Question

Why girders are given l shaped

Answer 1

giders have i shaped cross section for strength and horizontal support of the structure.
                       hope you understood;-)

Answer 2

    I beams have very high moment of inertia  for the same volume of the given material.  So they have high stability in case of bending moments.  The two horizontal parts (called flanges) of the I beam can bear high bending and shearing stress.  
 
   That means they do not get twisted and tilted easily.   That is why they are used in girders, and as rails on the railway tracks.