PLATE GIRDER PROPERTIES

Question

Hi all, 
 I am working on assessment of a steel railway bridge. The bridge is made of four plate girders interconnected with cross beams (standard sections). I have decided to use grillage method of analysis. I have following questuions regarding the properties of the elements: 
 
 1. Main girders being plate girders, instead of conventional method of calculating properties of plate girder manually and entering it in user table, I decided to create a user table with "wide flange" section type where for additional top plate i used option of "additional composite flange with modular ratio of 1. and for additional bottom plate there was already option in this selection. I have attached snapshot of it for clarity . - Am i doing it right ? 
 
 2. cross girders are only connected with steel deck plate, hence Instead of manual property calculations I used wide flange section type in Usertable options. I gave composite additional flange with modular ratio of 1. - Am i doing it right? 
 
 both questions are similar upto some extent. i would like to know your expert opinions before I proceed. 
 
 Thanks

Sye · Answer

The composite option is only meant for defining sections with steel beam and concrete flanges. I would suggest adding these sections to existing wide flange section database in STAAD.Pro and then use the options to add steel cover plates as needed. Please refer to the picture below 
 
 A video on how to add a new section to the existing section database can be downloaded from 
 http://communities.bentley.com/products/structural/structural_analysis___design/m/structural_analysis_and_design_gallery/258994

Sye · Answer

Design of these sections are supported for many codes like the AISC 360-05 and AISC 360-10. If for any particular code, design is not supported, appropriate messages will be provided in the analysis output file.

Sye · Answer

For carrying out a buckling analysis for the load case you have defined, you need to replace the PERFORM ANALYSIS command by the buckling analysis command PERFORM BUCKLING ANALYSIS MAXSTEP 15. 
 Also noted that in your user table definitions you defined a number of cross sections with negative values for cross sectional areas and some other properties and these are leading to the error messages during analysis. For example here is a sample section specification in your file 
 &hellip; 
 D-MIDDLE 
 -0.0638708 1.6002 0.01905 0.4572 0.0381 0.0268763 0.000607743 2.03693e-005 - 
 0.0304838 0.0232258 0.4572 0.0381 
 -0.2032 0.2032 0.0381 1 
 0.2032 0.2032 - 
 0.0381 
 &hellip; 
 The negative sign used at the beginning is making the cross sectional area negative which is not correct. Similarly you do not need a negative sign at the beginning of the fourth line above. You should rather define the above section properties as shown next 
 D-MIDDLE 
 0.0638708 1.6002 0.01905 0.4572 0.0381 0.0268763 0.000607743 2.03693e-005 - 
 0.0304838 0.0232258 0.4572 0.0381 0.2032 0.2032 0.0381 1 0.2032 0.2032 0.0381 
 You need to make similar changes for the other sections as well.

Sye · Answer

On troubleshooting the model further, we found the reason behind the zero buckling factors. If you carry out a PERFORM ANALYSIS with the loads you have applied, you will see that there are no axial forces developed in any of the members in your model and hence there is no buckling. To get the buckling factors, you may introduce a couple of small nodal loads in the plane of the grillage ( say add nodal loads of 1KN each at joints 105 and 106 ) 
 
LOAD 1 LOADTYPE None TITLE SELF WEIGHT 
SELFWEIGHT Y -1 
JOINT LOAD 
105 106 FX 1 
 
The buckling analysis should now produce buckling factors and since you are using advanced solver, you would also be able to see the buckling modes.

Sye · Answer

Post buckling behavior cannot be simulated using the buckling analysis in STAAD.Pro. Nonlinear Analysis would definitely be a better option to use if you are trying to account for the changes in stiffness due to deformed geometry and stress stiffening effects. However please note that STAAD.Pro Nonlinear analysis accounts for the geometric nonlinear effects ( large displacements) but does not consider material nonlinearity and it cannot account for post buckling stiffness of members.