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






RE: Need help with design for a sign (required embedment depth)
RE: Need help with design for a sign (required embedment depth)
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.
RE: Need help with design for a sign (required embedment depth)
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!
RE: Need help with design for a sign (required embedment depth)
RE: Need help with design for a sign (required embedment depth)
thread507-254169: Sign Foundation
RE: Need help with design for a sign (required embedment depth)
BA
RE: Need help with design for a sign (required embedment depth)
RE: Need help with design for a sign (required embedment depth)
RE: Need help with design for a sign (required embedment depth)
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
RE: Need help with design for a sign (required embedment depth)
RE: Need help with design for a sign (required embedment depth)
RE: Need help with design for a sign (required embedment depth)
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.
RE: Need help with design for a sign (required embedment depth)
BA
RE: Need help with design for a sign (required embedment depth)
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
RE: Need help with design for a sign (required embedment depth)
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
RE: Need help with design for a sign (required embedment depth)
For a structure of these proportions I can't imagine that the centre post would fail and the end posts not.