HOW IMPRESSIVE IS THE GULF STREAM TANK AT THE PHILLIP AND PATRICIA FROST MUSEUM OF SCIENCE?

Very much indeed!

This extraordinary tank looks like a tilted martini glass suspended above the museum’s viewing gallery. Visitors can view marine life inside the tank through large round acrylic viewing windows built into the walls of the tank.  Unlike a conventional tank design where the walls are designed for horizontal pressure, the walls of this tank had to be designed for lateral pressure and the vertical load of 4.5 million pounds of saltwater within the tank.  The tank is supported by a single circumferential beam over a series of concrete columns.  The concrete walls, therefore, had to be designed as members supporting the hydrostatic loads from the saltwater as well as flexural stresses created from the tank support system of beams and columns.  The acrylic viewing windows – or oculi – interrupt the continuity in the tank walls.  All of these factors resulted in extraordinary structural complexity.  The available post-tensioning (PT) software packages were well suited to design the beams and flat slabs; however, no PT software package could model a structure of this complexity.

 The specialty design team opted to use SAP 2000 to model the tank.  This software allows for the modeling of complex and intricate shapes, and it can insert PT tendons into the wall; however, the PT modeling feature was primitive.  The specialty designer had to insert tendon coordinates throughout the length of each tendon in the tank.

Aside from meeting all code requirements in ACI 318 and ACI 350, it was important to the design team to have concrete that was in a state of compression when subjected to the water load.  In areas of high demand, extreme fiber stresses were not allowed to exceed the cracking limit.  Post-tensioning concrete provided an elegant solution to reducing concrete tensile stresses throughout all portions of the tank.  Because of the high concentration of PT and the desire to have the highest level of durability, the specialty design team opted to use bonded post-tensioning.  The tendons were primarily 12-0.6” dia. strands inside a corrugated plastic duct with encapsulated end anchorages for enhanced corrosion protection.  The multistrand tendons are arranged in a tri-directional pattern in the tank walls.  The tri-directional pattern was critical to ensure a uniform distribution of prestress in all areas of the tank including around the oculi.  Most of the tank wall is 28 inches thick, with locally thicker areas at the perimeter and oculi.  The entire tank consisting of 1200 yards of concrete was poured monolithically in approximately 24 hours. 

Among the primary challenges in the construction of this tank were the installation of formwork and the installation of the PT stress end anchorages.  Formwork consisted of 400 custom-made panels to form the complex geometry of the tank.  A low slump concrete was used to avoid the need for an inside wall form.  It took over 5 weeks to install the formwork.

Stress end anchorages had to be bolted to the formwork at exacting angles to avoid having kinks in the duct at the anchorages.  The specialty design team set the anchorages into their design model and provided the construction team with the stress end Blockout formwork dimensions to make construction seamless.

USE OF POST-TENSIONING IN THE GULF STREAM TANK’S PROJECT: 

The tank uses a total of 50 – 12 strand tendons and 30 – 4 strand tendons for a total of approximately 50 tons of PT.  Because of the high concentration of prestress throughout the tank walls and to provide enhanced long-term durability, the specialty design team chose to use a bonded PT system.  The post-tensioning system provided enhanced corrosion protection with a plastic duct and encapsulated anchorages. 

For this project, post-tensioning offered significant advantages over any other method of construction.  Post-tensioning provided the following benefits and enhancements:

• The concrete is in a state of residual compression under all load cases.  This reduces the possibility that the concrete will crack.

• The extreme fiber concrete stresses are not allowed to exceed the cracking limit.

• This reduces the possibility that the concrete will crack.

• Post-tensioning resulted in reduced concrete wall thickness and reduced conventional reinforcing quantities compared to alternative designs.  The relatively thin concrete walls provided the aesthetic form and shape that the architect was trying to achieve.

• The reduction in concrete, formwork, and reinforcing steel quantities ultimately resulted in cost savings to the owner.

• Post-tensioning allowed for earlier stripping of formwork since the formwork can be stripped soon after stressing.

SOME INTERESTING FACTS…

Project Timeline:

• Design of the tank commenced in March 2013. 

• Tank construction commenced in August 2014. 

• Tank construction completed in May 2015.

• Museum Project completion in May 2017.

Size and total footage of structure:

100-foot wide. A 9,500-surface area in square feet which supports 4.5 million pounds of water. The tank walls vary between 28-in (71 cm) and 56-in (142 cm) of thickness and the structure also includes 200 tons of epoxy coated steel reinforcement and 50 tons of high-strength post-tensioning cables to prevent it from cracking. Approx. 120 ft. diameter at the top and 27 ft. in height. 

Building the vessel required a continuous concrete pour that lasted 25 hours, distributed 1,200-cubic yards of concrete, and involved 131 cement trucks.

Approximate cost of total project: $305 million.

If you need a post-tensioning expert or have any questions as you prepare for your next building project, consult an expert!

Photos credit to ©Phillip and Patricia Frost Museum of Science, Baker Concrete, Grimshaw Architects, and Kline Engineering & Consulting.