Timber Structure Deflection To AS1720

[aenger]

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

 

[Settingsun]

Long-term: Multiply the dead load deflection by the j2 factor for >1 year. Multiply the live load deflection by j2 for five months (item b in clause 2.4.1.1). Eg for moisture content <15%, 2*dead deflection + 1.85*live deflection = 2*10+1.85*5 = 29.25mm.

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.

 

[aenger]

Thanks for your reply steveh49. (sorry I have no idea how to reply to ur post directly on this website so I will just post here).

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.

 

[Settingsun]

I think all posts just go one under the other which can make long, popular threads a bit confusing. To top it off, there's no unique reference number for each post that can be used to indicate what you're responding to.

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.