Introduction: Modular Interlocking Steel Staircase

Over the last couple months we have been working with our instructor and Autodesk to design a steel staircase that utilizes the potential for custom cuts of steel with a waterjet cutter combined with off the shelf materials. The concept was to design a staircase that can be fabricated to any desired height with a system of interlocking parts.

Our system uses the waterjet cutter for unique cuts of 3/8" plate steel for the stringers and oak hardwood for the stair platforms.

Our steel stair system is designed to be expanded to any size. The system is comprised of an inner and outer stringers that support each other by through bolts at each sides at interlocking points. The system also incorporates a capping system of footers and wall mounting brackets.

Step 1: 3D Design

We modeled all of the components of this staircase in 3D using Rhinoceros 5. This method ensured that accurate tolerances were accounted for fitment after having them waterjet cut.

Step 2: Setting Up Waterjet Cut Files

Each piece was separated and made as 2D line work to be edited for waterjet cutting.

To set up pieces for waterjet cutting, where pieces connect to each other a tolerance of .005" is required. In our model we offset these connection as such to ensure proper fit during assembly. The pieces were then exported in DXF format to be sent to the waterjet cutter for production.

Step 3: Building Wood Stair Treads

We purchased 4 boards of white oak 6/4 thickness from the lumber yard. The boards were surfaced in the wood shop using the planer and jointer. First cut the boards to more manageable sizes of just more than 6' with a rotary arm saw. Next we jointed one face on the joiner, and one edge as well before planing to thickness on the planer. Afterwards we used the table saw to rip the threads to width of approximately 5". Last we cross cut on the table to length of 3'. In order to get a tread thickness of 13" we had to glue up several of these 5" boards. The glue clamps should remain on for an hour, and do not agitate aggressively for 24 hours in order for proper bonding to occur.

Later we sanded down to a 240 grit and added a reveal all the way around the bottom of the thread using a router.

Step 4: Steel Stringer Assembly

Once we had our steel pieces from the water jet, we cleaned off the steel with mineral spirit or use denatured alcohol. We checked the fit and the tolerance was slightly off. We used a file to shave off just a hair so we could get a snug fit.

Afterwards we lined up all our parts to make sure we drilled or tapped the correct sides as the next step. We were building two separate stringers and each stringer consists of two laters: the interior which is hidden from view, and the exterior which is exposed.

We used a counter sink drill bit for the hole on the interior of our stringer so the bolt heads would sit flush with the steel. Then we used a tap wrench to thread the exterior stringer. Once drilling and tapping was down, we bolted the two layers together (interior and exterior) with the bolt head on the interior and the shaft sticking out the other side.

Last we cut the steel tube which spans the stairs to the correct length. The tube sits on and is welded to the interior part of the stringer.

Step 5: Welding

To stabilize the two stringers we use the steel tube to span across the two and join it all together. The tube and the stringer were MIG welded on the inside of the stringer and hidden from view.

Next we used the angle grinder with a cutting wheel to cut the ends of the bolts that were sticking out. We grinded and buffed down these cuts until the bolt shaft was flush with the end of the stinger.

Step 6: Finishing

We used a black magic patina from Sculpt Nouveau to get our desired finish. To apply the patina effectively first clean the steel with denature alcohol or muriatic acid. Then spray on the patina and rinse after 1-3 minutes depending on desired colouring. If blotchiness occurs use a scotch brite pad and go over the patina. Apply another coat for best results then clear coat the steel to prevent it from rusting.

Step 7: Final Assembly

We only mocked up this stair as three steps, however this design could be expanded as far as desired before mounting to a wall.

Note: Ensure stable mounting to structure inside of wall before use.