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2015 IRC table 403.1

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vegeta82

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I have been reviewing the 2015 IRC and have a question. they have new width and thickness tables. Mostly they appear straight forward. . But their is a footnote that applies to the whole table which states "Based on 32 foot wide house with load bearing center wall that carries half of the tributary attic, and floor framing. For every 2 feet of adjustment to the width of the house add or subtract 2 inches of footing width and 1 inch of footing thickness (but not less than 6 inches thick)"... So questions what is a house width? I'd assume a 60'x80' home has two widths 60' and 80'.. so using a light frame home with brick veneer on 2000psf soil with a live load of 20 psf for a 1 story slab on grade the footing should be 12"x6"... if the home is 60'x80' then applying the footnote the footing on the 80' sides would be 60" wide and 32" deep... is this right???
 
Didn't look at the section, but it is speaking mainly to bearing walls (front to back in a typ ranch lets say) I would use depth as a better description in this case. The "length width" then is almost irrelevant because there is no real loading there...Any clearer?
 
The width is where the roof and floor loads bear. Normally the narrow portion of the house is the width the longer portion the length

I would not use the table and never permit less than a 10" thick footing.
 
A 60' x 80' framed home would not have 'a center bearing wall, more than likely 2 or 3. The tables are a prescriptive option, not the be all and end all. An Engineer would probably design a more standard footing based on the calculated loads.

But I do agree that the width is generally the lesser dimension.
 
Building width should be the span member direction. You can have a 30x40 box with roof frame and floor frame either direction; if you choose 30 as the width and the span members are at 40, you will miss the required footing size from the charts.
 
mtlogcabin is correct; footnote for girder and header spans defines the width is measured perpendicular to the ridge.

I think JBI is on that the same tables prescriptive to 36 ft. if more has to be designed.
 
Wow,

Every time i post on a national code forum I get a reminder how what I see is the outlier.

First the width question. The code should define width in black and white especially if it is going to dictate footing sizes. This notion the code implies width in a floor framing section footnote and now it applies to chapter 4 foundations is not ok with me.. I personally do not see a home having one width. I suppose Francis Vineyard's sentiments about perpendicular to the ridge makes sense. But that really only works for square gable end homes. What about hip roofs? Do you all not see roof configurations with multiple ridges, in different directions? because in most homes I see one could almost never just go perpendicular to the ridge because their are multiple ridges oriented different ways? (again i maybe the outlier)... Is their a better definition of width that is not a footnote in a floor framing chapter? If their is I am not aware of it..

JBI you implied one cannot prescriptively build a 60'x80' house. Talk like that here in Oklahoma would have you searching for a new field. Not being rude just letting you know the reality of the building world. We have a good many homes with dimensions well in excess of 50' on a daily basis. I do challenge the notion that any width in excess of 36' means you are not prescriptive. The footnote makes the provision to use the table in excess of 36'. Why have the footnote if you cannot apply it as prescriptive. For record we also never build basements. It is almost 100 percent footing with a slab and stem wall.
 
Basically the footing and foundation sizes are based on the load it carries and soil capacity. According to the Significant Changes "in the new tables, footing size increases for homes with a crawl space or basement (this addresses previous discussion about support the additional weight of story or portions below grade).

The tables are based on the following load case:

Total load (TL) equal to dead load (D) plus 75% of the snow or roof live load (S, Lr)

TL = D + .75(S or Lr)

In combining the two loads, a reduction is allowed in the live load.

Footnote b is similar to the girder and header for center bearing floor systems.

Although not specifically stated in the code, for the 2012 IRC Table R403.1, footing size and depth were based upon the following assumptions:

Snow load of 50 psf

20 ft. of tributary roof area

16 ft. of tributary floor area

10 ft. first-floor height

8 ft. second- and third-floor heights"

vegeta82 welcome to the forum and thanks for bringing this up for discussion.

Hope this helps.
 
vegeta82,

I'm well aware of what goes on in the real world as I was in construction for over 15 years and in direct enforcement for 17 years.

Not all projects are 'prescriptive', period.

The intent of the prescriptive provisions is to allow design and construction of 'regular' buildings of limited size.

There are a lot of reasons I'd be run out of town in most mid-west communities... proper application the ResCode would be pretty far down on the list. ;-)
 
Do you ever build any ICF houses? They are limited in size



R611.2 Applicability limits.

The provisions of this section shall apply to the construction of exterior concrete walls for buildings not greater than 60 feet (18 288 mm) in plan dimensions, floors with clear spans not greater than 32 feet (9754 mm) and roofs with clear spans not greater than 40 feet (12 192 mm). Buildings shall not exceed 35 feet (10 668 mm) in mean roof height or two stories in height above-grade. Floor/ceiling dead loads shall not exceed 10 pounds per square foot (479 Pa), roof/ceiling dead loads shall not exceed 15 pounds per square foot (718 Pa) and attic live loads shall not exceed 20 pounds per square foot (958 Pa). Roof overhangs shall not exceed 2 feet (610 mm) of horizontal projection beyond the exterior wall and the dead load of the overhangs shall not exceed 8 pounds per square foot (383 Pa).

Walls constructed in accordance with the provisions of this section shall be limited to buildings subjected to a maximum design wind speed of 130 miles per hour (58 m/s) Exposure B, 110 miles per hour (49 m/s) Exposure C and 100 miles per hour (45 m/s) Exposure D. Walls constructed in accordance with the provisions of this section shall be limited to detached one- and two-family dwellings and townhouses assigned to Seismic Design Category A or B, and detached one- and two-family dwellings assigned to Seismic Design Category C.

Buildings that are not within the scope of this section shall be designed in accordance with PCA 100 or ACI 318.

As are SIP homes

R613.2 Applicability limits.

The provisions of this section shall control the construction of exterior structural insulated panel walls and interior load-bearing structural insulated panel walls for buildings not greater than 60 feet (18 288 mm) in length perpendicular to the joist or truss span, not greater than 40 feet (12 192 mm) in width parallel to the joist or truss span and not greater than two stories in height with each wall not greater than 10 feet (3048 mm) high. All exterior walls installed in accordance with the provisions of this section shall be considered as load-bearing walls. Structural insulated panel walls constructed in accordance with the provisions of this section shall be limited to sites subjected to a maximum design wind speed of 130 miles per hour (58 m/s), Exposure A, B or C, and a maximum ground snow load of 70 pounds per foot (3.35 kPa), and Seismic Design Categories A, B, and C.

And Trusses

R802.10.2.1 Applicability limits.

The provisions of this section shall control the design of truss roof framing when snow controls for buildings not greater than 60 feet (18 288 mm) in length perpendicular to the joist, rafter or truss span, not greater than 36 feet (10 973 mm) in width parallel to the joist, rafter or truss span, not greater than two stories in height with each story not greater than 10 feet (3048 mm) high, and roof slopes not smaller than 3:12 (25-percent slope) or greater than 12:12 (100-percent slope). Truss roof framing constructed in accordance with the provisions of this section shall be limited to sites subjected to a maximum design wind speed of 110 miles per hour (49 m/s), Exposure A, B or C, and a maximum ground snow load of 70 psf (3352 Pa). For consistent loading of all truss types, roof snow load is to be computed as: 0.7 p g.

And Steel Roof Framing



R804.1.1 Applicability limits.

The provisions of this section shall control the construction of cold-formed steel roof framing for buildings not greater than 60 feet (18 288 mm) perpendicular to the joist, rafter or truss span, not greater than 40 feet (12 192 mm) in width parallel to the joist span or truss, less than or equal to three stories above grade plane and with roof slopes not less than 3:12 (25-percent slope) or greater than 12:12 (100 percent slope). Cold-formed steel roof framing constructed in accordance with the provisions of this section shall be limited to sites subjected to a maximum design wind speed of 110 miles per hour (49 m/s), Exposure B or C, and a maximum ground snow load of 70 pounds per square foot (3350 Pa).
 
Vegeta....Last I checked, you cannot prescriptively build a deck, handle a point load, or a column in the IRC (I am still on 09)....Some places find ways to make it work with other documentation, some require engineering, it all comes down to what you can justify and sleep at night with...
 
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