Stability Analysis of A L-Shaped Building with Mat Foundation Under Soil-Structure Interaction Approach NAW THAW THI OO 1, DR.

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1 ISSN Vol.03,Issue.08, May-2014, Pages: Stability Analysis of A L-Shaped Building with Mat Foundation Under Soil-Structure Interaction Approach NAW THAW THI OO 1, DR. KAY THWE TUN 2 Abstract: This paper presents the stability analysis of a tall building with mat foundation over sandy soil under soil-structure interaction approach. Selected building for this study is superstructure of a eight-storeyed reinforced concrete hotel belong to seismic zone four. ETABS software is used for the analysis of the superstructure. Most of the civil engineering structures involve some type of structural element with direct contact with ground. When the external forces, such as earthquake, act on these systems, neither the structural displacements nor the ground displacement, are independent of each other. The process in which the response of the soil influences the motion of the structure and the motion of structure influences the response of the soil is termed as soil-structure interaction (SSI). All reinforced concrete members are designed with ultimate strength design basis using ACI Load consideration and checking of the stability of superstructure are based on UBC 97.From superstructure analysis, both fixed based and spring based foundation results are described for interaction such as different storey drift, torsional irregularity and overturning moment. The structure loads are so high and the soil conditions are sandy soil that spread footing would be large. If spread footing would cover more than 50% of the building footprint area, a mat foundation will be more economical. So, mat foundation is used for substructure. Mat foundation is designed by SAFE software. Punching shear capacity ratio is check, soil liquefaction and immediate settlements are checked for the stability of the substructure. Keywords: Design Code: ACI , ETABS Software, Seismic Code: UBC-97, SAFE Software, Eight-ed R.C Building. I. INTRODUCTION The rapid growth of the urban population and the consequent pressure on limited space has considerably influence city residential development. Nowadays, developing technology, urban most area are been taking place with tall building. From a structural stand point, one of the major distinguishing characteristics of high-rise building is the need to resist large lateral forces due to wind or earthquake. Moreover, irregular buildings have been widely used during these days. Irregular buildings have significant physical discontinutics in configuration or in their lateral force resisting systems. Therefore, in constructing any building, it should be designed to resist not only gravity loading but also horizontal loading. High wind pressure on the sides of tall building produced overturning moments. The substructure, or foundation, is the part of a structure that is usually placed below the surface of the ground and that transmits the load to the underlying soil or rock. The gravity and lateral forces on the building's structural behavior and on the soil-structure interaction forces. Because of its height, the loads transmitted by the columns in a proposed building can be very heavy. So, it is necessary to consider foundation settlement and soil-structure interaction. On the selection of a suitable foundation system for a proposed building, various factors must be taken into consideration. Among them are soil conditions, load transfer pattern, shape and size of the building, site constraints and/or services, environmental issues, etc. There are two basic types of foundations; Shallow foundation and Deep foundation. In order to determine which foundation is the most economical, engineer must consider the superstructure load, the subsoil condition and the desired settlement. For high-rise structure, a computer analysis may be the most economical and saving means to obtain a reliable design. II. PREPARATION FOR PROPOSED BUILDING A. Superstructure for Proposed Building Tall building is a relative matter, and tall buildings cannot be defined in specific terms related just to height or to the number of floors. The tallness of a building is a matter of a person's or community's circumstance and their consequent perception; therefore, a measurable definition of a tall building cannot be universally applied. From the structural engineer's point of view, a tall building may be defined as one that, because of its height, is affected by lateral forces due to wind or earthquake actions to an extent that they play an important role in the structural design. There are three kinds of loading considered in the design (see fig 1). For superstructure, the investigations will be considered including P-Δ effect whether the selected structure is suitable 2014 SEMAR GROUPS TECHNICAL SOCIETY. All rights reserved.

2 NAW THAW THI OO,DR. KAY THWE TUN or not in the lateral force resisting system. drift is the lateral displacement of one level of a structure relative to the level above or below. drift should be determined using the maximum inelastic response displacement, Δ M, which is defined as the maximum total drift or storey drift caused by the design level earthquake. For the stability of the structure, it is also necessary to resist the lateral earth pressure together with dead and live load. Where, q all = allowable bearing capacity of supporting soil S.F= safety factor D. Settlement of Mat Foundations and liquefaction Settlement occurs, when the loads are applied to the soil. The vertical download is usually the greatest load acting on foundations and the resulting vertical download movement is usually the largest and most important movement. This vertical download movement is called settlement. Only immediate settlement is considered. Immediate settlement can be computed by an equation as follow: Fig1. 3D View of Proposed Building. B. Bearing Capacity of Soil Allowable bearing capacity is defined as the ultimate bearing capacity divided by a factor of safety. It is also the maximum pressure which may be applied to the soil. There are several methods for calculation of bearing capacity of soil. Among them, bearing capacity is calculated by using three methods. They are Meyerhof's method, Hansen's method and Vesic's method. C. Mat Foundation and Modulus of Sub-grade Reaction Mat foundation is primarily shallow foundations. Mat foundation is a thick reinforced concrete slab which supports all the loads-bearing walls and column loads of a structure or a large portion of structure. Mat foundation is required when the loads are heavy and the soil is very weak or highly compressible. So, mat foundation may be used where the soil has a low bearing capacity and the column loads are so large that more than 50 percent of the area is covered by conventional spread footings. The modulus of sub-grade reaction is a conceptual relationship between soil pressure and deflection that is widely used in the structural analysis of a foundation member. The modulus of sub-grade reaction represents the elastic stiffness of the soil. The higher value, the stiffer will be the soil. III. ANALYSIS AND DESIGN OF SUPERSTRUCTURE A. Data of the Proposed Building The data of the proposed building are shown by the following items. Type of structure Type of occupancy Size of Buildings Shape of buildings Height of structure : Eight storey R.C building : Hotel : Maximum Length = 108 ft : Maximum width = 65 ft : L- shape : Overall height ft Base to Ground floor - 12 ft Typical Floor height - 10 ft B. Material Properties Material properties used for the proposed structure are as follows: 1. Analysis Properties Data Weight per unit volume for concrete = 150 pcf Modulus of elasticity = 3122 Ksi Poisson's ratio = 0.2 Coefficient of thermal expansion = in/in/. F 2. Design Property Data Bending Reinforcement yield stress (fy) = psi Shear Reinforcement yield stress (fy) = psi Crushing strength concrete, (fc ) = 3000 Psi C. Loading Considerations Loads applied to structural members may consist of the following, alone or in combination of dead, live, earth pressure, wind, or earthquake loads and constraining forces. Loading consideration is according to Uniform Building Code There are three kinds of loading considered in the design which are gravity load, wind load and earthquake loads. 1. Gravity Loads Unit weight of concrete = 150 pcf 4½ inches thick wall weigh = 55 pcf (Including plaster) 9 inches thick wall weigh = 100 pcf (Including plaster) Weight of ceiling& finishing = 25 psf Unit weight of water = 62.4pcf k = q/ω (1) Live load on residential area = 40 psf Where, Live load on stairs = 100 psf q = bearing capacity of supporting soil Live load on roof = 20 psf ω= settlement For 1 in settlement, k = 1 2 S.F q all

3 Stability Analysis Of A L-Shaped Building With Mat Foundation Under Soil-Structure Interaction Approach 2. Lateral Loads TABLE II: Drift In Y-Direction (Spring Based Data for wind loads which are used in structural analysis are as follows Exposure Type = Type C Basic wind velocity = 100 mph SR Effective Height = 92 ft R Importance factor I w = 1.0 7Floor Pressure coefficient C q = 0.8 for windward 6Floor for leeward 5Floor Data for earthquake load are as follows; 4Floor Seismic zone = Zone 4 3Floor Soil profile type = S D 2Floor Seismic zone factor = 0.4 1Floor Structural system = Dual System GFloor Importance factor, I = 1 Response modification factor, R = 8.5 TABLE III: Drift in X-Direction (Fixed Based D. Coefficient of Sub-grade Reaction In the performance of the analysis for the structural design of soil-foundation interaction approach, it is required to know the principal of evaluating the coefficient of sub-grade reaction, k. If a foundation of width B is subjected to a load per unit area of q, it will undergo a settlement,. The coefficient of subgrade modulus, k, can be defined as k = 12 S.F q all = 12 x 2.5x 1.55x2.24 = K/ft 2 -ft E. Analysis and Design Results of Superstructure The design results for members are carried out 38 types of load combinations based on ACI (318-02).Beam sections are 10x14,10x16, 10x18, 12x18, 14x18, 14x20,14x24 and16x20. Column section for proposed building is 24x24, 20x20, 18x18, 16x16 and 14x14. The beam and column sizes are in fixed based foundation. The beam sizes are a little change in spring based foundation. Column sizes are not change. F. Structural Stability Consideration Checking for the structural stability such as P-Δ effect, storey drift, torsional irregularity, overturning moment and sliding check is expressed in this article. 1. Check for Drift TABLEI: Drift In X-Direction (Spring Based SR R Floor Floor Floor Floor Floor Floor Floor GFloor SR R Floor Floor Floor Floor Floor Floor Floor GFloor TABLE IV: Drift in Y-Direction (Fixed Based SR R Floor Floor Floor Floor Floor Floor Floor GFloor Check for Overturning Moment The distribution of earthquake forces over the height of a structure causes structure to experience overturning effects. For fixed based foundation, Overturning moment = k-ft X-direction centre of mass = ft Resisting moment =0.9 Total Dead Weight XCCM = k-ft

4 Factor of Safety = NAW THAW THI OO,DR. KAY THWE TUN >1.5 OK = 5.107> 1.5 OK Overturning moment = kip-ft Y-direction centre of mass = ft Resisting moment =0.9 Total Dead Weight YCCM = Factor of Safet y= > 1.5 OK = > 1.5 OK For spring- based foundation, Overturning moment = k-ft X-direction centre of mass = ft Resisting moment = 0.9 Total Dead Weight x XCCM = k-ft Factor of Safety = =9.299 > 1.5 OK > 1.5 OK Overturning moment = kip-ft Y-direction centre of mass = ft Resisting moment = 0.9 Total Dead Weight YCCM = Factor of Safety = =9.86 > 1.5 OK 3. 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 For spring based foundation, Sliding Force, V x = k ips = kips Safety Factor = > 1.5 OK Sliding Force, V y = kips = kips Safety Factor =3.824> 1.5 OK For fixed based foundation, Sliding Force, V x = kips = kips Safety Factor = > 1.5 OK Sliding Force, V y = kips = kips Safety Factor = > 1.5 OK 4. Check for torsional irregularity TABLE V: Torsional Irregularity At PT 1(Fixed Based Foundation ) Displ;X Displ; Y Drift X Drift Y SR R Floor Floor Floor Floor Floor Floor Floor GFloor TABLE VI: Torsional Irregularity at PT 28 (Fixed Based Displ;X Displ;Y Drift X Drift Y SR R Floor Floor Floor Floor Floor Floor Floor GFloor TABLE VII: Torsional Irregularity at PT 1 (Spring Based Displ; X Displ; Y Drift X Drift Y SR R Floor Floor Floor Floor Floor Floor Floor GFloor

5 Stability Analysis Of A L-Shaped Building With Mat Foundation Under Soil-Structure Interaction Approach TABLE VIII: Torsional Irregularity at PT 28 (Spring IV. ANALYSIS AND DESIGN OF SUBSTRUCTURE Based A. Type of Foundation Displ; Displ; In general, foundations may be classified based on where Drift X Drift Y X Y the load is carried by the ground producing: Shallow SR foundations the depth is generally D/B 1 but may be somewhat more. If satisfactory soil directly underlies the R structure, it is merely necessary to spread the load by footing or other means. Such substructures are known as spread 7Floor foundations and types of spread foundations are wall footing, isolated or single column footing, strip footing and mat or 6Floor raft foundation. 5Floor Floor Floor Floor Floor GFloor From the above result and fig 2, for fixed based foundation in X-direction, Δ max is obtained then Δ avg is calculated. The ratio of Δ max to Δ avg is compared with limitation (1.2). In Y-direction, Δ max is obtained then Δ avg is calculated. The ratio of Δ max to Δ avg is compared with limitation (1.2). The ratio is not exceeding limit, torsional irregularity does not exist in the building. From the above result, for fixed based foundation in X-direction, Δ max is obtained then Δ avg is calculated. The ratio of Δ max to Δ avg is compared with limitation (1.2). In Y-direction, Δ max is obtained then Δ avg is calculated. The ratio of Δ max to Δ avg is compared with limitation (1.2). The ratio is not exceed limit, torsional irregularity does not exist in the building. Fig2. Selection point (Point 1&28) of Torsional Irregularity Check. Deep foundation; drilled piers of drilled caissons, where L p /B 4. If adequate soil is not found immediately below the structure, it becomes necessary to use deep foundations to transmit the load to deeper firmer layers. B. Mat Foundation The second type of shallow foundation is the mat foundation. A mat is essentially a very large spread footing that usually encompasses the entire footprint of the structure. They are also known as raft foundation. It is common to use mat foundations for deep basements both to spread the column loads to a more uniform pressure distribution and to provide the floor slab for the basement. Mat foundations are sometimes preferred for soils that have low load-bearing capacities, but that will have to support high column or wall loads. C. Allowable Bearing Capacity Calculation The bearing capacity is calculated by three methods. Among them, the smallest value is used. TABLE IX: Bearing Capacity of Two Bore Hole Bore Hole Meyerho f Method (ton/ft 2 ) Hansen Method (ton/ft 2 ) Vesic Method (ton/ft 2 ) BH BH After calculating, the allowable bearing capacity is taken the minimum value from three methods. So, the average allowable bearing capacity is 1.55 ton/ft 2 from Meyerhof Method. D. Analysis and Design of Substructure Mat foundation for proposed building was analyzed and designed by using SAFE software. Assumed data for the calculation of mat foundation are as follows: Modulus of elasticity = 3122 ksi Poisson's ratio = 0.2 Unit weight of concrete = 150 pcf Thickness of mat = 3' X - cover Top = 3" Y - cover Top = 3" X - cover Bottom = 4" Y - cover Bottom = 4" Allowable bearing capacity = 1.55ton/ft 2 Modulus of subgrade reaction =104.16K/ft 2 - ft

6 NAW THAW THI OO,DR. KAY THWE TUN Projection beyond main building Crushing strength of concrete (f c ') Shear reinforcement yield stress (f y ) = 6 ft = 3000 psi = psi E. Design Results of Substructure After designing mat foundation (3 ft) with SAFE software, the resulted punching shear capacity ration check diagram is shown below fig Allowable settlement is 2in. So, settlement will not occur in the building because total immediate settlement mat foundation is between limitations. Fig3. Punching Shear Capacity Ratios F. Check for Settlement of Foundation Checking for settlement of proposed building foundation are shown in Table (10) TABLE X: Settlement of Proposed Building Foundation Points Settlements (in) V. DISCUSSIONS AND CONCLUSIONS In this study, performance of proposed buildings located in seismic zone 4 is checked, in terms of stabilities and strengths. Translation springs in vertical is used to model base restraint. ETABS software is used as a design aid for numerical completion of analysis of the model. Equivalent static analysis procedure is used for considering of the lateral loads such as wind, earthquake and design of superstructure is UBC - 97 and (ACI ). For earthquake resistance design, minimum required concrete strength is 3000 psi according to ACI Code. Then, the analysis and design of superstructure is checked. And then, the base reactions resulting the analysis and design of superstructure are used in foundation design. Considering in the stability analysis, storey shear of the spring base building with soil-structure interaction approach is not equal that of the building with fix base analysis. According to the finding from this study storey drift of spring base foundation is not same the storey drift of fix base foundation. Torsional irregularity is different. In which, the ratio of displacement of the buildings with spring base foundation is different that building with fix base analysis. According to the analysis results in this study the overturning moment of the building with spring base foundation is larger than that of fix base analysis. The sliding force of the building is also larger than that of the fix base analysis. P-Δ effects need not to be considered because of under limitation. Bearing Capacity estimation is based on equations proposed by Meyerhof, Hansen, and Vesic. It is found that bearing capacity estimates based on Meyerhof values are the smallest. For proposed structure, allowable bearing capacity is taken from Meyerhof. Bearing capacity is estimated for soil layer at foundation level. For the foundation of tall buildings, mat foundations are mostly used in Myanmar. Mat foundation is a suitable type of foundation for the structure with enough available space and bearing capacity of the soil. The substructure was analyzed by SAFE software. Then, the upward pressure caused by load

7 Stability Analysis Of A L-Shaped Building With Mat Foundation Under Soil-Structure Interaction Approach combinations is checked. Foundation soil in spring stiffness calculation is presented. If borehole data are available for corresponding influenced zone specially, compatible soil springs can be predicted as closely as the real soil performance. If it exceed the allowable bearing pressure of soil, the projection and thickness of the mat slab is increased and the slab is analyzed again. The increase in foundation area and thickness reduces the soil pressure. So, the mat slab is tried with various thickness and projection to meet the required strength of soil. The punching shear capacity ratio is checked whether it is less than 1 or not. For settlement checking, maximum settlement of mat foundation is obtained by SAFE software. VI. ACKNOWLEDGEMENTS The author is deeply indebted to his Supervisor, Dr. Kay Thwe Htun, Associate Professor of Civil Engineering Department, Technological University (Mandalay) for her 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 his deepest gratitude to teachers from Technological University (Mandalay) for their help and share of experience. Finally, the author would like to thanks his parents for his years of help and support, encouragement to attain his destination without any trouble throughout his life. VII. REFERENCES [1] Bowles, Joseph E: 1996, Foundation Analysis and Design, 5 th ed. Mc Graw-Hill Companies, Inc. [2] BRAJA M. DAS: 2002, Principles of Foundation Engineering, 5 th ed. PWS publishing. [3] Bryan S.S. and Alex C.: 1999, Tall Building Structures: Analysis and Design, Jhon Willy and Sons, Inc. [4] Jhon, P. W.: 1985, Dynamic Soil-Structure Interaction, Printice-Hall, Inc., Newjercy, USA. [5 ] Nilson, A.H. 1997, Design of Concrete Structure, 12 th ed. Singapore: Mc Graw-Hill Companies, Inc.