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circular stair - flat bar stringer
i have a circular stair that spans from the first floor to the second floor which is essentially 360 degree turn. the outside radius is 15'-0" and the inside radius is 9'-10". the maximum span between supports is 23 feet (arc length). i have designed the stringers using either a flat bar or a hss member. to satisfy deflection limits a 1" x 12" flat bar is required. does this flat bar seem unreasonable? also, what are industry standards for flat bar stringers?
thank you,
val courtney, pe
valstone engineering, inc.
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the width of 12" doesnt seem out of the normal range at all. nor does the 1" thick plate.
joe tank
hss sounds like a better section to me than a flat plate. it would be much better for torsion.
well my client wanted me to give him a stringer size using a flat bar or a hss section. he flipped out when i told him that a 1" by 12" flat bar would be required. he stated that the thickness seemed really big compared to what they have done in the past. i did a structural model of the stair to determine the stringer sizes. just wanted to make sure i was not going insane.
val courtney, pe
valstone engineering, inc.
what is your hss size? i would think it would work much better.
hss12x2x3/16
val courtney, pe
valstone engineering, inc.
consider a "standard" c channel for a emergency exit stair going the same net rise and run: there, for a (somewhat smaller) stair tread width, you need a 1-1/2 or 2" "flange" (missing from your flat bar) and a 8-10 inch deep web stiffened at both ends with 1/4 thick steel.
just a flat bar 1x12 seems pretty reasonable in comparision, doesn't it?
try this: if your building code allows it, add a center flange between the two outside stringers. that will let you reduce the depth of the outside stringers slightly.
second, carry the 1x12 plate up past the ends of the stair trends to form an archt. feature that reduces the apparent height of the outside plate. the stair treads will go into the "side" of the stringer plate, rather than going over the top of the plate, but the higher side plate merely serves as a toe plate instead. this may be your intention already.
when i roll square tube steel for decorative swept/spiral iron stairs, i need to create three bends simultaneously: the obvious one is in the "plan view": the stair curves in a circle. superimposed on this is a spiral as the stringers go down: if the rails and horizontal members are round (1-1/4 sch 40 pipes are common) you can roll the pipes in both curves at once and get a good aproximation of a fit.
if the stairs are swept vice a constant radius spiral, then each sweeping section has to be templated and rolled manually. stick to no more than 10 foot sections in any case. that way, if the roll screws up, the previous and next 10 foot section can still be used. weld, and grind smooth, the final stringers to get a look of one smooth piece of steel.
but, if the rails and horizontal members are rectangular - as they are here, you will need to "twist" the member to keep its top "flat" and its sides "vertical" as they go down the spiral. the mental image of a screw threads is a bit mispleading because an actual screw thread isn't "square" - it is too small to see the twisting effect at that scale anyway. try a couple of flexible (plastic) 1/4 x 2 trim molding pieces from home depot, at home bend them into a spiral approximating the scale stairs you have.
two options: i usually use 1x1 11 ga tube steel for residential iron stairs, most of the time these are less than a slope length of 14-15 feet so i roll two 8 ft sections, cutting off the 12" waste at each end of the rolled piece. (the rolling machine can't bend the last 8" of the
hss is structurally an better shape, certainly it will weight less.
but they are very, very difficult to roll, twist, and bend into a spiral (particularly if you need a high, narrow shape!) without bending and "dimpling" the side plates of the hss.
with a solid plate, there is no chance of compressing and "flattening" the member you are trying to twist. if you screw up the roll, or the bend, or the twist while trying to get the spiral, you can always heat it up and chainfall it back into position. also, if two sections are welded butt-butt to make a single
could you clarify the section regarding a 1x1 11ga tube steel?
val courtney, pe
valstone engineering, inc.
on a stair that i did the steel fabricator built the stringers on site with 1/4" thick rolled steel plates and then welded them together to form the hss shape i actually analyzed in risa. as you say, it is tough to roll a tube into a 3d shape.
1x1x 11 ga tube steel.
remember, i was building the wrought (decorative welded steel ballusters and railing) on an existing wooden stringer stair.
the wooden stringer was my strength member. most stairs shops that do spirals/swept stairs build them on site from thinner laminate strips glued together:
thinknig about his comment, i agree with structural aggie that four 1/4 x 12 thin plates, welded top and bottom with a series of button welds in between (maybe every 12" ?) would be much easier to handle and move into shape: almost doing the job by hand and c-clamp rather than shipbuilding-sized rolls and cranes. you face the expense of on-site welding then, but don't need expensive off-site templates. and even more expensive on-site templating, off-site template building, and on-site template rebuilding and on-site re-repairing of the final product.) but i disagree that welding them into an hss shape (top, inside, bottom, and outsdie) is easier than making a single 1x12 final member out of four identical 1/4 x 12 plates that are free to "slide" into place next to each other as they get moved into position.
my favorite spiral swept stair (email me, i'll send some pictures) was made of two 1x1x11 ga sq ts members that formed an upper and lower rails. a 1-3/4 molded hadrail was welded to the upper sq tube steel: it also had to be rolled into shape, twisted down and clamped to the rial, then welded underneath to the tube steel. joints were tough: but the 1-3/4 molded rail was relatively flat compared to its width so it was "pliable". again, i had to be real careful not to "squish" the hollow tube steel as i "coaxed" each end into position.
the decorative iron forged shapes were welded between the
the verticals (on every third step) were also 1x1 11 ga, with a 1/2" square bolt welded inside as a bolting surface. the verticals were bolted to the stringers with 6" long, 3/8" dia bolts going through the 1/2" dia hole in the sq nut and were about 30" apart. as a reinforced spiral, the net result was very stiff: even though the individual members were not very strong. in a straight rail, i needed 1-1/2 11 ga sq tube steel for the verticals because i had no curve to increase rigidity.
the forged steel decorative iron between the verticals didn't really add anything to the strength of the rails against getting pushed over, but they were needed to meet the 4" building code rules.

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