Need help with design for a sign (required embedment depth)

[Cap07]

Hi,

I need some help with a design for a sign. It is a 4’x8’ sign, supported by pressure treated wood posts. Please see the attached sketch. The jurisdiction requires that the sign and supporting structure be designed for a wind load of 30 psf. If I use tributary widths, I calculate a load on the center post of 480 lb:

Post load = (2’ + 2’)(4’)(30 psf) = 480 lb

The bottom edge of the sign is 3’ off of the ground, so I am calculating a moment of 2400 ft-lb at the post base:

Mpost = (3’ + 2’)(480) = 2400 ft-lb

I calculate that I need a 6x6 posts.

The thing that I am not sure about is the post embedment. I am calculating this using 2006 IBC section 1805.7. Specifically I am using the nonconstrained equation in section 1805.7.2.1:

d = 0.5A{1 + [1 + (4.36h/A)]½} Eq. 18-1

I don’t have any soil properties, so I am assuming the worst and using 100 psf for my lateral bearing value (per IBC Table 1804.2). If I use a 1.5 ft diameter concrete footing for the post, I calculate a embedment depth of almost 6.5 ft. Please see the attached spreadsheet. The calculation is an iterative process, so I assumed an initial depth, then continued until the assumed depth equaled the calculated depth.

My question is - does my calculational approach seem reasonable and does the depth I’m coming up with sound reasonable? It seems awfully deep to me.

Thanks in advance for any help or advice you can provide. It is most appreciated!

Cap

 

[Cap07]

Here's the spreadsheet

 

[nutte]

Your numbers look correct to me.

There's a note, I believe just prior to the equations, that says you can double your allowbale lateral bearing pressure if you can live with a 1/2" of movement of the sign at the ground. Use that to your benefit and reduce the embedment depth.

 

[Cap07]

Hi nutte,

Thanks for looking at my numbers. I can probably live with 1/2" of movement of the sign at the ground. I can't find the note you're referring to - can you point me to the IBC section that it is in?

Thanks for your help!

 

[MiketheEngineer]

Look at 1804.3 and 1804.3.1 -- Some interesting help there

 

[asixth]

this is an earlier post on the topic

thread507-254169

 

[BAretired]

If you remove the central post, the remaining two posts will each carry the same wind load as you have calculated for the central one.

BA

 

[Cap07]

Thanks everyone for your replies. I'll check out sections 1804.3 and 1804.3.1, along with the other post on this topic. Good point BAretired I'll check the sign framing and see if that will work.

 

[bdruehl]

haha..good one BAretired. thats something i would also miss as i stare at a project. i think ill give you a star!

 

[Cap07]

Hi,

I recalculated the loads for the sign using ASCE Fig. 6-20. For a 4x8 sign, 1 ft off the ground supported by (2) 4x6 posts, I am getting an embedment length of approximately 3.5 ft (2 ft diameter concrete footings). My wind speed is 105 mph and I am assuming a lateral bearing value of 100 and doubling that to 200 (per ASCE 1804.3.1).

Does the embedment length I'm getting seem like it's in the ballpark? I've double-checked all of my numbers and I think they are right, but size of the footings seems like overkill to me.

Thanks,
Cap

 

[asixth]

H/5 is a good starting point, so a 7ft sign would require 1.5ft embedment. I would not include the top 3ft as effective to allow for erosion or any soil shrinkage away from the pile that may take place from varying moisture changes. I haven't run the numbers to prove that the sign works but it would be advisable to get a geotech out there to dig a borehole and tell you what type of soil it is and whether the 200psf is achievable.

 

[miecz]

I have a few ways to calculate embedment depth, with widely varying results. See the attached workwheet. The first method follows the IBC. It agrees with the original excel worksheet. There is another method derived from Brom's method and published in AASHTO Standard Spec for Luminaires, etc. Article 13.6.1, that results in less than 4 ft embedment.

 

[Cap07]

Thanks asixth and miecz for your replies. I agree asixth, it would be a good idea to have a geotech take a look at the soil, but this is a very low budget project and the jurisdiction that I'm in specifies the use 1500 psf / 100 pcf in Table 1804.2 unless you can justify better numbers with an investigation. And since this is a sign, I'm going to use the factor of 2 allowed in 1804.3.1 (as pointed out by nutte and MiketheEngineer).

miecz - thank you very much for running the numbers. When using Fig 6-20 in ASCE 7-05, my highest post load is now 350 lb. Also, I've decided to lower the sign so that it is 1 ft off the ground (h is now 3 instead of 5). Those are the parameters that I'm getting the 3.5 ft embedment for. From your calculations it looks like that number is in the ballpark.

 

[BAretired]

The cost of drilling a couple of holes a bit deeper is trivial. If you don't know the soil properties, err on the safe side.

BA

 

[msquared48]

I would start with two poles and a 4 foot burial with 2' diameter footings for the size and height shown.

Verify the depth using 200#/ft2/ft doubled to 400 for the passive pressure.

The final result will not vary from this much.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Cap07]

Mike,

Are you looking at the original jpeg that I posted? I've revised that a bit - I now have the bottom edge of the sign 1 ft off the ground and I am using the loading from ASCE 7-05 (Fig. 6-20), wind speed 105 mph. Using ASCE I'm getting a max load on the post around 350 lb.

For the passive pressure, I was using the minimum in IBC Table 1804.2 which is 100 psf/ft, then doubling that to 200. I am using the minimum because the jurisdiction I'm in specifies to use the minimum table value unless you can justify higher numbers.

Thanks for your help.

Cap

 

[apsix]

If still using 3 posts I would use a load width of 8'/3, assuming that the sign structure is continuous over the 2 spens.
For a structure of these proportions I can't imagine that the centre post would fail and the end posts not.