Generically speaking, in considering floor framing design should joist sizes be chosen just by span charts using the proper chart for single vs multiple spans, desired L/xxx condition, live/dead loads, etc? What about where unequal multiple spans are involved? Do I choose a continuous joist size that is sized for the larger of the two spans and then the short span has a stiffer floor area? (assuming supporting member is below joists.)
To complicate it further, how about a non-continuous joist set up where the supporting member is at the elevation of the joists instead of below them with the joists coming into the sides of the supporting member? In this case, I would think that each span has different sized joists to try to match the deflection characteristics as close as possible to each other instead of using the larger joist size for each span. This design could play havoc with floor finishes having a line down the middle of the floor with the rigid supporting member.
Does material type have an effect i.e. engineered lumber vs. dimensional vs. light ga steel? Does each material type have different characteristics?
Thanks in advance.
GregT
Replies
What you are asking takes a whoile book to explain totally.
Briefly - the span tables are the minimum so with unequal spans, make you life simple by sizing them all to the largest needed.
The thing not covered in span tables is vibration. Deepeeer members in long spans have less vibration than the smaller will. For instance, if span tables will let you do a job with 2x10 @ 12"OC or 2x12@ 16"OC, use the deeper member to avoid vibration.
You next Q is about what is called a flush set beam. With dimensional lumber, setthe joists about 3/16" high so when they shrink they don't telegraph where the beam is, and layout the plywood so a full sheet spans this space.
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