Steel deck and open web steel joist combination is a roof and floor framing system widely used in every kind of buildings.  The most efficient application is in warehouses, distribution centers and super stores, where the repetitive configuration of bay sizes takes most advantage of the nature of joists – mass production.  In North America, this system is very popular in virtually all kinds of commercial, industrial, and institutional buildings.  In residential sector, this system finds its applications in condominiums.  The marriage between steel joists and steel deck is a good match making best use of their advantages.  Today, most steel joist fabricators produce steel deck too, and steel joists and deck are often bid and shipped in a single package. 

Steel Joists and Joist Girders                                                                               

The first edition of Standard Specifications for Steel Joists was adopted by Steel Joist Institute (SJI) in 1928.  The definition of steel joist in this edition was: “Any steel beam or truss shaped steel member suitable for supporting floors and roofs when used for floor filling between the main supporting girders, beams or walls, shall be known as a steel joist.  Such joists may be made of rolled shapes or strips of sheet steel, round bars, angles or specially rolled bars riveted or welded together, or by expanding rolled shapes, or by any other method complying with the requirements of this article.”  This edition covered two types of joists: “(a) Steel joists having solid webs shall be designed as beams;” and “ (b) Steel joists built up of bars or other sections and those fabricated by expanding rolled sections shall be designed as trusses.”

In 1933, 1941 and 1946 revisions of the specifications, the definition of steel joist was changed as: “Any steel member suitable for supporting floors and roof s between the main supporting girders, trusses, beams or walls when used as hereinafter specified shall be known as a ‘steel joist’. Such steel joists may be made of hot or cold formed sections, strip or sheet steel, riveted or welded together, or by expanding.”  Both beam type and truss type were covered by these revisions.

The 1949 revision kept the same definition of steel joist, but dropped the solid web, beam type joists from its scope and changed the name of the specifications as “Standard Specifications for Open Web Steel Joist Construction.”  Since then, the term “steel joist” in SJI domain has been implicitly interpreted as Open Web Steel Joist (OWSJ).  OWSJ is also frequently referred to as bar joist.

The first load table for OWSJ was adopted by SJI in 1929.  In this very first version of load table, the joist span was limited to 32 feet, and depth to 16 inches.  In 1952, the depth was expanded to 20 inches, and span to 40 feet.

The introduction of longspan steel joists in 1953 is a conceptual breakthrough to the traditional open web steel joists.  The maximum span for this kind of light weight steel structure was expanded to 96 feet, with depth ranging from 18 inches up to 48 inches. Today, these longspan steel joists are designated as LH-series joists. In 1970, in response to the demand for longer span and higher capacity, the deep longspan steel joists were introduced.  The maximum span was expanded to 144 feet, with depth ranging from 52 inches to 72 inches.  These deep longspan steel joists were designated as DLH-series, and this designation remains the same to date.

The introduction of joist girders in 1978 marked yet another milestone for steel joists.  The SJI Standard Specifications for Joist Girders brought steel joists into the domain of primary structural members.  Joist girders are designed to support secondary structural components like open web steel joists, longspan steel joists, etc.  Joist girders can be designed either as simply supported members; or as members with continuity forming rigid frames.

In the mean time, the capacity of “traditional” open web joists kept growing.  In 1959, the span was increased to 48 feet, and depth to 24 inches.  In 1972, maximum span was expanded to 60 feet, with depth ranging from 8 inches to 30 inches. These joists are known today as K-series open web steel joists.  In 1994, KCS joists were introduced as a part of the K-series Specification in response to the need for a joist with a constant moment and constant shear.

Today, steel joists are much more versatile and sophisticated than initially invented.  The boundary between joists and structural truss is no longer a clear cut.  The profile of joists has been expanded from traditional parallel chords to various geometries, such as scissor, arched chards, bow string, and gable, etc.  The maximum span has been expanded from initial 32 feet to 144 feet in SJI load tables.  Some joist manufacturers have further extended standard load tables.  For example, Nucor Vulcraft’s super longspan SLH-series joists have a maximum span 240 feet in the load table.  Even longer spans are possible through careful engineering and fabrication.

Steel Deck

A steel deck unit is a corrugated panel cold formed from sheet steel.  Steel deck is widely used in roof framing system to support finished roofing materials and to resist wind uplift load.  Steel deck is also widely used in floor framing system.  There are two types of floor deck applications: form deck and composite deck.  Form deck serves as a permanent form for concrete slab.  Composite deck serves dual purposes: during the construction stage, the deck serves as a form for wet concrete; after the concrete has hardened, the composite deck is bonded to concrete and becomes positive reinforcement for concrete slab.

In addition to its primary structural function to support gravity loads, steel deck also provides lateral structural capacities.  First, steel deck provides lateral support for steel joist top chord, eliminating lateral braces needed for compressive stability; second, steel deck is frequently designed to act as horizontal shear diaphragm, with the steel deck forming the web, interior steel joists forming the web stiffeners, and the perimeter structural members on all four sides forming the flanges of the diaphragm.  This shear diaphragm may be used to transfer wind and seismic loads to lateral load resisting components, replacing part or all of conventional structural bracing systems.

Since 1939, the Steel Deck Institute (SDI) has provided uniform industry standards for the engineering, design, manufacture, and field usage of steel decks.  SDI members are manufacturers of steel floor and roof decks.  Associate members are manufacturers of products used in the production or erection of steel decks.  Continuing SDI functions include preparation, review, and distribution of literature, referral of inquiries to appropriate sources, coordination of research and testing, and liaison with other construction industry associations on matters of common interest.

For engineers and architects steel deck offers design flexibility.  The system can be used for flat, pitched or arched construction on almost all type of building.  There are varieties of steel deck available from the deck fabricators in various depths and rib spacing, with or without stiffening elements, with or without acoustical material, cellular and non-cellular, and in varying material thicknesses.  Most deck fabricators have detailing staff working on the projects, and the deck sheets are supplied cut to length per job specifications.  The versatility and extensive choices make steel deck applicable to a wide range of projects.

Steel deck is made from light gage sheet steel.  The properties of steel are used with maximum efficiency in the deck design and fabrication, resulting in economical system with a high strength-to-weight ratio.  Steel deck has an attractive appearance and can be painted with different colors.  When exposed to view, steel deck is esthetically pleasing.  Steel deck can be erected in most weather conditions with fairly fast speed.  Because of its light weight nature, steel deck can be easily carried and laid by workmen, resulting in fast jobsite handling.  Once installed, the deck immediately provides a reliable surface for the continuing work of all tradesmen, and also protects workmen on lower levels.  In normal built environment, properly protected steel deck won’t rust or decay, exhibiting excellent durability and requiring minimum maintenance.  Steel deck is non-combustible building material. Most steel deck products are fire rated.