Help with intuitively understanding Post Loading on top of basement foundation wall.

[E M]

A "modern" house I'm currently designing has 2 stacked front porches 10' deep by 35'-0" long (which is the total width of the house.... narrow lot). They are covered with a simple rectangle 3.5 in 12 hip roof (trusses) that is the same roof that covers the main body of the house. Because of the style of the house, I'm keeping my front columns at the porches supporting the roof to 3. So, I have one span 23'-0" +/- and another span 10 or so feet (yeah I know it doesn't add up). What I would like to do is take posts (for all practical purposes) all the way down from the roof and through the 2 porches and have the posts bear on top of a 10" thick cast in place basement foundation wall. Assuming that my footings at these point loads (under the basement wall) are sized properly to support the posts, am I right in my thinking that so long as the posts are fully on top of the basement wall (not necessarily centered but fully on top) that the the basement wall will be able to handle this transfer to the footing without needing to thicken up the basement wall at the areas of the posts? If so, would I need additional reinforcing at these areas? Right now have drawn in 5.5 x 16" glulams as a starting point for both spans and 5-1/2" Posts. Basement walls are 10" thick..... possibly dropping to 8" if that wouldn't affect this "detail".
Just trying to understand this conceptually right now. Will be designed by an engineer.

 

[BAretired]

Sounds reasonable to me.

BA

 

[E M]

Thanks BA Retired for your input- that's encouraging. Hope some others will chime in as well.

 

[jayrod12]

That's how I typically do mine as well.

 

[E M]

Thanks JRod for the reinforcement!

 

[JAE]

...so long as the posts are fully on top of the basement wall (not necessarily centered but fully on top) that the the basement wall will be able to handle this transfer to the footing without needing to thicken up the basement wall at the areas of the posts?

Hmmm. - not necessarily a simple answer - If you have an eccentric load on top of a basement wall, with lateral earth pressures on the wall as well, you could have some significant local bending in the wall and need to add either additional rebar or a pilaster to counter it - depends on the side the eccentricity is on and the magnitude of the load, the height of the wall, the thickness and typical reinforcement in the wall, and the lateral soil pressure. So not a clean and cut answer here.




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[Once20036]

"...assuming that my footings at these point loads (under the basement wall) are sized properly to support the posts"

Personally, I have a hard time justifying the point footings under the concrete wall.
The compression force is going to distribute out through the wall, roughly 45degrees each direction. Assuming you have a 9' high basement wall, this will load an 18' width of wall footing. Will a point load help to resist that load? Are you analyzing a short width of wall and detailing the reinforcing like a column in compression? Seems odd to me.
I tend to check the width of the wall footing and adjust as necessary, skip the point spread footings.

As JAE says, think about the eccentricity/bending of the wall too.

 

[oldestguy]

As to changing wall thickness, each municipality I have lived in has minimum thickness for basement walls in their building code.

 

[BAretired]

If the wall is 8" thick and the post is 5.5" square, the greatest eccentricity would be 1.25", almost exactly on the kern of the wall. If the eccentricity is to the inside of the wall, it would actually help in resisting earth pressure. In any event, this will be reviewed by an engineer before it is constructed.

One of the posts will be at the corner of the house, so a spread footing may be required there. The other post could likely be carried by a strip footing.

BA

 

[E M]

Thanks everyone for the additional insight.
10'-0" Wall Height. This particular wall of this basement will not have any backfill placed against it. It's a walkout basement of sorts.... but used for a garage and therefor a "driveout" basement.

JAE- Not sure how what you're describing would apply to the wall without any back fill as I've clarified above. Would this help (or complicate) the situation? Regardless, IF the load is eccentric (worse case scenario loading on only half of the wall.... 5.5" column on 10" wall)then this could potentially be corrected with additional rebar vs a pilaster? Curious however if I were to have 9' of soil pressure on outside of wall and post was located at interior face of 10" wall how this would play out with the loads.

Once20036- I'm not a structural engineer as stated and much of what you're talking about is over my head but trying to understand/ get a handle on what my options are here. I don't think I was necessarily saying that the footing under the post/ point load would be sized differently than the rest of the footing under the wall (which I think is what you're describing as probably not a good idea), but guess I really figured that it COULD be done that way and I can understand how what I wrote could read that way. Guess I would hope that the typical footing required for the rest of the basement job could handle the load.... maybe even to the point of overengineering the typical footing somewhat for simplification.... I think this is what you're suggesting.... correct? Does this seem reasonable?

Oldestguy- Yes, code dictates that this wall be 8" min with proper reinforcement for that width since it's a 10'0 wall.

 

[E M]

BA Retired. Our last posts overlapped so missed yours initially. Doing a little research on the "kern" but need to think on this some more. Think I'm following about the spread footing possibly being needed at corners of house. Thanks for your insight.

 

[BAretired]

When a load falls within the kern of a wall, the stress is always compression. If it falls exactly on the edge of the kern, stress varies from zero on one face to maximum compression on the other. In your case, the load falls slightly outside the kern, so there will be some tension in the wall but I believe it will be minor, easily resisted by a few vertical bars near the post on the tension side and not requiring a wall thickening. If the wall is retaining earth, the eccentricity of the post can either increase or decrease moment but since your wall is not retaining earth, that is not a factor.

If a post occurs ten feet from the corner, the wall can act as a deep beam to distribute the load to a strip footing. If a post occurs on the corner, the strip footing may still be enough but it may be preferable to add a square or rectangular footing in that location; your engineer will make that call based on the magnitude of load and proximity to the property line.

BA

 

[JAE]

If the eccentricity is towards the earth side of the wall...even if still within the kern, and there's lateral earth pressure where the moments are additive, then you could still have a need for additional rebar - this being residential and I've seen how little (if any) rebar is placed in these walls. That's all I'm saying. The OP didn't state how much eccentricity there is. I've seen residential posts beyond the edge of the bearing support before.


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[E M]

BARetired. Thanks for the thorough explanation. It's really helping me to "understand" some of this. Never really had any structural background but have always been interested.
JAE- Part of my concern is that this is a residential project and there's not really any telling what is going to happen once I hand off the drawings with my engineers recommendations. It certainly will be drawn with the post completely within the wall albeit edges of wall and post could be flush.... not certain exactly what I need to happen yet to get everything else to work out.

Out of curiousity (JAE), when you say "eccentricity towards the earth side" does that mean the way the wall would "bend" before breaking/ failing at the tension side if proper amount of rebar (or thickening) is not incorporated? I'm thinking in my case the fracturing at the tension side of the wall (outside at earth side)if the post is on the interior (basement side)of the wall. Thinking what BARetired is saying is that if this is the case then the earth actually would help push back against the wall and keep it from breaking in tension. If on the other hand the post was on the outside face of the wall (the earth side), then nothing would be pushing back against wall.... which is what you are describing as kind of a worse case scenario.... other than post not being on top of wall.

All of this discussion has me thinking about a couple more factors that I can recognize probably play into this now.
Someone mentioned that the point load is distributed at 45 degree angle downward from top of wall to the footing and BARetired you mention the 10'-0" distance from corner then strip footing could probably handle. Well.... this all has me thinking and may change some things because I don't really have any of those dimensions available.
Sorry but I failed to mention in this thread that this basement and the wall we are talking about that the posts would be bearing on is a walkout basement. Actually it's partially a "driveout" basement with an 18'-0" garage door, because it's on the front of the house (tricky lot). 2 of the posts will be on a corner next to the garage door and another one of the posts will be on a spread footing or tall "pedestal?"... Incorporated into slab?... that the post bears on. The area under the covered front porch where the basement wall turns that is storage was originally slated to be a slab on fill, but trying to find out now (also here in another thread as well) if it could make sense to turn this into storage. Think this will probably get expensive so probably will just be some sort of spread footing below the slab (if I'm thinking about this right as well).

Easier just to show you so decided to include some preliminary plans and a front elevation in case you're curious. Note that I recogonize that I also have some loads that I need to contend with at the front columns that will be clad in either cast stone (heavy) or some sort of porcelain tile.

 

[E M]

For some reason could only upload one file so here's the elevation...