Seismic response of CFS shear walls sheathed with nailed gypsum panels: Experimental tests

Highlights

• Monotonic and cyclic tests on panel-to-steel connections were carried out.

• The effect of assembling procedure was considered in connection tests.

• Four tests on shear walls were conducted under monotonic and cyclic loads.

• One wall was tested with all finishing materials.

• Test-based evaluated behaviour factor values range between 3.43 and 4.31.

Abstract

Among the several available building systems, constructions involving cold-formed steel (CFS) profiles represent an efficient and reliable solution. These systems are very suitable to be used in pre-fabricated modular constructions, thanks to their lightness and possibility to automate the building process. In a framework of the European project ELISSA (Energy Efficient LIghtweight-Sustainable-SAfe-Steel Construction), which was devoted to the development and demonstration of CFS modular systems, an experimental campaign aimed at investigating the seismic response of this system was carried out at University of Naples Federico II. Specifically, the studied system was a sheathing-braced CFS solution, in which the seismic resistant elements were made of CFS stud shear walls laterally braced with gypsum-based panels. The sheathing panels were attached to the CFS frame by means of ballistic nails, whereas clinching points were used for steel-to-steel connections. This paper shows the results of tests performed on shear walls and on the relevant ballistically nailed panel-to-steel connections. In particular, four full scale shear walls were tested, in which the influence of the aspect ratio, the type of loading and the effect of finishing was investigated.

Introduction

In recent years, the use cold formed steel (CFS) systems for residential low-rise building (housing) is spreading in North America, Australia and New Zealand. The reason behind the growing use of these system is their capability to ensure high structural, technological and environmental performances. In particular, their main advantages include: the high quality of products, thanks to the production in controlled environment; the economy in transportation and the easy handling due to their light weightiness; and the shorter execution times [1]. Therefore, CFS systems represent a suitable and competitive solution to the demand for low-cost high performance houses.

The structural behaviour of CFS systems, with particular reference to the seismic actions, is defined by the in-plane response of floors and walls, which can be designed by using two different approaches: “all-steel” and “sheathing-braced”. In the case of the “all-steel” approach, only steel elements are considered as part of the load-bearing structure and the lateral bracing system is usually made with flat straps. In the “sheathing-based” design approach, the bracing contribution is provided by the interaction between the steel frame and the sheathing panels [2]. In this case, the efficiency of the bracing effect provided by sheathing panels is guaranteed by the connections with the steel frame, which strongly influence the lateral/seismic response of walls.

On this topic, the University of Naples Federico II was involved in the research project named "Energy Efficient LIghtweight-Sustainable-SAfe-Steel Construction" (Project acronym: ELISSA), which was funded by European Commission under the Seven Framework Programme (www.elissaproject.eu). The project is devoted to the development and demonstration of enhanced prefabricated lightweight CFS skeleton/dry wall constructions with improved thermal, seismic and fire performance, resulting from the inherent thermal, structural efficiency and fire spread prevention properties. In particular, the University of Naples was directly involved in structural/seismic behaviour assessments. From the structural point of view, the research was focused on the seismic response of the walls sheathed with gypsum panels. The peculiarity of the investigated system was the use quick connecting systems. Clinching for connections among profiles and ballistic nails for panel-to-steel connections were selected, with the aim of optimizing the assembling operations toward a more efficient level of prefabrication. The seismic response of the system was investigated by means of a comprehensive experimental activity involving tests on connecting systems, seismic resistant system and full-scale prototype.

In the last decades, several experimental research programs studied the seismic/lateral response of CFS walls sheathed with gypsum panels and generally connected by means of self-tapping screws [3], [4], [5], [6], [7], [8], [9], [10], [11]. Serrette and Ogunfunmi [3] performed monotonic tests on walls sheathed with gypsum wallboards considering the effect of the edge distance of sheathing-to-frame connection and the combination with flat strap bracings in X configuration. The test results showed that the failure was always governed by panel connections with a low influence of the edge distance, whereas the contribution of bracings gave an increase of about 20% in strength and a reduction of permanent lateral deflection. Serrette et al. [4] performed monotonic tests on shear walls and sheathing-to-frame connections considering different sheathing types (gypsum, OSB, plywood and fiberbond). Based on test results they concluded that the strength of gypsum sheathed walls was relatively low with respect to walls sheathed with other materials. Lange and Naujoks [5] tested walls sheathed with different materials, among which gypsum wallboard and fiberboard, under the vertical and horizontal monotonic loads. They also performed tests on sheathing-to-frame connections used in wall tests. On the basis of experimental results, they developed a design procedure involving the stabilizing effect of sheathing on steel studs. Moghimi & Ronagh [6] investigated the effects of gypsum boards on the structural response of strap-braced walls subjected to cyclic actions and vertical loads. The results showed that the effect of the gypsum sheathing on strap-braced walls entailed an increase of lateral strength, which was higher than the sum of the strength obtained for strap-braced walls and gypsum sheathed walls. This especially occured for large displacement (interstorey drift higher than 1%), due to stabilizing effect against local and distortional buckling of chord studs provided by panels. Pan and Shan [7] performed monotonic tests on wall sheathed with different panel materials (gypsum, OSB and calcium silicate), considering the effect of the aspect ratio (1 and 2, evaluated as height/length) and presence of sheathing panels on one or two sides. They observed that: the gypsum boards showed smallest values of strength with respect to the other materials; the wall sheathed on one side had a strength about 50% less than the walls sheathed on both sides; and the strength of walls with an aspect ratio of 2 was about 35% less than those of the walls with aspect ratio of 1. Peck et al. [8] tested gypsum sheathed walls with different aspect ratios (1 and 2), panels orientation (parallel or perpendicular to studs) and sheathing-to-frame connection spacings under monotonic and cyclic lateral loads. The authors observed that: the aspect ratio did not affect the wall strength; the connection spacing reduction resulted in an increase of strength in wall with perpendicular oriented panels; and the effect of cyclic loads did not produce significant strength variation with respect to monotonic loads. Ye et al. [9] performed cyclic tests on walls sheathed with gypsum boards in combination with calcium silicate board or Bolivian magnesium board, whereas Wang and Ye [10] extended this research by considering the effect of RHS studs reinforced with concrete. The shear wall strength in case of concrete filled studs increased due to the restriction to the panel screws tilting produced by the concrete core, while similar strength values were observed for wall sheathed with gypsum wallboards or Bolivian magnesium boards. Accorti et al. [11] performed monotonic and cyclic tests on CFS walls having different steel bracing configurations (diagonal strap bracings, deep vertical truss at each wall ends, trussed frame bracing to incorporate a window opening). For walls braced with diagonal strap bracings and deep vertical truss at each wall ends,the effect of the presence of sheathing (gypsum fiberboard or cement boards) was also evaluated though further tests. In addition, walls braced with only sheathing panels were also tested. Finally, they carried out ancillary tests on panels and panel connections. The main conclusions were that the best performance was provided by diagonal braced walls and the sheathing can significantly contribute to the wall lateral response. Experimental activities involving the use of nails in CFS walls were carried out by Tissel [12] and Serrette and Nolan [13], on walls sheathed with OSB and plywood panels connected by means of steel pins.

On the seismic behaviour of CFS systems, many research activities were also undertaken at the University of Naples Federico II, such as experimental tests on full-scale wall prototypes and their components [14], [15], [16], [17], evaluation of behaviour factor [18] and the definition of design procedures [2], [19].

The present paper illustrates the results of the experimental activity carried out on full-scale shear walls and relevant panel-to-frame connections made with ballistic nails. Four different wall tests were carried out, in order to evaluate the influence on the wall lateral response of different parameters, such as the wall aspect ratio, the type of loading protocol and the effect of finishing materials.