Thank you Doug for the response.
I have a further question regarding structures other than bridges which although covered by AS 5100 have no earthquake loading given in AS 5100 for them.
In Section 14 Ëarthquake Design" loading of AS 5100.2, the loading described in the section is for the design of bridge and associated structures such as approach retaining wall.
Bridge Design standard also applies to structures other than bridges such as sign gantries (see AS 5100.1, Section 1). In designing sign support structures, earthquake loading described in Section 14 of AS 5100.2 is not suitable as these structures are not bridges and neither are they bridge associated structures (e.g. approach retaining walls). For sign support structures, should one be using AS 1170.4 for earthquake loading instead of using AS 5100?