ISSN Vol.03,Issue.09, May-2014, Pages:

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1 ISSN Vol.03,Issue.09, May-2014, Pages: Comparative Study on K and V-Inverted Bracings in Steel Irregular- Shaped Building NYEIN MYINT THU 1, DR. ZAW MIN HTUN 2 1 Dept of Civil Engineering, Mandalay Technological University, Mandalay, Myanmar, nyeinmyintthu100@gmail.com. 2 Dept of Civil Engineering, Mandalay Technological University, Mandalay, Myanmar. Abstract: This paper presents comparative study on K and V-inverted bracings in steel irregular-shaped building. In this study, computer-aided analysis and design of superstructure for this building is carried out by using ETABS software. The building is a fifteen-storeyed L-shaped steel building. It is composed of special moment resisting frame (SMRF) in seismic zone 4. Dead loads, superimposed dead loads, live loads, wind loads and earthquake loads are considered based on UBC-97. All structural members are designed according to AISC-LRFD Wide flange W-sections are used for frame members. Structural steel used in building is A572 Grade 50 steel. Structural stability checking (overturning moment, sliding, storey drift, torsional irregularity and P- effect) are carried out for the stability of the superstructure. After checking the stability, the proposed building is analyzed with dynamic response spectrum case. Suitable bracing types such as K-type bracings and V-inverted bracings are used in this case. In this paper, storey drift, storey shear and storey moment of model A and model B in dynamic response spectrum case are compared. Keywords: AISC-LRFD 1999, ETABS Software, UBC-97. I. INTRODUCTION Our country is a developing country in Southeast Asia. It is populated more and more and so many high-rise buildings are needed to be constructed. Steel frame has many advantages and so steel frame is a suitable choice for highrise building. In this study, the proposed building is selected as a steel frame structure. High-rise buildings are more affected by lateral forces than low-rise buildings especially in seismic zone 4. For this lateral stability and stiffness, steel frames are constructed with bracings. So, using braced frames is safer than other systems. The proposed building is located in seismic zone 4. In this study, the proposed building is constructed with braced frames for dynamic response spectrum analysis. It is expected to construct more and more steel braced frames in the future. II. PREPARATION A. Data for Proposed Building The structure is fifteen-storeyed, L-shaped steel residential building. The structure has two normal stairs and two elevators. Type of structure = Fifteen-storey steel building Type of occupancy = Residential Length of structure = 102ft Width of structure = 81ft Ground floor height = 12ft Typical storey height = 10ft Stair roof height = 8ft Overall height = 172ft Typical floor plan, beam plan, column plan, three dimensional view and elevation view of proposed building are shown in Figure 1, 2, 3, 4, 5 and 6 respectively. Figure 1. Typical Floor Plan of Proposed Building. B. Material Properties The strength of a structure depends on the strength of the materials from which it is made. Weight per unit volume of concrete = 150pcf Yield stress, F y = 50ksi 2014 SEMAR GROUPS TECHNICAL SOCIETY. All rights reserved.

2 NYEIN MYINT THU, DR. ZAW MIN HTUN Figure 2. Beam Layout Plan of Proposed Building. Figure 5. Model A with K Bracing. Figure 3. Column Layout Plan of Proposed Building. Figure 4. 3D View of Proposed Building. Figure 6. Model B with V-Inverted Bracing. Tensile stress, F u = 65ksi Modulus of elasticity for steel, E s = 29x10 6 psi Coefficient of thermal expansion = 6.5 x 10-6 in / in per F Poisson s ratio, µ = 0.3 C. Loading Consideration The applied loads in this study are gravity loads that include dead and live loads, superimposed dead loads, lateral loads that include wind and earthquake loads. Design load combinations are also used. 1. Gravity Load All mass are attracted toward the center of the earth by the gravitational force. Loads are defined as these attracting forces acting upon their corresponding masses. There are two different gravity loads: (i) Dead loads and (ii) Live loads.

3 Comparative Study on K and V-Inverted Bracings in Steel Irregular-Shaped Building Dead Load: Dead loads consist of the weight of all material and fixed equipments incorporated into the building. 4.5"thick brick wall = 55 lb/ft 3 9"thick brick wall = 100 lb/ft 3 superimposed dead load = 25 lb/ft 2 unit weight of steel = 490 lb/ft 3 unit weight of concrete = 150 lb/ft 3 Live Load: Live loads are gravity load produced by the used and occupancy of the building and do not include dead loads, construction load, or environmental loads such as wind and earthquake loadings are based on to UBC-97. live load on residential area = 40 lb/ft 2 live load on roof = 30 lb/ft 2 live load on stair case = 100 lb/ft 2 live load on landing area = 100 lb/ft 2 unit weight of water = 62.4 lb/ft 3 2. 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 = 172 ft Method used = Normal Force Method Windward coefficient = 0.8inward Leeward coefficient = 0.5inward Importance Factor = 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 = S D Structure = Special Moment Resisting Frame Zone Factor, Z = 0.4 Importance Factor, I = 1.0 Response Modification Factor,R = 8.5 Seismic Response Coefficients, C a = 0.44 C v = 0.64 C t value = D. Load Combinations Design codes applied are AISC-LRFD 1999 and UBC-97. There are 18 numbers of load combinations which are used in the structural dynamic analysis DL +1.4SD DL + 1.2SD + 1.6LL DL + 1.2SD + LL + 1.6WX DL + 1.2SD + LL 1.6WX DL + 1.2SD + LL + 1.6WY DL + 1.2SD + LL 1.6WY DL + 1.2SD + 0.8WX DL + 1.2SD 0.8WX DL + 1.2SD + 0.8WY DL + 1.2SD 0.8WY DL + 0.9SD + 1.6WX DL + 0.9SD 1.6WX DL + 0.9SD + 1.6WY DL + 0.9SD 1.6WY DL + 1.4SD + 0.5LL + SPECX DL + 1.4SD + 0.5LL SPECX DL + 1.4SD + 0.5LL + SPECY DL + 1.4SD + 0.5LL SPECY III. DESIGN SECTIONS OF PROPOSED BUILDING FOR STATIC ANALYSIS This model is firstly analysed by static case. Design sections of beam and column for static analysis are shown in table 1 and 2. TABLE I: DESIGN SECTIONS FOR BEAM Column Section Column Section B1 W12x14 B13 W14x34 B2 W12x16 B14 W14x38 B3 W12x19 B15 W14x43 B4 W12x22 B16 W14x48 B5 W12x26 B17 W14x53 B6 W12x30 B18 W14x61 B7 W12x35 B19 W14x68 B8 W12x40 B20 W16x31 B9 W12x45 B21 W16x36 B10 W12x50 B22 W18x35 B11 W14x26 B23 W18x40 B12 W14x30 TABLE II: DESIGN SECTIONS FOR COLUMN Column Section Column Section C1 W12x65 C12 W14x99 C2 W12x72 C13 W14x109 C3 W12x79 C14 W14x120 C4 W12x87 C15 W14x132 C5 W12x96 C16 W14x145 C6 W12x106 C17 W14x159 C7 W14x61 C18 W14x176 C8 W14x68 C19 W14x193 C9 W14x74 C20 W14x211 C10 W14x82 C21 W14x233 C11 W14x90 C22 W14x257 IV. STABILITY CHECKING OF PROPOSED BUILDING FOR STATIC ANALYSIS In this study, the stability of the proposed building is checked in static analysis. Structural stability checkings are checking for overturning moment, checking for sliding, checking for storey drift, checking for torsional irregularity and checking for P- effect.

4 A. Checking for Overturning Moment For X-direction M y = lb-ft Total dead weight = lb Cumulated Center of mass in X direction, XCCM = ft Resisting Moment, M = 0.9xTotaldeadweightx XCCM = 0.9 x x = lb-ft Factor of safety = Resisting moment Overturning moment = 6.17 > 1.5 OK For Y-direction M x = lb-ft Total dead weight = lb Cumulated Center of mass in X direction, YCCM = ft Resisting Moment, M R = 0.9xTotaldeadweightx YCCM = 0.9 x x = lb-ft Factor of safety = Resisting moment Overturning moment = 9.59 > 1.5 OK B. Checking for sliding For X direction Sliding force, V x = lb Friction coefficient, μ = 0.25 Resistance due to friction = μ 0.9 Total dead weight = = l Factor of safety = 1.61 > 1.5 OK No Sliding occur in X direction Satisfactory For Y direction Sliding force, V y = lb Friction coefficient, u = 0.25 Resistance due to friction = μ 0.9 Total dead weight = = lb Factor of safety = 2.06 > 1.5 OK No Sliding occur in Y direction. C. Checking for Storey Drift UBC-97 Storey Drift Limitation T 0.7sec 0.02h (2% of story height) M M = 0.7 R s R = 8.5 (seismic zone 3 and 4) T = C t (H) 3/4 = (45) 3/4 = s < 0.7 s limit = 0.025h NYEIN MYINT THU, DR. ZAW MIN HTUN Sto r-ey TABLE III: CHECKING FOR STORY DRIFT Height(in) sx sy Mx My (in) (in) (in) (in) limi t (in) SR GL D. Checking for torsional irregularity For point 16, Drift X = in, Drift Y = in For point93, Drift X = in, Drift Y = in For X direction Maximum Drift Ratio, max = in Average displacement of two points, avg 2 = in max avg = 1.06 < 1.2 OK For Y direction Maximum Drift Ratio, max = in Average displacement of two points, avg 2 = in max avg = 1.08 <1.2 OK Therefore, torsion irregularity does not exist in the building. E. Checking For P- Effect

5 Comparative Study on K and V-Inverted Bracings in Steel Irregular-Shaped Building For SMRF buildings, P- effects need to be considered C9 W14X90 whenever the storey drifts do not satisfy the following C10 W14X99 criterion. C11 W14X109 C12 W14X120 For X direction C13 W14X132 Drift ratio = s/h C14 W14X145 s/h 0.02/R C15 W14X / /8.5 C16 W14X OK C17 W14X211 b) For Y direction C18 W14X233 Drift ratio = s/h C19 W14X257 s/h 0.025/R C20 W14X / /8.5 C21 W14X OK C23 W14X342 V. DESIGN SECTIONS OF PROPOSED BUILDING IN C24 W14X370 DYNAMIC ANALYSIS (RESPONSE SPECTRUM VI. COMPARISON OF ANALYSIS RESULTS BY CASE) DYNAMIC ANALYSIS (RESPONSE SPECTRUM In this study, model A and B are analysed with K and V- CASE) inverted bracings respectively. And then, structural member Analysis results of model A and model B by using sections of model A and B are shwon in table 4 and 5. There response spectrum case are compared in this paper. are 30 numbers of K-bracings and 26 numbers of V-inverted Comparison of storey drift, storey shear and storey moment bracings which are used in response spectrum case. for model A and B are described in figures 7, 8, 9, 10, 11 and TABLE IV: DESIGN SECTIONS OF MODEL A 12. Column for model A Section Bracing for Section model A C1 W12X27 BR1 W12x120 C2 W12X79 BR2 W14x120 C3 W12X87 BR3 W14x132 C4 W12X96 BR4 W14x145 C5 W14X61 BR5 W14x159 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15 C16 C17 W14X68 W14X74 W14X82 W14X90 W14X99 W14X109 W14X120 W14X132 W14X145 W14X176 W14X193 W14X211 TABLE V: DESIGN SECTIONS OF MODEL B Column for model B Section Bracing for model B Section C1 W12X27 BR1 W12x120 C2 W12X79 BR2 W14x120 C3 W12X87 BR3 W14x145 C4 W12X96 BR4 W14x159 C5 W14X61 C6 W14X68 C7 W14X74 C8 W14X82 A. Comparison of storey drift Comparisons of storey drifts in X and Y directions for model A and model B are shown in Figure 7 and 8 respectively. Figure 7. Comparison of storey drift in X direction.

6 NYEIN MYINT THU, DR. ZAW MIN HTUN Figure 10. Comparison of storey shear in Y direction C. Comparison of storey moment Figure8. Comparison of storey drift in Y direction According to above figures, it can be found that the storey drift is critical at storey 10 for both directions. The maximum storey drift of model B in X-direction is 97% more than that of model A. The maximum storey drift of model B in Y- direction is 83% more than that of model A. Figure 11. Comparison of storey moment in X direction. B. Comparison of storey shear Comparisons of storey shears in X and Y directions for model A and model B are described in Figure 9 and 10 respectively. In the figure 9 and 10, the maximum storey shear can be found at GL(ground level) for both directions. The maximum storey shear of model B in X-direction is 99% more than that of model A. The maximum storey shear of model A in Y-direction is 39% more than that of model B. Figure12. Comparison of storey moment in Y direction Comparisons of storey moments in X and Y directions for model A and model B are shown in Figure 11 and 12 respectively. In figure 11 and 12, the storey moment values in X and Y-directions are critical at GL(ground level). The maximum storey moment of model B in X-direction is 22% more than that of model A. The maximum storey moment of model B in Y-direction is 99% more than that of model A. Figure 9. Comparison of storey shear in X direction. VII. DISCUSSION AND CONCLUSION In this study, the proposed building is analyzed and designed for the superstructure. The design of superstructure for wind speed 80 mph was done. The structure is located in seismic zone-4 and static procedure was analyzed according to UBC- 97. The stability checkings such as P- Δ effect, story drift, overturning moment and sliding were also checked in the design calculation. K-type bracings and V-inverted bracings are used for lateral stability in response spectrum case. It is found that there is an upward trend in storey shear from SR(stair roof ) to GL(ground level). There is also an upward trend in storey moment from SR(stair roof) to GL(ground level). Thus, maximum storey shear and storey moment can be occurred at GL(ground level). According to figure 7, 8, 9 and 10 the maximum storey drift and the maximum storey shear can be occurred in X-direction.

7 Comparative Study on K and V-Inverted Bracings in Steel Irregular-Shaped Building According to figure 11 and 12, the maximum storey moment can be found in Y-direction. Storey drift, storey shear and storey moment in model A are less than those of model B. Column sections of model A are less than those of model B. So, K bracing is a more suitable choice than V-inverted bracing for proposed building. VIII. ACKNOWLEDGEMENT The author wishes to express her deep gratitude to his Excellency, Minister Dr. Ko Ko Oo, Ministry of Science and Technology, for opening the Master of Engineering course at Mandalay Technological University. The author is very thankful to Dr. Myint Thein, Pro-Rector of Mandalay Technological University, for his in valuable permission and kind support in carrying out this research work. The author wishes to record her thanks to Dr. Kyaw Moe Aung, Associate Professor and Head, Department of Civil Engineering, Mandalay Technological University, for his guidance, suggestions and necessary advice. The author is deeply indebted to her supervisor, Dr. Zaw Min Htun, Lecturer, Department of Civil engineering, Mandalay Technological University, for his careful guidance, necessary advice and encouragement. The author also wishes to thank all her friends for their helps and advices on her studying. Finally, the author would like to express grateful thanks to all teachers and parents for their supports, kindness and unconditional love. IX. REFERENCES [1] Charles G. Salmon, John E. Johnson, "Steel Structures Design and Behavior, 3rd Ed. [2] Roger L. Brockenbrough, Frederick S. Merritt: Structural Steel Designer s Handbook, 3 rd Edition., New York; Mc Graw Hill Co. Inc., (2007). [3] Uniform Building Code, Volume 2. "Structural Engineering Design Provisions". 1997, 8th Ed. International Conference of Building Officials. [4] Jack C. Mc Cormac and James K. Nelson, Jr. Structural Steel Design LRFD Method, 3 rd ed.