Did You Know? Beams Has a Limit Of Slenderness!!

A simply supported or continuous CONCRETE beam shall be proportioned in such a way that it should be laterally restrained with in 60*b or 250*b*b/d which ever is less where b is width of compression face midway between the lateral restraints and d is effective depth

For a cantilever beam the clear distance from free edge to the lateral restraint shall not exceed 25b or 100b*b/d

If you take an example of a 200 x 450 continuous beam across a room of 20m, b = 200 d = 450  – 25 – 8 – 20/2 = 407 (considering 1 layer of 20 dia bar)
60*b = 12000mm
250b*b/d = 250*200*200/407 = 24570mm
This means that the beam should have a lateral restrain at 12m intervals.
Lateral restraint can be a secondary beam entering the beam which provides lateral stability.