Response Spectrum Analysis_Queries

I am performing Response Spectrum Analysis in Staad Pro 2007. I have some queries regarding obtaining base shear and designing the members for seismic forces.

To start with, I have defined various gravity loads in load case No 1 to 5. In load case 6, I have defined applicable masses (in terms of loads) in all three directions X, Y and Z. The spectra in X direction is defined as follows:

Combination Method: CQC

Spectrum Type: Acceleration

Spectrum Table: Plugged in site specific spectra as defined in our seismic report.

Interpolation: Liner

Damping: 0.05 (5%)

Scale: 9.8 (as the values of acceleration provided in table are in terms of %g)

Direction: X: 0.385. Please see below how I defined this factor.

Query No. 1:

I believe the response modification factor for the type of frame (R ) and Importance Factor (I) are not explicit inputs to Staad Spectrum Analysis; but these are to be defined in the form of factors X, Y and Z in the input stream. Please clarify.

In my case, my structure in X direction is Ordinary Steel Concentrically Braced Frames. Therefore, R = 3.25 as per ASCE 7-05, Table 12.2-1. I value for Occupancy category III in my case is 1.25 as per ASCE 7-05, Table 11.5-1.

As per ASCE 7-05, Cl. 12.9.2, the base shear should be divided by R/I, which means the base shear in X direction should be multiplied by 1 / (R/I) = 0.385 as per ASCE 7-05. I, therefore, assume that the factor in direction X for Spectrum Analysis in Staad should be X=0.385. Please confirm that this is correct. If not, please advise what factor for X should be used in Staad.

Similarly my Z direction, being Special Steel Moment Frame, R = 8, as per ASCE 7-05, Table 12.2-1. I value for Occupancy category III is 1.25 as per ASCE 7-05, Table 11.5-1. Hence base shear in Z direction should be multiplied by 1 / (R/I) = 0.156. I, therefore, assume that in Staad Spectrum Analysis the factor Z should be 0.156. Please confirm whether or not this assumption is correct; and if not, please advise the correct value to be used.

 

Query No. 2:

This relates to the load combination. As suggested in Staad technical manual, the response loads will be absolute load. So, we need to consider two cases, Gravity + Seismic and Gravity - Seismic. Please clarify.

In my model, load case 7 is Spectra in Z Direction.

As per ASCE 7-05, Cl. 12.5.3 (a), we need to combine two orthogonal seismic forces for seismic category D. (100% seismic X +or - 30 % seismic Z and vice versa). How to do this in response load case as I get combined seismic force effect in a single load case only?

Query No. 3:

My third question is regarding distribution of seismic base shear in terms of vertical and horizontal distribution? How does Staad take care of it? In ASCE 7-05, this is calculated based on a function of the Effective Seismic Weight at each floor and the height above base. Please refer to equation 12.8-11 & 12.8-12 in ASCE 7-05.

Query No. 4:

My last question is regarding design of various members in Staad for this response spectra load. How should the load combinations be defined, and how the design parameters should be defined?

Thanks & Regards

Himali

Parents
  • Hello Himali,

    Sorry for the delay in getting back to you. Please find the responses as below:

    Answer to Query Number 1:

    You are correct when you say that the factors R and I are not explicit inputs in the response spectrum data. The way you have included the effects of R and I in the spectrum cases, as directional factor for directions X and Z would yield the correct responses as defined in the code. However, going by the definitions of the various spectrum parameters, it would have been more appropriate to have included this effect as the scale factor, which could have been specified as (I*g/R), where g = acceleration due to gravity.

    I also notice that you have used the generic response spectrum specification. In case you are using recent versions of STAAD.Pro, we have already incorporated the response spectrum specification as per IBC 2006 (or ASCE 7-05). The convenience of using this method is that the program would automatically calculate the spectrum data, instead of the user specifying the spectrum data himself.

    Answer to Query Number 2:

    If you have the following load cases:

    Load 1
    Gravity Loads
    Load 2
    Response Spectrum Load Case in X direction
    Load 3
    Response Spectrum Load Case in Z direction

    I would have the following combination in place:

    LOAD COMB 1
    1 1.0 2 1.0
    LOAD COMB 2
    1 1.0 2 -1.0
    Load COMB 3
    1 1.0 3 1.0
    LOAD COMB 4
    1 1.0 3 -1.0

    The above combinations will capture the maximum positive and negative results when considering combinations of the Response Spectrum Results with the gravity results.

    In addition to the above the following combinations may be considered if the structure falls in Seismic Category C:

    LOAD COMB 5
    2 1.0 3 0.3
    LOAD COMB 6
    2 1.0 3 -0.3
    LOAD COMB 7
    2 -1.0 3 0.3
    LOAD COMB 8
    2 -1.0 3 -0.3
    LOAD COMB 9
    2 0.3 3 1.0
    LOAD COMB 10
    2 0.3 3 -1.0
    LOAD COMB 11
    2 -0.3 3 1.0
    LOAD COMB 12
    2 -0.3 3 -1.0

    I think the above captures all the possible scenarios.

    I could not interpret clause 12.5.3a saying anything on the combination of the seismic load with the gravity loadings.

    Answer to Query Number 3:

    These equations do not extend to the response spectrum seismic analysis and is limited to Equivalent Lateral Force Procedure only. The base shear calculation in a particular direction for a particular mode in a response spectrum analysis is calculated as the product of the following:

    A * B * C * D

    Each of the terms above are defined as below:

    A = mass participation for the concerned mode in the concerned direction
    B = total mass specified in the concerned direction
    C = spectral acceleration in the concerned direction multiplied by the scale factor
    D = directional factor in the concerned direction

    The total base shear in a particular direction is the sum of the base shears calculated for all the modes.

    Answer to Query Number 4:

    Generally, LOAD COMB 1 through LOAD COMB 4 should capture the maximum positive and the maximum negative results and this is used for structural design. The response spectrum has no bearings on the selection of design parameters though.

    Sudip Narayan Choudhury

  • Hi Dan/Sudip

    Thanks for sharing the info on this forum.

    Regards

    Himali

Reply Children
No Data