Depth of bollard embedment

[ajk1]

Does anyone have an expression, or a procedure, or a spreadsheet for determining the depth of drilled concrete pier to use to take the force and moment from a bollard post that extends down into the drilled pier, when there is asphalt paving at the surface that can resist horizontal force? I am trying to resist about 3000 pound horizontal load applied at 20" above the asphalt.

 

[bridgebuster]

TheIBC has a formula in the foundation section for drilled shaft depth, or go to thread256-359373. The info you need is there.

 

[ajk1]

bridgebuster (Civil) - Thanks. That was what I needed.

 

[bridgebuster]

you're welcome.

 

[ajk1]

Hi again Bridgebuster - Can you help me understand their definition of "N"? "Post deformation at d divided by maximum post movement (at ground level for non-constrained post)...". But they don't say where the moevemnt is to be calculated for the "constrained" post! Also, what do they mean by "deformation"?

 

[jayrod12]

Below figure 3 it states that the maximum post movement for a constrained post is at y=d therefore for constrained N=1 always.

I've only ever used this standard for wood posts so I got to cheat and use table 2 for dmax.

 

[hope9010]

jayrod12 - Somehow I missed that note! That's why I don't like reading from the screen without printing out. If N is always 1 for constrained posts, why does N appear at all in the formula for constrained posts? And if "post deformation" and "post movement" mean the same thing, then why do they use two different terms?

Another question:
For wind and for isolated posts, they say that allowable soil pressures may be doubled for isolated posts. I suppose that there may be a further increase in allowable soil pressure for vehicle impact type loading?

 

[jayrod12]

Those are good questions I do not have an answer for unfortunately. I eagerly await the response from someone in the "know"

 

[bridgebuster]

ajk1,

here's a nomograph for pole embedments. It's the same as what you would find in an AASHTO Luminaire manual. According to AASHTO it can be used for small poles but they don't define how small is small. You can use it to compare results.

 

[ajk1]

bridgebuster (Civil) - yes I am aware of that nomograph and equations; many years ago I made a spread sheet of them, but that is without base restraint. In my current case I have base restraint available and would like to make use of it.

I notice that there is an equation for pole with base restraint in the Unifirm Building Code 1997 edition (that is the latest edition that we have in the office). But it is a different equation from the one that you sent me from the ASAE. I will try to compare the answers from the 2 equations. If anyone has a more recent edition of UBC, maybe you can let me know if there is any change. The equation in the 1997 edition is:

d^2 = 4.25 P h / (S3 b),
where
P = applied lateral load,
h = distance from ground to point of application of P,
S3= allowable lateral soil-bearing pressure as set forthin Table 18-I-A based on a septh equal to the depth of the embedment,
b = diameter of round post or footing or diagonal dimension of square post or footing.

 

[azcats]

ajk1 - that formula is also in the IBC and it's stayed the same.

I would be careful assuming that the asphalt provides complete lateral restraint...or at least justify it. The 3000# force will probably need about 5000# of restraint at the surface. I know I've seen bollards in parking lots that have deformed the asphalt when hit.

 

[ajk1]

azcats (Structural)- yes right, good point. I expect that the impact resistance of the asphalt is significantly greater than its static bearing resistance but I don't know for sure.

If we allow for the load spread into the large area of surounding asphalt, perhaps we can use a factor of 2 on the permissible bearing stress (analogous to the 2 factor used in such situations in concrete).
Then 5000 = 2 x fbrg x 18" pier diameter x 3" asphalt thickness. Then we required a bearing strength of asphalt = 46 psi. I think most asphalt can take 46 psi without significant permanent deformation, since trucks with tires inflated to 100 psi drive over it sometimes.

jayrod 12 - please note that N is defined for restrained posts in figure 12 of the ASAE publication. It is NOT 1. If you are using N=1 you are on the unconservative side. Note that by "deformation", they mean the lateral movement of the bottom of the flexible post - see figure 12. In my opinion, they have used a poor choice of words. But I wanted to point out to you that N is not 1.

 

[ajk1]

bridgebuster (Civil) -

I find the ASAE paper incomprehensible. Where does one find nh? It says see Table 1, but I see nhSv there but not nh alone, and I see no definition of Sv. Also the way N is defined in figure 12, it would be zero for an infinitely rigid post, which would mean that the more rigid the post, the lesser the permissible dmax. That is the opposite of what I would expect.

I think that I will stay with the UBC procedure!