Seismic Loading

Question

Can someone please give me the STAAD input text for seismic to UBC and to BS EN 1998 - I find the STAAD help editor difficult to follow. 
 I am assuming that I will use CQC for the modal analysis, but what about SRSS etc. 
 In the example I have pulled together, I get 85% mass participation after 6 modes - how do I increase the number of modes in order to raise the mass participation to what I think I need of around 97% 
 
 Thanks

Marc Thomas · Answer

Hi Keith, 
 Please repost in the RAM | STAAD forum , you will find the right experts there.

Modestas · Answer

Some example models can be found in the folder C:\SProV8i SS6\STAAD\Examp\US (UBC example) and C:\SProV8i SS6\STAAD\Examp\Eur (Eurocode 8 examples). 
 You should use CUT OFF MODE SHAPE command to increase the number of modes to be considered in analysis. The program by default calculates only 6 modes. If you use, let's say, command CUT OFF MODE SHAPE 10, then 10 modes will be considered. You can increase this value till you get the required mass participation. You can add this command by going to Commands -> Miscellaneous -> Cut Off Mode Shape.

Modestas · Answer

Files UBC_ELEMENT_WT.std or UBC_FLOOR_WEIGHT.STD can usually be found in folder C:\SProV8i SS6\STAAD\Examp\US (unless you are using a different STAAD.Pro version or you installed STAAD.Pro in other than default location). Some examples files starting with Eurocode8 name can be found in C:\SProV8i SS6\STAAD\Examp\Eur. 
 CUT OFF MODE SHAPE command should be inserted before the loading data in the input file. 
 You can select type 1 or type 2 using the RS1 or RS2 parameters in the input file or changing the Load Type to Type I or Type II Response Spectrum load dialog.

Sudip Narayan Choudhury · Answer

You asked: 
 
 The example STAAD files use SRSS whereas CQC is the default. Can you tell me why SRSS was used rather than the CQC default method? 
 Answer: 
 
 I do not think the Modal combination method in the examples were chosen with anything specific in mind. 
 
 The EN 1998-1: 2004 permits the use of SRSS method if the Period of the subsequent mode in lower than 10% of the Time period of the former (Clause 4.3.3.3.2 (2)). 
 
 In the event of closer periods in two consequitive modes, the CQC methods should be adopted. 
 
 But in the event where you do not know the frequencies or the time periods, adoption of CQC is always safer and a better option over the SRSS. 
 
 You asked: 
 
 The example STAAD files specify 'f1=1', however 'f2 and f3 =0'. Why are f2 and f3 specified as zero in the examples? 
 
 Answer: 
 
 f2 and f3 are the direction factors respectively in the Y and Z directions. As per clause 4.3.3.5.1 in EN 1998-1:2004 &ndash; 2(a) one can evaluate the structural response to each component separately. Once this is done, 2(b) says that you can do a SRSS Load Combination to evaluate the simulatenous effect of two combinations. 
 
 However, as per (3), you can provide f2 and/ or f3 as 0.3 &ndash; or you can also combine the individual components (with f2 =0 and f3 = 0) as full effect from one and 30% of the other. 
 
 You asked: 
 
 I am looking at seismic action in Uganda and I have a value from a web search of a Peak Ground Acceleration (PGA) of up to 0.256g whereas the example STAAD files use f10=1 or 2 (reference clause 5.32.10.1.3 of the Technical Reference Manual. 
 
 I am using an Importance Factor of 1.0, thus the Ground Acceleration value f10 is the same as the Peak Ground Acceleration value. 
 
 Eurocode 8 states that the DESIGN GROUND ACCELERATION = PGA x Importance Factor (clause 3.2.1(3). 
 
 Does therefore STAAD 'map' the f10 values (1, 2 etc) to values of the actual ground acceleration at a location? 
 
 Answer: 
 
 Yes, having the Design Ground acceleration of 1g or 2g is impractical &ndash; as this means huge earthquake beyond our wildest imagination. If the Peak ground Acceleration is 0.256g, the value of ALPHA should be specified as 0.256. But note that if the PGA data is obtained from a site specific spectrum, it will not be necessary to multiply the same with a Soil Factor. So, it would be appropriate to use TYPE A ground in such cases. 
 
 No, the f10 values are supposed to be a factor of g and f10*g represents the Peak Ground Acceleration of Soil Type A.

Sye · Answer

The screenshots did not get attached. By the way did you try the solution that I referred to in my last post ?