# Virtual Sections for Skewed Plates

Case 1 Simple Beam and Plate comparison
Parameters
Simply Supported, 10m length and 10kN force added at the mid of the Beam

Beam - Offset Center-Center

So, Moment should be Mmax = 100*10/4 = 250 kNm

Plate - Supports placed at the center

Created Groups of each part of Beam one by one

Then we add all the groups in Properties > Section for Resultant Forces:

Now, when we check the Results for Resultant Forces, we get same results:

Case 2: Composite Section Comparison

Beam:

Area of I section : 0.1189962 m^2, Density of Steel : 77.09 kN/m^3, so Mmax due to self weight of Beam : 46.9767 kNm
Area of Slab : .5 m^2, Density of Concrete : 23.56 kN/m^3, so Mmax due to self weight of Slab : 147.25

Total Mmax : 194.2267

So, we get the results as :

for Plate +Plate model :

The supports were shifted to the centroid of the section determined from the beam model.

Again we select groups one by one...

If we check the results it comes nearly the same:
(actually its coming a little more because at the flanges and web connections the thickness is considered 2 times as they intersect the same line)

Then if we change the node positions of the plate in such a way that it become skew...
(the Group information does not change as we are not deleting any nodes and neither does the Resultant Section Infromation)
We get results which are correct.

But if we remove the entries from Resultant Sections and reenter it, it doesn't show the results correctly:

As it is now taking i and j nodes differently:

So, when you see the results

So, you cannot use the Group method to enter Virtual Section infromation for skewed bridge. So, one method is you create virtual sections for non skew slab and then move the nodes so as to keep the correct information of i and j.

Another way to do that will be : We can use the Polygon Select method for slab instead. I created a video. Please follow each step carefully. I only isolated skewed slab now. You can follow the same procedure to select the slab elements separately after you do the beam elements:

Now in our case we are having slab and beam connected by rigid links. So, what we can do is follow the Structure Group method for beams and Polygon method for slabs.

And we need to give the position where we want the results to be calculated. For our case I gave .816 from bottom of the girder ( the centroid position found from the equivalent composite section ) . So , in the virtual sections tab , I replaced all the z coordinates by .816.

Now when we check the results :

So, Mmax = 106.552+89.758 = 195.735 (exactly the same as before).

You can also add the results in the Tables if you wish to.
Please go thorough all the materials. Hope the reply is clear enough.

Thanks and Regards,
Bodhi Rudra
Creation date: 9/4/2016 10:23 AM      Updated: 8/8/2018 9:01 AM