Question: In our STAAD the deflection value in the member query box shows that“Seismic load involved dynamic analysis, displacement results will not be available”. Kindly please inform how to apply the limiting displacement value in STAAD for RCC members.
Answer: You are trying to get the local deflection values of a beam using a member query facility from a Response Spectrum load case. The very basic theory of the Response Spectrum analysis is that a static analysis is performed for each mode considered on dynamically displaced modes. The structural responses obtained from each mode are combined using a modal combination like SRSS, CQC etc. The sign of the responses are lost in the modal combination. Relative nodal displacement obtained from a Response Spectrum load case is meaningless. The reason is that in response spectrum analysis the joint displacements represents the maximum magnitude of the response quantity that is likely to occur during seismic loading. Any response quantity like relative nodal displacement should be calculated from actual displacements of the dominant mode considered during analysis.
We can find out the dominate mode from the “Participation Factors” table provided in theoutput report as shown below.
It can be see that 1st mode is contributing 81.04 % mass participation and 2nd mode iscontributing 80.39 % mass participation. We can conclude that 1st Mode is the dominant modeof vibration for X directional Response Spectrum load case and the responses obtained fromthe modal analysis for this 1st mode would govern the responses for X directional ResponseSpectrum load case. Similarly, we can conclude that 2nd mode would dominate the responsesobtained from Z directional Response Spectrum load case.
In order to check the local deflection of a beam we can think of checking it from the individualload case created for dominant mode rather than using the actual Response Spectrum loadcase. As discussed earlier, the results obtained from a Response Spectrum load case arealways unsigned and a quantity like relative nodal displacement computed from unsignedabsolute maximum nodal displacement would be a vague quantity which has no meaning inreal sense.
STAAD.Pro can be used in this purpose to get the responses of an individual modal load case. The program has been equipped with this facility. You can tell the program to generate individual modal load cases. The program generates individual modal load cases as primary load cases with the load case numbers defined by the users. You have used Response Spectrum load cases like the following:
LOAD 16 LOADTYPE Seismic TITLE SEISMIC + X
------------------------------------
-----------------------------------
SPECTRUM CSM 1893 X 0.105248 ACC SCALE 1 DAMP 0.05
SOIL TYPE 3
You can change the commands in the following manner.
LOAD 16 LOADTYPE Seismic TITLE SEISMIC + X
------------------------------------
-----------------------------------
SPECTRUM CSM 1893 X 0.105248 ACC SCALE 1 DAMP 0.05 IMR 1 STARTCASE 200
SOIL TYPE 3
Graphically, it would look in the following manner.
On doing this, you are instructing the program to generate individual modal load case for 1st mode which will be the dominant mode for the X direction. This load case will have load case number as 200.
You can choose this load case to check local beam deflection checking purpose. Please refer to the attached picture.
