Timber Beam Long Term Deflection 1

[DAR5052]

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!

 

[msquared48]

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

 

[DAR5052]

Here is the elevation of the pergola. Note that the beam to post connection will be different from what is shown in the sketch.

 

[msquared48]

So it's knee-braced, making it closer to a 16 foot span, or is this the change in beam to post connection you are talking about?

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

 

[DAR5052]

Yeah, I did the member design for wet use conditions since it will be exposed to the elements outside. As for the knee braces, I plan on keeping them but didn't consider them to reduce my span since I was just using them for lateral stability. But you are certainly correct in saying that they will reduce the span.

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?

 

[msquared48]

Yes, there will. I would allow for a minimum of 1.5X the dead load deflection.

Mike McCann
MMC Engineering

 

[hokie66]

Yes, the deflection will be noticeable, and objectionable.

 

[Agent666]

For 'green' timber with long term loads (greater than 12 months), in New Zealand we have to take 3 times the short term deflections for normal timber. It's 2 times I think for laminated beams. The definition of green is related to the moisture content of the timber, not the fact that it's simply outside.

 

[DAR5052]

So I thought about this a little more last night and came to the following conclusion: The initial deflection of the beam will be about 3/8" after I set everything but since I am installing the knee braces, that will help reduce the span for the long term deflection... So at most, the deflection might be 2X the initial deflection bringing the total up to about 3/4" which would be about L/384. I think it would be difficult to see 3/4" of deflection over a 24' span but again, I'm not familiar with what is actually noticeable.

Yes, I am referring to the timber as "green" since the MC will be above 19% when I install it.

 

[tony1851]

In Eurocode, long-term deflection due to permanent load is increased by a factor of 3.0 (external timber). Even if you had 1" deflection, you probably wouldn't see it on that span.

 

[Splitrings]

Appendix F of NDS gives a method of calculating an adjusted E so long term deflection can be estimated.

 

[msquared48]

If you have vines growing on the arbor, as in Wisteria or Clematis, you should not notice the deflection, except in the winter.

Mike McCann
MMC Engineering

 

[DAR5052]

lol if I notice the deflection and it bothers me I'll cover it in vines.

Thanks for the help!

 

[Sawbux]

How about jacking it up in the center for about 6 months until it seasons to induce an upward crown?
Another option: Hang a weight at the center to bow it down-ward, then flipping it over?

 

[racookpe1978]

I vote also for the "let it sag and then flip over" process - If it is needed at all after the ivy grows over.

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.

 

[Agent666]

Racookpe1978, the deflection in that case will be exactly the same if it's only the self weight of the beam. Stiffness is directly proportional to the width, as is the self weight after all?

 

[DAR5052]

I like the idea of flipping the beam over after it develops the bow... could be a lot of work but its an option. I read something in NDS the other day that said for unseasoned timber, you should multiply the initial deflection by 2X to account for creep. If that's the case, I have nothing to worry about in terms of noticing the deflection.

 

[Tomfh]

Option A: Live with an inch of deflection

Option B: Demolish pergola and rebuild with beam flipped over.


Hmmm, tough choice...