TR.31.2.13 IBC 2000/2003 Load Definition

The specifications of the IBC 2000, and 2003 codes for seismic analysis of a building using a static equivalent approach have been implemented as described in this section. Depending on this definition, equivalent lateral loads will be generated in horizontal direction(s).

General Format

There are 2 stages of command specification for generating lateral loads. This is the first stage and is activated through the DEFINE IBC 2000 or 2003 LOAD command.

DEFINE IBC ( { 2000 | 2003 } ) (ACCIDENTAL) LOAD

 ibc-spec

weight-data

Refer to Common Weight Data for information on how to specify structure weight for seismic loads.

Where:

ibc-spec = { SDS f1 SD1 f2 S1 f3 I  f4 RX f5 RZ f6 SCLASS f7 (CT f8) (PX f9) (PZ f10) }

Parameter

Description

SDS f1

Design spectral response acceleration at short periods. See equation 16-18, Section 1615.1.3 of IBC 2000 and equation 9.4.1.2.5-1 of ASCE7-02

SD1 f2

Design spectral response acceleration at 1-second period. See equation 16-19, Section 1615.1.3. of IBC 2000 and equation 9.4.1.2.5-2 of ASCE7-02

S1 f3

Mapped spectral acceleration for a 1-second period. See equation 16-17 of IBC 2000, and 9.4.1.2.4-2 of ASCE 7-02

I f4

Occupancy importance factor determined in accordance with Section 1616.2 of IBC 2000 and 2003, and section 9.1.4 (page 96) of ASCE 7-02

RX f5

The response modification factor for lateral load along the X direction. See Table 1617.6 of IBC 2000 (pages 365-368) and Table 1617.6.2 of IBC 2003 (page 334-337). It is used in equations 16-35, 16-36 & 16-38 of IBC 2000

RZ f6

The response modification factor for lateral load along the Z direction. See Table 1617.6 of IBC 2000 (pages 365-368) and Table 1617.6.2 of IBC 2003 (page 334-337). It is used in equations 16-35, 16-36 & 16-38 of IBC 2000.

SCLASS f7

Site class as defined in Section 1615.1.1 of IBC 2000 (page 350) & 2003 (page 322).

Table 1. Values of IBC soil class (`SCLASS`) used in STAAD
STAAD Value	IBC Value
1	A
2	B
3	C
4	D
5	E
6	F

CT f8

Optional C_t value used to calculate time period, T_a. See section 1617.4.2.1, equation 16-39 of IBC 2000 and section 9.5.5.3.2, equation 9.5.5.3.2-1 of ASCE 7-02. If specified, it is your responsibility to provide the value in the correct system of units. Refer to Table 9.5.5.3.2 of AISC 7-02 for values.

If the value of C_t is not provided, then the program computes the average value of the modulus of elasticity of the model,

E_{a v g} = \sum E / M

(where M is the number of members) and uses this to determine the structure type:

E_avg < 4,000 ksi, the program uses a C_t for a moment-resisting concrete frame.
E_avg > 10,000 ksi, the program uses a C_t for a moment-resisting steel frame.
4,000 ksi ≤ E_avg ≤ 10,000 ksi, the program uses a C_t value for "all other structural systems".

Note: It is your responsibility to ensure that the structure type used actually matches the description for the automatically determined structure when C_t not specified. Refer to the IBC/ASCE 7 code for detailed descriptions.

Table 9.5.5.3.2 of ASCE 7-02 also includes "Eccentrically braced steel frames". STAAD.Pro does not select this value automatically. For this structure type, you must specify C_t.

PX f9

Optional Period of structure (in sec) in X-direction to be used as fundamental period of the structure instead of the value derived from section 1617.4.2 of IBC 2000, and section 9.5.5.3 of ASCE 7-02.

PZ f10

Optional Period of structure (in sec) in Z or Y direction to be used as fundamental period of the structure instead of the value derived from section 1617.4.2 of IBC 2000, and section 9.5.5.3 of ASCE 7-02.

Note: For additional details on the application of a seismic load definition used to generate loads, refer to TR.32.12.2 Generation of Seismic Loads.

The seismic load generator can be used to generate lateral loads in the X & Z directions for Y up or X & Y for Z up. Y up or Z up is the vertical axis and the direction of gravity loads (See the SET Z UP command in TR.5 Set Command Specification). All vertical coordinates of the floors above the base must be positive and the vertical axis must be perpendicular to the floors.

The implementation details of the respective codes are as follows:

IBC 2000

On a broad basis, the rules described in section 1617.4 of the IBC 2000 code document have been implemented. These are described in pages 359 thru 362 of that document. The specific section numbers, those which are implemented, and those which are not implemented, are as follows:

Table 2. Sections of IBC 2000 implemented and omitted in the program
Implemented sections of IBC 2000	Omitted sections of IBC 2000
1617.4.1 1617.4.1.1 1617.4.2 1617.4.2.1 1617.4.3 1617.4.4 1617.4.4.4	1617.4.4.1 1617.4.4.2 1617.4.4.3 1617.4.4.5 1617.4.5 1617.4.6

IBC 2003

On a broad basis, the rules described in section 1617.4 of the IBC 2003 code document have been implemented. This section directs the engineer to Section 9.5.5 of the ASCE 7 code. The specific section numbers of ASCE 7-2002, those which are implemented, and those which are not implemented, are shown in the table below. The associated pages of the ASCE 7-2002 code are 146 thru 149.

Table 3. Sections of IBC 2003 (ASCE 7-02) implemented and omitted in the program
Implemented sections of IBC 2003 (ASCE 7-02)	Omitted sections of IBC 2003 (ASCE 7-02)
9.5.5.2 9.5.5.2.1 9.5.5.3 9.5.5.3.1 9.5.5.3.2 9.5.5.4 9.5.5.5 Portions of 9.5.5.5.2	9.5.5.5.1 9.5.5.5.2 9.5.5.6 9.5.5.7

Methodology

The design base shear is computed in accordance with Eqn. 16-34 of IBC 2000 and Eqn. 9.5.5.2-1 of ASCE 7-02:

V = C_sW

The seismic response coefficient, C_s, is determined in accordance with Eqn. 16-35 of IBC 2000 / Eqn. 9.5.5.2.1-1 of ASCE 7-02:

C_{s} = \frac{S_{D S}}{R / I_{E}}

C_s need not exceed the limit given in Eqn. 16-36 of IBC 2000 / Eqn. 9.5.5.2.1-2 of ASCE 7-02:

C_{s} = \frac{S_{D 1}}{(R / I_{E}) T}

C_s shall not be taken less than the lower limit given in Eqn. 16-37 of IBC 2000 / Eqn. 9.5.5.2.1-3 of ASCE 7-0:

C_s = 0.044 S_DSI_E

In addition, for structures for which the 1-second spectral response, S₁, is equal to or greater than 0.6g, the value of the seismic response coefficient, C_s, shall not be taken less than the limit given in Eqn. 16-38 of IBC 2000 / Eqn. 9.5.5.2.1-4 of ASCE 7-02:

C_{s} = \frac{{0.5 S}_{1}}{R / I_{E}}

For an explanation of the terms used in the above equations, please refer to the relevant IBC and ASCE 7-02 codes.

Procedure Used by the Program

Steps used to calculate and distribute the base shear are as follows:

The Time Period of the structure is calculated based on section 1617.4.2 of IBC 2000, and section 9.5.5.3 of ASCE 7-02 (IBC 2003) This is reported in the output as T_a.
The period is also calculated in accordance with the Rayleigh method. This is reported in the output as T.
you may override the Rayleigh based period by specifying a value for PX or PZ depending on the direction of the IBC load.
The governing Time Period of the structure is then chosen between the above two periods, and the additional guidance provided in clause 1617.4.2 of IBC 2000, section 9.5.5.3 of ASCE 7-02 (IBC 2003) or section 12.8.2.1 of ASCE 7-05 (IBC 2006). The resulting value is reported as "Time Period used."
The Design Base Shear is calculated based on equation 16-34 of IBC 2000, equation 9.5.5.2-1 of ASCE 7-02 (IBC 2003) or equation 12.8-1 of ASCE 7-05 (IBC 2006). It is then distributed at each floor using the rules of clause 1617.4.3, equations 16-41 and 16-42 of IBC 2000. For IBC 2003, using clause 9.5.5.4, equations 9.5.5.4-1 & 9.5.5.4-2 of ASCE 7-02.
If the ACCIDENTAL option is specified, the program calculates the additional torsional moment. The lever arm for calculating the torsional moment is obtained as 5% of the building dimension at each floor level perpendicular to the direction of the IBC load (clause 1617.4.4.4 of IBC 2000, and section 9.5.5.5.2 of ASCE 7-02 for IBC 2003 ). At each joint where a weight is located, the lateral seismic force acting at that joint is multiplied by this lever arm to obtain the torsional moment at that joint.

Example 1

DEFINE IBC 2003 LOAD
SDS 0.6 SD1 0.36 S1 0.31 I 1.0 RX 3 RZ 4 SCL 4 CT 0.032
SELFWEIGHT
JOINT WEIGHT
51 56 93 100 WEIGHT 1440
101 106 143 150 WEIGHT 1000
151 156 193 200 WEIGHT 720

Example 2

The following example shows the commands required to enable the program to generate the lateral loads. Refer to TR.32.12 Generation of Loads for details.

LOAD 1 ( Seismic Load in X Direction )
IBC LOAD X 0.75
LOAD 2 ( Seismic Load in Z Direction )
IBC LOAD Z 0.75

The Examples manual contains examples illustrating load generation involving IBC and UBC load types.