As the name implies, a double tee or double-T beam is a precast, prestressed, load-bearing structure that resembles two side-by-side capital letter Ts. The two vertical leg sections are called webs or stems while the horizontal section is known as the deck or flange. Combined, these create a structure that offers flexibility in design and construction and is an ideal choice for superstructures requiring long, uninterrupted spans with high load carrying capabilities.
Manufacturing Double Tees
The pre-manufacturing of the double tees shortens construction time and the material-saving shape helps to reduce dead load, with the overall design providing a stiffer member.
Double tees are generally manufactured on pretensioning beds ranging from 61 to 152m long. Beds are manufactured in the same orientation as double tees are. The stems and lower surfaces of the flange are form finished while the top surface is finished manually. The typical sizes of double tees are up to 4.6m for the flange width, up to 1.5m for the web depth and up to 24m or more for the span length.
Stressed prior to casting, prestressing tendons are situated in the forms. These tendons consist of high tensile strength multiple strand cables. Each double tee’s flange is cantilevered from the stem. Reinforcing steel is then placed in the flange to help resist tensile and flexural stresses from out-of-plane loads. The reinforcing steel also provides protection from drying shrinkage.
After the concrete cast into the forms reaches a specified strength, the tendon ends are cut, placing the stems in compression which shortens the lower section of the double tee and which results in an upward camber to counteract live loads, dead loads and creep deflection. Once the double tees are welded together the joints are treated and, if applicable, a topping added. Waterproofing membranes can also be used to protect the double tee from water penetration.
Double Tee Advantages
Double tees offer many advantages, making them suitable for construction of elevated parking decks as well as the substrate for waterproofing, roofing systems and buildings.
- Initial low project cost and minimal maintenance throughout the life of the product.
- Quicker construction process as well as faster fabrication and erection.
- Simplified frame, design and construction.
- Use of precast elements in frame is possible
- A column-free solution.
- Openings for service pipes or ducts incorporated.
- Higher durability to withstand wind, road salt, earthquakes and even fire
- Sound resistance.
- Combined with robust reinforcing steel, the depth of the legs/stems can support loads over long spans.
Applications for the Double Tee
Double tees are an ideal solution for projects such as industrial buildings, bridges, parking decks, flooring and roofing systems and more.
Modern multi-parking structures are built from precast / prestressed concrete system with the floor systems being mostly built from pre-topped double tees. The concrete is then poured at the top of the tees during the construction process to create the floor surface.
For this double tee structure, the top concrete is usually added in a factory as an integral part of the pre-casting process. The benefit of pre-topped double tees is a higher quality concrete which creates a stronger and more durable surface, significantly reducing wear by traffic.
Prefabricated bridge designs are becoming increasingly common as this practice greatly reduces construction time. The double tee structure is a great alternative for short to medium spans between 12 and 27m. Concerns around the use of double tees in bridge construction include bridge deck longitudinal cracks. The addition of back rods, steel bars, welded plates and grouts are helping to resolve these issues.
Whatever the application, double tees are spanning longer distances and performing at higher levels than ever before.
Steelmor is the leading supplier and manufacturer of stainless steel in South Africa. Give us a call today on 011 747 5700 for advice and products.