Support options for the bottoms of wraparound stair stringers
markledbury
| Posted in Construction Techniques on
I’m designing/building a deck for my brother. He and the Mrs. want wraparound stairs on two sides of the deck. I’ve been trying to figure out the best way to support the bottom part of the stair stringer. I’d rather not have to pour a concrete ribbon for the stringer to sit on (I’ll have about 35 feet of stairs). I thought about putting in a 4×6 beam and connecting the bottom of the stringer to it with a joist hanger (see sketch). Looking at this, I’m not sure the stringer will be strong enough with the support at the end of the stringer, rather than under it.
Any better Ideas are welcome.
Thank you very much for your time 🙂
Mark
Replies
Hey Mark, The best way is to install a continuous pad of poured concrete, but it sounds like that is something you are trying to avoid. Another way to do this would be excavate and pour individual footings every 6 feet or so. Then build the bottom step as boxes that span between the footings. We often build wrap around deck steps in a series of boxes that stack - wedding cake style, although usually on a poured pad.
For our deck, with stairs that didn't wrap around but were sort of "suspended", our archie called for a sort joist/beam (double 2x10, I think, without going out to look) projecting outward from under the deck. Since there were matching stairs on both sides the beam ran all the way across the deck and projected out on both sides. The beam is 2-3 feet up from the bottom, leaving the last 5 feet or so of stringer cantilevered. (But these are full 2x10 stringers, not notched.)
Still standing about 30 years later.
Really?
Dan,
I'm not surprised it's still standing after 30 years; apparently an architect put his professional experience and reputation on the line for your specific solution. However, how does it apply to (or help) the OP'er here?
Are you really suggesting he suspend cantilevered beams for a 4 rise wrap around set of steps?
How would a cantilevered beam, of sufficient size and resonably placed, be unable to support a stair run? The beam runs under the deck and has plenty of anchorage, and only needs to project about 3 feet.
@DanH-
I didn't say or infer that it couldn't be done. Heck, you can do just about anything if you put enough material, effort, and engineering behind it. However, it is not (IMO) a practical suggestion or approach. To quote from your last reply it's "under the deck and has plenty of anchorage". Two points I'll make regarding that part of your statement:
1. You conviently left out the "suspended" part of the cantilevered beam concept. If you would like to provide a conceptual sketch of such structual gymnastics to show how really practical this is, I'm open to that learning experience. Trust me, with an architectural degree, I should be able to understand it.
2. I'd rather work outside the porch deck while creating structure for a wrap around stairway rather than under it. As a carpenter, that real world aspect of your structural suggestion alone makes me cringe.
no continuous footer probably means an engineer
You can think of stair stringers as notched beams set at an angle---much like rafters w/o notches. The effective rafter depth in the case of stringers would essentially be the narrow dimension at the notch. YOu have a short rafter run here, so the size you have drawn probably is not an issue. However, this "rafter" load must be resolved (shared) between the porch deck and the ground with half the load resolved at the porch and half resolved in the ground with a proper footing.
As others pointed out, you can pour a continuous footing (essentially a subterrainian beam) or you can dig piers and span between piers with a girder that catches each beam (aka, in this case, as a stringer). How many stringers you can catch depends on the size and capacity of the girder you can accomodate in your specific design.
If your goal is to minimizing excavation and concrete, than pouring piers and maximizing the span of your girder (post and beam structure) is your best bet. That is easier said than done. You can presumably use the last step as a girder location (with ground contact rated lumber) and connect your stringers with appropriatly detailed hangers or connectors (from perhaps Simpson or USP) just as you will do at the porch. Since you have 4 risers I believe you will also need a handrail by code...which means you will have to accomodate the structure of handrail anchoring posts in your design as well. This link will help you properly consider issues involved with anchoring handrail posts:
http://www.thisiscarpentry.com/2013/11/29/attaching-bottom-deck-posts/
If you are not qualified to design the structural specifications (or able to follow prescriptive deck building codes for your local jurisdiction) for the stairs of your client's wrap around porch, then I suggest consulting with a professional engineer that can.
Thanks, everyone, for your responses :-)
It appears that the best solution is to pour a concrete pad and have the bottom of the stringer sitting on that. how thick should the concrete be? is 4 inches deep enough, and 1 foot wide, or do I need to go down 12 inches (frost depth in western WA)?
If I did go the route of using a beam at the end of the bottom step cut-out, attaching the stringer to the beam witha joist hanger (see blue rectangle with X in picture), do you think the 5 inch width of the uncut portion of the stringer ( see green line in picture) would be strong enough to support people walking on it? Or, is that a question for an engineer?
Thanks again,
Mark
you probably should consult with an engineer
if you are using a beam at the end of your stair stringers to distribute part of the load to piers, then you don' t need a continuous footing /pad at the bottom. One or the other. Either way concrete footings should be adequately sized for the load and be below the frost line.
Mark, you haven't given us all the relative information needed to design your project. And it doesn't matter 'cause your not going to get any definitive and professional course of action for free here anyway. The main thing is that you identify areas of concern (which you have) and address them professionally.
From your comments (like guessing at structural conditions & footing dimensions), you seem unsure about structure. That's okay. Every carpenter isn't expected to be an engineer. In your case, I suggest getting your permit set of drawings approved by your local building department. They very well may have prescriptive code deck details that you can incorporate into your design which will work just fine. Your may also have to consider problem soil coditions in your area that will affect footing size. Who knows? The fail safe course of action is always to consult with a professional engineer (at least for structural issues beyond your ability to specify). Piece of mind in this case should not be that expensive.
I know it's your brothers family and all, but you probably still want to be invited over for Thanksgiving dinner for many years to come:)
Cheers!
The problem with a concrete pad is that, in colder climates, frost heave will lift the pad (and the end of the stair) while the rest fo the deck remains where it was (because its footings go down to the frostline). This is why my archie (who happened to be my brother-in-law) specifically DID NOT want me to use a concrete pad at the foot of the stairway. However, this may not be a concern where you live.
On our deck the beam came through BELOW the (uncut) stringer, with the stringer then resting on the beam. (I don't recall the finer details of this, though.) I suppose it might work as you've illustrated to put the beam at the end of the stringer and "hang" the stringer on the beam, but that's not the way we did it.
Don't rule out concrete pads
A concrete "pad" can be supported by pier footings and (when designed appropriately) can function as a grade beam as well. It's done all the time; even in cold climates. This would be done if you wanted to incorporate the pad as a finished landing as well as a structural component.
I think the joist hanger and beam could easily handle the loads. The trouble is that the hanger would catch mostly the short grain of the bottom notch, and the stringer would be subject to splitting. You'd be better off putting the beam further back and letting the last step cantilever. Or you could just pour a footing under each stringer - With a 12 in. frost depth, you could simply drop in a couple of solid concrete blocks. This really isn't that big a deal.
Small stature?; or small minded?
The original post does not contain a diagram of what you're proposing. And what your proposing is not "simple". If you don't think so, then try getting that detail approved under the prescriptive code. Good luck with that. Depending on how many stringers you intend to catch with your cantileverd beams, the uplight force on the inboard side of the deck will be substantial. You don't show how that is "simply" resolved. But I'm not surprised because your defense of this "simple" solution is just as off the cuff as your initial response.
BTW my cringing of working under a deck has nothing to do with the quality of construciton. It has everything to do with the fact that I am 6' tall (as many carpenters are). You must be of smaller stature.
I'm about 5-10 and the deck is about 4 feet above grade. Yes, it was unpleasant working under there, but placing the canti beam was the least of that. So you refuse to work in unpleasant working conditions?
And I'm a bit confused as to how you can believe that a beam that spans 12 feet between anchorage points and only cantis 3 feet would not be able to handle the load of a short staircase.
a bit confused?
@DanH
To answer your question: No. I'm just not that stupid.
Trying to prove that a beam can span (or not span) for your cantilever advice is a moot point. The original poster already indicated he's building steps for a wrap around porch. Which part of "wrap around" didn't you understand? Even if he could suspend framing beneath his deck to support a bottom tread beam catching the stringers for one side of the porch , how do your intend to have him "simply" cantilver beams for the other adjacent side(s) in the same plane?
I can see the wheels in your head spinning now...and Is that a sucking sound I hear coming from between your teeth?
Let me give you some advice your pappy must not have passed on to you: When you find yourself trying to get out of a hole, the first rule of thumb is to stop digging.
A "wrap around" stair will
A "wrap around" stair will have different flights at different levels. If this technique were used to support the entire length the beams would be at different levels and hence would not interfere with each oher.
You'd understand this if you didn't simply hate the suggestion because I made it.
Wishful thinking...
A wrap around will have differnet flights at different levels?? Where are you getting that from the original post?
You're still digging yourself a hole Dan. Why don't you draw your proposed framing layout for us (including what you're calling "suspended" beams)? Then compare that contraption to what you could build more easily and effecient outboard of the porch. You will then see how ridiculous your idea* really is. It may have worked in a limited scope for your porch, but it's not an effecient approach for this person's project.
* note that I'm addressing your idea, not you personally. I couldn't care less about you.
This seems pretty simple to me. Drop a 6" X 6" PT post down below the frost line on the inside bottom of each stringer. I've built hundreds of outside stairs and have never poured a slab under a single one.
A 6x6 for each stringer of a wrap around? Why bother?
If the OP'er is using PVC for treads, then he will most likely be required to run his stringers run 16" O.C. by the decking manufacturer. Digging 6x6 posts w/ concrete footings at 16" O.C. for eachj stringer? Why bother? Each post footing would practically be touching one another.
YOu could spread things out a bit by using wood decking and framing stringers 24" o.c. However, you could save yourself plenty of 6x6 lumber and use less concrete by pouring piers and using grade beams (concrete or wood). This way you do less digging and leave yourself both decking finish options.