An overview of UBC-97 for Earthquake Load and its calculations

2. DEFINITION OF TERMS
• BASE SHEAR, V
- The total design lateral force or shear at the base of a structure
• DESIGN SEISMIC FORCE
- Is the minimum total strength design base shear
• DUAL SYSTEM
- Is a combination of moment-resisting frame and shear walls or braced frames

3. DEFINITION OF TERMS
• LATERAL-FORCE RESISTING FRAME
- Part of the structural system designed to resist the design seismic forces
• MOMENT RESISTING FRAME
- Is a frame in which members and joints are capable of resisting forces primarily by flexure
• ORTHOGONAL EFFECTS
- Are the earthquake load effects on structural elements common to the lateral force resisting
systems along two orthogonal axes

4. DEFINITION OF TERMS
• STORY SHEAR, Vx
- Is the summation of lateral forces above the story under consideration.
• STRUCTURES
- Is an assemblage of framing members designed to support gravity loads and resist lateral forces .
may be recognized as building or non-building.

5. OCCUPANCY CATEGORIES
For purposes of earthquake resistant
design, each structure shall be placed in
one of the occupancy categories listed
in fig. which assign importance factors, I
and Ip, and structural observation
requirements for each category. The
importance factor (I) is used to increase
the margin of safety for essential and
hazardous facilities.

6. SITE GEOLOGY AND SOIL CHARACTERISTICS
Soil profile types SA, SB, SC, SD and SE are defined in fig
and soil profile type SF is defined as soils requiring site-
specific evaluation as follows:
1.) Soils vulnerable to potential failure or collapse
under seismic loading, such as liquefiable soils, quick
and highly sensitive Clays, and collapsible weakly
cemented soils.
2.) Peats and/or highly organic clays, where the
thickness of peat or highly organic clay exceeds 10 feet
(3048 mm).
3. Very high plasticity clays with a plasticity index, PI >
75, where the depth of clay exceeds 25 feet (7620 mm).
4.) Very thick soft/medium stiff clays, where the depth
of clay exceeds 120 feet (36 576 mm).

7. SITE SEISMIC ZONE & HAZARD CHARACTERISTICS
Seismic hazard characteristics for the site shall be established based on the seismic zone and
proximity of the site to active seismic sources, site soil profile characteristics and the structure’s
importance factor.
Each structure shall be assigned a seismic zone factor Z, in accordance with Table 16-I.

8. SEISMIC ZONE 4 NEAR-SOURCE FACTOR
In Seismic Zone 4, each site shall be assigned a near-source factor in accordance with Table 16-S.
Near source factors are used in determining base shear when analyzing lateral forces in designing
structures. Na (near source coefficient-acceleration) and Nv (near source coefficient-velocity)
account for the proximity to the nearest active fault.

9. SEISMIC RESPONSE COEFFICIENTS
Each structure shall be assigned a seismic coefficient (acceleration) Ca and a seismic coefficient
(velocity), Cv.

10. STRUCTURAL SYSTEMS
Structural systems shall be classified as one of the
types listed in Table 16-N

11. DESIGN BASE SHEAR
Base Shear (V) is the total design lateral force or shear at the base of the structure. The design base
shear in a given direction is specified by the formula:
V= Cv I
where:
T: Fundamental Period of the structure in the direction under consideration.
I: Seismic importance factor.
Cv: Numerical coefficient depends on the soil conditions at the site and the seismicity of the region.
W: Seismic Dead Load
R: Factor which accounts for the ductility and over-strength of the structural system
W
RT

12. The design base shear is depending on the seismic zone factor (Z). The base shear specified
previously is subjected to three limits:
The total design base shear need not exceed:
V=2.5Ca I
The total design base shear shall not be less than the following:
V= 0.11 Ca I W
In the highest zone seismicity (Zone – 4) the base shear must be greater than:
V= 0.8 ZNv I W
R
W
R

13. STRUCTURE PERIOD
The building period may be determined by analysis or using the empirical formulas.
Method A:
For all buildings, the value T may be approximated from the following formula:
T= Ct (hn)3/4
Where:
Ct= 0.035 (0.0853) for steel moment-resisting frames.
Ct= 0.030 (0.0731) for reinforce concrete moment-resisting frames.
Ct= 0.020 (0.0488) for all other buildings.
hn= Height of structure

14. Method B:
The fundamental period T may be calculated using the structural properties and deformational
characteristics of the resisting elements in a properly substantiated analysis. The value of T from
Method B shall not exceed a value 30 percent greater than the value of T obtained from Method A
in Seismic Zone 4, and 40 percent in Seismic Zones 1, 2 and 3. The fundamental period T may be
computed by using the following formula:
fi = applied lateral force
δi = elastic deflection
g = gravity

15. DISTRIBUTION OF LATERAL FORCES (FX)
The base shear V, as determined earlier, is distributed over the height of the structure as a force at
each level Fi, plus an additional force Ft (that portion of the base shear consider concentrated at
the top):
An additional force at the top Ft is determined by:
where:
Ft= 0.07TV ≤ 0.25V
Ft= 0 if T≤0.7sec

16. The remaining portion of the total base shear (V – Ft) is distributed over the height, including the
top, by the formula:
where:
w: weight at a particular level.
H: height of a particular level above the shear base.
At each floor, the force is located at the center of mass.