Light gauge metal frames

[bruinboy]

I am trying to analyze an existing carport that uses CEE sections (8" x 2-1/4", 14 gauge) as columns and a beam of a frame. The height is about 8' and the frame width is 10'. The columns are embedded in concrete. The beam is connected to the side of the columns with screws (no moment is transferred). This is in California and I believe that free-standing carports are supposed to be designed for seismic forces as well. Because of the complex geometry of the sections, I'm having a hard time with the analysis. Do I use the same beam-column interaction formulas from ASD? Does anyone have any section properties that they can send me? I'm going to assume a rigid connection at the beam to column node. How would I achieve this connection in the field?

 

[shepherd]

CEE is a light gage metal stud that I believe is made by Dale/Incor. The standard structural studs have either a 2" or 2 1/2" flange. The dale/incor website I checked was unavailable. The CEE is the manufactures description of the flange width. The equivalent standard SSMA (steel stud manufacturers association) shape that this relates to would be a 800S20068 (800=8",S200=2" wide structural shape, 68=14 gage) or 800S25068. See ssma.com for section property charts in the technical info section. IBC specifies "AISI, North American specification for cold-formed steel structural members" be used for cold form design (I suspect all other model codes do as well). For allowable stress design typically the effective section for design after local buckling occurs is found to get maximum allowable forces. Most manufactures list allowable moments based upon the effective sections. I've never designed nor seen a moment frame made of light gage members and am not aware of any literature. Again I'd check the ssma.com website. Could you not just design it as an inverted pendulum system cantilevered from the footing? Could x-bracing straps be used to take lateral loads?

Regards

 

[bruinboy]

shepherd, thanks for the info.
I can't use x-bracing because it would run straight through the parking areas. By inverted pendulum system, are you saying that I can just leave the connection as pinned at the top of the column and assume that it acts like a cantilever instead of acting as a frame?

 

[UcfSE]

You need to design the sections in accordance with the AISI spec for cold-formed steel, NOT the AISC structural steel spec. The lastest is the 2001 edition. I would make note that in my area at least standard structural studs are a 1.625-inch flange and 2-inch or 2.5-inch are special order, typically.

At any rate, you use the AISI spec to get your effective properties. This is because light-gauge members are very prone to local instability and you have to find the "effective" properties at the load for which you are designing. In addition you still have to consider lateral-torsional buckling and so on. Because these are thin-walled members you will also have different equations to find the nominal strength than those you will find in the AISC spec.

You will not find much literature on moment connections with metal studs, especially with self-drilling screws only. Also, do not assume to do an elastic analysis on the screw connection to find the moment the screw connection can handle. You have no way that I know of to check the stud web in addition to the screws. I would definitely assume the beam to column is pinned from what you describe. Try checking the columns as cantilevers from the base instead of checking the them as part of a moment-resisting frame. When I need to make a frame I use instead braced frames or knee braces. Those can help considerably. You just have to watch your connections carefully then.