Chapter: Civil : Prefabricated Structures : Prefabricated Components


Domes are construced with many planes so they ressemble the facets of a diamond.




Domes are construced with many planes so they ressemble the facets of a diamond.The structural problem in designing these shells is to provide enough angle between the planes so that an actual rib is formed which will be stiff enough to support the plane surface.Usually it is best to start with a spherical translation surface.


Folded plate dome:


This makes use of tapered folded plate slanting to the centre in the form of a tent.It can be built so that each of the triangular elements is self supporting during construction except for possibly a single shore at the crown.To obtain natural light the top may be cut off & a ring inserted with a sky light.The arch thrusts are taken through this ring & the difficult forming of the narrow plate at the crown is avoided.If the structure is large there would be very high bending stresses due to the curvature & the ring would be very large.


Multi facet dome:


There may be discontinuous in the layout of intersections which make or destroy the visual effect & make the structure more difficult to design.This dome can be of much greater span than the previous example because the span of the individual slab element is less.A dome hexagonal in plain can be made continuous with adjacent units if it is necessary to cover a large area.


Half sphere- vertical walls:


A half sphere for a dome of revolution does not require a thrust ring at the base so it can be placed on vertical walls & made continuous with a walls.This design is used for tanks because the roof becomes a part of the tank.The vertical portion of the sphere is not difficult to construct if




pneumatically applied shotcrete or a similar process is used.The structure with arched openings an a plastic dome on the crown has a rather oriental feeling.The most serious problems in the architecture of dome is acoustics.In a domed ceiling the sound may reverberate as many as 20 times unless there is acoustical treatment.


Domes-Square in plan:


This structure is a spherical dome with portions sliced off to form a square or rectangle.Most areas to be covered are rectangular so a circular dome is not always a good solution to the planning requirements.This dome is supported by four rigid frame & would only be suitable for small span because the frame would get quire in large.For long spans it is necessary to place a tie between the knees of the frame.These ties can be made a part of the windows.Stresses in the shell are direct compression stresses except across the corner where there are direct tensile due to the outward spread of the forces.The arches or rigid frames pic up the shell forces by shear parallel to the arches which are zero at the top & maximum at the bottom.There is no component of force in the shell perpendicular to the arches.


Multiple Domes:

The domes is rectangular & is continuous with the adjacent domes.The edges of dome are supported by tied arches or brousing trusses.If windows are needed in these arches,the mullions may be made to serve as vertical hangers for the bottom chords of the arch.In constructing the shell,each one of the dome elements is an independent structural unit so the forms may be moved without shoring all or part of the dome already cast.The shell thickness of this type of dome does not need to be greater than a circular dome except at the triangular corners.Membrane action ceases to exists & the corner should be designed as a slab.


Translation domes:


This structure looks very much like the square dome.A translation shell is generated by a vertical curve sliding along another vertical curve.The curvs can be circles,ellipses or parabolas.Therefore the vertical sections are all identical as opposed to a circular dome in which all vertical sections vary in height.This is a big advantage in construction of the formwork.This method can provide a rectangular dome with the same height of arch on all sides,thus making a rectangular dome feasible.


Most of the load is carried by the side arches with some coming directly to the corners.A tie at the springing of the arches but usually this will be covered by the walls.Such shells are suitable for quite long spans with some interior lighting furnished by skylights in the shell.


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