## 1 Introduction
Besides, it has been pointed out that, specifically for soft soils in the proximity of an active fault in Mexico City, the effects of the vertical component of ground motion should be incorporated in codified design, [PERSON] et al. [10]. Other studies including the vertical component are given in [11; 12; 13; 14; 15; 16; 17], and some others related to bridge engineering with the vertical component are [18; 19; 20; 21]. The effects, damage and possible collapse of structures under the action of the vertical component of ground motion due to earthquakes have been reported in the literature. For instance, the contribution of the vertical component to the response of structures is indicated by [PERSON] et al. [22], who found that the demand due to this component can be significant. The observed evidence, after large seismic events, of structural damage induced by the vertical seismic component is reported for the 1994 Northridge, California and the 1995 Kobe, Japan historical earthquakes [22]. More recently, similar observations are also reported for the 2009 L'Aquila, Italy earthquake [22]; it was indicated that the axial load in reinforced concrete columns was very large (more noticeably in compression), if both components were considered in the seismic analysis. It was stated that both components should be incorporated in the evaluation of the response of building frames, more markedly in the near-fault cases. Compressions in columns above the balance load, as well as tensions in isolation systems and columns (over the balanced load too) have also been reported for buildings ([PERSON] et al. [23]) when the horizontal and vertical components of near-source earthquakes are considered. Other study related to seismically isolated systems subjected to seismic excitations from horizontal and vertical components found that including the latter is of key importance to designing pendulum isolators for two-degree of freedom systems ([PERSON] et al. [24]). In addition, regarding base-isolated structures (irregular ones with friction pendulum), it has been found that when the vertical component is significant (near-source sites), uplift could occur if the effect of the vertical component is not accounted for ([PERSON] et al. [25]). Collapse-prone frame structural types under the action of the seismic vertical component have been identified in terms of the ground motion characteristics, when the structures are subjected to multi-directional excitations ([PERSON] et al. [26]). The Housner intensity (Housner [27]) can be used to assess the influence of the vertical seismic component in the collapse of structures, especially structures more vulnerable to such component, as buildings with many cantilevered members and buildings which columns are subjected to large load effects. Therefore, investigating the impact of including the earthquake ground motion vertical component in the earthquake resistant design and analysis of buildings is needed. In this study, the seismic response of steel buildings subjected to horizontal and vertical ground motions during Mexican earthquakes is presented. The main objective of this study is to assess the impact of including the vertical component of essentially near-fault earthquake ground motions in the performance level of steel beams, by carrying out non-linear dynamic analyses on steel-frame buildings. Comparisons are given against the analysis by considering only the horizontal component, including the case of a steel frame designed by an experienced practicing engineer.