added spring.m

spring.m

+clear
+clc
+k1 = springstiff(200);
+k2 = springstiff(300);
+k3 = springstiff(400);
+k4 = springstiff(100);
+K = zeros(4);
+K([1,3],[1,3]) = K([1,3],[1,3]) + k1;
+K([1,2],[1,2]) = K([1,2],[1,2]) + k2;
+K([2,3],[2,3]) = K([2,3],[2,3]) + k3;
+K([3,4],[3,4]) = K([3,4],[3,4]) + k4;
+
+kread = K([2,3],[2,3]);
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