Modelling of slab supporting on piles

RE: Modelling of slab supporting on piles

Ze Yang Lee — Fri, 17 Jul 2020 01:42:26 GMT

Dear Sye,

Really appreciate your reply sir. I have included dummy elements (high stiffness) now at the support location (one side only to allow us to see if the dummy beams help).

As you said, there is no much of a difference there. When I run this in SAFE, I do not see such a high shear at the edge of the slab like what is showing here. I would expect the stress will be high at the support position but not along the edges. The out of plane shear stresses is still 1.27MPa ish , with 600mm thickness, it is about 770kN ( a lot larger than all other points at the supports), and this makes my slab really thick >1m thick. Not sure I am interpreting the stress correctly?

I have tried different mesh size (refine it further but does not help).

Appreciate if you can advice sir.

communities.bentley.com/.../0081.Launch-chamber.STD

RE: Modelling of slab supporting on piles

Sye — Thu, 16 Jul 2020 20:48:41 GMT

The stress value I see from your plot are 1284 kPA which is only 1.284 N/mm2. When you say high stresses, what stresses are you expecting ? On a side note, the DUMMY material you defined has a very low E value ( should be 1000 times that of the material you used for the other entities). However it may not make much of a difference in your case as I do not see high stress anyway.

RE: Modelling of slab supporting on piles

Ze Yang Lee — Thu, 16 Jul 2020 15:53:04 GMT

Dear sir,

I have attached my model with the dummy members. From the results, you can see there is no much difference. I am not sure how should I resolve the edge high stresses.

communities.bentley.com/.../1581.Launch-chamber.STD

RE: Modelling of slab supporting on piles

Ze Yang Lee — Thu, 16 Jul 2020 10:31:58 GMT

communities.bentley.com/.../1106.Launch-chamber.STD

Hi sir, the model attached here is without the dummy members in it, I have removed them and did not save the model when I realize the edge (not at the support location) still has high stresses. However, the way I have added the dummy members is by creating it a the support, forming a T shape around the support. The support stresses are fine, but at the edges shown in the photos in the previous post is still there (not shown in SAFE model.)

I still have my dummy materials created in the model and appreciate if you can provide some advice.

I have been cracking head on this.

RE: Modelling of slab supporting on piles

Modestas — Thu, 16 Jul 2020 10:08:48 GMT

Please attach the model where dummy members were added.

RE: Modelling of slab supporting on piles

Ze Yang Lee — Thu, 16 Jul 2020 09:36:47 GMT

Dear sir,

I have tried to model similar structure in SAFE, but did not witness such a high stress on the edge of the plate. I am not sure how to resolve this issue in STAAD still.

RE: Modelling of slab supporting on piles

Ze Yang Lee — Tue, 14 Jul 2020 13:51:52 GMT

Dear sir,

Thanks for your suggestion. I have tried to included the dummy members as follows. However, I still facing issue at the left edge of the slab. I am still not sure how to resolve this issue and suspect whether this really manifest in the slab. For your information, this is a slab to resist uplift force and the piles around are actually tension piles.

Do you think we can include the dummy members at the left and right edges too for better load distribution? Really thanks a lot for your reply sir.

RE: Modelling of slab supporting on piles

Modestas — Tue, 14 Jul 2020 08:29:21 GMT

Think about the support nodes as a concentrated load on the mat. You do not need to create dummy members around the perimeter. Instead, create a dummy member between the support node and some nodes around that support node.

RE: Modelling of slab supporting on piles

Ze Yang Lee — Tue, 14 Jul 2020 08:25:33 GMT

Dear sir,

Yes, I have read about that previously. However, my load is now applied as a plate load instead of a concentrated point load. Do I need to create dummy beams around the perimeter of the slab as my supports are mainly at the 2 sides of the perimeter to help on this situation in this case?

Best regards,

Ze Yang Lee

RE: Modelling of slab supporting on piles

Modestas — Tue, 14 Jul 2020 08:04:25 GMT

Check a 2nd point in this Wiki: https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/29385/avoiding-stress-concentrations

As it suggests, you may connect the support node with adjacent nodes by using the dummy members, which would have a material with high E value and low density. This would spread the stresses and reduce it.