Base Shear and beam forces

[Chster6]

In a building, the lower floors should have lower horizontal seismic storey forces acting compared to the upper floors from the base shear. But in Etabs, the beams have larger moment at ground floors than upper floors. How come?

 

[AELLC]

Shear is cumulative as you go down story by story

 

[Chster6]

https://courses.cit.cornell.edu/arch264/calculators/seismic-wind/index.html

but see above where shears is highest at top storey and lesser as you go down so how can you say otherwise?

Also when changing column sizes to larger ones, the beam moments increased so this means oversizing columns at ground floor is not good enough to avoid large moments at beam?

 

[AELLC]

You are getting confused by story shear vs accumulated shear at each level as you go down, starting at roof. See the attached and study the FBD

 

[Chster6]

So the storey shear at second floor may be lower than the 10th floor yet the accumulated shear in the second floor is more than the 10th floor hence the columns and beams would attract more forces in the second floor?

Another thing. When I oversize the Columns at etabs. I noticed the beams framing into it has larger moments. We normally don't take this into account in manual computation. So as the column size increase, the moments of the beam framing into it indeed increase during seismic movement?

 

[AELLC]

Of course the second floor shears, moments and axial forces are larger than at the roof. The building is secured to its foundations at ground level. Are you sure you understand basic engineering concepts, FBD's and all?

I don't know what you learned in University, but the seismic story shear (with the simplest first harmonic) is a function of magnitude of seismic acceleration, mass of the story, and distance above ground. That is why those seismic shears increase as you go upward, and the roof shear may be proportionately a bit smaller because the roof may be lighter than the floors.

Stiffness attracts moment, so yes the moment in the beams increases , and the story drift decreases because of the larger EI's of the beams and columns.

 

[Chster6]

AELLC, of course the roof being lighter than lower floor means it has smaller moments in beams. You should have used example of floors just below the roof.

Also remember the displacement is distributed in each storey of the building so the shear is not really accumulative at lower floor. Any reference about this?

I just design static system but curious about seismic design and trying different configurations in the Etabs.

Also about stiffness attracting more moments. In columns braced against sway. It is more stiff.. hence beam moments are less at the support. So in what sense are you talking about stiffness attracting more moment?