Moments values on beam under gravity loads
Moments values on beam under gravity loads
(OP)
Hi
When we have high rise building consist of columns and beams ,and we apply the vertical gravity load only.we will notice that the moment values for the same beam will differ form floor to floor.
So I think this true, but can anyone give interpretation for this behavior please.
When we have high rise building consist of columns and beams ,and we apply the vertical gravity load only.we will notice that the moment values for the same beam will differ form floor to floor.
So I think this true, but can anyone give interpretation for this behavior please.






RE: Moments values on beam under gravity loads
The columns and beams will take their stiffness' portion of the moment at each connection. Larger total moment at the connection would result in larger moments going into each member.
RE: Moments values on beam under gravity loads
- Analysis considering only gravity loads
- Beams rigidly connected to columns.
In a high-rise building, I would expect the columns to get larger, and stiffer, on the way down. As such, I would expect the gravity moments in the beams to:
- Decrease in the middle.
- Increase at the supports.
Is that what you're seeing perfect access?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Moments values on beam under gravity loads
RE: Moments values on beam under gravity loads
Like constructicons, we combine to form a single, technically proficient, eloquent, Canadian structural engineering juggernaut! Of course, we use our powers to further the forces of goodness and light.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Moments values on beam under gravity loads
Actually ,I think even if the columns were all the same size,( lets assume for the sake of argument),thus the amount of loads and moments for columns in low level is higher than top levels, thus as you said:
beams
negative moment increase way down
beams positive moment decrease way down
and if the columns become bigger in low level(which is not my case now,but could be )this fact will be more clear.
This behavior is called frame action and interaction(Am I right guys?)
Thanks guys.
RE: Moments values on beam under gravity loads
I don't think that we've yet come up with a satisfactory explanation for the results that you're seeing. My theory wouldn't apply if the column sizes remain constant throughout the height of the building.
Some alternate explanations:
1) Differential axial shortening of vertical elements.
2) Thermal effects in exterior columns.
3) Reduced flexural stiffness in members carrying greater compression due to P-delta effects.
Items two and three would only manifest themselves is a pretty sophisticated model however. And item one would only meaningfully impact a very tall building.
Can you provide a bit more information about your specific case:
1) Are you running a linear elastic model?
2) Is there a significant imbalance in the moment delivered to one side of your columns versus the opposite side? A plan framing sketch would be helpful.
3) How many stories is the building that you're investigating?
3) Is this a concrete building or a steel building?
4) Do you see the influence on all floors or just a few floors near the roof or ground floor?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Moments values on beam under gravity loads
1.No thermal Effect on columns
2.I am taking P-Delta in Analysis
3.I have a round 20 typical stories
4.I have Reinforced Concrete Building
5. the sketch I attached have all same shear walls,and same beams size,but negative moment is decrease upward
as you can see is not frame system ,but shear walls_beams is similar behavior
6.Only 1.2 D+ 1.6 L is consider
http://files.engineering.com/getfile.aspx?folder=8...
RE: Moments values on beam under gravity loads
1) look at joint displacements at the lower floors to see if there is relative axial movement between walls and columns.
2) Make both walls and columns axially rigid and see if the effect goes away.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Moments values on beam under gravity loads
In the previous case the negative moment is decreasing upward ,and this is logic(the axial deformation commutative is higher when you go up)
Now,
What I did is multiplying the axial stiffness by 50,so the axial deformation become very marginal,
the value of negative moment increase,become larger which is logical.the trend also become different the negative moment increase when you go up(don't know why now?)
see attachment
http://files.engineering.com/getfile.aspx?folder=7...
RE: Moments values on beam under gravity loads
Are both your walls and columns modelled as fixed at the ground floor?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Moments values on beam under gravity loads
RE: Moments values on beam under gravity loads
Thanks
RE: Moments values on beam under gravity loads
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Moments values on beam under gravity loads
RE: Moments values on beam under gravity loads
If you model a column as fixed at the base and laterally restrained at all floors, that column will be stiffer in response to floor joint rotation as one moves closer to that fixed joint. With normal column proportions, the effect is not very pronounced. However, your walls are basically acting as monstrously stiff columns, exacerbating the effect.
Possible tests:
1) Fix the tops of the walls and columns against rotation to see if you get more vertical symmetry in your results.
2) Change the walls to columns of a similar proportion to the interior columns and see if the trend is less pronounced.
3) Pin your walls and columns at the base and add some dummy lateral restraint to the building to keep things stable.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Moments values on beam under gravity loads
I fixed all the walls as you required, but nothing changed(same behavior)
See attachment
http://files.engineering.com/getfile.aspx?folder=d...
I reckon that maybe your first point ,of changing axial stiffness is in the same time missing the flextural (could be the answer,but not quite sure)
RE: Moments values on beam under gravity loads
Actually, I'd only wanted the very tops of the two wall systems fixed. Just fixity at the roof. And that fixity would need to be modelled in such a way that it still allowed for axial displacement at the top of the wall.
It would be very nice if we could eliminate differential shortening from the model entirely so that we could tease out other effects. I'll noodle on that. I believe that what your seeing in the latest run is pretty much pure differential shortening effect.
Perhaps you could post your model so that Jayrod can tinker with it.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Moments values on beam under gravity loads
And honestly, I would perform Koot's third option as my first, pin the bases of the columns. If the model becomes unstable, add lateral supports to the columns until the stability returns, and then check the beam results. I'm going to bet it has to do with the fixed base modelling as well.
RE: Moments values on beam under gravity loads
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Moments values on beam under gravity loads
I pin the model ,put the behavior doesn't change.
I attached the model on 2015 version,sorry I don't have 2013
http://files.engineering.com/getfile.aspx?folder=1...
RE: Moments values on beam under gravity loads
BA