huangyhg

deflection limits (ibc) for roof framing system
i know the deflection limits for each member are l/360, 240, 180...for each member.
but if the rafter deflects 2" at midspan and purlin deflects 1.5" at midspan. then the total deflection is 3.5"
is there any part in the code (or any code) about the combined effect of deflections?
adam
i don't consider the combined affects of deflection. especially live load. the only situation i might consider combined deflection would be snow load or fluid load (any load that is always there and everywhere) and even then it would have to be long spans.
not that i know of --- that said - i do get a bit worried about tiled floors or roof ponding on low sloped roofs. also - i suppose a complex lintel system would need a double check.
when i design roof, if it is flatter than 3/12 slope i do l/480 deflection. that is just my preference. i've never considered combined deflection.
if the rafter deflects 2" and purlin 1.5" does necessarily mean it is 3.5" deflection. just draw it. it is like simply supported beam (draw it with little deflection), then drop one support a little and keep the same curvature of the beam. how low is the mid point of the beam? i dont think it changes that much. i hope i am making any sense. i wouldnt worry about it if i were you.
most people wont check for this as the deflection criteria are based on visible curvature.
the purlins also sag the same amount at the eaves, so it is usually not an issue. if you really wanted to check the combined deflection then you would check the 3.5" over the diagonal.
there are a few situations where i probably would check this:
1. a 2 way flat slab with fragile finishes or sensitive equipment.
2. a very flat roof that may pond (as noted above).
3. where partitions or similar are attached to the roof and the actual maximum deflection is important.
thank you...this helps.

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