beam deflection question 1

[AlpineEngineer]

Hi guys,
I have a situation where the contractor needs a beam sized and only has so much headroom to work with. I've sized the beam that fits and it is more than adequate in shear, moment, bearing length, ect.. but the beam is slightly over on deflection (0.55" allowable and 0.61" actual) The beam will be exposed (glu lam) thus no one is worried about cracked drywall. If we size up to the beam that meets deflection limits he looses his headroom and we have to go to a steel beam which would be a lot more work. I'm thinking 1/16" over on deflection things will be just fine, do you all agree?

And for future situations, I wonder if it is negligent to undersize beams with respect to deflection when space is tight. Even if the beam did deflect the allowable 0.55" there would still be cracked drywall in a drywall situation. I guess I am assuming I am still within the elastic limits when I go over the allowable deflection.

The beam is a roof beam, not a floor beam which might create more bounciness..

Your thougths are appreciated.
Thanks,

 

[desertfox]

hi AlpineEngineer

I am not a structural engineer but is it possible you could
weld a plate to the beam to strengthen it and bring the deflection down.

Regards

desertfox

 

[csd72]

Can you provide some fixity/continuity to the ends to reduce deflection? It is amazing how little fixity you need to make dramatic reductions in deflection.

 

[AlpineEngineer]

CSD72. I suppose it could be argued there is some fixity, I hadn't thought of that. The beam is dropped just below the top plates and it will extend 5" into the stud wall, thus it is 'built in' a bit, not to mention its the width of the wall and there will surely be some osb nailing into it, plus roof dead load/truss load on the ends.

Is there a typical textbook fixed end condition in wood construction? I always picture a concrete beam pocket for fixed end conditions.

Thanks a ton.

 

[csd72]

No the text book will tell you to design timber as pinned connections - this is where you have to use engineering judgement.

I have found that any fixity from the weight of floors above is negligible. Can you provide some definate fixity between the beam and the wall studs under to give it some portal frame action?

I would also check this beam for vibration issues.

http://www.awc.org/HelpOutreach/faq/FAQfiles/Vibration.html?areaID=1

 

[rday]

Camber the glu lam.

 

[MiketheEngineer]

Are you gettng any help from a plywood roof??

What are you using for live loads?? Can you get any help form it being highly pitched.

In many applications - I wouldn't worry about it.

 

[RMunoz1361]

You should not deviate from the deflection limit criteria for a load bearing header. It sounds like you can easily see the beam before you entire a room and if the span is not long enough people will notice it as an issue.

Options:

1. Use a flitch beam comprised of (2)-LVL’s on the exterior and a steel plate in the middle.
2. Try a live load reduction based (ASCE 7-05 or IBC) if the situation allow for it.
3. Or like someone else said early, “camber” the beam.

 

[UcfSE]

That's only about 10% over. Sharpening the pencil might take care of it, or get it within your comfort zone.

 

[AlpineEngineer]

Thanks guys.
The glu lam is a camber beam, still over the allowable deflection. I could reduce snow loads w/ ACE 7-05 if I wanted to but most folks around here don't like it, we had a big snow in '03 that collapsed a lot of roofs.

I think the answer is to give it some fixity with the portal frame action like csd72 talked about, pretty easy to do in this application. I suppose the flitch beam would work as well, I hadn't looked at that yet.
thanks a bunch.

 

[Lion06]

Can you specify a higher E value for the glulam?

 

[msquared48]

Not for the glulam, but if you went to a parallam, the E is 2M compared to the 1.8M for glulams - what you need to get the deflection you need.

If the Architect is worried about the look since it is exposed, just face the parallam with a wood veneer - no worries.

Mike McCann
MMC Engineering

 

[msquared48]

As a corollary, you could also use a wider glulam of the same depth and stress grade. Not as economical, but it works.

Mike McCann
MMC Engineering

 

[hokie66]

If the beam is cambered and still sags below level, camber it some more. Deflection is a serviceability issue, and if the deflection does no harm other than visually and you can take care of that, then the problem goes away.

 

[PEinc]

Is your calculated deflection based on DL + LL or just LL? If you just use LL to calculate the deflection, does that help get below 0.55 inch?

 

[abusementpark]

I have always heard that wood typically deflects more than predicted by the thin beam equations.

 

[swearingen]

How about some decorative knee braces at the ends to help out with end fixity? Even small decorative chocks under the beam/column interface would help a bunch by reducing the effective length. The equation does depend on L^4 after all...



If you "heard" it on the internet, it's guilty until proven innocent. - DCS

 

[rday]

Alpine,

If you ran the beam at non-reduced snow and it works for strength considerations, I see no problem taking the allowable reductions to check deflections.

You have considered the most conservative case for life safety.

There are a lot of good options given herein for you to consider.

'03 storm? Colorado front range?

 

[MiketheEngineer]

However - all that said - is long term creep a problem??

Wood will creep and "take" a set...