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To what others have said, I would add the following:- 1. Beam width will generally depend upon one or more of the following factors: a. Architectural requirements, mostly requiring that beam width should not be more than the thickness of wall, to be provided above or below the beam. b. Required number of rebar layers, which in turn depend upon beam span & loading to be supported. I personally limit maximum number of rebar layers to 3, and mostly to 2. c. Seismic requirements imposed by the adopted design code. For example, ACI 318-08 requires a minimum beam width of 10 inches, for structures located within seismic zones 3 & 4 (corresponding to seismic design categories D, E & F) d. General beam detailing requirements of adopted design code. For example, according to ACI 318 a minimum beam width of 8" is required for an RC beam reinforced in flexure with 2-#8 bars & #4 stirrups. [Refer to Table A.7 in Appendix A of 'Design of Concrete Structures', 14th edition, by Nilson et al. 2010] 2. Therefore, the actual beam width would depend upon which of the above mentioned conditions governs in the case under reference. Regards.

Yes. Practically however the force or point load is resolved into rectangular components (one parallel to & the other perpendicular to the member axes), before applying to the member. Regards.

I haven't read the book by Susil Kumar, but the reason I think that he has an empirical formula is that with the increase in length of the beam, the amount of reinforcement in it would increase as well. Although there are other factors that are more sensitive to causing increase in reinforcement like load intensity but lets focus on span and reinforcement . Naturally a longer span would require more rebars and more rebars would require a wider beam. For the typical design cases we see in Bangladesh and Pakistan where there is a slab on top of beams, lateral stability can be ruled out because of beam top stability due to slab. I think the important item to highlight here is that if your beam is part of a frame, make sure the width of the beam is wide enough to provide enough development length to other frame beams that cross or terminate together with the beam in question. In order to understand my statement, please visualize edge of the frame where 2 beams meet. If the beam in question in not wide enough, the beam merging in this beam would never get enough length to develop top bars. Hope this helps.