Abstract. Local and perhaps regional vernacular reinforced concrete building construction leans heavily against designing slabs with imbedded hidden beams for flooring systems in most structures including major edifices. The practice is distinctive in both framed and in shear wall structures. Hidden beams are favoured structural elements due to their many inherent features that characterize them; they save on floor height clearance; they also save on formwork, labour and material cost. Moreover, hidden beams form an acceptable aesthetic appearance that does not hinder efficient interior space partitioning. Such beams have the added advantage of clearing the way for horizontal electromechanical ductwork. However, seismic considerations, in all likelihood, are seldom seriously addressed. The mentioned structural system of shallow beams is adopted in ribbed slabs, waffle slabs and at times with solid slabs. Ribbed slabs and waffle slabs are more prone to hidden beam inclusion due to the added effective height of the concrete section. Due to the presence of a relatively high reinforcement ratio at the joints the sections at such location tend to become less ductile with unreliable contribution to spandrel force resistance. In the following study the structural influence of hidden beams within slabs is investigated. With the primary focus on a performance based analysis of such elements within a structure. This is investigated with due attention to shear wall contribution to the overall behaviour of such structures. Numerical results point in the direction that the function of hidden beams is not as adequate as desired. Therefore it is strongly believed that they are generally superfluous and maybe eliminated altogether. Conversely, shallow beams seem to render the overall seismic capacity of the structure unreliable. Since such an argument is rarely manifested within the linear analysis domain; a pushover analysis exercise is thus mandatory for behaviour prediction under strong seismic events. In such events drop beams have the edge.
Building structures with a soft storey are gaining widespread popularity in urban areas due to the scarcity of land and due to the pressing need for wide open spaces at the entrance level. In earthquake prone zones dynamic analysis based on the Equivalent Static Lateral Load method is attractive to the novice and the design codes leave the choice of the analysis procedure up to the discretion of the designer. The following is a comparison of the said method with the more elaborate Response Spectrum Method of analysis as they apply to a repertoire of different structural models. The results clearly show that the former provides similar results of response in structures with gradual change in storey stiffness; while it is over conservative for a bare frame structure. It is however less conservative for structures with a soft storey.
The inclusion of a soft storey in multistory concrete buildings is a feature gaining popularity in urban areas where land is of exorbitant cost. In earthquake prone zones, this feature has been observed in post earthquake investigations. Although engineers are prepared to accept the notion that a soft storey poses a weak link in Seismic Design, yet the idea demands better understanding. The following study illustrates the importance of the judicious distribution of shear walls. The selected building is analyzed through nine numerical models which address the behavior of framed structures. The parameters discussed include, inter alias, the fundamental period of vibration, lateral displacements, axial and shear forces. It is noticed that an abrupt change in stiffness between the soft storey and the level above is responsible for increasing the strength demand on first storey columns. Extending the elevator shafts throughout the soft storey is strongly recommended.
A widely followed practice in the local vernacular reinforced concrete construction industry is to build major edifices on levelled terrains; this is because most urban areas in Palestine enjoy hilly topography. Moreover, city planning regulations mandate, quite often, setbacks at upper floor levels. Thorough seismic analysis is a requirement imposed by local governments and since Palestine lies in an active earthquake prone region, the subject of adequate scrutiny of the present construction practices demands added attention. More often than not, building facades are primary constructed of stone with lean concrete backing, so called infill walls. At times, such walls are constructed centered at the column lines of the periphery columns or they may be constructed offset outward. Towards this objective, a repertoire of bare framing patterns are investigated along with judiciously placed masonry walls. The selected structures cover a spectrum of forms frequently adopted by architects. In four of the selected forms, the structure enjoys slight yet gradual vertical irregularity; one structure forms the basis for comparison. Furthermore, horizontal geometric symmetry is intentionally maintained in all floor plans together with square section columns; this confines attention to the impact of the general structural configuration and façade infill-wall forms on the present narrative. The comparison of the different construction systems and between the various structural numerical models includes, inter alias, the standard dynamic parameters in addition to the lateral deflection and force distribution. Standard Seismic Analysis is conducted based on the Response Spectrum Method. The present study scrutinizes the adequacy of the common construction trends and offers prudent design recommendations.
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