Sign Foundation 5

[rittz]

I have been asked to design a a concrete foundation and structute for a sign (piles and Pile Casp ?) Sign 8 to 10 ft wide 30 ft high. Not having done this before looking for source of information or other help . I would expect a geotech test would be a priority. The sign supports would be 2 30 ft WF's 10 ft apart with solid signage between.

 

[BAretired]

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

 

[JedClampett]

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.

 

[rittz]

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.

 

[asixth]

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

 

[csd72]

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.

 

[BAretired]

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

 

[asixth]

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.

 

[asixth]

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

 

[BAretired]

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

 

[rittz]

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.

 

[apsix]

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).

 

[apsix]

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