Calculation of Negative Moment
Calculation of Negative Moment
(OP)
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.
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.






RE: Calculation of Negative Moment
((.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.
RE: Calculation of Negative Moment
More accurate model - more accurate results.
Mike McCann
MMC Engineering
RE: Calculation of Negative Moment
RE: Calculation of Negative Moment
Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin
RE: Calculation of Negative Moment
RE: Calculation of Negative Moment
Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin
RE: Calculation of Negative Moment
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.
RE: Calculation of Negative Moment
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
RE: Calculation of Negative Moment
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.
RE: Calculation of Negative Moment
RE: Calculation of Negative Moment
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.
RE: Calculation of Negative Moment
BA
RE: Calculation of Negative Moment
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
RE: Calculation of Negative Moment
RE: Calculation of Negative Moment
RE: Calculation of Negative Moment
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
RE: Calculation of Negative Moment