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Sign Foundation

Sign Foundation

Sign Foundation

(OP)
This is an interesting situation:

I am working on a project where the site engineer wants to excavate an area for retention pond. However, in the same area exists a big billboard sign. The sign MUST remain. It appears that the sign is supported via a drilled pier system. All data about the sing is not available.  

My civil engineer wants to excavate 7 feet on one side and slope the banks of the pond on two sides. My company does not want the liability for the sign foundation integrity. However, I am recommending that the contractor drive sheet piles all around the drilled pier square in plan. I also intend to starting the sheet piles about 6 feet from the face of the sign pole.  

My question,

1. If any one encountered a situation like this what would you do?
2. Is the six-foot enough to eliminate the pressure influence from the foundation? If not, is there a method to calculate the safe set back dimension?

By the way, I intend on contacting the sing owner and have them approve my approach and sign of on it. In other words, have them come back and address our approach.

Your input and thoughts would be appreciated.

Regards,
Lutfi

RE: Sign Foundation

There should be codes and standards for you to
follow in your State.  Check with the Department of
Transportation for signs and billboards.  There
might even be city codes that you must follow.

RE: Sign Foundation

(OP)
dimjim,

I live in Florida and I designed many overhead signs. However, I never faced a situation like the one I am describing.

I already contacted FDOT and I could not get anyone to respond since it is close to Thanksgiving holiday.

Do you have a reference that provides guidance as to how much earth needs to be at the pressure and the kick of a pole type foundation?


Regards,
Lutfi

RE: Sign Foundation

I would first the engineer doing the retention pond design to adjust the pond configuration so that the sign foundation is not affected.

If you can't relocate the foundation then you need to get as much information about it that you can. Get the plans and design data. If that is unavailable hire a geotechnical engineer do a field investigation and testing to confirm the foundation type, depth and capacity. This is critical. It may cost some money, but compare that cost to the costs associated with the sign falling over and hurting someone, not to mention all of the other costs associated with a failure.

Sheeting around the foundation may be acceptable, but if it were me I would want more information about the foundation before making any recommendation.

RE: Sign Foundation

Lutfi - I would consider using a continuous top wale around the sheet pile perimeter (kind of a cofferdam turned inside-out). Sheet pile interlock is not very good at resisting the tension that the sign foundation would apply as it tried to overturn under wind load. A top wale might be conservative, but for the small size of the structure, would relatively inexpensive.

Whether 6 feet is a good distance or not...
I defer to the Geotechs. You could take a look at "Pole Building Design" on my website - I lot of the info in that (old) publication comes from the Outdoor Advertising Industry.

www.SlideRuleEra.net idea

RE: Sign Foundation

Lufti

Let me add that you might want to get a geotechnical consultant involved anyway to help you negotiate FDOT geotechnical requirements. The geotech might be able to give you some advice that will help you on what has been done in the past for similar situations.

RE: Sign Foundation

If they are going to be driving the sheet piling, the vibration may affect the sign foundation.

Dik

RE: Sign Foundation

I see something similar to this done several times where they excavate around large transmission towers and leave like 15 feet or so around the base. I am not sure exactly how they come up with the amount to leave in place. It must surely depend on the soil characteristics, the amount of the pole that is left below the excavation line, and the loads.

RE: Sign Foundation

(OP)
Many thanks for responding. I am going to gather all I can on the sign before final plans are released.

SRE, Your thinking lines up with mine. I am going to add a whaler. I am intent on using concrete sheet piles for aesthetics. I am being told the pond will be dry.

I downloaded the pole file from your site a while back. It is a good one. I also have the outdoor advertising institute "old" handbook as well.

I already have geotech on the project and I will ask him. The time is bad because almost everyone is about to leave for the holidays.

I am just curious as to the cut off where the soil adds not value to the lateral load resistance. If I find it, I will be sharing it with all.

Regards,
Lutfi

RE: Sign Foundation

Lutfi - This is a very interesting problem. Perhaps one way to look at it is use the analogy of the interaction of piling in a pile group. In this case, you are looking for the pile spacing where interaction becomes negligible - each pile in a group approximates the behavior of an isolated pile.

Here is a paper that looks at interaction of piling at various spacings:
http://www.asce-sf.org/downloads/clay_analysis.pdf

See pages 1, 13 and 19.

Consensus seems to be that at a horizontal distance of 6.5 to 8 pile diameters the surrounding soil does not "know" that a pile is being laterally loaded.

www.SlideRuleEra.net idea

RE: Sign Foundation

Lutfi-

It seems to me that this is an issue that the geotech is paid to solve.  I would dump it on his/her lap.

Jim

RE: Sign Foundation

Lutfi...taking 6 feet of soil away from a drilled pier could be risky.  The pier is designed to have a certain depth to achieve moment fixity for the wind loads.  Let's say you have a drilled pier with a depth of 36 feet.  If you take 6 feet away, you take a significant percentage of the lateral load resistance away (not 1/6, since the resistance at the top is less than at the bottom, but could easily approach 10 percent), particularly in your soil conditions (assuming this one is near your location).

The lateral distance relationship is not the same as for a shallow foundation.  You might consider having them place a ringwall around pier to provide some top stability.  I designed one about 50 miles to the west of you and we HAD to have the ring around the top to get it to work for wind loads (it was a cell tower foundation).  Your wind loads are even higher and your soil conditions probably not any better, except that you might have a competent clay layer down there.

RE: Sign Foundation

Lufti,
It might be worth thinking about re-profiling the pond to make it shallower near the sign. Obviously there is no chance of moving the pond completely or you wouldn't have posted, but shallower near the sign and deeper further away may help some.

One more to consider - While the pond may be dry most of the time it presumably is a design expectation that it will also be full sometimes. You may need to consider the effect of the water on the dry soils around the sign and then conversely saturated soils around the sign and an empty pond after drawdown. Clearly more work for the geotech boys...

RE: Sign Foundation

(OP)
SRE, many thanks for the paper. I had a conversation on Friday with a good friend of mine who is a geotechnical engineer. He recommended to me to leave earth equivalent to not less than 5 times the pile diameter. His rational made sense to me for the spacing. The ASCE paper in pages 13 and 19 corroborates what he told me.

The problem is the area where the sign is located is very limited. It is on a 5 acres site and the sign happens to be located in the tightest retention zone. The sign is owned by a third party and the property owner gave them easement that has been recorded for few years.

I cannot locate or gather any information on the sign. I made contact with FDOT and gave them the sign permit numbers. The guy who is in charge will be gone till Monday. I hope they have plans for the sign structure and foundation.

I spoke with other engineers in my office and other offices regarding this problem. No one seems to have the right answer. One suggestion was to project a line from the bottom of the pier at 45 degrees. In other words, if the pier is 20 foot deep, then retain 20 feet of earth all around the sign. This may sound excessive. However, if  the sign has 3 foot diameter pier this will require 24 feet of earth all around using 8 times the pier diameter.

Jimiec, I agree about getting the geoteh involved. However, I do not feel comfortable on dumping on his/her lap for two reasons. The first is I want to see how this problem would be solved. Secondly, I think it is my duty to ensure that the solution is proper. I owe this to the owner.

Pba, the pond will be re-shaped. However, the civil engineer has his work cut out for him to achieve his volume as well.

I will keep all posted. This problem is indeed challenging and the solution should be the best technically and economically.

 

Regards,
Lutfi

RE: Sign Foundation

Lutfi - Perhaps one solution would be to use pile group interaction to your advantage. Working with your Geotech, calculate a pile type, size, depth, spacing, etc. that creates a traditional pile group with the sign foundation at the center.

A hypothetical example could be a ring of concrete piling (say each being 16" square, spaced at 4' on center). These surround the sign foundation, at a reasonable radius. Then, as the sign receives lateral load, the soil transfers some of this force to these surrounding piling - no pile cap or anything else, just driven piling.

This should allow the "island" to be somewhat smaller. Cost for a few spaced driven piling should be much lower than a sheet pile enclosure. Also you mentioned the aesthetics - the installed pile butts could be just below ground level - not visible.

www.SlideRuleEra.net idea

RE: Sign Foundation

As I see it the solution must be based on the depth of excavation ie. a simple 'width = x times dia.' can not be right, although it could give a minimum limit.
Wouldn't a line projected down from the top of pier (at GL) make more sense?
I favour the sheet piling tied with waler solution.

RE: Sign Foundation

Until you get the sign details
you cannot get a decision that
would be meaningful.  What a way
to spend the Holiday.  You can
calculate the wind loads, etc.,
but you have to know how it is
sunk into the substructure and
what that is.  Why hire a
Geotechnical Engineer if he does
not become responsible for his
analysis.

RE: Sign Foundation

Lufti,

I've thought again about your proposed sheet pile and waling solution - If you don't get the information you need on the existing sign foundation could you consider a modification of the sheet piling design so that it acts as a new foundation?

RE: Sign Foundation

(OP)
Well, one radical solution would be to fully take the sign down and design a new foundation for it that is flush with the bottom of the pond.

This whole matter is going to boil down to economics! The most economical solution will win.  I want to make this least painful to the client.

I am not sure about the rest of you but I think this situation calls for intuitive thinking and demands engineering problem solution. Not the usual beam or column design. I kind of like it!

I hope all of you have a great holiday. Do not eat to much turkey.

Regards,
Lutfi

RE: Sign Foundation

Lufti,
I wonder if pooring a pad would help stabilized
the sign and give you a great assurance that it
would not topple and also help to assure water
run off.

RE: Sign Foundation

Perhaps the sign owner would not object to addition of guys to stabilize the sign.  Two pair of cables at right angles leading from deadmen to the pole would replace the soil resistance to lateral loads.

RE: Sign Foundation

Lufti, I agree the most economical would win, and since I don't know anything about the soils, sheet pile cofferdams might end up being more expensive than other methods. Dik makes a point about vibration issues if you are 5 or 6 feet from the sign, and you should consult your geotechnical engineer to see if vibration-induced settlement would be a significant problem.

Sheet piles can support the surcharge loads from the foundation but you can greatly reduce cost by getting a pile with a smaller section modulus if you move farther away from the structure.

Now don't quote me on this, but with 7 feet of excavation sheet pile structures can be cantilevered.  Of course if the geotech feels that the moment would be too high and cause deflection of the wall would occur with a cantilevered design, add the walers and you should be fine.
On a side note, adding walers decreases the length of the pile you need to drive and could save you money on the additional weight of steel and installation costs necessary for a cantilevered system.

For aesthetics, you can always paint the exposed portion of the pile or place falsework on the pile head. Or you can cut the exposed portion as mentioned before by SlideRuleEra.

Do the math on alternatives, but if sheet pile is most economical, my company can eliminate the issues of vibration and the risk of litigation from noise if you're in a residential or urban environment. We're located in Orlando.

WePushPiles
http://www.gikenamerica.com

RE: Sign Foundation

I can't recall the number, but there is a spec in the IBC that says you have to notify the neighbor prior to an excavation up to 10' deep and he is responsible to shore his own foundation.  The NY bldg code even says piles have to be designed for adjacent excavation to 10'.  I never understood how this came into the code, but this might be a good time to use it.

RE: Sign Foundation

Lufti's project is in Florida because he's going to FDOT to pull permits on the sign.  Pslem has a point though, hit the books and maybe the law will save you from doing anything about the sign altogether(except your lawyer's billable hours).

WePushPiles

RE: Sign Foundation

I cannot see how any code can require a neighbor to shore up his own foundation.  Those costs must be borne by the person disburbing the foundation.

Don Phillips
http://worthingtonengineering.com

RE: Sign Foundation

Technically, you are only disturbing your own property.  That your neighbor chose to gamble and use your soil for lateral restraint was not a problem.  But now that you wish to develop it, he is restricting your usage.  It should be up to him to reinforce his footing as necessary, not you. Section 3307 "The person making or causing an excavation to be made shall provide written notice to the owners of adjoining buildings advising them that the excavation is to be made and that the adjoining buildings should be protected.  Said notification sahll be delivered not less than 10 days prior to the scheduled starting date of the excavation."  

RE: Sign Foundation

That version is not exactly what I was hunting for.  There is one that specifically mentions the 10' and under is his, over is yours.  It might have been an earlier version or IRC.

RE: Sign Foundation

(OP)
All,

Let me share with you what I find out and what I wound up doing:

1. There are not published papers or documents that deal with this matter. This brings to mind what my college professor used to say “Use your engineering judgment”.
2. Contacting geotechnical engineer would be of no use or value at this juncture since I do not know the sign foundation type, size and depth.
3. Florida DOT only issue sign permits. They do not review or issue construction permits for signs.
4. Florida DOT has a state wide data base for every sign in the state. The data base is impressive. It ahs lots of data and even pictures of the sign.
5. This is a link to the site in case any one is interested: http://www2.dot.state.fl.us/RightOfWay/dbhome.asp
6. I finally made contact with the sign owner. It turned out they did not design the sign. They bought it from another owner. The researched their records for me. I also had to work with them to coordinate the access to the sign for maintenance.
7. I performed my own wind load calculations and did quick pole embedment depth calculations. I came up with 19.5 feet. For simplicity I used 20.0 feet.
8. Sign owner said that for that particular sign they use 16 foot depth embedment.
9. Based on my conversations with respected geotechnical engineer friend of mine (he is PhD and PE and author of text book) he recommends using a minimum of 5 times the pole diameter. This seemed reasonable to me.
10. SRE provided me with an excellent paper that I read and found to be very interesting and informative. It came to similar conclusion to that my PhD colleague suggested. I think 8 times the pole is more suitable for the paper’s type of soils.
11. Another approach that was reached amongst me and three other engineers. The approach was to use a radial line that is taken from the bottom of the pole at an angle equal to repose with the vertical. This would provide the end of “no disturb zone”. To be conservative, I opted to use 45 degree angle.
12. Using the five times the pole diameter which is 4 feet I come up with 20 feet of “no disturb zone”.
13. Based on my quick pole embedment calculation of 20 feet and using 45 degree, my “no disturb zone” comes up to be 20 feet.
14. I opted to construct “L-shaped” retaining wall on three sides of the sign with the face of the wall being 20 feet from the center line of the sign. The footing will be constructed towards the sign.
15. The civil engineer had to re-work his retention areas and volume of the pond and re-grade all around the sign.

This has been very interesting and challenging task. I appreciate all of the input from everyone.

Some comments on recent input:

A. Use of guy cables is not an option since the sign will be in the front of a shopping/retail mall.
B. There are no issues with neighbors and set back. All we have to maintain is access/easement for the sign company since the easement was deeded.
C. IBC is not applicable in Florida (at least not yet). We use Florida Building Code 2004 edition with 2006 revisions. The 2006 revisions went into effect this month.
D. Sheet piles would have been costlier and the vibration effects were not appealing.

Regards,
Lutfi

RE: Sign Foundation

Here is the full 2003 IBC code text
3307.1 Protection required. Adjoining public and private
property shall be protected from damage during construction, remodeling and demolition work. Protection must be provided for footings, foundations, party walls, chimneys, skylights and roofs. Provisions shall be made to control water runoff and erosion during construction or demolition activities. The person making or causing an excavation to be made shall provide written notice to the owners of adjoining buildings advising them that the excavation is to be made and that the adjoining buildings
should be protected. Said notification shall be delivered not less than 10 days prior to the scheduled starting date of the excavation.


And the commentary text:
This section emphasizes the need to protect all existing
public and private property bordering the proposed construction or demolition operations. The term “property”
only alludes to existing buildings. As such, any building
element or system must be provided with a safeguard that will limit the damage that could be caused from the processes involved to the equipment and materials
used. Additionally, soil erosion and land disbursement
control resulting from the construction or demolition operations must be provided to prevent spillage and
spread of disturbed soil debris. The site must be graded
in accordance with Sections 3303.4 and 3303.5 for demolition
and must be maintained in a similar manner while there is construction taking place. The owner or owner’s agent has the responsibility to provide a written notice 10 days in advance for any demolition or construction activities that may warrant bordering lots to be protected from damage.


As a building official, the way I interpret this is the construction documents being submitted for a permit must show the proposed protection to existing building.  It would be unreasonable to put the adjacent owner on notice that his building will be in jeopardy and has 10 days to protect his building. It would take 10 days to get an engineer on contract.

I think the intent is to place that design responsibility upon the owner doing the work and the notification is intended to ensure that vibrations caused by pile driving, or any other activity that is unusual and may affect the building occupants be addressed.  Rattling glass is one thing, filing cabinets falling over and killing people are another.

Don Phillips
http://worthingtonengineering.com

RE: Sign Foundation

(OP)
Don,

Florida is way ahead of the rest of the country when it comes to property protection and storm water management. As a matter of fact we are regulated to the hilt when it comes to storm water and erosion. In general, I am in agreement with the IBC.

The sing is not any where near property of any one. It is set back well within the project site.

Regards,
Lutfi

RE: Sign Foundation

Lutfi,
A big thanks for posting you final solution to this problem. It's nice to get close out on these!

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