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DOMO STRUCTURES
ADVANTAGES
OVER CONVENTIONAL STRUCTURES
1. DOMO STRUCTURES
Domo structures are made of modern axi-symmetrical sheeting
in reinforced and/or pre-stressed concrete, perfectly
insulated from the outside and that can adopt a wide variety
of shapes to adapt to different uses, with great potential
appearance-wise.
2. CONSTRUCTION PROCESS
They are built by a unique process using a highly resistant,
pressurised PVC membrane as an auxiliary part that becomes
the outer finish. A 2 cm. layer of rigid polyurethane foam
is then sprayed onto the inside of the membrane.
In successive stages, layers of concrete are subsequently
sprayed onto the inside in a continuous, self-balancing
process and the corresponding steel structures are fitted
until the sections defined in the corresponding construction
project are obtained.
3. TYPOLOGICAL CONSIDERATIONS
3.1
DOME-SHAPED STRUCTURES
The typology we call domo structures is an amalgamation of
very old and very modern parts that leads to a completely
new type of structure with basic advantages over
conventional structures designed for the same purpose.
Domes were used in the past and acquired notable development
in Roman times, with a new peak during the Renaissance
period. Some of the more notable examples still stand today,
the hundreds or even thousands of years since they were
built proving their durability, even in areas affected by
earthquakes.
Historic domes had limitations due to the materials used,
with no suitable parts to withstand high levels of traction.
This problem is now correctly solved by using reinforced
concrete, lead to the possibility of thin domes with a high
level of both tensile and compression strength.
3.2 MODERN MATERIALS AND STRUCTURAL PARTS
3.2.1 Support system
The use of modern elastomeric support appliances means that
these modern concrete domes can be supported on the
foundations in the most appropriate manner in each case, in
order to avoid, for example in the case of a silo, heavy
horizontal load on the foundations and significant bending
moments on the structure and the foundations, which would
arise if they were rigidly connected. Moreover, were the
action due to an earthquake, an elastic support would
decrease its effects on the structure. Furthermore, with
this type of support it would be possible to correct any
differential settlement, reducing the possibility of damage
to the structure.
All this leads to a more favourable behaviour and a
reduction in cost.
3.2.2 PVC Membrane. Rigid polyurethane foam
The construction of Domo structures is possible thanks to
the use of modern materials such as PVC membranes and rigid
polyurethane foam.
High-resistance, pre-stressed structural textile membranes
become the outer finish and provide additional protection
from dampness.
The spraying of a layer of rigid polyurethane foam onto the
inside of the membrane provides excellent damp and heat
insulation.
This is extremely important when storing products at high or
low temperatures, as the stress acting on the wall of the
structure is greatly decreased and heat contamination is
eliminated.
The inside of the structure is perfectly insulated, thus
eliminating all kinds of condensation, a vital aspect when
storing clinker or cement, and maintaining a greater heat
balance.
Insulation is so important that it is even possible to
create special atmospheres inside where required.
These properties of the Domo structures are much better than
those of conventional silos. In fact, conventional silos
are often cylinders, generally made of concrete with a metal
casing.
The metal casing always leads to a lack of airtightness
given that there it is not a monolithic structure with the
shaft and different structural parts with different
behaviour and made of different materials are used, leading
to dust emissions, seeping water and condensation that may
affect the material stored and, furthermore, frequent
maintenance and repair work is required.
The Domo structure, therefore, has important advantages
regarding insulation and airtightness.
4. STRUCTURAL BEHAVIOUR
Structurally, it can be easily understood why a cylinder
offers worse structural behaviour than a dome, especially
when there is non-uniform, horizontal stress, such as when
the material is not arranged uniformly, when silo emptying
includes eccentricity or when wind or earthquakes play their
part.
Appendix 1 shows a comparative study of stress on a
conventional silo and a dome produced by seismic action.
Appendix 2 compares the effects caused on both types:
conventional and dome, when the product is stored at high
temperatures.
With the Domo construction system, larger spanned structures
are possible given that the problem of the roofing
construction has been perfectly solved. Our experience
includes silos with a diameter of 70 m. and projects of 120
m. storage areas for raw materials or coal.
A larger diameter of silo may be an option when the
supporting capacity of the foundation ground is smaller.
The direct loads of the stored material are reduced when
distributed over a larger area and any action transmitted by
the structure onto the foundations is reduced.
In large diameter storage areas, our structures show
excellent aerodynamic behaviour due to both their shape and
the lack of roughness on their surface, solving assembly,
presentation, instability and fatigue problems that may
affect metal structures.
5. CONCRETE INSTALLATION. CONSTRUCTION
A basic advantage is the excellent installation of concrete
compared with other travelling or conventional systems.
The only additive in the concrete, with a high cement
content and a maximum aggregate size of 12 mm., is silica
fume.
It is installed by spraying to make it extremely compact.
It is sprayed on in thin 2-3 cm. layers to notably reduce
cracking due to initial retraction.
Furthermore, there are very favourable conditions of
temperature and dampness inside the structure being built
for concrete curing and setting due to the microclimate
created inside by the insulation of the outer membrane and
the polyurethane in relation to the outer atmosphere.
Another basic advantage is that, by being built from the
inside with the majority of the building machinery and
materials required being inside the area it occupies, the
occupation of the outer space is reduced to a minimum.
Lastly, completion times are reduced notably given that
construction is completed in a single stage instead of the
three required for conventional structures: shaft, roofing
and, where applicable, post-tightening.
6. ADVANTAGES. SUMMARY
Its main advantages can be summarised:
a)
Functional
-Complete airtightness, with the possibility of storing
water or gas.
-Complete insulation from the outside.
- Elimination of condensation.
- Better protection of stored products
- Possibility of storing products at high or low
temperatures.
- No interior columns.
- Loading directly onto the walls is possible.
b) Environmental
-Dust emission is suppressed.
-Thermal and acoustic contamination is eliminated.
-Its appearance improves the look of the surrounding area.
c) Use
- Maximum durability
- Low operating costs.
- Possibility of automatic extraction of 100% of the stored
products
- Reduction in staff required for operations
d) Structural
-Monolithic structures with single or double curve
- Excellent behaviour during:
High or Low Temperatures
Earthquakes
Hurricanes
Fires
Wind
- Reduction of loads on foundations
- Possibility of large clearance.
-Versatility in the way it is joined to the foundations or
the infrastructure.
-Resistance capacity of heavy specific loads.
-Any localised settlement is easily corrected.
e) Constructive
- Shorter completion times.
-Excellent installation of concrete
- Smaller area of occupation
d) Financial
-
The Domo solution is around 10% cheaper than the convention
method.
In short, it can be said that the Domo system allows for
monolithic, ecological
structures thanks to its perfect
insulation and its lower operating and maintenance
costs,
with greater structural behaviour and better protection of
stored materials.
APPENDIX 1
SEISMIC ANALYSIS
The response of two silos for clinker with the same 50,000 t
capacity is compared.
The first, the conventional silo, cylindrical with a radius
of 19 m and 25 m in height. The second, a dome silo,
rounded with the centre at a height of 6.75 m and an
interior radius of 23 m.
Both structures are excited by a uniform movement on their
base in the form of the elastic spectra responding to the
horizontal acceleration defined in the Seismic Resistance
Construction Standard NCSE-94, using a value of 0.12 g for
basic seismic acceleration.
The clinker mass is distributed uniformly on the wall of the
silo, as indicated in the Règles Professionnelles de
Conception et de Calcul des Silos en Béton Armé ou
Précontraint.
60 initial frequencies and the corresponding vibration modes
are obtained and the square root SRSS method of the sum of
the squares is used as a technique to combine the values of
the sliding in order to estimate maximum response stress,
which is the most common method, considering a value of 0.10
for the cluster factor.
The following figures show the stress values obtained in
both cases, observing that the stress peak reaches a value
that is 3 times higher in the case of a conventional silo
and is limited to a very specific area in the case of the
dome. Furthermore, in the case of the conventional silo,
most of its surface area is subjected to stress between the
factors of 1 and 4 and in comparison with the most
significant value in the extended area of the dome.
The most favourable response of the dome is first due to the
fact that it is a dome structure that is closed at the top,
compared with the open cylinder, and with extraordinary
behaviour compared with non-centred loads. This has been
historically proven by the duration of these structures,
often dating back to the ancient world.
In our case, the response is even more favourable because
the dome is elastically supported on the foundations using a
combined system of banded elastomeric material supporting
appliances and angular guides to lessen the effect of an
earthquake.
This leads to an increase in the natural frequencies of the
structure, which decreases the values of spectral
acceleration and, therefore, the overall response.
Fig 1. Dome Silo. Seismic Action. SRSS Stress
Fig 2. Conventional Silo. Seismic Action. SRSS Stress
APPENDIX 2
THERMAL EFFECTS
The effect produced on the wall of the silo when clinker is
stored inside at high temperatures is considered.
A reference temperature Tr when the clinker enters the silo
of 150ºC is used.
The Règles Professionnelles de Conception et de Calcul des
Silos en Béton Armé ou Précontraint are taken into account.
The effects produced by the temperature in the most
unfavourable part of the silo will be studied, which is the
part of the wall that is not in direct contact with the
clinker but with the irradiated air, reaching a temperature
of 0.72Tr, according to said standard.
On the part of the wall in direct contact with the clinker,
the temperature is lower due to the insulating power of the
clinker in its surrounding area (0.51 m2 ºC/W), which
insulated the clinker further inside and reduces the
temperature on the inside of the wall to a value below that
of the case of a wall without clinker touching it.
The difference in behaviour between a conventional silo and
a dome silo lies in the presence of a 2.5 cm insulating
layer of rigid polyurethane foam in the latter.
The distribution of temperatures on the wall is studied,
bearing in mind concrete conduction coefficient values Lb of
1.75W/m ºC, 0.11 m2 for internal wall surface resistance
1/hi and 0.06 for external 1/he and a conduction coefficient
Lp of 0.03 for the polyurethane. (See (1)).
With a constant heat flow and an exterior air temperature of
0ºC for a concrete wall 30 cm. thick, the difference in
temperatures between the interior and exterior sides of the
concrete wall of the silo is around 54º in the case of the
conventional silo, whereas it is reduced to 15º for the dome
silo.
This is extremely significant for both the cost and
durability of the structure, as the conventional silo is
subject to the constant activity of bending movements
proportional to this difference in temperature, which are 3
times above in the case of conventional silos.
N.B.:
The unit flow is expressed as follows:
(Tai-ti)/(1/hi)=(ti-te)/(e/Lb)=(te-tp)/(ep/Lp)=(tp-Te)/(1/he)
(1)
Tai=interior temperature
ti=interior temperature of concrete wall
te=exterior temperature of concrete wall
Te=exterior temperature
tp=exterior temperature of polyurethane wall
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