# How to Determine Seismic Design Category, IBC

In IBC, structural classification and

*category depends on parameter, mapped***earthquake design***at 1-S period (S***spectral response acceleration**_{1}). When S_{1}is equal or greater than 0.75, the occupancy category I,II and III are assigned. The structure is also assigned to seismic design category E.Structures located in regions where parameter mapped spectral response acceleration, S

_{1}is equal or greater than 0.75, the occupancy assigned to them is occupancy category IV, also the structure is assigned following requirements of seismic design category F.Other structures not categorized as E and F, are categorized depending on occupancy category and also in co-efficient design spectral response acceleration, S

_{DS}and S_{D1}. The determination procedure of S_{DS}and S_{D1}are as follows:S

_{DS}stands for design spectral response acceleration at five percent damping at short period and S_{D1}stand for design spectral response acceleration at five percent damping at 1 second period.S

_{DS}and S_{D1}determine as follows:S

_{DS}=2/3X S_{MS }(1)S

_{D1}=2/3 XS_{M1 }(2)Where

S

_{MS}= Maximum seismic spectral response acceleration taking into account for short period.S

_{M1}= Maximum seismic spectral response acceleration taking into account for 1 second period.S

_{MS}and S_{M1}are determined based on mapped spectral acceleration for respective periods. Dear reader we will discuss about determination of S_{MS}and S_{M1}in our next post.The category A, category B, category C and category D are more severe design category and can be categories according to Table 1 and Table2.

Table 2. Seismic design category for 1-S period response acceleration |

We can easily understand that table 1 is based on response acceleration at short period and table 2 is based on response acceleration 1 second period.

Now we will learn about structures located in regions when S

table 1 when following all are applicable:

_{1}<0.75. In this case we can usetable 1 when following all are applicable:

A. Approximate fundamental period of structure in each orthogonal direction (each of two)

T_{a}<0.8T_{s}

T

_{s}and T

_{a}are determined as per 11.4.5 and 128.2.1 of asce7.B. Fundamental period of structure in each orthogonal direction (each of two) that are used to determine story drift<

T_{s}

C. Seismic response co-efficient is determined as per ASCE-7

S_{DS} =As per Part-1

R=Response modification factor

I=Occupancy importance factor

D. Diaphragms in structure are rigid. For flexible diaphragms distance between vertical element≤40ft.

In IBC a simplified procedure for seismic design also provided when seismic design category will be determined as per ASCE7.