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Civil Engineering/Subgrade reaction in mat foundation

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Question
I want to model a mat foundation in etabs. For this i assign modulus of subgrade reaction as a area spring constant at direction 3 and restrain the Trans x,Trans y and Rot z direction. But  percent force and moment error at origin, in global direction is very high. Is this ok ?  Please help me.

PERCENT FORCE AND MOMENT ERROR AT THE ORIGIN, IN GLOBAL COORDINATES

   LOAD          FX          FY          FZ          MX          MY          MZ
DEAD        0.737756    0.483366    2.00E-10    1.21E-10    5.80E-10    0.000108
LIVE        0.495937    0.226577    5.92E-10    3.22E-10    2.82E-10    0.000236
WX          0.885080    3.870909    1.10E-08    5.39E-09    4.46E-09    0.005341
WY          4.122153    1.360607    1.05E-09    5.99E-10    1.10E-09    0.019386
EX          1.655842    3.343302    1.10E-08    5.16E-09    4.52E-09    0.015163
EY          4.518052    1.536668    1.05E-09    3.34E-10    1.16E-09    0.011590
TRIBUTAR    0.461645    0.224859    5.27E-10    2.90E-10    1.63E-10    0.000154

Answer
Hi Ahetesham,

It is difficult to say without interrogating the model, but I would suggest you try the following changes, one at a time, and see how the output is affected:

1. Remove the restraint for rotaion about the Z-axis.

2. Move the origin to a point near the centre of the matt foundation.

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Arshad Khan

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I can answer any questions to do with civil and structural engineering consultancy and construction industry in East Africa and the Middle East, and specifically with the analysis and design of reinforced concrete structures. My particular expertise is in the aseismic design and optimisation of tall buildings.

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.Fellow of Institution of Structural Engineers (UK) .Fellow of Institution of Civil Engineers(UK) .Member of the Institution of Engineers, Kenya .Registered Engineer, ERB, Kenya .Member of the Architectural Association of Kenya (Engineers Chapter) .Chartered Engineer (UK)

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•1984: International Conference on the Art and Practice of Structural Design, London •1994: 3rd Int. Kerensky Conference in Structural Engineering, Singapore •2008: International Conference on High-Rise Towers, Abu Dhabi •2013: IEK International Conference, Kisumu, Kenya

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BSc, 1st Class Hons, in Building Engineering, University of Bath, UK MSc in Concrete Structures and Technology, University of London. Diploma of Imperial College, UK.

Awards and Honors
•Science Congress Special Award (for 2-seater Hovercraft - 1968) •Institution of Civil Engineers Award for outstanding performance at Bath University (1975) •Concrete Society Postgraduate study Bursary Award (1976) •Consular Representative for British High Commission, Nairobi. (1995 to 1998) •Examiner for Institution of Civil Engineers Professional Interviews, Nairobi. (1997 to 1998) •Branch Representative in Vienna for PI assessment for Inst. of Struct. Engrs. (1999 to 2010)

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