Diaphragm Forces Due to Live Load

[Signious]

Hello All,

I have encountered a few structures which are racking back-and-fourth due to live loads (mostly pin-pin connection deck columns which were field built). I know many different ways of dealing with it structurally, but haven't found many resources for actually putting numbers to it.

Anyone have anywhere to point me to / ways you actually quantify this? Thanks.

 

[msquared48]

Are y0ou talking cyclical loading from compressors or mechanical units that are response frequency related?

Mike McCann, PE, SE (WA)

 

[Signious]

Just occupancy. The one I checked out last week would visibly move 2-3 inches from 3 (bigger) guys walking across the deck.

You get that thing up to full occupancy loading and everyone does the time warp, it is going to start AND end with the jump to the left.

Built completely by Part 9 specs by framers, no eng involvement except from geotech for screw pile installation on the structure (30' long, 10' spans, 8' joists w/ 1'6 overhang).

 

[msquared48]

I would just do the F=ma approach and reinforce from there...

Mike McCann, PE, SE (WA)

 

[Signious]

Had considered just going to base principles for it, a lot of assumptions though. Just hoping someone would have a 'rule of thumb' they go by for LLx(factor) = shear force (psf).

 

[msquared48]

You could always assume 30% of the live weight as a lateral force.

The same as for non-building structures. After all, humans are not buildings, but they are structures, right?

Seriously, I have taken this approach too in the past and been accepted.

Mike McCann, PE, SE (WA)

 

[thaidavid40]

You could try adapting the lateral loads from chapter 3 of the "2012 ICC 300 STANDARD | BLEACHERS, FOLDING SEATING & GRANDSTANDS". It gives some design live loads on a per-foot basis which might help.
Dave

Thaidavid

 

[KootK]

Sounds to me as though you've got a diaphragm stiffness problem rather than a diaphragm strength problem. If so, that's even tougher to quantify. Knee braces on the columns or horizontal, under deck bracing back to main structure I would think.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jayrod12]

Koot beat me to it. My bet is there's no lateral system bringing the mezzanine lateral down to something real. Talk them into a cross brace in each direction and I bet you'd be fine.

Fwiw I usually design all mezzanine for 10% of the live load as a notional load.

 

[Triangled]

this, for time warp loading, pg 15

 

[Signious]

Thanks for the input all.

Trangled: The is exactly what I was looking for, thanks.

Kootk / Jay: Exactly the problem here, the homeowner actually went ahead and manufactured some thick water cut T and L brackets for post and beam connection that did a bit of help but there is still a stiffness issue at the base where the columns meet the supporting screw piles in a pinned connection. I know the, 'It has worked in the past' solution, just started to get on my nerves that I did't have much in the ways of quantifying it.

David: Gonna have to find a way to expense that standard... 'I am running a quote for a stadium project' sounds like a good line. Thanks haha.

Edit. To summarize the report from Trangled:

-12psf (plan area) lateral load is a good design assumption (based on 40lbs live for cyclic; 10lbs live for impulse loading)
-Unzipping at the ledger board wasn't an issue (surprising)
-Nails shouldn't be used for ledger boards due to withdraw loading (unsurprising)

They exclude decks with a larger than 1:1 ledgerboard length: deck length; makes sense.

 

[TLHS]

Where is your lateral load actually going to, as far as a load path? I haven't heard it mentioned. You talked about deck posts, so I'm picturing a deck connected to the building on one side and on pin-pin posts on the other side. In this case, your posts have no lateral load path, so you need to follow it through the deck diaphragm back to the main structure. The diaphragm is acting as a cantilever. If it's moving, it's likely that the connection to the structure isn't up to snuff (or the main structure can't take the loads...). It needs to meet stiffness and strength against shear and against tension/compression from moment.

Is there a good tension load path in the connection to take the moment coming out of the diaphragm? I feel like that's the most likely culprit.

Regardless of the cause, the best way to deal with it is to put braces on your posts, or at least little knee braces to turn it into a moment frame. Then you're just passing pure lateral shear into the two baylines.

I use the same 10% that jayrod uses. It's a strength level that doesn't usually require more than a nominal cost and it ensures you're actually checking it. I've dropped below that before, but it's my starting point.

 

[KootK]

Were I to speculate on the cause of the problems, my guess would be that one or more of the vertical framing members is abnormally out of plumb. Vertical load on that/those members is translating into a P-Delta lateral load on your diaphragm. I'm especially suspicious because of the use of screw piles. A residential screw pile can easily be 6" out of tolerance. Maybe the deck didn't move to suit and the builder just "connected the dots" resulting in a skewed post.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.