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Diagram of double tee beam

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.[1]

History

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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.[2] 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.[3]

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.[3]

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.[4]

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.[1]

Manufacturing process

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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.[1]

Applications

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Roofing

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Double-tee roof structure of an indoor swimming pool

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).[5]

Parking structures

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Precast parking structure showing an interior column which supports two girders, left and right. Double-tee beams hang onto the girders.

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.[6]

A benefit of pre-topped double tees is a higher quality concrete for more durable surface to reduce traffic wears. Factories can produce the topping with minimum concrete strength of 5,000 psi. In some areas, the strength can be 6,000-8,000 psi. This compares to the field-placed concrete topping with the lower concrete strength of 4,000 psi.[6]

Typically, the double-tees are hung over a supporting structure. This is done by having dapped ends at the webs of the double tee (pictured). The dapped ends are sensitive to cracking at the supporting area. A recommendation to prevent cracking is to include reinforcing steel in the double-tee design to transfer the loads from the bearing area (the reduced-depth section) to the full-depth section of the web.[6] In case that the cracks are developed after the parking structure is already in use, other methods to provide external support to the double-tees are needed. One of such alternatives is to use externally bonded carbon fiber reinforced polymer (FRP) to provide reinforcement.[7]

Bridges

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The first NEXT Beam bridge, in York, Maine, which uses four double tees to form a bridge span

Prefabricated bridge designs have been used in many bridge constructions to reduce the construction time. In the United States, there are efforts to come up with Prefabricated Bridge Elements and Systems in many states. Double tee structure is an alternative for short to medium spans between 40 and 90 feet (12 and 27 m). There are many standards such as double-tee beam of Texas Department of Transportation and the Northeast Extreme Tee (NEXT) Beam of the Northeast.[8]

A benefit of using double tees for bridge replacements is to shorten the construction time. Texas has a goal of shortening short-span bridge replacements to one month or less instead of 6 months in traditional bridge constructions.[9]

NEXT Beam development started in 2006 by the Precast/Prestressed Concrete Institute (PCI) North East to update regional standard on Accelerated Bridge Construction (ABC). The NEXT Beam design was inspired by double-tee designs that have been used to build railroad platform slabs. The use of double tees with wide flange permits fewer beams and to have them stay in place to form the deck, resulting in a shorter construction time. The first design was introduced in 2008 called "NEXT F" with 4-inch (10 cm) flange thickness requires 4-inch (10 cm) topping. This was used for the construction of the Maine State Route 103 bridge that crosses the York River. The seven-span 510-foot (160 m) long bridge was completed in 2010 as the first NEXT Beam bridge. The second design was introduced in 2010 for Sibley Pond Bridge at the border of Canaan and Pittsfield, Maine. The design was called "NEXT D" with 8-inch (20 cm) flange thickness that does not require deck topping, allowing the wearing surface to be applied directly on to the beams. The combination of F and D called "NEXT E" was introduced in 2016.[10][11]

Concerns of using double tees in bridge constructions include bridge deck longitudinal cracks. As the connection points between the double tee beams are longitudinally along the traffic flow, any lateral movements of double tees can cause the road surface to crack longitudinally. These include differential rotation of double-tee flanges that can cause asphalt surface to raise or crack. A separation of the flanges can cause asphalt to sag into the gap forming a reflective crack. To reduce these problems, many methods have been developed to manage the lateral connections of the double tees. The materials used in the connections are backer rods, steel bars, welded plates, and grouts.[12]

Walls

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A double-tee retaining wall
Warehouse/office building with double tee wall panels

Double tees have been used in vertical load-bearing members such as exterior walls,[3] and retaining walls.[13]

When using load-bearing double tee wall panels, it can significantly reduce construction time as a large area of walls can be covered in a short amount of time. Using load-bearing double tee wall panels in conjunction with double tee roof can reduce the amount of interior columns because double tee roof members can have long spans and the ends are connected to double tee walls to transfer the loads. Additionally, the ceiling can be raised higher as double tee wall members can have long spans also. This is suitable for warehouses as a large area with high ceiling is needed but without windows. This type of construction has been used since the 1970s.[14] Precast Prestressed Concrete Institute included double tee wall panels in its PCI Design Handbook between 1971 and 2010.[15]

References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Double tee

View on Grokipedia
from Grokipedia
A double tee, also known as a double-T beam, is a precast, prestressed concrete structural member characterized by a wide top flange connected to two parallel stems or webs, forming an efficient cross-section that resembles two inverted T-beams joined side by side. This design allows for long spans, typically up to 100 feet, while maintaining relatively lightweight construction and high load-bearing capacity, making it a staple in modern building systems. The double tee originated during the early , with the first design—a 4-foot-wide by 12-inch-deep unit—developed by engineers Harry Edwards and Paul Zia in 1951 and produced commercially in 1953. It evolved from earlier precast forms like channel sections and ribbed slabs, driven by advancements in that enabled greater spans and efficiency; by the late , spans reached 80 feet, and widths expanded to 16 feet by the 1970s through standardization by the Precast/ Institute (PCI). Key innovations include variations like the Northeast Extreme Tee (NEXT) beam for bridges and the Mega-Tee for wider applications, with ongoing developments incorporating high-strength concrete and larger prestressing strands to push spans toward 160 feet. Recent advancements as of 2025 include applications in for modular efficiency and with carbon-fiber reinforced (CFRP) grids for improved durability. In terms of design, double tees feature prestressing strands embedded in the stems for compression, a flange thickness of 2 to 4 inches (often topped with additional concrete in the field), and depths ranging from 18 to 48 inches, depending on load requirements. They are manufactured off-site under controlled conditions, ensuring quality and allowing for rapid on-site erection, which reduces construction time compared to cast-in-place methods. Double tees are widely applied in structures, where their inverted orientation supports vehicles over multiple levels; roofing and systems for commercial buildings like offices, warehouses, and gymnasiums; and bridge components such as girders and pedestrian walkways. They excel in environments requiring fire resistance (up to 4-hour ratings based on flange thickness) and against , with minimal maintenance needs due to the protective over prestressing elements. Among their primary advantages are from fewer components and optimized material use, enhanced stability during handling and compared to single tees, and versatility for both horizontal and vertical load-bearing roles. Additionally, they provide cleaner interior spaces by allowing mechanical systems to pass through the webs rather than below the structure, and their lower profile reduces overall building height relative to alternatives.

Description

Components

A double tee is a precast, composed of two parallel vertical stems, or webs, connected by a wide horizontal top , forming a akin to two T-beams joined side by side at their tops. The stems serve as the primary load-bearing components, transferring vertical loads from the flange to supports below while resisting and shear forces through their depth and prestressing. These vertical elements are typically 4 to 6 inches wide—tapering from about 6 inches at the top to 4 inches at the bottom—and range from 12 to 60 inches deep, with provided by multiple high-strength prestressing strands embedded longitudinally. The flange functions as the horizontal deck surface, distributing loads across the stems and providing the flooring or roofing plane in building applications. It is usually 4 to 8 inches thick—often comprising a 2- to 4-inch precast portion with additional field-applied topping—and spans 8 to 16 feet wide, incorporating mild reinforcing steel to handle tensile stresses and control cracking. At the junctions where the stems meet the , haunches or localized thickenings are incorporated to enhance stress distribution and prevent concentration of forces in these critical areas. The used in double tees achieves a typical of 4,000 to 6,000 psi at 28 days, enabling efficient load resistance in precast form. The prestressing consists of low-relaxation strands with a yield strength of 270 , tensioned before placement to induce compressive stresses that counteract service loads.

Types and Dimensions

Double tees are classified into several types based on their configuration and intended application. Standard double tees, featuring two prestressed stems supporting a wide top , are primarily used for and roof systems in buildings such as structures and commercial facilities, providing efficient long-span support. Inverted double tees, where the stems extend downward from the flange, are adapted for bridge girders and shallow beam systems, offering enhanced stability for cast-in-place toppings in transportation . Hollow-core variants incorporate voids within the stems to reduce self-weight while maintaining structural integrity, suitable for applications requiring lighter members without sacrificing span capability. Standard dimensions for double tees vary to accommodate diverse requirements, with overall lengths typically ranging from 20 to 120 feet, flange widths from 8 to 16 feet, and stem depths from 24 to 60 inches. A representative example is the 8DT48 configuration, denoting an 8-foot flange width and 48-inch stem depth, commonly used for spans up to 80 feet in parking garage applications. In the United States, the Precast/Prestressed Concrete Institute (PCI) provides guidelines for double tee design and fabrication, including standardized section properties and load tables that support spans up to 100 feet for configurations with 8-foot flange widths and typical thicknesses of 2 to 4 inches. Custom variations enhance adaptability, such as lightweight double tees produced with lightweight aggregates to reduce dead load and accelerate construction in high-rise projects. Additionally, those incorporating ultra-high-performance concrete (UHPC) enable longer spans exceeding 100 feet by improving tensile strength and durability, often minimizing reinforcement needs. Per PCI and ACI standards, net deflection under service loads is limited to span/360, with prestress-induced camber designed to counteract expected downward deflections for levelness and compatibility with adjacent members.

History

Origins and Development

The development of the precast, double tee emerged in the early as part of the broader post-World War II push for efficient, standardized building components in the United States, building on advancements in techniques pioneered by engineers like Eugène Freyssinet in the 1930s and introduced to through Gustave Magnel's work, including the 1950 Walnut Lane Memorial Bridge in . The double tee's conceptual roots lay in adapting single and channel sections for longer spans and , addressing the need for rapid, economical amid and demands. Initial designs focused on a 4-foot-wide by 12-inch-deep configuration using pretensioned strands, enabling spans starting at 25 feet. The first double tee was designed in 1951 by structural engineers Harry Edwards and Paul Zia in , with production beginning in 1953 at a plant in that state; independently, a similar design was developed in late in by Nat Sachter, George Hanson, Jack Perlmutter, Leonard Perlmutter, and Michael Atenberg. These early efforts were spurred by the limitations of and the advantages of precasting for and speed, though initial challenges included underdeveloped high-strength concrete and strand technology, restricting spans to under 40 feet and complicating lifting and transportation with available equipment. Edwards played a pivotal role in advocating for the component, co-founding the Precast/Prestressed Concrete Institute (PCI) in 1954 to standardize designs and promote industry growth. By the late 1950s, refinements in prestressing allowed spans to extend to 50 feet, with double tee depths increasing to 24 inches and widths to 8 feet, facilitating broader in low-rise buildings. The introduction of long-line casting beds in the early further enabled efficient production of units over 60 feet and up to 80 feet, overcoming earlier issues through continuous prestressing along extended forms. These advancements solidified the double tee as a versatile element for and systems, emphasizing stem-flange integration for optimal load distribution.

Adoption and Evolution

Following the initial development in the , double tees experienced rapid adoption in the United States for commercial buildings and structures, driven by their efficient pretensioning and the expansion of the , which facilitated faster erection times compared to cast-in-place alternatives. By the late , double tees had become a staple in precast , with spans evolving from 25 feet to 50 feet, enabling broader application in multi-story facilities where they remain the most common component today. Key technological evolutions in the included the introduction of higher strengths and deeper sections—up to 30 inches—which supported consistent application of longer spans exceeding 80 feet in various designs and, by the late , some reaching over 100 feet. Post-1994 Northridge earthquake observations of diaphragm failures in precast structures prompted significant seismic enhancements, including new provisions in the 1997 Uniform and 1999 ACI 318 for improved topping slab diaphragms and connection detailing to better distribute seismic forces. The Precast/ Institute's first Design Handbook in 1971 further standardized double tee dimensions and load tables, promoting consistent industry-wide implementation. Globally, double tees spread to Europe in the 1960s through variants like TT-beams, which incorporated flange-supported details for simplified erection in flooring systems. By the 2000s, adoption extended to Asian high-rise construction, where precast systems addressed rapid urbanization demands for efficient, long-span floors. In recent decades, has influenced double tee evolution, with PCI guidelines in the 2020s incorporating recycled aggregates and high-strength concretes (over 5,000 psi) to reduce environmental impact while maintaining . Since around 2010, integration with (BIM) software has streamlined design through improved interoperability standards like IFC 2x3, enabling precise coordination of precast elements. More recent advancements as of 2025 include the use of ultra-high-performance concrete (UHPC) to optimize double tee flanges for greater and spans up to 160 feet, as well as new connection designs enhancing seismic resilience in precast structures.

Design Principles

Structural Mechanics

The stems of a double tee primarily resist shear forces and moments through the axial compression provided by prestressing strands, which are typically tensioned in the stems to induce an upward camber and counteract tensile stresses under load. The top , acting as a wide compression zone, distributes uniform distributed loads across the member's width and provides the structural topping or finish surface for or systems. This configuration allows double tees to efficiently span long distances, such as 40 to , while maintaining composite behavior when topped with additional . Bending stresses in double tees are analyzed using the standard formula for members: σ=MyI\sigma = \frac{My}{I} where σ\sigma is the bending stress, MM is the applied moment, yy is the distance from the to the of , and II is the gross of the section. Prestressing is applied as initial force Pi=ApsfpiP_i = A_{ps} f_{pi} at transfer, where ApsA_{ps} is the area of prestressing and fpi≤0.94fpuf_{pi} \leq 0.94 f_{pu} for low-relaxation strands (ACI 318-22 Section 20.3.2.3), with effective prestress Pe=ApsfpeP_e = A_{ps} f_{pe} after losses, where fpef_{pe} is typically around 0.6 fpuf_{pu}. Service-level tensile stresses are limited in design practice to 12fc′12 \sqrt{f_c'}
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