Stairs play an important part in many projects. Providing both access and egress, stairs can be out of sight and infrequently used—such as in high-rise buildings—or they may be among the most prominent features at the gateway to a site. Regardless of whether their primary function is life safety or looks, they must be well-executed to meet code requirements and to offer optimal functionality. For concrete stairs, that begins with getting the formwork right.
Ed Fallin, project executive for Coastal Construction on the 55-story Trump Royale now under construction in Miami, says on this project they are pouring the stairs with each floor. “That way our stairs are going up with the structure, which helps the crews not have to rely so much on ladders.”
Fallin says that because the building codes have tight requirements on stair dimensions, it's best to have some of the better formwork carpenters on this part of the job. For example, on the Trump Royale the difference in riser heights on a run of stairs, from one landing to the next, is limited to 1/8 inch. The overall dimension of each flight of stairs, also has to stay within 3/16 inch.
On some of its other projects, Coastal has used precast stairways that are flown into place and steel pans that are filled with concrete after having been anchored in-place. “With precast you can control your riser heights very closely,” Fallin says, which addresses one of the parameters that inspectors tend to focus on. “The building department checks them all.”
With steel pans, Fallin says the concrete placement usually lags a floor or two below the working deck. Crews install plywood in the treads or pans to prevent tripping when using those stairs until the concrete is placed.
On the 40-story 1450 Brickell project—a class-A office tower under construction in Miami—Coastal is site-casting most of the concrete stairs, says general superintendent Chuck Hilliard. Frequently done with metal forms, the stairs can be cast and cured in a controlled environment before the tower crane lifts them into place. Hilliard says that although this is a little more costly than forming and pouring in-place—it requires crane time and setting up a casting yard onsite, for example—it minimizes rework. A credit from the finishing contractor on this project also helped offset the additional cost.
“The stairs can be formed with the treads up, or on their side, and with or without nosings,” Hilliard says. The consistency in both dimensions and finish is advantageous, but the fact that they're already cured when they're put into place is also an advantage, he notes. “With fresh cast-in-place concrete you have to be more careful of spills. For example, concrete falling from pouring the next deck will stick to stairs that are fresh concrete, so protection becomes more important for cast-in-place stairs.”
On the 1450 project, Hilliard says they are likely to use site-cast stairs for most of the top 35 floors. “The stairs from ground level to the second floor will be cast-in-place and installed later, after the expected settlement has occurred,” he says. Stairs in other areas where something is nontypical also will be cast-in-place. This includes such things as where there are obstructions, where the alignment must be adjusted, and access for machine rooms.
Although forming and pouring interior stairs presents a number of challenges, the variability only increases when considering exterior stairs. Sometimes the stairs are constructed to match other existing structures, and may include serving as the transition between surfaces that are not perfectly aligned. In other cases, the stairway stands alone in the landscape.
In either case, good planning is very important. The first challenge is to fully understand where your stairs are going to start and stop. You generally begin with a landing at the top or at the bottom. It should be square and level in the direction parallel to the stair treads but
Next consider the overall rise and run of the stairway to determine the rise and run of each stair. Unless there is a good reason for doing otherwise, the rise should be between 6 ½ and 7 ½ inches. One rule of thumb for stairs in that range says maximum user comfort results when the sum of the rise and tread, excluding nosing, is 17 ½ inches.
Take as an example, then, a stairway with an overall rise of 2 feet 11 inches. If we assume four risers, dividing that dimension by 4 makes each rise 8 ¾ inches, which is outside the suggested guidelines. Assuming five risers would make each 7 inches. Note that when planning new work, the top or bottom elevation likely can be adjusted to arrive at such a convenient number, but in matching existing work you might end up with a much less convenient dimension. Just remember to keep the height of each riser the same.
Applying the rule of thumb from above, the ideal tread turns out to be 10 ½ inches, and gives an overall run of 3 feet 6 inches from the front of the bottom riser to the front of the top riser. Now that you know what profile of the completed stairs should look like, the framing can begin.
One helpful method is to use some type of a jig to help minimize any discrepancies in the repetitive work of framing each stair. Two products developed by experienced concrete contractors recently were introduced to do just that.
A handy tool developed by concrete contractor Chuck Blankenship allows one person to form a set of stairs. Beginning with the formwork already set for the top landing, for example, a pair of these adjustable frames hangs the next riser board at the proper position. Then stakes can be driven, the riser anchored to them, and bracing installed.
The frames are set to the predetermined rise and run, so each riser board is set at exactly the correct location. These devices are built and sold by Redding, Calif.-based Bolt-Rite Products, a division of Northwest Plastics.
Leroy Rellergert, Perryville, Mo., has been in concrete construction since 1977. In the past few years he has developed a system for suspending the riser boards from stringers, which gives finishers much more room to finish each tread and reduces the number of obstructions they have to work around. It also eliminates the need to fill in or patch holes left in the stairs by stakes or bracing, and enables the reuse of most of the forming materials.
Rellergert says the way he learned to form stairs was by using 2x12s for the stringers with a reverse cut out for each step. These were located every 36 inches along the length of the stairs, with the riser boards attached at the top of the notches. That required cutting quite a bit of lumber, which afterward was only reusable for forming stairs with the same profile. It also left a section of the tread inaccessible to finishers until the forms had been stripped.
Today rather than cutting 2x12 stringers, Rellergert lays out the stair pattern on 2x6s, adding an extra 5 inches at the bottom end to raise the stringer above the tread surfaces. Angle iron extensions, designed and patented by Rellergert and now sold by Symons as the Concrete Step Wizard, are attached to the stringers at each riser location. The angle is lined up with the riser mark on the 2x6, and upper and lower stops ensure that the riser will be attached at the proper location.
“This system simply eliminates cutting reverse stringers,” Rellergert says. “It gets the stringer up off the top of the riser by 5 inches so the finisher can work without anything in the way.”
To accommodate a toekick, where the riser surface is tipped forward to provide a slightly longer tread, the framer simply redraws the vertical line on the 2x6 with the proper orientation. The angles then are attached along that set of lines.
The stringers should be set on 36-inch centers, with shorter spacing at each end as required by the overall stair length. After the stringers have been assembled, they are attached to the top riser, which has either been set in-place or attached to the existing structure.
One useful feature of this approach is that the stringers can easily be cut to match up with variations in existing grade. For example, Rellergert formed a stairway at Washington University in St. Louis that began at an existing sloping sidewalk at the bottom of the stairs and ended level at a predetermined upper elevation where the future construction would be, simply cutting each stringer to the required span.
“That project showed how critical it is to check the in-field dimensions,” Rellergert says.
The riser boards can be ripped from any good, straight forming lumber, keeping in mind that imperfections on the surface of the boards also will appear on the stair risers. To allow finishers to have access as far back on the tread as possible, Rellergert suggests cutting the riser at a 45-degree angle. If a fall is required for drainage, the risers also can be cut to automatically incorporate that. Based on the example considered the 7-inch riser could be cut at 6 ¾ inches to provide a built-in ¼-inch fall on each stair.
When all the stringers are in place, the riser boards are attached to the extension angles. After applying form release to the riser boards and the extension angles, the formwork is ready for concrete.
Another feature of this system is the speed and ease of stripping the forms and preparing them for reuse. All the angles can be detached from the stringers, leaving convenient handles on the riser boards for tipping them away from the stairs. Alternatively, the screws can be removed from the risers, leaving the stringers set for setting up the next set of forms.
Building concrete stairs is demanding and not an easy task. But at least as far as the formwork goes, it's a lot easier than it used to be.