Moment Frames permitted in Single Family Homes

[ampersand]

The Subject sounds like a dumb question. I have designed ordinary moment frames in countless single family homes over the years. But today I found myself carefully reading section 12.3.1.1 of ASCE 7-05 (it's the only copy I own. It states that a plywood diaphragm my be idealized as flexible "in structures in which the vertical elements are steel or composite steel and concrete braced frames, or concrete, masonry, steel, or composite shear walls." Note that it does not include "moment frames" under the vertical elements.

The next sentence allows the flexible diaphragm assumption in single family homes that are "of light-frame construction".

When I brace homes with steel frames, then the home is not completely "light-frame construction".

I may be splitting hairs. I know we all include moment frames in single-family homes. But has anyone noticed this before? Ever had it come in in plan check?

 

[structSU10]

It is only talking about idealization of the diaphragm. A moment frame is generally flexible itself, so the diaphragm may no longer be flexible in relation to the bracing system - The diaphragm may be rigid or need to be analyzed as semi-rigid. Beyond that, you can use them all you want.

 

[MotorCity]

I hear what you're saying and never gave it a second thought, but even with steel moment frames I believe that single family, wood, stick built homes are still light frame construction. A steel moment frame is simply a steel element of the overall construction and does not make up the majority of the building. There are other steel elements commonly found in residential construction: steel beams supporting the first floor, steel lintels supporting brick, flitch plate beams or other steel headers, etc. If the gravity system is light frame, the building is light frame (the gravity system usually comprises a much larger percentage of the construction than the lateral system).

 

[sandman21]

You might also notice that it does not list wood shear walls either. The exception permits you to idealize the diaphragm as flexible, even thou its likely that the diaphragm is rigid, when using MF I confirm the assumption, I also do an overlap analysis for mid-rise wood buildings as well. The addition of steel moment frame will now require the connections be designed for omega level loads, ASCE7-10 12.10.2.1. Chapter 11 has the definition of light frame construction, if the structure is primarily comprised of light frame materials, it is light framed.