Consider the following statements:
$$1.$$ On a principal plane, only normal stress acts
$$2.$$ On a principal plane, both normal and shear stresses act
$$3.$$ On a principal plane, only shear stress acts
$$4.$$ Isotropic state of stress is independent of frame of reference.

Which of the above statements is/are correct ?

In the theory of plastic bending of beams, the ratio of plastic moment to yield moment is called

The degree of static indeterminacy of a rigidly jointed frame in a horizontal plane and subjected to vertical loads only, as shown in figure below is

The value of lateral friction or side friction used in the design of horizontal curve as per India Roads Congress guidelines is