Wood Column Splicing

[jayrod12]

Hello all,

I've searched the forum and haven't found the answer to my question (I may not have looked hard enough but I did use the search function).

I have a job where we are using 4 ply 2x8 columns that are sunk 6 feet into the ground and extend to 19' above ground giving a total column length of 25 feet. Obviously they cannot get 25 foot laminations.

Does anyone have any references to designing a column for combined loading when none of the laminations are the full length? I scoured the NDS (And the Canadian equivalent as this is being built in Canada) and some other references that were posted in the forum for other questions that were similar to this one.

I have a old job in our office that was done by an engineer over 15 years ago. He has that in the spliced region (1/4*L before the first lamination splice to 1/4*L after the last lamination splice with no lamination splices lining up with each other) the allowable bending stress is 0.42 times the allowable of the unspliced region.

I cannot find where he would've gotten that number. The engineer no longer works at this office and I'm sure would no longer remember where that number came from.

Any help would be appreciated.

 

[boo1]

AWC has info on build up wood columns

http://www.awc.org/pdf/WSDD/C3.pdf

 

[jayrod12]

Yes but that one is only for full length laminations. and they are only looking at vertical capacity and not bending (In my situation bending due to wind load governs)

 

[msquared48]

Seems to me that depending on the moment diagram in the column, you could start your splices above the levels where three, or two members would work. I would have to detail this and think about it for a while here...

Mike McCann
MMC Engineering

 

[jayrod12]

I'm gonna throw up the formula the other engineer used as I've been trying to find where he got it but I can't. He titled this section of the design as Interlay Shear Capacity.

ISC= Fbu*d*(0.0024 + A*d/L^2 - MoE/B)

I'm assuming the Fbu is unspliced allowable bending stress. but I don't know where this formula came from. Is it a standard structural analysis formula for interface shear between plies?

 

[jayrod12]

And our column is almost considered completely free at the top so there is barely an inflection point on the moment diagram allowing for the use of your other method Mike.

The reason I say almost is I've determined the proper method for this post-frame building analysis is to assume there is a spring at the top of the column frame that has been calibrated to provide the an allowable shear resistance of around 100 plf. This makes the deflected shape look more realistic and very similar to a k=1.5 (Fixed bottom and semi-pinned top).

Since there is only slight lateral restraint at the top the positive moment seen by the column is very small magnitude and acts over a small area. The majority of the column is in negative bending.

I did find a US DoA Forestry Service Research paper on spliced nail-laminated posts that suggest a spliced post has strength that is equivalent to 48% of an unspliced post with all the same other characteristics. So that explains the 0.42 factor in my original post

 

[jayrod12]

The 100 plf allowable is due to the 29 ga metal roof sheathing

 

[dhengr]

Jayrod12:
Actually, there are suppliers for GlueLam columns. The butt joints are finger jointed, and the lower portions are pressure treated and then transition to untreated a few feet above grade.
Off the top of my head, three of the suppliers for the laminated columns are:
>> Timber Technologies, LLC
>> Rigidply Rafters,

http://www.rigidply.com

>> Gruen-Wald Engineered Laminates, Inc.,

http://www.gruen-wald.com

And, I would bet that there are others, regionally. Most any laminator should be able to do it, although they may not want to mess around with small quantities. These laminated columns are promoted mostly for post-frame agricultural bldgs. and the like. The biggest issue is that they must be treated for structural use in continuous contact with the ground and moisture, to some distance above grade.

There are also a number of variations on the theme for precast conc. posts/piers which extend below frost and then terminate with some column connection hardware, 6" or a foot above grade. The biggest problem with these is that they be light enough so you can handle them, and still big enough in dia. so I can get some lateral soil resistance out of them, in the hole, and assuming well compacted backfill.

 

[jayrod12]

Those are basically exactly what we're doing, treated below the splice, standard lumber above. They don't want to pay the premium for someone else to create the posts. I've determined that the posts will work as long as I can get the splice details to work.

There are done all the time as has been said in other places and so someone has had to have done this kind of design before.

 

[RFreund]

If you wanted to analyze the connection you could do something like this:

http://www.eng-tips.com/viewthread.cfm?qid=333686

Basically an elastic vector distribution method (using the moment and shear forces) See how it compares.

EIT

http://www.HowToEngineer.com

 

[focuseng]

You are looking for ANSI/ASAE/ASABE paper EP-559 published Feb '03 and revised EP-559.1 Dec 2010. ASAE is now ASA&BE

______________
MAP

 

[jayrod12]

Thanks! That was the one.

 

[boo1]

That equation only applies to assemblies with overall splice lengths equal to or greater than the ANSI/ASAE EP559 table 3 minimums to calculate level II values. Level II values apply to the splice region of all columns with simple butt joints.

 

[jayrod12]

Yes I am aware thanks, my splice length is 6 feet with a splice region of 9 feet. And besides as per sentence 5.2 the table is strictly a recommended splice length for high moment areas however splices smaller than those prescribed in the table can be used where the splice occurs in regions of low moment.