The Islamic University of Gaza Department of Civil Engineering ENGC 6353 Design of Spherical Shells (Domes)
Shell Structure A thin shell is defined as a shell with a relatively small thickness, compared with its other dimensions.
Shell Structure
Four commonly occurring Shell Types: Barrel Vault Dome Hyperbolic Paraboloid (Hypar) Folded Plate
What is a shell structure? To answer this question, we have to investigate some important notions of structural design.
Two-dimensional structures: beams and arches A beam responds to loading by bending the top elements of the beam are compressed and the bottom is extended: the development of internal tension and compression is necessary to resist the applied vertical loading. An arch responds to loading by compressing. The elements through the thickness of the arch are being compressed approximately equally. Note that there is some bending also present.
Plate Bending A plate responds to transverse loads by bending This is a fundamentally inefficient use of material, by analogy to the beam. Moreover, bending introduces tension into the convex side of the bent plate.
Plate bending vs. membrane stresses Note: this is an experiment you can try yourself by folding a sheet of paper into a box. This slide shows a concrete plate of 6 thickness, spanning 100 feet, resisting its own weight by plate bending If the plate is shaped into a box, then each of the sides of the box resists bending by the development of membrane stresses. The box structure is much stronger and stiffer!
Domes A shell is shaped so that it will develop membrane stresses in response to loads The half-dome shell responds to transverse loads by development of membrane forces. Note that lines on the shell retain approximately their original shape.
Domes The primary response of a dome to loading is development of membrane compressive stresses along the meridians, by analogy to the arch. The dome also develops compressive or tensile membrane stresses along lines of latitude. These are known as hoop stresses and are tensile at the base and compressive higher up in the dome. Meridional Compressive Stress Circumferential Hoop Stress (comp.) Circumferential Hoop Stress (tens.)
In this figure, the blue color represents zones of compressive stress only. The colors beyond blue represent circumferential tensile stresses, intensifying as the colors move towards the red. A dome that is a segment of a sphere not including latitudes less than 50 does not develop significant hoop tension. The half-dome shell does develop membrane tensile stresses, below about 50 north latitude. These are also known as hoop stresses
Barrel Vaults A barrel vault functions two ways compression In the transverse direction, it is an arch developing compressive membrane forces that are transferred to the base of the arch Arch (compression) tension When unsupported along its length, it is more like a beam, developing compressive membrane forces near the crown of the arch, and tensile membrane forces at the base.
Barrel Vaults A barrel vault is a simple extension of an arch shape along the width. It can be supported on continuous walls along the length, or at the corners, as in this example. If supported on the corners, it functions as an arch across the width, and as a beam, with compression on the top and tension on the bottom in the long direction. This form is susceptible to distortion.
Barrel Vault, continued As with any arch, some form of lateral restraint is required-- this figure shows the influence of restraining the base of the arch--the structure is still subject to transverse bending stresses resulting from the distortion of the arch.
Folded Plates Folded plate structures were widely favored for their simplicity of forming, and the variety of forms that were available. Perpendicular to the main span, the shell acts as short span plates in transverse bending In the main span direction, the shell develops membrane tension at the bottom and compression at the top, in analogy to a beam in bending
What s wrong with this Folded Plate Structure? Compare to the discussion of barrel vaults, and see if you can tell what key element is missing from the folded plate shown. It is missing transverse diaphragms, especially at the ends.
This animation shows the effect of adding a diaphragm at the two ends and at midspan. The folded plate shell distorts much less.
Thin Shell Structures Two type of stresses are produced: 1. Meridional stresses along the direction of the meridians 2. Hoop stresses along the latitudes Bending stresses are negligible, but become significant when the rise of the dome is very small (if the rise is less than the about1/8 the base diameter the shell is considered as a shallow shell)
Thin Shell Structures Assumption of Analysis 1. Deflection under load are small. 2. Points on the normal to the middle surface deformation will remain on the normal after deformation 3. Shear stresses normal to the middle surface can be neglected
Spherical Shells Internal Forces due to dead load w/m 3 Consider the equilibrium of a ring enclosed between two Horizontal section AB and CD The weight of the ring ABCD itself acting vertically downward The meridional thrust N per unit length acting tangentially at B The reaction thrust N +d N per unit unit length at point D E N C A F B D N +dn H r a N a d
E N C A r F B D H r a N +dn Meridional Force N Surface area of shell AEB A EF a 2 a EF 1cos W w A 2 a EF D W w a D 2 2 (1 cos ) N r w a N 2 (2 ') sin D 2 (1 cos ) r' asin N D a w D a (1 cos ) w D a (1 cos ) w Da 2 sin (1 cos )(1 cos ) 1 cos d +ve compression -ve tension
Spherical Shells C A F E N B D H r a N +dn Hoop Force N N a The difference between the N and N dn which respectively acts at angles and with the horizontal give rise to the hoop force. Hoope force = N ad The horizontal component of N is N cos N causes hoop tension N a cossin D W similarly N +dn d d +d cos cos The horizontal component of N +d N is N +dn cos d N +d N causes hoop tension N N d a d N B
Spherical Shells C A F E N B D H r a N +dn N a d When increasein issmall dn tends to be zero N ad d N a cos sin where N w D a (1 cos ) 2 sin 2 1 cos cos1 N w D a cos- w D a 1cos 1cos d N B D W N +dn
Spherical Shells HoopForce N 1 N w D a cos- 1 cos wa At crown 0 N ( compression) 2 At base 90 N wa ( tension) when N for for o ' 0 51 49 o ' 51 49 N will be compressive o ' 51 49 N will be tensile
Summary In order to make the Negative sign for compression and +Positive sign for tension for Meridional and Hoop forces The previous equations can be rewritten as follows: N w Da 1 cos 1 N w D a cos 1 cos
Spherical Shells
Spherical Shells Ring Force H H N cos w a D cos 1 cos o ' at 51 49 N 0 & H is maximum H 0. 382 w a max D
Spherical Shells Internal forces due to Live load (w L /m 2 )horizontal Meridional Force T W w r w a sin L y a(1 cos ) r asin 2 2 2 L N 2 a sin sin w a sin 2 2 L w La N 2 Hoop Force N w L a N 2 Ring Tension cos 2 cos H N cos w L a 2 N H 35 w o at 45 0 & H is maximum max 0.35 L a
Spherical Shells
Ring beam design Design of the Circular Beam Horizontal Load A s T 0.9f Vertical Uniform load ( w ) N sin o. w 2 r w Ultimate Load T H r Vertical Load y Span length l P M C P r V 2 r # of supports max ve 1 (see the tables of circular beams M max ve C 2 P r V (see the tables of circular beams) )
Edge Forces In flat spherical domes, bending moments will be developed due to the big difference between the high tensile stress in the foot ring and compressive stresses in the adjacent zones It is recommended to use transition curves at the edge and to increase the thickness of the shell at the transition curve. Bending moments can avoided if the shape of meridian is changed in a convenient manner. This change can be done by a transition curve, which when well chosen gives a relief to the stress at the foot ring. In order to decrease the stress due to the forces at the foot ring, it is recommended to increase the thickness of the shell in the region of the transition curve.
Edge Forces In flat spherical domes, bending moments will be developed due to the big difference between the high tensile stress in the foot ring and compressive stresses in the adjacent zones It is recommended to use transition curves at the edge and to increase the thickness of the shell at the transition curve.
Ring Beam At the free edge of the dome, meridian stresses have a large horizontal component which is taken care of by providing a ring beam there. This ring beam is subjected to hoop tension. In case of hemispherical domes, no ring beam are required since the meridional thrust is vertical at free end
Reinforcement Steel is generally placed at the center of the thickness of the dome along the meridians and latitudes. If all the meridional lines are led to the crown, there will be a lot of congestion of bars and their proper anchorage may be difficult. To overcome this problem, small circle is left at the crown and all the meridional steel bars are stopped at this circle. Area enclosed by this small circle at the top is reinforced by a separate mesh.
Example: Design of a spherical dome Design a spherical shell roof for a circular tank 12m in diameter as shown in the figure. Assume the following loading: Covering material = 50 kg/m2 and LL= 100 kg/m2 Use ' 2 2 f 300 kg / cm and f 4200 kg / cm c y r=6m y=1.4m a
2 2 a r a y 2 2 2 2 a r a y 2ay 2 2 2 2 2 r y 6 1.4 Radius of the Shell a 13.56m 2y 2 1.4 6 sin = 0.442 13.56 26.23 cos 0.896 tan 0.493 r=6m y=1.4m Loading on roof Assume shell thickness = 10 cm Own weight = 0.1(2.5)= 0.25 t/m 2 Covering materials = 0.05 t/m 2 LL= 0.1 t/m 2 Note: the live load is considered as loading per surface area a
Design of Ring Beam: Wu= 1.2(0.2+0.05)+1.6(0.1)=0.52 t/m 2 Total load on roof = 2ayWu=2(13.56)(1.4)(0.52)=62 ton Vertical Load per meter of cylindrical wall =62/(2*6)=1.645 ton/m Outward horizontal force =1.645/tan=3.337 t/m Ring tension in beam T H r 3.3375 20 tons A s T 20*1000 5.35cm f 0.9 4200 use 8 10 mm y 2
N s Design of the Shell Meridian Force Meridian force per unit length of circumference Wa u 1 cos at 0 Wa u 0.52*13.56 N 3.52 t / m ' (compression) 2 2 at foot cos 0.896 0.52 13.56 N 3.72 t / m ' (compression) 1 0.896 Use minimum reinf. ratio = 0.0018 A 0.0018(10)(100) 1.8cm use 5 8 mm/m 2
Ring (Hoop) Force 1 N w u r cos 1 cos wr 0.52(13.56) At crown 0 N 3.52 t / m ' (compression) 2 2 At foot cos 0.896 N 2.59 t / m ' (compression) A 0.0018(10)(100) 1.8cm s use minmum reinf. 5 8 mm/m 2
Bending Moment Assume that the thickness at the foot = 15 cm x 0.6 at 0.6 13.56 0.15 0.85cm Fixing moment M 2 W 0.52 0.85 u x 2 2 0.188 t / m d 15 3 12cm 6 0.85 300 1 2.6110 0.188 1 0.0003 2 4200 10012 300 use minmum reinf. 5 8 mm/m 2 min
Example: Design of a spherical Dome Reinforcement details
Spherical Shells under General Loading Internal Forces Due to Others Loading