Many codes (ICBO 1997; AIJ 1999;
SEAOC 1999; IBC 2003; EC 2004; ASCE 2005; ECP-203: 2004, 2007; ECP-201: 2008) recognize that the period of vibration from the simplified period-height equation is more realistic, having been directly obtained from the measured periods of vibration of buildings subject to earthquake ground motions (Goel, Chopra 1997; Ghosh, Fanella 2003), but that when higher modes are important (in tall and/or irregular structures) the modal response spectrum method gives a more realistic profile of the lateral forces.