Timber Structure Deflection To AS1720
Timber Structure Deflection To AS1720
(OP)
Hi Guys,
I have a question regarding the deflection of timber structures.
Lets say we have a timber floor beam and its deflection under dead load is 10mm and 5mm for live load.
So as per AS1720, what's its short term and load term deflection? I am confused by the creep factor j2.
Thank you guys :)
I have a question regarding the deflection of timber structures.
Lets say we have a timber floor beam and its deflection under dead load is 10mm and 5mm for live load.
So as per AS1720, what's its short term and load term deflection? I am confused by the creep factor j2.
Thank you guys :)






RE: Timber Structure Deflection To AS1720
Short-term: Depends what you mean by short-term... If you just want the instantaneous deflection due to live load only to check if the floor is too springy, then it's 5mm. If you want to know the total deflection when the live load is applied the same day the beam is installed, it's 15mm. If you want to know the total deflection somewhere between day 1 and long-term, use reduced j2 factors appropriate for the time in question.
RE: Timber Structure Deflection To AS1720
When we comp the long term deflection, shouldn't we add the long-term factor 0.4 (combination 1.0DL + 0.4LL) which give us the long term deflection 2*10+0.4*1.85*5 = 23.7mm?
By short term deflection, I thought it would be 1.0DL+0.7LL, which means the deflection would be 2*10+0.7*1.85*5 = 26.475mm, which would be always greater than the long term deflection. So I was confused by the point checking the long term deflection...
If the short term refers to what you mentioned -- LL, then the short term deflection will be smaller than the long term one in most cases (unless LL>>DL). So it seems to me there's no point in checking short term deflection then.
RE: Timber Structure Deflection To AS1720
I thought your question was how to apply the j2 factor to loads of different duration so didn't think it all the way through to be honest. Have a look at Appendix B of AS1720.1. It suggests the following cases unless there's a good reason to check more stringent cases:
Long term: G + (psi.L)*Q = 23.7mm
Short term: (psi.S)*Q = 3.5mm
These are used to check different things so different limits apply. Eg you might want long-term deflection < span/250, so 23.7mm would be acceptable for spans > 5925mm; and also want short-term < span/800 (>2800mm). The long-term deflection governs in this case.
If you wanted total long-term deflection under short-term load increment, I'd go:
j2(permanent)*G + j2(5-month)*(psi.L)*Q + (psi.S - psi.L)*Q
Hope that last bit makes sense.