Slabs with Hidden Beams, Facts and Fallacies
Keywords:Drop Beams, Hidden Beams, Flat Slabs, Seismic Behavior
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 popular in both framed and in shear wall structures. Hidden beams are favored structural elements due to the many inherent features that characterize them; they save on floor height clearance, they also save on formwork, labor and material cost. Moreover, they form an acceptable esthetic appearance that allows for efficient interior space partitioning.Â Hidden beams have the added advantage of clearing the way for horizontal electromechanical ductwork. However, seismic considerations, in all likelihood, are seldom 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.
In the following study the structural influence of hidden beams within slabs is thoroughly investigated. The investigation tackles, inter alias, deflection, bending moment distribution between beam and slab as well as its impact on relevant seismic parameters during earthquake ground excitation thus assessing the vulnerability of such structural systems. Furthermore, the following parametric study is extended to focus on medium size three reinforced concrete structures that differ in their respective flooring systems. The present study is a comparative one among various slab systems that include slabs with drop beams, ribbed slabs and solid slabs with hidden beams. Natural frequencies and Mass Participation Factors are compared; both values are fundamental for the number of characteristic modes necessary for inclusion in the analysis.
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.Â Such argument is rarely manifested within linear analysis domain; a pushover analysis exercise is thus mandatory.
ACI 318-08 (2008) Building Code Requirements for Structural Concrete and Commentary, American Concrete Institute.
Popov, E.P, Cohen, E, Koso-Thomas, K., Kasai, K. (1992) Behavior of narrow and Wide Beams, ACI Structural Journal, 89 (6). 607-16
Dominguez, D., Lopez-Almansa,F., Benavent-Climent, A., Seismic Vulnerability Analysis of RC Buildings with Wide Beams, Engineering Structures (2013), Vol. 46 pp. 687-702.
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