option here. Most of the interior will be gutted anyway so it won’t be a problem to remove some of the flooring.

Is that what you had in mind and if so how hard would it be to dig from that position? The soil is a hard clay.

[Don_P]

Yup, that's what I was thinking. It won't be fun but might be the easiest way. I use a digging bar (tamping bar) and post hole diggers, dirt into a mortar pan and then outside. We only have to go down 2', what is your frost depth?

[sajoco]

This property is in upstate New York on the east shore of Lake Ontario (just a few miles south of Canada). I don’t know the frost depth for sure yet but I’ve heard around 42 inches. It’ll take a while especially in clay but I think that’s what we’ll have to do.

I appreciate all of your suggestions and advice, it’s been very helpful. Thank you.