Calculation of Negative Moment 5

[sb79]

Can someone help me in calculating the negative moment (or moment at support)

I am designing a pedestal with a circular slab of diameter 50 feet supported on a circular wall of 8" thick and 38 feet outer diameter. The thickness of the slab is 27"

Loads:
Self Wt of Slab = (150 lb/ft^3)* thickness (2.25') = 0.34 kips/ft^2
Wt of Water = 438000 Gal = (438000*8.33)/(pi/4*48^2)=2.02 kips/ft^2 ------------- (8.33 is the conversion of Gal to lb and 48 is the diameter of the steel tank). This load as the area load over 48' diameter

Wt of Steel Tank = 40750 lb = 40750/(pi*d) = 40750/(pi*48) = 0.27 kips/ft
Wt. of Snow load = 30 psf (Act as a line load along the circumference of the tank) = (30*pi*48^2)/(pi*48) = 0.360 kips/ft

Note: For simplicity snow load is taken as dead load

Load Combination = 1.4*(Dead load+Water Load)

Hope this will clearly explain my whole problem

I am trying to design this in Risa and my negative moment (or moment at support) is 65k-ft.

 

[hokie66]

65 k-ft looks right to me, based on your numbers.

((.34 x 6 x 3) + (2.02 x 6 x 3) + (.27 x 6) + (.36 x 6)) x 1.4 = 64.8

Is that all you are asking? Just a simple summation of the moments on the cantilever. I suppose you might get a little more picky about the pie shaped area contributing, and maybe the 6' cantilever dimension should be modified a bit for the different loadings and not taking the load right at the edge of the wall, but that won't make a big difference.

 

[msquared48]

Actually, since this is a radial geometry, and the area to resist the exterior forces decreases moving in from the outer radius, I think you need to multiply the 65K' by 1.31 to get an upper bound for this of around 85 K'. Probably something less due to the trapezoidal rather than rectangular loading, but 85k' should be the max.

More accurate model - more accurate results.

Mike McCann
MMC Engineering

 

[hokie66]

Mike is correct that radial geometry should be considered, but his number is overly conservative because the tank does not extend to the edge of the slab. I did it again, and got 57.4 k-ft. So the 65 k-ft is a conservative estimate.

 

[paddingtongreen]

Strange, my first take was that sb79 was asking about the moment at the centre support.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[hokie66]

The way I read it, there is no centre support, just the perimeter wall, which is 38' in diameter.

 

[paddingtongreen]

You are right, the word "pedestal" threw me.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[sb79]

Hokie66: Ya you are right, the tank does not extend to the edge of the slab. So I got 1.4*(0.34*6*3 + 2.02*5*2.5 + .27*5 + 0.36*5)=48.3

And then should I multiply 1.31 to get an upper bound as msquared48 described?

That will make it a total of 62.8 k-ft.

 

[BAretired]

Why extend the slab one foot beyond the steel tank wall? That seems to be a waste of concrete.

Why maintain the same 27" slab thickness for the 6' cantilever unless it is required for shear? That seems to be a waste of concrete.

The factor 1.31 is the ratio 50/38 and does not apply to all components of load, so it is not appropriate to multiply by 1.31 except perhaps to obtain a very rough upper bound.



BA

 

[dhengr]

Does anyone wonder why we are doing basic moment calcs. for someone who is pretending to be an engineer, on a fairly complex design problem, but doesn’t know how to do those basic calcs. himself. He has a RISA, whatever that is; and however to use it, and however he might enter data into it.... that could be a real mystery. He might contact the people who sold him the ‘little black box’ called Risa and ask them how to calc. the negative moment, and if they will be the EOR on this project, for him. They are the only ones who have made any money so far in making him an engineer, and they offer damn few guarantees. This is pretty scary, and not many of us should want to be involved in giving the guy little bits of info. so that he can continue pretending he knows what he is doing. The questions he has asked so far don’t show much engineering education, experience or good judgement, on his part, and without some of this he could get in big trouble, and be downright dangerous.

sb79.... you really should go ask your boss to help you with this design problem. He/she should know what you know and what you don’t know about what you are trying to do, so they can help keep you out of trouble. They can look at the same plans you are, do sketches for you, and explain the other things to look out for, etc. If you are being forced to do this work without some more experienced supervision, you really should demur.

E-Tips seems to have pretty much degenerated into a bunch of people pretending to be engineers, who have almost no basic engineering knowledge or experience, asking questions about structural design problems which they have no business working on, given their lack of a fundamental understanding of the problem. Give them a computer program and there is almost nothing they won’t tackle, just don’t ask them what they are doing and how it works, never mind that kind of nonsense, you know, fundamental understanding of the problem. The ‘little black box’ can do all.

 

[CTW]

Standing ovation for that post dhengr!

 

[hokie66]

dhengr,
You don't need to reply if the problem is too simple for you. It was interesting to me at the time, so I gave it a shot. Sometimes, doing the numbers is the only way to get to the bottom of an issue.

sb79,
You still don't get it. Mike's 1.31 applies only to the concrete load. For the other loads, the number is 1.26. But you should not be using either one as a final fudge factor. Do the geometry, including the triangular part of the loading, and you should get the number I gave.

A more interesting and complicated issue with your slab is the positive bending moment to be used in the design.

 

[BAretired]

hokie, I am not sure the problem is as simple as this thread implies, although I don't suppose a theoretically correct solution will make a great deal of difference to the negative moment. Furthermore, I think it errs on the safe side which is never a bad thing, but it seems to me that the circumferential stresses have a positive effect on the negative radial bending moments. If I have to prove it, it may take a little time.



BA

 

[msquared48]

I agree that I was conservative, an upper bound, a check on reality, but that's my middle name.

A more sophisticated analysis is obviously in order here using RISA 3D or similar. The results would be interesting to say the least.

Mike McCann
MMC Engineering

 

[haynewp]

I get 48.3'k at the support based on your load position descriptions.

 

[sb79]

Thank you all for your comments. As Mike suggested I used risa foundation and design my slab as foundation for a quick check

 

[hokie66]

Some inconsistency in the numbers.

For what it is worth, my arithmetic follows:

Slab moment (.34*6*3 + .34*6*.31*4/2) * 1.4 = 10.34 k-ft

Water (2.02*5*2.5 + 2.02*5*.26*3.33/2) * 1.4 = 41.47 k-ft

Tank .27*5*1.26 * 1.4 = 2.38 k-ft

Snow .36*5*1.26 * 1.4 = 3.18 k-ft
__________

Total 57.4 k-ft

 

[haynewp]

I didn't read all the other posts before I posted. The difference is likely those radial geometry factors I missed.