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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 shortened.
The developments of double tee were started in the 1950s by two independent initiatives, one by Leap Associates founded by Harry Edwards in Florida, and the other by Prestressed Concrete of Colorado. They designed the wings to expand the structural channel in order to cover more area at a lower cost. In 1951, Harry Edwards and Paul Zia designed a 4-foot (1.2 m) wide prestressed double tee section. Non-prestressed double tees were constructed in Miami in 1952 followed by prestressed double tees in 1953. Separately, engineers of Prestressed Concrete of Colorado developed and constructed the first prestressed double tee which was 6-foot (1.8 m) wide called "twin tee" in late 1952. The early twin tee spans were between 20 feet (6.1 m) and 25 feet (7.6 m). Those double tee spans were first used for the first time to build a cold storage building for Beatrice Foods in Denver.
The early double tee spans of 25 feet (7.6 m) had grown to 50 feet (15 m) quickly. The Precast/Prestressed Concrete Institute (PCI) published the double tee load capacity calculation (load tables) for the first time in the PCI Design Handbook in 1971. The load tables use the code to identify double tee span type by using the width in feet, followed by "DT", followed by depth in inches, for example, 4DT14 is for 4-foot (1.2 m) wide, 14-inch (36 cm) deep double tees. In its first publication there were seven double tee types from 4DT14 to 10DT32. The list included 8DT24 that were proven to be the most popular double tee type used for 60-foot (18 m) spans for several decades. Currently, the common double tee type is 12DT30 with 4 inches (10 cm) pretopped surface on the flange. This type has been included in the PCI Design Handbook since 1999.
The first building with all pre-stressed concrete columns, beams, and double tees was 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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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 shortened.
The developments of double tee were started in the 1950s by two independent initiatives, one by Leap Associates founded by Harry Edwards in Florida, and the other by Prestressed Concrete of Colorado. They designed the wings to expand the structural channel in order to cover more area at a lower cost. In 1951, Harry Edwards and Paul Zia designed a 4-foot (1.2 m) wide prestressed double tee section. Non-prestressed double tees were constructed in Miami in 1952 followed by prestressed double tees in 1953. Separately, engineers of Prestressed Concrete of Colorado developed and constructed the first prestressed double tee which was 6-foot (1.8 m) wide called "twin tee" in late 1952. The early twin tee spans were between 20 feet (6.1 m) and 25 feet (7.6 m). Those double tee spans were first used for the first time to build a cold storage building for Beatrice Foods in Denver.
The early double tee spans of 25 feet (7.6 m) had grown to 50 feet (15 m) quickly. The Precast/Prestressed Concrete Institute (PCI) published the double tee load capacity calculation (load tables) for the first time in the PCI Design Handbook in 1971. The load tables use the code to identify double tee span type by using the width in feet, followed by "DT", followed by depth in inches, for example, 4DT14 is for 4-foot (1.2 m) wide, 14-inch (36 cm) deep double tees. In its first publication there were seven double tee types from 4DT14 to 10DT32. The list included 8DT24 that were proven to be the most popular double tee type used for 60-foot (18 m) spans for several decades. Currently, the common double tee type is 12DT30 with 4 inches (10 cm) pretopped surface on the flange. This type has been included in the PCI Design Handbook since 1999.
The first building with all pre-stressed concrete columns, beams, and double tees was 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.