story masses

[Saedhalteh]

What is difference between choosing to include half mass of the columns and walls above and below OR to include them only below ?
Is there is some specific cases where should I use each of these options ?

Note : I am using RAM Structural system program

 

[r13]

I guess the inclusion/exclusion of masses above and/or below is for purpose of seismic analysis and design. The user's menu regarding seismic force shall have instructions and explanations.

 

[rscassar]

It is where it distributes the mass of these elements to the diaphragms.

If using AS1170.4 which I think you are the use the half mass top & bottom. There is a diagram in AS1170.4 which indicates this

 

[Saedhalteh]

thanks for replying, I am using the IBC code .
what I want to ask is there some situation where we should include the mass of walls and columns only below or half above and half below ?

 

[wannabeSE]

In my area, 1/2 above and 1/2 below is typically selected. I doubt the code specifically mentions anything. The engineer needs to understand the lateral load path, then the answer will be clear.

 

[rscassar]

I can't think of one unless the column or wall cantilevers above the floor/diaphragm. Half mass top and bottom would be typical but I sure there is a condition where an exception to this rule would be appropriate.

 

[JoshPlumSE]

I can't think of an engineering reason why you'd want the full load of those items at the floor below. However, I can wager a guess based on my experience with similar software. Keep in mind that I have virtually no experience with RAM.

My guess is that some previous version of RAM lumped all column, and wall masses at the floor below. This could make sense if the optimization of column and wall sizes is done last for the floor. Then, if users objected to this programming simplification, they added the ability to split the mass up between the two floors. But, they put this in as an option because they didn't want an existing model to give dramatically different results when users ran it in the new version.

Note: I have no knowledge that this is actually what happened in this case, I'm just giving a wild guess based on the type of decisions I have seen made on the structure software packages that I have been involved with developing.

 

[r13]

According to RAM, the masses in the right side column are used for P-Delta analysis purpose only. Does this sounds something?

 

[Agent666]

Reading the explanation is seems like its taking the full height of the storey below and lumping any associated mass at the level above. In most cases given that the walls or columns are the same weight above or below it won't really make much difference except at the top storey.

Obviously if the columns/walls were different in a storey then it could make a difference to the mass distribution, hence a difference to the period (dynamic response), and hence some difference to the seismic loads.

Run both and for most structures as they get taller I can't see there being much difference in the results from a global perspective. However given the choice I'd be doing the half above/below option as the other option isn't analytically correct.