Q: When trying to select cold formed steel framing members, it appears that there are several options for any given situation. For example, you can select (in very simple terms with a bit of rounding) a 5.5 inch deep stud with a 33 mil base metal thickness (gauge), or a 4.00 inch deep stud with a 68 mil base metal thickness, or a 3.62 inch deep stud with a 97 mil base metal thickness, all with a 1.62 inch flange width, all on 16-inch centers, with the same load requirement, for use with approximate height/length of 29.5 feet. How do I decide whether to select the framing member with the greater base metal thickness or the greater depth?
A: This question requires expert help, so I consulted Robert Grupe, the market development manager for the Steel Framing Industry Association. He said, "The goal simply stated is to find the thinnest gauge steel at the greatest spacing that meets the height/span requirements.
"Ideal stud and joist design is a balancing act between performance and economy, using the least amount of steel possible. Structural performance of cold-formed steel framing members is a combination of the material properties of the steel, and the shape of the stud or joist. Material properties include the yield stress of the steel itself, which is an indicator of the framing member’s ability to bear load without permanent deformation. The higher the yield stress, the greater the potential strength. However, strength is also governed by the physical shape. The wider flanges on some members are there to maximize the allowable stress that the profile can accommodate.
"Stiffness of walls or floors is achieved through the depth of the framing member and the elasticity of the framing member material. The deeper the framing, the stiffer the wall will be. In wall and floor design, there is a limit put on how much the assembly can deflect. This limit is imposed in the design to assure that the intended finish materials (plaster, drywall) will not crack. The downside of deep framing members is they are more susceptible to web buckling at the supports.
"Published limiting height or span tables are developed by looking at controlling deflection, allowable stress in bending of the steel, and web buckling at the supports. Typically, higher yield strength (50 versus 33 ksi) is used in structural applications, either axial load bearing or in curtain wall conditions. Higher strengths combined with wider flanges provide bending and axial load carrying capacity. Narrower flanges and lower yield strengths are better suited for non-structural applications. The exception is the newer generation of non-structural framing members. Manufacturers have manipulated yield strength and physical shape to maximize performance while using less steel.
"The required wall height and thickness cannot be altered. Wall thickness is critical to the design of the overall building (from rentable floor space to door frames), and the floor-to-floor height is from the structural design. The thinnest mil thickness at the maximum framing spacing is ideal for the most economical solution. Wall height and thickness may dictate tighter framing spacing and/or thicker mil thickness of the steel. If it is determined that this (thicker steel and tighter spacing) will have to happen to meet the height, then analyze the negative impact on acoustical performance.
"A quick review is to look at the limiting heights tables and focus on the design load (say 5 psf) and limiting deflection (l/240 for example). Scroll down the column labelled L/240 and find the first height at 24” o.c. that exceeds your required height. Looking along that row will tell you the stud profile. If the stud is too deep, then look at the stud that will meet the wall thickness.”
Q: We want to replace a non-functioning radiant heat ceiling system. The old system appears to have had the heating element embedded in the gypsum ceiling panel. We have been unsuccessful in finding replacement panels for this project. Do they still exist? Is there another way to build such a system?
A: I looked and asked around, and it seems as if several larger manufacturers once offered these panels but don’t any longer. There is still a specialty supplier who offers them, but I have to wonder why the big guys don’t any more.
ASTM C1546, Standard Guide for Installation of Gypsum Products in Concealed Radiant Ceiling Heating Systems, offers instruction on replacing heating elements behind or above gypsum panel ceilings and replacing the heating elements embedded in gypsum plaster ceilings. There are also several heating elements for use in ceiling heating systems available on the Internet, so it is reasonable to assume that you can come up with a replacement system that uses either gypsum board or plaster as part of the ceiling. As always, I strongly recommend double checking with the manufacturers of any products that might be used in such a system before the installation.
Lee G. Jones is AWCI’s director of technical services. Send your questions to email@example.com, or call him directly at (703) 538.1611.