A geotechnical investigation is ALWAYS a good idea for any foundation design.  For design of the sign itself, see ASCE 7-05 Figure 6-20 which addresses solid signs specificially.

You have 2 choices without a geotech report.


1.)  Go to ASCE 7-05 and get the appropriate wind loading.  Use that wind loading along with IBC 2006 1805.7 to determine the required embedment of the WF's.

2.)  Go to ASCE 7-05 and get the wind loading.  Use that wind loading along with IBC 2006 1805.7 to determine the required embedment of a sonotube pier.

Thanks for the info.   We are in Canada and are not Familiar with  ASCE 7-05 ...Sounds like American Society of Civil Engineers ...right ?
Ritz

Yes, ASCE 7-05 is the structural code in the US defining minimum load requirements on buildings and other structures.  In Canada, the code requirements may be different.

Sign

I am thinking toward the columns base plate bolted to a the conc pile cap as opposed to straight embeddment of the cols

WinPost will help you analyze your embedded posts. The program will perform a stress analysis on the posts and determine how deep the posts must be embedded into the ground. The program has a simple section property calculator, soil database and Wind load calculator. You also have the option to enter a single lateral load on the posts as well as an axial bearing load.


Google Archon Engineering. Moderately priced.

good luck

In Canada, climatic information is provided in the various provincial codes.  You will need Commentary I "Wind Load and Effects" in the User's Guide to determine appropriate coefficients.

I would strongly suggest enlisting the help of a geotech to  establish criteria for pile depth.

BA

Direct embedment of the poles is the best way to go.  

Piles, pile caps, and anchor bolt foundations will far too expensive to consider unless the soil conditions dictate this is the only way to go.

If you give them the design using piles and pile cap, my guess would be they would find another engineer that would provide direct embedment and use their design instead.

enginerding,

I don't agree with your comment at all.  The majority of signs I have seen are steel HSS or WF members with base plates.  

The pile cap is handy for placing anchor bolts.  It is usually formed with a sonotube of the same diameter as the pile.

BA

Agree with BAretired... Star.

B.Eng (Carleton), P.Eng (Ontario), MIPENZ (Structural-New Zealand)
Working in Canada, and missing my adoptive New Zealand family... at least I brought the little Kiwi with me!

Burying the posts is by far the easiest and cheapest today.  But you will never be able to monitor corrosion or condition of the post.  So if its for one year - sure.  If is for 20 years - forget it.

I am just telling you from my experience in the states.

Please let me know how you monitor corrosion or condition of the piles?  

You have all the same problems (or more - e.g. anchor misplacement, bent bolts, jacked-up threads etc.), but you pay more than triple for the foundation at the start!  

Drill the hole, drop the steel column to just above the bottom, pour concrete around it, done.

engineerding:

That's different from what I inferred from your first post...  Obviously it's not what you expected to imply, but I (and I'll hazard to guess BAretired et al as well) though you meant naked driven column shafts as the foundation...

If you cast the column into the soil an pour concrete around it, I wouldn't call that direct embedment.  I would agree, however, that it is an excellent solution and a good way to go.  The solution is predicated on the ability to source and handle long enough columns to get the embedment required.

At 30ft out of the ground, and I'm guessing about 6ft into the ground, we're just about pushing for length.  Also this commits the project to field work on site unless you can get the fabricator to deliver the fully assembled sign (not out of the question, but I've seen and heard of trouble in this regard).

A star for your excellent idea, now that I actually see what you meant! lol

Regards,

YS

B.Eng (Carleton), P.Eng (Ontario), MIPENZ (Structural-New Zealand)
Working in Canada, and missing my adoptive New Zealand family... at least I brought the little Kiwi with me!

rittz,

Instead of posting for your thread under Concrete Pile Engineering I will post here because there seems to be much more discussion going on.

My design had an overturning moment of 340kN-m (250kip-ft), it was socketed 2m (7') into DW rock. So the passive pressure I needed to resist the overturning was 300kPa (6kips/ft^2). Can someone please correct me if my SI-US conversions are wrong.

I spoke with a geotech who recommended that the passive resistance is 9*Cu where Cu is an undrained shear strength of the soil. Once on site they drilled a borehole and certified that the DW rock had a Cu of 100kPa (2kips/ft^2) which gave the design a high enough safety factor.

This sign is slightly larger than yours and used W20 columns with a 1-1/2" baseplate. From memory there was 8-10 anchors (1-1/4" dia) embedded 2' into the pile.

Pile was reinforced with 0.5% longitudinal reinforcement.

I think YS is using a H/5 rule as a quick estimation of pile embedment for large sign structures which is a good rule of thumb.

• http://files.engineering.com/getfile.aspx?folder=29354cf4-af88-4fa1-a066-04

9*Cu is ultimate.  You should use a factor of safety on that - usually 3.0.

 

rittz,

I'm curious as to the ultimate passive pressure distribution in the little sketch to the right of your diagram, especially when the soil is softer than sandstone, say clay.  

I have added a sketch showing my guess at the pressure distribution under rigid body rotation.  For soft soils, you would need a considerable pile depth to develop the moment resistance you require.
 

BA

• http://files.engineering.com/getfile.aspx?folder=8d3e4c65-e549-42a0-95b5-0d

In our area, I have seen a backhoe just simply dig a deep trench approximately the length of the sign to whatever depth is required by calcs, lower a cage of resteel in and fill with concrete. This can be done with steel columns or anchor bolts embedded into the foundation.

Quick and simple!

asixth:  Exactly right.  From there I use a set of modified Terzaghi equations to do the embedment.

As for actually calculating the depth of embedment there's some very good information for light poles (both timber and other materials) into all sorts of materials available.

Cheers,

YS

B.Eng (Carleton), P.Eng (Ontario), MIPENZ (Structural-New Zealand)
Working in Canada, and missing my adoptive New Zealand family... at least I brought the little Kiwi with me!

asixth and/or BA
How do you get  sketches into your message

rittz,

You need a PDF or JPG file in your computer (perhaps other formats will work too).  Click on the words:
 "...or upload your file
to ENGINEERING.com" at the bottom of the message area.  Then follow directions.
 

BA

Whenever I get a problem like this, I like to check the nearest DOT website and see what they have for a typical detail.  They have light poles, signs, etc.  Some of them have accessable computer programs that you can plug in your parameters and it will spit out a design.  
To me this has two advantages;  DOT's are notoriously conservative, so I get a top end design.  Plus, if (when)the contractor questions your design, you can respond, "That's the way the DOT does it." to shut them up.  

ASIXTH .....In your post you showed two 4' dia  concrete piers  7 ft deep. You had calculated the overturning moment to be 250 ft-kips. Would you please show the calculations that led to the result showing a passive pressure of  6 kips/ft^2 was nedeed. You can do that in either SI units or imperial.

If we say the overturning resistance of the piles is:

Mres=w*b*(D/2)^2

where w is the earth pressure on the pile, b is the width of the pile and D is the overall embedment depth. We know b to be 4ft, the embedment length to be 7ft and the overturning moment to be 250kips-ft. We can back calculate the soil pressure:

w=Mres/(b*(D/2)^2)
 =250/(4*3.5^2)
 =5.1kips/ft^2   

rittz,

Some very good comments above but also a few misleading ones.

I have designed signs all over the US for a man who did nothing but signs.

Any comments that this or that type of foundation are cheaper are irrelevant unless taken in the context of your project. Some situations make different methods infeasible or just plain expensive.

The main methods of support are as follows:

Spread footing - generally the cheapest for good soil conditions with sufficient space. Since you are talking about piles here then I expect this is not an option in your case.

Piled footing (or piles and pile cap) - this is an extension of the above with the addition of piles. I would recommend using screw piles for this as these tend to be cheaper and are just as good in tension as they are in compression. This also requires a good spread between the piles in order to work.

Vibratory caisson - where the steel pile is literally vibrated into the ground. This works only for granular soils where there are no nearby structures.

Concrete caisson - bored hole and either reinforced with anchor bolts at top or with a steel tube inserted and surrounded by concrete. Only really works if the ground is self supporting.

I have also used a thin reinforced base with rock anchors over a hard rock base.

You need to take into account the surrounding conditions. For example if the sign is adjacent to a basement then you need to ensure that additional load is not exerted on the structure.

Also, the geotechnical details will govern which methods are the most feasible.



 

asixth,

What is the source of your formula Mres=w*b*(D/2)^2?  How can it be justified?  A rigid body rotation should produce variable pressure on the side of the pile, not constant.  Also, passive pressure cannot act all the way to the surface.

The moment from a sign comes from wind pressure, so there must be horizontal shear as well as moment.  The formula you have used is not correct.

BA

BAretired

The reference for these calculations comes from a chapter in a book titled "Foundation Engineering Handbook" edited by Manjriker Gunaratne which uses the theory which was presented in Brom's paper for designing laterally loaded piles in the 1960's to the ASCE which assummed for the purpose of design that there is a uniform pressure distribution developed over the pile (Gunaratne's text can be viewed on EngNetbase for those who have access). I have been inclined to take this assumption because despite the fact that we are assumming the pile to act as a rigid body (which it will for the stiffness and design loads that we have), it is another assumption to assume that the soil behaves as an elastic materail. If you don't think it is valid to assume that the passive soil distribution is uniform can you please reference the paper or text because I would like to do some further reading on this. Maybe my approach is incorrect, I just haven't seen anything published in any code or handbook which suggest otherwise.

For the second point you raised. I didn't include solving for the unbalanced horizontal forces because they would not be a significant contributor to the design of these piles. Resolving the unbalanced moment would be governing the design. I have attached calcuations to shown how the horizontal shear force is reacted, assuming we have 10kips acting at 25ft to give the overturning moment of 250kips-ft.

• http://files.engineering.com/getfile.aspx?folder=7ffad5d5-5adc-4bf2-9286-8d

I don't know why but it didn't load.

• http://files.engineering.com/getfile.aspx?folder=0433d8ca-dbd4-4b19-8298-67

Sorry asixth, I cannot cite a good reference on the subject, but the assumption that the uniform pressure near the surface can exceed the available passive pressure in the soil seems wrong to me.

Unfortunately, this thread has not drawn any response from the geotechnical community.  Where is BigH when you need him?

BA

Try "Bridge Substructure and Foundation Design" By Petros P. Xanthakos.(Prentice Hall)(ISBN 0-13-300617-4) It seems to be on track if I could ever wade through the analysis.   

Brom's theory is still regarded as a valid design method AFAIK. Note that the 9*Cu 'passive' pressure starts at 1.5*dia down (hence no maximum lateral resistance at the surface), and a Factor of Safety (usually 3 for ASD) is applied. This is only valid for cohesive soils, non-cohesive soils are limited by the passive resistance.

It was included in the old AS piling code (see attached) and a version is given in the current 'Design of portal frame buildings' (by Woolcock et al).

 

• http://files.engineering.com/getfile.aspx?folder=69a233e2-b3ba-4132-a839-30

The Brinck-Hansen method is another one used in Oz. I haven't used it and don't know how it compares to Broms.

• http://files.engineering.com/getfile.aspx?folder=536dd722-c1ed-413b-9add-54