Confusing reaction values

[Anonymous]

Ive been trying to make a model for a 30+storey building and ive noticed a bit odd results in it. The model itself is a bit too big with calcs to put here but I give my best to explain it.

I have two different files. One has whole structure and other one has just floor slab to get the bending moments and reactions (faster to do changes and finer mesh).

I took the biggest load bearing column out of the smaller slab file, got the reaction from 1 floor and bascially multiplied it with the count of floors.

When trying to compare it to the big model reaction the difference is 2 times. Big model has 9k kN and smaller "hand-made" calculation does give 18k kN.

I understand that loads can redistribute and so on but two times difference seems odd for me to belive them.

Does anyone know what could be the reason behind it or what could I have done wrong. 

I appriciate any help I can have.

[Artur.Kosakowski]

Please check: http://forums.autodesk.com/t5/robot-structural-analysis/load-sequencing-construction-stages/m-p/4568...

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Anonymous]

Read it through but it did not touch my problem area.

The problem is that when all the floors are identical then why are the reactions different when I sum them up by hand? is this 2x difference load redistribution or error in model?

[Artur.Kosakowski · Accepted solution]

First of all I assume that you defined the "big" model correctly meaning that your total sum of vertical reactions is equal to the total sum of reactions in a 'small" model multiplied by the number of stories from the 'big' one.

If so, I think that what can possibly happen is that the bottom columns of the 'big' model get 'shorter' due to dead load from the upper floors and which causes the floors to 'sink' in these locations which in turn 'attract' more load to be 'assigned' to these columns. The post I indicated describes the way of avoiding such effect which doesn't exist when you analyse a selected story separately. 

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Anonymous]

Thank you for explaining the case. 

This topic is answered but just out of curiousity, should I count it in or not ?

I understand that real construction is done one storey at the time. 

But what is the correct thing to do from Engineering perspective. 

[Artur.Kosakowski]

I think that the closest to reality is to follow the steps from the post 3 of the topic I indicated before.