Residential Strip Footing Questions 1

[generick]

Hey all,

I got a project where owner wants to incorporate sliding doors with a pocket wall. I'm having some doubts in assessing the existing foundation in Enercalc wherein:

1) For assessing existing strip foundations, would the axial loads from the trimmers/jacks to represent a localized point load onto a strip footing (similar to how column footings are designed)? Does the bottom plate play any part in distributing the point load into a larger surface area onto the foundation? I can imagine these strip footings are sized via stud wall as a "line-load" rather than a localized point load.

2) Is there a way to strengthen existing footing to take these new axial loads? I'd assume with 3 #4 bottom bars in the footing will need more flexural resistance. I imagine it would be a mess having to cut through existing reinforced foundation. UNLESS, these are mainly for T&S whereas the only concern for this would be moreso bearing.

3) Is the soil-bearing pressure taken as "uniform" distribution under the footing as opposed to a triangular distribution due to its eccentricity on-top of the footing itself? I imagine that the slab on grade tied into-it would counter-act this eccentricity.

4) Would out-of-plane wind load acting on king studs normally be taken care of via shear of anchor bolts/hold-downs (dashed circle)?

Thanks and appreciate any insight.

 

[JStructsteel]

Start by looking at the load, how much of the footing width would be needed to support the load. Since you are at a corner with each load, I would say that it would give you some extra distribution.

Then figure out the bearing pressure. Keep in mind the soil has been consolidated for the life of the structure. You have a 2'-6" deep beam, granted not alot of reinforcing.

 

[milkshakelake]

1) To figure that out, you need to do a complicated FEM model with the soil (like with using SAFE) and make sure all the stiffnesses of the slab and soil is set correctly. You probably don't have enough info about the soil to do that. So I'd just assume a certain length of footing for being effective for bearing pressure, and calculate the bending moment / one way shear for that. Maybe 3' x 2.5'. In reality, it's continuous, but this is a simplification that could work. Stud walls are line loads, but posts/jambs for large openings are concentrated loads. It looks like about 15' opening here. If I'm misinterpreting it and it's a few small openings, then yes, you can use a line load.

2) For 30" deep footing, the flexure may actually work. You have a huge d (that's what she said). If it doesn't work, there is a possibility of analyzing it as a deep beam with a strut/tie model. If you want to reinforce it, it's a lot of trouble, but you can pour new isolated footings around it and dowel it with rebar and epoxy. This will reduce the applied bearing pressure.

3) The slab on grade will not counter the tension and prevent rotation that much, unless there's a lot of rebar tying it in. However, you do have two grade beams connecting it to prevent the torsion. So I'd consider it uniform instead of triangular.

4) Yes, generally one anchor bolt or holdown next to it is enough. Even if it's not, the bottom plate will distribute the rest of it. For large openings, I'd check if the standard IBC nailing can handle the shear to bottom plate; generally, it's sufficient, but there's always an option to add a clip connector.

 

[generick]

Thanks for all your input!

 

[BigH]

Is this a residential structure - one to two stories? How much does the door weigh? Thomas Jefferson had pocket doors in Monticello and I doubt that a FEM analysis was run. I guess I just am having a hard time grasping if this question(s) is an overkill. For, say airport hanger doors, okay - but residential?? Oh, and I would think that the strip footing would be with a minimum width, say 600 to 750 just as a matter of course . . . but, this is just me I guess

 

[XR250]

I mean, how much is your point load?

 

[generick]

Apologies if I left that info out from my initial question. This is a residential structure with an existing 2x8 stud wall that goes up 21 feet tall. 2-story house but this wall in particular has no intermediate floors due to a high-ceiling layout. The multi-slide configuration should weigh roughly ~600 lbs. See existing elevation as a reference.

I am coming up with 6.75 k DL and 2.85 k RL for my post reactions.

I have analyzed the strip footing as a 4' x 2' x 2.5' (L x W x D) and concluded that it is sufficient to take the point loads.

I do have another question pertaining to the sliding door manufacturer's deflection criteria:

"The header must be constructed in such a manner as to allow a maximum deflection of 1/8" over the span of the opening when the header is fully loaded."

Does this entail the loading of DL + RL or just RL? I'm thinking that the dead load will already be loaded onto the header whereas the live load on roof would only happen if you'd have people on top of the roof for maintenance or whatever.

Thanks

 

[manstrom]

Step 1.

Stop overthinking it.