Question about footing under a mono post/lally column on basement slab

husemn

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Hello, maybe someone can help answer and provide their opinion....

I am installing a 125 gallon cube tank on the second floor along an exterior bearing wall. Tank will be 5" off of the exterior wall. The tank will rest on three joists and the joists are 9" high and span 15' to a beam in the middle of the house that rests on a bearing wall. A couple of those floor joists have already been sistered with a second floor joist that I installed many years ago when I finished the basement. I am anticipating around 1600 lbs final weight when factoring in, tank, stand, water, rock hood etc...

It is a finished basement and I plan on installing a header beam of doubled up LVLs perpendicular to the floor joists about 40" out from the basement bearing wall to help support the front of the tank. Now to support that header I install I am planning on installing 2 mono posts under the header.

My question... The basement is a poured concrete slab so there will be no foundation under the mono posts. Just the slab. Do I need to cut the slab to pour footings for the two mono posts?

My thought process is that each post will be supporting 800lbs and the supporting post plate on the concrete slab is 4"x5" so 20 square inches/ 800lbs= 40PSI. Isnt' concrete rated for 2500PSI thus I should be fine in not cutting up the concrete to add a pier foundation?

What am I missing?
 
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lapin

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I think your fine.
Not all the weight will transfer to the posts. Even if it did your ok.
Im just a hack but the concrete floor will support the posts.
.
 

LRT

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Hello, maybe someone can help answer and provide their opinion....

I am installing a 125 gallon cube tank on the second floor along an exterior bearing wall. Tank will be 5" off of the exterior wall. The tank will rest on three joists and the joists are 9" high and span 15' to a beam in the middle of the house that rests on a bearing wall. A couple of those floor joists have already been sistered with a second floor joist that I installed many years ago when I finished the basement. I am anticipating around 1600 lbs final weight when factoring in, tank, stand, water, rock hood etc...

It is a finished basement and I plan on installing a header beam of doubled up LVLs perpendicular to the floor joists about 40" out from the basement bearing wall to help support the front of the tank. Now to support that header I install I am planning on installing 2 mono posts under the header.

My question... The basement is a poured concrete slab so there will be no foundation under the mono posts. Just the slab. Do I need to cut the slab to pour footings for the two mono posts?

My thought process is that each post will be supporting 800lbs and the supporting post plate on the concrete slab is 4"x5" so 20 square inches/ 800lbs= 40PSI. Isnt' concrete rated for 2500PSI thus I should be fine in not cutting up the concrete to add a pier foundation?

What am I missing?

Chances are your slab was poured with much stronger psi. Most likely 3500-4500 psi. After cure you can likely X that by minimum of 12%.
Also your soil was likely engineer filled, compacted before pour, and your slab thickness is probably 5 or 6 inches and your distributing the weight over the area of posts.
Truth be told ive built a helicopter pad the same way.
 
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husemn

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I think your fine.
Not all the weight will transfer to the posts. Even if it did your ok.
Im just a hack but the concrete floor will support the posts.
.
Thanks and that was also what I was thinking. That some of the weight will be carried by the exterior bearing wall, some will be carried by the internal bearing wall and current joists and some will be carried by the new header and posts. The floor the tank will rest on is already slightly bouncy so trying to minimize that as much as possible when i get the tank filled.
 
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husemn

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Chances are your slab was poured with much stronger psi. Most likely 3500-4500 psi. After cure you can likely X that by minimum of 12%.
Also your soil was likely engineer filled, compacted before pour, and your slab thickness is probably 5 or 6 inches and your distributing the weight over the area of posts.
Truth be told ive built a helicopter pad the same way.
Thanks... forgot that concrete gets stronger after curing. The slab was poured in 1980 if that matters at all.
 

LRT

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Thanks... forgot that concrete gets stronger after curing. The slab was poured in 1980 if that matters at all.
In the 80's when they had pride and cared about craftsmanship they most likely used even stronger psi concrete. Probably reinforced it with steel, pumped and vibe it free of air pockets and produced a much better product anyway.
I say your good to go slab wise.
 

Gtinnel

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The best answer I can give is that in my utility room I put in a couple jack posts to help secure a section of a bearer. I'm sure mine is holding up way more weight than what you would be to help secure a tank.

My best guess is that you are worrying about it way too much. If the joists are 9" high are you saying they're 2x10 (which are 9.25") and with some of them being sistered you probably don't need to worry about any posts below the tank anyway.
 
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husemn

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The best answer I can give is that in my utility room I put in a couple jack posts to help secure a section of a bearer. I'm sure mine is holding up way more weight than what you would be to help secure a tank.

My best guess is that you are worrying about it way too much. If the joists are 9" high are you saying they're 2x10 (which are 9.25") and with some of them being sistered you probably don't need to worry about any posts below the tank anyway.
Understood and I am absolutely worrying too much... I tend to do that:) My concern is that the 3 joists the tank will be resting on are a 15 foot span and currently are attached to the interior load bearing beam by joist hangars. They don't rest on top of the beam due to a weirdly designed 80's themed sunken living room floor. So i am concerned that all of that weight will be putting too much load on those three brackets and I will have too much sag and bounce in the floor. You are probably right and I would really like to avoid adding the new posts in the basement but just don't know how else to get around it.
 

Gtinnel

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Understood and I am absolutely worrying too much... I tend to do that:) My concern is that the 3 joists the tank will be resting on are a 15 foot span and currently are attached to the interior load bearing beam by joist hangars. They don't rest on top of the beam due to a weirdly designed 80's themed sunken living room floor. So i am concerned that all of that weight will be putting too much load on those three brackets and I will have too much sag and bounce in the floor. You are probably right and I would really like to avoid adding the new posts in the basement but just don't know how else to get around it.
Everytime I've seen or used joist hangers there are still nails/screws that go through the joist into the bearer behind it, so it's probably not just the hangers that are holding the weight, although I do agree joist hangers aren't the ideal way to transfer the load. I completely gutted and rebuilt (the inside at least) of my home which was built in the 30s, so I get how over the years there were some less than ideal methods used for construction.
 

snorklr

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if you're really concerned with the load of the mono posts on the slab you could lay a pressure treated 4x6 or 6x6 across the floor for the mono posts to sit on to spread out the load...or add a third mono post...hopefully these will be adjustable to level the floor and so you can readjust them as the wood shrinks
 
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