Question for Engineers (both shadetree and papered)

I come from a long line of farmers.  Most farmers I've known could ace the PE exam and many are also rural certified welders, veterinarians, doctors, and philosophers.  My Dad has built everything from barns to houses using only a hammer, chainsaw, and a length of string.  He used a chainsaw to cut the tin for our barn roof.  I'm saving that story in case there's ever a 'Nearly a Darwin Award" thread.

I have a 16'x12' granary on my farm that straddles a small stream.  It sits on concrete block piers, with one in the run that supports the middle of the 16' span.  Whoever designed it, built more of a dam and as a result the pressure from the occasional high water has washed out the footer and it is leaning.  I'm afraid it will eventually push the building off the supports, so I plan to put in an I-beam corner to corner and remove the middle support.

You can see the poor design and lean of the middle support below.  The other side is the same way.

And here it is during the flood that washed out the footer.  I have no idea how the building stayed on the piers.

I mentioned the issue to my Dad and told him I need to find some I-beams.  He offered some C channel he had and said I could weld them together to make an I-beam.  So my question is:

Welded web-to-web  ][   to make an I-beam  or leg-to-leg   []   to make a tube - which is stronger for this application?  He didn't figure it mattered.

Does it matter on paper? Probably.

Does it matter in real life? No.

I would run it as an I beam to take advantage of the dbl thickness of the main support structure. Welding into a c tube is going to be difficult as with c channel thins out away from center.

Just my AAS mechanical engineering tech degree and just figuring things out in life plugged nickel.

I-beam will be stronger than tube, but tube might be enough depending on size and load. Does that make sense?

my 2c

+1 for I beam. Less chance of it holding moisture. 

The I beam seems like it would have more weldable surface area than the tube.

It appears what you are wanting to consider is distributed weght beam loading for an beam runing from the front to back under the floor in the 1st picture.  

I'm going to for simplicity assume that your beam is sufficiently strong for the loading case so I am only answering the question you are asking.

What you care about is the Moment of Inertia for the cross section.   For that there will be no difference between a hollow rectangle or a I beam of identical sizes and material thicknesses, oriented with the C-channel sections vertically face to face or back to back.  

Moment of Inertia for both shapes will be identical.  They will have identical performance.  

Practically it will probably be easier to bolt through the flange of an I beam than anchor the wood to a closed tube.  I wouldn't bother welding it together, just put some bolts through the web of both C-channels.  Welding would have MINIMAL impact on the strength and stiffness of the beam.

In all honesty, it doesn't really matter from a strength standpoint.

I would do them web-to-web, because you don't want water sitting inside your impromptu tube and rusting it out from the inside.

^ What nocones said. I'd put put a set of bolts at each end and at the 1/4, 1/2, and 3/4 points along the beam.  

Does it matter? It's going to wash away with the next higher flood anyway...

I like how you went from " farmers are basically the smartest people evar" to " I don't know how the building is still standing". Not that I disagree.

I kid, I kid. 

If you are going to all that work to put I-beams under that, why not use the I-beams and a tractor to tow it to higher ground?  Or at least away from where the water is flowing.

While I agree about the I beam, there is no way to seal the seam. All you can really do is treat the surface the best you can before putting them together. 
 

BTW, I would bolt them before welding- buildings are done both ways, but bolting seems more common and there is no risk of warping. 

Many thanks for the input.  Homemade I-beam it is.  To answer the questions:

Farmers are smart.  Farmers didn't build that foundation.  Though my great-grandfather bought the place in 1911 and it's been in the family since, there was a time before my stewardship started in 2016, when it was not owned by rural folk.  G-grandpa had it on posts, which it typical outbuilding construction in those parts.

Why not drag it to flat ground?  What flat ground?  There isn't much in that part of WV and what we do have either already has a building on it or is a garden spot.

In reply to Apis Mellifera :

Some farmers are smart. farmers are not engineers, nor would they pass anything on a PE exam. I digress. 

strength is not a factor with steel channel, and your application. You don't even need to weld them together, as a C channel would be completely adequite for your requirements. If you're welding them for stability to rest on concrete blocks, then that makes sense. 

In reply to lnlogauge :

It's been a while, but I guess the PE no longer has a section on hyperbole?  I also digress.

Not an Engineer and I can't see in the pictures, is it possible to put a turn in the brook and have it pass in front of the granary instead of under it?

My concern with attaching the two C channels together back to back is that area where they mate becoming a prime location for corrosion to occur.  I believe alfadriver alluded to that.  I also believe we're both mechanical engineers, and I know in my job corrosion is something we deal with regularly.  

I also live on a farm type property.  Parts of the house predate the War Between the States.  Like you, I have Seen Some E36 M3 that farmers did.  Depending on how close the property is/was to a railroad line, you may well find bits of old track, RR ties, and tie plates used as structural members in your buildings...

If it were me, I probably wouldn't bother attempting to attach the two C channels together.  Instead I would use them intentionally separate, thus distributing the load across more area, and eliminating as much as possible any additional potential for corrosion.  

I like the "farm use" truck in the first pic.  It looks new.  Is that a 2023 model?

Edit:  Oh, yeah.  I vote I-beam, just seems like it would be easier/more convenient.

Structural PE here.    Web to web is the way to go to make an I-beam shape.  The weld will be easier for one.

A box, as others pointed out, is a place for moisture to collect. 

The Vertical Section modulus and Moment of Inertia will be virtually the same for both approaches. 

The box section will be stiffer horizontally but you aren't going to see any lateral load worth worrying about.

Either approach of welding them will be more stable from rotating about the horizontal  axis than placing the channel sections next to each other without welding.  

volvoclearinghouse said:

My concern with attaching the two C channels together back to back is that area where they mate becoming a prime location for corrosion to occur.  I believe alfadriver alluded to that.  I also believe we're both mechanical engineers, and I know in my job corrosion is something we deal with regularly.  

I also live on a farm type property.  Parts of the house predate the War Between the States.  Like you, I have Seen Some E36 M3 that farmers did.  Depending on how close the property is/was to a railroad line, you may well find bits of old track, RR ties, and tie plates used as structural members in your buildings...

If it were me, I probably wouldn't bother attempting to attach the two C channels together.  Instead I would use them intentionally separate, thus distributing the load across more area, and eliminating as much as possible any additional potential for corrosion.  

 

If rust is a concern then spray the backsides with weld thru primer, just like the body shop uses.

I would jack the building up and put 2-3 more courses of block in to raise it above flood level. That's what my dad did with one of the buildings on the farm. It was near a creek and prone to the occasional flood. A steel beam is going to be pretty unhappy if it's wet. I have see the results of that type situation in some loading docks. Let's just say the bottom foot of the w beam columns holding up an awning for 18 wheelers completely dissolved in less than 20 years because they were mounted directly on the concrete. The ones that were on mounted on concrete risers, 2ft above grade were perfect. You have to get it up and out of the water. Also, you probably have a lot of rotten wood in the floor structure and sills. That's going to be some work. 

 

Mechanical engineer, son of a farmer turned sailor, turned mechanic...