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1. Structural Design 1.1. Introduction Structural design aims to design a safe structure against expected loads to come in the life period of the structure due to the dead weights of the structural components, occupancies, wind, earthquakes, snow, settlements etc. This phase consists of mainly two stages, first is the Analysis, and next is the Design and detailing of the structure and structural elements to achieve the safest, economically viable at par with the latest Nepal Building Code and in some part of the design preferred Indian Standard codes. The work will be comprised of following components: a) Review of the Architectural drawing b) Structural Analysis and design and detailing with standard commercial Finite element based software. The Structural analysis method adopted is linear- static analysis. Nepal lies in seismically the active zone. So there is always the risk of large earthquakes. However the Earthquake Resistant Design doesn’t mean to design a damage-free structure against earthquake shaking, but has the following basis: o o o
Minor Earthquakes: No damage at all Moderate Earthquakes: Non-structural elements may get damaged, but Structural Elements are not yet affected. Major Earthquakes: Structural Elements may also be damaged, but collapse is prevented, thereby, saving the lives and properties.
Thus the structure is so designed as to prevent sudden collapse and remain serviceable against the design shaking as per codal requirements. This is achieved by performing the proper analysis for seismic loads, and proper design, maintaining the ductility in the structure. The Earthquake resistant design is done for the structure performing the Seismic-Coefficient as well as Dynamic Analysis. Following assumptions have been made: a. Earthquake cause impulsive ground motion which is complex and irregular in character, changing in period and amplitude each lasting for small duration. b. Earthquake is not likely to occur simultaneously with wind. Structure design and analysis has been performed using Etab2015, one of the most popular commercial structural analysis software. We have found these tools as user-friendly & reliable. In spite of their efficiency and reliability, we don’t hesitate to accept and realize that there is always space for improvements.
1.2. Structural Arrangement The structural system adopted in the building is the ductile moment frame system. The structural system is believed to be preformed best under seismic event. To ensure the ductile response of the building during the seismic event the overall structure is detailed according to the latest code provision (SP 34). To ensure the better response of the structure ductile detailing of the joints has been adopted to protect the failure against the severe impulsive dynamic loads caused by seismic events.
1.3. Analysis To represent the structural system correctly, the most simple to the most sophisticated design approach has been adopted as per requirement. Detailed finite element model of the structure is created using the latest software and evaluation of the system is conducted at different level of complexity. 1.4. Software For the Analysis, Etab15 software is used to design and detailing. 1.5. Codes a. Materials: 1. IS 1983 2. Brick: (IS 1077, IS 2212) 3. Cement: ( IS 8112, IS 12269) 4. Admixtures: (ASTM C 494/C,494/M, IS9103) 5. Reinforcing Steel: (IS 432, IS 1139, IS 1786) b. Loadings: 1. Dead Loads: (NBC 102, IS 875 part I) 2. Live loads: (NBC 103, IS 875 part II) 3. Wind loads: (NBC 104) 4. Seismic loads: (NBC 105) c. Design Codes: 1. Reinforced concrete: (NBC 110, IS 456, SP 16) 2. Ductile detailing: (IS 13920, SP34)
1.6. General Structural Arrangement and Design Procedure 1.6.1. Structural Frame: The structural frame consists of RCC columns and beams. Beams are provided at plinth level and floor level and it is assumed that there will be no beam in the staircase landing level. The beams are provided at each level to form a closed rectangular geometry. The partitions are of full and half brick masonry. The frame system is designed for gravity loads and seismic loads. 1.7. Determination of Seismic forces The seismic loads are calculated using seismic coefficient method for this structure. The basic seismic coefficient 0.08, seismic zoning factor (Z) = 1.0, Importance factor = 1.0, Structural performance factor = 1 is considered . 1
II. Design Method: Limit State Design III. Analysis Method 3D- SMRF IV. Analysis and Design Tool: Etab-15. V. Load Combinations for Analysis and Design of concrete frame [NBC 105-1994] a. 1.5(DL+IL) (DCON 2) b. DL+(1.3IL±1.25EL) (DCON 3, DCON 4, DCON 5, DCON 6) c. 0.9DL±1.25EL (DCON 7, DCON 8, DCON 9, DCON 10) As per cl.22.214.171.124 of IS1890:2000 The following Nine load combination is cross checked in the analysis.
1.5 ( DL + IL )
1.2 ( DL + IL + ELx )
1.2 ( DL + IL - ELx )
1.2 ( DL + IL + ELy )
1.2 ( DL + IL - ELy )
1.5 ( DL + ELx )
1.5 ( DL - ELx )
1.5 ( DL + ELy )
1.5 ( DL - ELy)
Where : DL
Earthquake Load n
( Once in +ve x and y dir ) ( Once in -ve x and y dirn )
3. Structural Loadings The building frames have been designed for various loads viz, dead load, live load (or imposed load) and lateral load (earthquake load). 3.1 Dead load Dead load has been assumed to be produced by slab, beams, columns, walls, parapet walls, staircase, and water tank. The weight of building materials are taken as per NBC 102 & IS: 875(Part 1)-1987).
Specific weight of materials [Ref: IS: 875(Part 1)-1987)] Materials
Common Burnt Clay Brick Masonry
18.85 ~19.00 KN/m³
Floor Finishing (Screeding & punning)
Cement Sand Plaster
Floor finishing (Marble)
3.2 Live load This load is assumed to be produced by the intended use or occupancy of a building, including the weight of movable partitions, distributed, concentrated loads, load due to impact and vibration, and dust load but excluding wind, seismic, snow and other loads due to temperature changes, creep, shrinkage, differential settlement, etc. The magnitude of live load depends upon the type of occupancy of the building. These are to be chosen from codes as NBC 103, IS 875:1987 (part 2) for various occupancies.
Imposed Floor Loads for Floors (Clauses 3.1, 3.1.1 and 4.1.1, IS: 875-1987, Part 2)
Uniformly Distributed loads (UDL), KN/m2
Toilets and Bathrooms
Corridors, passages, staircases including fire escapes and store rooms
Roofs Flat, sloping or curved roof slopes up to and including 10 degrees a) Access provided
b) Access not provided except for maintenance
Vertical loads on the beams due to Brick masonry a)
Full Brick, 230 mm thick wall Sample calculation 9” thick Wall of ht. 8’ with ½” finishing both sides, Overall height= 2.5 m Clear height = 2.5 m – 0.35 m =2.15 m Wt. density = 19 KN/m3 UDL without opening = (0.23+0.025)m*(2.5-0.35)m*19kN/m3 = 10.5 kN/m
Half brick wall Sample calculation 9/2” thick Wall of ht. 8’ with ½” finishing both sides, Overall height= 2.5 m Clear height = 2.5 m – 0.35 m =2.15 m Wt. density = 19 KN/m3 UDL without opening = (0.23/2+0.025)m*(2.5-0.35)m*19kN/m3 = 5.5 kN/m
3.4 Earthquake load This load on a structure depends on the site of the structure, maximum earthquake intensity or strong ground-motion and the local soil, the stiffness design and construction pattern, and its orientation in relation to the incident seismic waves. Building experience horizontal distortion when subjected to earthquake motion. So, buildings should be designed with lateral force resisting system. For designing purpose, the resultant effects are usually represented by the horizontal and vertical seismic coefficients α h & α v. Alternatively, a dynamic analysis of the building is required under the action of the specified ground motion or design response spectra. Since the probable maximum earthquake occurrences are not frequent, buildings are designed for such earthquakes so as to ensure that they remain elastic and damage-free may not be feasible, instead reliance is placed on kinetic energy dissipation in the structure through plastic deformation of elements and joints. The design forces are reduced accordingly. Thus, the main philosophy of a seismic design is to reduce collapse of structure rather than a damage free structure. To achieve a greater degree of protection, the critical and important buildings are designed for higher seismic factors by using an Importance Factor I, (NBC 105).
Methods adopted in this analysis for calculation of Earthquake loads: a. Seismic coefficient Method Design Base Shear VB = Cd *W
= Design horizontal seismic coefficient as per 8.1.1 NBC 105 =CZIK Z=
Zone Factor = 1for Kathmandu
Importance Factor =1 for normal Building
Basic seismic coefficient.
Structural performance factor = 1.0 for SMRF
0.09 h/√d (7.3 NBC 105]. For moment resisting frame with infill panel
Height of building (m) above ground level
Base dimension of building at the plinth level in m, along the considered direction of lateral force.
Seismic Weight of Building, that includes total dead load plus appropriate amount of live load. - Percentage of live load to be taken for calculating seismic weight =25% for live load intensity upto and including 3.0 KN/m2 and 50% for live load intensity above 3.0 KN/m2. -the live load on roof need not be considered for calculating the seismic weight of the building.
3.5 Storey Drift: The storey drift on any storey due to the specified design lateral force, with partial load factor of 1.0, is not exceeding 0.004 times the storey height.