Development and Application of Pretension Guidelines for the Z Block Modular Connection

Z Block cast steel connectors are used for intermodular and intramodular connections of steel modular buildings designed and built by modular construction company, Z Modular. These connectors use threaded steel bolts complying with the 2022 ASTM standard ASTM A574-21, Standard specification for alloy steel socket-head cap screws (ASTM A574 bolts), for mounting and connecting two adjacent modules in the construction site. Pretensioning of these bolts is required to reduce the separation between two successive building modules. However, currently there exist no design guidelines for pretensioning ASTM A574 bolts. Hence, this study was designed and executed to develop pretension guidelines for ASTM A574 steel bolts and demonstrate the application of the developed pretension guidelines. This study was completed in two phases. Phase I included experimental and analytical studies. Ten specimens comprising two different test setups, the Skidmore test and the Z Block test, were tested for the development of the pretension guidelines. Two pretension design equations were developed for the application of calibrated wrench method and the turn-of-nut method. Subsequently, in Phase II of this study, two full-scale column-to-column specimens were built and tested under axial tension load to demonstrate and validate the turn-of-nut pretension guideline developed in this study. The first specimen did not have any pretension, while specimen 2 was subjected to a pretension load in ASTM A574 bolts. This phase of the study comprised experimental and numerical methods. It was found that the failure of the full-scale column-to-column specimens occurs due to tensile rupture in the ASTM A574 bolts. Pretension in specimen 2 produced a beneficial effect of reducing the separation in the Z Block by 39%, which is substantial. However, as anticipated, the maximum tensile load capacity of the pretensioned column-to-column connection reduced though the reduction was only 0.4%.