Foundations for Large Sign

[asixth]

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.

 

[kslee1000]

haynewp:

You pozzled me on "base plate", did I mention it? Maybe you were trying to point out my previous "T shape foundation" suggestion, which was intended to overcome excessive OT while utilzing both soil pressures around the shaft, and bearing pressure under the cap. Or you were questioning "I would provide minimum steel on the upper few feet of the footing, maybe down to local frost level." in my latest response, the footing here was meant the "drilled hole" as mentioned by cmm1080. So which one is your question pointing to?

 

[haynewp]

Neither, I was saying if you embed the pole into the concrete foundation a big base plate design is not required. So I was attempting to agree with you.

 

[swivel63]

wait.

1000 year return interval?

 

[kslee1000]

I see, and sorry for the mis-interpretation. Thanks for the clarification. I agree, that (embed the pole without mess around with base plate assemblage) may be the most cost effective way in many occasions for this type of structures.

 

[cmm1080]

Thanks for the suggestions thus far. The design for this particular type of sign does not consist of a base plate. The structures are usually located just outside of the state r/w (typically along the interstate). I design under the assumption that a rebar cage will be dropped into the hole, pole placed, and concrete poured.

 

[csd72]

asixth,

I would then go with a 5x5m pad footing with 4 helical (screw) piles each side 7m into the sand. Load on the piles is approx 50kN plus or minus the pad which will give you the minimum size anchor.

If the load tsts on these dont get what you expect then you can always add another one.

I would imagine that where you are, a 2m caisson or bored pier would be uncommon and expensive.

 

[apsix]

"1000 year return interval?"

The Australian codes don't factor the wind loads. The appropriate return interval is selected according to structure type (eg farm shed or nuclear facility etc) and limit state (serviceability or strength/stability).

I would have used a 500 year return interval for a sign.

 

[aayjaber]

How about a large and deep enough in the rock vertical pier to accommodate the main pole of the sign. Then 4 slanted(45 degrees) piers all tied to the main pier around the grade level.

 

[Ron]

cmm1080...due to the high moment in many of these foundations, the steel requirement will usually exceed the minimum requirements.