Foundation (footing) and floor
Depending on the purpose of the building and taking into account the loads, the foundation for the dome structure is designed, as a rule, either in the form of a ring footing band (ring beam) or as a monolithic circular plate (slab). In the latter case, the plate may be functioning for sustaining loads from the material stored in the dome — fertilizers, ash, slag, cement, potash, grain and other bulk commodities. At the same time the plate often serves as a pile work, forming the upper structure of a piling field.
The foundation is reinforced by circular and radial reinforcement (steel rebar) in strict accordance with the project. Along the perimeter of the ring special side steel bars outputs with external thread are arranged. The airform edges are secured to this ring foundation system later. For smaller domes these steel bars outputs may be omitted. In such a case the airform shall be secured to the foundation with a steel band and special screws.
In addition to the side steel bars outputs for securing of the airform the foundation of the dome has vertical steel bars outputs embedded in the ring of the foundation along the perimeter that shall be later attached to the vertical steel reinforcing of the dome shell itself.
Before placing the airform inside the ring, these vertical steel bars outputs are bent to the floor and then, after the airform is inflated and the polyurethane foam is applied, they are bent backwards – to a vertical position.
An airform is a tent fabricated to the proper shape and size of the future construction and made of architectural reinforced PVC (polyester vinyl) fabric. Along the perimeter of the bottom the airform has a cord, sewn into a circular loop, for securing the airform to the foundation, and a spout for connecting the airlock through which people’s access to the dome shall be later carried out. There is also a spout for connecting the «inflator» — a powerful system (blower fans) for filling the airform with the air (inflating the airform) and for further keeping the air-overpressure inside the dome during the construction period.
Before the beginning of the second stage construction materials (sand, steel bars, fittings, wire knitting, film, stickers, etc.) and construction equipment (crane, forklift, scaffolding, hoses, hand tools, etc.) are staged inside the footprint of the dome.
The second stage begins. The airform is placed to the center of the footprint (ring base) by crane. Then the airform is unfolded over the top of staged materials and the airform edges (circular loop with a cord inside) are secured to the ring foundation system. Using the blower fans (inflators) that are connected through a spout to the airform and when activated inflate the airform to provide sustained pressure within the airform. The inflator is equipped with a twin drive system and an automatic adjustment of the air supply. It means that the power supply of the powerful centrifugal pneumatic generator engine from the electrical supply network of the construction site is duplicated by the work of an autonomous diesel generator throughout the construction period. In case of power outage, the system will allow almost instantly (without delay) to switch the work of pneumatic generator to self-contained power supply from diesel generator. On the other hand, in case the airform is damaged in the working process that results in an unauthorized pressure drop inside the shell, the system will automatically increase the air flow and the shape stability deformation will be prevented.
When the airform is inflated and the pressure inside reaches the operating value (0.5-0.8 kPa), the air supply is reduced to the minimum, the airlock is connected to the spout allowing the access of staff inside the shell to start inner works. The second stage is completed and from this moment onwards the majority of works on the construction of the dome will be maintained inside the airform, thus climate impacts have little effect on the speed of erection.
Later, when the construction is completed, the airform will serve as the outer cover of the dome. Therefore, the color and quality of the fabric should be chosen taking into account the requirements for the outward appearance of the building and aesthetics point of view.
Once the inflation of the airform is completed a coat of specialized primer (as a rule — two-component polyurethane mixture) is applied to the interior side of the inflated airform by the construction workers using a crane, and/or a swing loader. Components of the mixture are fed under pressure separately through the hoses inside the dome by a powerful foam pump located outside the building. In this case, one group of workers is applying foam to the interior surface of the airform while the other one is providing the continuous operation of the foam pump outside.
Once the foam is hardened (solidified) the so-called «Stickers» (special brackets, clamps for the mounting steel bars) are embedded in the foam insulation surface over the entire area. Then the second (last) layer of polyurethane foam is applied, that firmly fixes stickers in polyurethane foam. Once the second layer of foam is hardened (solidified) the third stage is completed.
Thermal insulating polyurethane foam layer is the first «strengthening» layer that serves as a reliable base for attaching the steel reinforcing rebar.
The total thickness of the insulating layer of the dome is defined by the project, depending on the specific purpose of the designed facilities and the climate conditions, and generally ranges from 5 to 8 centimeters. In some cases, depending on the requirements of the customer, the insulation thickness may be increased up to 10-15 centimeters.
Steel reinforcing rebar
Once the polyurethane foam is hardened (solidified) the workers bend steel bars outputs to a vertical position along the whole perimeter of the shell. The vertical and horizontal assembly reinforcing rebar of the 1st reinforcement tier is attached (tied) to the stickers of the polyurethane layer.
Once the assembly reinforcement is completed the installation of main (principal) reinforcement of the 1st tier takes place. First the vertical rods are fixed to the steel bars bends of the foundation and to the circular assembly reinforcement. Then circular (hoop) horizontal rods are fixed from the bottom up — so that the weight of the overlying rods would be transferred more to the foundation, than to the shell.
Once the 1st reinforcement tier is accepted by the supervisor, the shotcrete spray-application around the perimeter of the building takes place.
Once the shotcrete is set the reinforcing of the next level begins. Then acceptance and shotcreting comes, and so on – up to the end, from bottom-up, adding more and more layers of shotcrete, embedding the steel rebar in the shotcrete and gradually increasing the carrying capacity of the entire structure.
For domes, experiencing besides their own weight the load of the overlying equipment (conveyor galleries, technological penthouses, pneumatic pipelines, etc.) or of the stored bulk materials, the installation of 2 or more layers of reinforcement may be required by the project.
Arrangement of openings (windows, doors, etc.) in the shell requires additional reinforcing around the perimeter of such openings. All these technical and operational features of the construction are defined by the project and strictly followed and supervised during the construction process.
A special cement-sand mixture of particular granulometric composition is delivered to the site by concrete mixers from the concrete products plant or is dosed on the site, loaded into the receiving bunker of the concrete pump located near the dome under construction and fed inside the shell through high-pressure hoses.
The group of workers located inside the shell, applies thin layers of mix uniformly on the steel grid, thus creating a protective layer around the rebar.
Once the shotcrete is set and cured (solidified), it forms together with the steel rebar a single high-strength reinforced concrete frame (shell) of the structure that is able to experience the calculated load with a large margin of safety and stability – solid and fireproof.
The thickness of the assembly (of the reinforced concrete layer) varies depending on the height and is based on the design parameters for the specific project. Usually it ranges from 20 to 60 cm at the bottom and from 10 to 30 cm at the top.
In case some technological equipment or superstructures (overbuilds) that create additional loads on the reinforced concrete shell are located on the top of the dome, the thickness of the upper part of the shell should be increased.
When the strength of the reinforced concrete shell reaches in total 75-80% of its design strength (at least 35-40 days after the start of shotcreting, but not earlier than the 100% completion of all rebar works) the air supply to the dome may be stopped by decision of the project manager. The blower fan (inflator) is turned off and the main entrance of the shell is opened. The waste materials and construction equipment can be removed from the dome through this opening..
As a rule, after the air supply is stopped, some finishing layers of shotcrete may be applied to achieve the project thickness and desired surface texture.
Once the shotcreting is finished the required openings are cut through the structure and finishing components and assemblies are integrated to the shell at these locations. The fifth and final stage of construction is completed.
Possible shapes of dome structures
There are the following options of domes shapes:
- Elongated ellipse
- Ellipse on vertical stem wall
- Hemisphere on vertical stem wall
- Elongated ellipse on vertical stem wall
- Low profile shape
- Combination – double dome
- Combination – triple dome
- Combination – Caterpillar
Cross sectional view of the dome Dome’s capacity example
Cost and time of implementation
Because of relatively low consumption of materials and high technological effectiveness of the construction process the domes are far ahead of silos and frame-type storages in terms of cost and construction time. The specific cost depends on the size of the order, the program of construction and local conditions.
Time of shells construction is very short: 2,5-3 months from the start of the second stage for shells with diameter ~ 40-45 m.
As a rule time for foundations construction is traditional and depends on the specific geological conditions, construction program and local situation with roads, infrastructure, etc.
LLC BaltTerminalStroy with 15 years of experience in the construction of domes carries out calculations, pre-design and design works, supply of equipment and materials, integrated construction of domes of various capacity and shapes both in Russia and Europe from concept and all the way through to the «turnkey» commissioning.