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Footing width at wall openings

F

fw.

Bronze Member
Joined
Oct 21, 2009
Messages
56
Location
Minnesota
When reviewing residential plans at what point do you start to consider the weight on the footing from the trimmers supporting a header?
I'm not finding anything specific in the text or the footnotes to the tables. Assuming the footings were designed to the appropriate table. Prescriptive design; no engineer.
Thanks
 
Prescriptive codes take that into account. Footing size is based on the bearing capacity of the soil, and the number of stories. The "weight on the footing" is accounted for based on the stories. If any element of the building exceeds prescriptive requirements, and engineer would have to do the math and change the design accordingly.
 
2018 IRC, Header span Table R602.7(1): Roof, ceiling, and two clear span floors (3 story slab on grade), building is 36' wide, ground snow load of 50#, soil bearing capacity 1,500#. With that table a 4-2x12 header will span 5'6".

Footing sizing Table 403.1 (1) for light-frame construction. It has a footing size for 3 story slab on grade, with a 50# snow load, and 1,500# soils. But the table is based on a 32' wide building with a center bearing wall for the attic and two floors.

So prescriptively I can't use close to 2/3 of the header table because they don't have a footing table that doesn't have a center bearing wall or wider than 32'.
 
fw. said:
2018 IRC, Header span Table R602.7(1): Roof, ceiling, and two clear span floors (3 story slab on grade), building is 36' wide, ground snow load of 50#, soil bearing capacity 1,500#. With that table a 4-2x12 header will span 5'6".

Footing sizing Table 403.1 (1) for light-frame construction. It has a footing size for 3 story slab on grade, with a 50# snow load, and 1,500# soils. But the table is based on a 32' wide building with a center bearing wall for the attic and two floors.

So prescriptively I can't use close to 2/3 of the header table because they don't have a footing table that doesn't have a center bearing wall or wider than 32'.
Click to expand...
Look at the footnotes...

1740685763536.png
 
I'm a BO, and this all started after a phone call from a contractor I respect. He submitted residential plans to a different jurisdiction, the plan reviewer noted on the footing plan that at the foundation bearing point for every opening that a pad footing was required. The size of the pad he required for the example I was given is oversized for the load and soils. I told the contractor I would get back to him with a code reference. The strip footings are oversized per the table. And while I appreciate the comments above about wall openings and their headers being built into the tables there's nothing I can find (besides common sense) that proves that.
 
Is the center bearing wall supporting the roof or floor above or ceiling only?
You would need to the answers in order to determine if you can still use the table
 
fw. said:
He submitted residential plans to a different jurisdiction, the plan reviewer noted... I told the contractor I would get back to him with a code reference.
Click to expand...
Shouldn't the plan reviewer from the other jurisdiction do that for him?
 
fw. said:
Thanks Steve, I did see that then promptly forgot it. But it doesn't resolve that the footing table only has a center bearing wall and half of the header table does not.
Click to expand...
So...you are doing a 36' clear span floor? I see what you are saying, the footing table allows a clear span roof ceiling, but not a floor....And exterior headers do...

1740747919960.png
 
Joe.B said:
Shouldn't the plan reviewer from the other jurisdiction do that for him?
Click to expand...
Yes, he should. My imagined response from the reviewer is going to be either 2018 R301.1 and/or R401.2 which are just general statements that the building and foundation be designed to accomodate all of the loads that will be affecting the structure. I was hoping to give him something more specific to respond with, or to tell him I learned something new, and that the reviewer is correct and this is why.
 
I wouldn't worry too much until the opening is wider than a single garage door and you have a couple stories bearing on it.
 
fw. said:
Yes, the two tables don't mesh. My examples are hypothetical.
Click to expand...
A few comments on IRC Tables 403.1(1) through (3):

(1) The three table titles are (1) ". . . for light-frame construction," (2) ". . . for light-frame construction with brick veneer or lath and plaster," and (3) ". . . with cast-in-place concrete or partially grouted masonry wall construction." So I understand the difference to be in the wall construction and hence the resulting wall dead load. But footnote (b) for all three tables says ". . . 12 psf wall assembly . . ." Seems like this must be an error, each table should have a different wall assembly dead load specified? What should the three values be?

(2) Does the IRC have any other prescriptive guidance on how to size continuous footings? E.g. something like "for X plf live load and Y plf dead load do the following." Seems like the width is easy enough to determine, it would be (X+Y)/(load bearing value of the soil) in feet, but that leaves open the thickness. For example, what procedure was used to determine the required thicknesses in the three tables?

(3) If my assumption in the previous question is correct, namely that the footing size can be determined from just the plf live and dead loads imposed on it, then it seems like we can draw inferences from the tables for conditions not directly listed, based on the loads specified in footnote (b)

For a simple example without any numbers, a "2 story slab-on grade" with a clear span floor would still have no more load on the perimeter footings than a "3 story slab-on grade". The latter has the correct floor load (eliminating the center support wall doubles the floor load), while it would also have a superfluous extra 9 ft of wall load.

For other cases we could calculate the net X and Y for the cases covered in the tables and for the case we actually have, and find a table case where both live and dead loads on the perimeter footing are greater than or equal to what we have. [Not sure how snow load is dealt with in this paradigm; why is 25 psf ground snow load equivalent to 20 psf live load, does the snow load get a 0.8 factor, so you use the larger of Lr and 0.8 * S?]

Cheers, Wayne
 
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