lateral loads on timber piles in coastal environments 1

[DZNC]

I am designing a home on the east coast that will be supported on 8x8 timber piles. After reading FEMA publications it appears the preferred method of bracing the piles is diagonal bracing. Does anyone have a design example of this. It seems like the bolted connection to the pile is the "weak link" with (2) 3/4" diameter bolts and I dont see any way I could possibly get more than a couple thousand pound tension load into the 2x10 brace which is only a small fraction of the lateral load. This is particularly problematic on the street front elevation as they want to park vehicles and there is really only room for one braced bay. Is it more realistic to add enough piles and design each of the piles for an equal amount of the lateral load as cantilevered columns?

 

[Lomarandil]

Sure, assuming the cantilever length (including depth to fixity below grade, with a healthy look at scour for a coastal environment) is not unmanageable.

 

[phamENG]

I'll preface this by saying there's about an 80% chance we're in direct competition. I service clients from Hatteras north, but would go out to Ocracoke if I got a call, and have a local client that hired me to design a building down toward Morehead City. I've done a couple of these, have assessed a few, and have one on my board right now. That said, I'd never try to sabotage anyone.

This is a really tough thing to do. First things first, how big is the house? Are you building one of the 12 bedroom monsters, or is this a more modest house for somebody to live in? I'm sure you're aware that the NC Residential Code has a special section just for beach front houses. USE IT. Especially for the piles. It was heavily influenced by the post disaster studies in the 90s from Fran and Floyd. If this house fits the prescriptive limitations, use the prescriptive pile spacing and sizing. The contractors know it and will call you out on anything else. If you don't meet those limits, then you'll have to do a fully engineered design. Make sure you have a geotech for that and yes, you'll likely be looking at cantilevered piles. It's not fun, especially since quite a few of those places have been built using the prescriptive pile design despite being 4 stories tall, and everyone knows it. Keep in mind that, if you're subject to wave action, cross bracing is prohibited (or at least strongly discouraged) as it can easily fail from out of plane loading either from waves themselves or from floating debris.

 

[SlideRuleEra]

...home on the east coast that will be supported on 8x8 timber piles?

Are these 8x8 timber piles driven, or are they 8x8 timber "posts" that are buried.

Edited: It is extremely unlikely that 8x8s are available that are long enough to be effective driven piles (that is, long enough to have adequate embedment to establish a point of fixity).

In other words, how long are the 8x8s and how much of that length is cantilevered.

 

[phamENG]

SRE - all the building suppliers along the beaches of the Outer Banks have a stock of 8x8 timber piles, and they can usually get up to 24ft pretty easily. The prescriptive requirement is 8ft embedment outside of Ocean Hazard Zones and 16ft or 5ft below MSL (whichever is less) in the Ocean Hazard Zones. So as long as you're outside of the OHZ, 8x8's are easy...if you're in it, sometimes switching to round timber makes sense if you need more length.

Edit: I am assuming this is in the Outer Banks. OP has NC in their user name and the profile says they are in NC. And everything out there is "on stilts".

 

[SlideRuleEra]

phamENG - Thanks, 24'... learned something today. I'll edit out the portion of my previous post about 8x8 availability.

Driven or buried, along with how much of the 8x8s are cantilevered and embedded remain relevant.

Beach houses on most of the SC coast are on "stilts" also... don't recall seeing driven 8x8s, only round driven piling. The 8x8s seem to only to be buried here.

 

[phamENG]

Vibrating is common here, a few people have wanted to jet them, but I haven't had it done on any of my projects.

 

[SlideRuleEra]

Vibratory hammers are ok for "light" loads (like a house), but there is no practical way to determine pile bearing capacity for "heavy" loads, other than using an impact hammer (briefly) after vibration is complete.

Agree, I wouldn't use jetting for a house... for piles is water (like a dock), sure. Jetting on land could compromise the piles lateral load resistance along with not knowing pile bearing capacity.

 

[DZNC]

Thank you all for the replies. They require all the piles to be driven to 16' here so I felt like creating fixity was not an issue. I ended up just adding a few more piles and taking out the lateral as point loads at the top of the columns.

 

[XR250]

Being an NC beach goer, I am always curious about the beach house foundations. I have been in a few that sway just from walking inside the house. I feel like the most robust foundations are those that use a mat slab and reinforced 16x16 CMI piers. Although, maybe allowing the house sway in the face of wind or wave loads is a better solution.

 

[phamENG]

I feel like the most robust foundations are those that use a mat slab and reinforced 16x16 CMI piers

Under serviceability conditions, I can go along with this. But storm surge and wave action could result in absolutely devastating scour around a mat slab. Even tightly spaced piles can cause 'group scour' that removes 5 feet of sand from under the house. FEMA has some pretty "neat" pictures.

I've been in plenty that sway like that, too. Bugs me to no end, and my wife just laughs at me. A little sway under design loads is fine. If the tractive force of my shoe on the floor is doing it...yikes.