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.

 

[dgkhan]

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.

 

[swivel63]

i was under the impression that these were imbedded posts or something to that effect.

 

[DaveAtkins]

A third option would be a drilled pier (also known as a caisson).

DaveAtkins

 

[haynewp]

I would start with trying to embed the shaft in concrete. Do you have a geotech report?

 

[jike]

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.

 

[csd72]

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.

 

[lkjh345]

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.

 

[asixth]

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.

 

[kslee1000]

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.

 

[msquared48]

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

 

[TpaRAF]

On Florida DOT projects, we have gone almost exclusively with drilled shafts (caissons). Given the overturning forces, I would think socketing the caisson into the rock would give you a good design. You mentioned weathered rock, so the socket would need to be in competent material.

Is this a cantilever sign? If so, torsional forces can be quite large (esp. given the 7.5m+ moment arm).

For this size panel, I'd think a 6ft (3m) caisson would do nicely.

Also note, design is just one leg of the triad. Materials and installation are just as important as your design... If this is atypical construction for your area, bump up the safety factors to account for poor workmanship.

Good luck on your project! RAF

 

[TpaRAF]

Arrgh -- 6ft => 2m RAF

 

[BigH]

You can always use tie-down rock anchors (passive - non-tensioned) to resist uplifts. Any soil/rock driller could drill these in for you and install.

 

[Ron]

Agree with DaveAtkins and RAF...drilled shaft. While you probably won't need the full depth to rock for overturning resistance, if you stop the heavy shaft and its heavy sign in the sandy clay, settlement might be a problem, so socketing would be good to handle both.

As RAF noted, your torsion component on this will likely be large.

 

[BigH]

I guess I'll point out the passive rock anchors again - we've used it on many occasions - to hold down tanks taken well under the water table, to tie large stack tie-wires to the ground for uplift. I would presume that a small portable or truck mounted drill rig is a lot cheaper to mobilize and demobilize that a drilled shaft rig - especially if it is intended to socket into rock for the uplift. It's what you used for the investigation! But, then, that is my take on it - we are using many rock bolts (dowels) for rock wall support where I am now working . . .

 

[asixth]

Thanks guys, I believe I have enough options to get back to the contractor with. I'll let them decide which will be the easiest way to construct.

 

[cmm1080]

I have a question relating to the original one. First off, I don't claim to be a structural engineer. We do have a couple on staff... hats off to all of you guys. I'm more of a site developement guy... dirt and water. Anyway, I have done a couple of signs and billboards and the question was brought about by a contractor who has drilled hundreds of mass footings. I called for the A_req or A_min of steel, whichever controlled. The contractor told my client that he has never placed steel for the erection of a sign. FYI... the foundation starts 2" below the ground surface, "slots" are made on opposite sides of submerged portion of the pole to allow for concrete flow in, the pole does extend to the bottom of the foundation, typical sign is 36'-6" x 10'-6" atop a 30" diameter monopole. Total height is 45'. I usally stay around 16-18 feet in depth with my foundation. Was it overkill for me to call for steel reinforcement? Do you guys count the submerged portion of the pole as reforcement? Thanks in advance.

 

[kslee1000]

My opinon:
Pole extended to the bottom of foundation could get away without structural reinforcing. I would provide minimum steel on the upper few feet of the footing, maybe down to local frost level.

 

[haynewp]

kslee

I think you mean the same as what we almost always did. Extend the pole into concrete pier almost to the bottom without any rebar. The whole foundation with pole functions to resist the lateral and you don't have to mess with a big base plate.