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8x8 post fixed base
5

8x8 post fixed base

8x8 post fixed base

(OP)
I can't recall to have ever fixed the base of wood post. I have a situation where I might have to do that. I couldn't find an appropriate simpson base anchor for that. Is a custom built base plate the way to go? I am thinking of a steel base plate with welded HSS to fit the 8x8 in.

RE: 8x8 post fixed base

Are you trying to prevent termite/rot damage to the post by elevating it?

Or trying to minimize the sideways movement of the bottom of the post by anchoring the steel widget down to the underlaying concrete (with a vertical through-bolt perhaps); so that you want the steel widget to be able to be bolted firmly to the wood post?   (That is, through bolt holds the steel to the concrete by resisting shear, and the secondary bolts hold the steel to the wood by resisting pull-out?

RE: 8x8 post fixed base

If you are looking for a moment transfer bracket - they don't exist.

A company call Sturdi-bracket makes something they say will transfer "some" moment.  Google that.  Be careful - it is capable in one direction only.

An HSS bracket might work - sort of.  If there is any "wiggle" in the timber to HSS - then you will lose some capacity.

RE: 8x8 post fixed base

Consider Simpson ABU88 and ABU88R
http://www.strongtie.com/products/connectors/AB-ABA-ABE-ABU.asp

These post bases feature 1" standoff height above concrete floors, code-required when supporting permanent structures that are exposed to the weather or water splash, or in basements. They reduce the potential for decay at post and column ends.

 

RE: 8x8 post fixed base

If this is for appearance only (you need it to look like wood), why don't you use a 5 inch square steel HSS and add wood facing?

RE: 8x8 post fixed base

(OP)
Actually after getting some more info, re-configuring a few things here and there, I don't need a fixed base for moment transfer....I know that would be a pain.

But I do need a base that works in uplift as well as shear in both directions. I should be able to find that in Simposn for 8x8, I think.

RE: 8x8 post fixed base

(OP)
Its good to know though that you can't really fix the base of a wood post and the idea of using steel and cladding it with wood is a good one for future reference.

RE: 8x8 post fixed base

IF you can bury the post - say 4'-5' down you will be able to generate a moment connection.  But for how many years??  Treated wood says 20???

And being in that industry - seems to work....

RE: 8x8 post fixed base

(OP)
Mike, this is the first time I heard about studi-bracket, thanks for that. If I ever need to fix the base, thats what I am using.
 

RE: 8x8 post fixed base

CB88 would work for your needs just fine.

Mike McCann
MMC Engineering

 

RE: 8x8 post fixed base

8x8 wood post is (nowdays) 7.5 x 7.5 inches.   

So, you'd need a short section of HSS with an inside dimension of 7.5 x 7.5 inches, then weld that to a backing plate (of 1/4 or 3/8 plate ?) 8.5 x 8.5 with a cheap, easy fillet fillet.  

To resist pullout, use 2x 1/2 bolts through the HSS.   

Alt:  Bend a 1/4 or 5/16 thick plate 8 inches wide into a U with inside width of 7.5 inches.  Use 1, 2, or 4 anchor bolts into the slab/foundation on the bottom of the "U", and clamp the two walls of the "U" into the wood post with through bolts.     

RE: 8x8 post fixed base

The trouble with any device to create a moment connection of a timber post is that wood shrinks across the grain, and depending on the species, the shrinkage can be quite a lot.  Unless you are going to tighten the bolts periodically, it is not a good idea.

RE: 8x8 post fixed base

I have designed brackets similar to the sturdi-bracket before, but only for very lightly loaded structures such as trellises.

Hokie's comment regarding shrinkage is valid. A bigger problem is the installation. We need the bolts to be snug to the wood in order to resist the moment. The bolt holes will almost always be overdrilled- killing the fixity.

RE: 8x8 post fixed base

I have used base plates and 1/2" knife plates with thru bolts for small shade trellises.
For higher load conditions, I would use two vertical plates welded together and to a base plate. Designed for the moment required. Bolt the post to the each plate with plate washers. Design the wood connection on perpendicular compression of wood against the plate washer to resist the moment in each direction. Using the bolts only in tension.
This would of course be too visual for most architects or owners.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

RE: 8x8 post fixed base

Teguci's effort deserves a tick.

RE: 8x8 post fixed base

(OP)
Teguci, wow thanks for the detail. That would certainly do it.

RE: 8x8 post fixed base

Why not scope the timber over a bit of steel SHS, then bolt the two together.  

How could you do anything so vicious? It was easy my dear, don't forget I spent two years as a building contractor. - Priscilla Presley & Ricardo Montalban
 

RE: 8x8 post fixed base

RE,
Cross grain shrinkage.  Teguci's solution goes a long way toward dealing with the loosening of the connection due to shrinkage of the timber.

RE: 8x8 post fixed base

using pressure treated lumber and glulam rivets in lieu of screws...
nice detail. The spacer at the bottom elevates the wood and avoids the rounded fillet...

Dik

RE: 8x8 post fixed base

Given the bolts are in shear, couldn't you allow a bit of shrinkage, maybe even spec a spring washer or similar.

I don't mind teguci, I just wouldn't want to be the poor guy that has to install this connection.  

How could you do anything so vicious? It was easy my dear, don't forget I spent two years as a building contractor. - Priscilla Presley & Ricardo Montalban
 

RE: 8x8 post fixed base

Installing through shear bolts connecting timber posts to insitu plates both sides is not an easily performed operation.  I have done it myself with U-shaped post bases, and aligning the bit is not easy.  If you get it misaligned and have to redrill, or if the holes in the steel are too big, then they don't work in shear.

With Teguci's detail, you can have the holes in the steel made to match the screw shank.  Predrill a bit smaller and run in the screws.  Done.  

RE: 8x8 post fixed base

Teguci's detail looks very good to me too for the intended purpose.  Either screws or timber rivets would work just fine, with due attention to spacing and edge distance in all directions and faces.  Generally, on a connection like this, more, smaller fasteners, into the wood, lead to a better performing connection.  And, I would make the standoff plate larger in plan, maybe the same width as the plate we see face on in the sketch.

The only issue I have is that it seems to me that he has detailed half the screws and nuts and anchor bolts to be left handed threads, and they may be hard to come by.  And, most certainly will confuse the hell out of the contractor's workers.   smile
 

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