Why Replace Pinned Support with Roller Guide for Qualitative Influence Lines?

[fonseh]

Homework Statement

For the highlighted part , why we need to replace the support at A with a roller guide ? We could see at A , it's a pinned support , right ?

Homework Equations

The Attempt at a Solution

Pinned support will resist horizontal and vertical reaction , why the author said it will not resist vertical force ? Is the author wrong ? [/B]

 

[CivilSigma]

I think you've completely missed the idea of constructing qualitative influence lines.
The trick is that to do so, you must replace the support at A with a roller, it's written in the text...