wood deck lateral load
wood deck lateral load
(OP)
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...
http://www.structuremag.org/article.aspx?articleID...






RE: wood deck lateral load
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.
RE: wood deck lateral load
RE: wood deck lateral load
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
RE: wood deck lateral load
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.
RE: wood deck lateral load
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....
RE: wood deck lateral load
RE: wood deck lateral load
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.
RE: wood deck lateral load
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.
RE: wood deck lateral load
Mike McCann
MMC Engineering
RE: wood deck lateral load
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.
RE: wood deck lateral load
Mike McCann
MMC Engineering
RE: wood deck lateral load
RE: wood deck lateral load
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.
RE: wood deck lateral load
RE: wood deck lateral load
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?