Shell Structure Domes
SHELL ROOFS
A shell roof may be defined as a structural curved skin covering
a given plan shape andarea, main points being: 1. Primarily a
structural element. 2. Basic strength of any particular shell is
inherent in its shape. 3. Quantity of material required to cover a
given place shape and area is generally less than other forms of
roofing.DOMES Domes are semi-spherical or semi-elliptical in shape.
They are used as roof structures. Constructed of stone or bricks or
concretes. They are supported on circular or regular polygon shaped
walls. Dome structures have within certain height and diameters
vary small thickness. Dome structures are generally used in
monumental works were roof are to be build on building circular or
hexagonal in plan. The domes can be either (1) Smooth shaped domes
(2) ribbed domes. Smooth shell domes can have either shell of
uniform thickness or with shell of uniformly varying thickness. A
dome can be constructed with or without lantern. In their simplest
form these consist of a half sphere but domes based on the ellipse,
parabola and hyperbola are also possible. Domes are double
curvature shells, which can be rotational and are formed by a
curved line rotating around a vertical axis or they can be
transitional domes, which are formed by curved line moving another
curved line. Space frame dome exceptionally light structures, which
permit the spanning of large distances with relative reduction
materials. The dome surface can be subdivided into a number of
triangles or other regular polygons the sides of which are hinge
bars. Any dome shell roof will tend to flatten due to the loading
and this tendency must be resisted by stiffening beams or similar
to all the cut edges. As a general grid domes which rise in access
of 1- 6 of their diameter required a ring beam. Timber domes like
their steel counter parts are usually constructed in a single layer
grid system and covered with a suitable thin skin membrane.
CONSTRUCTION DETAILS
Any dome shell roof will tend to flatten due to the loadings and
this tendency must be resisted by stiffening beams or similar to
all the cut edges. As a general guide domes that rise in excess of
one- sixth of their diameter will require a ring beam. Timber domes
like their steel counter parts are usually constructed on a single
layer grid system and covered with a suitable thin skin
membrane.
TYPES OF DOMES ADVANTAGES OF DOMES
Domes are very strong, actually getting stronger as they are
constructed larger. The basic structure can be erected very quickly
from lightweight pieces by a small crew. Domes as large as fifty
meters have been constructed in the wilderness from rough materials
without a crane. The dome is also aerodynamic, so it withstands
considerable wind loads, such as those created by. Solar heating is
possible by placing an arc of windows across the dome: the more
heating needed the wider the arc should be, to encompass more of
the year. Today there are many companies that sell both dome plans
and frame material with instructions designed simply enough for
owners to build themselves, and many do to make the net cost lower
than standard construction homes.
DISADVANTAGES OF DOME HOMES:
The shape of a dome house makes it difficult to conform to code
requirements for placement of sewer vents and chimneys.
Off-the-shelf building materials normally come in rectangular
shapes. There can be considerably more scrap, left from cutting
rectangles down to triangles, than with a conventional building
approach, thus driving costs up. Fire escapes are problematic;
codes require them for larger structures, and they are expensive.
Windows conforming to code can cost anywhere from five to fifteen
times as much as windows in conventional houses. Professional
electrical wiring costs more because of increased labor time.
However, even owner-wired situations are costly, because more of
certain materials is required with a dome versus conventional
construction. Air stratification and moisture distribution within a
dome are unusual, and these conditions tend to quickly degrade
wooden framing or interior paneling. Privacy is difficult to
guarantee because a dome is difficult to partition satisfactorily.
Sounds, smells, and even reflected light tend to be conveyed
through the entire structure. As with any sloping shape, the dome
produces wall areas that can be difficult to use and leaves some
peripheral floor area with restricted use due to lack of headroom.
Circular plan shapes lack the simple modularity provided by
rectangles. Dome builders find it hard to seal domes against rain,
because of their many seams and because solar heat flexes the
entire structure each day as the sun moves across the sky. The most
effective waterproofing method with a wooden dome is to shingle the
dome. One-piece reinforced concrete or plastic domes are also in
use, and some domes have been constructed from plastic or waxed
cardboard triangles that overlapped in such a way as to shed
water.
MONOLITHIC DOME.
Schematic cutaway of the layers of the final Monolithic
Dome.
FOUNDATION - The Monolithic Dome starts as a concrete ring
foundation, reinforced with steel rebar. Vertical steel bars
embedded in the ring later attached to the steel reinforcing of the
dome itself. Small domes may use an integrated floor/ring
foundation. Otherwise, the floor is poured after completion of the
dome.
AN AIRFORM fabricated to the proper shape and size is placed on
the ring base. Using blower fans, it is inflated and the Airform
creates the shape of the structure to be completed. The fans run
throughout construction of the dome.
POLYURETHANE FOAM is applied to the interior surface of the
Airform. Entrance into the air-structure is made through a double
door airlock which keeps the air-pressure inside at a constant
level. Approximately three inches of foam is applied. The foam is
also the base for attaching the steel reinforcing rebar.
STEEL REINFORCING rebar is attached to the foam using a
specially engineered layout of hoop (horizontal) and vertical steel
rebar. Small domes need small diameter bars with wide spacing.
Large domes require larger bars with closer spacing.
Shotcrete a special spray mix of concrete is applied to the
interior surface of the dome. The steel rebar is embedded in the
concrete and when about three inches of shotcrete is applied, the
Monolithic Dome is finished. The blower fans are shut off after the
concrete is set. DETAILS
Reinforcement details
WALL AND DOME DETAILS CASE STUDY - Millennium Dome, Richard
Rogers
The dome is the largest of its type in the world. Externally, it
appears as a large whitemarqueewith twelve 100m-high yellow support
towers. In plan view it is circular, 365m in diameter. The canopy
is made ofPTFE-coatedglass fibrefabric, a durable and
weather-resistant plastic, and is 52m high in the middle. Its
symmetry is interrupted by a hole through which a ventilation shaft
from theBlack wall Tunnelrises.
SECTION DETAILED SECTION PLAN
MEMBERS SUPPORTING THE DOME
Dome roofs, destined for placement 150 high, are assembled on
the ground to help maximize safety and speed assembly time. Three
domes are each 37 9 in diameter.Dome Roof Installation,
Annapolis
Dome roofs, destined for placement 150 high, are assembled on
the ground to help maximize safety and speed assembly time. Three
domes are each 37 9 in diameter.Dome roofs, destined for placement
150 high, are assembled on the ground to help maximize safety and
speed assembly time. Three domes are each 37 9 in diameter.Domes
are assembled one panel at a time, with a crew of 3 or 4, depending
on dome size.
Page | 11
