The basics of mezzanines: dimensions, design criteria, materials
While mezzanines, like customers, can be as varied as they are many, there are really only three basic areas of critical importance: dimensions, design criteria and materials. Let’s take them each in that order.
A mezzanine is essentially an elevated area, or partial floor, between a floor and a ceiling. Generally speaking, it usually doesn’t take up the entire floor area but only a part of it. Mezzanine dimensions fall into two general categories: horizontal and vertical. The initial horizontal dimensional information is the exact size and location of the mezzanine footprint within the overall building system. Most mezzanine designers prefer to have this information communicated via sketch.
The old cliché, “A picture is worth a thousand words” is not lost in these situations. If there are any openings in the mezzanine floor, show these on the sketch and note the locations and the exact dimensions of each of the openings. Also, for each opening, it’s helpful to identify their purpose and if there are any specific edge conditions required.
Vertical dimensions include the exact elevation of the mezzanine, usually identified as the elevation of the top of the finished floor relative to a base elevation, as well as any clearances required either below or above the mezzanine. Vertical clearances below the mezzanine are usually noted to be from a base elevation to the lowest point where the mezzanine framing is allowed, which could be the beams, the joists, or both.
Clearances above the mezzanine are usually noted to be from the mezzanine finished floor to the lowest point where metal building framing is allowed, which is typically the rigid frame rafters, the roof purlins or both. The actual thickness of the mezzanine flooring (e.g., concrete or deck thickness) can be a critical dimensional detail needed to ensure proper clearances. While not a true vertical clearance dimension, one should give due consideration to the potential interference of a tapered rigid frame sidewall column; if this is not allowed or undesirable, it’s important to note on the sketch and in the metal building system order documents so a straight column may be provided.
The most important design criteria needed is the floor live load, typically reported in pounds per square foot (psf). This is usually dictated by the intended purpose of the mezzanine, e.g., storage space, offices, etc. The building code has tables stipulating minimum live load requirements for various purposes. In all editions of the International Building Code (IBC), live load tables can be found in Chapter 16. The mezzanine dead load, also expressed in psf, is needed; the configuration or composition of the mezzanine framing will likely establish the dead load of the mezzanine; concrete flooring is significantly heavier than plywood decking.
It’s important to note if there will be any partitions present on the mezzanine as their presence represent loads in addition to the typical floor live and dead loads. Identify any materials to be suspended below the mezzanine framing such as ceilings, HVAC machinery and/or ductwork, sprinkler systems, etc., and provide the dead loads either as uniformly distributed loads (psf) or as concentrated loads (pounds). If there are any concentrated loads to be located on the mezzanine (e.g., machinery, extraordinarily heavy materials, etc.), provide their specific loads and any special support requirements. For all concentrated loads, either above or below the mezzanine, provide their exact dimensional locations so the mezzanine designer can account for their presence and give due consideration to their effects on the mezzanine framing.
Because mezzanine framing materials can be provided by numerous sources other than the metal building supplier, identify exactly which material(s) you want the metal building systems manufacturer to design and supply. It’s not uncommon to ask the manufacturer to only design for the loads from a mezzanine and to provide no materials at all. In this case, the completeness and accuracy of the dimensional and design criteria become increasingly important to ensure the mezzanine is supported safely and properly.
However, it’s more likely some or most portions of the mezzanine framing will be designed and provided by the metal building systems manufacturer. These can include such things as the mezzanine beams and their connections to the metal building framing, the floor joists and bridging, the metal floor decking for concrete floors and related fasteners, the pour stops or edge angles, and any additional support columns.
As a typical rule, the metal building systems designer will determine the sizes and profiles of any material(s) provided by the manufacturer. If there are specific requirements for any of the materials (e.g., specific beam sizes, type or gauge of decking, etc.), it’s imperative these be explicitly noted on the sketch and in the order documents.
Stephen J. Reiners, P.E., SECB, M.ASCE, M.NSPE, M.AWS, is the assistant general manager of Behlen Building Systems, Columbus, Neb., and has been in the metal building systems industry for 38 years. To learn more, visit www.behlenbuildingsystems.com.