TR.31.2.19 CFE (Comisión Federal De Electricidad) Seismic Load
The purpose of this command is to define and generate static equivalent seismic loads as per Manuel de Diseño por Sismo - Seismic Design Handbook Comisión Federal De Electricidad - Electric Power Federal Comission - October 1993 (Chapters 3.1, 3.2, 3.3 and 3.4) specifications. Depending on this definition, equivalent lateral loads will be generated in horizontal direction(s). This is a code used in the country of Mexico.
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.
General Format
DEFINE CFE (ACCIDENTAL) LOAD
cfe-spec
weight-data
Refer to Common Weight Data for information on how to specify structure weight for seismic loads.
Where:
cfe-spec = { ZONE f1 QX f2 QZ f3 GROUP f4 STYP f5 (REGULAR) (TS f6) (PX f7) (PZ f8) }
The optional parameter REGULAR is entered to consider the structure as a regular structure. By default, all structures are considered as irregular.
Generation of CFE Seismic Load
To provide a CFE Seismic load in any load case:
LOAD i
CFE LOAD {X | Y | Z} (f)
Where:
Parameter | Description |
---|---|
LOAD i | load case number |
CFE LOAD { X | Y | Z | f | factor to multiply horizontal seismic load. Choose horizontal directions only |
Methodology
Seismic zone coefficient and parameter values are supplied by the user through the DEFINE CFE LOAD command.
Program calculates the natural period of building T utilizing Rayleigh-Quotient method. If time period is provided in the input file, that is used in stead of calculated period.
The acceleration a is calculated according to the following:
Where:
- c = Seismic coefficient is extracted from table 3.1
- a0, Ta, Tb, and r are obtained form table 3.1
The ductility reduction factor Q’ is calculated according to section 3.2.5.
If not regular, then Q’ = Q’ x 0.8
If the period Ts of the soil is known and the soil type II or III Ta and Tb will be modified according to section 3.3.2.
Lateral loads for each direction are calculated for:
When T ≤ Tb, Eq. 4.5. Section 3.4.4.2 is used:
When T > Tb, Eq. 4.6/7/8. Section 3.4.4.2 is used:
Where:
q = (Tb/T)r |
The base shear are distributed proportionally to the height if T ≤ Tb or with the quadratic equation mentioned if T > Tb.
The distributed base shears are subsequently applied as lateral loads on the structure.
Example
UNIT KGS METER DEFINE CFE LOAD ZONE 2 QX .5 QZ 0.9 STYP 2 GROUP B TS 0.2 SELFWEIGHT MEMBER WEIGHT 1 TO 36 41 TO 50 UNI 300 JOINT WEIGHT 51 56 93 100 WEIGHT 1440 101 106 143 150 WEIGHT 1000 FLOOR WEIGHT YRA 11.8 12.2 FLOAD 400 - XRA -1 11 ZRA -1 21 LOAD 1 ( SEISMIC LOAD IN X DIRECTION ) CFE LOAD X 1.0 LOAD 2 ( SEISMIC LOAD IN -Z DIRECTION ) CFE LOAD Z -1.0