When it comes to constructing buildings and structures, the materials and designs used play a crucial role in ensuring strength, stability, and functionality. One key element often employed in modern construction is the Open Web Steel Joist (OWSJ). In this blog post, we’ll delve into the definition of OWSJs, their significance in steel structure, and explore the various types available.
Defining Open Web Steel Joists (OWSJ)
Open Web Steel Joists, often abbreviated as OWSJ’s, are structural elements commonly used in building construction. In structural engineering, the open web steel joist (OWSJ) is a lightweight steel truss consisting, in the standard form, of parallel chords and a triangulated web system, proportioned to span between bearing points.
They are designed to provide support for floors and roofs, distributing the weight of the structure and any applied loads evenly across the supporting framework. OWSJ’s are particularly valued for their efficiency, versatility, and cost-effectiveness.
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Significance of Open Web Steel Joist in Construction
OWSJs offer a range of benefits that make them a preferred choice for many construction projects:
- Lightweight Yet Strong: OWSJs are engineered to be lightweight while maintaining a high degree of strength. This makes them ideal for supporting heavy loads without adding excessive weight to the overall structure.
- Open Design: The “open web” design of these joists consists of a series of interlocking triangular shapes. This design reduces the overall weight of the joist, allows for easy passage of mechanical systems, and simplifies installation.
- Flexibility: OWSJs come in a variety of sizes and shapes, offering flexibility in design. They can be customized to meet the specific load-bearing requirements of a project.
- Cost Efficiency: Due to their lightweight design and efficient use of materials, OWSJs are often cost-effective, helping to reduce overall construction expenses.
- Ease of Installation: The open design of OWSJs makes them relatively easy to install, potentially saving time and labor costs during construction.
Types of OWSJs
Steel Joists are standardized regarding depths, spans, and load-carrying capacities.
K-Series Joists:
K-Series are light in weight – they possess an exceptionally high strength-to-weight ratio in comparison with other building materials. These are among the most common steel joists and are known for their versatility. Coupled with their low price per pound, they contribute significantly to lower building costs.
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| Fig: K series joist |
K-Series Joists are standardized regarding depths, spans, and load-carrying capacities. There are 63 separate designations in the Load Tables, representing joist depths from 10 inches (254mm) through 30 inches (762 mm) in 2-inch (51 mm) increments and spans through 60 feet (18,288 mm).
Standard K-series joists have a 2 ½” (64 mm) end bearing depth so that, regardless of the overall joist depths, the tops of the joists lie in the same plane. Seat depths deeper than 2 ½” (64 mm) can also be specified.
Designation: 12K4
12 – Depth in inches
K – Joist series
4 – Designation within series (the higher the number, the heavier the joist)
LH-Series Joists:
LH-Series joists are designed to support larger spans and heavier loads. They are often used in applications where the need for longer spans is a priority.
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| Fig: LH and DLH series joist |
Long span series joists have been standardized in depths from 18 inches (457 mm) through 48 inches (1219 mm), for spans through 96 feet (29,260 mm).
DLH-Series Joists:
DLH-Series joists are a variation of the LH-Series, designed for even heavier loads and longer spans. They are ideal for projects that demand high load-bearing capabilities.
Deep long span series joists have been standardized in depths from 52 inches (1321 mm) through 120 inches (3048 mm), for spans up through 240 feet (73,152 mm).
The depth of the bearing seat at the ends of underslung LH- and DLH- Series Long Span Joists has been established at 5 inches (127 mm) for chord section number 2 through 17. A bearing seat depth of 7 ½” (191 mm) has been established for the DLH Series chord section number 18 through 25.
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| Fig: Steel Joist configuration |
Long Span and Deep Long Span steel joists can be furnished with either under-slung or square ends, with parallel chords or with single or double pitched top chords to provide sufficient slope for roof drainage. Square end joists are primarily intended for bottom chord bearing. Sloped parallel-chord joists shall use span as defined by the length along the slope. The joist designation is determined by its nominal depth at the center of the span and by the chord size designation.
Joist Girders:
Joist Girders are open web steel trusses used as primary framing members. They are designed as simple spans supporting equally spaced concentrated loads for a floor or roof system. These concentrated loads are considered to act at the panel points of the Joist Girders. Joist Girders have been designed to allow for a growing need for longer span primary members, coupled with a need for more efficient steel usage.
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| Fig: Girder joist |
Joist Girders are furnished with underslung ends and bottom chord extensions. The standard depth at the bearing ends has been established at 7 ½” (191 mm) for all Joist Girders. Joist Girders are usually attached to the columns by bolting with two 3/4-inch diameter (19 mm) A307 bolts.
A loose connection of the bottom chord to the column or other support is recommended during erection in order to stabilize the bottom chord laterally and to help brace the Joist Girder against possible overturning.
Top chord widths for Open Web Steel Joist
| Sr.no | Joist Type | Top Chord Widths |
| 1 | K-series | 4” to 5” wide** |
| 2 | LH-series | 5”to 7” wide** |
| 3 | Girders | 11”to 13” wide** |
NOTE – Actual seat widths and top chord widths will not be available until after final design is complete. Steel detailer can request actual width from joist supplier through fabricator.
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OSHA standards for Open Web Steel Joist
A vertical stabilizer plate shall be provided on each column for steel joists. The stabilizer plate shall be a minimum of 6 inch by 6 inch (152 mm by 152 mm) and shall extend at least 3 inches (76 mm) below the bottom chord of the joist with a 13⁄16 inch (21 mm) hole to provide an attachment point for guying or plumbing cables.
Except for steel joists that have been pre-assembled into panels, connections of individual steel joists to steel structures in bays of 40 feet (12.2 m) or more shall be fabricated to allow for field bolting during erection.
Conclusion
In conclusion, the main function of an Open Web Steel Joists (OWSJ) is to provide direct support for roof or floor deck and to transfer the load imposed on the deck to the structural frame i.e., beam and column. In order to accurately design an OWSJ, engineers consider the joist span between bearing points, joist spacing, slope, live loads, dead loads, collateral loads, seismic loads, wind uplift, deflection criteria and maximum joist depth allowed. Many steel joist manufacturers supply economical load tables in order to allow designers to select the most efficient joist sizes for their projects.