I’ve got a stair project that we’re looking at right now that went back to the architects for some redesign and on of the changes that was made was that one of the two story stairwells was changed from a U-shaped stair-landing-stair-landing-stair design to a two curved stairs one above the other. I think it’s a pretty neat idea but it poses some unique problems that I thought I’d put out here for everyone to see and chime in on. I scanned a parts of the elevation and plan so everyone could see what I’m talking about. The blue lines I drew show what I expect in the way of the critical forces at play.
This is two curved curved stairs one right above the other and the elevation below (click on it for a larger view) shows that there is nothing underneath this stair holding it up. It sticks out 7′-1-1/2″ out from the floors it attaches two so the only support for the stair is where it connects to the floor. Not a problem with a straight run but it obviously a different case here. The lower floor rests on a basement floor but the second floor stair sits on top of floor system just like the one shown in this elevation(section) of the top floor. So what do ya do to hold a stair like this up? I got a couple of ideas but before I contaminate the idea pool with what I’m thinking about I thought I’d open it to the “floor” for discussion.
I recall a while back a conversation I think Stan once started regarding a stair supported by tensioned cable and there was a lot of kool thinking around that so thought this might stir up some of that again.
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Okay I just found that old discussion I was deferring to. Stan had called it
Curved
Stairway/freestanding/structur... You can click to open it up and read the
whole thing if you like. I'll quote here some of what Stan said in his first
post there since it's very relevant. (The numbering I added.)
all means available to make it structurally sound.
a shorter run than the outside stinger.
to support the dead load of the stairs and any live load.
style. Then you have the stringer being weakened by the riser/tread cutouts.
the strength of this outside stringer. These techniques have been gathered
by personal experience, visits to other stairshops, and questions I have asked
of others.
are NOT on my list.
Like Stan there are surely ideas that aren't on my list either so that's why
I'm bringing it all up again. Regarding what Stan wrote back in April of 2001
Hmmnn, I don't know, while I have a lot of ideas about how I might go about
doing this, the working drawing I'm going to have to submit for this are not
getting submitted without the approval of an engineer.
just makes making the riling a little more difficult, that's bad)
need to connect to up top.
Okay so what new ideas are out there?
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Edited 10/25/2002 6:38:27 PM ET by Jerrald Hayes
Jerrald: Very interesting stairs......I had a 180 degree freestander that went 154 inches floor to floor. It was totally freestanding and the owners were very particular and this baby had to be solid. In fact..it had 19 treads and they tested it by putting a 200 pound guy on each tread....it passed.
I had a welding outfit come in and weld box beams to a triangle frame that was through bolted to the floor at the bottom of the stairs. This had a cantilever force that transmitted half the stair load to the floor. Then...the top had box irons cantilevered out of the second floor system...then more box irons were on site welded inside the belly of the stairs. By the way...the stringers were lapped over each other to give extra depth...each stringer had ten 5/16 inch poplar laminates..that were all scarf jointed so as to have no joints whatsoever in the stringer.
I had toyed with the idea of putting cables and pulleys inside this and tensioning them tup..but the box beams worked beautifully. ....so I never tried the cables.
Jerrald....in your case...I would suggest the inside stringer have a laminated beam built inside the stringer...and arranged so as to half lap with an adjoining beam going on up into the second stair system. There are lots of little tricks that different people can come up with...my advice is to incorporate as many of these tricks so as to have an accumulating strength..to where your stairs is very strong.
I have not had much luck with structural engineers able to advise on what it takes to make a freestander strong. There is nothing like good ol seat of the pants engineering in my book.
The belly of the stairs was plastered in...and unfortunately I never was back to get finished pictures. Here is a poorly lit picture of the freestander..excuse the construction clutter.
Edited 10/25/2002 7:18:33 PM ET by Stan Foster
Recall a FHB aricle (maybe saw it someplace else) in the 80's about a curved stair frame being made up of bent pipe and rebar, then welded together into one big structural element. Welded on clips to attach the woodwork and hide the steel. Seemed like my type project to use surplus pipe and short rebar pieces.
Stan-"...and they tested it by putting a 200 pound
guy on each tread....it passed" I test all of our stairs, I weight
290.
We are the welding outfit in our case. As far as stairs and railing are concerned
I'm in a joint venture with another Ironworks and Fence Contractor and we get
a lot of jobs because we do both the metal and the woodwork. In fact that's
why this contractor called us. The other three story stair that's part of this
project has some tricky ironwork in it holding some of the transverse sections
of it up.
Not exactly sure at all what your saying about what you built but the idea
of constructing a "box beam" does click
with what I was thinking of doing. I'm maybe confused by "a
triangle frame"?? Metal or wood and why triangular? Got any drawings
or sketches of what your talking about?
Questions "laminated beam built inside the stringer."???
The stringer is a laminated beam although I do plan on using thicker stingers
than I would normally. Is that what you were inferring? Ya know a potential
trick I've been thinking about for years was to sandwich fiberglass (with epoxy)
between wood laminations but I've never been able to find any information as
to whether I'd really be gaining any kind of significant strength or stiffness
doing it. I've got about 3/4 of a ton of fiberglass still left around from an
exhibit project we built years ago that I would mind using up. I swear it seems
like I use some of it every other week for something but the stack just doesn't
get any smaller. We used it to make mountains and a volcano for an exhibit environment
that had 25 full-sized Dinamation robotic dinosaurs living in it around the
time Jurassic Park first came out.
I'm not really looking for structural engineer to "tell me what to"
do I use a structural engineer "to approve what I want to do". It's
a liability thing. I am certainly not an engineer but I'm not clueless by any
stretch of the imagination. I know how I "think" I want to make a
box beam out of those floors and I want an engineer (possibly their engineer)
to okay and sign off that I'm correct in my assumptions. I use "seat of
the pants engineering' all the time but in a court when something does go wrong
the "seat of the pants" engineering is often full of...
When I get back tomorrow I going to create an SK of what I want to do and I'll
post that here tomorrow sometime. In the meantime lets hope we hear some other
troops chime in.
By the way how's your son doing nowadays? Everything going along better now?
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Jerrald...the 200 pound guys....19 of them..is what I meant to say were each on one of the 19 treads at the same time...and they all were told to jiggle a little up and down...there was minimal movement.
The stringers were each 1.25 inches thick and lapped over each other..making it actually 2.5 inches thick and a width of around 17 inches.
The stairs had a large footprint on the floor...by that..I mean that the first first four treads were totally enclosed to the floor. Inside this "footprint" of the stairway..I had some beefy angle irons welded into a triangle frame that was secured through the fllor system with large bolts and headered off under the floor system. This gave tremendous support to the box beams that then were welded to these triangles..and followed the hollow cavity under the treads. Little tread supports were welded on and these had a bead of construction glue applied under each tread. With this coming up from the floor..the first 9 treads were easily supported...and vice versa from the cantilevered boxbeams coming down from the top.
You are right about needing a structural engineer to sign it off....to avoid a lawsuit....but our test of 19 / 200 pound guys assured me there would never be a lawsuit. Its the least of my worries.
My son Jeff has had nothing but a remarkable recovery..he has been through 14 months of hell..and its all over now. He is back to work..looks good,...feels good..and is good.. thanks for asking.
Edited 10/25/2002 9:04:02 PM ET by Stan Foster
Great to hear about Jeff Stan, You guys keep on going.
19 guys my size is is called the NY Jets!
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Looks like some solid advice you posted. From some previous frestanding circular stair experience I found it was possible to build up the stringer laminations to 2" or 21/2" with either 1/4" fir ply (inside stringer) and 1/2" fir ply (outside stringer) to achieve a very strong staircase. The housed stringers are going to be the strongest, as opposed to a cut out stringer and having full risers will also be the strongest as opposed to an open riser. I have made treads out of two layers of 3/4" laminated fir ply for treads covered in carpet. For finished wood treads I use an overlay of re-sawn 1/4" oak glued to the 1 1/2" ply and a soid nosing. The use of ply for the treads allows me to house the treads into the stringers very tightly with glue and screws from the outside, no wedges showing underneath and no shrinkage to worry about.
The last lamination can then be done on site out of a finished 1/4" ply to cover the screws. Just a few items off the top of my head.....not sure if this is any help.
One thing I noticed was that the treads on your plan view showed straight treads evolving to tapered treads and back to straight treads...is this O.K. code wise in your area?
Jeff Up North
...oh great!...."20 200 pound guys "jiggling"....."....how do i get the created graphic out of my head?
Cheers,Phil..If it is to be, 'twil be done by me..
That's an interesting mix of materials. When you mention plywood laminated treads are you crewing into the end grain of them or using a pocket screw setup?.
Excellence is its own reward!
Jerrald: There are several little tricks that when all used result in accumulative stength.
Another one that comes to mind is having deep risers that connect the inner and outer stringers. These provide more leverage and also can be cut to form the belly of the stairway as well. Cross bracing also is effective..as well as making the inside sringer extra thick. This can then be used to help support the outside stringer.
Obviously having stringers wider makes a difference also..say having a stringer laminated ouf of 16 -18 inch wide laminates instead of typical 11.25 inch ones.
I am not quite certain how your landing between the two stairs is constructed..but if it can have cantilevered beams supporting it...this will give a good foundation for supporting the stairs above and below...especially the outside stringers.
The inside stringer is easy to make very strong..its just that outside stringer that keeps us challenged.
The plywood has scarf joints to create one single lamination piece. Never used pocket screws but a good idea.....I usually screw (glue) into the ends of the treads from the outside of the stringers. When everything is assembled I apply a final finished lamination over the outside of the stringer of 1/4" oak ply or a veneer. Hides the screws, clamp marks etc. This assumes when laying up the laminations for the stingers on the forming walls that the first lamination layer on the inside of both the stringers is a finished ply (oak or ...) The final step is to finish the top (and bottom) edge of the stringers with either a bent lam. of solid, a segmented (multiple mitres) of solid or a veneer.
Jeff Up North
Edited 10/28/2002 9:25:08 AM ET by JEFFUPNORTH
Jerrald: I dont believe I mentioned or stressed how much strength is added to a stairway just by pocket screwing the risers to the bottom of the treads. This not only is strong enough by itself...but also makes a perfect tight joint for the glue to have its maximum bond. This is much better than most stairs that just have glue blocks stapled on underneath. A bonus is that there are also no visible fasteners.
This really makes the riser/tread joint like an I-beam....
Edited 10/28/2002 6:16:30 PM ET by Stan Foster
Stan-"I don't believe I mentioned or stressed how
much strength is added to a stairway just by pocket screwing the risers to the
bottom of the treads. This not only is strong enough by itself...but also
makes a perfect tight joint for the glue to have its maximum bond."
Couldn't agree with you more on that one! Absolutely and what kind or kinds
of glues do you use?
With the drawings I posted here's something I surprised no one has picked up
on or argued with. You'll notice I made the circle around the beam that the
stairs connect to indicating the torque forces that would be placed on that
beam when there is a downward force (load) on the stair at it's mid point. I
was being intentionally misleading with that in that in reality I feel that
rotational torque force is negligible to the point of it really not even worth
considering. The force or load to be concerned with is really straight down.
That rotational force that I indicated is held in check by the stair being fastened
securely the the beam or floor underneath.
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I'm going down to this job site tomorrow to check for what may be the most
important construction issue of all. As I mentioned these stairs were originally
two sets of U-shaped stair-landing-stair-landing-stair assemblies. Getting all
those units into the home is really no problem. However two fully constructed
curved stairs is another thing altogether. I know Armin has said that he builds
stairs and then breaks them down completely into treads risers and carriages
and then assembles them again on the job site. Have you or do you ever do it
that way?
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Jerrald: First of all...that little circle you had drawn gave me no clue at all what you were after..I still dont know what the exact situation that you have going there.
That freestanding stairway that I posted a picture of was completely built in my shop..and torn down and then reconstructed on site...only because it was logistically impossible to have moved it inot position. It took up a space of 13 feet by 13 feet by 13 feet.....I could not even have hauled it down the road without special permits.
Just hauling the 29 foot outside stringer down the road caused me to worry about getting a ticket. Then it took 8 guys just to screw this outside stringer through the front door.
This was the only curved stairway I have ever had to assemble on site.
Stan I wasn't specifically addressing that rotational torque issue to you (although I just knew there was a real good chance you would probably be in on any discussion about stairs). As for the "situation I having going" with this project; that plan and elevation view is pretty much all there is to it. The only difference between what I scanned and posted here and the drawings I'm working from is size. The same elevation and plan just repeats itself since it's a two story stair. What else were you thinking of that might be important or critical?
What we have done while not breaking down stairs into treads, risers, and carriages we have built them so that they break down into two smaller curved stairs that we then mate and join together on site. I'm not nuts over that idea with respect to this project because there aren't any walls to support the stair so the strength of the stairs is very dependent on the inherent strength of the carriage (stringer).
Twenty something years ago on one of the first few house I ever worked on as a carpenter I saw a curved stair installed that was delivered on a flatbed truck that they then lifted with a crane and then lowerd corkscrewing it through an opening they had made in the roof. For years that was how I though all curved stairs were delivered and installed.
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Jerrald: I didnt mean to sound like you were directing that at me..sorry. After rereading what I had posted..it sounded like I was pointed..and I didnt mean it that way.
Usually my stairs screw the front door.. .....It looks impossible sometimes..but most curved stairways will actually go through a 20 inch wide door.
Jerrald,
You're post has brought up a question that's been in my mind. I am building a landing and two runs of stairs for a guy building his own house. The landing is independent of the main deck. It will be about 72" above grade, with the two runs perpendicular to each other. 4'4 platform. I told him that lateral force produced by the lower run of stairs will significantly push the platform in the opposite direction and the lack of bracing or other means of lateral strength will make for a bad situation. His argument is that the load will down, and as long as the base of the run is securely attached to the slab at grade then there will be no lateral force. He installs vinyl siding for a living so of course he is quite qualified for armchair engineering.
My question is what percentage of force would be transferred laterally. Assuming a typical rise/run, would half of the weight be pushing out and half down? 1/3 & 2/3? I am not looking for exact numbers, just some words from experience (Stan, Armin, Piffin, etc. feel free to chime in).
In response to your question, I also did not notice the circles in the original post. I don't think the torsional force would be a concern unless the beam is a cantilever. I'm still not entirely sure from your posts if this is the case or not.
Jon Blakemore
Jon, if I understand the conditions you are describing correctly he (your client)
is correct ...sort of. And so are you,...sort of. If you install any straight
run stair and secure it at it's base it will exert a lateral force against whatever
the top of the stair is "leaning" against. I'm thinking that that is the force
you were thinking of when you said "...that lateral
force produced by the lower run of stairs will significantly push the platform
in the opposite direction..." and I think you are correct in that
regard however you go on to say "...and the lack
of bracing or other means of lateral strength will make for a bad situation."
My thinking and understanding, and it's been true for all the stairs I've done,
is that when that stair that was leaning against that platform whether it's
a landing or a floor when it's also securely attached at the top it becomes
a triangulating brace to that landing or floor. The stair is then holding
that landing in place (along that plane). I'll make a note that we're not just
nailing our stairs in place, we attach them to the floors and/or platforms with
flanges that are lagged or bolted to those structures.
If I'm not making sense let me know and I'll draw out what I think you are
talking about and we'll see if 1.) I understand what you are describing and
2.) If I am correct about the forces being applied.
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Jerrald,
Let me try to clarify my question and the situation. Imagine a 4'x4' platform 6' above grade with a run of stairs on one of the sides. The force in question would be pushing the platform away from the side that the stairs are on. Will it be wiggly? Or will the triangle bracing action that you mentioned be sufficient? I can certainly understand if there were two runs, on opposite sides of the landing (much like rafter/ridge/rafter) but there's nothing there.
As to your thoughts, I understand what you are saying but am not sure that you are right, althought I have my reservations with saying this. Geometrically I can see that we have a right triangle (stringer, posts, and grade), and in order for the triangle to distort (the angle between grade and posts become >90°) the stringer would have to increase in length. Real world I am not sure that it's that easy, but don't have much concrete proof to back it up. It's getting built either way and if there's a problem I'll deal with it then. Thanks for you insight.
Jon Blakemore
Jon here's a sketch to show you what it was I thought (and still think) you
are saying.
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When you say "The force in question would be pushing
the platform away from the side that the stairs are on. " I
think what you are saying is that a stair secured a Point A will exert
a Force Y at Point B that will cause Motion Z (or Force
Z)? Am I correct? What I'm saying is that when the stair is firmly secured
at Point A and at Point B that Motion Z (or Force Z)
doesn't happen. The Stair AB acts as a triangulating brace. With
that firmly secured connection at both Points A and B the load
is then transferred straight down through the stud underneath Point B
and directly down to the surface at Point A.
What you then said "in order for the triangle to
distort (the angle between grade and posts become >90°) the stringer
would have to increase in length." is absolutly true. I think you
are just doubting yourself a little,... don't. You're absolutly correct with
your thinking on that.
If the stair wasn't attached at Point B then it would "swing"
down on that Arc Y and slide down the stud at B forceing Z
to move to the right. In other word the stair would be "leaning" against
the platform. When you attach it at Point B it's no longer "leaning"
on the platform it's "hanging" from it.
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Jerrald,
Exactly right. Thank-you for the drawing (BTW, what program do you use?). We both understand each other, and I will test out our understanding and report when it's finished.
Jon Blakemore
Jerrald,
As promised, I have a report on the stairs. Finished up on Saturday and there are no significant bracing issues needing to be addressed. The stairs are fine just as they are built, so it looks like you and the client were both right.
Thank-you for your help.
Jon Blakemore
Jon glad to hear it worked out okay and I thought it would. If you scoll back
you'll recall that I thought both you and your client were correct,...sort
of
I think you were just over thinking the problem a little and that got you to
doubting yourself (like that never happens to me, geez!).
I'm going to have to call this builder about that project I illustrated here.
I haven't heard anything from them in just a little over a week now. I'll have to see
what's up with that. Thanks for the reminder.
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