function [s]=spectra(a,xi,dt,f)%[s]=spectra(a,xi,dt,f)%-------------------------------------------------------------% PURPOSE%  Compute seismic response spectra for elastic design.%% INPUT:%    a : acceleration time history at %        equal time step dim(a)= ntimes x 1%    xi: damping ratio, dim(xi)= 1 x 1%    dt: time step dim(dt)= 1 x 1%    f : frequency vector, dim(f)= nfreq x 1%% OUTPUT:%    s : complex response matrix %        dim(Y)= m x nw, m : number of dof's%-------------------------------------------------------------% LAST MODIFIED: H Carlsson 1993-09-21   % Copyright (c)  Division of Structural Mechanics and%                Department of Solid Mechanics.%                Lund Institute of Technology%-------------------------------------------------------------[np nr]=size(f);for i=1:np  r=sqrt(1-xi*xi);  w=2*pi*f(i);  wt=w*dt;  e=exp(-xi*wt);  c1=2*e*cos(r*wt);  c2=e*e;  c3=e/r/wt*sin(r*wt);  c4=[-c3 2*c3 -c3];  c5=[1 -c1 c2];  y=filter(c4,c5,a);  s(i)=max(abs(y+a))end  %--------------------------end--------------------------------