1.  In Chapter 3 of the Technical Reference Manual (American Concrete Design), under Section 3.5, in the 4th paragraph, you state that a PDELTA ANALYSIS is the appropriate way to account for slenderness effects during column design.
2. Can I do a Pdelta analysis using STAAD for Response Spectrum load cases?
3. When the PDELTA ANALYSIS command is specified with the CONVERGENCE (m) parameter, the output file contains a message with a convergence error. What does this number represent ?
4. When doing P-Delta analysis for the design, what does the message "Load Case = 101 Convergence Error (SRSS) = 3.6603E-02 INCH" in the output file mean?
5. Are there any validation examples for the Direct Analysis method implemented in STAAD ?

1. In Chapter 3 of the Technical Reference Manual (American Concrete Design), under Section 3.5, in the 4th paragraph, you state that a PDELTA ANALYSIS is the appropriate way to account for slenderness effects during column design. You have the following additional statements in that paragraph :

 "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.

2. Can I do a Pdelta analysis using STAAD for Response Spectrum load cases?

 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.

3. When the PDELTA ANALYSIS command is specified with the CONVERGENCE (m) parameter, the output file contains a message such as:

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 then
that PDELTA load case is converged.
Mathematically this is the Euclidean norm of the vector of displacement changes.

|| {DISPi} - {DISPi-1} ||

SRSS=0.00
do iDOF=1,nj6
SRSS=SRSS+(dj(iDOF,ipdload)-olddj(iDOF,ipdload))**2
enddo
SRSS=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.

4. When doing P-Delta analysis for the design, what does the message "Load Case = 101 Convergence Error (SRSS) = 3.6603E-02 INCH" in the output file mean?

 "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.

5. Are there any validation examples for the Direct Analysis method implemented in STAAD ?

The AISC published two benchmark problems to ascertain suitability of a second order analysis method for use in Direct analysis. These are available in the Appendix 7 Commentary section 7.3 of the AISC 360-05 specifications. STAAD.Pro direct analysis has been validated against these two benchmark problems and the results are found to be satisfactory. A pdf document with details of the two benchmark problems along with the two STAAD models used for verification, can be accessed using the link below.

Verification Documents

See Also

Product TechNotes and FAQs

Structural Product TechNotes And FAQs

External Links

Bentley Technical Support KnowledgeBase

Bentley LEARN Server

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