Studi Numerikal Perilaku Perubahan Frekuensi Natural Bangunan Baja 2D dengan Sistem Concentric Braced Frame dan Korelasinya dengan Damage Index dari Struktur menggunakan Software OpenSEES = Numerical Study of the Changes in the Natural Frequency of 2D Steel Structures with a Concentric Braced Frame System and its Correlation to the Damage Index of the Structure using OpenSEES