Sizing footings for covered

[darcar]

I want to see what each person comes up with in calculating footing sizes.

This is a simple design for trusses attached to the side of an existing building with the other end supported by beams and posts. The posts are spaced 8 foot on center. End post covering 28sqft (7'x4') and center peirs covering 56sqft (7'x8') roof only - no floor load.

The building is located in Iowa with a ground snow load of 30 PSF.

What criteria do you use to calculate?

 

[Glennman CBO]

What is your roof snow load? If I recall, ground snow load must be converted to roof snow load.

 

[pwood]

darcar,

you have 20# roof snow ? wind speed ? soil bearing? call george roberts collect! he'll give you a size.

 

[darcar]

Wind speed 90mph - soil bearing either 1500 or 2000 - roof snow...?

I multiplied 56sqft x 40 psf (30psf + 10) = 2240

By your charts, what size "round" post footing is required?

and what charts are you using?

 

[mtlogcabin]

What are the size of the post and are you using base plates?

 

[mn joe]

Roof snow load=ground snowX.7. So in your case roof snowload is 30 X .7 = 21psf. you still need dead load and soil bearing. say you allow for 9psf dead load you now are back to 30psf total load. Your intermediate posts are supporting 56s.f. 56X30psf=1680 lbs. divide that by your soil bearing and you have your footing size. Make sure you have any overhangs figured in to your sizing.

Joe

 

[darcar]

overhangs are factored in... 4x4 posts spaced 8 foot on center anchored with base plates

see if I got this right...

if I calculate as above and take 1680lbs / 1500 soil bearing = 1.12 which would equate to about 13"-14" round peirs.

pwood... how does wind speed factor in? as a multiplier and how much?

The reason I'm asking this is to see how each jurisdiction calculates footings for decks, etc. Is there a standard calc? It seems each person has their own way of sizing these

 

[globe trekker]

mn joe ( or others ),

I'm kinda slow at this, ..please bear with me. Assuming darcar has a 2,000 lb. soil bearing

capacity, ...using your 1,680 lbs. total load = 2,000 lbs. / 1,680 lbs. = 1.1904 lbs.

Please explain how 1.1904 lbs. equals a footing size.

Thanks! :?:

.

 

[darcar]

i assume it is 1.1904 feet

 

[DRP]

GT check your units and the direction of your calc

1680 pound load/2000 pounds per square foot bearing capacity= .84 square feet (or greater) footing required.

I use ground snow load from the builder end.

 

[pwood]

darcar,

gravity loads are easy to figure as noted above. you are building a big sail which will blow away in a strong wind unless properly anchored to the properly sized footings. gravity loads may require a 12"round 12" deep footing but a 90 mile per hour exposure C wind may require a 2' round by 2' deep footing. like i said earlier "call george roberts collect". i would not do engineering over the internet, but maybe rick astoria could!:mrgreen:

 

[globe trekker]

DRP,

mn joe stated:

"Your intermediate posts are supporting 56s.f. 56 X 30 psf =1,680 lbs. divide thatby your soil bearing and you have your footing size. Make sure you have any overhangs

figured in to your sizing."

To me, this statement is asking for the soil bearing capacity to be divided by the loads from the postsand roofing, which equals 2,000 lbs. per sq. ft. of soil bearing capacity / 1,680 lbs. per sq. ft.

equals 1.1904.

I may be wrong, which is why I am asking for help in doing the calcs. and trying to

figure this conundrum out. I still do not know what the " 1.1904 " is supposed to

represent.

Maybe someone on here can help this "mathematically challenged" individual out!

.

 

[cboboggs]

Gt, you are sort of correct. Using the soil bearing capacity of 2000 psf the equation would be 1680 psf/2000 psf = .84 square foot of bearing area. So you would need a pier or footing pad about 10.25 inches in width.

 

[DRP]

In the spirit of trying to do prescriptive math,

Uplift;

From the OP description above I'm assuming its a 10' deep porch with posts on 8' centers, 2' overhang.

Looking at the table, 4 of the 7' is in zone 2 or 3, -25.6 psf, multiplying that by 1.29 for exposure C=-33 psf. 3' is in zone 1 works out to -17 psf.

-33 psf x (4'x8') sf=-1056 lbs uplift from zone 2 or 3

-17 psf x (3x8')sf= -408 lbs uplift from zone 1

Total uplift 1464 lbs

Material weights roof and deck (10psf roof+10psf deck) x 56= 1120 lbs

You need a 344 lb boat anchor

Concrete weighs 150 pounds per cubic foot

344/150 pcf=2.29 cubic feet of concrete

 

[High Desert]

globe trekker, the 2,000 number represents a pounds per square foot number. If you divide that by actual pounds, the resulting number tells you how big the footing needs to be to in square feet based on the soils ability to resist that amount of weight. That's as close as I can get to explaining it.

 

[globe trekker]

DRP,

Where are you getting the " 344 " from?

.

 

[FredK]

Not enough info to figure out other loads. Like Pwood stated gravity is one.

Under the old UBC you had to check wind and snow combinations. Now I do neither but ask for engineering on the structure (pole, connections and depth of footing).

 

[globe trekker]

High Desert,

If the resulting number gives a " per square foot " amount, how will someone know how

deep to create said footing. Shouldn't the resulting number be a cubic ft. amount, or

is there a table in the IRC to guide us "unknowledgables" ?

Thanks!

.

 

[darcar]

Not enough info to figure out other loads. Like Pwood stated gravity is one. Under the old UBC you had to check wind and snow combinations. Now I do neither but ask for engineering on the structure (pole, connections and depth of footing).

You ask for engineering for residential decks?

 

[MarkRandall]

Globe,

For uplift you'd calculate cubic feet (to determine weight need to resist uplift), but for vertical loads of soil pressure bearing capacity it is a square foot calculation. Someone with more knowledge than I can maybe chime in with how to determine thickness. Obviously, it has to be thick enough not to fail when load is applied. My engineers usually spec between 8" to 12", the thicker may be because of uplift requirements.

 

[FredK]

You ask for engineering for residential decks?

Yes indeed.

 

[pwood]

Not enough info to figure out other loads. Like Pwood stated gravity is one. Under the old UBC you had to check wind and snow combinations. Now I do neither but ask for engineering on the structure (pole, connections and depth of footing).

we also check for snow combined with seismic!

 

[pwood]

Gt, you are sort of correct. Using the soil bearing capacity of 2000 psf the equation would be 1680 psf/2000 psf = .84 square foot of bearing area. So you would need a pier or footing pad about 10.25 inches in width.

square root the .84 sq.ft and multiply the result times 12 for the footing size in inches by 12"deep. 10.99 "sq. won't work for uplift.

 

[DRP]

GT, conservatively the total uplift of the deck is 1464 lbs at 90 mph. The dead weight of the roof and deck is 1120 lbs. 1464lbs-1120lbs= 344 lbs additional dead load needed at each post to balance the uplift. The snow load will be gone in a 90 mph wind, Iowa is seismic A.

The footing minimum thickness is at least the projected width beyond the post. From the edge of the post up top to the edge of the footing down below there needs to be a minimum 45 degree angle to avoid punch through. You'll find the wording in the foundation chapter R403.1.1.

So far we are still prescriptive IMO. That is a $350 engineering bill here for what is spelled out in the book.

The truss sheets will also provide much of this info.