The secret revealed by the recent large quakes in South America is that the
seismic designs on the US west coast should actually be for 9 + quakes. What
happens in a quake cannot necessarily be generalized because not all sites are
the same. A site can vary from solid rock to something that in a quake is
essentially gelatin. New York City buildings can can be pinned into granite.
Mexico City buildings are sitting on jello hundreds of feet thick. Is NZ rock
or volcanic? What happens in a quake at the juncture between the site and the
building, which is what I think you are referring to, is a function of the
building's foundation mass and the springiness of the ground around it that
connects to the overall infinite mass of the earth. How much of that
surrounding site to consider as spring and how springy it is and how the
building foundation mass is dampened to that spring is stuff that the local
earthquake folks know how to estimate. Ask those folks about what to consider.
Wha
t might have been mentioned by others is a design where the building proper is
isolated from its foundation. Those designs might allow for many inches of
relative movement between foundation and building. That movement can be handled
in a few ways but it does nothing for the service entrance where it first
enters the basement or first floor. That topic was briefly mentioned here from
a different angle when someone asked about sleeving the ground floor as a
requirement. As an aside, just ask your structural engineer if the ground floor
slab is an essential part of his or her structural design. It is structure.
It sounds like you are saying the construction for a new building is a 30 inch
thick concrete foundation mat sitting on an even thicker rock mat. I don't know
what the soil types are like in NZ or what type of building is this, but I
would be wondering how much settlement is expected to happen in the next 20
years because that type of foundation design might be taking something else
into consideration besides seismic and you are probably going to need to
account for some settlement happening somewhere in addition to quake
considerations.
There are ball and socket type fittings for underground and these can be used
just prior to entering a basement wall. I think these might need inspection
during their lives. A section of HDPE piping might also do the same, but take a
close look at what goes on at the HDPE to steel connection. The pipe might be
flexible enough but it might just pop off its connection. It would make sense
for the underground piping to be generously backfilled with a clean graded,
rounded sand that would become fluid like in vibration to allow pipe movement,
but it may need to be wrapped in a filter fabric to prevent smaller particles
from eventually migrating into the sand and chocking it into a solid mass.
If anything remember it is foolish to think about calculating quake forces.
They are basically infinite. Think deflection instead.
Allan Seidel
St. Louis, MO
On Mar 3, 2013, at 11:06 PM, Nicky Marshall <[email protected]> wrote:
> With rebuilding starting to get underway in Christchurch a number of items
> are being looked at a little closer.
> One of them is whether we should be allowing for movement due to seismic
> events between underground services and the ground when they rise into the
> building to feed building services.
> One building we are working on has a gravel concrete flooring and foundations
> 750mm thick sitting on a 1.2m deep gravel raft reinforced with geo-grid.
> Has anybody looked into this? What solutions have you come up with and how
> much movement does it allow for?
>
> Looking forward to your ideas.
> Thanks
>
> Nicky Marshall
> Branch Manager (Blenheim)
>
> Protech Design
> Specialist Fire Protection Consultants
> 03 579 5577
> 021 433 488
> www.protechdesign.co.nz
> skype: nicky-marshall
> 105A Alabama Rd, Redwoodtown, Blenheim 7201, New Zealand
> PO Box 4022, Redwood Village, Blenheim 7242, New Zealand
>
>
>
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