wood deck lateral load 1

[a2mfk]

This article from the most recent Structure Magazine addresses this oft-discussed topic that doesn't have a lot of good code/spec guidance...

http://www.structuremag.org/article.aspx?articleID=1797

 

[jittles]

Interesting. They recommend using a 12 psf lateral live load, which, in general, will control over seismic or wind forces.

My gut feeling is that the cyclic loading test they did there seems highly unlikely to occur in reality, but maybe I am just not seeing the analogous real-life loading scenario there. I could see the impulse loading occurring with people goofing off on a deck.

 

[Archie264]

...maybe from a bunch of drunk fraternity boys singing "Louie, Louie" in unison and stomping their feet at the keg party?

 

[msquared48]

Also interesting to note that 12# is 30% of 40# live load. In the older codes for snow loads of 30 psf or more, 25 to 30% of the snow load was to be included with the dead load for seismic. This is totally in line with past thinking here.

The only thing missing in the analysis is the absence of a sloshing factor, similar to the seismically induced waves for a water storage tank, due to the beer consumed prior to staging this ridiculous stunt.

Mike McCann
MMC Engineering

 

[a2mfk]

Most deck failures I have read about occur during parties, but I can't say these were from lateral load or from excess live load (I think the latter). Also, how decks get connected to existing structures I think is probably one of the bigger construction errors out there, and one of the biggest achile's heels of decks.

I do agree the activities they tested are not likely to occur, but that is what structural engineering is about in my mind (earthquakes and hurricanes don't happen very often either). Predicting events that are unlikely to occur or maybe only have a once in ten years occurrence interval. The deck failures you read about are likely due to the largest load the deck experienced in its lifespan. So that one raging kegger at the Delta Fu house where someone lights off some fireworks on one end of the deck and 20 people run to one side and then abruptly stop- there you go!

Plus I think a few knee braces would probably take care of this anyway.

 

[a2mfk]

I just remembered that in college my Timber Design class project was designing a deck where it supported a hot tub and “party” live loads. I think we used a factor well over the required residential load, and when our professor asked us about that, my buddy and project teammate said something like, "You haven't been to a Panamanian party." He got some laughs for sure. I have, and there is a LOT of lateral movement!

That same class, my otherwise deadpan professor described the difference between live and dead loads and said his mother-in-law was an example of a dead load since she never moves....

 

[a2mfk]

OK, last comment after further studying that paper, I see one type of test they left out. Assume the edge of the deck furthest from the structure is braced by knee braces, or a stair, etc. and the ledger along the structure is bolted. Repeat the load tests.

 

[LorenAWC]

A few notes about this research.

1) The deck is extremely overloaded at 12 psf. Horizontally sheathed shear walls have an allowable shear value of 70 plf. This is at 144 plf. The research notes that the large deflections (7 in.) allowed the people to rock in motion together and create a larger load. If the deck was properly sized to the load, would the load be as large?

2) This only gives loading for one size of a deck with two different stiffnesses. It showed how sensitive the load is to the stiffness (diagonal sheathing has an associated load of 7 psf), so it is reasonable to assume that the load would also vary based upon deck size, just as the stiffness does.

3) It is extremely conservative. If the research team could have figured out a way to make a larger load, they would have done it. This should be viewed as a worst case for a 12 x 12 deck.

Overall this is extremely useful and interesting research, but it doesn't "close the door" on what lateral occupancy loads on decks should be. It is really just the start.

 

[a2mfk]

Loren- where did you get your shear wall number of 70plf? That is super low for a shear wall, and I don't see anyone placing plywood horizontally on houses.

But I agree with you on this being too high and conservative, and agree this is just a start.

12x25 foot deck, 12psf(12')(25')= 3600lb of total lateral!

There would be piles of sticks and bodies all around the country if that was accurate. I think there would have to be some type of upper limit, or a reduction with the size of the deck, or the loading should be expressed in terms of PLF applied laterally to the two sides of the deck. Perhaps a table that would give you different values based on the dimensions of the deck or something would be more accurate and realistic.

If you have 40 people on your 12x25 deck, they cannot realistically all move in unison and get any type of momentum built up. They certainly are not going to be able to generate 3600lb of total lateral force.

 

[msquared48]

You are forgetting the beer factor...

Mike McCann
MMC Engineering

 

[LorenAWC]

a2mfk:

From the 2008 Special Design Provisions for Wind and Seismic (SDPWS), Table 4.3D gives the nominal values of horizontal lumber sheathed shear walls as 140 for resisting wind. Divided that by 2.0 per 4.3.3 to arrive at an ASD value of 70 plf. Diaphragms also have a nominal value of 140 plf, but this really is a horizontal shear wall.

We could also have the conversation if the wind or seismic resistance value should be used. I chose wind since it is larger and we are clearly beyond the elastic range.

I agree that people aren't going to be putting plywood on decks. There is a 4x increase in stiffness by going to diagonal sheathing. This research may be the end of large, horizontally sheathed decks.

 

[msquared48]

Or to quote Mr. Gump, "Stupid is as stupid does."

Mike McCann
MMC Engineering

 

[hokie66]

Diagonal decking? Not on my deck. Decks fail due to failure of ledgers and poor connection of structural members, or else due to deterioration, not due to racking of decking members.

 

[a2mfk]

Loren- GOTHCYA! I for some reason was thinking you were talking about the vertical shear wall of the structure that the ledger was attached to, not that you were talking about the horizontal shear diaphragm of the deck itself.

At least this study is a start. However, the more I think about it, the more I agree with Hokie (who was essentially agreeing with my earlier post ).

It seems the largest lateral force that you could realistically generate would be with FEWER people on the deck rather than more people. A crowded deck with people will not allow everyone to move quickly to one direction and suddenly stop in unison, the mass of the people will act as a mass dampener against the stopping force. A test with one person every two or three feet on center like they did, run to one end and stop, and this would give you a good lateral load that you would equate in terms of PLF applied one at a time in each direction. I do not buy the equivalent PSF in terms of the total area of the deck.

 

[bigmig]

I have seen decks fail from lack of x bracing (i.e. lateral racking). It is a real thing. I was on one last night with my 5 year old. He was running back and forth. The entire deck (8' x 30') was on 4 columns equally spaced along the outer edge. The sway in the deck (parallel to the long axis) was clearly noticeable. And there were only 2 people on it. I got home and read that same article. Coincidence?

 

[a2mfk]

Sounds like your 5 year old, if a boy, is a linebacker in the making! That is an odd timing of events though.

Was this deck along the back of a house and extended out 8ft, with the long direction parallel to the back wall? And the decking then went parallel to the long direction?