"Positive" moments

[2much4any1]

Hello,

Task: I need to design a building with 6 stories.

Problem: After calculation, I've got "positive" (bending moments on bottom side) moments on beams above columns (supports) with vertical load.

Possible reason: Shear walls are more rigid than columns so due to uneven vertical displacements I got this untypical moment diagram.

Question: How to solve this problem? How can I design frame on seismic forces?

Thank you in advance..

 

[BAretired]

Please take a bit more time to define the problem. It is not clear as stated.

BA

 

[Nonlinearer]

What is the seismic Zone you are designing for? Also which software you are using? What is your lateral force resisting system? For (gravity + Seismic) load combinations, you might get reversal moments at supports.


IR

 

[2much4any1]

Building is in Novi Sad (Serbia), according to our regulations it's VIII seismic zone (ag=0.2g).
I think that's unimportant, my confusion is unusual moment diagram under gravity load.
Lateral force resisting system are frames with shear walls.
Software: Radimpex Tower (

http://www.radimpex.rs)

Maybe i wasn't clear enough, under gravity loads I get positive moments above columns (supports).

Link

Thank you for your answers..

 

[Nonlinearer]

I have seen your frame's BM diagram. This column is between the shear walls and shouldn't have this unusual behavior. I do not know your software but I can suggest to check the column properties and section input in to your software. It looks like a hanging column, check its connectivity with the foundation. It should have proper restraints similar to other columns.

IR

 

[2much4any1]

I totally agree with you.
I checked all those things and got nothing.
Planar support is rigid so there should not be any "hanging" of columns.
I just wanted to check, have this ever happened to you?

 

[Adawi]

I think this happens because of geometry.
For example if the beam spans not equal, Therefore the positive moment that applied on the middle span is vary large according of the deffirance in spans lengths, so the moment wont totaly convert to nigative above the support that exsist at the short span but it will decrease the value. However, the moment will stay at positive zone .

Try to insulate the beam with same geometry and spacing and make a 2D design at the same software ..i think the results will be the same you got in 3D modeling.

Note : Please inform me if you got more logical explanation for this problem .

 

[BAretired]

I am still not quite clear on the issue here. The typical moment diagram for each floor shows parabolic curves with peaks over the columns. The peaks are shown above the floor. Does this mean positive moments at each column? If so, then you may have entered the typical load as a positive value. If the vertical axis is upward, then gravity loads must be entered as a negative value.

The second span from the right on the first floor above grade shows a different behavior than all the rest. It appears there is an upward concentrated load applied right of midspan.

Check the sign of your load input. Positive loads are up. Negative loads are down.

BA