International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
[(]
Comparison of High Rise Steel Building under
Seismic Hazards
Mya Sandar Win, Dr. Zaw Min Htun
Abstract— This study deals with studying on time history analysis and design of twenty-storeyed steel superstructure in seismic zone (4) and it is solved by using ETABS Software. All structural members are designed in accordance with ACI 318-99.The building is considered the wind for of 80 mph.Seismic forces are essentially considered in this proposed building. All members are designed with concrete having compressive strength of 5000 psi and reinforcing yield strength of 65000 psi. Seismic load is considered as external lateral load in static analysis procedure.The behaviour of structural members in the proposed building are analyzed due to Elcentro Earthquake from Default in software and Iwate Miyagi Nairiku Earthquake in Japan. Finally, response of the structural members due to these earthquakes are compared.
Index Terms - ACI 318-02, ETABS Software, Seismic Code: UBC-97, ETABS Software, twelve- storeyed steel Building.
I. Introduction
Nowadays, the high-rise buildings are widely used in Myanmar and population is also increasing more and more .So, high-rise buildings are needed to be constructed .Myanmar is saturated in a secondary belt which is the junction of two major belts called Alps-Himalaya and Circum-Pacific Belts. Mandalay area is saturated the Myanmar Largest active fault , the Sagaing fault which has the high level of seismic actively. Therefore, it is essentially to design buildings to withstand moderate earthquake without damage and sever earthquake without collapse.
From a structural engineering standpoint, one of the major distinguishing characteristics of high-rise building needs to resist large lateral forces due to wind or earthquake. . The proposed building for this study is selected as a steel structure in seismic zone 4 by UBC-97. Structural elements are designed according to AISC-LIFD. In this study, twelve-storeyed steel superstructure is designed by using ETABS software.
II. Preparation for Proposed Building
A. Site Location and Structural Framing System
Location : Seismic zone 4
Type of Structure : Twelve-storeyed steel
Building
Type of Occupancy : Residential
Plan Dimension : X direction = 120ft
Y direction = 70ft
Height of Structure : 141 ft
Typical story height : 10 ft
Bottom story height : 12 ft
Figure 1. First Floor Plan of the Proposed Building
Figure 2. Second to Twelve Floor Plan of the Proposed Building
Figure 3. 3D View of the Proposed Building
B. Material Properties
The strength of a structure depends on the strength of the materials from which it is made.
Analysis property data
- Weight per unit volume = 490 pcf
- Modulus of elasticity for steel = 29×106 psi
- Poisson's ratio = 0.3
- Coefficient of thermal expansion = 6.5×10-6
Design property data
-Concrete strength, fc ' = 3 ksi
- Yield stress, Fy = 50 ksi
- Tensile stress, Fu = 65 ksi
C. Load Consideration
Gravity Load
The gravity loads considered in this design are dead
load and live load. The lateral loads of wind load and
earthquake load are calculated according to UBC-97.
1) Dead Load
Dead loads consist of the weight of all material and fixed equipment incorporated into the building.
- 4.5thick brick wall = 55 lb/ft3
-9 thick brick wall = 100 lb/ft2
-unit weight of concrete = 150 lb/ft3
-superimposed dead load = 25lb/ft
2) Live Load
Live loads are gravity load produced by the used and occupancy of the building and do not include dead loads, construction loads, or environmental loads such as wind and earthquake loadings are based on to UBC-97.
-unit weight of water = 62.4 pcf
-live load on residential = 40lb/ft2
-live load on roof = 20 lb/ft2
-live load on stair case = 100 lb/ft2
-live load on lift = 100 lb/ft2
3) Wind Load
The wind pressure on a structure depends on the wind response of the structure. Required Data in designing for wind load:
- Exposure type = Type B
- Basic wind velocity = 80 mph
- Total height of building = 130 ft
- Method used = Normal Force
Method
- Windward coefficient = 0.8
- Leeward coefficient = 0.5
- Importance Factor = 1.0
4) Earthquake Load
The purpose of seismic design is to proportion the structures so that they can withstand the displacements and forces induced by the ground motion.
- Seismic zone = 4
- Seismic Source Type = A
- Soil Type = SD
- Structure = Special Moment
Resisting Frame
- Seismic Response Coefficient, Ca = 0.44 Na
- Seismic Response Coefficient, Cv = 0.64 Nv
- Near Source factor, Na = 1
-Near Source factor, Nv = 1
- Zone Factor = 0.4g
- Importance Factor, I = 1.0
- Response Modification Factor, R = 8.5
- CT value = 0.035
IV. Analysis And DESIGN Results of Structure
A. Design Results for Column Sections
Wide flange W-sections are used in column sections. Design results of columns section for the structure are shown in Table 1.
Table I
Design Sections of Columns
Story 1 / C 1 / C 2 / C 3
Story 2 to 3 / W 14x176 / W 14x193 / W 14x233
Story 4 to 6 / W 14x120 / W 14x132 / W 14x176
Story 7 to 9 / W 14x109 / W 14x120 / W 14x90
Story 10 to 12 / W 14x99 / W 14x109 / W 14x90
Roof / - / W 14x99 / -
Figure 4. Columns Layout Plan of Proposed Building
B. Design Results for Beam Sections
Wide flange W-sections are used for floor beams. Beams are classified as B1, B2, roof beams and stair roof beams according to load effects. Design results of beams section for the superstructure are shown in Table 2.
Table II
Design Sections of Beams
Story 1 to 12 / Roof Floor / Stair RoofBeams
B 1 / W 10x30 / B 1 / W 10x30 / W 10x30
B 2 / W 10x22 / B 2 / W 10x22 / -
B1 (red line ), B2( blue line )
Figure 5. Beams Layout Plan of Proposed Building
V. Stability Checking of Structure with static analysis
In design of structure, stability checked is required. In this study, the stability of structure such as overturning moment, sliding, story drift, torsion irregularity and P-r effect is checked.
A. Overturning Moment
In checking for moment, the ratio of resisting moment to overturning moment of the (the safety factor) is greater than 1.5. As the safety factor for both X and Y directions are 10.49 and 6.171 respectively, so these are greater than 1.5. Therefore, the structure is capable of resisting overturning effect. The distribution of earthquake forces over the height of a structure causes structure to experience overturning effects.
For X-direction,
Overturning moment = 420432.1 k-ft
Total Dead Weight, WD = 6803 kips
X-direction center of mass = 720.731 ft
Resisting moment =0.9×Total Dead Weight × XCCM
= 4412819.694 k-ft
Factor of Safety = >1.5
= 10.49 > 1.5 OK
For Y-direction,
Overturning moment = 420033kip-ft
Total Dead Weight, WD = 6803kips
Y-direction center of mass = 423.317 ft
Resisting moment = 0.9×Total Dead Weight× YCCM
= 2591842.996 k-ft
Factor of Safety = > 1.5
= 6.171 > 1.5 OK
i. Check for sliding resistance
To check the sliding force, the structure must be resisted at least 1.5 times the base shear. Friction coefficient is 0.25.
For X-direction,
Sliding Force, Vx = 458.84 kips
Friction coefficient = 0.25
Resisting Force = 0.9 × WD × 0.25
= 1495.5413 kips
Safety Factor = 3.259 > 1.5 OK
For Y-direction,
Sliding Force, Vy = 458.84 kips
Friction coefficient = 0.25
Resisting Force = 0.9 × WD × 0.25
=1495.5413 kips
Safety Factor = 3.259 > 1.5 OK
B. Sliding
In the checking for sliding, when the ratio of resistance due to friction to sliding force, V is greater than or equal to that of 1.5. The safety factor for sliding in both X and Y directions is 3.254, so these are greater than that of 1.5. Therefore, there is no sliding occurs in the structure.
C. Story Drift
In checking for story drift, it is found that story drift for all stories do not exceed limit. Based on the analysis, all story drifts are within story drifts limitation. The maximum story drift in X and Y directions are 0.001315 and 0.00124, so these are less than 2.4. Therefore, the structure is stable.
D. Torsion Irregularity
In proposed structure, the maximum drift at one end of the structure transverse to its axis is not more than 1.2 times the average story drifts of both ends. Therefore the effect of torsional irregularity can be neglected.
E. P-rEffect
P-reffect does not exist when the ratio of story drift height (story drift ratio) does not exceed 0.02/R in seismic zone 3 & 4. The maximum drift ratio in X and Y directions are 0.001315 and 0.00124. So these values are greater than 0.02/R = 0.02/8.5 = 0.002353, P-r effect can be neglected.
VI. Display Time history Function
Time History Functions for Elcentro Earthquake is shown in figure 6 (a). Time History Functions for Iwate Miyagi Nairiku is shown in figure 6 (b).
Figure 6 (a). Time History Function for Elcentro Earthquake
Figure 6 (b). Time History Function for Iwate Earthquake (Sine Function)
Table III
Specify Load Case And Load Factor For The Remaining Of Load Combination
Load Comb. Name / Load Case/ Load FactorDEAD / LIVE / WIND X / WIND Y / EQX / EQY
CON 11 / 1.05 / 1.275 / - / - / 1.4025 / -
CON 12 / 1.05 / 1.275 / - / - / -1.4025 / -
CON 13 / 1.05 / 1.275 / - / - / - / 1.4025
CON 14 / 1.05 / 1.275 / - / - / - / -1.403
CON 15 / 0.9 / - / - / - / 1.43 / -
CON 16 / 0.9 / - / - / - / -1.43 / -
CON 17 / 0.9 / - / - / - / - / 1.43
CON 18 / 0.9 / - / - / - / - / -1.43
VII. Comparison Results For Time History analysis
Story drift, story shear and story moment for different seismic hazards in both directions are presented in comparison of time history analysis.
A. Comparison of story drift
The story drift values for different seismic hazards are shown in Figure 7 and 8.
Figure 7. Comparison of story drift in X-direction for different seismic hazards
Figure 8. Comparison of story drift in Y-direction for different hazards
From the above figure 7 and 8 ,maximum story drift in X and Y direction is occurred at Elcentro Earthquake. The story drift of Elcentro Earthquke is 1.169 times greater than Iwate-Miyagi Nairiku Earthquake.
B. Comparison of story shear
The story shear values for different seismic hazards are shown in Figure 9 and 10.
Figure 9. Comparison of story shear in X-direction for different seismic hazards
Figure 10. Comparison of story shear in Y-direction for different seismic hazards
From the above figure 9 and 10,maximum story shear in X direction is occurred at Elcentro Earthquake.The most value of story shear in Y-direction is found atIwate-Miyagi Nairiku Earthquke.The story shear of Elcentro Earthquke is 1.02 times greater than Iwate-Miyagi Nairiku Earthquake.
C. Comparison of story moment
The story moment values for different seismic hazards are shown in Figure 11 and 12.
Figure 11. Comparison of story moment in X-direction for different seismic hazards
Figure 12. Comparison of story moment in Y-direction for different seismic hazards
From the figure 11and 12, maximum story moment in X and Y direction is occurred at Elcentro Earthquake.The story momentof Elcentro Earthquke is 0.925 times greater than Iwate-Miyagi Nairiku Earhquake.
VII. Discussions And Conclusion
In this study, the structural behaviour of an twelve- storeyed steel building is checked under seismic performance. For lateral earthquake loads, Time history analysis is used based on UBC-97, and all structural members are designed according to ACI 318-99. Structure of the building is analysed by applying ETABS version 9.7.1 software.
First of all, structural stability which includes sliding, overturning, storey drift, storey shear, and P-Δ effects is checked under peak ground acceleration of 0.4g. All of them are within limitations. After all checking, seismic data are considered based on the extreme earthquakes such as Elcentro Earthquake and Iwate Miyagi Nairiku Earthquake. In comparison of analysis results, the most value of story drift, story shear and story moment are found at Elcentro Earthquake. Elcentro Earthquake is almost 1.2 times greater than Iwate-Miyagi Nairiku Earthquake. According to these comparisons, Elcentro Earthquake is the most affect than Iwate Miyagi Mairiku Earthquake. So Elcentro Earthquake is chosen as the most extreme earthquake than Iwate Miyagi Mairiku Earthquak.
In conclusion, the designer should consider the extreme earthquake strike such as Elcentro Earthquake,if we design the structure with time history analysis in the future.
Acknowledgement
The author is deeply indebted to her Supervisor, Dr. Zaw Min Tun, Associate Professor of Civil Engineering Department, Technological University (Mandalay) for his careful guidance and necessary advice. The author also wishes to extend special thank to Dr, Kyaw Moe Aung, Associate Professor and Head of Civil Engineering Department, Technological University (Mandalay), for his kindness to share ideas and knowledge. The author expresses her deepest gratitude to teachers from Technological University (Mandalay) for their help and share of experience. Finally, the author would like to thanks her parents for her years of help and support, encouragement to attain her destination without any trouble throughout her life.