Stairs and rails for a town library: the measure that decides everything

The tape is still on the bench from the morning check, a 25 footer with the hook worn bright, and beside it a stringer that came back from the galvanizer tagged 13 ft 4-1/2 in. The shop drawing calls out 13 ft 4 in. Half an inch is a rounding error on a highway bridge. On a stair that has to land on a nosing somebody else poured, it is the whole job.

This is the note for a town library: two flights, an intermediate landing, guard rail on the open side, handrails both sides of both flights, and a short picket panel at the bottom that the architect labeled "decorative metal, see finish schedule." It is a public building, so the building code has opinions, and the code's opinions arrive in inches.

The field measure comes before anything is drawn

Protocol first. Before the shop put a line on paper, two of us walked the building with a 25 ft tape, a 4 ft level, a plumb bob and chalk line, and a laser distance meter. We read top of the lower slab, top of the landing slab, and top of the upper floor at three points each, both ends and the middle, because a slab that is flat to a quarter inch over 20 ft is a good slab, and a slab that is level to a quarter inch is a different thing entirely.

Then the number that decides the job: nosing face of the bottom step to nosing face of the landing, read in four places. The plan dimension was 11 ft 3 in. The tape read 11 ft 3-1/4 in on the north side and 11 ft 3-5/8 in on the south. The laser read 11 ft 3-1/2 in and 11 ft 3-3/4 in. We took the tape as the number of record, since the laser was shooting past a column corner at the south end, and kept the laser as a check on the tape rather than the other way around.

Rail height is measured from what, exactly?

Every guard rail question starts with a reference line. The guard on the open side of the landing was measured with the 4 ft level laid on the finished floor and the tape hooked on the level's top edge, top of rail down to the finished surface. That is the reference the International Building Code uses for a guard, and the number it holds is 42 in minimum. Our first post read 42-3/8 in, which is fine. The second read 41-15/16 in, over a floor that dipped under it. That one got a 3/8 in shim plate under the post base and a re-read at 42-5/16 in.

Handrails on stairs use a different reference: measured from the nosing of the tread, not the floor, and the band is 34 in to 38 in. We set ours at 36 in at the nosing, which put the rail 35-1/4 in over the tread line at the top of the flight and 36-3/4 in at the bottom, all of it inside the band. The 1-1/2 in schedule 40 pipe has a 1.900 in outside diameter, so a hand can close on it.

How far apart do the pickets go?

The picket panel and the landing guard both had to hold a 4 in sphere. Not a nominal 4 in, an actual 4 in ball, and it has to fail to pass anywhere in the opening, not only at the widest point you happen to like. We laid the pickets out on the bench at 4-5/16 in on center using 1/2 in square bar, which leaves a clear opening of 3-13/16 in. The welder asked for the odd fraction. The odd fraction is the point: 4-1/4 in on center with 1/2 in bar gives 3-3/4 in clear, and then the galvanizer adds zinc at roughly 3 to 5 mils per surface, and it is still fine, but nobody wants to find that out at the railing.

Stair guards carry a second rule that flat guards do not: the triangle formed by the tread, the riser, and the bottom rail cannot pass a 6 in sphere. Our bottom rail sat 2-1/4 in above the tread nosing on the flight, which closes that triangle well short of 6 in.

The dimension that had to be cut twice

The landing edge. The stringer's top cut is notched to sit on a bearing plate welded to the landing beam, and that plate was set in the field by somebody else, from a chalk line, before we got there. The plate elevation read 9 ft 11-1/4 in, not the 9 ft 11-1/2 in on the drawing. That quarter inch moved down the whole stair, and the first stringer we cut took the full quarter out of the top notch rather than splitting it. It fit. It fit with the first riser at 7-3/16 in instead of 7-1/16 in, which is inside the code allowance on variation between the tallest and shortest riser in a flight, but it also put the handrail at 36-7/8 in over that nosing, closer to the top of the band than any of us wanted on a building full of kids.

So we cut a second stringer, split the difference, 1/8 in out of the notch and 1/8 in out of the bottom cut, and re-read every riser in the flight. Tallest 7-3/16 in, shortest 7-1/16 in, a 1/8 in spread across nine risers.

The bolts, and the wrench that signs off

The stringer to landing beam connection is 3/4 in A325 bolts, and the drawing calls the joint pretensioned. The RCSC Specification for Structural Joints Using High-Strength Bolts governs how you get there. We used the calibrated wrench method, wrench set at 300 lb-ft and calibrated that morning against a Skidmore. Six bolts in the joint. All six turned without the wrench slipping, and none moved after. The inspector watched the last two.

Fit-up on the shop floor: the AISC Code of Standard Practice puts fabrication tolerances in numbers, and for member length and squareness this shop held everything inside 1/8 in, read with a tape and checked with a framing square on the bench. The plasma ran a kerf of about 0.060 in on the 1/4 in picket plate and the 5/16 in stringer reinforcement, which is the cut, not a defect, and it gets dressed before paint.

What this note does not cover

This note is about dimensions and fit-up on one stair in one building. It does not cover the structural design of the landing beam the stringer bears on, which was somebody else's sealed drawing. It does not cover the concrete the base plates anchor into, beyond the fact that the anchors went in and the plates sat flat. It does not cover the finish: the paint system, the coating thickness beyond what the galvanizer's certificate states, or the color. It is not a certification of anything, and it does not represent that a building department has signed off. It records what a tape, a level, and a torque wrench read on the days we were there.

The thing to do with this

If you are a GC or an owner's rep and a stair is coming, get the field measure done before the shop draws, and get it in writing, with who read it and against what. If you are in another shop: read the landing plate elevation yourself. Do not take the drawing's word for a plate that somebody set from a chalk line. Half an inch was nothing. A quarter inch cost a stringer.