Dear sir,

I made two models
1-the first model is:  a simple beam  its properties (depth =0.70 m,width =0.3m ,length =6.00m) ,with SBETA material ,with vertical uniform line load =38.25 KN/m with 4 nodes quadrilateral element by using ATENA ENGINEERING 2D

2-the second model is: the same as the the first model but using 8 nodes quadrilateral element BY USING ATENA SCIENCE

in the first model the failure occurred at concrete compression stress= -21.65 N/mm2 and
in the second model the failure occurred at concrete compression stress= -4.24 N/mm2

why the element type affected on results ?

Dear samuelhanna93, 1. from your message, I still doubt if the BCs are set correctly (see also Troubleshooting, 2.1.12 Strange deformations in a corner or on an edge). If you still have the unexpected mesh dependence, please send your models (Troubleshooting, 2.1.1).

2. As already explained, this option allows to use linear elements for selected materials when quadratic are selected in GiD (and used for all the other materials). As GiD the only has the global linear/quadratic option, the quadratic elements generated by GiD are replaced with linear ones for the selected materials at the time of writing the ATENA Input FIle (.inp). Therefore, you can only see quadratic elements in GiD.

Dear sir,

1- Refining the linear mesh didn't get a similar response as switching to quadratic,but when i adjust the boundary conditions( add horizontal constraint at the middle node between the two corners constraint nodes),the displacement at middle node between the two corners of the cantilever equal (3.68 E‐07 meter) near to the model with one quadrilateral element with 4 nodes(normal) in which (displacement = (4E‐07 meter)) but the displacement at the corners nodes of the cantilever equal (7.35 E‐07 meter).

2-For The "Non-Quadratic Element,for one material (selected materials),I check on the option "Non-Quadratic Element" (linear)and then  the number of nodes of the model are the same as without check on this option(Quadratic).

Dear samuelhanna93,
1. without seeing your models, I suspect you have applied your loading in 2 corner points? If so, mesh dependency is to be expected (and refining the linear mesh would get a similar response as switching to quadratic). Please check the deformed shape and adjust the boundary conditions if needed.

2. The "Non-Quadratic Element" allows you to use linear elements for one material (selected materials) while quadratic elements are used for all the other materials.

Dear sir,

1-for Quadratic type (quadratic )(8-nodes)

I modeled a simple beam with  cantilever its properties (depth =1.00 m,width =0.25m ,length =1.00m) ,with elastic material ,with horizontal load =10 KN at top and bottom of the beam.
-I used one quadrilateral element with 4 nodes(normal) and used in another model one quadrilateral element with 8 nodes (Quadratic)
-the horizontal displacement for model with one quadrilateral element with 4 nodes(normal) = (4E‐07 meter) which equal to the hand calculation =PL/EA
and for model with one quadrilateral element with 8 nodes (Quadratic)=(1.51 E‐06 meter)

why the element type affected on results ?

2-for your reply (For mixing quadratic and linear elements in a single model, please see the option "Non-Quadratic Element" at the "Element Geometry" tab of the material dialog).

I check on the option "Non-Quadratic Element" (linear)and then  the number of nodes of the model are the same as without check on this option(Quadratic).

Dear samuelhanna93, the supported options for Quadratic type are "Normal" (=linear) and "Quadratic" (which I have confusingly called "normal quadratic" in my previous answer, not having a GiD window in front of me at that time). The element type "Quadratic9" are not supported.

For mixing quadratic and linear elements in a single model, please see the option "Non-Quadratic Element" at the "Element Geometry" tab of the material dialog.

Dear sir,
1-Do you mean that the (QUADRATIC9)(9nodes) and quadratic (8 nodes) element types are not defined in ATENA?

-but for quadratic (8 nodes) element type there is no error appeared and the processing of all steps completed.
2-according to your reply(only the "normal" quadratic elements from GiD are supported in ATENA ),will the quadratic (8 nodes) element type give wrong results?
thanks a lot.

Dear samuelhanna93, please understand only the "normal" quadratic elements from GiD are supported in ATENA (apart from the linear ones, of course).