Preload–Pioneers of prestressed concrete tank design and construction

More than 70 years ago Preload designed and constructed the first-ever wire-wound prestressed concrete tank, revolutionizing the liquid storage tank industry. The company’s vision then–and now–is to design and build the highest quality liquid storage tank structures that:

• Provide excellent service for decades, even withstanding extreme weather and seismic conditions
• Do not require coatings or scheduled maintenance, making them virtually maintenance free
• Can be used for many applications across many industries
• Are economically priced

 

Industry leader. Since our beginnings in the 1930’s, Preload has designed and built more than 3500 prestressed concrete tanks. Many of our design and construction techniques have been adopted as industry standards by American Concrete Institute (ACI) and American Water Works Association (AWWA) publications.

Specialized Preload design and construction expertise. To assure you the highest quality tank designed and built for the long-term, we provide our customers “singular responsibility” for all projects. Preload engineers perform all of the design and detailing work, and Preload construction crews build the tank to ensure the highest quality throughout the entire process.

INNOVATIVE DESIGN

Preload has a long history of innovation in the prestressed concrete tank industry

innovativedesign

Not only did Preload build the world’s first wire-wound prestressed concrete tank in 1941, but we kept on innovating.  Our engineers and construction crews work together to develop ingenious ideas that keep us at the forefront of the industry.   Some of our biggest contributions to the industry are:

  • Wire-Winding
  • Diaphragm
  • Sliding Base
  • Restraint Cables

WIRE-WINDING

High strength prestressing wire keeps the tank in compression

Picture2Preload developed an innovative method to wind high-strength prestressing wire around concrete tanks in a continuous spiral. This innovation allows for much more efficient use of materials over conventional cast in place concrete tanks. Throughout the 1940’s Preload patented various methods and machines for wire-winding.

DIAPHRAGM

The steel diaphragm creates a watertight tank

Picture1

Preload built upon the success of the first wire-wound prestressed concrete tank by patenting the use of a steel diaphragm in the wall in 1943.  The diaphragm creates an impervious barrier to prevent water leakage through the wall.  Over 75% of the 10,000 AWWA D110 tanks that have been built to date utilize a steel diaphragm.

SLIDING BASE

Sliding base joint reduces wall stresses

Wall Panles and Bearing PadIn 1947, Preload patented the sliding base joint utilizing a rubber bearing pad. A sliding base joint removes the fixity at the floor/wall joint which greatly reduces the shear and bending moments in the wall. Prior attempts at removing the restraint at the floor/wall joint proved unreliable. By using rubber bearing pads under the wall, Preload was able to reliably calculate the deflection and stresses in the wall.

RESTRAINT CABLES

Earthquakes and other large-scale disasters are a design priority for citizens, state, and local officials – and Preload engineers and designers

Picture3Preload has been involved with seismic design for over 70 years. In fact, Preload was the first concrete tank builder to use seismic restraint cables, specifically designed to improve the performance of such structures under extreme conditions.

Seismic restraint cables allow a sliding-base concrete tank to resist earthquake-induced base shears.  Expert tests early on confirmed the great ductility and energy absorption capabilities of restraint cables.  As a result this detail was universally adopted as the prime mechanism for the seismic resistance of sliding-base tanks, and was incorporated in industry standards.

We’re proud to say that the use of base cables by all AWWA D110 tank manufacturers today is a continuation of Preload’s pioneering innovation. While specific cable layout details may differ from one manufacturer to another, the basic concept remains the same: it allows a sliding-base concrete tank to safely resist earthquake-induced base shears and over turning, while fully maintaining the tank’s serviceability.

Today, every AWWA D110 tank that withstands large earthquake forces is, in large part, an affirmation of the effectiveness of Preload’s ground breaking design.

SUPERIOR CONSTRUCTION

Our construction methods reflect Preload’s more than 80-year history as industry leader

FrameThe construction of a prestressed concrete tank requires special knowledge that only comes from building these types of tanks. At Preload, all of our construction personnel are trained in specific techniques of prestressed concrete tank construction which have been refined over our 80 year history. Owners and Consulting Engineers can rest assured that when Preload constructs your tank, you’re getting industry leading construction expertise.

FOUNDATION

Preload tanks start with a solid foundation

Picture7Tank construction begins with the foundation. A solid tank foundation requires thorough subbase preparation. Our engineers carefully review each project to ensure that geotechnical issues are correctly addressed throughout the subgrade and subbase preparation.

After the subbase is prepared, a minimum 6” thick leveling course of granular material is placed on top of the subgrade. This allows for a free-draining work area to facilitate construction, and ensures close tolerances in the construction of the finished floor.

The Preload Team’s expertise and experience in tank foundation design and construction are unequaled in the industry.

PIPING

Piping for watertight transmission

Picture5

Uninterrupted liquid transmission to and from the tank is vital to the operation of the storage facility. The Preload Team uses time-tested design and construction details that ensure

  • Proper installation of the piping to exacting tolerances,
  • Exceptional concrete encasement of underground and internal pipes, and
  • The flexibility required to allow differential settlement or movement of the pipeline relative to the structure.
Our detailing and installation of the tank piping results in high quality and watertight service.

FLOOR AND FOOTING

Watertight liquid containment

Preload constructs a highly reinforced concrete membrane floor that transfers the liquPicture8id load directly to the subgrade and contains the liquid in the tank. This type of floor system, in conjunction with a custom-designed PVC waterstop, provides watertightness and the ability of the floor to settle differentially without being subjected to high secondary bending stresses. The result is a reliable, economical, and durable tank structure.

Where site conditions necessitate the use of a structural floor, we choose design details and construction compatible with the pile supports, rock anchors, or other deep foundation systems.

In either case, our construction methods call for installation of reinforcing steel, and consolidation of properly prepared and tested concrete mixes that render high quality, waPicture9tertight tank structures.

RESTRAINT

Built to withstand the forces of nature

Picture10In high-seismic areas or sites with uneven backfill elevations, restraint cables are utilized to minimize lateral displacement of the cylindrical wall relative to the tank footing.

Preload pioneered the use of restraint cables in prestressed concrete tank structures. Our restraint system is proven to assure superior tank performance in any situation.

WALL

PRELOAD – The Original Inventor of Wire-Wound Prestressed Concrete Tank Walls

Sup Con_Wall_cut out bottom rt

PRELOAD engineered the first wire-wound prestressed concrete tank wall more than 70 years ago and has been at the forefront of continuous industry innovation ever since. Today’s industry standards for specifying and constructing prestressed concrete tanks are largely due to the technology, methods and practices PRELOAD brought to the industry. These include industry-accepted publications AWWA D110 and ACI 372R, used to guide the design and construction of prestressed concrete tanks throughout many parts of the world.

Whether it is an AWWA D110 Type I, Type II, or Type III tank, PRELOAD has the experience and capacity to construct your storage structure.

Wall – Type III

Type III Tank Walls – The Superior Choice in Prestressed Concrete Tanks

Picture1The Type III tank wall represents an innovation in prestressed concrete tank design and construction methods. PRELOAD’s Type III tanks provide maximum quality control during construction and greater durability. Our advanced methods for precasting wall panels and unequaled level of quality control make a Type III design the best option.

Type III tank wall panels are precast by the PRELOAD Team on-site and shaped to the proper curvature of the tank radius. Panels are constructed in casting beds and then stack-cast in an efficient assembly line process.

A steel diaphragm is used as the bottom form in fabricating each panel. The steel diaphragm is vertically ribbed with reentrant channels, providing a mechanical keyway anchorage to the concrete. The individual diaphragm sheets are mechanically lock-seamed. This process, invented by PRELOAD, produces an impervious watertight membrane throughout the entire wall area.

Next, side forms are positioned to form the edges of the panels, vertical reinforcing and lifting inserts are placed, and the concrete is cast and finished. As the concrete is cast, the thickness and cover over the diaphragm and reinforcing steel are positively controlled by the side form system. This process ensures a minimum of 4” of densely consolidated concrete cover over the steel diaphragm and proper cover over the reinforcing steel. Each panel is then cured in a high-humidity environment.

Once all wall panels are cast, the individual panels are rotated vertically and erected along the perimeter tank footing. The panels are then temporarily braced as the wall construction continues to assure safe construction in nearly any weather condition. The diaphragm is then joined together and joints between the erected panels are filled with shotcrete or concrete. Once the wall construction is complete, the circumferential single wire prestressing is applied.

If required, PRELOAD can embed inserts to support internal tank accessories. Wall sleeve penetrations that accommodate permanent manholes and through-the-wall piping are made watertight by sealing them to the steel diaphragm.

Picture2

Wall – Type II

Type II Tank Walls that Exceed Industry Requirements

Picture1The construction methods used for Type II tank walls were developed by PRELOAD in the 1940’s. While the basic methods remain largely unchanged today, advancements in materials and construction techniques have allowed PRELOAD to improve the tank’s quality characteristics.

A galvanized steel diaphragm with vertically ribbed reentrant channels is at the core of each PRELOAD Type II tank wall, providing a mechanical keyway anchorage to the shotcrete. The steel diaphragm is seamed and sealed to form a watertight, impervious barrier from the base to the top of the tank. Mild reinforcing steel is then placed in the wall to resist stresses due to differential temperature gradients and vertical bending moments.

Picture2Permanent manholes and through-the-wall piping, if required, are accommodated through penetrations made watertight by sealing them to the steel diaphragm.

Finally, shotcrete is pneumatically applied to the interior and exterior of the steel shell diaphragm to build out the tank core wall. This process allows efficient construction of the tank wall and is performed by one of our specially trained, ACI Certified Shotcrete Nozzelmen.

If required, PRELOAD can attach internal tank accessories to the tank wall using embedded or post-installed anchors.

ROOF

No matter the project requirements, the Preload Team has you covered

Picture11The Preload Team’s expertise allows us to assist with the selection and construction of the right cover for your storage tank.

As a leader in dome roof construction, our time-tested construction methods enable us to construct a high quality dome for your tank for maximum long-term durability.

For projects with height restrictions or for fully buried tanks, we utilize a column-supported flat-slab concrete roof.

On projects where a roof isn’t required, our open-top tanks are a perfect choice.

Dome Roof

Most Preload tanks feature a spherical shell dome roof

The dome roof design is unmatched in its economy and DomeRoofdurability. The design results in uniform compression throughout the spherical shell, allowing relatively thin sections of concrete to span large tank diameters.

The efficient dome shape provides a low profile as well. Rise-to-diameter ratios of 1:8 to 1:12 are standard, depending on tank diameter. Other rise-to-span ratios are available on request.

In constructing the roof, reinforcing steel is placed in perpendicular directions throughout the dome shell. Then, the concrete is placed onto the form, finely screeded and finished to the required spherical shape. A thickened fillet section is incorporated at the dome perimeter to resist the bending moments resulting from shell edge discontinuity.

Preload’s dome designs include venting for adequate draft, and an access hatch for future entrance or the use of equipment

Flat-slab Column-Supported Roof

Reinforced concrete, flat-slab roofs for prestressed concrete tanks with special form or aesthetic requirements

At times a flat-slab column-supported roof is required foFlat-slabroofr fully buried tanks, for a project’s height restrictions, or for aesthetic reasons. Preload’s flat-slab roof is supported by circular columns, each with a footing sized for the allowable soil-bearing capacity. To economize the design, we use column capitals or drop panels that are compatible with the roof design.

Our flat-slab roofs are designed and constructed to meet local and project requirements; and are designed in accordance with provisions of the AWWA D110 Standard, ACI 372R, and the ACI 350 Code for Environmental Structures.

Open Top

When a roof isn’t a requirement

For certain applications, Preload’s prestressed concrete tanks are copenroofonstructed with an open top. Like our dome or flat-roof tanks, Preload’s open-top tanks consist of

  • A circular, cast-in-place, reinforced, concrete floor slab
  • A precast, circumferentially prestressed, concrete wall incorporating a continuous mechanically bonded steel diaphragm.

Preload’s open-top tanks are constructed to meet the requirements of the AWWA D110 Standard, ACI 372R, and our own high standards for tank construction.

Our quality-centric construction methods result in an open-top tank with tolerances that will allow the incorporation of any process equipment necessary for your project.

PRESTRESSING

Circular prestressing is our business

Circular, single-wire prestressing is the time-proven method Preload invented for constructing wire-wound prestressed concrete tanks. Prestressing places the entire tank wall into a state of permanent compression. This prevents the cracking and leaking associated with other types of tank structures. Prestressing the dome edge keeps the dome in compression, enabling the use of large free-spanning concrete domes.

The prestressing wire is applied around the tank in a continuous helix, using specially designed machines capable of exceeding industry standards and meeting our own stringent quality requirements. These machines allow us to maintain a consistent force in the wire while also maintaining a minimum clear distance between individual prestressing wires to ensure that every wire is fully encapsulated in shotcrete.

Encapsulating individual wires and applying them helically eliminates the need for buttresses and anchorage zones. This benefit is found only in wire-wound prestressed tanks, resulting in superior reliability and durability.

COVER COAT

Quality and beauty through the final step

Shotcrete-Each-Layer-of-Wire-Winding-Prestressing
The final step in the construction of the tank structure is the exterior cover coat. Concrete is applied pneumatically in a process called shotcreting. The concrete is applied or “shot” onto the wall with air pressure, typically in layers.

The cover coat is applied by our skilled ACI certified shotcrete nozzlemen, thus ensuring high quality construction in the finished tank structure.

The final surface of the cover coat is architecturally finished by applying either a natural gun or a steel troweled finish to produce a durable and aesthetically pleasing tank structure for the community.

ACCESSORIES

Accessories are provided to enable safe, efficient use of the tank.

After the construction of the tank is complete, accessories are added to complement the tank structure based on specific project requirements.

Accessories such as exterior and interior ladders, roof handrail systems, vents, natural and forced draft aerators, personnel and equipment access hatches, and wall manholes are available and easily incorporated into the design of the tank.

Accessories are fabricated from the highest quality materials available, to provide durability and low maintenance for the life of the tank structure. Like the tanks themselves, accessories are designed to comply with all applicable local, state, and federal regulations.

ARCHITECTURAL TREATMENTS

When it comes to water storage tank design, different communities have different aesthetic needs

That’s why we offer a wide range of design possibilities, enabling you to choose the look that best suits your environment.

We build Preload tanks in diverse configurations — including at grade, partially buried, or fully buried — to satisfy your profile preferences and accommodate specific site conditions.

Additionally, with a Preload tank you can select from a wide range of colors and geometric alternatives, as well as exterior treatments to blend with nearby structures and the surrounding environment. Whatever your location, we’ll build the right tank to suit your community’s needs.

A PERFECT BLEND

With a wide range of exterior treatments and coatings, you can select the right combination to suit your needs. Choose from such architectural treatments as:

Frederick,-MD-Water-Storage-Tank-2.0MG

Pilaster

Arched Pilaster

Exterior Insulation and Finishing System (EIFS)

Roof Perimeter Panels

Face Brick

External Cladding

We can also coat your tank’s surface in a color of your choice, or retain its natural color. Whichever treatment you choose, you can count on Preload to build the tank that fits best in your environment.

WE’RE READY. ARE YOU?

Start Your Project