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Homeowner Forum / Construction / April 2006Free standing deck
Want to construct 12' X 16' second story free standing deck to masonry home. Contractor doesn't think free standing is safe. Any pros, cons, advice? In a previous post riverhome wrote... > Want to construct 12' X 16' second story free standing deck to masonry > home. Contractor doesn't think free standing is safe. Any pros, cons, > > advice? A properly designed and properly constructed free standing deck can be just as safe as (or safer than) any deck connected to the house. You may wish to have an engineer review the design and offer recommendations.  SignatureBob Morrison, PE, SE R L Morrison Engineering Co Structural & Civil Engineering Poulsbo WA bob at rlmorrisonengr dot com > Want to construct 12' X 16' second story > free standing deck to masonry home. > Contractor doesn't think free standing is safe. As long as the deck structure is properly sized and braced, it should be just as "safe" as an attached deck. The big advantage of a free standing deck is that it doesn't require a connection to your house. So there's very little chance of water getting behind a ledger board or something and causing rot. In your case, it would mean you wouldn't have to drill into the masonry construction of you house. Another benefit is if you decide to remove the deck years from now, you won't be left with holes in your walls where the deck structure attached. Every deck I have built has been free standing, just for the reasons mentioned above. Anthony This will be 2nd story deck, 12' m/l high. What techniques have you used to insure structure is safe? In a previous post riverhome wrote... > This will be 2nd story deck, 12' m/l high. What techniques have you > used to insure structure is safe? The deck must be adequately braced. I repeat my notation that you should probably have an engineer look at the design. For less than $500 you can have someone who understands the forces involved make sure the the framing and bracing are properly sized. SignatureBob Morrison, PE, SE R L Morrison Engineering Co Structural & Civil Engineering Poulsbo WA bob at rlmorrisonengr dot com > This will be 2nd story deck, 12' m/l high. What techniques have you > used to insure structure is safe? The highest deck I've built was 5 feet off the ground, but the techniques are fairly basic. Proper footings, posts/beams/joists that are properly sized for their spans and loads, lots of diagonal bracing, and BOLTED not nailed or screwed connections on the main posts/beams/braces. Another issue is the railing, especially with a 2nd story deck. Most decks I see add the railing almost as an afterthought. It usually doesn't take much to push the railing out of the way, which seems very unsafe to me. I prefer to extend the deck posts all the way up to support the railing. This results in a much more solid railing. For railing posts that don't have a deck post below, I like to sandwich the post, bolting it between joists before laying the decking. If you have basic building skills, I'd recommend picking up some deck building books from a bookstore or library. They should provide all the info you need to build a deck. I like to oversize my deck structure from the bare minimum's, but that's just my personal preference. If you have any doubts, I'd agree with Bob, check with an engineer. You should also get a permit to build the deck, and the inspectors will verify that you meet some of the basic standards too (though they DO overlook things from time to time). Anthony In a previous post HerHusband wrote... > Another issue is the railing, especially with a 2nd story deck. Most decks > I see add the railing almost as an afterthought. It usually doesn't take [quoted text clipped - 3 lines] > deck post below, I like to sandwich the post, bolting it between joists > before laying the decking. Anthony: IRC/IBC require that a residential rail be able to take a horizontal load of 200 pounds or 20 lbs/ft (whichever is more) at the top of the rail. This doesn't sound like much until you factor in the 42-inch lever arm that is the typical railing post. The connection at the deck level can get very beefy. My favorite way to deal with this is to connect the top bolt of the bolted post to a horizontal hold-down that is attached to the side of a deck joist. To push the post off you must almost complete destroy the deck. SignatureBob Morrison, PE, SE R L Morrison Engineering Co Structural & Civil Engineering Poulsbo WA bob at rlmorrisonengr dot com > My favorite way to deal with this is to connect the top bolt of the > bolted post to a horizontal hold-down that is attached to the side > of a deck joist. To push the post off you must almost complete > destroy the deck. The idea here is that if you push outward on the top of the railing, the fulcrum point is below this topmost bolt, and the horizontal holddown can resist the outward force, despite the mechanical disadvantage? E.g. a 36" high railing on a roughly 9" floor system, if the top bolt is 6" above the fulcrum, then the mechanical disadvantage is 7.5 times, so a 200 lb point load would require a 1500 lb horiztonal holddown. Is that the idea? Thanks, Wayne In a previous post Wayne Whitney wrote... > The idea here is that if you push outward on the top of the railing, > the fulcrum point is below this topmost bolt, and the horizontal [quoted text clipped - 3 lines] > disadvantage is 7.5 times, so a 200 lb point load would require a 1500 > lb horiztonal holddown. Is that the idea? Wayne: You have summed up the idea very well. Just think of how difficult it would be to get a single bolt to resist that 1500 pounds. And also how much cross-grain bending that 1500 pounds would induce in the deck perimeter member if you don't use the HD. SignatureBob Morrison, PE, SE R L Morrison Engineering Co Structural & Civil Engineering Poulsbo WA bob at rlmorrisonengr dot com > In a previous post Wayne Whitney wrote... > [quoted text clipped - 8 lines] > You have summed up the idea very well. Just think of how difficult > it would be to get a single bolt to resist that 1500 pounds. How does the analysis go if two bolts are used, at different heights? The applied torque about the fulcrum is 200 lbs * 45" = 9000 in-lbs. So does one simply divide this torque equally between the two bolts? Then each bolt would have to withstand 4500 in-lbs of torque. So if, e.g. one bolt is 4" above the fulcrum and one is 6" above the fulcrum, the higher bolt connection would need to be rated for 750 lbs (4500/6), and the lower bolt connection for 1125 lbs (4500/4). Is that correct? Thanks, Wayne In a previous post Wayne Whitney wrote... > How does the analysis go if two bolts are used, at different heights? > The applied torque about the fulcrum is 200 lbs * 45" = 9000 in-lbs. [quoted text clipped - 4 lines] > (4500/6), and the lower bolt connection for 1125 lbs (4500/4). Is > that correct? The simple (but not rigorous)analysis goes like this: M = PxL, where L = railing height + distance from top of deck to center of bolt group So for 2x decking and 2x10 framing, 42-inch rail L = 42 + 1.5 + (9.25/2) or about 48 inches M = 200# x 48 = 9600 #-in If the bolts are 3 inches apart, then tension on top bolt T = 9600/3 = 3200# This is a conservative result because it assumes compression in one bolt and tension in the other. In other words the wood has no part in the result. Obviously, in this procedure you can reduce the tension in the top bolt by spacing the bolts as far apart as possible. The problem with trying to figure out where the "fulcrum point" is located, and then ratioing the bolt tensions is that there are too many unknowns in the equations. This is akin the problem of determining the exact steel stress values in a concrete beam that has two layers of rebar. In the latter instance we can reasonably estimate these stresses, because we use a standardized compression block of related to 0.85f'c. No such thing exists for wood. SignatureBob Morrison, PE, SE R L Morrison Engineering Co Structural & Civil Engineering Poulsbo WA bob at rlmorrisonengr dot com > In a previous post Wayne Whitney wrote... > [quoted text clipped - 10 lines] > where the "fulcrum point" is located, and then ratioing the bolt > tensions is that there are too many unknowns in the equations. OK, the "fulcrum point" with two bolts is indeterminate, because the wood will bend or compress, e.g. the band joist member. So your analysis basically treats the lower bolt as the fulcrum point and assigns the entire tension load to the other bolt. Is the two-bolt analysis I originally suggested valid if the wood members were replaced with something infinitely rigid? Both in terms of locating the fulcrum point and then distributing the load between the two bolts by dividing the applied moment equally between them. Lastly, what about the case of overturning moment of wood structural panel shear wall segments? It seems standard to use the outside corner of the shear wall segment as the "fulcrum point" in that situation. So if you have a shear wall segment that would normally require two holddowns of, say, 6000 lbs (ignoring eccentrity), could one instead use 4 holddowns at the panel third points? For rotation about one corner, just consider the furthest two holddowns as resisting the overturning moment, and divide the moment equal between the two holddowns. The outer holddown would need to be 6000/2 = 3000 lbs, and the inner holddown at the 2/3s mark would need to be (6000/2)/(2/3) = 4500 lbs. Is this valid? Thanks, Wayne > So your analysis basically treats the lower bolt as the fulcrum > point and assigns the entire tension load to the other bolt. Actually, I guess your analysis treats the midpoint of the two bolts as the fulcrum and assigns half the moment to the upper bolt as a tension load and half the moment to the lower bolt as a compressive load. In practice, will the lower bolt experience a compressive load comparable in magnitude to the tension load in the upper bolt? Thanks, Wayne In a previous post Wayne Whitney wrote... > Actually, I guess your analysis treats the midpoint of the two bolts > as the fulcrum and assigns half the moment to the upper bolt as a > tension load and half the moment to the lower bolt as a compressive > load. In practice, will the lower bolt experience a compressive load > comparable in magnitude to the tension load in the upper bolt? Probably not. In reality it will probably have a minor tension load as you have suggested in your previous posts. However, this is difficult to calculate. Thus the reason for the less mathematically rigorous solution. SignatureBob Morrison, PE, SE R L Morrison Engineering Co Structural & Civil Engineering Poulsbo WA bob at rlmorrisonengr dot com In a previous post Wayne Whitney wrote... > OK, the "fulcrum point" with two bolts is indeterminate, because the > wood will bend or compress, e.g. the band joist member. So your > analysis basically treats the lower bolt as the fulcrum point and > assigns the entire tension load to the other bolt. Yes that is correct. As I said I said it is NOT a rigorous analysis, but one of convenience that gives a conservative result. > Is the two-bolt analysis I originally suggested valid if the wood > members were replaced with something infinitely rigid? Both in terms > of locating the fulcrum point and then distributing the load between > the two bolts by dividing the applied moment equally between them. You need a compression force as part of your moment resisting couple. It is the determination of that compression force that is the difficult part. If the post were steel and the deck concrete, you have the same indeterminate design problem. This would be like a column base plate with an overturning moment. The "traditional" method has been to simply use the distance between the anchor bolts as the resisting couple. > Lastly, what about the case of overturning moment of wood structural > panel shear wall segments? It seems standard to use the outside > corner of the shear wall segment as the "fulcrum point" in that > situation. I typically use an effective length for overturning that is 6 inches less thatn the overall shear wall length. > So if you have a shear wall segment that would normally require two > holddowns of, say, 6000 lbs (ignoring eccentrity), could one instead [quoted text clipped - 6 lines] > > Thanks, Wayne SignatureBob Morrison, PE, SE R L Morrison Engineering Co Structural & Civil Engineering Poulsbo WA bob at rlmorrisonengr dot com > You need a compression force as part of your moment resisting couple. It > is the determination of that compression force that is the difficult part. OK, thank you. I see now that in the free body diagram of the post, the floor system must apply both an outward force and an inward force to the post to balance the outward force at the top of the railing. So my original one-bolt analysis was too simplistic, although it did easily illustrate the magnitudes of the forces involved. > > Lastly, what about the case of overturning moment of wood structural > > panel shear wall segments? It seems standard to use the outside [quoted text clipped - 3 lines] > I typically use an effective length for overturning that is 6 inches less > thatn the overall shear wall length. My understanding is that a tension/compression post is typically a 3x member (2.5"), and that holddown hardware typically requires an anchor whose centerline is 1.5" from the face of the tension post, for a total of 4" in reduced length. Are you suggesting that the "fulcrum" should not be considered to be the far corner of the compression post? I guess the compressive forces there will be spread out over the base of the compression post, so perhaps the "fulcrum" should be roughly the midpoint of the compression post? Thanks, Wayne In a previous post Wayne Whitney wrote... > My understanding is that a tension/compression post is typically a 3x > member (2.5"), and that holddown hardware typically requires an anchor [quoted text clipped - 4 lines] > of the compression post, so perhaps the "fulcrum" should be roughly > the midpoint of the compression post? That is correct. Again, this is not a rigorous analysis. I'm looking for "rules of thumb" in my design practice that are conservative and easy to remember. For most shear walls, whether the "fulcrum length" is reduced by 5.5 inches or 6.5 inches doesn't make much difference in the HD force, so I opt to use an easy to remember number of 6 inches. One can spend enormous amounts of time calculating the forces in all the elements of a shear wall. When you are on a fixed time budget, then the use of shortcuts to get the work out the door becomes paramount. The trick is to use them effectively so that your client gets a safe design without costing him an arm and a leg for construction. SignatureBob Morrison, PE, SE R L Morrison Engineering Co Structural & Civil Engineering Poulsbo WA bob at rlmorrisonengr dot com In a previous post Wayne Whitney wrote... > So if you have a shear wall segment that would normally require two > holddowns of, say, 6000 lbs (ignoring eccentrity), could one instead [quoted text clipped - 4 lines] > the inner holddown at the 2/3s mark would need to be (6000/2)/(2/3) = > 4500 lbs. Is this valid? You probably could do that, but would need to install 4 pieces of hardware instead of two. I think it would be less expensive to install only 2. SignatureBob Morrison, PE, SE R L Morrison Engineering Co Structural & Civil Engineering Poulsbo WA bob at rlmorrisonengr dot com > You probably could do that, but would need to install 4 pieces of > hardware instead of two. I think it would be less expensive to > install only 2. Right, just wondering if that was a valid alternate strategy when the design load exceeds what can readily be achieved with a single holddown. Of course, with two anchors resisting the tension load, then any anchor spacing load reduction factors must be applied to each anchor. Cheers, Wayne > IRC/IBC require that a residential rail be able to take a horizontal > load of 200 pounds or 20 lbs/ft (whichever is more) at the top of the > rail. Well, you and Wayne are way over my head with the calculations... :) I just look at things from a common sense point of view. I've seen lots of decks where the railing posts were just bolted (even a couple that were just nailed on!) to the outside of the deck joists. One was on a small second story deck at my sister-in-laws apartment building. I leaned on the railing to see the yard below and the thing started moving outwards. Scared the living daylights out of me! :) It doesn't take an engineer to see how easy it is to pull nails out with a 36" long post as a lever. Even if the post is bolted to a single joist, it wouldn't be too difficult to twist the joist by pushing the post outwards, unless there's blocking or something to prevent the twisting. Like I said, my preference is to extend the deck support posts up through the deck to support the railing too. The only way that post is going anywhere is if you push the entire deck over. Obviously, the deck structure needs diagonal bracing to prevent that from happening. I always install two bolts per post, one near the top of the joist, and another near the bottom. That way there's no obvious pivot point for the lever. For situations where I need a railing post in the middle of the deck, I sandwich the post between two joists, and lock it in place with the double bolts. Combined with the posts that extend all the way to the footings, this results in a very strong railing. I weigh 225 pounds and couldn't budge the railing over a 12' distance. > connect the top bolt of the bolted post to a horizontal hold-down > that is attached to the side of a deck joist. I saw that in a magazine recently. Sounds like a handy tip to keep in mind for the future, though I'm hoping I won't be building any more decks any time soon... :) Take care, Anthony |
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