post and pier vs slab

[Rob Tucker]

Because the demand for non-wood package homes isn't there. Most everybody wants wood. Home Depot contacted me a few years ago wanting to do Castleblock package homes and I pursued that through Honolulu, through the L.A. office and onto Atlanta. Problem I ran into was the management at HD was getting head hunted - hired away. Every two weeks there would be a new face at the desk. The other issue was product liability. If I provided roofs and walls and HD provided windows, doors, etc. Whose product was it? Couldn't get that answered with the turnover in their Atlanta office. I did get an interesting comment from HD management though: "We hate wood."

2x4s out of recycled material is kinda the holy grail. No one has come up with one that has any tensile strength.

[MarkP]

quote:

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Originally posted by ericlp

with the earthquakes we get around here ... I'm thinking wood will outperform steel. As wood will flex unlike steel. But what's just me. There is a lot info on the net. That's also a concern with cracking of slabs during a nice jolt.

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Steel is strong and stretchy while concrete is weak (in tension) and stiff. Unless the steel is pre-stressed, cracking is normal and inevitable and the steel is there to hold the chunks together. This works well enough most of the time. In cases where a certain amount of cracking is unacceptable pre-stressing of the steel is done. Pre-stressed post-tensioned slabs are not uncommon in certain parts of the country where the soil expands and contracts a lot over the course of the season. I personally think that spending the money for the extra materials to make the slab strong enough to span any crack that forms underneath it is the way to go, plus put it on top of a double thick layer of crushed stone fill to isolate it from the ground. Make sure it is NOT keyed to the rock underneath. Now build whatever you want on top, pinned to the slab. The ground can move underneath it for all you care. The whole bit about houses on piers walking in a productive way has always struck me as deep rationalization.

Steel reinforcement of concrete is a mature technology. Ships have been made of the stuff that have lasted for decades. Those that last are very well built with extreme attention to steel placement and cover. I have never seen a house foundation built with that sort of attention to detail. I don't know the specifics of the Castleblock and similar systems but I bet that either the components are factory made with good quality control or for example in the case of ICFs the design of the block makes it very difficult for the builder to get the re-bar wrong.

[DrWho]

some post and pier houses will do better than slab houses in earthquakes. it depends on the frequency of the vibration. if your structure reaches a resonance you are screwed. this means sometimes slabs will do better than post and pier. so to make it simple....it depends on the earthquake more than the construction.

the doc is in...

[Rob Tucker]

Thank you Doctor.... that was quite enlightening. Some will, some won't. What do you suppose is the difference between those that will and those that won't?

[DrWho]

the earthquake make more difference than the way you build. thats all im saying.

for fast shakes a house on piers will do better since it has the ability to sway.
fast shakes will crack the foundation on a slab house since there is no give.

for slow shakes a house on slabs will do better since a house on piers will sway MORE a fast shake and that will topple the house on piers. the slab keeps the house intact during slow shakes.


so either method is ok, nobody knows which will do better unless they know what earthquake will come...fast or slow.

the doc is in...

[kander]

Sounds like I should whip up some seismic dampeners to sit between the ground pads and the steel posts.

[Rob Tucker]

Doc. I'm not sure what you are a doctor of. Stick to it and try to not be an amateur engineer.

Engineering design is to a given seismic event level. Building codes are the MINIMUM standard of construction allowed by law, not the maximum. I don't work with minimum standards. Most everyone else does. I do tell all my clients that no matter how well we design and build, ultimately there is nothing made by man that cannot be defeated by nature.

[DrWho]

i specialize homeopathic medicine. i find building interesting and so thats what i am going to do for the time being. having said that after doing a bit i research i found that earthquakes are indeed different almost every time. the code covers this i suppose but i wonder how they address the differing types of earthquakes. anyway maybe your right maybe i should focus on medicine LOL.

i still like post and pier better than slab.

the doc is in...

[Rob Tucker]

So do I.

[DanielP]

Being raised by a structural engineer who specialized in earthquakes relating to concrete design, doing projects like the San Mateo Bay Bridge, Hospitals, and many large projects around the world, it was explained to me that the primary focus of engineering for earthquakes was about preventing catastrophic failures which risk human life. If a structure cracks and crumbles during a large earthquake, but doesn't collapse, killing the inhabitants then the design can be deemed sufficient.

Slabs generally don't add structural integrity. The footings, grade beams, piers and lateral bracing are what transfer vertical, lateral, and horizontal loads to the ground; not the slab.

Piers set on compacted base course act more as a flex for lateral loads and bearing against vertical loads. Footings above grade actually "grade beams" also act as a lateral cushion, while tying the foundation together to resist the lateral loads. Footings and Piers "pinned" or placed into native soil or rock will effectively tie any lateral loading to the ground without allowing any lateral flex.

My amateur engineer perspective
Dan