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Sizing footings for covered

Discussion in 'Residential Foundation Codes' started by darcar, Nov 1, 2010.

  1. FredK

    FredK Gold Member

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    Yes indeed.
     
  2. pwood

    pwood Platinum Member

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    we also check for snow combined with seismic!
     
  3. pwood

    pwood Platinum Member

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    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.
     
  4. DRP

    DRP Gold Member

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    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.
     
  5. globe trekker

    globe trekker Sawhorse

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    DRP ( and others ),

    Thank you for your explanation and code section reference.

    I'm still digesting all of this info... :D

    .
     
  6. GHRoberts

    GHRoberts Silver Member

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    My neighbor built a car port. Beautiful structure. The first wind picked the 6x6 supports off of the footings and dropped the whole thing on the ground. Well, on top of the car parked in the car port.

    The footing sizes computed above seem reasonable. I am not sure of the required hold down forces. But 1000+ pounds at each post seems reasonable. Proper shear strength (bracing) at the top and bottom of the posts seems necessary.
     
  7. TimNY

    TimNY Platinum Member

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    DRP- thanks for the uplift calc lesson.. really helps me!
     
  8. peach

    peach Sawhorse

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    so why aren't you asking for the RDP calcuations? We make alot of assumptions based on assumed soil bearing, etc.?

    I would guess good soil, good bearing, but for all we know you're in the one seismic zone with soil subject to becoming plastic...
     
  9. texasbo

    texasbo Platinum Member

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    Peach is exactly right. This is all prescriptive, IF you are certain of soil bearing capacity. 1500-2000? That's a 33% difference.
     
  10. peach

    peach Sawhorse

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    In reality, you could probably bear all the posts on a 4" slab and be ok. the reason screen porches sometimes end up in the driveway or pool is because they aren't connected at the base (simpson and others make very nice, easy to use column bases for just this application) for wind. Don't always assume the roof snow load is less than the ground snow load; if it's a flat roof, that's not necessarily true.
     
  11. DRP

    DRP Gold Member

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    That is the reason I said I use ground snow load... it can actually be higher due to drifting at times.

    I've been biting my tongue since my previous post but this is education so, what the hey. The deck is also a sail, I forgot to add it in. The resistance of the footing to uplift also indludes the cone of soil over the footing. We have enough weight below ground to handle the deck's uplift but the required connection strength just went up. 99 times out of 100 the failure is going to be the connections.

    For a benchmark, blueboard is normally rated at 15psi. Multiply by 144 square inches per foot= 2160 psf bearing capacity.
     
  12. peach

    peach Sawhorse

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    The Codes base everything on ground snow load; not actual snow load.. blowing, drifting against walls, melting, sliding all enter into the picture. Is it an elevated deck (the space between the boards will alleviate some of the uplift against the floor, but not the roof.. it is slab mounted covered screen room? In most parts of the country, you'll look at the major limiting factors. Like it or not, except in the mountains, it'll be wind, not snow.
     
  13. Phil

    Phil Sawhorse

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    DRP

    Without more information, a 20 psf dead load is not conservative when used to resist wind uplift. Also, does the IRC specify load combinations? The IBC requires the strucutre to be evaluated with a 0.6D + W load combination (this is an ASD load combinations, LRFD combinations include 0.9D+1.6W).
     
  14. DRP

    DRP Gold Member

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    Phil,

    The information given by the OP was 90 mph, I used prescriptive table R301.2(2).

    From that section as per Fred K's comments earlier, I do not believe load combinations are required to be investigated below 110 mph. I am interested in your ciphers on this.

    Peach I'm in that SWR known as the Blue Ridge :)
     

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