Shell structure.pdf
- 2. INTRODUCTION A SHELL STRUCTURE IS A THIN CURVED MEMBRANE OR SLAB USUALLY OF REINFORCED CONCRETE THAT FUNCTIONS BOTH AS STRUCTURE AND COVERING. THE TERM “SHELL” IS USED TO DESCRIBE THE STRUCTURES WHICH POSSESS STRENGHT AND RIGIDITY DUE TO ITS THIN, NATURAL AND CURVED FORM SUCH AS SHELL OF EGG, A NUT, HUMAN SKULL, AND SHELL OF TORTISE.
- 4. APPLICATIONS Architecture and building. The development of masonry domes and vaults in the Middle Ages made possible the construction of more spatious buildings. In more recent times the availability of reinforced concrete has stimulated interest in the use of shells for roofing purposes. Power and chemical engineering. construction of suitable boilers. These thin shells were constructed from plates suitable formed and joined by riveting. Structural engineering. An important problem in the early development of steel for structural purposes was to design compression members against buckling. A striking advance was the use of tubular members in the construction of the Forth railway bridge in 1889: steel plates were riveted together to form reinforced tubes as large as 12 feet in diameter, and having a radius/thickness ratio of between 60 and 180. Vehicle body structures. The construction of vehicle bodies in the early days of road transport involved a system of structural ribs and non-structural paneling or sheeting. The modern form of vehicle construction, in which the skin plays an importatnt structural part, followed the introduction of sheet-metal components, preformed into thin doubly curved shells by large power presses, and firmly connected to each other by welds along the boundaries. The use of the curved skin of vehicles as a load bearing member has similarly revolutionized the construction of railway carriages and aircraft. In the construction Composite construction. The introduction of fiberglass and similar lightweight composite materials has impacted the construction of vehicles ranging from boats, racing cars, fighter and stealth aircraft, and o on. The exterior skin can be used as a strong structural shell. Miscellaneous Examples. Other examples of the impact of shell structures include water cooling towers for power stations, grain silos, armour, arch dams, tunnels, submarines, and so forth.
- 5. SINGLE OR DOUBLE CURVATURE SHELLS SINGLE CURVATURE SHELL: ARE CURVED ON ONE LINEAR AXIS AND ARE A PART OF A CYLINDER OR CONE IN THE FORM OF BARREL VAULTS AND CONOID SHELLS. DOUBLE CURVATURE SHELL: ARE EITHER PART OF A SPHERE, OR A HYPERBOLOID OF REVOLUTION. THE TERMS SINGLE CURVATURE AND DOUBLE CURVATURE DO NOT PROVIDE A PRECISE GEMOETRIC DISTINCTION BETWEEN THE FORM OF SHELL BECAUSE A BARREL VAULT IS SINGLE CURVATURE BUT SO IS A DOME. THE TERMS SINGLE AND DOULBE CURVATURE ARE USED TO DISTINGUISH THE COMPARITIVE RIGIDITY OF THE TWO FORMS AND COMPLEXITY OF CENTRING NECESSARY TO CONSTRUCT THE SHELL FORM. CONOID DOME HYPERBOLOID PARABOLOID BARREL VAULT
- 6. TYPES OF SHELL STRUCTURES: FOLDED PLATE SHELLS: THE DISTINGUISHING FEATURE OF THE FOLDED PLATE IS THE EASE IN FORMING PLANE SURFACES. A FOLDED PLATE MAY BE FORMED FOR ABOUT THE SAME COST AS A HORIZONTAL SLAB AND HAS MUCH LESS STEEL AND CONCRETE FOR THE SAME SPANS. THE PRINCIPLE COMPONENTS IN A FOLDED PLATE STRUCTURE CONSIST OF : 1) THE INCLINED PLATES 2) EDGE PLATES WHICH MUST BE USED TO STIFFEN THE WIDE PLATES 3) STIFFENERS TO CARRY THE LOADS TO THE SUPPORTS AND TO HOLD THE PLATES IN LIN 4) COLUMNS TO SUPPORT THE STRUCTURE IN THE AIR. FOLDED PLATE TRUSS Z SHELL TAPERED FOLDED PLATES CANOPIES THREE SEGMENT FOLDED PLATE
- 7. TYPES OF SHELL STRUCTURES: CYLINDRICAL BARREL VAULTS: BARREL VAULTS ARE PERHAPS THE MOST USEFUL OF THE SHELL STRUCTURES BECAUSE THEY CAN SPAN UPT O 150 FEET WITH A MINIMUM OF MATERIAL. THEY ARE VERY EFFICIENT STRUCTURES BECAUSE THE USE THE ARCH FORM TO REDUCE STRESSES AND THICKNESSES IN THE TRANSVERSE DIRECTION. MULTIPLE BARRELS - CORRUGATED CURVES UNSTIFFENED EDGES OUTSIDE STIFFENERS THE LAZY S
- 9. TYPES OF SHELL STRUCTURES: DOMES OF REVOLUTION: A DOME IS A SPACE STRUCTURE COVERING A MORE OR LESS SQUARE OR CIRCULAR AREA. THE BEST KNOWN EXAMPLE IS THE DOME OF REVOLUTION, AND IT IS ONE OF THE EARLIEST OF THE SHELL STRUCTURES. EXCELLENT EXAMPLES ARE STILL IN EXISTENCE THAT WERE BUILT IN ROMAN TIMES. THEY ARE FORMED BY A SURFACE GENERATED BY A CURVE OF ANY FORM REVOLVING ABOUT A VERTICAL LINE. THIS SURFACE HAS DOUBLE CURVATURE AND THE RESULTING STRUCTURE IS MUCH STIFFER AND STRONGER THAN A SINGLE CURVED SURFACE, SUCH AS A CYLINDRICAL SHELL. HALF SPHERE SPHERE SEGMENT DOMES - SQUARE IN PLAN
- 10. MOST SUITABLE MATERIAL THE MATERIAL MOST SUITED FOR CONSTRUCTION OF SHELL STRUCTURE IS CONCRETE BECAUSE IT IS A HIGHLY PLASTIC MATERIAL WHEN FIRST MIXED WITH WATER THAT CAN TAKE UP ANY SHAPE ON CENTERING OR INSIDE FORMWORK. SMALL SECTIONS OF REINFORCING BARS CAN READILY BE BENT TO FOLLOW THE CURVATURE OF SHELLS. ONCE THE CEMENT HAS SET AND THE CONCERETE HAS HARDENED THE R.C.C MEMBRANE OR SLAB ACTS AS A STRONG, RIGID SHELL WHICH SERVES AS BOTH STRUCTURE AND COVERING TO THE BUILDING.
- 11. TYPICAL SHELL ROOF FORM L<S/2 =S/6 =S/9
- 12. TYPICAL SHELL ROOF FORM W/8 W/5 R=W S/20 S/7
- 14. CONSTRUCTION OF BARREL VAULT THE BARREL VAULT IS THE MOST STRAIGHT FORWARD SINGLE CURVATURE SHELL CONSTRUCTION. IT IS THE PART OF A CYLINDER OR BARREL WITH SAME CURVATUREALONG ITS LENGTH. ANY NUMBER OF CONTINUOUS BARRELS OR CONTINUOUS SPANS ARE POSSIBLE EXCEPT THAT EVENTUALLY PROVISION IS MADE FOR THE EXPANSION OF THE JOINTS IN A LARGE STRUCTURES. THE BARREL VAULTS ARE USED AS PARKING, MARKET PLACE, ASSEMBLY HALL ,ETC. TYPES OF BARREL VAULTS 1. SHORT SPAN BARREL VAULTS 2. LONG SPAN BARREL VAULTS
- 15. BARREL VAULT SHELL ROOF SHORT SPAN BARREL VAULT SHORT SPAN BARREL VAULTS ARE THOSE IN WHICH SPAN IS SHORTER THAN ITS WIDTH. IT IS USED FOR THE WIDTH OF THE ARCH RIBS BETWEEN WHICH THE BARREL VAULT SPAN. LONG SPAN BARREL VAULT LONG SPAN BARREL VAULTS ARE THOSE IN WHICH SPAN IS LARGER THAN ITS WIDTH. STRENGTH OF THE STRUCTURE LIES AT THE RIGHT ANGLES TO THE CURVATURE TO THAT SPAN IS LONGITUDINAL TO THE CURVATURE. USUAL SPAN OF THE LONGITUDINAL BARREL VAULT IS FROM 12-30 M WITH ITS WIDTH BEING ABOUT 1/2 THE SPAN AND RISE IS 1/5 OF THE WIDTH. TO COVER LARGER AREAS MULTIBAY ,MULTI SPAN ROOFS CAN BE USED WHERE THE ROOF IS EXTENDED ACROSS THE WIDTH OF THE VAULT AS A MULTIBAY .
- 16. ROOF LIGHTS: TOP LIGHT CAN BE PROVIDED BY DECK LIGHT FORMED IN THE CROWN OF VAULT OR BY DOME LIGHT. THE DECK LIGHT CAN BE CONTINUOUS OR FORMED AS INDIVIDUAL LIGHTS.ROOF LIGHTS ARE FIXED TO AN UPSTAND CURB CAST INTEGRALLY WITH THE SHELL. ADVANTAGE OF THE SHELL IS THAT ITS CONCAVE SOFFIT REFELECTS AND HELPS TO DISPERSE LIGHT OVER AREA BELOW. DISADVANTAGE IS THAT TOP LIGHT MAY CAUSE OVER HEATING AND GLARE. ROOF COVERING: SHELLS MAY BE COVERED WITH NON-FERROUS SHEET METAL, ASPHALT, BITUMEN FELT, A PLASTIC MEMBRANE OR A LIQUID RUBBER BASE COATING. ROOF INSULATION: THE THIN SHELL OFFERS POOR RESISTANCE TO TRANSFER OF HEAT. THE NEED TO ADD SOME FORM OF INSULATING LINING ADDS CONSIDERABLY TO COST OF SHELL. THE MOST SATISFACTORY METHOD OF INSULATION IS TO SPREAD A LIGHT WEIGHT SCREED OVER THE SHELL. DIFFICULTIES OF PROVIDING INSULATION AND MAINTING THE ELEGANCE OF CURVED SHAPE MAKES THESE STRUCTURES LARGELY UNSUITED TO HEATED BUILDINGS IN TEMPERATE CLIMATE.
- 17. STIFFENING BEAMS AND ARCHES: UNDER LOCAL LOADS THE THIN SHELL OF THE BARREL VAULT WILL TEND TO DISTORT AND LOSE SHAPE AND EVEN COLLAPSE IF THE RESULTANT STRESSES WERE MORE. TO STRENGTHEN THE SHELL AGAINST THIS POSSIBILITY, STIFFENING BEAMS OR ARCHES ARE CAST INTEGRALLY WITH THE SHELL. THE COMMON PRACTICE IS TO PROVIDE A STIFFENING MEMBER BETWEEN THE COLUMN SUPPORTING THE SHELL. DOWNSTAND STIFFENING RCC BEAM IS MOST EFFICIENT BECAUSE OF ITS DEPTH, BUT THIS INTERRUPTS THE LINE OF SOFFIT OF VAULTS, FOR THIS UPSTAND STIFFENING BEAM IS USED. THE DISADVANTAGE OF UPSTAND BEAM IS THAT IT BREAKS UP THE LINE OF ROOF AND NEED PROTECTIONS AGAINST WEATHER.
- 18. EDGE AND VALLEY BEAMS: DUE TO SELF WEIGHT AND IMPOSED LOAD THE THIN SHELL WILL TEND TO SPREAD AND ITS CURVATURE FLATTEN OUT. TO RESIST THIS RCC EDGE BEAMS ARE CAST BETWEEN COLUMNS. EDGE BEAMS MAY BE CAST AS DROPPED BEAMS OR UPSTAND BEAMS OR PARTIALLY AS BOTH. IN HOT CLIMATE THE DROPPED BEAM IS USED WHEREAS IN TEMPERATE CLIMATE UPSTAND BEAM IS USED TO FORM DRAINAGE CHANNEL FOR RAIN WATER. IN MULTI-BAY STRUCTURES, SPREADING OF THE VAULTS IS LARGELY TRANSMITTED TO THE ADJACENT SHELLS, SO DOWN STAND AND FEATHER VALLEY BEAM IS USED.
- 19. EXPANSION JOINTS: THE CHANGE IN TEMPERATURE CAUSES THE EXPANSION AND CONTRACTION IN CONCRETE STRUCTURES, WHICH CAUSES THE STRUCTURES TO DEFORM OR COLLAPSE. TO LIMIT THIS CONTINUOUS EXPANSION JOINTS ARE FORMED AT THE INTERVAL OF ABOUT 30M, ALONG THE SPAN AND ACROSS THE WIDTH OF THE MULTI-BAY AND MULTI-SPAN BARREL VAULT ROOFS. LONGITUDINAL EXPANSION JOINTS ARE FORMED IN A UP STAND VALLEY.
- 28. CATENARY SHELL • Cheesecloth and plaster shells were dried upside down creating forms in pure tension. When flipped, shells rest in pure compression, resulting in lightweight forms that could be built from thin concrete.
- 33. ADVANTAGES AND DIS-ADVANTAGES OF SHELLS: ADVANTAGES: 1. VERY LIGHT FORM OF CONSTRUCTION. TO SPAN 30.0 M SHELL THICKNESS REQUIRED IS 60MM 2. DEAD LOAD CAN BE REDUCED ECONOMIZING FOUNDATION AND SUPPORTING SYSTEM 3. THEY FURTHER TAKE ADVANTAGE OF THE FACT THAT ARCH SHAPES CAN SPAN LONGER 4. FLAT SHAPES BY CHOOSING CERTAIN ARCHED SHAPES 5. ESTHETICALLY IT LOOKS GOOD OVER OTHER FORMS OF CONSTRUCTION DIS-ADVANTAGES: 1. SHUTTERING PROBLEM 2. GREATER ACCURACY IN FORMWORK IS REQUIRED 3. GOOD LABOUR AND SUPERVISION NECESSARY 4. RISE OF ROOF MAY BE A DISADVANTAGE
- 34. NUBIAN VAULT
- 36. GUNA TUBE VAULT