Open Web Steel Joists Research Articles

Bracing of Beams, Trusses, and Joist Girders Using Open Web Steel Joists

Designers typically use joists as a part of the lateral bracing for the compression flange of beams, or for the lateral bracing of the top chord of trusses and joist girders. Joists serve as the connection to the primary members to deliver the brace force to a diaphragm or horizontal bracing system. The attachment of the joist seat to the primary member is an important element of the bracing system. Traditionally joist seats have been attached to primary members (beams, trusses, and joist girders) by welding. The OSHA Steel Erection Standard, Part 1926.757 - Open Web Steel Joists states: (i) Except for steel joists that have been pre-assembled into panels, connections of individual steel joists to steel structures in bays of 40 ft (12.2 m) or more shall be fabricated to allow for field bolting during erection; (ii) These connections shall be field-bolted unless constructability does not allow. The purpose of this paper is to discuss possible alternatives to welding the joists to the primary members in order to provide bracing to the primary members.

Strengthening Open-Web Steel Joists

The paper presents procedures and suggested details for the strengthening of open web steel joists. Strengthening of open web steel joists is often required due to the addition of roof top units, under hung conveyors, or other loading increases not contemplated in the original specification for the joists. Three basic methods of strengthening a joist or joist system are discussed: load redistribution, adding new joists or beams, and reinforcing existing joists.

Strength and Design of Open Web Steel Joists with Crimped-End Web Members

Open web steel joists are prefabricated truss-type members economical for use in floor and roof support systems for lightly loaded structures. Joists are designed in accordance with the specifications and standard load tables published by the Steel Joist Institute. An experimental study was conducted to determine strength and general flexural behavior of open web steel joists having L1 in.×1 in.×1/8 in. single angles with crimped ends for web members. Crimping the ends of the web members allows them to be aligned symmetrically with respect to the centerline of the joist, thereby eliminating the effects of eccentric loading in the web members. The purpose of the tests was to observe overall flexural behavior based on failure loads, deflections, and strains in the top and bottom chords. Outcomes from the study show that there is considerable rotational restraint in the web members at the panel points. Based on the parameters tested, an effective length factor is recommended that is less than that corresponding to a pin ended column. Finally, using the more accurate measure of rotational stiffness at the joints, revised service loads are recommended.

Composite Steel Joists

Composite steel construction has been recognized for a number of years as one of the most economical systems for constructing building floors. Three options have evolved over the years to meet the requirements for building height limitations and the need to run complex heating, ventilating, electrical, and communication systems: (1) Composite wide flange beams with web openings; (2) Stub girders; and (3) Open web steel joists and joist girders. This paper focuses on the advantages associated with composite open web steel joists, development and testing of open web steel joists, behavior of welded shear studs observed while testing composite joists, case histories of projects using composite joists, unique vibration considerations associated with joist floors, and current status of composite joist specification development.

Experiments on the Effects of Power Actuated Fasteners on the Strength of Open Web Steel Joists

A series of 10 full scale tests were conducted on roof subassemblages consisting of open web steel joists and roof deck. In these specimens the roof deck was fastened to the joists by using either 5/8-in. diameter puddle welds or by using power-actuated fasteners (PAFs). PAFs are small high strength nails pneumatically or powder driven through the roof deck into the joist top chord angles. In these tests, the joists and roof deck were loaded to failure under downward acting vertical loads. The purpose of these tests was to determine if the presence of the PAFs produced any detrimental effects on the gravity load capacity of a joist roof system. The test results showed essentially identical performance for specimens using puddle welds as for specimens using PAFs. The PAFs produced no detrimental effects on the load capacity of the joists in these tests.

FRC-Steel Joist Composite Frames for Seismic Resistance

The research reported in this paper is concerned with development of fiber-reinforced concrete-encased open web steel joist composite frames for seismic resistance. This new and innovative system completely eliminates the need for any shear connectors between steel joists and surrounding concrete as well as that for conventional longitudinal and transverse reinforcement, all of which are quite labor intensive. In the initial phase of the study tests were carried out on half-scale beam specimens subjected to monotonic and reversed cyclic loading. The results showed that the presence of steel fibers as well as the open web steel significantly enhances the shear capacity of the system thus making it very suitable for seismic resistance. In the final phase, a half-scale, one-story, one-bay frame was designed by strong column-weak beam concept. The results showed ductile and stable hysteretic behavior. The study indicates that this type of composite framing system has excellent potential in seismic regions.

Design of Steel Beams in Cantilever-Suspended-Span Construction

In cantilever-suspended-span construction, secondary framing members such as open-web steel joists are likely to enhance markedly the stability of the beams that support them. Properly made joist-beam connections enable the joists to provide both lateral and torsional restraints to the beam top flange at discrete locations. This restraint, in turn, is transmitted through the web to the bottom flange of the beam, which is in compression in negative moment regions. Therefore, the buckling modes are generally characterized by the occurrence of web distortion. Current design procedures for this very common form of construction are inadequate as they do not consider web distortion and the beneficial effect of torsional restraint. They may also neglect the detrimental effect of the height of load application above the shear center. This results in an inaccurate assessment of the lateral-torsional buckling resistance. A finite-element model, corroborated by tests, is used to develop a simplified design procedure for these beams.

Station Square revisited: distortional buckling collapse

After failure of the roof of the new Save-on-Foods store at the Station Square development in Burnaby, British Columbia, Canada, the government of British Columbia established a commissioner inquiry to investigate the causes of collapse. Collapse was attributed to an undersized W610 × 113 beam in the cantilever-suspended span arrangement and inadequate buckling resistance of the beam-column assembly. The analysis of the lateral-torsional buckling resistance of the collapsed beam in the commissioner's report did not take into account two counteracting effects: the detrimental effect of the load applied above the shear centre and the beneficial effect of the lateral and torsional restraints provided by the open-web steel joists to the collapsed beam. A distortional buckling finite element program is used herein to determine the moment resistance at buckling of the collapsed beam. This program takes into account web distortion, height of load application, inelastic behaviour, and actual restraint conditions. The moment resistance so obtained is in good agreement with the moment applied to the beam at failure. Further analyses show that even with improved restraint details at the critical beam-column location, the beam was inadequate to support the factored loads. A W610 × 195 or even a W610 × 174 beam could be considered adequate. Key words: steel beams, Station Square, lateral-torsional buckling, cantilever-suspended span, web distortion, restraints.

Dynamic testing of an open web steel joist floor

A non-composite open web steel joist warehouse floor was tested to determine its dynamic characteristics and maximum strain levels under actual operational conditions. Altogether 28 tests were carried out. The strain and acceleration information for each test was recorded on strip chart and magnetic tape recorders. The recorded data were examined and analyzed through a dual channel spectrum analyzer and the results were compared with calculated values. Good agreements are noted.

Elastic Buckling of a Column Under Varying Axial Force

When a truss or open-web steel joist is used as a frame member, parts of the bottom chord near the ends carry compressive forces and adequate bracing must be provided to prevent lateral buckling of the chord. The compression in the chord varies from panel to panel; the problem is to determine the buckling load factor for a given loading and assumed location of the bracing point. Although the problem can be solved exactly, a simple procedure is given herein which may be used in the design office for rapid, hand computation of the buckling load factor in the elastic range.

End moments in open web steel joists

A study is made of the behaviour of open web steel joists having both top and bottom chords connected to a column when the end is subjected to negative bending moments. Six tests are described, three of which examine the behaviour with typical standard connections as detailed and supplied by manufacturers; the other three involve modifications to these connections in an attempt to minimize connection eccentricity. It is shown that the eccentricities inherent in the standard connections examined can have a significant influence on the behaviour and strength of a joist, whereas if the eccentricities are eliminated, reasonably predictable behaviour, based upon the member resistances under axial loads, can be achieved. These results are examined in relation to tie joists, which are designed to be simply supported under gravity load but have the bottom chord extended and attached to a column, and also in relation to joists designed as framing members.

Uniform-load testing of open-web steel joists

A technique involving the use of an inflatable rubber bag to apply a uniform load to a single open-web steel joist, which could also be used in the testing for any similar two-dimensional structural member, is described and compared to the method of applying a dead load of known weight. The advantages of this technique over concentrated-load testing and the dead-load method are briefly discussed.

Composite Action of Concrete Slab and Open Web Joist (Without the Use of Shear Connectors)

The advantage of composite action of concrete slab and steel beam in increasing load carrying capacity and structural stiffness has been well recognized. The American Institute of Steel Construction in its 1963 Specification has given the explicit methods of analysis. However, it restricts the composite action to only fully encased beams or beams with properly designed shear connectors. In the case of concrete slab and steel open web joist, limited study of composite action has been made. As demonstrated in the Discussion of Results, composite action exists in the K-composite system of reinforced concrete slab and open web steel joist construction. This system acts compositely and therefore reduces deflections and member stresses appreciably. Comparison of data illustrates that mid-span deflections were reduced to 39.8 percent and the maximum lower chord stresses were reduced to 78.1 percent of a comparable noncomposite structure. Based on the results of these tests, corrugated sheet construction exhibits some composite action up to a lower load level, depending on the strength of the plug welds between the welding washers and the top flange of the joist.