## Response Spectrum Analysis in STAAD explanation Video

## Methodology

The design lateral shear force at each floor in each mode is computed by STAAD in accordance with the IS: 1893 (Part 1) -2002

(equation 7.8.4.5c and 7.8.4.5d.)

Qik = Ak*fik*Pk*Wi

## STAAD utilizes the following procedure to generate the lateral seismic loads.

User provides the value for Z/2xI/R as factors for input spectrum

Program calculates time periods for first six modes or as specified by the user.

Program calculates Sa/g for each mode utilizing time period and damping for each mode.

## How STAAD Calculates of Base Shear in RS Analysis

VB = X*Y*Z*D

where

## BMD for RS Load Case

The sign of these forces cannot be determined because the method used to combine the contribution of modes cancels the signs.

Force values do not necessarily occur at the same instant of time.

STAAD plots a straight line joining the max. bending moment values at start and end joints of member, absolute (positive) values.

## What is the Scale Factor to be provided when specifying a Response Spectra?

Spectrum data consists of pairs of values which are Period vs. Accn. or Period vs. Displacement.

The acceleration or displacement values obtained for that site may have been provided to you as normalized values or un-normalized values.

Normalization means that the values of acceleration or displacement have been divided by a normalization factor.

One of the commonly used normalization factors is ‘g’, the acceleration due to gravity.

## Direction Factor in STAAD

SPECTRUM SRSS X 0.7 Y 0.5 Z 0.65 DISP DAMP 0.05 SCALE 32.2

Period is determined for each mode.

Spectral displacement (Sa/g) for that mode is read by interpolating the response spectrum plot.

Spectral displacement “Sa/g” for each direction is multiplied with associated Direction factor. D

The X direction spectral displacement = “Sa/g” * 0.1822

## How to obtain critical value from the design standpoint?

LOAD COMB 3

1 1.1 2 1.3

LOAD COMB 4

1 1.1 2 -1.3

Use the critical value from amongst these 2 load combination cases for design purposes.

Thus considering a positive effect as well as the negative effect of the spectrum load case.

## Obtain vertical distribution of the total base shear in a RS analysis, like in Static Procedure.

No direct method to obtain lateral force distribution. Since the values from a RS analysis are absolute quantities.

We may add up the shears in the columns above that level for an approximate estimate.

## Global Reaction does not equals Global Base Shear.

- Suppose we have two modes and 4 supports in the x direction & in SRSS combination method the results are computed as follows:

Note that SRSS base shear (39.8) does not equal the sum of the SRSS reactions (18.0+19.6+46.2+12.4=96.2).

This is due to the fact that the individual maximums would not occur at the same time and not necessarily with the same sign. So the base shear magnitude is usually much less than the sum of the reactions.