Double-tee

A double tee or double-T beam is a load-bearing structure that resembles two T-beams connected to each other side by side. The strong bond of the flange (horizontal section) and the two webs (vertical members, also known as stems) creates a structure that is capable of withstanding high loads while having a long span. The typical sizes of double tees are up to 15 feet (4.6 m) for flange width, up to 5 feet (1.5 m) for web depth, and up to 80 feet (24 m) or more for span length. Double tees are pre-manufactured from prestressed concrete which allows construction time to be shorten.

The first application long-span double tees was on a two-story office building in Winter Haven, Florida, designed and built in 1961 by Gene Leedy. Leedy experimented when building his architectural office by using structural elements of prestressed concrete and designing the new "double-tee" structural elements. In their early days, the applications of double tees were limited to multi-story car park structures and roof structures of buildings, but they have now been used in highway structures as well.

Double tees are manufactured in factories. The process is the same as in other prestressed concrete manufacturing by building them on pretensioning beds. The beds for making double tees are of the typical sizes of the area that double tees will be used. In most cases, the lengths of the pretensioning beds are of about 200 to 500 feet (61 to 152 m) long.

In non-residential buildings, the roof structure may be flat. Structural concrete is an alternative for flat roof construction. There are three main categories for such method: precast/prestressed, cast-in-place and shell. Within the precast/prestressed concrete roofing, the double tees are the most common products used for roof span up to 60 feet (18 m).

Modern multi-story parking structures are built from precast/prestressed concrete systems. The floor systems are mostly built from pre-topped double tees. This system evolved from the earlier use of tee systems where the flanges of the T-beams were connected. The concrete is then poured at the top of the tees during the construction to create the floor surface, hence the process is called field-placed concrete topping. In double-tee structures, the top concrete is usually made at the factory as an integral part of the precast double tee structure. Double tees are connected during the construction without topping with concrete to create the parking structure floor surface.

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Wikipedia