Beamer overprint environment altering vertical spacing of items

Question

I have a frame with an itemize environment where each \item contains an equation. Some of the equations will change, however, so I wrapped them in an overprint environment, so that one equation appears and is then replaced by another. This works just fine, but the vertical spacing of the items is not ideal, as can be seen below. Compare the space between the \phi_a equation and the next item and that of the \phi_f equation and the next item. How can this be corrected? Or does it need to be done manually with \vspace or something of the sort?

Here's the MWE:

\documentclass[utf8]{beamer}
\begin{document}
\begin{frame}
\begin{itemize}
\item A NBR 6118 define $\phi$ como composto por três parcelas:
\begin{equation*}
\phi = \phi_a + \phi_f + \phi_d
\end{equation*}
\item $\phi_a$ representa a deformação rápida;
\begin{equation*}
\phi_a = 0,8\left(1-\beta_1\left(t_0\right)\right)
\end{equation*}
\item $\phi_f$ representa a deformação lenta irreversível;
\begin{overprint}
\onslide<1>
\begin{equation*}
\phi_f = \phi_{f\infty}\left(\beta_f\left(t\right)-\beta_f\left(t_0\right)\right)
\end{equation*}
\onslide<2>
\begin{equation*}
\phi_f = \phi_{f\infty}\left(\beta_f\left(t_i\right)-\beta_f\left(t_{i-1}\right)\right)
\end{equation*}
\end{overprint}
\item $\phi_d$ representa a deformação lenta reversível.
\begin{overprint}
\onslide<1>
\begin{equation*}
\phi_d = 0,4\frac{t-t_0+20}{t-t_0+70}
\end{equation*}
\onslide<2>
\begin{equation*}
\phi_d = 0,4\left(\frac{t_i-t_0+20}{t_i-t_0+70}-\frac{t_{i-1}-t_0+20}{t_{i-1}-t_0+70}\right)
\end{equation*}
\end{overprint}
\end{itemize}
\end{frame}
\end{document}

Answer 1

overprint only copes well with fairly simple cases. I suspect that the use here does not count as sufficiently simple, at least without some sort of adjustments.

Simpler to use overlayarea for the entire itemize environment:

\documentclass[utf8]{beamer}
\begin{document}
  \begin{frame}
    \begin{overlayarea}{\linewidth}{.75\textheight}
      \begin{itemize}
        \item A NBR 6118 define $\phi$ como composto por três parcelas:
        \begin{equation*}
          \phi = \phi_a + \phi_f + \phi_d
        \end{equation*}
        \item $\phi_a$ representa a deformação rápida;
        \begin{equation*}
          \phi_a = 0,8\left(1-\beta_1\left(t_0\right)\right)
        \end{equation*}
        \item $\phi_f$ representa a deformação lenta irreversível;
          \only<1>{%
          \begin{equation*}
            \phi_f = \phi_{f\infty}\left(\beta_f\left(t\right)-\beta_f\left(t_0\right)\right)
          \end{equation*}}
        \only<2>{%
          \begin{equation*}
            \phi_f = \phi_{f\infty}\left(\beta_f\left(t_i\right)-\beta_f\left(t_{i-1}\right)\right)
          \end{equation*}}
        \item $\phi_d$ representa a deformação lenta reversível.
        \only<1>{%
          \begin{equation*}
            \phi_d = 0,4\frac{t-t_0+20}{t-t_0+70}
          \end{equation*}}
        \only<2>{%
          \begin{equation*}
            \phi_d = 0,4\left(\frac{t_i-t_0+20}{t_i-t_0+70}-\frac{t_{i-1}-t_0+20}{t_{i-1}-t_0+70}\right)
          \end{equation*}}
      \end{itemize}
    \end{overlayarea}
  \end{frame}
\end{document}