Deck Uplift in Hawaii

[rspaul]

My daughter wants to build a yurt on the island of Kauai (one of the Hawaiian islands). A yurt is basically a tent with vertical side walls and is circular in shape. This yurt will be placed on a deck, or platform, is 30' in diameter, and will be 2' off the ground. This deck will be supported by beams, resting on posts, which are anchored into a number of pier footings approximately 18"x18"x18" and are 12" below grade. The code for Kauai says that all structures must be designed to handle 105 mph winds.

The yurt itself is a tent and is a temporary structure and not subject to permitting, but I want the platform to to be able to withstand this loading. With these parameters, should I be overly concerned with uplift and/or overturning? I can minimize the number of footings by using parallams, but am concerned that I will lose a lot of dead load from the concrete footings that will help hold down the platform in a good wind. I've looked at the IBC and ASCE 7-05, and don't really see any specific reference for platforms or decks. Should this structure for code purposes be considered an open building with a 2' high flat roof? What is the magnitude of the uplift loading are we looking at?

Can anyone shed some light on this issue and maybe steer me in the right direction? I don't want to over engineer the footings but I also don't want the structure to blow away.

 

[msquared48]

The vast majority of the uplift will come from the Yurt and it's connection to the deck, assuming that there are such connections. You should design the deck structure to take these loads, unless you consider the Yurt expendable.

It is true that the deck could become an airfoil, but at two feet off the ground, any vegetation around it should deflect the wind, minimizing the tendency.

Mike McCann
MMC Engineering

 

[MiketheEngineer]

If it was mine - I would consider it a "permanent" building and design as such. If the yurt is strongly connected to the deck - then it too will have a problem.

My guess is that the cost will not be that much greater.

Either way - check it for all possible conditions. At least you will get an idea when it will blow away. Is it OK to lose the yurt yet keep the deck?? Your call - you are the engineer.

If I remember correctly, Kauai had a major hurricane about 20-22 years ago that pretty much devastated the island. We were headed for vacation there and it was strongly suggested that we change plans - we did. Sorry we never got there...

Of course the other side of the coin is that if you design for 105 mph - a 150 mph hurricane will eat it up anyway....

 

[rspaul]

Thanks Mike(s) for responding. I'm not sure of the hold-downs that are supplied with the yurt. The manufacturer says yurts are stable in high winds because of their circular design, but I have doubts how it would perform in a hurricane. I'm thinking of breaking this down into smaller components, starting with the deck and then analyzing the yurt's additional requirements.

Regarding ASCE 7-05, I'm a little confused by the "Load Case A & B" column in Figure 6-18a. Are these load cases analyzed separately and designed for the worst case scenario, or are they analyzed concurrently and added together? For the life of me, I can't find any reference anywhere on this.

 

[msquared48]

That's BS. Manufacturers will say anything to make a sale. Don't believe it for a minute. He probably thinks high winds are those above 50 feet. Typical salesman...

Run you own numbers and just do what your guts tell you.

Mike McCann
MMC Engineering

 

[BigH]

Ah, sorry if I digress a bit, Yurts. . . . takes me back 12 to 15 years ago when I lived in Xinjiang China. We were outside of Urumqi with Heavenly Lake and the mountains very close by - which is where we saw all the yurts with the nomadic uighers for the summer grazing. Had tea in them a couple of times. Very interesting structures - lots of room.

Have no idea about wind - but none of the ones I saw were "tied down" to anything permanent - so the question is, why are you putting in such a "base" or deck. Why not just put in a good well graded crushed stone base, then use bricks or similar for flooring? - can put a more "permanent" floor on that. Can build the pad up a bit higher to ensure drainage.

 

[MiketheEngineer]

I am sure more than a few Indians (North American) had to chase down their tee-pees once in a while.

One of my old pieces of software is saying that 10'high x 30' dia at 105 mph - you might get about 23 psf. Seems about right.

Which begs the question - how do you chase a yurt and catch it??

 

[cvg]

here is your traditional Mongolian yurt outside view and ceiling. The ropes are used to help hold it down