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Creating a Continuous Wood Beam by Sistering

Corey7320 (Structural) (OP)
14 Jul 06 13:16
I want to design a very long continuous wood beam for use as main girder of a 80' long pier structure. There will be two girders with floor beams spanning between the girders. I want to sister 3x10's that are 20'long and alternate splices at 1/4 points of 20' spans. I would like to know how to design the bolt pattern and spacing connecting the two sistered 3x10's. Is it just the shear in the beam that the bolts need to transfer? The shear flow equation VQ/I doesn't make sense to me in this instance since the two 3x10's have the same neutral axis. Also, i would like to design a cover plate bolted through the 3x10's at the splice of each 3x10 to handle the additional moment and shear that the continuous 3x10 alone can not handle at the 1/4 point. Any help is appreciated.
RARWOOD (Structural)
14 Jul 06 17:09
Are you asking how to join double  3 x 10's togther with splices between the supports so they act as a composite sections?  If so I don't know of a reliable way to do that.  I would be interested in learning more if anyone out there knows of a design method.


I am assuming you want to create an 80' long continous beam with supports every 20'.  Why not use treated Southern Pine glulams 6 3/4" x 9 5/8" with no camber spanning 20'?  You might even be able to use 40' long glulams.  I think though that at 40' that small of glulam might be too flexible out of plane to produce and handle.

With the Southern Pine you could use 26F-V4 material with 2600 Fb. top and bottom.

O
Corey7320 (Structural) (OP)
18 Jul 06 8:33
I did think of a glulam but none of the manufacturers that i talked to can recommend that glulams be used in a marine environment. Also you'd still need some way to join them continuously. I have also considered sistering three members rather than two and varying splice points so you'd always have 2/3 the strength of the composite member at any  required splice. There is also i product i've seen, but don't know the technical name of, that resembles the plates they use for connections on prefab trusses. The product has teeth punched in both directions so you can press two pieces of wood together and transfer shear. I can't find any info online to see what its intended use is though.
SlideRuleEra (Structural)
18 Jul 06 12:29
I gave you the suggestions on using three members in this crossposted thread507-159988

So, rather than continuing there, take a look at page 47 of the TECO Catalog for the spike connectors you mentioned, might work for your application. Here is the link
http://www.clevelandsteel.com/divisions/construction/PDF/Entire_Catalog_print.pdf

Also see the TECO Design Manual at
http://www.clevelandsteel.com/divisions/construction/PDF/Design_Man_Teco_print.pdf

www.SlideRuleEra.net idea

boo1 (Mechanical)
21 Jul 06 14:15
SRE is my mentor
Corey7320 (Structural) (OP)
26 Jul 06 10:00
Based on comments here and some additional reading, i have decided to use a built up section using four 2x12's and varying splice points so the section always has 3 out of 4 2x12's continuous. Also the built up section will have a more consistent treatment throughout the section since larger members get minimal treatment in the center of the section. The 2x12's are also a better grade of wood. I will use stainless screws for building up the member. Does any one know if the treatments used for wood react with certain fastener materials?
SlideRuleEra (Structural)
26 Jul 06 11:36
Hot-Dipped Galvanized and Stainless Steel hardware/fasteners are acceptable for use with commercially available wood preservative treatments such as
ACQ, CA, Borate Compounds, CCA, and Oil Based Preservatives (like as Creosote).

www.SlideRuleEra.net idea

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