Footing capacity after 35 years

[JStructsteel]

Would an existing wall footing (or really the soils) have additional capacity after 35 years from consolidation? I have a wall footing I want to add some load to. Current design pressure is 2000 allowable bearing. Its maybe got 900lbs/ft currently (including footing weight).

I want to dowel into existing footing, and thus add about 2K/ft bringing the pressure to 2.9K/ft. Would the soil have consolidated enough?

Conversely, The existing wall footing is continuous, 3'-0"x1'-6" so If I said the load was distributed over my new footing width (6.5') plus the wall footing width of 9.5' i could say my pressure would be down to 2000psf along the wall.

sound reasonable?

 

[ANE91]

Would an existing wall footing (or really the soils) have additional capacity after 35 years from consolidation?

Yep

Your situation is the opposite of the above, assuming we’re dealing with clays here.

I’d get a geotech to comment on the numbers. Best case scenario, you induce additional settlement, which may not be tolerable. That’s not an insignificant surcharge. Definitely leverage the new footing; Boussinesq can inform how it stresses the soil underneath the existing footing, doweled or otherwise.

 

[Tomfh]

To me, this looks like double dipping. (Assuming the wall is still in service). Your 900 lbs has already caused some settlement - and another 2000 lbs will cause more settlement - more than 2000 lbs would have.

Allowable bearing capacities aren’t about ultimate “capacity” — they’re about limiting settlement.

I’d simply spread the load across both the new and existing footings. I don’t see why you’d force it all back onto the existing one.

 

[JStructsteel]

To me, this looks like double dipping. (Assuming the wall is still in service). Your 900 lbs has already caused some settlement - and another 2000 lbs will cause more settlement - more than 2000 lbs would have.

Allowable bearing capacities aren’t about ultimate “capacity” — they’re about limiting settlement.

I’d simply spread the load across both the new and existing footings. I don’t see why you’d force it all back onto the existing one.

Im not forcing all of it onto the new one, just the left side. its a pretty even distribution under the combined footing, but unless I say the existing is spreading it out more (the 9'-6") ,then the combined is 2900psf at the footing on the left side.

Maybe the question should be, do you think the existing footing at 3'-0" deep is stiff enough to justify spreading out the load to the 9'-6" dim. Structurally its adequate for the load. I think its stiff enough to distribute the load alot further than the 9'-6", but need 9'-6" min to stay within bearing pressure.

Thanks

 

[Tomfh]

How much it can spread depends on the footing and wall's strength and stiffness. If it is stiff and strong enough to dilute the bearing pressures then you are fine. If the wall is continous the wall loads can redistribute elsewhere anyway...

 

[gte447f]

How much load on the new column? Why dowel them together? In situations like this I prefer not to dowel them together to avoid inducing additional settlement into the existing construction.

 

[JStructsteel]

How much load on the new column? Why dowel them together? In situations like this I prefer not to dowel them together to avoid inducing additional settlement into the existing construction.

Its a 54K load. With the eccentric load, the new footing would exceed the bearing pressure at the joint.

 

[Ron247]

Are you assuming the new footing load to the existing footing is going to cause new eccentricity of the old footing. The new footing must settle some first to "catch up" to the old one in terms of initial settlement. Problem is, the dowels should make the shared load eccentric to the old footing. If the old is 18" wide, but 3' deep, i assume you are in a cold climate. 18" wide isn't very wide to resist overturning. I would check how much of the 54k must transfer to the old, and then check that for increased soil stress due to overturning of the old at a minimum.

I don't know how many footings you have to do, but soil anchors might be an option to consider. Not counting eccentricity, your new footing would have 3.3 ksf soil bearing if not using the old footing.

 

[JStructsteel]

Existing footing is 3' deep, 18" wide. Im trying make it one footing that takes the load, and if need be spread down the wall length a little.