design of beam in etabs 3

[structech08]

Hello,

Why is it that the beam bending moments in ETABS gives lesser value when compared to other engineering software? Beam steel rebars are much lesser if we allow the ETABS to design it.

Thanks,

structech08

 

[slickdeals]

Are you checking moments at centreline or at face of column? ETABS automatically computes column width and provides moment at the face of the column.

 

[structech08]

Hi slickdeals,

Im checking the moment at the face of column.

I observed too that when I used slab as membrane type, I got more rebars compared to slab as shell type. But amazing, when I compared to manual calculations, the results are far from the slab as in membrane and shell.

Would you please clarify the differences?

Thanks
Structech08

 

[slickdeals]

A membrane will give you more beam bending because the slab element has no out-of-plane stiffness, and all loads are dumped back to the beam. I don't know why you are getting a difference. Can you post your model?

 

[osquro]

From "CSI Analysis Reference Manual For SAP2000®, ETABS®, and SAFE™"

"The Shell element is a three- or four-node formulation that combines membrane and plate-bending behavior..."

"For each homogeneous Shell element in the structure, you can choose to model pure-membrane, pure-plate, or full-shell behavior. It is generally recommended that you use the full shell behavior (Shell - thick) unless the entire structure is planar and is adequately restrained..."

"Shell Section Type

For Shell sections, you may choose one of the following types of behavior:
• Membrane – pure membrane behavior; only the in-plane forces and the normal (drilling) moment can be supported; homogeneous material
• Plate – pure plate behavior; only the bending moments and the transverse force can be supported; homogeneous material
• Shell – full shell behavior, a combination of membrane and plate behavior; all forces and moments can be supported; homogeneous material
• Layered – multiple layers, each with a different material, thickness, and location; provides full-shell behavior, all forces and moments can be supported

It is generally recommended that you use the full shell behavior (Shell - thick) unless the entire structure is planar and is adequately restrained."

 

[dollarbulldog]

The way etabs computes load pattern when using membrane element is based on simplified yield line pattern. Have you check a simple example with hand calculation? I think Etabs did nothing wrong.

 

[Skicat]

I think you mean to say Shell - thin. Thick shells are only to be used for very thick elements, not necessarily slabs and normal width walls.

 

[structech08]

Hello Guys,

Thank you for the valueble inputs. I will revise and make a simple model of slab to easily verify using the moment coefficient method.

Good day,

structech 08

 

[structech08]

Hi,

I modelled 3 continuous beam with hinge support in etabs and compared it to manual calculations. The etabs and manual calculation almost have the same results for bending moment and steel reinforcement.

The only difference is when we modelled the slab as shell type, it gives less bending moment and membrane type slab has more bending moment.

When do we use the shell type slab and membrane type slab? I really appreciate if you could give a better explanation or example.

How do we model gravity columns in ETabs?

Thank you so much!
Structech08

 

[dollarbulldog]

Hi
Has anyone experienced the problem in calculation of excess rebar in ETABS? I found that ETABS always multiplies the required flexural reinforcement with 4/3 whether or not the calculated flexural rear is greater than the minimum value stated by ACI.

 

[ATOOMUS]

Hi,
I am Eng. Mu'men Atoom from Jordan.
I studied this case. I saw:
- when we have solid slab ( slab on drop beams ), we must use membrane slab because it is more accurate than shell slab. ( membrane transfer loads from slab to beams by tributary area method then from beams to columns).
-but when we have flat slab ( without drop beams ), we must use shell slab because it is more accurate than membrane slab. ( shell transfer loads from slab to columns directly by tributary area method ).

Note: membrane transfers loads to horizantal boundary elements while shell transfers loads to boundary nodes of shell slab.

good luck.
Atoomus

 

[structech08]

Hello Atoomus,

Thank you for sharing your observation.


Regards,
structech08