Basement wall in etabs 1

[Ryan_C]

Hi,

I'm modelling a tower with 20 floors and 5 basements in Etabs. No seismic load need to be applied. Only wind load is needed. My question is that whether should I model basement wall as panel or as frame with close spacing. My senior suggested me to model as frame so the wind load with attract to the corewall. However, another senior suggested to model it as panel. Anyone has experience about this? Could you suggest which one is more acceptable? Thanks before hand.

 

[MrHershey]

I don't know that the frame idea is a realistic assumption unless the wall will actually be constructed as individual elements with no structural continuity. It keeps the shear and moment in the core walls, which is certainly an admirable goal. But I wouldn't imagine it accurately reflects what the actual behavior will be if the wall is built continuously like one would normally build a wall.

In the end, the behavior of the building is going to depend on what is actually constructed, not what is in the model. That's important to keep in mind when making modeling decisions and make sure our detailing and our modeling assumptions are in sync. During an earthquake or hurricane, the building is not going to consult the structural model to determine how it should behave.

 

[cyrosmith85]

Hi,

Above the basement all shear will be taken by the core wall. Once the accumulated shear reaches the basement top slab, this slab will act as a support to the cantilever beam that is your building (with a distributed wind load acting on it). This is generally what happens when the soil surrounding the basement can be considered stiff enough as to prevent any significative displacements.. plus, if your building area is much smaller than your basement area and the basement walls are very large (which is common), the walls alone can act as supports restraining the basement from any significative displacement. So either way, it is important that you put restraints to your model at the basement-top-slab level. Like MrHershey said, you should think about what is most likely to happen in reality, most of the times you can determine this by visualising the building as a cantilever beam with any kind of forces acting on it.

I use ETABS for analysis. I guess that the "panel" would equal "shell" in ETABS. If so, I think that is the way to go, since all shear collected above basement-top-slab level will be transferred to the basement walls by the basement top slab and the basement walls will transfer it to the soil around and under them. So you do not need of any special technique or modelling to capture this. Using shell elements to model the basement walls would be more realistic, simple and accurate.

Hope it helps.

 

[Enrique2015]

The frame idea is completely unrealistic, if you are assuming that below grade level all lateral forces will be transferred to surrounding soil foundation , then you can use lateral restraints at base. In many case or when you are designing in big cities , the city code does not allow to use soil as lateral support, in case someone builds next to you, your building should be capable of resisting forces, hence is usual to use basement walls as part of the Lateral resisting system. You can also assign a very small stiffness modifiers to your basement walls (f12) then their lateral in plane stiffness will be negligible.