Structural Calculation of Post

[greencircle]

All,

I need help from you guys to recognize few steps from the below calculation for a lighting post. Unfortunately I have no drawing, but quite simple to visualize.

I am clear that the [highlight #729FCF]blue color[/highlight] highlighted are need to be updated to current values. The gust multiplier of 1.32 is outdated & believe I will have to use 1.14
The Wind pressure formula also should be modified I believe.

I don't understand the [highlight #F57900]orange color[/highlight] highlighted area. I would appreciate if any one of you help me identify these values/formula used.

[ol 1]
[li]In the formula for FP, I don't follow why its 4/12. I think this is got to do with the conversion of in to feet. But not sure what & why?[/li]
[li]Similarly in the FB calculation also. what's 12 + 6, where did these come from? I can see 12 is the height of the pole; but whats 6 for?[/li]
[li]Under "To Check Anchor Bolts"; whats the 36 for? I guess its KSI value of steel AB? But I am not sure. But I dont understand whats the 1.4 for?[/li]
[li]Then under calulation for T, whats the purpose of multiplying with 0.012[/li]
[li]To check Base Floor : What is M? I can see that M is calulated by ARM x T[/li]
[li]The next one is the calulation for S. It appears, it's a calculation of section modulus. then why /6?[/li]
[/ol]



Thanks,
GC

 

[andriver]

This is where the term garbage in garbage out comes from.

Somehow you want us to reverse engineer this problem without a diagram. You literally just put calculations down and asked where the stuff is coming from. All we can do is see the inputs you already have, you should have this information not us.

The division by 12 is clearly to convert ft to inches (although this is not done on his 2.33' which makes no sense). The 36 is steel yield strength like you assumed. M is a moment if it is a distance multiplied by a force. S is the section modulus of what appears to be a base plate - dividing by 6 is part of the formula for a rectangular section modulus bd^2/6 (although he is showing units of in^2 which is not correct)

This calculation packet is poor and this is a good example of why clearly displaying your units and having a diagram are important.

 

[greencircle]

Andriver,

I apologize. Here is a marked up sketch that gives the details.

I know that FP & FB are the Force calulation on Pole & Base. Generally I would think its AREA X WIND PRESSURE X Cd. I couldn't figure the where the 4' & 12'+6' came for inches conversion & why?

Thanks,
GC

 

[Archie264]

.012 converts ft-lbs to in-kips. (It's 12/1,000.)

I haven't chased it all out but I think there are some errors in there. For example:

S is the section modulus but if it's a rectangular section (as it appears to be with W = 2.5" and t = .75") then it should be S = bh[sup]2[/sup]/6, not bh[sup]3[/sup]/6. Also the units are wrong; they should be in[sup]3[/sup], not in[sup]2[/sup].

And is the base really only 2.5"? Did they mean 2.5'? The one seems too small and the other seems too big.

As noted the 36 is 36 ksi, the .5 looks like it's the 1/Ω=2.0 ASD reduction factor, but the 1.4 looks like an (if so, incorrect) LRFD load factor...and further "if so", the value should be divided by that, not multiplied by it. I'm not sure that's what they were trying to achieve.

In any case I think you'd be better served doing this by hand rather than trusting a spreadsheet that, from this vantage point, seems to have errors.

 

[greencircle]

Thanks a lot for explaining the in-kips conversion Archie..
Sounds like the base section modulus calculation is incorrect then. There for, I could quickly create a drawing of the base section near anchor bolts...
Hope this helps ...

Thanks,
GC

 

[Archie264]

Whoops, our posts crossed in the mail.

I think the 4/12 refers to a 4" diameter pole, converted to feet. It's the wind acting on the projected area.

If that's indeed the case then the (12+6)/12 is probably an incorrect attempt to do the same thing with the average diameter of the pole base. If so then it's another error; it should be [(12+6)/2]/12.

In short, the spreadsheet has problems; don't trust it. It's not you; it's him. It's probably a good idea to inform whoever wrote it of these issues. With ASTM/SEI 7 and a steel book it could be corrected but I wouldn't try; I'd just reject and ask for a resubmission.

 

[Archie264]

I'm guessing the 1.02 is left over from a prior project. In any case, 1.02/.18 ≠ 6.14, even accounting for rounding errors. If he left info in there from a prior project that would explain why his base plate dimensions don't resemble what you've shown there, unless he's trying to take some specific portion of it but I think that's unlikely.

Don't trust this spreadsheet; it needs a lot of work.

 

[greencircle]

Thanks a lot Archie. This is a very old calc on file (1996) that I am trying to breakdown. (Appears it's already broken LOL)

From FP and FB calculation its is clear, that he is doing

Projected Area X wind pressure X Drag

Particularly in FB, like you, I can now see he is trying to take average but forgot to /2. However do you think if I use a precise area from CAD would be OK?

On the anchor bolt calculation I found a formula from another project. From this it's clear that 1.14 is from (2)[sup]0.5[/sup].

On Base Floor Thickness check his formula is still very unclear. I really need some more help with this please. I don't want to use his values because they might also be a leftover from his previous project. How would you check the base floor thickness? Is there a generic formula for it? The bottom of the base is round like shown on the CAD screen shot above.

Also, I see he is missing the calculation of CSR. I think this kind calculation MUST include it.


Thanks,
GC