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Chapter IV: Seismic Analysis I.
Introduction
The seismic analysis and design of buildings has traditionally focused on reducing the risk of loss of life in the largest expected earthquake. Building codes have based their provisions on the historic performance of buildings and their deficiencies and have developed provisions around life safety concerns, i.e., to prevent collapse under the most intense earth- quake expected at a site during the life of a structure. These provisions are based on the concept that the successful performance of buildings in areas of high seismicity depends on a combination of strength, ductility manifested in the details of construction, and the presence of a fully interconnected, balanced, and complete lateral-force-resisting system. In regions of low seismicity, the need for ductility reduces substantially. In fact, in some instances, strength may even substitute for a lack of ductility. Very brittle lateral-force-resisting systems can be excellent performers as long as they are never pushed beyond their elastic strength. The seismic analysis, that is carried using “ETABS”, aims to choose the most suitable conditions for safety of the “ B-CENTRAL-RESIDENTIAL TOWER” against earthquakes loading. In general, most earthquake code provisions implicitly require that structures be able to resist: 1. Minor earthquakes without any damage. 2. Moderate earthquakes nonstructural damage.
with
negligible
structural
damage
and
some
3. Major earthquakes with some structural and nonstructural damage but without collapse. The structure is expected to undergo fairly large deformations by yielding in some structural members. II.
Basis for Design:
The procedures and the limitations for the design of structures shall be determined considering seismic zoning, site characteristics, occupancy, configuration, structural system and height. Structures shall be designed with adequate strength to withstand the lateral displacements induced by the Design Basis Ground Motion, considering the inelastic response of the structure and the inherent redundancy, over strength and ductility of the lateral-force resisting system. The minimum design strength shall be
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III.
Earthquake Loads
The earthquake load, which is also called seismic load, is a lateral load caused by ground motions resulting from earthquakes (Sudden movement and rupturing of crust plates along fault lines). The magnitude of earthquake load depends on building’s mass and the acceleration
caused by the earthquake. As the ground moves suddenly, the building attempts to remain stationary, generating the inertia induced seismic forces that are approximated by the static lateral force procedure covered here. This procedure is introduced in UBC ‘97 1629.8.3 and discussed in detail in UBC ‘97
1630. The static force procedure is limited to use with regular structures less than 73m in height and also to irregular structures 19m or 5 stories in height.
Regular structures are symmetric, without discontinuities in plan or elevation.
The building plan is generally rectangular.
The mass is reasonably uniform throughout the building’s height.
The shear walls line up from story to story.
Irregular structures include both vertical irregularities (UBC Table 16-L), or plan irregularities (UBC Table 16-M). These irregular features include:
Reentrant corners.
Large openings in diaphragms.
Non-uniform distribution of mass or stiffness over building height (e.g. soft story).
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Figure 4.1: Flowchart Showing Steps Taken for UBC-97 Analysis
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B-CENTRAL IV.
Design base shear:
1997 UBC static lateral method considers both horizontal movement and vertical ground movement.
UBC base shear design equations, as given below, where each equation is a function of the building weight and some form of an acceleration factor. The total design base shear in a given direction is to be determined from the following formula: V
C v I W R T
(3.9)
The total design base shear need not exceed the following: V
2.5 C a I W R
(3.10)
The total design base shear shall not be less than the following: V 0.11C a IW
(3.11)
In addition, for Seismic Zone 4, the total base shear shall not be less than the following: V
0.8 Z N v I W R
(3.12)
The minimum design base shear limitation for Seismic Zone 4 was introduced as a result of the ground motion effects observed at sites near fault rupture in 1994 Northridge earthquake. Where: V = total design lateral force or shear at the base.
W = total seismic dead load
In storage and warehouse occupancies, a minimum of 25 % of fl oor live load is to be considered.
Total weight of permanent equipment is to be included.
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2
Where a partition load is used in floor design, a load of not less than 50 kg/m is to be included.
I = Building importance factor given in UBC 97 Table 16k. Z =
Seismic Zone factor, shown in UBC 97 Table 16I.
R =
response modification factor for lateral force resisting system, shown in
UBC 97 Table 16N. C a = acceleration-dependent seismic coefficient, shown in UBC 97 Table 16Q
C v = velocity-dependent seismic coefficient, shown in UBC 97 Table 16R. N a = near source factor used in determination of C a in Seismic Zone 4, shown in
UBC 97 Table 16S. N v = near source factor used in determination of C v in Seismic Zone 4, shown in
UBC 97 Table 16T . T
= elastic fundamental period of vibration, in seconds, of the structure in the direction under consideration evaluated from the following equations:
For reinforced concrete moment-resisting frames, T 0.073 hn
3/ 4
(3.13)
For other buildings, 3/ 4
T 0.0488 hn
(3.14)
Alternatively, for shear walls, 3/ 4
T 0.0743
h n
Ac
(3.15)
hn = total height of building in meters 2
Ac = combined effective area, in m , of the shear walls in the first story of the structure,
given by
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Ac
A
i
D 0.2 h
2
e
n
, De / hn
0.9
(3.16)
De is the length, in meters, of each shear wall in the first story in the direction parallel
to the applied forces. 2
Ai = cross-sectional area of individual shear walls in the direction of loads in m
V.
Seismic Parameters
Occupancy Category Importance Factor
Occupancy Category
Seismic Importance Factor, I
1-Essential facilities
1.25
2-Hazardous facilities
1.25
3-Special occupancy structures
1.00
4-Standard occupancy structures
1.00
5-Miscellaneous structures
1.00
Table 4.1: Occupancy Category Importance Factor (UBC1997-Table 16-K) Although this structure is a residential building. According to table 16-K:
Seismic Importance Factor: I = 1
Seismic Zone Factor
The seismic zone factor Z, given in table 16-I, is the code estimate of the applicable site dependent effective peak ground acceleration expressed as a function of the gravity constant g. The tower is located in Beirut: Zone 2B, then Z =0.2
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1
2A
2B
3
4
Z
0.075
0.15
0.2
0.3
0.4
Table 4.2: Seismic zone factor Z (UBC1997-Table 16-I)
Site Category and Soil Characteristics
According to table 16 –J (section 1629.3): Soil profile type is S , Rock (According to the geotechnical report). B
Average Soil Properties For Top 30 m Of Soil Profile Standard Soil Profile
Soil Profile Name/Generic
Type
Description
Shear Wave Velocity, v s m/s
Penetration Test,
N
Undrained Shear Strength, S u kPa
(blows/foot) S A
Hard Rock
> 1,500
S B
Rock
760 to 1,500
Very Dense Soil and Soft
S C
Rock
---
---
360 to 760
> 50
> 100
S D
Stiff Soil Profile
180 to 360
15 to 50
50 to 100
S E
Soft Soil Profile
< 180
< 15
< 50
Soil Requiring Site-specific Evaluation
S F
Table 4.3: Site Category and Soil Characteristics (UBC1997-Table 16-J)
Seismic Response Coefficients
Soil Profile Type
Seismic Zone Factor, Z Z =0.075
Z = 0.15
Z = 0.2
Z = 0.3
S A
0.06
0.12
0.16
0.24
0.32 N a
S B
0.08
0.15
0.20
0.30
0.40 N a
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Z = 0.4
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0.09
0.18
0.24
0.33
0.40 N a
S D
0.12
0.22
0.28
0.36
0.44 N a
S E
0.19
0.30
0.34
0.36
0.36 N a
See Footnote
S F
Table 4.4: SEISMIC COEFFICIENT Ca (UBC1997-Table 16-Q)
Soil Profile Type
Seismic Zone Factor, Z Z =0.075
Z = 0.15
Z = 0.2
Z = 0.3
Z = 0.4
S A
0.06
0.12
0.16
0.24
0.32 N a
S B
0.08
0.15
0.20
0.30
0.40 N a
S C
0.13
0.25
0.33
0.45
0.56 N a
S D
0.18
0.32
0.40
0.54
0.64 N a
S E
0.26
0.50
0.64
0.84
0.96 N a
See Footnote
S F
Table 4.5: Seismic Response coefficient Cv(UBC1997-Table 16-R)
This structure shall be assigned a seismic coefficient: Ca = 0.2 (Z=0.2 & Soil profile type S ), in accordance with Table 16-Q.
B
Cv = 0.2 (Z=0.2 & Soil profile type S ), in accordance with Table 16-R. B
Response Modification Factor R
The structure response modification factor R given in UBC97 Table 16-N is the ratio of the seismic base shear, which would develop in a linearly elastic system, to the prescribed design base shear. It is a measure of the ability of the system to absorb energy and sustain cyclic inelastic deformations without collapse. Lightly damped structures: Constructed of ductile materials assigned low values of R Highly damped structures: Constructed of ductile materials assigned larger values of R
According to table 16 –N, structural systems: Lateral force resisting system description: Shear wall- frame interaction systems
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R = 5.5
Table 4.6: Structure Response Modification Factor R (UBC97) VI.
Input data for seismic loads in ETABS:
In our model, four seismic loads are defined for the static analysis in the ETABS: EQPX: in the x-direction + 0.05 eccentricity ratio in the y-direction
EQNX: in the x-direction - 0.05 eccentricity ratio in the y-direction
EQPY: in the y-direction + 0.05 eccentricity ratio in the x-direction
EQPY: in the y-direction - 0.05 eccentricity ratio in the x-direction
The input data for ETABS for the static analysis are shown in the four windows below: 64
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Figure 4.2: Static seismic data in ETABS in the X-direction (EQPX & EQNX)
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Figure 4.3: Static seismic data in ETABS in the Y-direction (EQPY & EQNY)
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B-CENTRAL VII.
Mass Source
Mass source is defined from self and specified mass and l oads such that, and superimposed dead load (SD) is multiplied by one.
Figure 4.4: Mass source definition
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