Stairs are an essential nonstructural system within buildings, providing egress to occupants as well as much needed access for emergency responders following an extreme event, such as an earthquake. Unfortunately, past earthquakes continue to reveal that these displacement-sensitive systems are highly vulnerable to damage and collapse. In this paper, high fidelity finite element models are developed and exercised in an effort to advance understanding of the seismic behavior of steel stairs under pseudo-static displacement loading indicative of earthquake-induced building movements. The proposed modeling approach is first validated through comparison with a set of experimental data and subsequently extended into a parametric study to broaden the range of stair configurations and details. In particular, the effect of story height, connection and landing details, and geometry on the behavior of the system is studied. Parametric analysis results indicate that the static force and displacement response of the stairs are sensitive to these key design parameters. Importantly, stair-to-buidling connections are subjected to large stress and strain demands under lateral displacement loading, as such their capability to maintain connectivity during an earthquake is crucial for robust seismic performance and hence continued functionality of the stair as a system.

Introduction

Stairs are a primary means of egress in buildings. They must remain operable following a strong-intensity earthquake and any ensuing postearthquake events to support occupant evacuation and emergency response [1,2]. Stairs typically span from floor to floor in a building and therefore are subject to multiple-support dynamic excitations induced by the building during an earthquake. However, their structural response is complex due to the variability in spatial geometry, material, and construction details. Stiff and heavy stairs may even detrimentally interact with the supporting structure and thus modify the seismic response of the supporting structure [3]. Although stairs within a building virtually perform as structural systems from the design perspective, they are often considered within the category of nonstructural components in practice. The seismic design strength of a stair system may be readily estimated using code provisions [4], however their seismic performance is more significantly dictated by the differential displacements induced by their multiple attachment points to the building. Detailing stair systems with sufficient deformability to accommodate the expected floor-to-floor seismic drifts, however, remains a challenge due to limited knowledge regarding their structural behavior under lateral loading. This is further complicated by their complex geometries and variations of specific connection details in practice. As a result, stair systems continued to suffer severe damage and even collapsed in past earthquakes (e.g., [5–۸]). Indeed, earthquake-induced damage to stairs continues to cause disruption of building functionality, delayed rescue operations, and even life safety hazards. Experimental investigations of the seismic behavior of stair systems have occurred only in a few recent efforts. These studies include pseudo-static component tests of full-height reinforced concrete straight-run stairs [9] and full-scale prefabricated steel stairs in a scissors configuration [10]. In addition, a recent shake table test program investigates the system-level seismic behavior of a prefabricated stair system installed within a full-scale building [11,12]. These studies have advanced the state of understanding regarding the seismic behavior of stair systems. Research of this kind, however, is limited in occurrence due to its tremendous cost. To complement these experimental efforts, computational studies are important as they offer a cost-effective alternative to expand experimental findings. Recent computational studies have incorporated stair systems into numerical modeling of the seismic response of buildings in an effort to understand the effect of the stair system on the building response (e.g., [13,14]).