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Executive Summary

This report presents the results of a research investigation aiming at understanding the behavior of steel connections employing both slip-critical bolts and welds in a single load sharing system. Steel connections are typically constructed with either bolts or welds. However, there are instances where it may be desirable to use combination connections with bolts and welds sharing the load. The need to combine bolts and welds may occur most commonly during the construction phase of a project when the design load changes, when there are unforeseen difficulties in make-up or matching of bolt holes, or in retrofit of existing structures. However, due to the different load-deformation behavior of the connecting elements, bolts and welds may not reach their maximum strength simultaneously. Accordingly, the nominal strength of the connection cannot be easily predicted.

The behavior of these combination connections has been investigated in literature; however, many knowledge gaps still exist. Furthermore, the influence of several critical variables on the capacity of connections with bolts and welds sharing load is not well understood. Examples of these critical variables include the effect of bolt grade, condition of faying surfaces, bolt pretensioning technique, load eccentricity, and connection size, among others. Therefore, this research report aims at quantifying the effect of these critical variables on the load-deformation behavior and capacity of concentrically and eccentrically loaded combination connections. The study presents and analyzes the results of experimental testing of 75 and 36 connections under concentric and eccentric loading conditions, respectively.

Based on the investigation results, it was found that when slip-critical bolts are combined with welds in a double-shear single load sharing system under concentric loading (e.g., double shear tension splice), the resulting connection experiences an increase in the ultimate load-carrying capacity compared to that of the bolted- or welded-only connection. The load-deformation behavior of the bolted- and welded-only connections can be used to characterize the behavior of the combination connection. In addition, the ultimate deformation at fracture experienced by the welds is significantly higher in the combination connections compared to welded-only connections. Furthermore, connections combining welds with slip-critical bolts installed on blast-cleaned faying surface (i.e., Class B) experienced significant improvement in ductility compared to their bolted-only counterparts. These bolted-only connections under concentric loading slipped suddenly at their ultimate capacity with some losing more than 50% of their capacity at this slip event. Finally, it was found that the current AISC model for concentrically loaded connections may overestimate the capacity of the connection at the slip limit of the bolts; accordingly, modifications are proposed to the current AISC equation to better predict the capacity of combination bolted and welded connections under concentric loading.

Similar improvement in the capacity was achieved in combination connections subjected to eccentric loading; however, to compute the capacity of these combination connections, it is recommended to use the instantaneous center of rotation considering all connecting elements to be in a single load sharing system with their appropriate load-deformation behavior