Dear ALL While carrying out seismic analysis by equivalent static method we calculate the forces manually at each floor level and apply them at each beam column junction as horizontal nodal force in both direction as required. After that we carry out the analysis and obtain the result. To avoid this manual calculation STAADPRO has introduced a reference mass loading which can be given as primary load case and the software calculates the masses at each floor level. Accordingly base shear is also calculated as per equivalent static method and nodal forces are generated at each member node irrespective of its loaction whether it is a beam column junction or it is the ends of a secondary beam. I have checked the masses considered for equivalent static method for both the cases are same and seismic base shear coefficient is also same. My query is whether the 2nd approach is a correct approach? I feel that if a mass is present at a node irrespective of it's position, seismic force shall be generated as the mass will be subjected to a acceleration due to the ground motion. Let us consider the case of a bin supported on four beams at mid point through bracket. In this case the entire load is transferd to the structure through the mid point of the supporting beam and inturn it goes to the column as beam reaction. Definitely the beam will be subjected to horrizontal forces due to seismic shock. I am attaching a ppt and sketch for your understanding. Request your expert comment in this
Arup Munsi
Firstly, we always used to automatically calculate the equivalent static seismic loads on a structure - even before we had introduced the mass reference load case. Earlier, we used to define the masses as seismic weights in the seismic definition which was directly used for the base shear calculations. The introduction of the mass reference load case only enabled us to save the mass modelling time if we are to do both the static seismic analysis and the dynamic analysis in the same model.
Now, coming to your main question - the generation of seismic loads on beam column junction is a very worthwhile assumption but not the reality. In reality the masses are distributed continuously - so you will have more exact results if beam elements are broken into multiple pieces and masses are lumped on intermittent nodes. However, we will not like to do this because this will unnecessarily augment the problem size. But, as you can understand, intermittent nodes with masses lumped increases the accuracy on the contrary.
In case you are not interested to have the masses lumped on the intermittent nodes, the load data corresponding to masses should be specified as joint loads instead of member loads. But in case of long span beam elements it would be a good idea to break up the span.
Sudip Narayan Choudhury
Thanks, for the reply I am also in the same page, if there is a mass there has to be a seismic force at that location. I was taking a second opinion so that I can break the conventional minds
ARUP MUNSI
Is it possible to create seismic loads for different opereating condition of equipment? generally we have an empty condition and an operating condition. For a complex industrial structure both of these cases shall be analysed, hence we will have 2 different seismic definition. Say for example reference load R1 is dead load, R2 live load, R3 equipment load in empty condition, and R4 equipment load in operating condition. Now I need to define one seismic load with R1,R2 & R3 another seismic load case with R1,R2 & R4. The problem is I can define seismic load once in STAAD. I can have 2 different file but that will lead to human error as in any industrial structure the load changes frequently in course of project and as you know copy-paste is the biggest error in cyber world
Can you try:
Type * in front of second seismic load in input text file. Run analysis with R123 comb and reverse: set * in front of first seismc load and run. This option you have to run the model manually twice by turn on & off * sign in your text file, at least you have results on the same file.
Just a suggestion.
Arup,
Sorry to disappoint you, but the program is currently limited to solving only one set of frequencies per model. We agree that different sets of masses to represent different operating conditions is needed, and it is on the list of enhancements with the software development group. So, we are hopeful that it will be available in a future release of the program.
For what it's worth STAAD(X) allows users to create multiple mass tables to use with multiple analysis runs.
Hi Kris,
Currently, I'm facing the same problem as mentioned by Arup. I have to prepare different STAAD files for the different set of Masses in operating condition.
I am using STAAD connect, is there any tool in this version of STAAD which take care of this problem & allow me to define a different set of masses in same STAAD file.