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"To take advantage of this analysis, all combinations of loadings must be provided as primary load cases and not as load combinations. This is due to the fact that load combinations are just algebraic combinations of forces and moments, whereas a primary load case is revised during the pdelta analysis based on the deflections."
Can you explain the above statements in further detail?
These statements are provided to emphasize the fact that if you wish to carry out a PDELTA analysis for a load case where 2 or more previously defined load cases are combined, as for example, one case involving DEAD load and another involving WIND load, the right way to combine them is through the means of a REPEAT LOAD command, and not a LOAD COMBINATION command. See Member Tension And Combination Load Cases for more details.
A response spectrum analysis by definition provides the user with joint displacements which are the absolute maximum displacements which can occur at the joint when the structure is subjected to dynamic loads represented by the spectrum. This also means that these displacements do not necessarily occur at all joints at the same instant of time. In other words, the absolute maximum at joint 1 and that at joint 2 are most probably occurring at different instances of time. Another fact to be considered is that the response of the individual modes is combined using either the SRSS or the CQC methods depending on the input provided by the user. Both of these are approximate methods. The result is that the joint displacements and consequently, the member forces cannot be used as a basis for obtaining the secondary forces on the structure. Hence, doing a P-Delta Analysis under such conditions does not make any sense.
Load Case = 1 Convergence Error (SRSS) = 2.790E-02 INCH
What does this number represent?
The SRSS is the square root of the sum of the squares of the change in displacement (from one iteration to the next) of every dof. If after an iteration, the SRSS falls below the SET DISP tol or default tolerance thenthat PDELTA load case is converged.Mathematically this is the Euclidean norm of the vector of displacement changes.
|| {DISPi} - {DISPi-1} ||
SRSS=0.00do iDOF=1,nj6SRSS=SRSS+(dj(iDOF,ipdload)-olddj(iDOF,ipdload))**2enddoSRSS=sqrt(SRSS)
The SRSS printed is for the last iteration performed for that load case and tends to be an upper bound on the amount of error in any one displacement.
"Load Case = 101 Convergence Error (SRSS) = 3.6603E-02 INCH" is NOT an error message. It is a measure of the convergence tolerance. If are getting a lot of these messages and you don't want to see so many of them you can set your tolerance to a lower value, say, 1/4 ". This can be done by adding "SET DISP 0.25" into the std file, provided the units has been set to inches.
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