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Thread: Sheil/Drewbie help me fix my deck!

  1. #11
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    That looks similar to the bolts on my deck (10x14) and my deck is about 6 more feet off the ground. Although I think mine is anchored directly to the joists that span the entire deck width.

  2. #12
    Tiny Dancer Drewbie's Avatar
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    They're small. We can go bigger. I have the bits and the bolts are cheap. How close are you to a large hardware store and when would you like me to show up with my drill?

  3. #13
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    Hold on here, instead of following Drewbie's "feeling" about your bolts, let's do some math.

    I'm going to estimate those to be 3/8" diameter bolts. The lowest grade steel commonly available in that size provides 3,975 lbs of shear capacity. Shear is the failure mode you're worried about here, basically cutting the bolt across the shaft. Ok, so you've got two bolts giving you 7,950 lbs of capacity for that connection.

    Now let’s look at the load on that connection. Based on the giant child in the first picture I'm going to overestimate that your deck is 12 ft x 18 ft. We’ll take that 216 SF and apply a standard 80 lbs/sf pedestrian live load. That comes to 17,280 lbs. or approximately 75 Drewbies. It’s hard to tell in the picture but I’m thinking there are three posts across the edge of your deck, which distributes the load across 3 supports and knocks our design load down to 5,760 lbs. or 25 Drewbies on this connection. (Note: we’re not distributing any load to the house, which is cutting the load in half again.)

    Let’s think for a minute about the possibility of 75 Drewbies standing on your deck, it would be crowded and you can’t afford that much beer. Design loads don’t always account for reality.

    Back to the post, based on our design assumptions that post needs to hold 25 Drewbies. Looking back at our bolt capacity of 7,950 lbs. (or 34 Drewbies) it would appear we have an extra 9 Drewbie capacity. Oops, I forgot the live load factor of 1.7, in case all 25 Drewbies drinking over that post decide to dance. That means 25 Drewbies x 1.7 = 43 Drewbies, which means we’re over capacity by 9 Drewbies. Working that out another way 34 Drewbies/1.7 = 20, in other words you can only allow 20 Drewbies to dance on that corner of your deck at one time.

    I suggest that you leave the bolts alone, and install a capacity sign to limit the number of dancing Drewbies to a maximum of 20 at one time. If the Drewbies are all calmly drinking you may safely crowd 34 of them into that corner of your deck.

    EDIT: I realized after the fact I figured this for one-way shear, really we have two way shear because both joists need to shear the bolt to fail, which doubles our capacity again.
    Last edited by nynnja; 08-05-2015 at 06:24 AM.

  4. #14
    Tiny Dancer Drewbie's Avatar
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    Quote Originally Posted by nynnja View Post
    5,760 lbs. or 25 Drewbies
    Hey! Are you calling me fat?

    Does math

    Shit. That's surprisingly accurate.

  5. #15
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  6. #16
    Darth Small Macheath's Avatar
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    Fuck man, there's other reasons to not invite too many Drewbies over.

  7. #17
    Senior Member sheil's Avatar
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    I'm home now so I will respond too.
    The bolts do look undersized to me as well. Get some 1/2" bolts and drill out the holes one at a time and run in a new bolt. 1/2" is pretty much the standard when it comes to beam to post connection. As long as you are planning on landing a harrier or something on the deck they will do just fine.
    The thing that I am seeing that is strange is that it looks like there is a horizontal board running along the length of the beam that the joists are bearing on. That isn't a normal design element.
    Good chance you probably have done this already considering it is the weekend now and you posted this several days ago. Let us know what you did.

  8. #18
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    I'll give you that 1/2" is standard, but it's also not required. I also saw that extra board, wonder if they put it in as a spacer? It would provide lateral stability and make the joists work together. However, I would have used screws instead of nails to attach it.

    Gnorlin, can you just measure the diameter of the bolts so we can stop guessing?

  9. #19
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    Quote Originally Posted by nynnja View Post
    Ok, so you've got two bolts giving you 7,950 lbs of capacity for that connection.
    What about the possibility that the upper bolt is holding a substantially larger load? I feel like this is a likely scenario

  10. #20
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    I haven't done anything yet, I was up visiting my parents over the weekend.

    I'm apparently completely retarded, because I forgot to mention the fact that this deck has a screened in porch with a shingled roof on top of it, which, I would imagine, drastically changes Nynny's math. I'll measure the bolts tonight and get back to you.

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