Moments values on beam under gravity loads 2

[perfectaccess]

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.

 

[jayrod12]

If it is structural frames, then it makes sense that as the total moment in the column increases on the way down, the amount of moment dumped back into the beams would increase proportionally.

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.

 

[KootK]

I'm assuming:

- 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.

 

[jayrod12]

Thanks for putting that more clearly than I could Koot. That's what I was attempting to relay.

 

[KootK]

Thanks for putting that more clearly than I could Koot.

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.

 

[perfectaccess]

Exactly Ktootk,

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.

 

[KootK]

Actually ,I think even if the columns were all the same size,( lets assume for the sake of argument)

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.

 

[perfectaccess]

Hi Ktootk

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/getfil...d-051f-4e29-be30-b4331f8f14c2&file=sadeq2.PNG

 

[KootK]

Neat! In my opinion, that reads like classic differential column shortening. The columns displace axially more than the more lightly loaded walls and the beam moments are impacted. Two ways to check:

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.

 

[perfectaccess]

Kootk

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/getfil...46a6-81ee-7f56e72add1e&file=Sadeq_Keewan2.PNG

 

[KootK]

Hmm. Messing with the axial stiffness of the walls seems to have also messed with their flexural stiffnesses and exacerbated things. Back to the drawing board I guess.

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.

 

[perfectaccess]

Yes Indeed, Fixed Support

 

[perfectaccess]

Just between the lines.How it will change flextural ?? Cant Understand it!!!!

Thanks

 

[KootK]

It may not change flexural. It all depends on how you've got the wall modelled.

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.

 

[perfectaccess]

It is modeled as pier

 

[KootK]

New theory:

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.

 

[perfectaccess]

Well, your last theory is not clear to me.
I fixed all the walls as you required, but nothing changed(same behavior)

See attachment

http://files.engineering.com/getfile.aspx?folder=d25209ba-20e1-43ba-bbd0-9e5b87c5485e&file=s3.PNG

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)

 

[KootK]

I fixed all the walls as you required, but nothing changed(same behavior)

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.

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

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.

 

[jayrod12]

Depends on the software, but I can take a look. I only have access to a select few at this time.

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.

 

[KootK]

You're a good sport Jayrod. I was just teasing. If the model's back saved to ETABS 2013, I can take a look on the weekend.

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.