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Analysis Inquiry

Analysis Inquiry

Analysis Inquiry

(OP)
Hi everybody,
I have an inquiry and I hope that all you guys help me out with an accurate solution
First, the problem started when I decided to analyze a (7m*7m) concrete foundation for a three legged, 50 m height self-supporting telecommunication tower (a hand drawing is attached in the post).

According to the foundation designer, the critical section for moment is calculated underneath
(Column c) when the wind is blowing in +y-direction, the maximum bending moment moment is calculated as following:
Maximum soil pressure = 100kpa
Mx = 100*0.5= 50 kn.m /m

Therefore, the reinforcement provided is equal to the minimum reinforcement of (0.0018) in both directions, corresponding to footing depth of (55 cm).

However, my analysis using sap2000 shows a similar value for (Mx) value, but the value of (My) underneath (column c) for the same loading condition is (180 kn.m/m). Accordingly, the reinforcement provided for this foundation is not enough to resist this 3X bending moment.
The required reinforcement to resist (My) is 0.0028 according to sap2000's analysis , In contrast the provided reinforcement is 0.0018 only.

Indeed, the results from sap2000 seem to be reasonable, because (My) has a larger arm than (Mx). But the designer of this project is working for an internationally recognized company, and this design was implemented many times for previously constructed towers around the globe.

So,is it correct to neglect the secondary moment (My) under column c as the designer did, especially if it considered a rigid foundation due to its large thickness?

RE: Analysis Inquiry

it seems odd to me that the maximum bending moment from the wind is dependent on the soil conditions ?

i'd've thought that you'd first calc the wind load from the exposed area and a wind loading obtaining the moment that the wind applies. then you'd consider the maximum moment that the structure can react, based on the soil conditions.

another day in paradise, or is paradise one day closer ?

RE: Analysis Inquiry

It certainly makes sense to me that you'd see a large moment about the y-axis in the foundation. You've got soil reaction across the full 7 meter width. Your column is square in the centre. So if you treat that column as your support, the foundation cantilevers out to either side. Those loads need to be transfered to the column location if you're going to use them. That has to happen in bending.

RE: Analysis Inquiry

the back calculation assumes the designer is using the full 100kpa across large sections of the footing, what is he isn't?, what if it is a distributed situation, with only some small areas of 100kpa.

http://www.nceng.com.au/
"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

RE: Analysis Inquiry

Based on the information provided OP, I wonder if the original designer didn't consider the reaction at column C to be resisted by a fictional, concentric footing within the larger mat. Perhaps a 2m x ??m concentric footing. Three things suggest to me that this might be the case:

1) The use of the allowable soil pressure rather than the applied load.

2) Were the entire mat relied on as a rigid foundation, I would have expected the maximum Mx demand to be negative (top steel) and occur in the interior of the footing.

3) It's reasonable. Given flexibilities in the soil and footing, assuming uniform pressure across the entire 7m width is not.

If I'd designed this foundation in such a simplified manner, I would have probably provided some nominal top steel for crack control.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Analysis Inquiry

(OP)
@ rb1959
I used the same overturning moment calculated by the designer to find the maximum bending moment in the concrete foundation .The calculated moment seems to be reasonable ( 2400kn.m ) and no need to recheck it.

RE: Analysis Inquiry

(OP)
@ rowingineer

The designer treated the critical section for the moment as cantilever loaded by the wind's soil pressure under that section.In the hand calculations, the designer took the average between the soil pressure in the edge of the foundation ( 100kpa) with the pressure under column c.But it doesn't make a significant difference than using 100kpa only to cal. Mmax.

RE: Analysis Inquiry

that would be true for Mx but not for My.

http://www.nceng.com.au/
"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

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