foundations for large sign

huangyhg

foundations for large sign
hi guys
i have a large sign (15m x 4m or 50' x 12') that is situated on 8m (24') of sandy clay which overlies weathered rock. i need to come up with the best foundation system to resist the large overturning moment.
i have decided on using either a very large pad footing (several cubic meters of concrete required to provide the required resistance to overturning). or provide a pile cap with either 2 or 4 piles that go down to rock.
are there any other foundation systems that others would like to recommend to resist such a large overturning moment as this one.
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what is the dead load of sign structure? how high it is?
or you tell me p and m. my gut feeling is going with single pile.
sign column mounted on pile top and piles extends to rock below.
make sure to extend pile 12" above grade.

i was under the impression that these were imbedded posts or something to that effect.
a third option would be a drilled pier (also known as a caisson).
daveatkins
i would start with trying to embed the shaft in concrete. do you have a geotech report?
if obviously depends how high this sign is, but what we usually see here is a big hole filled completely with concrete. i refer to this as a big mass footing or block foundation. it is a little more concrete but a one step operation which says money, no backfill, no piers, etc. use the mass to resist the overturning.
i am trying to remeber the heirarchy for sign footings, i think it went pad,caisson,piled.
pads are straight forward and can be done by any ground contractor. usually only 4 foot deep.
i doubt a caisson would work in this instance due to the short embedment in the clay down to the rock. the rock wont stop this rotating in the clay and you dont want to bore 15' into the rock.
for piled foundations i usually would use helical piles, but once again i dont think it would work in this case.
if you were to use piles i would suggest raked traditional h piles. but i would think this would be overkill.
if at all reasonable, we usually go with the mass footing, similiar to what jike is describing.
takes no specialized equipment, just dig a hole and fill with concrete (a lot of tconcrete).
sometimes mobilizing a cassion or pile rig is more expensive than just pouring concrete.  
dgkhan,
the dead load of the sign is 5tonne and the overturning momenet is around 800kn m for a 1000yr wind event. as you can see, it is a very large overturning moment and requires a lot of concrete to resist overturning.
try a t shaped foundation. big core shaft with enlarged thick regid reinforcing cap. you get hlep from both the lateral soil resistance and bearing under the cap beyond the shaft. you may even install tension ties under the cap.
i'd try 4 - 18" diameter augercast piles with an "x" grade beam system, piles at the ends of the "x", and a small slab on top to set the structure on as needed.  the "x" leg lengths can be set so that there is no uplift on the pile.  the pile could be extended to bedrock if not too deep.
mike mccann
mmc engineering