Timber Beam Long Term Deflection
Timber Beam Long Term Deflection
(OP)
Hi All,
I have a quick question for individuals experienced in timber design. I work as a structural engineer but most of the structures I design are steel so I'm not great with timber. Here is my scenario: I am building a pergola in my back yard with a pretty large span so I'm a little concerned about the long term deflection. The span is 24' and I plan on using a 6"x12" rough sawn white pine beam from a local mill so it will be "green" when I install it. I calculated the initial deflection of the beam to be about .375" which is due to a 50PLF load due to selfweight and the rafters. The question I have is what should I expect for a long term deflection since the beam is green and will go through a drying process over the next few years? and do you think that the long term deflection will be noticeable on that span? Any other hints or suggestions would be greatly appreciated. Thanks!
I have a quick question for individuals experienced in timber design. I work as a structural engineer but most of the structures I design are steel so I'm not great with timber. Here is my scenario: I am building a pergola in my back yard with a pretty large span so I'm a little concerned about the long term deflection. The span is 24' and I plan on using a 6"x12" rough sawn white pine beam from a local mill so it will be "green" when I install it. I calculated the initial deflection of the beam to be about .375" which is due to a 50PLF load due to selfweight and the rafters. The question I have is what should I expect for a long term deflection since the beam is green and will go through a drying process over the next few years? and do you think that the long term deflection will be noticeable on that span? Any other hints or suggestions would be greatly appreciated. Thanks!






RE: Timber Beam Long Term Deflection
24 feet is an awfully long span for a 6X12.
Can you post a framing plan of what you are dealing with?
Mike McCann
MMC Engineering
RE: Timber Beam Long Term Deflection
RE: Timber Beam Long Term Deflection
Remember that the beam will take on the deflected shape it sees under load, permanently, as it dries, developing a memory. And it will seasonally dry and re-moisturize with the seasons. I would design the member for wet use conditions in any case.
Mike McCann
MMC Engineering
RE: Timber Beam Long Term Deflection
Won't there also be a long term deflection (creep) in addition to the initial deflection due to the permanent loading since the timber is green?
RE: Timber Beam Long Term Deflection
Mike McCann
MMC Engineering
RE: Timber Beam Long Term Deflection
RE: Timber Beam Long Term Deflection
RE: Timber Beam Long Term Deflection
Yes, I am referring to the timber as "green" since the MC will be above 19% when I install it.
RE: Timber Beam Long Term Deflection
RE: Timber Beam Long Term Deflection
RE: Timber Beam Long Term Deflection
Mike McCann
MMC Engineering
RE: Timber Beam Long Term Deflection
Thanks for the help!
RE: Timber Beam Long Term Deflection
Another option: Hang a weight at the center to bow it down-ward, then flipping it over?
RE: Timber Beam Long Term Deflection
But, much of the sag is due to the weight of the suspended middle section. Can you slice it so the mass is less, while maintaining the depth of the member. The load is the beam itself in this unique case, so a pair of members just as deep but tied together with a few cross-bolts and separators would be lighter, but just as stiff. So you would have two 2x12's with spacers rather than a single 6x12 beam.
RE: Timber Beam Long Term Deflection
RE: Timber Beam Long Term Deflection
RE: Timber Beam Long Term Deflection
Option B: Demolish pergola and rebuild with beam flipped over.
Hmmm, tough choice...