RE: Pier Depth

[Dennis]

Thanks for this information. It definitely has added to the understanding. Our lot is mostly Ah-ah lava and we will need to rip the house pad. We want to leave as many trees as possible and clear just enough area for the home, drive and a small front and back yard. You are correct in how post & pier is described. We are trying to be a little off the ground.

[Wao nahele kane]

Yep... rip and roll it with a D-9 where you intend to build as it's a must in Ah Ah.
After that... if you want to be off the ground, pour some round or square footings about 8-16" into the lava rubble and bolt some steel posts atop the footings or pour some more small columns atop the footings and or do a monolithic pour for the columns and footings in one shot.
It's a very simple process although I believe if using posts & footings only - I swore that I read while skimming through a revised code a few months back, the new code stipulates a rafting pad be poured to connect the footing blocks and that’s probably applicable to any solely based footing block and column base structure. In some measure or another, a pad or direct connect foundation is required to a particular point now.
I just finished up the 4th or 5th design for our house and it will use several footing blocks and columns but it’s all based around and atop a 26’X38’ turn down footing pad so rafting out to the footing blocks will not be necessary though there are exterior pads that do by default connect most of them.


E ho'a'o no i pau kuhihewa.

[Hazen]

allright you got me, what's a rafting pad? does it float? otherwise i'm 100% in agreement with Wao nahele kane's statements.

[MarkP]

The reason for individual posts/piers is economy and minimal use of materials. The reason not to use same is that there is nothing to ensure that all the posts will move in the same direction at the same time. If they do there is a much better chance of the house surviving. If they don't and they move in different directions, the house will fall apart. By fixing the posts to the underlying rock you are tying the posts together so that they all move with the underlying bedrock. This is beneficial to some degree but has the drawback that in an earthquake it is the underlying rock that is the source of the damaging motion, plus you are counting on the bedrock to not have a crack in it that runs under your house. So, the underlying rock is a poor man's slab that you assume is solid but may not be, that has no steel in it, and that is an intimate part of the largest shaker table known to mankind. Keep in mind that while tofu block plantation style houses have a certain charm, like many facets of local Hawaiian culture, they are not necessarily the best design. They were built that way because they were cheap. Some people make excuses like "Yea, after a quake you can walk around with a sledge and knock each post back into place a little at a time." Wouldn't it be better to not have the house sprawling all over like a klutz on roller skates in the first place? I say, and I have talked to builders who agree, put it on a good well reinforced slab with a foot of crushed rock underneath. Remember, you don't WANT it to move during a quake.

[DanielP]

"Rafting pads" sounds like what we, in that other place I was, called grade beams.

[Dennis]

In looking at various homes for sale, you can obviously tell that some are built on a slab and others appear to be sitting on posts. I understand that just putting this posts on the ground do nothing if the earth moves and your house can fall off. That being said, I can't believe that the county would even permit that type of construction. I have seen other pictures on this site of others homes being built that seemed to have holes dug/drilled and subsequently filled with cement and then post attached to these piers? Am I missing the basics here?

We are trying to design something a bit off the ground and able to stay together if the earth moves. Possible?

[DanielP]

Dennis,

Are you in soil, Pahoehoe, A'a or ripped lava?

Dan

[Wao nahele kane]

Mark P gave a good explanation as to why a rafting pad is used.

Sorry… I wasn’t watching the thread for a few days there.
Rafting is unfortunately thought a generalized term these days and nearly lost is it's origin meaning but when properly used, it indicates connectedness (in a boating situation, a cluster of boats may “raft” (tie) to one another “rafting” from the the Old Norse meaning to Tie and or Connect to one another many members) and such a pad should not be considered a floating or spreading load pad, in the case of a house, we’re talking about rafting the posts and or footings to one another through a concrete pad thus deemed a “rafting pad” and not a garage pad/slab or turndown footing pad/slab.


A grade beam is typically a concrete reinforced beam established at grade level and serves as a foundation. Such beams may be established under several differing circumstances. A typical established grade beam might be placed atop piles spanning from one pile to another along the ground hence “grade”. There are other scenarios also. Sometimes the ground may be too soft or subject to light flooding and there are a handful of other reasons for this too but most deal with soil conditions.

Is there a way to keep the house together under a seismic event? Certainly.
Hawaii isn't really known for heavy earth quakes moderate perhaps but certainly not heavy. The current seismic regulations in the IBC are more than adequate for Hawaii; winds are a greater concern in Hawaii.

What's the grade slope(ground angle) of your house location Dennis? Will you be creating a flat level area for the house or are you on a slope? You've already stated you're going to rip and roll the aa so I'd gather there's no critical slope. If you can level your house footprint area you should be able to rely on a simple rafting pad for your post footings to be tied to one another if you have deep earthquake concerns.

I opted for a slab garage with turn down footing perimeter to serve as my houses main foundation. Everything will be built atop and over hanging to a degree above that. I specified a 55" high storage vault above the garage putting the first main occupied space at 15' above grade to provide some additional security with the only exterior decks on the next floor above at 25' above grade. All of that was to establish an ocean view and provide some additional security to the house along with minimizing the rafting pads necessary etc. I’m trying to kill a bunch of birds with one stone so to speak.


E ho'a'o no i pau kuhihewa.

[Dennis]

We have a single lot in Nanawale. It is all rough A' A' lava that will need to be ripped. We are looking at putting a single story ranch with a wrap around lanai on this land. Would like to lift it off the ground a bit. Considering ICF & light gauge steel construction. Trying to avoid using wood if possible. The land is fairly level. Hope this helps.

[mr. bill]

My lot in HPP has a raised area on the back 100', 5 to 6 feet above the rest of the lot. That area rises abruptly and is almost flat with some pukas running cross lot. I want to build on it starting with formed and poured fondation wall about 30" high, bonded to the rock with epoxied rebar. It would span one or as many as two cracks.I got conflicting opinions from graders and contractors.Any views on it would be very very appreicated. Thanks

I'd rather be fishing "WITH BAIT"