Executive Summary

Special moment resisting frames (SMRF) depend on large inelastic strains in the beam-to-column connections to dissipate seismic energy and protect buildings from collapse. The ANSI / AISC 341-10 Seismic Provisions for Structural Steel Buildings define the ends of the beams in a SMRF as protected zones at the locations where large inelastic strains are expected. In the protected zone, AISC 341-10 prohibits any decking attachments that penetrate the beam flange and any welded, bolted, screwed, or shot-in attachments for other purposes. However, these restrictions are based primarily on a lack of data showing that such fasteners will not cause premature low-cycle fatigue fracture in the moment connection.

A series of full-scale beam-to-column moment connection tests were conducted to evaluate the effects of powder-actuated fasteners (PAF) applied in the protected zone on the seismic behavior of steel moment connections. The primary objective of the testing program was to determine if beam-to-column connections with deck attachments using PAF’s satisfy the SMRF qualification criteria of AISC 341-10. The test matrix consisted of twelve full-scale specimens with varying parameters to represent both common and extreme conditions. Variations in fasteners included control specimens with no fasteners, PAF at 12 in. spacing representing typical deck attachment, puddle welds at 12 in. spacing, and PAF in a dense grid over the protected zone with 1in. spacing to any edge. The powder actuated fasteners were Hilti X-ENP-19L15 type fasteners and were selected because they were some of the larger fasteners used for attachment to structural steel members. Both W24x62 and W36x150 beams were tested with flange thicknesses of 0.59 in. and 0.94 in. respectively. The test matrix included reduced beam section (RBS) connections and non-RBS connections that are considered representative of the range of currently prequalified moment connection types.

The experimental configuration consisted of one-sided (exterior) moment connections. The column was twelve feet tall to approximately simulate the distance between inflection points of two adjacent stories. The beam specimens were connected to the column using extended end plate connections to allow the reuse of the column section for all tests. A displacement protocol in accordance with the AISC 341-10 Chapter K SMRF qualification criteria was applied using an actuator connected to the cantilever end of the beam specimen at a distance of 17 ft from the centerline of the column. Additional cycles were performed at 4.7% story drift.

All twelve specimens passed the SMRF qualification requirements by maintaining 80% of the specimen nominal plastic moment strength through the first cycle of 4% story drift. Furthermore, in comparing results from specimens with and without PAF, there was negligible difference in the cyclic envelope, energy dissipation, and strength degradation prior to fracture. The general progression of limit states included yielding at the extreme fibers, spread of plasticity through the depth of the section, and local buckles formed at story drifts of 3% or larger. Ductile tears initiated during the 4% or 4.7% story drift cycles at the flange tips (common in the W24 RBS specimens), on the flange surface at the inside of a local buckle (common in the W36 RFS specimens), at the junction of the stiffener and the flange (in the W36 specimens), and at the PAF.

Fracture that caused significant loss of strength in specimens with PAF generally occurred during a similar cycle or during an earlier cycle than specimens with no fasteners, although it is noted that all fractures occurred at the 4.7% story drift level after qualification was complete. Because the difference was not sizable and the data set relatively small, it was not possible to quantitatively evaluate the difference in a statistically significant manner. Since all of the specimens including three specimens with a grid of PAF satisfied the qualification criteria, it is expected that SMRF beams in configurations such as the ones tested in this study, with PAF applied in the protected zone with 1 in. spacing to edges or welds, would satisfy SMRF qualification criteria and thus be expected to produce ductile SMRF seismic performance.

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