3D plate modelling STAAD Pro 1

[shashank_b]

Dear all,

i am trying to model 3D plate model with the application of compression springs both at base and side walls and have some doubts regarding that:

1) Initially i was trying to apply "load combination" command to add the respective loads but after going through staad literature i found that "repeat loads" were to be used in this case. Upon the application of repeat loads i noticed close to 50% decrease in the bending moment . Isn't this decrease too large or am i making some mistake? The command i have used is as follows:
LOAD 102 DL+EXTERNAL LOAD CASE 102
REPEAT LOAD
1 1.0 2 1.0
LOAD 103 DL+INTERNAL WATER PRESSURE LOAD CASE 103
REPEAT LOAD
1 1.0 3 1.0
LOAD 123 DL+EXTERNAL+INTERNAL LOAD CASE 123
REPEAT LOAD
1 1.0 2 1.0 3 1.0
PERFORM ANALYSIS PRINT STATICS CHECK

2) In order to reduce bending moments, i applied joints using the release option in plate specifications for the whole structure. But to my dismay, i see no perceptible decrease in the maximum bending moment (rather an increase in bending moments?? why is this happening?

3) Also regarding application of release, i am confused about direction in which they are to be applied. File links show 2 directions (option 1 and 2) in which releases can be applied, which one of them is correct? i need Moment Mx to be zero for the purpose of design (To me option 1 seems right, but Mx is coming close to zero with option 2)
Option 1: file's link is:

https://files.engineering.com/getfi...33-94af-26dd5b8ecaf7&file=point3_option_1.png

Option 2:

https://files.engineering.com/getfi...8b-a566-84b4b22cff4a&file=point3_option_2.png

4) Also, i would need some advice on the support conditions to be employed. Should i use fix boundary conditions at corners of base (with intermediate nodes being attached to spring) or should i go for Pinned/fixed boundary condition on top of side wall as shown in attachment
File link: Your file's link is:

https://files.engineering.com/getfi...41de-9b05-41620043e9bd&file=support_doubt.png

 

[Sye123]

Your questions have been answered on the Bentley Communities forum. For the records I have reproduced the responses below

1. Using REPEAT LOAD is the correct approach. You need not compare the results with the LOAD COMBINATIONs as the later would not be correct anyway for a compression only analysis.

2. Have to look at the model to answer this one

3. Couple of things to note here. Plate releases are specified with respect to local axes of plates. You may refer to section 5.1 Plate and Shell Element for details on how the local axes are defined for plates and how the Mx and My are oriented with respect to the local axes.

4. I assume you have used PLATE MAT type support which is the correct way to define soil support. You may apply a PINNED support at the top of the wall, if the wall is completely retrained against translation in all 3 directions

 

[Sye123]

Couple of additions with respect to the point 3.

If you are trying to release the MX moment ( for plates MX is moment ABOUT the local Y axis) then option 1 would be the way to go. However I see that you have applied the release to 2 of 4 plates that are meeting at every node. You may want to apply the MX release at the other 2 plates too.

Having said that I would like to add that MX at joints would not be perfect zeros though as there is some amount of approximation involved in deriving the corner stresses and also stress continuity across elements in not guaranteed in a finite element analysis.

 

[Shu Jiang PEng SE]

Question 2. Staad does not consider membrane effect for the plate element. You cannot release the bending feature of the plate.
Question 4. How to apply boundary condition when soil spring is involved. You have consider the actual structural behaviour under corresponding loads.