NEAR SOURCE FACTORS AND SEISMIC COEFFICIENTS

Near source factors are used in determining base shear when analyzing lateral forces in designing structures. Na and Nv account for the proximity to the nearest active fault. Although the NSCP specified interpolation of factors depending on the location of the structure, most designers consider the lumped location of structures whether located in a near field or far field.

 = near source coefficient (velocity)

 = near source coefficient (acceleration)

NSCP 2015 introduced the less than 2km distance. Although still debatable, most designers work on using this range for their designs instead of interpolating the actual value for the structure in the maps provided.

NEAR-SOURCE FACTOR Na

SEISMIC SOURCE TYPE	CLOSEST DISTANCE TO KNOWN SEISMIC SOURCE
	< = 2 km	< = 5 km	> = 10 km
A	1.5	1.2	1.0
B	1.3	1.0	1.0
C	1.0	1.0	1.0

NEAR-SOURCE FACTOR Nv

SEISMIC SOURCE TYPE	CLOSEST DISTANCE TO KNOWN SEISMIC SOURCE
	<= 2 km	< = 5 km	<= 10 km	>= 15 km
A	2.0	1.6	1.2	1.0
B	1.6	1.2	1.0	1.0
C	1.0	1.0	1.0	1.0

From those values taken from nearness factors, the parameters for seismic coefficients can thus be identified.

 = Seismic coefficient (acceleration)

 = Seismic coefficient (velocity)


SEISMIC COEFFICIENT, Cv

SOIL PROFILE TYPE	Seismic Zone, Z
	Zone 2	Zone 4
Sa	0.16	0.32Nv
Sb	0.20	0.40Nv
Sc	0.32	0.56Nv
Sd	0.40	0.64Nv
Se	0.64	0.96Nv
Sf	Note 1

SEISMIC COEFFICIENT, Ca

SOIL PROFILE TYPE	Seismic Zone, Z
	Zone 2	Zone 4
Sa	0.16	0.32Na
Sb	0.20	0.40Na
Sc	0.24	0.40Na
Sd	0.28	0.44Na
Se	0.34	0.44Na
Sf	Note 1

NOTE 1: 

Site-specific geotechnical investigation and dynamic site response analysis shall be performed to determine seismic coefficients.

RELATED ARTICLES:

Design Base Shear
Solving for Total Seismic Design Load
How to Determine the Coefficient of Over Strength and Ductility Capacity, R
Identifying the Importance Factor, I
What do Seismic Zones mean?
Philippine Seismic Source Types
Near Source Factors and Seismic Coefficients
Soil Profile Types
Elastic Fundamental Period of Structures