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Civil Engineering/i have problems in etabs about unstability

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Expert: - 10/31/2010


Question
Dear Sir
Am working on a tower of 32 floor with 5 basment , i do the auto mesh method and after the run i found that my building is unstable and found lot of warning wich are :* * * W A R N I N G * * *
THE SOLUTION LOST  9.7 DIGITS OF ACCURACY FOR DOF U1 OF CONSTRAINT      42  
LOCATED AT X =    0.480448, Y =   -9.093111, Z =    3.000000,
STIFFNESS MATRIX DIAGONAL VALUE =  1.3172E+17


* * * W A R N I N G * * *
THE SOLUTION LOST  8.0 DIGITS OF ACCURACY FOR DOF U2 OF CONSTRAINT      42  
LOCATED AT X =    0.480448, Y =   -9.093111, Z =    3.000000,
STIFFNESS MATRIX DIAGONAL VALUE =  3.3176E+15


* * * W A R N I N G * * *
THE SOLUTION LOST  8.8 DIGITS OF ACCURACY FOR DOF R3 OF CONSTRAINT      42  
LOCATED AT X =    0.480448, Y =   -9.093111, Z =    3.000000,
STIFFNESS MATRIX DIAGONAL VALUE =  1.7971E+19


* * * W A R N I N G * * *
EXCESSIVE LOSS OF ACCURACY WAS DETECTED DURING THE SOLUTION OF EQUATIONS
NUMBER OF DIGITS LOST EXCEEDED 11.0


* * * W A R N I N G * * *
THE STRUCTURE IS UNSTABLE OR ILL-CONDITIONED !!
CHECK THE STRUCTURE CAREFULLY FOR:
- INADEQUATE SUPPORT CONDITIONS, OR
- ONE OR MORE INTERNAL MECHANISMS, OR
- ZERO OR NEGATIVE STIFFNESS PROPERTIES, OR
- EXTREMELY LARGE STIFFNESS PROPERTIES, OR
- BUCKLING UNDER P-DELTA LOADS (IF ANY), OR
- AN EIGEN SHIFT (IF ANY) ONTO A NATURAL FREQUENCY  

Answer
Hi Hussein,

Without seeing the model it is difficult to advise. One possible problem may be due to autoline contraints invoked when running pdelta analysis. Choose either pdelta or auto line constraints, but not both at the same time, and see if that stops the instability.

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Arshad Khan

Expertise

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.

Experience

Employment history: 36 years in Construction and consultancy in the UK, Kenya, Tanzania, Uganda, Ethiopia, Eritrea, South Africa, Somalia, Zambia, Austria, Bahrain, Kuwait, Doha and the U.A.E.

Organizations
.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)

Publications
•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

Education/Credentials
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 date)

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