Cantilevered timber poles supporting timber Deck

[micks87]

Hi All,

Really simple query,what deflection limit would you use when designing cantilevered timber poles supporting a timber deck. As the decks as sitting on posts / poles (2.4-3m high), they may be some apparent sensory vibration.

Traditionally, the 'codes' [Aus/NZ] here do not give any guidance and thus there appears to be a bit of finger licking? I would check serviceable loads against a height/250.

Does anybody have any references for deflection limits or any comments?

Cheers

 

[KootK]

You might Google Frank Woeste. If anybody's studied this, it's surely him. I think h/250 seems reasonable but that's just instinct talking.

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[MIStructE_IRE]

As a minimum, height/300 for sway. However, this results in 10mm sway... Sitting on that deck with a pina colada will be like sitting on a cruise ship.. And that’s before creep kicks in.

Can’t you brace it?

 

[countervail]

It's not the deflection limit that gets you it's what horizontal load you use to generate the deflection, In the UK we have a horizontal load based in a proportion of dead and live load as a minimum horizontal load (great for buildings) but this has been found to be to small for mezzanines and there is specific guidance for such structures (if wind loads are applied these would generally govern). As for a limit H/300

 

[micks87]

Thank you for the advice. H/250-300 or 10mm seems reasonable I guess.

MIStruct_IRE - The Deck cannot be braced underneath, however I have strapped it back to the floor of the house which should really take any vibration out of the floor / posts.

Countervail / KootK - thank you for your comments.

 

[Tomfh]

Check people if people can get it moving back and forth too.

You don’t want some kids seeing how far they can get it moving.

 

[KootK]

n the UK we have a horizontal load based in a proportion of dead and live load as a minimum

Out of curiosity, what percentage is used for this?

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[Agent666]

You can also diagonally brace just under the decking or underside of the Joists with some strap braces corner to corner. This may ensure you are at least trying to engage all posts against any sidesway, rather than perhaps engaging a single line since any decking is a pretty poor diaphragm.

I've been in a number of houses supported on poles of dubious construction with no subfloor bracing down to ground and they do tend to move a fair bit, over time it tends to get worse as connections in timber framing lossen up in the subfloor. Depending on location you are in the house, you can get all sorts of weird effects like feeling someone walking down the other end of the house being felt.

I feel in these things that it's prudent to provide some forewarning to the client that it's possible given the type of construction with no bracing, and height of the poles that they may perceive some vibration/movement, and if it's noticible there may be things you can do to lessen the perceived vibrations after the fact so they know they have options. They may have different expectations after all that it will be totally rigid or something.

 

[oldrunner]

If you have strapped the deck to the house, it now becomes a anchored diaphragm. If you are in a seismic zone, you should check the capacity of the ties versus whatever the diaphragm weight with the seismic factor. The deck should also be attached to the house at the face of whatever house framing you have to take the horizontal shear at that connection. Actually now, you don't care how high your poles are because the house is resisting the full lateral loads. You just have to make sure that the diaphragm has stiffness with plywood or metal cross straps under the deck. If you just have timber planks for flooring, there can be some movement. Very common design in California.

 

[Tomfh]

Indeed.

is this deck attached to a structure?

 

[Lomarandil]

In the UK we have a horizontal load based in a proportion of dead and live load as a minimum

Out of curiosity, what percentage is used for this?

I'm also curious. In construction platforms, 2% is a common rule of thumb. (Actually, it's probably codified in ASCE 37 now).

----
just call me Lo.

 

[oldrunner]

Review micks87 9 Aug 23:07 post -"however I have strapped it back to the floor of the house which should really take any vibration out of the floor / posts."