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knsheth123
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PostPosted: Sun Feb 26, 2017 12:26 pm    Post subject: Reply with quote

Regret for a correction in the equation, kindly use the updated one.

K. N. Sheth



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Corrected-Seismic-Footing-Proportionin.pdf
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Correction for Mx/Z and Mz/Z

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BVRAO
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PostPosted: Sun Feb 26, 2017 6:29 pm    Post subject: Reply with quote

'

Last edited by BVRAO on Tue Feb 28, 2017 3:01 pm; edited 1 time in total
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sknsrinivasan
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PostPosted: Mon Feb 27, 2017 3:45 am    Post subject: Reply with quote

Dear K.N Sheth sir

  I have one doubt in your basis3. Basis for Proportioning:
𝐑𝐲 + (ϒ𝐛 𝐱 𝐃𝐟)∗𝟏.𝟏𝐋𝐟 𝐱 𝐁𝐟+𝟔 .𝑨𝒃𝒔(𝑴𝒙)𝑩𝒇 .𝑩𝒇.𝑳𝒇+𝟔 .𝑨𝒃𝒔(𝑴𝒚)𝑩𝒇 .𝑩𝒇.𝑳𝒇 ≤(𝒒𝒏𝒂 .𝑭𝒔+ϒ𝐛 𝐱 𝐃𝐟)

in this one Mz is major axis moment of column and My is minor axis moment of column. Lf is parallel to x axis Bf is parallel to Z axis is it is right the proportioning will be

   6xMz/(BfxLfxLf) + 6x my /(LfxBfxBf) I think it should be like this  I want to know which one is wright


Regards
N Srinivasan     




knsheth123 wrote:
Regret for a correction in the equation, kindly use the updated one.

K. N. Sheth
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kishorpss
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PostPosted: Mon Feb 27, 2017 4:59 am    Post subject: Reply with quote

Dear Er. K. N. Sheth Sir,
Thanks for your active participation. Still I have some doubts in addition to doubt raised by Er.N Srinivasan.

Please refer staad global axis for my question.

1. Are ABS(MX), ABS(MZ) to be considered or value of MX & MZ with their actual sign ?
2. Moment generated due to FX & FZ should be considered or not ?
Generated MZ=(FX* Df) & MX=(FZ*Df)
3. When FY with negetive sign(upward force). Same equation will hold good or equation to be modified ?
4. 0.9DL + 1.00EQ combinations are to be considered for foundation or not ?

I also request all respected senior Sefians for the active participation on this subject.


Thanking you.

Brgds,

KISHORE
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knsheth123
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PostPosted: Mon Feb 27, 2017 2:37 pm    Post subject: Reply with quote

Dear Er. Srinivasan,

The Reaction from Staad Analysis : Mx is Moment about X axis.
The Footing has Dimension Lf parallel to X axis, Bf perpendicular to X axis.
Hence  pressure = 6. Mx / (Bf.Bf.Lf).

Dear Er. Kishoreji,


1. Are ABS(MX), ABS(MZ) to be considered or value of MX & MZ with their actual sign ?


For given Ry, Mx and Mz values (+ve or -ve) we get pressure intensity at the four corners of the footing. Maximum Compressive Pressure should be be compared with SBC and Minimum pressure should remain compressive (i.e. greater than zero) irrespective of the qudarant location of the corner.

Hence Abs values of Mx, Mz are considered once additive for maximum compression case. Similarly for Minimum both values will be subtracted.

2. Moment generated due to FX & FZ should be considered or not ?
Generated MZ=(FX* Df) & MX=(FZ*Df)

For the Analysis of Super Structure the Depth of Fixity is considered placement Depth of Footing i.e bottom of footing. Thus Moment due to Horizontal Reactions are accounted for.

3. When FY with negetive sign(upward force). Same equation will hold good or equation to be modified ?

For -ve Fy, There will be resulting Tension in the footing at least at one corner. The footing shall be treated as in case of Transmission Line Tower Footings/Poles. Refer IS 4091.


4. 0.9DL + 1.00EQ combinations are to be considered for foundation or not ?

Tall Buildings with small plan dimensions shall be verified for Overall stability against Overturning. Overturning forces (EQ forces) 1.40 time and Restoring forces (DL) as 0.90 times. Thus there should be a FoS against overturning is 1.55 for the whole building.

Individual footings under overturning are not checked. I suppose, this has been discussed several times by Variyani Sir and others on SEFI.  

With Regadrs,

K. N. Sheth
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sknsrinivasan
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PostPosted: Tue Feb 28, 2017 3:54 am    Post subject: Reply with quote

Dear sir
   In Staad Mx is Torsion that means local X axis parallel to node to node. Local Z and Y axis major and minor axis moment Respectively  My opinion the footing major and minor axis should coincide with column major and minor axis so the upward soil pressure should calculate what i have mentioned yesterday in my post. This My opinion only Whether that is right or wrong

Regards
N.Srinivasan          



knsheth123 wrote:
Dear Er. Srinivasan,

The Reaction from Staad Analysis : Mx is Moment about X axis.
The Footing has Dimension Lf parallel to X axis, Bf perpendicular to X axis.
Hence  pressure = 6. Mx / (Bf.Bf.Lf).

Dear Er. Kishoreji,


1. Are ABS(MX), ABS(MZ) to be considered or value of MX & MZ with their actual sign ?


For given Ry, Mx and Mz values (+ve or -ve) we get pressure intensity at the four corners of the footing. Maximum Compressive Pressure should be be compared with SBC and Minimum pressure should remain compressive (i.e. greater than zero) irrespective of the qudarant location of the corner.

Hence Abs values of Mx, Mz are considered once additive for maximum compression case. Similarly for Minimum both values will be subtracted.

2. Moment generated due to FX & FZ should be considered or not ?
Generated MZ=(FX* Df) & MX=(FZ*Df)

For the Analysis of Super Structure the Depth of Fixity is considered placement Depth of Footing i.e bottom of footing. Thus Moment due to Horizontal Reactions are accounted for.

3. When FY with negetive sign(upward force). Same equation will hold good or equation to be modified ?

For -ve Fy, There will be resulting Tension in the footing at least at one corner. The footing shall be treated as in case of Transmission Line Tower Footings/Poles. Refer IS 4091.


4. 0.9DL + 1.00EQ combinations are to be considered for foundation or not ?

Tall Buildings with small plan dimensions shall be verified for Overall stability against Overturning. Overturning forces (EQ forces) 1.40 time and Restoring forces (DL) as 0.90 times. Thus there should be a FoS against overturning is 1.55 for the whole building.

Individual footings under overturning are not checked. I suppose, this has been discussed several times by Variyani Sir and others on SEFI.  

With Regadrs,

K. N. Sheth
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sakumar79
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PostPosted: Tue Feb 28, 2017 7:34 am    Post subject: Reply with quote

Dear Er Srinivasan,
     In STAAD, member forces are in local axes, but reactions are in global axes. MX is torsion when you are looking at member end forces, but in reactions, it is moment about global x axis. It can be either bending about major axis or about minor axis depending on the orientation of the column.

Yours sincerely
S Arunkumar
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