POSTED ON June 11, 2012
Exterior stairs should be designed and constructed such that the service life of the stair assembly is no less than the life expectancy of the building or structure to which they provide access, and are maintainable without non-visible damage occurring. Increasing the ability of exterior stair assemblies to withstand prolonged exposure to moisture and other deteriorative elements will inevitably extend the service life of the assembly. It is uneconomical to continuously attempt repairs, or to replace stair assemblies that have degraded to the point that they can no longer function as originally intended. A typical failure of exterior stair assemblies occurs in stair designs that incorporate the use of a steel pan with concrete fill at the treads or landings. Manifestation of the damage to this particular type of stair assembly appears as corrosion of the steel liners, freeze-thaw damages to the concrete fill, and structural weld and connection damage. The corrosion will inevitably progress, causing unaesthetic conditions, decreasing structural performance, and ultimately threaten the public’s safety. These negative attributes are further exacerbated when the stair system is designed with closed riser and stringer components.
Stair assemblies constructed with steel pans filled with concrete are inappropriate for exterior construction in areas where exposure to rain, snow, and de-icing salts is expected. Without the incorporation of a drainage mechanism, the steel tread pans act as permanent formwork for the cast-in-place concrete fill, creating a “bathtub” like condition where water accumulates and is held within the tread or landing assembly. Additionally, the joint located at the interface between the steel riser and the concrete tread often experiences separation due to prolonged exposure to variant weather conditions. Constant wetting and exposure to ice management salts, supplemented by freeze-thaw cycles, promotes an ongoing separation between the two materials. This condition promotes further corrosion by directing increased amounts of moisture, now a salt solution, to the bottom of the steel pan. The short- and long-term exposure to moisture creates ideal conditions for corrosion of the steel tread and landing pans. In addition to compromising the structural integrity of the stair assembly, the corrosion results in unsightly rust staining. The problems associated with this particular stair construction can be prevented by protecting the steel and providing expedient drainage mechanisms, or can be avoided completely by utilizing a variety of assemblies that are properly designed for an outdoor environment.
In the case of exterior stair assembly designs that incorporate steel pans with concrete fill, preventative measures can be taken to protect the steel from corrosion that results from exposure to moisture and salts. Two basic methods of protection—barrier and cathodic—can be utilized to prevent steel from experiencing corrosion. Applying an appropriate paint or coating to the steel to form a barrier protection is only effective if the barrier has no penetrations, and the coating is designed to survive for the life of the structure as it cannot be maintained after the concrete is installed. Any inconsistencies of the coating application, which include the exposure of any steel components not covered by the coating, would allow moisture introduction and thus corroding the underlying steel. The coating application does not solve the “bathtub” type construction, and the damage due to freeze-thaw conditions to the concrete surfaces will still occur. To avoid this, coatings utilized in such applications should be of a type that is designed for submersed water conditions. Because exterior stairs are considered to be areas that experience high levels of foot traffic, the exposed portions of coated steel are subject to scratches, chips, or other damage after the application of the coating. Additionally, this assumes that the coating was applied such that all areas of the steel that will be exposed to the elements are completely covered. One method of cathodic protection can be implemented by dipping the steel in a bath of primarily molten zinc, which metallurgically bonds to the steel. This creates galvanized steel, which consists of an outer layer of zinc that serves as a sacrificial protective layer for the underlying steel. Despite the protective properties of galvanized steel against corrosion, constant exposure to moisture and salts will still significantly decrease the service life of the steel components. Additionally, in order for the protective attributes of a galvanized stair assembly to function as intended, the galvanic coating must be free of any imperfections. The coating must be designed for the anticipated use and life of the structure.
Several exterior stair design alternatives exist that will not experience the severe corrosion that results from exposure to inclement weather conditions. Depending upon the desired aesthetic appearance, concrete, steel, or aluminum can be utilized as the primary material used to comprise stair assemblies with either open or closed risers. Steel reinforced concrete stair assemblies can incorporate open and closed risers, which can consist of pre-cast or cast-in–place elements. Steel reinforced concrete treads connected to galvanized steel stringers is an example of an open riser design, while a closed riser design may consist of a single reinforced concrete element that spans from one landing to another. Alternative designs for steel stair assemblies can consist of either open or closed risers, although if closed risers are utilized, it is necessary to incorporate a drainage mechanism such that the accumulation of water is prevented and the ability of water to drain away from the system is promoted. Open riser designs include those that incorporate grated or perforated diamond plate treads connected to stingers. Aluminum or synthetic materials can also be considered and utilized as an alternate to steel or concrete, many of which utilize slip resistant materials atop the treads. Local codes and below stair uses may dictate open or closed tread and riser designs.
When choosing an exterior stair assembly design for your next project, ensure that the life-cycle cost of the selected design is beneficial to the particular project. While the initial cost of one specific design may be less than another, it is important to take into consideration the anticipated cost of repairs and maintenance that will be required in the future. Unless costly maintenance and repairs are budgeted and planned for with a stair design that was incorrectly selected for exterior use, one should choose a stair assembly that is capable of withstanding prolonged periods of moisture for the life expectancy of the building or structure to which it provides access.
Figure 1 – Comparison of exterior stair assemblies. Precast reinforced concrete assembly shown on the left, steel pans with concrete fill on the right. Note the severe corrosion that has occurred to the approximately 15 year old steel pan assembly.