Pushover Analysis - ERROR : MASS PARTICIPATION IS LESS THAN 75 PERCENT

Question

Hello Kris Sathia , 
 When I ran the pushover analysis it stops, giving me the error of the participation mass less than 75%. When I look at the output file the mass participation factors are all 0. How can I find these participation factors in order so I can use the command "MODE SELECT" to do the analysis? 
 When I do the frequency analysis it gives me Mode 5 with the highest participation factor in Z direction. However, when I use this in the pushover I still get the error. 
 I have uploaded the file via secure file upload - file name: "Pushover" 
 
 Kind Regards, 
 Andi

Kris Sathia · Answer

Hi Andi, 
 
When I ran your model, I found that, as the program goes through the pushover analysis, some beams fail due to plastic hinge formation, and some columns fail because the axial force exceeds the Euler buckling load based on a PINNED-PINNED condition of the ends. The number of such failures reaches a point where the overall structure becomes unstable and the analysis is terminated. 
 
I did not get a chance to increase the size of the members and re-run the analysis. 
 
What is the error message you get? What version of the program are you running?

Kris Sathia · Answer

The MODE SELECT feature for pushover analysis was introduced in 
 
STAAD.Pro V8i SS5, Build 20.07.10.64 ( 22 July 2014) 
 
which happened some 3 years after SS3. 
 
See item A)04 at the following link 
 
 communities.bentley.com/.../16036.staad-pro-v8i-ss5-build-20-07-10-64-22-july-2014

Amitava · Answer

Hi 
 
In absence of Kris let me answer few of the queries you have raised - 
 
Q: "Can you please take me through how the "Number of push load steps" definition works?" 
 
A: Pushover analysis is an incremental static analysis. Structure is pushed with incremental load and corresponding force and displacement are noted. In the second step, the structure is again pushed and corresponding force and displacement are noted. In this way the analysis goes on until the target displacement is reached. The push load that will act as incremental load on the structure = the lateral load / number of push load steps. If number of push load steps is small w.r.t. the lateral load, the incremental push load will be high enough to make the structure jump to nonlinear stage without capturing the structural behavior while passing from linear to nonlinear stage. On the contrary, if number of push load steps is high w.r.t. the lateral load, the incremental push load will be so small that it will perform several analyses and store huge data against each incremental load step. This will consume much time. Hence, user needs to perform some trial and error method to find what should be the ideal number of push load steps for the model under consideration. Also, please note that once loading on the structure varies (4.3.1 Program Defined Push Load Distribution Pattern) number of push load steps will also vary. 
 
Q: "I am a bit confused because I specify the method to "Push upto defined displacement" say 2m, and set a certain number of load steps, say 10, but the load steps that I get in the Capacity Curve are never equal to the "Number of push load steps" (10) and the displacement does not reach 2m as specified." 
 
A: You may need to increase the number of push load steps from 10. It seems for the model you are using, this value is too low. 
 
Q: "what does the accuracy of the results depend on? Does it depend on the "Number of push load steps"? How many should I consider in general?" 
 
A: Accuracy depends on lots of factors: 
 1. Types of frame & material properties 
 (4.2.1 Type of Frame, 4.2.2 Expected Yield Stress, 4.2.3 Effective Length Factor of Member) 
 2. Hinge properties (4.5 Define Hinge Property) 
 3. Loading pattern (4.3 Define Loading Pattern) 
 4. Number of push load steps 
 
There is no general rule to Number of push load steps. However the value of 10 seems to be too low for any real life model. The program default is 100.