radius stair stringer

huangyhg

radius stair stringer
i inherited a steel stair design that was using a mc12x10.6 stair stringer. the fabricator already has the stair built and the eor is requiring the steel stair to be designed by a registered engineer. well the short story is that i ended up adding a vertical plate to the channel for torsional stifness. i am wondering if anyone could point me in the right direction for determining the correct stitch weld pattern since my stair stringer has axial, shear, torsion, and flexure forces. please help me in this matter. thank you.
val
val courtney, pe
optimal engineering, inc.
i will take a stab at it.
if the axial load is the same as each of the member and it goes into the channel first and you would like the plate  to share in that load, then provide end welds to transfer that portion of axial load to be carried by the plate.
for flexure, the flexural shear is a measure of the change in bending along the   
if this is an exterior stair, stitch welding can become an issue, allowing water to get inside the section.  i have seen cases where the resultant rusting (rust is about six times the thickness of the parent steel) has actually jacked apart and broken the weld (this was in corten steel, none the less).  i am speaking as an architect (gulp!) who runs a metal fabricating business, and not as an engineer.  i know that continuous welding can be a real pain as it will change the radius of the stringer.
thanks for the advice jike and abelgus!! i really do appreciate it. the radius stair was an interior stair and what i ended up doing was adding a plate to the channel stringer and using stitch welds for the attachment. i also added 2-1/2" schedule 40 pipe at 2'-0" spacing between the two stringers to help stiffen the two stringers. i strongly suggested to the steel fabricator to always call me up before they build or even detail a radius steel stair to ensure that what they are using will work, becasue from my experience a steel channel usually does not work for a radius stair. the steel fabriactor listened to my advice and gave me an elliptical stair to design. anyways, i appreciate the advice!!!
val
val courtney, pe
optimal engineering, inc.
elliptical forms are very interesting to build.  if you use any sort or steel profile (rectangular tube) for the stringer, keep in mind that rolling equipment for curving the stringer works only on a fixed radius curve.  the work-around is to draw the ellipse in autocad, take one typical quarter of it and divide it into a series of blended radii.  you may need only three or four to complete the quarter ellipse.  sometimes the divide command is useful because it will break the ellipse into equal segments - and the curves can then be formed of, for example, one, then three, then five segments, or whatever works.  of course the hard part is moving from the flat into three dimensions - which on second thought would recommend a flat plate stinger, which would be developed out of the elliptical "helix".  the cut out shape would be fed into the rollers at an angle, so that the axis if the stair and radius of curvature would be parallel to the floor.  each segment of curve would be rolled separately, then welded, if rollers are used.  the other possibility is brake forming the curves  by bumping.  the advantage of the former is that each curve can be rolled quickly and uniformly, but requires 18" to 24" of extra material at each end.  brake forming is slower but requires little extra material at the segment end.  also in brake forming, depending on the size of the piece and radius of the curve, sometimes two segents can be blended in one work piece by changing the depth of the stroke to change the radius, but handling large shapes in a press brake can also be very awkward.  the fabricator may know all of this, but it helps to get these things on the air in the design phase.
courtnvm:
just a question, wouldn't the stringer pans brace the stringers for torson?
flat plate stringers are used frequently for the spiral stairway on oil storage tanks.  i am told by the draftsman that there is an equivalent circular radius for the rolling of the stringer.  they often have a twinkle in their eye when they say this...  because they know you have not a clue about what they speak of.  many years ago i think i had a copy of an often used page of equations for sorting through that mess.  it is long gone now.  it seems to be one of those magic secrets of the tank world, but maybe someone here may know how it is done.
steve braune
tank industry consultants