Comparison of Different Beam Sizes Based on Deflection and Bending Moment

Beams are the principal horizontal members of a reinforced concrete framed building. They collect loads from slabs and walls and transfer them to columns, and their size has a direct influence on the strength, stiffness, headroom, and cost of the structure. The depth of a beam in particular controls its flexural stiffness, and a small change in depth can produce a large change in deflection. In practice, beam sizes are often chosen from span-to-depth thumb rules and reused across projects without checking how the choice affects bending moment distribution, deflection, and reinforcement. The present study compares different beam sizes in an RCC frame on the basis of deflection and bending moment. A G+4 storey RCC framed building with a plan of 18 m by 12 m, typical beam span of 6 m, and storey height of 3 m is modelled in STAAD.Pro. Five beam cross sections are considered, namely 230 mm by 380 mm, 230 mm by 450 mm, 300 mm by 450 mm, 300 mm by 525 mm, and 300 mm by 600 mm, while column sizes, slab thickness, loading, and material grades of M25 and Fe500 are kept the same. Dead and live loads are applied according to IS 875 (Parts 1 and 2), and seismic loads for Zone II are applied according to IS 1893 (Part 1):2016. For each beam size the frame is analysed under limit state load combinations, and the maximum sagging and hogging bending moments, shear forces, mid-span deflections, and lateral storey displacements are recorded. The beams are designed according to IS 456:2000, and the required reinforcement and the deflection check based on span-toeffective-depth ratio are evaluated. The short-term elastic deflection from analysis is compared with the permissible limit of span divided by 250. The results show that increasing the beam depth reduces the mid-span deflection sharply, approximately in inverse proportion to the cube of the depth, while increasing the width has a much smaller effect. The bending moments change moderately with beam size, because a stiffer beam attracts a larger share of the joint moment from the columns and its self-weight increases. Deeper beams require less tensile reinforcement for the same moment, and the smallest sections fail the span-to-depth deflection check or require compression steel. The study concludes that a beam depth of about one twelfth of the span, combined with a width of 300 mm, gives a balanced solution for the building considered, satisfying deflection limits with a moderate steel percentage. The comparison demonstrates how analysis software can be used to select beam sizes rationally rather than relying on thumb rules alone.